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Studies of the oxidativedegradation of butyl rubber
in tyre inner tube
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• A Doctoral Thesis. Submitted in partial fulfillment of the requirementsfor the award of Doctor of Philosophy of Loughborough University.
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Publisher: c© E.A. Hatam
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STUDIES OF THE OXIDATIVE DEGRADATION
OF BUTYL RUBBER IN TYRE INNER TUBES
by
EKBAL AWAD HATAM, BSc
A Doctoral Thesis submitted in partial fulfilment
of the requirements for the award of Doctor of Philosophy of
Loughborough University of Technology
December 1984
Supervisor: Claude Hepburn, BSc, MSc(Cantab), ANCRT, FRSC, FRPI, PhD
Institute of Polymer Technology
® by Ekbal Awad Hatam, 1984
This thesis is respectfuZZy dedicated to
MY PARENTS
to whom I owe so much
1
" ACKNOWLEDGEMENTS
The author wishes to express her deep and sincere gratitude to Dr C Hepburn for his friendly and considerate guidance, advice and willingness to assist at any time throughout the preparation of this thesis.
The author acknowledges also with gratitude the Iraqi Government - especially the-State Enterprise for Rubber Industries - for their financial support to complete this project.
The author wishes to present her sincere gratitude and thanks to her parents, brothers and sisters for their patience and moral encouragement.
Acknowledgements are also presented to the following people:
- Technicians and staff of the Institute of Polymer Technology
- Mr Waad S Yousif who kindly offered instruction on the use of the computer system and provided advice in the preparation of the graphics presented in this thesis
- Dunlop International Projects Ltd for undertaking the oil analysis in their laboratory
- Mrs Janet Smith who typed this thesis with great efficiency.
ii
" ABSTRACT
Tyre inner tubes made of butyl rubber have been found to have poor heat resistance when used in the environment of Iraq. Research investigations were undertaken to establish the causes of the
problem: these consisted of examining the individual contributions of oxidative scission processes in both rubber hydrocarbon and the various processing oils. The principal method used to monitor the degradation processes was ageing in hot air using the reduction in
strength for the rubber and change in chemical composition of the
processing oils.
This preliminary study established that the processing oil made in Iraq was found to be of different composition from the common European rubber processing oils and contributed considerably to the poor life of the butyl inner tubes. Analysis of the oil from Iraq identified the presence of trace amounts of copper (3 ppm) known, from the literature, to adversely affect the resistance of many rubbers to elevated temperatures and, as found in this work, to also degrade butyl rubber.
Replacement of the butyl rubber by chlorobutyl rubber overcame the problem of inner tube degradation and an antioxidant system based on a combination of an acetone/diphenylamine (ADPA) plus mercaptobenzimi- dazole (MBI) with MgO was found effective in minimising chlorobutyl inner tube degradation. However, unexpectedly, the research also established that trace quantities of copper were useful as stabilisers in enhancing the heat ageing properties of chlorobutyl rubber and the addition of a particular copper salt (copper sulphatel in small proportions (3 ppm) as an anti-degradant was found beneficial.
111
Another method studied to improve the resistance of halogenated butyl rubber to heat induced oxidative degradation tried to use ZnO as the curative, as recommended by the manufacturers of chloro- butyl rubber; however it was found that with this technique higher degradation was obtained in chlorobutyl than with the standard ZnO/
sulphurless curing system (i. e. ZnO/TMTD in proportions 5: 1 phr).
It was concluded from these studies that:
1. Reducing the oil level in butyl inner tubes from 25 phr to 15 phr enhances heat ageing properties of the rubber. Factory trials confirmed this finding.
2. Adding a copper inhibitor (ZDC) in the proportions 1-2 phr in butyl inner tubes was found successful in minimising oxidative degradation.
3. The existence of elemental copper in a*chlorobutyl inner tube rubber formulation in the proportion 3 ppm improved the rubber properties at elevated temperatures.
iv
" CONTENTS
Page No
CHAPTER 1: A REVIEW OF POLYMER DEGRADATION AND AGEING 1 1.1 Polymer Degradation ..... ..... 1
1.1.1 Historical Note ..... 1 1.1.2 Introduction ..... ..... 2 1.1.3 Modes of Polymer Degradation 3
1.1.3.1 Historical note on thermal oxidative degradation 3 1.1.3.1.1 Thermal degra-
dation 4 1.1.3.2 Mechanical degradation 6 1.1.3.3 Oxidative degradation 7 1.1.3.4 Ultrasonic degradation 10 1.1.3.5 Photodegradation 10 1.1.3.6 Biological degradation 11 1.1.3.7 Ozone degradation ' 11 1.1.3.8 Chemical degradation 14 1.1.3.9 Mechanical fatigue degra-
dation - Wohler curves 14 1.1.3.10 Degradation by dehaloge-
nation ..... 15 1.2 Ageing and Weathering of Rubber..... 17
1.2.1 Introduction ..... ..... 17 1.2.2 Classification of Oxidative Ageing
Process ..... ..... 17 1.2.2.1 Shelf ageing or (normal)
oxidation ..... 18 1.2.2.2 Metallic poisoning 19 1.2.2.3 Heat ageing ..... 20 1.2.2.4 Light ageing ..... 20 1.2.2.5 Flex cracking ..... 21 1.2.2.6 Atmospheric cracking 22
V
Page No
1.2.2.7 Humidity ageing..... 22
1.2.3 Methods of Assessing Ageing 23
1.2.3.1 Air-oven test ..... 25
1.2.3.2 Oxygen bomb test ... 25
1.2.3.3 Air bomb test ..... 26
1.2.3.4 Ozone test ..... 26
1.2.3.5 Cell oven test ..... 27
1.2.3.6 Stress relaxation and creep test .....
27
1.2.3.7 Spectroscopic test 28
1.2.3.8 Humidity and steam test 29
1.2.3.9 Light ageing test 29
1.2.3.10 Swelling in solvents test 30
CHAPTER 2: A REVIEW OF BUTYL RUBBER TYPES DEGRADATION 31
2.1 Butyl and Halogenated Butyl Rubber Degra- Mechanisms ..... ..... dation 31
2.1.1 Butyl Rubber Degradation..... 31
2.1.1.1 Thermo-oxidative degradation 31
2.1.1.2 Photo degradation 35
2.1.1.3 Ozone degradation 36
2.1.2 Halogenated Butyl Rubber Degradation 39
2.1.2.1 Thermo-oxidative degradation 39
2.1.2.2 Ozone degradation 41
2.2 Methods of Assessing Ageing of Butyl and Halogenated Butyl Rubber .....
43
2.2.1 Butyl Rubber ..... ..... 43
2.2.1.1 Air, steam and oxygen oven 43 ageing ..... 2.2.1.2 Stress relaxation 52
2.2.1.3 Swelling in solvents 60
2.2.2 Halogenated Butyl Rubber 64
2.2.2.1 Air, steam and oxygen ageing .....
64
2.2.2.2 Stress relaxation 70
2.2.2.3 Swelling in solvents 70
vi
Page No.
CHAPTER 3: COMPOUND PREPARATION AND TESTING 77
3.1 Introduction ..... ..... 77
3.2 Materials ..... ..... 77
3.2.1 Rubber ..... ..... 77
3.2.2 Compounding Ingredients ..... 77
3.2.2.1 Curatives ..... 77
3.2.2.2 Carbon black ..... 77
3.2.2.3 Processing oil ..... 77
3.2.2.4 Other compounding Ingre- dients .....
7°
3.3 Preparation of Test Specimens .:... 80
3.3.1 Mixing of Rubber Compound 80
3.3.2 Mixing Cycle ..... sofoo 80
3.4 Testing of Unvulcanised and Vulcanised Compounds ..... .....
81
3.5 Experimental ..... ..... 82
3.6 Results and Discussion .... 82
3.6.1 Vulcanisation Characteristics of Unvulcanised Compound .....
82
3.6.2 Stress-Strain Properties 83
CHAPTER 4: HOT AIR AGEING PERFORMANCE OF BUTYL AND HALOGENATED BUTYL VULCANISATES .....
98
4.1 Introduction ..... ..... 98
4.2 Hot Air-Oven Ageing ..... ..... 99
4.2.1 Significance ..... ..... 99
4.2.2 Experimental ..... ..... 99
4.2.3 Results and Discussion ..... 99
4.2.4 Conclusions ..... ..... 102
4.3 Stress Relaxation ..... ..... 122
4.3.1 Basic Principles ..... 122
4.3.2 Experimental ..... ..... 122
4.3.3 Results and Discussion ..... 126
4.3.4 Conclusions ..... ..... 130
vii
Page No 4.4 Determination of Development of Vulcanisate
Acidity due to Heat Ageing (Using the Ace- tone Extraction Technique) ..... 132 4.4.1 Introduc tion ..... .... 132 4.4.2 Experimental ..... ..... 132 4.4.3 Results and Discussion .....
151
4.5 Factory Trials Tyre Inner Tubes 161
4.5.1 Butyl Polysar 301 Inner Tubes 161
4.5.1.1 Introduction ..... 161
4.5.1.2 Compounding ..... 161
4.5.1.3 Results and Discussion 162
4.5.1.3.1 Unvulcanised Properties 162
4.5.1.3.2 Vulcanisate Characteristics 162
4.5.1.3.3 Stress-strain properties 162
4.5.1.3.4 Hot air-oven ageing 164
4.5.1.3.5 Tube life - wheel test 165
4.5.2 Chlorobutyl Inner Tubes ..... 167
4.5.2.1 Introduction ..... 167
4.5.2.2 Compounding ..... 168
4.5.2.3 Results and Discussion 168
4.5.2.3.1 Hot air-oven ageing 168
4.5.2.3.2 Tube life - wheel test 170
4.6 Conclusions ..... ..... 171
CHAPTER 5: INVESTIGATION OF HEAT RESISTANT CROSSLINKING SYSTEMS IN CHLOROBUTYL RUBBER ..... 172
5.1 Introduction ..... ..... 172 5.2 Experimental ..... ..... 173
5.2.1 Results and Discussion ..... 174 5.2.1.1 Curing Characteristics of
the Unvulcanised Compounds 174
viii
Page No
5.2.1.2 Compound Physical Properties 176 5.2.1.3 Hot-air Ageing ..... 177 5.2.1.4 Swelling in Solvent 179
5.2.1.4.1 Basic principles 179 5.2.1.4.2 Results and dis-
cussion 180 5.2.1.5 Determination of Vulcanisate
Acidity After Heat Ageing (Using the Acetone Extrac- tion Technique) 182
5.3 Conclusions 0 .... ..... 183
CHAPTER 6: INVESTIGATION OF USE OF ANTIOXIDANT IN PROTECTING HALOGENATED BUTYL RUBBER 184
6.1 Literature Review ..... ..... 184
6.2 Mechanism of Antioxidant Protection 184
6.3 Experimental ..... ..... 189
6.3.1 Results and Discussion ..... 190
6.3.1.1 Unvulcanised and Vulca- nised Properties 190
6.3.1.2 Thermal Oxidation Process 192
6.3.1.2.1 Hot-air ageing 192
6.3.1.2.2 Stress relaxation 193
6.3.1.2.3 Swelling in solvent 196
6.3.1.2.4 Determination of vulcanisate acidity development after hot-air ageing (using acetone extraction technique) 198
6.3.1.3 Dehydrochlorination Process 201
6.3.1.3.1 Introduction 201
A: Measurement of Hydrogen Chloride Evolved 201
A-1 Experimental 201
A-1.1 Results and 203 iscussion A-1.1.1 easurement of
HCl evolved 203
ix
Page No
A-1.1.2 Compound's Physical properties after dehydrochlorination process 204
A-1.1.3 Swelling in solvent 205 A-1.1.4 Determination of
vulcanisates acidity development after dehydrochlorination process (nitrogen atmosphere) using the acetone extraction technique 208
B: Observation of the colour changes which occurred as a result of the dehydrochlorination process 209
B-1 Experimental 209
B-1.1 Compound preparation 209
B-1.2 Measurement of changing the colour 211
6.4 Conclusions ,,,, ,,,,, 215
CHAPTER 7: INVESTIGATION OF RUBBER PROCESSING OILS CONSTITUENTS 226
7.1 Introduction ... ... 226
7.2 Changes in Processing Oil Composition Due to Heat Ageing ..... ..... .....
226
7.3 The Action of the Copper on the Heat Ageing 227 Properties of Rubber ..... ..... .....
7.3.1 Literature Review ..... ..... 227
7.3.2 Compound Preparation ..... :.... 229
7.3.2.1 Butyl Rubber ..... ..... 230
7.3.2.1.1 The effect on the heat ageing properties of using a copper inhibitor in butyl rubber compound 231
1. EDTA's copper inhibitor action 233
1.1 Curing characteristics of 233 unvulcanised compound 1.2 Hot air oven ageing 234
X
Page No
2. ZDC's copper inhibitor action 234 2.1 Unvulcanised and vulcanised
properties 234 2.2 Stress-strain properties 238 2.3 Hot-air ageing 239
-2.4 Stress relaxation test 241 7; 3.2.2 Chlorobutyl Rubber ..... 241
7.3.2.2.1 Heat ageing properties 241 7.3.2.2.2 Stress relaxation 243
7.4 Conclusions ..... ..... ..... 246
CHAPTER 8: GENERAL CONCLUSIONS AND RECOMMENDATIONS FOR FURTHER WORK .:... ..:.. .....
254
8.1 Introduction ..... ..... ..... 254
8.2 Conclusions ..... ..... ..... 254
8.2.1 Effect of Variation of Oil Level and Source on the Heat Resistance of the Butyl Rubber Types ..... .....
254
8.2.2 Enhancing the Compound's Heat Resistance 255
8.2.2.1 Effect of the vulcanisation system on the heat resistance of CIIR 256
8.2.2.2 The beneficial effects obtained by using antioxidants in chloro- butyl rubber subjected to ageing at high temperatures .....
256
8.2.2.3 Effect of the rubber processing oil's composition and origin on rubber ageing ..... .....
257
8.3 Recommendations for Further Work ..... ..... 257
APPENDICES: 259
Appendix I: A: Testing of Unvulcanised Rubber ..... 259
1. Curing characteristics ..... ..... 259
2. Viscosity ..... ..... 261
3. Mooney scorch ..... ..... 262
4. Vulcanisation ..... ..... 262
xi
Page No B: Testing of Vulcanisates ..... ..... 264
1. Physical Testing .:... ..;.. 264
1.1 Tensile Strength ..... ..... 264
1.2 Elongation at Break ..,.. 265
1.3 Modulus at a Given Tensile Strain 265
1.4 Tear Strength ..... ;.... 266
1.5 Hardness ;:... ..... 266
1.6 Compression Set at Constant Strain (25%) 266
Appendix II: Nomenclature of the Polymers and Ingredients 268
Appendix III: Protective Mechanism of MBTS as a Peroxide Decomposer Type Antioxidant .:... .....
272
References ..... ..... ..... 274
xii
LIST OF FIGURES
Figure No Page No
2.1 Rate of oxygen absorption by various rubber types at 130°C ..... ..... 32
2.2 Absorption of oxygen by SBR and IIR in the presence and absence of light at 100°C 37
2.3 Effect of vulcanisation time and polymer unsaturation on ozone rate of IIR. 38
2.4 Effect of plasticizer proportion on ozone resistance of IIR vulcanisates ..... 38
2.5 Effect of carbon black on ozone resistance of IIR vulcanisate ..... ..... 39
2.6 Variation of ozone resistance with cure system forCIIR ..... ..... 42
2.7 Influence of the curative system on IIR inner tubes compound ..... ..... 48
2.8 Effect of ZnO and sulphur on IIR vulcani- sates stability when exposed to dry and wet heat ..... ..... 50
2.9 Appearance of thin gum vulcanisate after ageing at 1500C ..... ..... 51
2.10 Continuous relaxation curves for extended periods for various rubber types at 130°C, 50% extension ..... ..... 53
2.11 Permanent set as a function of time of IIR gum vulcanisate at 130°C, 50% extension 53
2.12 Permanent set as a function of time of an IIR filled tread vulcanisate at 1300C and 50% extension ..... ..... 54
2.13 Stress relaxation of sulphur and non-sulphur cures of four elastomers at different chain structures at 130°C ..... ..... 55
xiii
Figure No Page No
2.14 Continuous stress relaxation of IIR at 140°C in N2 and 02 ..... ..... 57
2.15 Stress relaxation of SBR and NR vulcani- sates at 120°C and 50% elongation 58
2.16 Stress relaxation of IIR vulcanisate at 120°C and 50% extension ..... 58
2.17 Stress relaxation in compression and tension versus deformation ..... 59
2.18 Stress relaxation in compression versus time ..... ..... 59
2.19 Effect of initial state of cure of sulphur vulcanisates on air oven ageing behaviour 61
2.20 Effect of unsaturation on air ageing of sulphur cured butyl ..... ..... 61
2.21 Effect of unsaturation on air ageing of IIR cured with p-quinone dioxime ..... 62
2.22 Effect of unsaturation on network density of resin cured IIR during air ageing at 1770C ..... ..... 62
2.23 Types of scission during air ageing of IIR vulcanisates cure systems (sulphur, quinoid, resin) ..... ..... 62
2.24 Variation of volume swelling of IIR vulcani- sates with different types of accelerators cured at 1210C, 1490C, 177°C and 205°C with respect to time ..... ..... 63
2.25 CIIR-sulphur donor curing and NR-sulphur cure heat resistant vulcanisation aged at 1210C for different periods of time ..... 65
2.26 CIIR-ZnO cure and IIR-sulphur cured heat resistant compound cured 8 minutes at 165°C and aged at different conditions ..... 66
2.27 Comparative heat resistance of HAF black filled CIIR compounds, as shown by tensile strength and elongation retention after air ageing 16 hours at 1930C ..... ..... 67
xiv
Figure No Page No
2.28 Modified CIIR-ZnO cure heat resistant vulcani- sate cured 8 minutes at 165°C, aged at different temperatures ..... .....
68
2.29 Effect of TCBQ level on CUR heat resistance compound, cured 8 minutes at 165°C and aged 96 hours at 176°C ..... .....
68
2.30 Effect of ZnO on CUR vulcanisate heat ageing, aged 8-96 hours at 1760C in a hot-circulating
69 air oven ..... ..... 2.31 Continuous stress relaxation of IIR and CUR
extended 50% at 25°C ..... ..... 75
2.32 Continuous stress rglaxation in IIR and CUR extended 50% at 100 C ..... .....
75
2.33 Average molecular weight of chain between cross- links ..... ..... 76
3.1 Vulcanisation characteristics (ODR trace) of IIR, CIIR and BIIR compounded without oil, cured at 1710C, 30 arc ..... .....
88
3.2 Vulcanisation characteristics (ODR trace) of IIR, CIIR and BIIR compounded with 5 phr Esso oil cured at 171°C, 30 arc ..... 89
3.3 Vulcanisation characteristics (ODR trace) of IIR, CIIR and BIIR compounded with 5 phr Iraqi oil cured at 1710C, 30 arc .....
89
3.4 Vulcanisation characteristics (ODR trace) of IIR, CIIR and BIIR compounded with 10 phr Esso oil, cured at 171°C, 3arc .....
90
3.5 Vulcanisation characteristics (ODR) of IIR, CIIR and BIIR compounded with 10 phr Iraqi oil, cured at 1710C, 30 arc .....
90
3.6 Vulcanisation characteristics (ODR) of IIR, CUR and BIIR compounded with 15 phr Esso oil, cured at 1710C, 30 arc .....
91
3.7 Vulcanisation characteristics (ODR1 of IIR, CIIR and BIIR compound8d with 15 phr Iraqi oil, cured at 171°C, 3 arc .....
91
3.8 Vulcanisation characteristics (ODR) of IIR, CUR and BIIR compounded with Esso oil (at level as indicated), cured at 171°C, 30 arc 92
xv
Figure'No Page No
3.9 Vulcanisation characteristics (ODR trace) of IIR, CIIR and BIER compounded with Iraqi oil (at level as indicated), cured at 171°C, 3° arc 92
3.10 Physical properties of butyl rubber Polysar 301 compounded with Esso and Iraqi oil 95
3.11 Physical properties of chlorobutyl rubber HT1066 compounded with Esso and Iraqi oil 96
3.12 Physical properties of bromobutyl rubber X2 compounded with Esso and Iraqi oil 97
4.1 Wallace extension stress-relaxometer 124
4.2 Block diagram of stress-relaxati on apparatus 125
4.3 Continuous stress relaxation of IIR with Iraqi oil at 1250C in air ..... .....
133
4.4 Continuous stress relaxation of IIR with Esso oil at 1250C in air ..... .....
133
4.5 Continuous stress relaxation of IIR with Iraqi oil at 125°C in nitrogen.... .....
133
4.6 Continuous stress relaxation of IIR with Esso oil at 125°C in nitrogen.... .....
133
4.7 Continuous stress relaxation of CIIR with Iraqi oil at 125°C in air ..... .....
134
4.8 Continuous stress relaxation of CIIR with Esso oil at 125°C in air ..... .....
134
4.9 Continuous stress relaxation of CIIR with Iraqi oil at 125°C in nitrogen .....
134
4.10 Continuous stress relaxation of CUR with Esso oil at 125°C in nitrogen .....
134
4.11 Continuous stress relaxation of BIIR with Iraqi oil at 125°C in air ..... .....
135
4.12 Continuous stress relaxation of BIIR with Esso oil at 1250C in air ..... .....
135
4.13 Continuous stress relaxation of BIIR with Iraqi oil at 125°C in nitrogen .....
135
xvi
Figure'No Pace No
4.14 Continuous stress relaxation of BIIR with Esso oil at 125°C in nitrogen ..... 135
4.15 Intermittent stress relaxation of IIR with. Iraqi oil at 1250C in air ..... 136
4.16 Intermittent st8ess relaxation of IIR with Esso oil at 125 C in air ..... 136
4.17 Intermittent stress relaxation of IIR with Iraqi oil at 125°C in nitrogen ..... 136
4.18 Intermittent stress relaxation of IIR with Esso oil at 1250C in nitrogen ..... 136
4.19 Intermittent stress relaxation of CUR with Iraqi oil at 125°C in air ..... 137
4.20 Intermittent stress relaxation of CUR with Esso oil at 125°C in air ..... 137
4.21 Intermittent stress relaxation of CIIR with Iraqi oil at 125°C in nitrogen ..... 137
4.22 Intermittent stress relaxation of CIIR with Esso oil at 125°C in nitrogen ..... 137
4.23 Intermittent stress relaxation of BIIR with Iraqi oil at 125°C in air ..... 138
4.24 Intermittent st; ess relaxation of BIIR with Esso oil at 125 C in air ..... 138
4.25 Intermittent stress relaxation of BIIR with Iraqi oil at 125°C in nitrogen ..... 138
4.26 Intermittent stress relaxation of BIIR with Esso oil at 1250C in nitrogen ..... 138
4.27 Calculated crosslink formation of IIR with Iraqi oil at 1250C in air ..... 139'
4.28 Calculated c rosslink formation of IIR with Esso oil at 125°C in air ..... 139
4.29 Calculated c rosslink formation of IIR with Iraqi oil at 125°C in nitrogen ..... 139
xvii
Figure No Page No
4.30 Calculated crosslink formation of IIR with Esso oil at 1250C in nitrogen .....
139
4.31 Calculated crosslink formation of CUR with. Iraqi oil at 125°C in air, .....
140
4.32 Calculated crosslink formation of CUR with Esso oil at 125°C in air .....
140
4.33 Calculated crosslink formation of CUR with Iraqi oil at 1250C in nitrogen .....
140
4.34 Calculated crosslink formation of CUR with Esso oil at 125°C in nitrogen .....
140
4.35 Calculated crosslink formation of BIIR with Iraqi oil at 1250C in air .....
141
4.36 Calculated crosslink formation of BIIR with Esso oil at 125°C in air .....
141
4.37 Calculated crosslink formation of BIIR with Iraqi oil at 125°C in nitrogen .....
141
4.38 Calculated crosslink formation of BIIR with Esso oil at 125°C in nitrogen .....
141
4.39 Continuous stress relaxation of IIR with Iraqi oil at 150°C in air ..... .....
142
4.40 Continuous stress relaxation of IIR with Esso oil at 150°C in air ..... .....
142
4.41 Continuous stress relaxation of IIR with Iraqi oil at 150°C in nitrogen .....
142
4.42 Continuous stress relaxation of IIR with Esso oil at 1500C in nitrogen .....
142
4.43 Continuous stress relaxation of CIIR with Iraqi oil at 150°C in air ..... .....
143
4.44 Continuous stress relaxation of CIIR-with Esso oil at 150°C in air ..... .....
143
4.45 Continuous stress relaxation of CUR with Iraqi oil at 1500C in nitrogen .....
143
xviii
Figure No Page No
4.46 Continuous stress relaxation of ° CIIR with Esso
C in nitrogen oil at 150 ..... 143
4.47 Continuous stress relaxation of BIIR with Iraqi oil at 150°C in air ..... .....
144
4.48 Continuous stress relaxation of BIIR with Esso oil at 1500C in air ..... .....
144
4.49 Continuous stress relaxation of BIIR with Iraqi oil at 150°C in nitrogen .....
144
4.50 Continuous stress relaxation of BIIR with Esso oil at 1500C in nitrogen ,....
144
4.51 Intermittent stress relaxation of IIR with Iraqi oil at 1500C in air ..... .....
145
4.52 Intermittent stress relaxation of IIR with Esso oil at 150°C in air ..... .....
145
4.53 Intermittent stress relaxation of IIR with Iraqi oil at 150°C in nitrogen ,....
145
4.54 Intermittent stress relaxation of IIR with Esso oil at 150°C in nitrogen .....
145
4.55 Intermittent stress relaxation of CIIR with Iraqi oil at 1500C in air ..... .....
146
4.56 Intermittent stress relaxation of CUR with Esso oil at 1500C in air ..... .....
146
4.57 Intermittent stress relaxation of CUR with Iraqi oil at 1500C in nitrogen .....
146
4.58 Intermittent stress relaxation of CUR with Esso oil at 150°C in nitrogen .....
146
4.59 Intermittent stress relaxation of BIIR with Iraqi oil at 150°C in air ..... .....
147
4.60 Intermittent stress relaxation of BIIR with Esso oil at 1500C in air ..... .....
147
4.61 Intermittent stress relaxation of BIIR with Iraqi oil at 1500C in nitrogen .....
147
xix
Figure No Page No
4.62 Intermittent stress relaxation of BIIR with Esso oil at 150°C in nitrogen ..... 147
4.63 Calculated crosslink formation of IIR with Iraqi oil at 150°C in air ..... ..... 148
4.64 Calculated crosslink formation of IIR with Esso 148 -oil at 150°C in air ..... .....
4.65 Calculated crosslink formation of IIR with Iraqi oil at 150°C in nitrogen ..... 148
4.66 Calculated crosslink formation of IIR with Esso oil at 1500C in nitrogen ..... 148
4.67 Calculated crosslink formation of CUR with Iraqi oil at 150°C in air ..... ..... 149
4.68 Calculated crosslink formation of CUR with Esso oil at 1500C in air ..... ..... 149
4.69 Calculated crosslink formation of CUR with Iraqi oil at 150°C in nitrogen ..... 149
4.70 Calculated crosslink formation of CUR with Esso oil at 150°C in nitrogen ..... 149
4.71 Calculated crosslink formation of BIIR with Iraqi oil at 150°C in air ..... ..... 150
4.72 Calculated crosslink formation of BIIR with Esso oil at 150°C in air ..... ..... 150
4.73 Calculated crosslink formation of BIIR with Iraqi oil at 150°C in nitrogen ..... 150
4.74 Calculated crosslink formation of BIIR with Esso oil at 1500C in nitrogen ..... 150
4.75 Acidity % of unaged and aged IIR, CUR and BIIR with Esso and Iraqi oils, after ageing 3,7 and 14 days at 125°C ..... ..... 153
4.76 Reaction scheme for the combination of sulphur with polyisoprene during vulcanisation 156
4.77 Chlorobutyl rubber reaction with ZnC12 159
4.78 Curing characteristics of a factory and laboratory inner tube compound ..... ..... 163
xx
Figure' No
5.1 Vulcanisation characteristics (ODR tracel of CIIR,. cured with. (ii different ZnO levels, (Ti) ZnO/TMTD system, cured at 171°C, 3° and
6.1 Vulcanisation characteristics (ODR trace) of CIIR. vulcanisates with and without antioxidant cured at 171°C, 30 arc ..... . .....
6.2 Continuous stress relaxation of CUR vulcani- sates with and without antioxidant at different temperatures.. in air ..... .....
6.3 Crosslink density of CIIR vulcanisates with and without antioxidant after ageing in air at temperatures of 1750C and 2000C .....
Page No
175
191
195
197
6.4 Acidity % of CIIR vulcanisates with and without antioxidant after ageing in air at temperatures of 175°C and 200°C ..... ..... 200
6.5 Apparatus used to measure dehydrochlorination by means of the HCl evolved from the CUR com- pound with and without antioxidant after heat ageing in nitrogen at temperatures of 175°C and 200°C ..... ..... 202
6.6 Crosslink density of CUR vulcanisates with and without antioxidant after dehydrochlorination in nitrogen at temperatures of 175°C and 2000C 206
6.7 Acidity % of CIIR vulcanisates with and without antioxidant after dehydrochlorination in nitro- gen at 175°C and 200°C ..... ..... 210
6.8 Crosslink density changes of CIIR vulcanisates without antioxidant after (i) ageing at 175°C and 2000C in air, (ii) dehydrochlorination in nitrogen at 1750C and 200°C ..... 217
6.9 Crosslink density of CIIR vulcanisates with antioxidant after (i) ageing at 175°C and 200°C in air, (ii) dehydrochlorination in nitrogen at 175°C and 200°C ..... 218
6.10 Acidity % of CIIR vulcanisates with antioxidant after (i) ageing at 175°C and 200°C in air, (ii) dehydrochlorination in nitrogen at 175°C and 2000C ..... ..... 219
xxi
Figüre'No" Page No
6.11 Acidity % of CIIR vulcanisates without anti- oxidant after (i ageing at 175°C and 200°C in air, (ii) dehydrochiori. nation in nitrogen at 175°C and 2000C ..... ..... 220
7.1 Continuous stress relaxation of a butyl rubber tyre inner tube compound contaminated with copper (3 ppml aged in hot air at 1500C and 100% extension ..... .....
232
7.2 Vulcanisation characteristics (ODR trace) of IIR vulcanisates compounded with EDTA as a copper inhibitor cured at 171°C, 3° arc 235
7.3 Effect of ZDC as a copper inhibitor on the vulcanisation characteristics (ODR trace) of IIR inner tube compound, cured at 171°C, 30 arc 236
7.4 Influence of ZDC as a copper inhibitor on the heat ageing properties of a butyl rubber tyre inner tube compound aged at 15000 and 100% extension ..... .....
242
7.5 Continuous stress relaxation of chlorobutyl rubber containing different percentages of copper (1500C and 100% extension) 245
7.6A In-situ generation of the antidegradant ZDC from TMTD during vulcanisation .....
247
7.6B Protective action of the copper ion on the heat stability of CUR vulcanisates .....
253
8.1 Summary of the research investigation 2 58/1
xxii
LIST OF TABLES
Table No Page No
1.1 Bond dissociation energies of various single bonds ..... ..... ..... ..... 8
2.1 High temperature ageing resin cured IIR 45
2.2 A comparison of vulcanisation systems on the heat ageing of IIR ..... ..... ..... 47
2.3 Comparison of heat resistant inner-tubes compound ..... ..... ..... ..... 49
2.4 CUR heat resistant inner tubes ..... 69
2.5 Formulations, reference compounds ..... 71
2.6 Heat resistance of common rubbers ..... 72
2.7 Evaluation of various curing systems in BIIR heat resistant vulcanisates ..... ..... 73
2.8 Evaluation of antioxidants in BIIR heat resis- tant vulcanisates ..... ..... ..... 74
3.1 Specific properties of IIR, CUR and BIIR 78
3.2 Typical properties of the processing oils used ..... ..... ..... ..... 79
3.3 Basic compound formulation used for heat resistant tyre inner tubes ..... ..... 80
3.4 Properties of vulcanised rubber compounds of IIR, CIIR and BIIR mixed with (1) Esso oil Flexon 845,, (2) Iraqi - paraffinic oil 86
3.5 Physical properties of butyl Polysar 301, chlorobutyl HT-1066 and Bromobutyl X2 compounds with Iraqi paraffinic oil ..... ..... 93
3.6 Physical properties of butyl Polysar 301, compounds chlorobutyl HT-1066 and bromobutyl X 2
with Esso Flexon 845 oil ..... ..... 94
4.1 Heat ageing properties of butyl Polysar 301 rubber with Iraqi paraffinic oil ..... 104
xxiii
Table No Page No
4.2 Heat ageing properties of butyl Polysar 301 rubber with Esso Flexon 845 oil ..... 107
4.3 Heat ageing properties of chlorobutyl HT-1066 rubber with Iraqi paraffinic oil ..... 110
4.4 Heat ageing properties of chlorobutyl HT-1066 rubber with Esso Flexon 845 oil ..... 113
4.5 Heat ageing properties of bromobutyl X2 rubber with Iraqi paraffinic oil ..... ..... 116
4.6 Heat ageing properties of bromobutyl X2 rubber with Esso Flexon 845 oil ..... ..... 119
4.7 Inner tube compounds formulation used in acetone extraction ..... ..... ..... 152
4.8 IIR inner tube factory trial compound formula- tion - ..... ..... ..... ..... 161
4.9 Unvulcanised properties of factory and labor- atory inner tube compounds ..... ..... 162
4.10 Physical properties of a factory trial IIR inner tubes compound with laboratory produced inner tube compound ..... ..... 164
4.11 Physical properties of IIR mixed in the labora- tory and mixed in the factory compared with the properties of the finished inner tubes. Ageing at 125°C in hot air ..... ..... 166
4.12 Butyl inner tubes wheel life testing result (truck type) ..... ..... ..... 167
4.13 CUR inner tube compound formulation used in the factory trial ..... ..... ..... 168
4.14 Physical properties of unaged and ag ed factory mixed CUR inner tubes, specimens cu t from cured inner tubes made in Iraq ..... ..... 169
4.15 CUR inner tubes wheel life testing result (passenger type). ..... ..... ..... 170
5.1 CIIR heat resistant tyre inner tube compound based on different curing systems ..... 174
xxiv
Table No Page No
5.2 Effect of curing system on the physical properties of CUR inner tube compound ..... 176
5.3 Percentage losses in physical properties of ZnO and ZnO/TMTD cured CIIR vulcanisates after 3 days ageing at 1500C and one day at 1650C 178
5.4 Crosslink density and swollen volume of unaged and aged samples of CUR cured with different levels of ZnO, compared with CUR cured with the ZnO/TMTD (sulphurless) curing system 181
5.5 pH value and acidity percentage of unaged and aged CUR cured with different levels of ZnO1 compared with CUR cured with ZnO/TMTD .....
183
6.1 Antioxidants and their evaluation in heat resistant BIIR vulcanisates ..... .....
185
6.2 Effect of TMTD-MBTS ratio on high temperature resistance of CIIR vulcanisates in the presence of the antiodixant 2246 .... ....
186
6.3 Antioxidant CUR black compound formulation 190
6.4 Effect of the antioxidant on unvulcanised properties of CUR compound ..... .....
192
6.5 Physical properties of CUR vulcanisates with and without antioxidant ..... .....
193
6.6 Percentage change in physical properties of CUR compound with 8nd without antioxidant after ageing at 175 C and 200 C in air .....
194
6.7 Effect of the antioxidant on the swollen volume and crosslink density of CUR vulcanisates after ageing in hot air at 175°C and 200°C 198
6.8 pH values of unaged and aged (in hot air) of , CIIR vulcanisates with and without antioxi- dants, after acetone extraction .....
199
6.9 Influence of antioxidant in CIIR vulcanisates on HC1 evolved after dehydrochlorination In nitrogen at temperatures of 175°C and 200ýC 204
6.10 Percentage change in physical properties of CUR vulcanisates after dehydrochlorination at 1750C and 200°C in nitrogen .....
207
xxv
Table No. Paae No
6.11 Effect of the antioxidant on the swollen volume and crosslink density of CUR vulcanisate after dehydrochlorination in nitrogen at 175 C and 2000C ..... ..... 208
6.12 pH value of CUR vulcanisates with and without antioxidant after dehydrochlorination in nitrogen at temperatures of 1750C and 2000C ..... ..... ..... ..... 209
6.13 White CUR compound formulation used to measure the changes in colour during the dehydrochlorination process ..... .....
211
6.14 Changes in colour of unaged and aged CIIR vulcanisates with and without antioxidant after dehydrochlorination in nitrogen at 175°C and 200°C ..... ..... .....
213
6.15 Changes in colour of unaged and aged CUR vulcanisates with and without antioxidant after heat ageing in air at temperatures of 1750C and 200C ..... ..... .....
214
7.1 Changes in processing oil compositions due to heat ageing ..... ..... .....
228
7.2 IIR and CIIR tyre inner tube compounds formu- lation used to evaluate the effect of trace amounts of copper on oxidative ageing 230
7.3 Influence of copper content on the ageing of a butyl rubber tyre inner tube compound at 150°C ..... ..... ..... .....
233
7.4 Influence of EDTA as a copper inhibitor on the heat ageing properties of IIR tyre inner tube compound aged 3 days at 150°C ..... 237
7.5 Influence of ZDC as a copper inhibitor on the characteristics of unvulcanised IIR inner tube compounds ..... ..... .....
238
7.6 Influence of ZDC as a copper inhibitor on the physical properties of butyl tyre inner tube compound ..... ..... ..... .....
239
7.7 Influence-of ZDC as a copper inhibitor on the heat ageing properties of IIR tyre inner tube compound aged 3,7 and 14 days at 150°C 240
7.8 Influence of increasing percentages of copper content on the ageing of a chlorobutyl rubber tyre inner tube compound at 150°C .....
244
r. HAPTFR I
A REVIEW OF POLYMER DEGRADATION AND AGEING
1.1 . POLYMER DEGRADATION
1.1.1 Historical Note
The degradation of polymers has interested scientists since the
natural materials rubber and gutta percha came into use during the
nineteenth century. It became important in the 1930's when, with the development of the modern plastics industry, it became essential to understand the nature of the deteriorative processes occurring in
the few materials available at that time.
Between 1945 and 1950, pure scientists began to take interest in the fundamental chemistry underlying polymer degradation (1-3)
. This interest has developed continuously to the present time. It has been
stimulated by the fact that the fine structure of polymer molecules has become better understood, and a greater variety of materials has become available for study caused by the explosive increase in the
use and application of synthetic materials.
In the last ten years the study of polymer degradation has received tremendous stimulus from the concerted efforts in various parts of - the world to develop thermally stable materials particularly for
applications associated with high-speed flight. As a result, the chemical structures necessary for optimum stability have been broadly defined and great efforts are being made to synthesize potentially useful materials.
2
1.1.2 Introduction
What is "polymer degradation"? It is the collective name given to various processes which degrade polymers i. e. deteriorate properties or ruin their outward appearance. Generally speaking, polymer degradation is a harmful process which is to be avoided or prevented. In classical chemical usage the term "degradation" means a breaking down of chemical structure; in terms of polymer chemistry "degrada- tion" seems to imply a decrease in molecular weight(').
Sometimes, although not often, polymer degradation may be useful. Depolymerisation leading to high purity monomers may be exploited for practical production of such materials. Another important field in which degradation is desirable is in mastication of the rubber where mechanical forces are employed to lower molecular weight. Degradation, which may be defined as the loss of desired properties, occurs when a polymer is affected by mechanical action, heat, radia- tion or the chemical action of such agents as, oxygen, ozone and other atmospheric pollutants, water, acid or bases, either singly or in
(4 -6). combination
Degradation(? may happen during every phase of polymer's life, i. e. during its synthesis, processing, and use.
During synthesis, various contaminants such as catalyser residues or other polymerization additives may become incorporated into the poly- mer and assist degradation.
Whilst being processed, the material is subjected to very high thermal and mechanical stress which may cause degradation.
In review most kinds of polymer degrade to some extent when subjected to outdoor and other ageing agencies leading to discolouration,
3
stiffening or softening, surface crazing or'cracking, reduction in
mechanical properties. '
In summary, we can conclude that degradation plays an important role in every phase of polymer life and hence the importance of studying its mechanism and other relationships.
1.1.3 Modes of Polymer Degradation
By definition, polymer degradation is mainly caused by chemical bond
scission reactions in macromolecules (2)
of either the main chains or the crosslinks, if present. For practical reasons, however, it is
useful to subdivide the broad field of degradation according to its
various modes of initiation:
1. Thermal degradation 2. Mechanical degradation 3. Oxidative degradation 4. Ultrasonic degradation 5. Photodegradation 6. Biological degradation 7. Ozone degradation 8. Chemical degradation 9. Mechanical fatigue degradation - Wohler curves
10. Dehalogenation.
1.1.3.1 Historical note on thermal oxidative degradation
Historically, mechanisms of degradation and stabilisation of organic compounds have developed concurrently and much of the supporting data
4 is interwoven( ). Though Bateman(8) has commented that the oxidation of hydrocarbons is the most extensively studied of all chemical reactions, there still remains much to be learned about the mechanisms
4
for both degradation and stabilisation of polyolefins. Certain key
aspects of these mechanisms were actually proposed as far back as the early 1900's. - Even in 1883, Burghardt observed that; the degree of deterioration (of rubber) may be roughly measured by the
proportion of oxygen absorbed; Backstrom in 1927 proposed that oxidative degradation is a chain reaction, and Ziegler in 1933
concluded that hydroperoxides are important intermediates in the oxidation process. Very early concepts of stabilisation against oxidation suggested that protective agents function simply by
preferentially absorbing oxygen. Negative catalysis was proposed by Tittoff in 1924 as a mechanism by which the protective agent destroyed or combined with a positive catalyst present in the oxi- dising system.
In 1924 Christianson established a key point in the degradation
mechanisms stating that "Retardation by an antiöxidant is a premature cutting of a chain reaction". The term 'antioxidant' goes back in its derivation to early work by Moureau and du Fraisse, who in 1921 introduced the term 'antioxygen' to explain their ability to retard oxidation reactions. (see no. 4 ).
1.1.3.1.1 Thermal degradation
The term "thermal degradation" refers to the case where the polymer, at elevated temperatures, starts to undergo chemical changes without the simultaneous involvement of other compounds.
For the purposes of making a systematic survey it is convenient to classify thermal degradation reactions of polymers into two groups:
1. Random chain scission(9) 0 (1 2. DepolymerizationOJ,
5
The first type, random chain scission, can be visualized as a reaction sequence approximating to the reverse of polycondensation. Chain rupture or scission occurs at random points along the chain, leaving fragments which are usually large compared to the original monomer unit. For all practical purposes-it can be assumed that
no monomer is liberated (as the weight loss of volatile products is
negligible). In such a case, each scission step produces one new polymer molecule, so the amount of degradation can be evaluated by
counting the number of polymer molecules.
The second type of degradation process is essentially a depolymeri-
zation process in which monomer units are released from the chain ends, so that at any intermediate stage in the reaction the product is similar to the parent material though presumably of lower mole- cular weight in the sense that the monomer units are still distin-
guishable in the chain. Such a process can be varied as the opposite of the propagation step in addition polymerization reactions.
Depolymerization does not necessarily require initiation at the chain ends of the original polymer molecule. Any weak linkage due to the
method of polymerization or prior exposure of the sample to oxidation during processing etc. can introduce sites which will trigger thermal degradation at elevated temperatures.
These two types may occur separately("') or in combination, and it is
possible to differentiate between the two processes in some cases by following the reduction in molecular weight of the residue as a function
of the extent of reaction. Molecular weight drops rapidly as random degradation proceeds but may remain constant in chain depolymerization,
as whole molecules are reduced to monomer which then escapes from the
residual sample as a gas. Examination of the degradation products may allow differentiations between the two processes; the ultimate product of random degradation is likely to be a dispersed mixture of fragments of
6
molecular weight of up to several hundred, whereas chain depolymeri-
zation yields large quantities of monomer.
1.1.3.2 Mechanical Degradation
A; reductionin molecular weight occurs when polymers are subjected to the action of mechanical forces(12). Due to the, long-chain, thread- like structures of linear polymers, it is quite reasonable to expect that due to the large shear gradients, encountered in such operations as extrusion, milling, and calendering, the polymers would degrade by
purely mechanical forces. Such occurrences are, well known and it has been shown that the polymer molecule can be degraded quite drastically to a minimum molecular size depending on the structure of the polymer and the severity of mechanical forces employed. In practice, the
questions raised concerning mechanical degradation are: is the process purely' mechanical or is the mechanical energy dissipated into thermal
energy, giving rise to thermal and/or oxidative degradation, or are all three processes occurring simultaneously? Probably in actual plant operations all do occur in most cases. Whereas thermal degradation
exhibits appreciable energies for the degradation process, purely mechanical degradation exhibits an activation energy of essentially zero. For such substances as cellulose and polystyrene, thermal degradation produces an increase in carbon content. However, purely mechanical degradation produces no such change. In addition, for a given mechanical degradation, there is a minimum molecular weight of the sample below which further degradation does not occur.
Bristow(13) has mentioned that bond scission during mechanical degrada- tion occurs preferentially near the centre of the molecules and that this leads to a sharp molecular-weight distribution rather than the broad distribution obtained by random degradation. The viscosity molecular weight-relationships which would result from random and used. non-random scission are also3experi. mentally to distinguish one type
(13) from the other.
7
1.1.3.3 Oxidative'Degradation
Early studies of chain scission in the oxidation of rubber were made by Bevilacqua(14-17). The oxidative stability of rubbers depends on the structure of the rubber and the impurities contaminating the
rubber. It is known that the rate of degradation increases with increased chain branching. This is expected as "branching" materials contain more tertiary hydrogen atoms which are more vulnerable to
radical attack than are primary and secondary hydrogen atoms. The
chemical composition (i. e. what kinds of chemical bonds in what sort of arrangement), is in itself a decisive factor. Bond energies between the same atoms are very different depending on the chemical groups to which the atoms belong. A few selected bond energy values are included in Tablel1 8).
The mechanism (6) for oxidation of simple hydrocarbons was established
many years ago and the basic mechanism is considered to consist of an initiation step in which hydrogen is removed from polymer molecules leaving the free radical "R.
Initiation:
by (Polyolefin) RH energy R' + H* (_1)
Though there is still some uncertainty as to how initiation occurs under mild conditions of oxidation: it is generally agreed that imperfections in a few polyolefin molecules are responsible. Commer-
cial polyolefins usually contain either traces of hydroperoxides or sensitising groups which activate hydrogens on adjacent carbon atoms.
Once initiation has started and a few radicals are formed then a propa- gation series of reactions is considered to be initiated as follows:
8
TABLE 1.1
BOND DISSOCIATION ENERGIES OF VARIOUS SINGLE BONDS (18)
Bond Broken A-B
Bond Dissociation
Energies g/mole
Bond Broken A-B
Bond Dissociation
Energies g/mole
C2H5-H 413.82 C6H5-CH3 392.92
n-C3H7-H 409.64 .
C6H5CH2-CH3 300.96
t-C4H9-H 380.38 CH3-C1 351.12
CH2=CHCH2-H 342.76 C2H5-C1 338.58
C6H5-H 430.54 CH2=CHCH2-C1 271.7
C6H5CH2-H 346.94 CH3-F 451.44
C2H5-CH3 346.94 C2H5-F 443.08
n-C3H7-CH3 346.94 HO-OH 213.18
t-C4H9-CH3 338.58 t-C4H90-OH 150.48_
The first is a rapid reaction of (R*) with oxygen to form a peroxy
radical (R00*)
Propagation:
R* + 02 + R00* (2)
R00* -+ RH -ý- ROOH + R* (3)
9
This is then followed by the rate controlling reactions, abstraction of (H) from the same or another polymer molecule by the (R00') radi- cal (i. e. equation 3). It is evident from reaction (3) that each propagation step produces another polymer radical initiation reaction. Hence this radical can react with oxygen in a similar way and the
resulting peroxy radical (R00*) can then abstract hydrogen from other polymer molecules.
(4) Thus oxidation
is a chain reaction as stated by Backstorm in 1927.
Furthermore, hydroperoxides which are shown in reaction (3) are the first molecular products and are important intermediates as pointed
out by Ziegler in 1933 This is evident in the branching step in
which hydroperoxides undergo homolytic cleavage into a variety of radicals. The formation of these hydroperoxides can also lead to branching as depicted by the following equations (4) and (5):
2ROOH ; R00' + RO' + HO' + H' (4)
R0'(R00*) + RH ; ROH(ROOH) + 'R' (5)
Each of the radicals formed by hydroperoxide homolysis could react with additional polymer molecules as shown in reaction (5), and the result would be the initiation of new oxidative chains. Hence as hydroperoxides accumulate and decompose into radicals the rate of oxidation accelerates. This is generally referred to as the auto- catalytic phase of the reaction. Obviously the reaction will stop in time by the process of "autotermination"; however, before this stage is reached severe degradation usually has occurred to the extent that the polymer has failed aesthetically, mechanically or dielectri-
cally. The oldest approach to stabilisation involved the use of amines or phenols, it is now recognised that compounds of this type function as chain termination agents.
10
In the propagation stage of oxidation the rate controlling reaction is the abstraction of hydrogen by alkyl, alkoxy, or peroxy radical. Phenols and amines function by virtue of their ability to donate hydrogen in competition to reaction (3), thus competing with polymer molecules for reaction with the propagating radical:
ROO* + HA + R00EL + A* (6)
In this reaction, the labile hydrogen donor or chain terminator (HA) reacts with the peroxy radical to form a molecule of hydro-
peroxide and, as a by-product, the free radical (A').
1.1.3.4 Ultrasonic Degradation
It is a general property of polymers in solution that a decrease in
molecular weight occurs when they are subjected to the influence of ultrasonic radiation(19). In natural polymers such as gelatin, gum arabic, and agar-agar, this may be detected by a temporary decrease in solution viscosity', which sometimes occurs when polymersin solution are subjected to mechanical forces.
In synthetic polymers it is usually a permanent effect and is due to chain-scission.
It has become clear that ultrasonic degradation is very similar in its
general properties to mechanical degradation, and may in fact be
considered to be a special case of this type of breakdown.
1.1.3.5 Photodegradation
This type of degradation concerns the physical and chemical changes caused by irradiation of polymers with ultraviolet or visible light (20).
11
In order to be effective, light must be absorbed by the substrate. Thus, the existence of chromophoric (light absorbing) groups in the
macromolecules (or in the additives) is a prerequisite for the initia-
tion of photochemical reaction.
The photo-oxidation of most polymers proceeds by a radical chain mechanism(21), which involves the various steps common to chain processes: namely, initiation, propagation, possibly branching and termination.
1.1.3.6 Biological' Degradation
Biologically initiated degradation also is strongly related to chemical degradation as far as microbiological attack is concerned. Micro-
organisms produce a great variety of enzymes which are capable of reacting with natural and synthetic polymers. The enzymatic attack of the polymer is a chemical process which is induced by the micro- organisms in order to obtain food (22). (the polymer serves as a carbon source).
1.1.3.7 Ozone Degradation
one of the major areas of degradation of polymers is that related to (23 , 24).
ozone cracking
It has been recognised that the tendency of a vulcanisate to produce cracks when stretched and exposed to an atmosphere of ozone is in some way related to the presence of double bonds in the polymer chain. Contrary to the reaction with saturated compounds, ozone reacts readily with olefinic double bonds causing the scissioning of these bonds, a process denoted as "ozonolysis".
12
Generally, the reaction of (031 with olefinic double bonds in polymers proceeds quite readily as long as the double bonds are accessible. Since ozone is electrophilic, its rate of reaction with a double bond increases if that bond is substituted with an electron donating
group and, vice versa, the rate decreases if the substituent is an electron acceptor.
The concept of the mechanism(25-27) concerning the reaction of (03)
with olefinic double bonds is based on the idea that five-membered
cyclic intermediates Cr) and (II) are formed:
/o\ o-o\
(I) (II)
According to Benson (26), (I) decomposes into a carbonyl and a biradi-
cal: / 0' 0'
0 0' 0/
(I) -ý -C-C` -ý j'+0=C
Criegee(27) assumed that Zwitterion/Ketone Pairs are formed inter-
mittently. In many cases these species recombine to (II)
0/0 \0e
0-10C=C` 034. C-OC/ ->ýC=O + C© -0-0ý A "*N _1010
j= 0+
j 0- 0 ý4. (II)
13
(II) decomposes at a later stage into free radicals, which are capable of abstracting hydrogen atoms, from neighbouring base units (either intra- or inter-molecularly).
The possibility of (03) directly attacking saturated hydrocarbons
at ambient temperature, has been occasionally investigated(25-29) A proposed mechanism is as follows:
0 HO' 0'
RH +0 [R' '0 /]
./ 0 +
R' + 'OH +'02
. -.. (. "--. IF'
R-0-0-OH
ROs +*0-OH
HO N
0+
0H+RH-> H20+R*
OOH + RH -º H202 + R*
R0' + RH -+ ROH +R
In the presence of molecular oxygen, radicals 0R will readily undergo the reaction:
Rý +02-+R-0-00
and, therefore, the direct attack of saturated hydrocarbons by (03) implies the initiation of autoxidation.
14
1.1.3.8 Chemical' Degradation
Many chemical reactants attack polymers and most reactions result in deterioration of useful properties. Oxygen, water and ozone are the
principal reactants present in the normal environment, and these
reagents are responsible for the failure of most polymers in service. Less common reagents that attack polymers include acids, bases, dyes
(30 of various types, adhesives, solvents, and nonsolvents,
31)
Chemical agents deteriorate polymers either by cleavage of primary chemical bonds or by the irreversible rupture of secondary valence forces.
Commonly, the rate of chemical reactions is strongly dependent on temperature, polymer structure, nature of the reactant, which implies that thermal and chemical processes overlap.
1.1.3.9 Mechanical Fatigue Degradation - Wohler Curves
Mechanical stress accelerates polymer degradation in many ways by the
application of a non-cyclic single stress either in tension, in shear, or in combinations of these characteristics of the duty performed by the component or structure, are usually referred to as static failures. Dynamic failures, on the other hand, which occur after the repeated application of stresses are lower than those for static failure and involve much more complicated mechanical behaviour.
One obvious difference between the two modes of failure is concerned with the two parameters used to specify fatigue strength, namely the stress level and the endurance (or life to failure).
15
The fatigue properties of a given material can be conveniently
expressed in the form of S-N diagrams, where S is the amplitude of
stress to produce fatigue failure after (N) cycles in a purely alter-
nating mode of stressing typical of a rotating, bending or reverse- bending fatigue test. This form of test is quite commonly called
(32j the Wohler test.
Deterioration under stress occurs only when a critical stress is
applied. The fatigue of vulcanised rubbers was concisely defined by
Gent et a1(33) as the gradual weakening of rubber specimens and even- tual fracture brought about by repeated deformations much lower than
the breaking strain, a definition that would apply equally well to fatigue of metals. Beatty 0341 in reviewing this subject took pains to emphasise the similarity between fatigue in metals and rubbers. He gave groove cracking in tyres, tread and ply separations and failures in motor mounts as typical examples of fatigue.
1.1.3.10' Degradation by Dehalogenation
This term of degradation is caused by elimination of a hydrogen halide
from the rubber molecule and is characterised by discolouration of the
polymer and unfavourable effects on mechanical, optical and electrical (35)
properties.
The most important part in the dehalogenation is the initial step which has been discussed by David(36I and Broun(37). The dehalogenation
mechanisms can be divided into two types as ionic reaction and radical reactions as follow:
16
1. Hydrogen halide is evolved from the inherent weak links such as tertiary halide and allyl halide adjacent to carbon-carbon double bond-. This mechanism is usually considered as an ionic
reaction rather than free radical reaction:
---CH 2- CH - CH 2- CH --- -ý ---CH - CH - CH 2- CH---->.
-Cl Cl H ---Cl Cl
--- CH = CH - CH2 - CH ---. + HC1
C1
2. The initial loss of hydrogen halide is initiated by the attack of radicals which are produced by the decomposition of residual
catalyst or peroxide structures in the polymer chain. In this
case dehalogenation proceeds by a free radical process:
- CH2 - CHC1 - -> - CH2 -' CH -+ Cl.
Cl* + CH2 - CH --ý-'CH - CH -+HC1 1I Cl Cl
*. C 1'+- CH = CH - - *CH - CH - -*. Cl*
Cl
17
1.2 AGEING AND WEATHERING'OF'RUBBER
1.2.1 Introduction
The ageing and weathering of rubbers are of supreme importance to the consumer and to the rubber industry. There is no doubt that many theories have been evolved to explain the ageing and weathering of rubber; in fact, there is a tendency for each rubber technologist to have his own theory to explain the ageing and weathering of his
particular products (38). This is understandable since rubber products
are used under widely different conditions and in most cases rubber is
exposed to a combination of chemical and physical agencies.
Most of the ageing in rubber occurs by a chemical process and it has been natural to assess ageing by chemical analysis.. The older methods of analysis, however, gave little useful information; the chemist, realising that it was the effect of age. ing on physical properties which governed the life of the article, turned to physical methods hence physical methods have been the tools most successfully used to detect the effect of each of the degradative factors.
1.2.2 Classification of Oxidative Ageing Processes
The ageing of rubber and rubber compounds has been the subject of study for many years from both scientific and technological standpoints.
It is generally agreed that oxygen attack is the chief cause of the degradation of rubber by the scission of the long chains of the mole- cular structure and also by causing variations in the crosslinking. The rate of ageing or degradation does however vary very widely depen- ding upon the conditions and circumstances prevailing and also upon the compounding of the stock.
18
So it is convenient to classify the various ageing processes (39
:
1.2.2.1 Shelf ageing or (normal) oxidation 1.2.2.2 Metallic poisoning 1.2.2.3 Heat ageing 1.2.2.4 Light ageing 1.2.2.5 Flex cracking 1.2.2.6 Atmospheric cracking 1.2.2.7 Humidity ageing.
It is suggested that in all these cases, the fundamental reaction responsible for the ageing is always the same, namely the oxidation
of the rubber hydrocarbon by oxygen, but in each case the reaction is "triggered" or activated by a different force, varying in nature and magnitude but selected from one or a combination of the following:
1. No activation (in the ideal case) 2. Catalytic activation 3. Thermal activation 4. Photo activation 5. Mechanical activation 6. Molecular activation.
It is realised that this assumption is an over-simplification; that
the basic reaction is not in fact a simple one, but consists of a series of reactions occurring, sometimes simultaneously, sometimes preferentially, depending upon conditions.
1.2.2.1' Shelf'Ageing'or'(Normal) Oxidation
Exact information on the "life" of a wide range of vulcanised rubber compounds under conditions of natural or shelf ageing is lacking but Dawson and Scott(401 studied the changes in various physical properties
19
of various kinds of rubber over a period of five years in the dark
and reviewed the literature.
1.2.2.2 Metallic'Poisoning
It is now an axiom in the rubber industry that copper, manganese and to a lesser extent cobalt, are catalysts of ageing and should be scrupulosuly avoided. Even so, in no field of rubber technology is there so much confusion, or so little real understanding of the
underlying reasons. The effect is not confined to copper and manganese
alone, for other heavy metals such as iron, nickel and cobalt also act as catalysts. The effect of copper on the ageing of rubber was first
studied by Miller(41) in 1865. Chovin(42), Villain(43) and Hansen et al
44) all studied the effect of metals on'the oxidation of rubber.
The dominant reaction is that proposed by Robertson and Waters (45)
ROOH + Mn+ ; R0* + OH + M(n+l)+
ROOH + M(n+l)+ + R00' + H+ + Mn+
which is equivalent to a bimolecular decomposition of hydroperoxide
2ROOH -* R0' + R00' + H2 0
Uri (46) suggested three possibilities of metallic poisoning mechanisms:
1. Reduction activation of traces of hydroperoxide already present in the system.
2. Direct reaction of the metal ion with oxygen
Mn+ + 02 + M(n+l)+ + 02-
20
3. Complex formation of metal compounds with oxygen and subsequent formation of HO 2* radical.
CO2+ + 02 -> CO 2+ + "02
CO2+ + '02 + CO2+(XH) ; CO3+X
+ H02. + CO 2+
In general, Uri hypothesised that there are always many trace metals
present in the substrate, therefore the autoxidation in its initial
stage is in fact always a trace-metal-catalysed reaction.
1.2.2.3 Heat'Ageing
The effects of heat and oxygen are normally studied by means of accelerated ageing tests such as oven and bomb ageing tests. Many
workers(39) have pointed out that in accelerated ageing tests at elevated temperatures oxidation occurs only in the outer layer causing surface hardening or softening dependent on the compounding ingredients, the state of cure, and the conditions of oxidative exposure. However, in natural rubber and synthetic polymer, crosslinking (hardening) and scission (softening) reactions normally occur simultaneously in
quantitatively significant amounts. Their balance determines whether oxidative hardening or softening takes place.
1.2.2.4 Light Ageing
Light catalysed oxidation produces an inelastic skin and discoloration
on the surface of vulcanised rubber, particularly in the case of white or non-black compounds. The layer of inelastic skin on the surface of
21
the rubber is resinous in nature and on deformation cracks in random directions and forms a pattern known as (crazing). Many workers
(47)
have investigated this cause of light stiffening and ways of preven- ting it.
Work on light ageing is complicated by the lack of correlation between
outdoor exposure and accelerated light ageing tests C48). The photo- stability of raw and cured rubbers depends not so much on their nature as on the optical properties of the ingredients and impurities that they contain
(49). Many ingredients of polymers, including the anti- oxidants used at the present time, are latent or potential photosen- sitizers.
1.2.2.5 F1ex'Cracking
In a review of the mechanism of flex cracking and flex cracking tests(49), two main agencies were listed as playing a part in flex cracking, namely fatigue and oxidation (including attack by ozone).
The fact that fatigue makes a definite contribution to flex cracking as measured on the De Mattia machine has been shown in the early work of Rainier and Gerke(50) and Buist(51), and more information on the
(52) practical aspects was reported by Springer.
Dynamic ozone cracking and flex cracking were recognised as two distinct
phenomena by Thornley(53). He pointed out that while strain was a common factor affecting both phenomena, the rate of development of flex cracks fell away very quickly below a certain strain whereas ozone cracking was apparent at quite low strains and, for the most part, did not increase with strain.
22
Lake(S'' b) has reviewed the fracture mechanics and the environmental factors which affect fatigue life. Lake and Lindley(55) differentia-
ted clearly between two types of cut growth in rubber, each of which could, occur under static or dynamic conditions:
a) mechanico-oxidative cut growth due to mechanical rupture at the tip of a flirr, and
b) ozone cut growth due to primarily chemical scission.
1.2.2.6 Atmospheric Cracking
There is some confusion on the subject of the cracks which are formed in stretched rubber when exposed to ozone from the outside atmosphere. Sun cracking or checking persist even today in spite of the fact that Van Rossem (56)
showed that this type of cracking occurs much more readily at night than it does during the day. Buist and Welding (57)
illustrated the differences in the appearance of atmospheric cracking and crazing as they occurred in the side wall of an aeroplane tyre. Under the same service and exposure conditions atmospheric cracking and crazing have been observed to occur side by side, sharply separated by the junction line of the two compounds.
1.2.2.7 Humidity Ageing
There has been differing opinions on whether moisture plays any part in the ageing of rubber. Buist(58I and Soden(59) show quite clearly that
moist heat causes greater deterioration than dry heat. Therefore it is important to differentiate between dry tropical conditions (low humidity) and moist tropical conditions (high humidity).
Buist(58) has investigated the effects of humidity on the ageing of a range of rubbers for periods of up to two years. Humidity and annealing
23
effects have been demonstrated in the laboratory by Shea and Cannons(60) (61)
and by Gardner and Martin.
With regard to other polymer systems recent reviews by Wright(62)
cover primarily the influence of absorbed moisture on the properties of composite materials based on epoxy resins.
1.2.3 Methods of Assessing Ageing
The ageing or breakdown of rubber under widely different conditions of use has been studied by innumerable investigators during the last 100
years and considerable effort has been devoted to the design of con- venient accelerated ageing tests(63). Over the years ten main types
of test have been used. These are:
1.2.3.1 Air oven testing 1.2.3.2 Oxygen bomb testing 1.2.3.3 Air bomb testing 1.2.3.4 Ozone testing 1.2.3.5 Cell oven testing 1.2.3.6 Stress relaxation and creep testing 1.2.3.7 Infra-red spectra changes 1.2.3.8 Light ageing testing 1.2.3.9 Humidity and steam testing 1.2.3.10 Swelling in solvent testing
The type of degradation produced in an oven, oxygen bomb, or air bomb, is that of oxidation; the essential difference between-the three methods is the rate factor. In general terms, the degradation
produced by years of natural shelf ageing, will correspond to weeks of exposure in the oven at 70°C, days exposure in the oxygen bomb at 70°C, with an oxygen pressure of 20 atmosphere, and two hours exposure in the air bomb at 126°C, with an air pressure of 5 atmosphere.
24
At the outset it should be said that the number and variety of ageing conditions met in service is legion and no single accelerated test
can be expected to simulate all types of service. If the type of service is known it is useful to try and analyse what will be the
main cause of failure in service.
The degree of acceleration produced (by increasing temperature and/or oxygen pressure) varies from one vulcanisate to another and also from
(64) one property to another. Consequences of this are:
1. Accelerated ageing tests do not truly produce the changes caused by natural ageing.
2. They do not always predict accurately-the relative natural or service life of different rubbers, thus raising the temperature
may tend to equalise the apparent life of rubbers which deterio-
rate at different rates under natural ageing conditions.
3. Different accelerated tests do not agree in assessing the relative life of different rubbers, and may even arrange them in different
orders of merit.
This situation appears to apply to all forms of accelerated ageing tests because amplification of any factor (temperature, oxygen or ozone concentration, etc) accelerates ageing changes to an extent which can and does vary from one vulcanisatO., to another.
There is evidence that raising the temperature may change the nature as well as rate of the oxidation process. Nevertheless, accelerated ageing tests have proved extremely valuable as a guide to ageing behaviour and methods of improving ageing resistance. Comparisons of different vulcanisates are less reliable the more they differ in com- positions and extrapolation of results is the more difficult the more the ageing conditions differ from the service conditions.
25
1.2.3.1 'Air'Oven Test
This is one of the oldest and still most widely used accelerated
ageing tests and consists of subjecting the rubber to air exposure
at an elevated temperature. The variables involved are(64):
1. - Control temperature 2. Control of air flow 3. Temperature used should be the lowest that gives accurately
measurable deterioration within an acceptable test period 4. Light must be excluded 5. The sample should be free from strain 6. Only similar rubbers should be aged together to prevent cross
migration of volatiles. So only vulcanisates containing:
a) same type of polymer b) same type of acceleration and accelerator/sulfur ratio
c) same type of stabilizer d) same type and amount of plasticizer
should be aged together.
In the Geer oven (65) it is essential that the temperature should not
rise above 70oC(66) , on account of the heterogeneous oxidation which
otherwise sets in. At higher temperatures the oxidation takes place too rapidly and the oxygen cannot penetrate into the interior of the test pieces in sufficient concentrations and so only a superficial ageing is achieved.
1.2.3.2\ Oxygen Bomb Test
To overcome the difficulty of uneven oxidation in air oven tests, due to inadequate oxygen diffusion, and also to accelerate the test
still further, Bierrer and Davis (67), replaced air at atmospheric pressure
by oxygen under high pressure, the test being carried out in a strong
26
I
steel vessel or "bomb". This. undoubtedly fulfils the two requirements indicated, and also appears to provide a sensitive test for poor ageing caused by metals. The necessity for accurate temperature
control is just as pressing in the case of bombs as in ovens. Bombs
are normally immersed in a medium such as water or oil: the temp-
erature variations in commercial equipment are normally less than in
air ovens.
Again the conditioning of aged tests pieces should follow that for
the oven test.
1.2.3.3''Air'Bomb'Test
Although these tests, like the oxygen pressure method, provide an increased oxygen concentration, it seems doubtful if they can produce uniform oxidation, because the temperature is raised so much (i. e. to
range 120-125°C) that with any normal rubber the oxidation rate must far outstrip the increased rate of diffusion.
The air bomb, developed by Booth(68) does not appear tobe widely used due to the high temperature used and deterioration occurs more rapidly than in an oxygen bomb (70°C), even the test is more rapid than the
normal oxygen-pressure test.
All other conditions could well correspond to those prescribed for the
oxygen pressure test.
r 1.2.3.4 'Ozone'Test
These tests are intended to estimate the resistance of vulcanised rubber to the development of surface cracks when it is stretched or otherwise
(59) deformed and exposed to outdoor air or other ozone containing atmosphere.
27
The most important factors to be controlled are(69):
1. Test piece size, shape and deformation 2. Production and control of very low ozone concentrations 3. Control of rate of flow of ozone over samples 4. Control of temperature 5. Control of humidity 6. Exclusion of light 7. Exclusion of ozone destroying materials.
Another complication is that some anti-ozonants are ineffective under dynamic conditions, therefore dynamic tests under ozone are necessary.
1.2.3.5 Cell Oven Test
To meet all the requirements in oven ageing, the cell type ageing oven was developed and it is to be preferred. The cell type is preferred because it is possible to age vulcanisates of different composition in
the same oven. This latter point is of great importance when comparing different antidegradants. If such compounds are aged in a normal cir- culatory air oven, there is a high probability of obtaining erroneous data due to intersample antidegradant migration. In the last few years cell ovens complying with the standards and using air as a heating
medium have become available commercially(? . Whilst operating in the
range up to 125 ± 1°C these ovens have the advantage of lower fire hazard, small weight and cost over other ovens using oil or aluminium
(7l as the heating medium
ý.
1.2.3.6 Stress Relaxation and Creep Test .
-Stress relaxation and creep measurements have been used in the study of ageing processes, since the 1940's, as a high speed ageing method. It was first recommended for this purpose by Tobolsky-76. Stress -72
28
relaxation and creep are two closely associated phenomena, very sensitive tests of changes in network structure and configuration(77).
Creep is the increase in deformation with timeundera constant load,
and stress relaxatfon is the decay in stress with time under conditions of constant deformation. The technique has been used to study the mech- anism of chain scission by providing information on the type of bonds
most susceptible to oxidative scission. Stress relaxation(78-80) can be carried out in two ways as continuous or intermittent stress relaxation. In contfnuous stress relaxation, a stretched rubber sample is kept at a constant elongation and the rate of decay of stress in a rubber strip at constant temperature is a direct measure of the breakdown of the network. While in intermittent stress relaxa- tion, the test piece is unstretched for most of the time and is only extended for a brief moment to measure the stress.
From the difference between these two methods is a measure of the (81
crosslinking reactions accompanying degradation .
1.2.3.7 Spectroscopic Test
Infra-red spectrometry (82)
and to a lesser extent UV spectrometry (83)
have proved valuable techniques for following the formation of functio-
nal groups in autoxidising systems and particularly in high polymeric materials where normal chemical techniques are not applicable.
Usually Infra-red (84,85 )
analysis of the samples is only a secondary method of subsequent investigation of oxidized polymers. Sometimes, however, Infra-red absorption spectroscopy is used as the main investi-
gative method of thermo-oxidative degradation.
29
The limitations of this method are however its inability to pick out non-absorbing groups of importance in auto-oxidation (e. g. ethers, epoxides, etc) and its failure to distinguish unequivocally different
chemical modifications of the same group (e. g. the hydroxyl group in alcohols and hydroperoxides or the carbonyl group in esters, ketones etc), particularly when a number of these are present.
1.2.3.8 Humidity and Steam Test
The ageing tests so far described have not involved deliberate increase
in the moisture content of the ageing atmosphere. However, high humidity can influence the course of ageing; thus even with a hydro-
carbon polymer, increasing theýrelative humidity to 100% in a 700C
air ageing test almost doubles the rate of deterioration, and poly- mers containing hydrolysable bonds can be especially liable to break
down under humid conditions. Hence tests have been introduced involving (86)
exposure of the rubber to hot moist air or steam
1.2.3.9 Light Ageing Test
This section deals with tests in which light and atmospheric oxygen are the sole deteriorating agencies, and which use artificial light
sources. The light sources used are intended to provide an accelerated test environmentas compared with sunlight, and the usual difficulties
associated with attempts to accelerate deterioration are met with. Thus, even intensifying sunlight itself by a system of mirrors can give misleading resul ts(87) . The visible effects are
(88):
1. Changes in colour 2. Development or alteration of surface bloom 3. Cracking, crazing, or in extreme cases, (chalking).
30
1.2.3.10 Swelling'in'Solvents Test
Another technique can supply valuable data about the sol-gel frac- tions and the crosslink density of the elastomeric network is swelling in solvent, used to study the ageing of the polymer to help elucidate the basic mechanism of its ageing(89190). However, as air is present, and rubber is more prone to oxidation when swollen by absorbed liquid, there must be considerable oxidation of the polymer, causing either degradation or crosslinking and thus altering its tendency to absorb liquid as well as its other physical properties
(88) . The solubility
of a rubber in a solvent depends on the free energy level of the rubber in the solid state and in the solution. Flory and Rehner(89) derived
an expression which gives the number of effective network chains per unit volume of gel in terms of the. volume fraction of polymer in
swollen gel in the swollen network.
31
CHAPTER 2
A REVIEW OF BUTYL RUBBER TYPES DEGRADATION
2.1 Butyl and Halogenated Butyl Rubber Degradation
2.1.1 Butyl Rubber Degradation
2'. 1-. 1.1 Thermo-oxidative degradation
The importance of olefin chemistry in the oxidation degradation of
polymers and crosslinked networks is now well known. Butyl rubber UIR)
contains carbon to carbon linkages just as does NR, SBR, CR, and-other
vulcanisable elastomers(91). It has been estimated that the carbon-
carbon bond is stable up to 3150C. Looking further at the structure
of butyl rubber, we find a much lower degree of unsaturation than is
found in NR and other synthetic rubbers, so this makes IIR much less
susceptible to oxidatiVe 'Influences. '
The large concentration of tertiary carbon atoms is an important
factor in the general oxidation of hydrocarbon chains and this must be
considered( 92). It is known that there are large increases in oxidi-
zability in going from primary to secondary and tertiary carbon atoms. Moreover, the methyl group is relatively easy to oxidize to a carbonyl
and associated factors resulting from this may be important in the
general problems of ageing and stabilization.
The relative oxygen absorption, of pure gum vulcanisates of NR, SBR, IIR,
CR andsiliconeN) rubber as reported by Nrsobian and Tobolsky(93) in
Figure 2.1, shows a wide variation of oxygen absorption rate in passing from NR to Q rubber; the conclusion that the presence of double bonds
and methyl side groups both enhance the absorption of oxygen, and thus
picked up more oxygen than any of the other polymers which contain other side groups such as chlorine, shows there is a retardation in the
rate of oxygen absorption.
32
I-
E 0 CL
0
a CL u ü
z
m O N m
z w LO r x 0
Fig. 2.1. Rate of Oxygen absorption by various rubber 'types at 130'C. (93)
IIR degrades more rapidly in the presence of oxygen than in an inert (94)
atmosphere and its degradation is due to chain scission. Madorsky
et al (95 )
examined the degradation of polyisobutylene above 300'C. After 30 min. at 3130C, 97.2% of polymeric residue remained, whereas after 30 min. at 4010C only 0.2% of the polymeric residue remained. Above 3400C, approximately 30% of the decomposition products were volatile at room temperature and they consisted largely of isobutene The remaining pyrolytic products were low molecular weight polymers containing an average 9 or 10 carbon atoms.
0 10 20 30 40 50 TIME (hours)
33
Thus it was concluded that polyisobutylene degrades in part by
"unzipping" depolymerisation from the terminal group and in part by
random scission. The oxidative degradation is reduced by use of (96-98)
selected vulcanisation systems . The vulcanisation of IIR has been reviewed by Kirkham(97) and earlier by Smith(98). As Edwards (96)
pointed out, the ageing of IIR generally results in a softening or "reversion" rather than hardening or crosslinking which predominates in NBR vulcanisates. The softening or reversion, due to crosslink scission, is prevented by the use of a low sulphur or sulphur donor
system (96-99) to . ve monosulphidic crosslink. Buckley and Clayton (100)
101) and other workers also studied the effect of typical compounding ingredients on the degradation of polyisobutylene at 2050C as a mode; for their effect on main-chain degradation in IIR. They found that ZnO and TMTD did not afford protection on their own, but together,
especially in the presence of MBTS, they re'duced the degradation rate. This protective action was assumed to be due to the protective action of ZMDC formed in situ for reaction between ZnO and TMTD.
Hydrogen sulphide(97), which is formed during vulcanisation, is known
to react with organic disulphides according to the following equation:
R-S-S-R+H2S-º2RSH+S
or R-S- R+ H2S-. 2RSH
ZnO effectively reacts with H2S thus inhibiting this reaction. So because of this reason 15-20 phr of ZnO is usually used in heat
(99) resistant formulations . Since heat degradation of IIR is the
result of the breaking of disulphide linkages, the rate of degradation (or u. seful life) will depend on the number of such linkages initially
present, 6
34
In butyl rubber-sulphur vulcanisates there are four chemically distinct (102).
sites for radical attack
1. The S-S bond in the disulphide or polysulphide crosslink. 2. The tertiary hydrogen (a) to the sulphur created during vulcani-
sation.,
3. - The allylic hydrogen in the isoprene unit. 4. The hydrogens-of the methyl groups in the polyisobutylene chain.
Many studies have been made on the vulnerabilities of these sites to
attack by various radicals. Pryor(103-105) has shown that phenyl radi-
cals will attack the S-S bond of alkyl disulphides by a free radical (H2S) displacement mechanism.
Phenyl radicals will also abstract hydrogens ajothe sulphur, such hydrogens being comparable to benzylic hydrogens in reactivity. In isopropyl disulphide it was found that 64% of the attack occurs on the S-S bond.
In IIR vulcanisates, hydrogen abstraction will be favoured to even a greater degree since the tertiary hydrogen will also be allylic.
From the work of Russell and Bridger(106) and Walling and Thaler (107) 9
it would be predicted that in IIR hydrogen abstraction at the tertiary hydrogen should occur from I to 2 times as often as attack on sulphur by a radical and a selectivity similar to the phenol and butoxy radicals at 600C and 400C. It would also be expected that attack at the tertiary hydrogen would occur 4 to 10 times as readily as at an allylic hydrogen,
assuming equal concentration. Hence the greater amount of attack will occur at the allylic hydrogens, the next at the tertiary hydrogen, and then at the-sulphur site; in the extreme situation, the ratios between the different organic groupings may vary from 1: 1: 1 to 4: 2: 1. Attýck
at a primary aliphatic hydrogen by a phenyl radical occurs about 1/30
as readily as at an allylic hydrogen.
35
In IIR there are approximately 75 times as many primary hydrogens as allylic hydrogens, so initial attack on the polyisobutylene portion of the chain should be twice that of
-allylic hydrogens in the isoprene
portion.
Zapp and Ford (108) and other w-orkers(109) found that metal peroxides
also prevented crosslink scission. They hypothesized that disulphides break down to produce thiols which are subsequently oxidised back to disulphides by the inorganic peroxide. Kirkham(97) discussed the
vulcanisation of IIR with quinone dioxime and pointed out that this has largely been superseded by resin systems.
Edwards (96) found a slow, steady degradation at 1770C in air whose rate was not affected by unsaturation level. He noted that the resin was similar in structure to established high temperature antioxidants such as 2,2-methylene bis (4-methyl-6-tert-butyl phenol) and concluded that the resin acted as an antioxidant by preventing the degradation, and subsequently repairing crosslink scission at the double bonds*'
Towney et al("O)., -' originally noted that resin cured IIR shows little reversion,,. Lee et al(l1l) stud, ied the influence of a number of metals on the ageing of several rubbers including nitrile and butyl. They found that the oxidation rate of IIR was-accelerated mostly by cobalt and was affected more by stearic acid than by any of the metals. Copper mildly accelerated the oxidation, nickel, manganese, zinc and iron inhibited it.
2.1.1.2 Photodegradation
Eby(112) studied the photochemical breakdown of pure gum vulcanisates of JIR exposed to light of various wavelengths in vacuum and in air. No appreciable breakdown of the vulcanisate was observed at any wave- length in vacuum. Breakdown in air occurred, but only when the wavelength was less than 460 nm.
36
It is of interest to note the similarity of the observations on IIR
relative to wave-length effects and those made by Bateman(113,114)
on NR. Also, it may be noted that there is great similarity of the
wavelength effects on the photochemical degradation of all hydro- (112)
carbons
Eby(112) examined a large number of antioxidants as protective agents against surface deterioration of IIR vulcanisates under various photo-
chemical conditions. He found certain aryl amines effectiVe a, s protec- tive agents, but they were unsatisfactory in light coloured rubbers because of the very intense discolouration they produced. Samples of
(115)
pure gum vulcanisates of SBR and IIR rubbers were aged at 70*C
and 1000C, in the presence and absence of light. The resulting oxygen
absorption curve at temperature IOOOC is shown in Figure 2.2.
A very interesting experiment was performed by subjecting samples ageing at different temperatures in an oxygen atmosphere to an inter-
mittent light source ay also no light present. The results show OW
evidence that the ratelabsorption was the same for the samples exposed to light, whether intermittently or not, and was much greater in these
cases than for the samples exposed in the dark(115).
2.1.1.3 'Ozone degradation
Because of its low'unsaturation, IIR has inherently good ozone resis- tance, although this ozone resistance is reduced by increasing mole- cular mobility through the action of plasticizers, or by increasing temperature, as was found by Edwards and Storey (116 )
and other (117)
workers
Buckley and Robinson(118,119) noted that a small amount of reversion during cure adversely affected ozone resistance presumably because of increased molecular mobility. Also they reported (Figure 2.3) the
37
I a
14 SBR in light w
12 m 0 n8
4 IIR in light
LLJ L7 1
>- IIR no light x SBR no light 0
011 0 12 16
TIME (hours) (115)
Fig. 2.2. Absorption of Oxygen by SBR and IIR in presence and absence of Jight cLt 1000C.
results of studying the chemidal unsaturation effects and the extent
of vulcanisation on the ozone resistance of butyl rubber.
Further efforts have been made to define the effect of plasticizer concentration on ozone cracking
(120,121 ). The overall effect of the
plasticizer is considered to be one of allowing greater chain mobility and thus increasing the amount of ozone which can permeate the sur7 face as shown in Figure 2.0119)
Carbon black type and loading haveaLconsiderable effect on ozone resis- tance, as shown in Figure 2.5(119). Reducing the total black loading from 120 phr to 80 phr enhances the ozone resistance considerably.
38
0
r- x
w F- cc
v cr U
0
Uw VULCANISATION (minutes at 16VC)
Fig . 2.3. Effect of vutcanisation time and Polymer (118) unsaturation on ozone cracking rate of IIR.
LU Lj +60
+40. Ln LO Q: +20 0 Phr
0 r -5 -P-h r- 1 10 Phr Control)
20-
LU -40. CONDITIONS. 20 Phr
-0.2% volume o., one
-6o- -6A9mm specimen -50 % elongation INCREASING PLASTICIZER
PROPORTION
Fig. 2.4. Effect of plasticiZer proportion on ozone resistance of ( IIR) vulcanisates. M relative change). (119)
m% Change = Minutes to break (sample) x Minutes to break (control)
x 100 Minutes to break (control)
39
Soo CONDITIONS-.
450. 0-2 % volume ozone
Li 400- 6xl9mm specimen 2-1 ý<( 350. SOY90ongation V) U-i 300- LU Ix 250- LLI
:Z CD 200- 4D C) Ln Lj
rI4 CD 150- Ln 0. U- Uj 0. a
100- C) U- Uj LIJ 50- Ln
CY% U-
90SRF 0
-50- t Control
-100- C. L L3 0ý CD
Fig. 2.5. Effect of Carbon black on ozone resistance of IIR vulcanisates. M relafive change),
(119)
% Change = Minutes to break (sample) - Minutes to break (control)
x 100 Minutes to break (control)
2.1.2 Halogenated Butyl Rubber Degradation
2.1.2.1 Thermo-oxiddtive degradation
The thermo-oxidative stability of chlorobutyl (CIIR) rubber has been (122)
reviewed by Zapp and Hous
sion does not exist in properly went would also apply to bromobi ZnO was claimed to be effective withstanding temperatures up to
They noted that the problem of rever- vulcanised CUR, and that this state-
utyl (BIIR). A combination of TMTD and in producing vulcanisates capable of 19311C.
40
Timar and Edwards(123) sought to optimize the heat resistance of BIIR
and the cure system consisted of 3 parts of ZnO and 0.3 parts of TMTD. This gave satisfactory ageing at temperatures up to 1750C.
Walker et al (124) described the vulcanisation'of BIIR with organic
peroxides. Brominati6n introduces the possibility of stable radicals being fo
, rMed on the chain and subsequently combining to produce a stable crosslink. Two possible routes to produce a stable crosslink in BIIR were postulated:
Br
2---C-CH--- R) N CH 2
2-- C- EH--- -, --2-- C= CH
11 J CH2 'CH2
CH2 tl
--CHC--- ---C - CH
I CH2
Br R Br IR0
--- II
--- C- CH--- C- CH --- 11 1 CH2 'wCH 2
(IX) (X)
(1)
(2)
(IX) + (X)
R Br Ii C- CH I CH2 I C- CH II
.CH2 Br
41
Berger (125 ) has attributed the difference between the heat stability
of CUR and IIR to the nature of the crosslinks in each polymer. The
crosslinks in CIIR are thought to be both mechanically and chemically more stable than those in IIR. This prevents crystallization on stretching and enhances high temperature stability.
2.1.2.2 Ozone degradation
Butyl polymers generally have excellent resistance to attack by ozone. (126)
CIIR is similar to unhalogenated grades of IIR in this respect The comparative ozone resistance ofCIIR'cured with various vulcani- sation. systems is shown in Figure 2.6. Zapp and Perry (127 ) have
suggested that this ozone resistance might be improved still further by the use of appropriate crosslinking systems which could saturate such double bonds as are present in IIR. They cited the crosslinking of CIIR
with phenol-methyl resins as a means of bridging the double bond.
Walker et al(124) found that peroxide-dimaleimide cured BIIR had
excellent ozone resistance. This was attributed to crosslinking through the double bond. It was also pointed out that the unsaturation in BIIR
might be pendent at, -the main chain and might remain so during this type of vulcanisation.
42
OZONE RESISTANCE
2
1
1
ui ix
uj m L-p Z
IA
LA UJ CX
uj cx
z I.. )
00 r-4
LU
0 Uj
CL- C5 a)
CLO a) aC -6
C = Lj
C3 -Ln
T Q - Cl
OW . 6. - E Z <0
0-4 lZ tA Wj Cc
LL. ý- - 1: ca 10)
CURE 0. cc 0: E 4ýj
ki 1) -d Uj L. C: j -6-- T ><. 2
= t-
Ca im X: :2
SYSTEM 0- te
r_ CL =) =
= CL 0
0 ". E a
1- .0 2= C14 aj aj E U- " -0 L- C: ) 0.
C: ý LLJ a=
tA LLS .
CD Cl Ln C L- M= -
Wx Q) La
a-CL. 0- 6 = 0. 1: 75 " ý*- LD U- E cD
I- L: C7% ý qN" CLO 0
2: = CP X- =C
CL IOL CL cN Ln in Gi CL
E C14 ca.
ao ca. E
0. CL
CURE AT 153 0 S (minutes) O 30 30 30 15 45 60. 30 60 30 60
Fig. 2.6. Variation of ozone resistance with c ure s ystem forClIR (127)
43
2.2 METHODS'OF ASSESSING AGEING OF BUTYL AND HALOGENATED BUTYL . RU=
2.2.1 Butyl Rubber
2.2.1.1 Air, steam and oxygen oven ageing
The superior ageing properties of IIR are usually attributed to its low
unsaturation. The behaviour of IIR(128) vulcanisates, prepared in sul- phur, quinoid, and resin systems, have been studied in hot air ageing. For sulphur cured vulcanisates aged in air for up to 96 hours at 1210C it was found that, the sulphur system showed appreciable improvements in ageing behaviour with increasing unsatoration. With the quinoid system, ageing studies were made at 1500C for 3 days owing to the
greater thermal stability of this system. It was found that hardness
and elongation at break were almost unaffected, whereas tensile strength and modulus decreased; there was no apparent inter-dependence on
unsaturation level.
Resin cured vulcanisates of low and high unsaturation (IIR) showed good stress-straih and hardness behaviour after ageing at high temperatures
of 200*C, and had better overall retention of physical properties; the lower unsaturation polymers are the preferred types (128). Detail s(130) of hot air ageing for resin curing data of a number of representative systems are. given in Table 2.1; the specimens were exposed in a cir- culating air oven at 1770C for periods up to one week.
In addition to its excellent hot-air ageing behaviour, IIR is also recognised as being superior in applications which must wifthstand steam, or steam and hot air, with vulcanisates cured using a quinoid system they
(128) found better retention of tensile strength as the unsaturation in the polymer increased.
The effect of severe hot air ageing was studied on two quinoid-cured black reinforced (IIR) vulcanisates
(122), one based on Polysar IIR 100,
44
the second on Polysar IIR 600; it was concluded that the more highly
unsaturated polymer showed a far superior retention of elastic modulus. Surprising results were obtained by Zapp et al
(122) who found that higher
levels of unsaturation in IIR gave better heat ageing performance; the
reason is believed to arise from competitive crosslinking reactions associated with the isoprene groups in the polymer which tended to preserve a useful level of network density during vulcanisation.
A comparison of the effect of vulcanisation systems with respect to air ageing has been made by other workers(129-131) where data is given in Table 2.2. In the search for improved heat resistant IIR inner tube
compouhds(132) , two accelerated heat ageing tests were employed, namely (a) ageing for 48 hours at 1380C under 5 atmosphere air pressure, and (b) ageing for 8-96 hours at 1750C in a circulating air oven at atmos- pheric pressure. Results are shown in Figure 2.7.
Table 2.3(133) shows the typical properties of IIR and BIIR inner tubes
afterageing at high temperatures.
Figure 2.8(131) shows the effect of ZnO and of sulphur on an IIR
vulcanisate's stability; -tensile specimens were aged using a moulding press* and also aged in a steam atmosphere using an autoclave for
various times at 160*C. Judging reversion by modulus decrease, the com- pound containing the
' lowest amount of ZnO showed the greatest rate of
reversion. Furthermore, the steam-aged specimens showed lower moduli than the press-aged specimens. This data indicates that the reversion tendency is reduced with incremental increase of ZnO up to 5 parts.
A general survey (134)
of the magnitude of the aggregative (cross- linking) and disaggregative (scission) reactions which occurs during the-heat-ageing of rubber. polymers may be presented. in a very, simple
Ageing'by press moulding implies an absence of Air.
45 '
TABLE 2.1: HIGH TEMPERATURE AGEING RESIN CURED IIR( 130)
Formulation .0
4-) M &. -r- to (U
C3. =M
V) -4
U7 S- C r- :3 (L) 0 > 4-)
C) to
CU 4.. )
r- M ., - r- --4 ta 0) (a r- S- 0- (1) 4-) X:,
V) -
C ZA 0
4J AS
(0 W t7) S- C .0 a r-- 4J LLJ (a
I to 4-) 4M--l
CO MM Oil.
#A rý =
= LLJ r- * 23 IA C -0 C> 0 0 4D, r,
r-
I (0 4-) CM, -*4
to C Ca 0 a-
(A r- M:
LU ý-- = "K I=
'a C) 0 0 CD -r-
M 4-)
4A C: C &n
(2) a) C S- -U 0 S- = fts V)
IIR Polysar 200 100 0 12.9 260 2.5 - 58 HAF Black 50 24 12.3 180 4.6 - 64 Stearic acid 1 48 8.5 160 4.1 - 64 Amberol ST-137 12 72 9.7 175 4 - 66 SnCl 2 2H 20 2.7 96 7.4 140 4.2 - 66 Dispersion
--------------------- -------- 120
------ 7.2
------ 150
------ 4.1
------- -
-------- 66
------- IIR Polysar 200 100 0 12.7 545 1.2 5.8 50 HAF Black 50 24 10.9 485 1.3 6.4 48 ZnO 5 48 8.1 420 1.3 5.7 49 Stearic acid 1 72 8.2 455 1.2 5.5 46 Amberol ST-137 12 96 5.4 365 1.2 4.6 44 BUR --------------------
10 -------
120 ------
5.6 -------
410 -------
1.2 -------
4.4 --------
45 -------
IIR Polysar 200 100 0 12.4 480 2.3 7.6 59 HAF Black 50 24 12 300 4 12 71 ZnO 5 48 7 195 4.3 - 77 Stearic acid 1 72 8.5 210 4.8 - 78 Amberol ST-137 12 96 5.5 165 4.3 - 80 NBR-W --------------------
10 -------
120 -------
5.2 -------
130 -------
4.6 -------
- --------
82 -------
IIR Polysar 200 100 0 12 370 3.7 10.3 70 HAF Black 50 24 8.8 280 4.3 - 82 ZnO 5 48 4.6 175 3.7 - 82 Stearic acid 1 72 5.2 180 4 - 84 Amberol ST-137 12 96 3.2 100 3.2 - 84 Hypalon 20
--------------------- 10
------- 120
------- 3.3
------- 80
------- ------- -------- 84
------- IIR Polysar 200 100 0 14.6 625 1.5 4.5 5-3. HAF Black 50 24 12 475 2.4 6.5 58 ZnO 5 48 10.3 340 2.4 6.4 58 Stearic acid 1 72 10.2 360 2.4 5.9 56 Amberol ST-137 12 96 7.7 250 2.4 - 54 Resin SP-1055
L ------------------- 4 ------- 120
L ------ 7.2
L ------ 230
L ------ 2.4
L ------ L ------ J 54
------ J
Continued...
46
TABLE 2.1 ... continued
Fomulation 60 4-) tn = 4J M S- arý (o W
CL 3:
4-)'Uý go S-
t7). -o (0
u r_ 0 (1) W > rý%
C>
4)-1--) r- M -fý C: -ý 0 (1) (0 C S- CL Q) 4-)
Ln
0 -r- 4-) M (o a) 0) S- C .0 C)
r- 4J LU to
1 to
4-) tmý r_ to 0 CL
(A r- M: = LLJ -ý
r- :3 W-Q C a CD o 0 C) -r-
4.3
1 to
4. ) M, -ý to Cm
om W r- M: = LLI --, o
r- = ZA r_ a C) 0 0 C> -r-
2: M 4J
4A< tA C) W r_ S- a0 S- = CO (n
IIR Polysar 200 100 0 15.3 520 1.5 7.7 53 HAF Black 50 24 11.4 295 2A - 58 ZnO 5 48 6.7 300 2.4 10.3 58 Stearic acid 1 72 7 305 2.4 9.7 56 Amberol ST-137 12 96 5.5 280 2.2 - 54
--------------------- -------- 120
------ 4.1'
------- 265
------- 2.4
------- ------- 54
------ IIR Polysar 600 100 0 14.2 355 2.7 12.5 58 HAF Black 50 24 , 9 235 2.8 - 58 ZnO 5 48 7.4 235 2.7 - 56 Stearic acid 1 72 5.9 200 2.8 - 54 Amberol ST-137 12 96 4.6 180 2.5 - 55
120 3.6 180 2.3 - 53
47
TABLE 2.2: A COMPARISON OF
Basic Recipe: Polysar Butyl
VULCANISATION SYSTEMS ON THE HEAT
301 100, N-220 (HAF) - 50, ZnO
ASEING OF IIR L129
a 5. Stearic acid I
Curing System 1 2 3 4 5 6 7 8 9
MBT 1.5 0.5 - - - - MBTS - 0.5 1 - - TMTD I I - - 3 - 4 4 CMDC 2 - - - - TEDC - 1.5 - Sulphur 1.25 2 1 1.5 - 2 Morpholin disulphide - 2 - GMF - 1.5 2 Dibenzo GMF 6 - Red Lead 10 5 Neoprene W - 5 Amberol ST-137 - 10
Cure temperature (OC) 160 160 160 160 170 150 150 170 190 Cure time (minutes) 25 25 45 30 35 12 11 30 30 Hardness, Shore A E6 65 62 64 55 64 64 64 64 Modulus at 100% Elonga- tion (MPa) 2.5 2.6 2.2 2.3 1.5 2.1 2.0 2.4 1.9
Tensile Strength (MPa) 16.6 16.3 17.3 15.4 7.9 12.8 15.7 18.1 15.8 Elongation at break % 530 - 480 540 460 630 400 560 430 590 Compression set 68 70 64 65 76 68 60 66 12 (70 hours at 1000C)
Aged in air at 26COC: Change after: 1.24 hours ageing:
Hardness (points) -14 -14 -11 -10 -3 -8 -2 +1 +8 Modulus at 100% Elonoation -64 -69 -54 -50 -47 -32 0 -12 494
Tensile Strength % -85 -88 -60 -62 -93 -62 -41 -31 -4 Elongation at break % +32 +44 +22 +17 -13 -15 -41 -15 -35
2.48 hours ageing: Hardness (points) -18 -23 -14 -14 -9 -14 .6 .6 -8 Modulus at 100% Elongation -76 -77 -68 -71 -47 -52 -30 -32 -100 Tensile Strength % -92 -96 -91 -90 -97 . 78 -65 -54 -15 Elongation at break % +9 +8 +11 +35 -60 -22 -32 -9 -41
3,72 hours_ageing: Hardness (points) .9 .8 -6 .1 -14 Modulus at 100% Elongation -60 -52 -30 -28 +105 Tensile Strength % -97 -78 -65 -54 -15 Elongation at break % -68 -25 -36 0 -46
4.96 hours ageing: Hardness (points) - -12 -12 .2 +17 Modulus at 100% Elongation -80 -45 -36 +100 Tensile Strength % -96 -89 -77 -34 Elonnation at break % -15 -28 .9 -44
48
16
14
12
STRESS10 (M Pa)
a
6
4
2
0
Tensile Strength
Modulus at Elongation
111
CONVENTIONAL 2' Sulphur 1.1 TMTO
LOW Su lphur' HIGH Accelerator
1: Su [phur 2. T MTO
Soft
Original A8 Original A
Fig. 2.7. Influence of the curative system on IIR inner-tubes compound. 032)
A: Aged 48 hours at 1380C under 5 atm. air pressure B: 'Aged 8-96 hours at 175% in circulating air oven
way, as shown in Figure 2.9. The samples of NR, CR, SBR, IIR and poly- butadiene gum vulcanisates were aged in an air oven at 150*C.
IIR was observed to become more tacky with ageing, indicating that in this case scission was more preponderant.
49
TABLE 2.3: COMPARISON OF HEAT RESISTANT INNER TUBES COMPOUND( 133)
phr phr
IIR Polysar 301 100 - BIIR-X 2 100 N-650 (GPF) Black 60 67.5 Stearic acid 1 1 Light Naphthenic Oil 25 22.5 ZnO 5 3
MBTS 2 0.75
'TMTD 1 0.25
Sulphur 1 - Maglite D (MgO) - 0.4
Mooney Scorch Time t5 >25 20 (mins at 1250C)
Cured 8 minutes at 1650C Hardness, Shore A 50 47 Modulus at 300% Elongation (MPa) 4.2 5.1 Tensile Strength (MPa) 11.2 10.0
Elongation % 670 560
Aged in air, 22 hours at 1650C Hardness, Shore A 65 Tensile Strength (MPa) Degraded 7.7 Elongation at break % 310
Aged in air, 70 hours at 1650C Hardness, Shore A 68 Tensile Strength (MPa) Degraded 2.5 Elongation at break % 320
50
10 TENSILE 7.5. STRENGTH 5
(Mpa) 2-5 0
1000 ELONGATION800'
(%) 600 400
MODULUS 4 3
AT 300 %2 ELONGAtION 1
0 MPa3
2-S ZnO S ZnO 10 ZnO 20 ZnO
--
-
C=D CZP Co
CN TIME OF AGEING (minutes)
Recipe: IIR 100; Carbon black 72; Sulphur 2.0; TMTD 1.0. (Specimens cured 30 minutes at 1600C)(131)
-- Aged in steam at 1600C Aged in press at 1600C
S= 0-5 S=1-0 SZ1.5 S=2-0 TENSILE 12-
STRENGTH 10:
(M Pa) 8- 6,
1000. ELONGATION 800. AT BREAK 600ý
400m 5.
MODULUS 4. 3.
. AT 3001/o FLONGATION A _ _
--
_I
rn (7,
,- TIME OF AGEING (minutes)
Recipe: IIR 100; Carbon black 72; ZnO 5; TMTD 1. (Specimens cured 30 minutes at 160*C)
Aged in steam at 160'C Aged in press at 160%
v
Fig. 2.8. Effect of ZnO and Sulphur on IIR vulcanisates stability when exposed to "dry and wet' heat (131)
51
1234 5- 56789
NR
CR
SBR (75125)
3 hours 10hours
IN 20 hours 40 hours
Po(ybutadiene
56 Tacky Rubbery Brittle
Fig. 2.9. Appearance of thin gum vulcanisate after ageing at 1500C (134)
52
Esso workers (135,136 )
reported data concerning the heat-resistance of
several elastomers compounded for best high temperature performance, IIR-218, EPM 404, EPDM 2504, CIIR (HT 1066) and CR. Heat ageing in
an air oven was monitored for one parameter only, the strain energy
at break. They found heat resistance to improve when moving from CR
to resin cured IIR.
2.2.1.2 Stress relaxation
The change in physical properties that occurs in hydrocarbon rubbers
at elevated temperatures is due to the simultaneous existence of chain
scission and crosslinking reactions induced by oxygen. Depending on
whether chain scission or crosslinking is faster in each individual
case, various rubber types soften or harden as a result of exposure to (72)
air at elevated temperatures
Continuous stress relaxation was carried out on (137) IIR, NR, SBRq
and CR as shown in Figure 2.10. In IIR vulcanisates which soften
progressively., the stress decay completely to zero and no increase
in stress whatever is observed in the continuous relaxation curves,
even when they are carried out for a very extended period. Similar data for IIR gum and IIR filled tyre tread at 1300C and 50% elongation are shown in Figures 2.11 and 2.12, which represent relaxation and permanent set showing that molecular scission takes place more rapidly than crosslinking.
In IIR a Pb021P-quinone dioxime cure was observed to give the same stability to oxidative scission as the sulphur cure; it-was concluded
(139) that chain scission was involved rather than crosslink scission It is interesting to note that the chemical relaxation time of IIR
was partially unaffected by the nature of the cure. The difference in
relaxation time between sulphur and sulphUrless cure systems is not (139), large . It is clear that for NR, SBR and IIR scission must occur
along the hydrocarbon chains otherwise much larger differences would be observed between sulphur and sulphurless curves, as shown in Figure 2.13.
53
fo
ix Li
, 0-
, 6-
-4-
-2-
. 0.
-4-
04 0 0: 1 1 10 ido 1000
TIME (hours)
Fig. 2.10. Continuous relaxation curves for extended periods for various rubber types af 130'C and 50% extension.
(137)
100
80
f6
TO
60PERMANENT
SET
40
20
0 TIME (hours)
Fig. 2.11. Permanent , set as a function of time of (137) IIR gum vu[canisates at 130"C and 50% extension.
54
11
0
c
fe fo
0
0
TIME (ho. urs)
0
0
PERMANENT SET
Fig. 2.12. Permanent set as a function of time of a IIR fitted tread vu[canisate at 1300C and 50% extension. (137)
55
3 TIME (hours)
1 ýý 1 -0 !
Fig. 2.13. Stress relaxation of Sulphur and non-Sulphur cures of four elastomers of different chain structures at 1300C. 0 39)
05 10 15 20 25 30 35 40 45 50 TIME (hours)
56
(138) Norl ing studied the stress relaxation of different crosslinking densities of IIR vulcanisates in nitrogen and oxygen atmospheres at 150*C. He observed that under thermal conditions the samples with the greater crosslinking density showed a greater vulnerability to scission in both atmospheres than the low crosslinking densities
vulcanisates (see Figure 2.14).
Other workers(140) found IIR stress relaxation behaviour similar to SBR vulcanisat6s (see Figures 2.15 and 2.16), the reason was attributed to the appreciable residual of unsaturation in IIR.
Mercurio and Tobolsky (141) studied the effect of the crosslinking nature
on the rate of stress relaxation. By comparing an IIR vulcanisate with an NR vulcanisate, they concluded that chain scission is involved in IIR in a similar manner to NR.
Eller(142) studied the relationship between stress relaxation in com- pression and in tension for IIR and SBR vulcanisates with respect to continuous ageing at room and elevated temperatures. Figure 2.17 shows the relationship between stress relaxation in both methods with respect to percentage of deformation; it can be seen that there is no signi- ficant change in stress relaxation to be observed for SBR and IIR speci- mens tested in tensio-n; the stress relaxation of both specimens was higher in compression than in tension at the same percentage of defor- mation.
IIR specimens in Figure 2.18 exhibited higher stress relaxation at 900C in both compression and tension than the SBR specimens whereas at room temperature the reverse was true.
57
le 0. 0. 0.
oý
fL- fo
0. ý
n
0.
Figures represent cure time.
I Np
02
0123456759 10 11312 13 14 SECONDS 00'
Fig. 2.14. Continuous stress relaxation of IIR at 1400C in N? and 0?. (138)
58
1.5
1.0 STRESS (Mpa)
0 '!
SBR Gum
NR Gum
0101111 0.001 0.01 0.1 1 10 100
TIME (hours)
Fig. 2.15. Stress relaxation of S. B. R and N. R (140) vulcanisates at 1200C and 50% extension.
0-8 -1 1
0-6
1- IIR B-3 High Black. 2-IIR B-3 Low Black 3-AIR B-1-45 High Black
STRESS 24. -1IR B-3 Gum M Pa) 0-4- 3
5-11R B-1-45 Gum 45 5 -IIR B-1-45 Low Black 66 0-2-
0 0-001 0.01 0 .11 10 100
TIME (hours)
Fig. 2.16. Stress relaxation of IIR- vulcanisate at 010 20 C and SO% extension. * (140)
SBR Trecaýid'
59
-100- 90-
:z C) 80- 'ý'IIR in compression
< 70- 0 x 60- IIR in tension
. LU Cr_ so- SBR in c mpression
V, V) 40. -------. &SBR in tension LU gr ý0-
Ln 20 101 0-
0 10 20 30 40 50 60 OEFORMATION M
Fig. 2.17. Stress retaxation in compression and tension
versus deformation. Specimen aged 46 hours
at 900C ±2"C. (142)
CD
LLJ Ix
L'I V) uj
Ln
1- IIR in compression at 900C. 2-SBR in compression at 900C 3-11R in tension at 900C 4-SBR in tension at 900C 5-SBR in compression at 230C 6-SBR in tension at 23% 7- IIR in compression at 23"C 8- IIR in tension at 23%
1
0-, - ��e
S 6
1 10 TIME (hours)
100
Fig. 2.16. Stress relaxation in compression and tension versus time. Specimen subjected to continuous cloeino at room temoerature and at 90'C t 2*C.
.a -0 1
Compression tests conducted at 20% deformation. Tension set at 40% deformation ý142)
60
2. '2.1.3 Swelling*in Soli/ents
The determination of 'network density by swelling measurements was (143)
chosen by Edwards . as a convenient and accurate method of obser-
ving the degradative process; his measurements were made on an IIR
gum,. vulcanisate and swelling undertaken in cyclohexane. Figure 2.19
shows a group of Purves obtained with sulphur vulcanisates for a polymer of intermediate unsaturation (1.4 mole percent), aged in
air for various periods at 1440C.
Network degradation (128,143) measurements bn gum vulcanisates have
shown a large improvement in air ageing with increasing unsaturation for sulphur and quinoid curves. Figures 2.20 and 2.21 have indicated
that the distinction is due to a relative increase in the rate of oxidative crosslinking.
A comparison of the same polymers cured by a resin system and aged at 177*C is shown in Figure 2.22; this indicates that the maximum net- work densities attained by the lower unsaturation polymers substan- tially exceed those found with the sulphur or, 'quinoid systems.
Solubility data shown in Figure 2.23 have provided evidence (143) that
crosslink scission occurs in sulphur cures, while relatively stable crosslinks are produced by quinoid and resin cures.
Zapp and Ford(144) studied the'effect of the org anic accelerators on the rate of IIR degradation. Figure 2.24 presents the percentage change in swelling volume of four curves depicting the course of vulcanisation at four'temperatures (1210C, 1490C, 1770C and 2050C)
and with three different types of accelerators (TMTD, TDEDC, BTMCS).
61
x 7- 0 10mins 6- o 20 mins
5- 40 mins
z 4- o 80mins LU C3 3. 1, ýý 2 cr
1 2,34 B -16 24 HOURS AT 149%
Fig. 2.19. Effect of initial state of cure Of IR sulphur vulcanisates on air oven (143) ageing behaviour. Cure time at 144C.
x
vi
LU ca
le er CD
Fig. 2.20. Effect of unsaturation on air ageing of sulphur cured butyl. (143)
Cure time 20 minutes at 144 oc
Red pe: Polymer 100, ZnO = 5, St. Ad d=1, MBTS TDEDC = 2, S=1 . 5.
10 20 30 40 50 60 70 80 90 100 110 120 HOURS AT 149"C
x
kA z w Q
62
Degrees of unsaturation in mole per cent 0 0-8 Q 1-4 A 2-2 a 2-8
'-1 z
20 40 60 80 100 120 TIME (hours at 17700
Fig. 2.21. Effect of unsaturation on air ageing of IIR
cUred with p-quinone dioxime (143,128)
Recipe: Polymer 100, ZnO = 5, MBTS = 4, S=2, Kenmix GMF (p-quinone- dioxime) 4. Cure = 80 mins at 1440C.
L4-
vi Z uj cm
Id cr- C3
uj z
Oegrees of unsaturation S- fn mote per cent 0 0.8 o 1.4 A 2-2 a 2-8
-0-- 2f Cb
j
0 20 40 60 to 100 120 140 TIME (hours at 177"C )
Fig. 2.22. Effect of unscituration on network density (143,128) of resin cured IIR during air ageing cit 177"C.
Recipe: Polymer = 100, Amberol ST-137 = 10, SnCl 2- 2H 20=2, Stearic Acid Cure = 80 mins at 1440C
m
C3 LLS I-- I Li L&J CY. I
0aa
A 2- theorerical
ýha he ore
ý ai sc , ss
fica in
ion sc ss
heore .1 ca I
D- chain scission a
&I d6 sciss. 0
0
theoretictai theoret IC al 0 crosslink
A scission
a00
CURE SYSTEMS
* Sulphur * Quinoid A Resin
Fig. 2.23. Types of scission during air ageing of IIR vulcanisates. (128)
63
3000 ui 2 OW ui (x Li F= 1000
800
60Q
400
0.1
(a) IIR vulcanisate with
2000- ui LA
cc $-. )1000-
U. 1
800 'ro
0 600-
400
0.1 1 10 100 1000 TIME (minutes)
N IIR vulcanisate with Tellurium Diethyl Dithiocarbamate accelerator. (TDEDC).
3000
2000
205 z .
1770C 1 4ý900 C 121 OC U. A
100 a0 0
0 600 >
69
400
011 1,1b 100 10,00 TIME (minutes)
(c) IIR vutcanisate with Benzothiazyt Mono Cyciosulphenaniide accelerator. 13 TMC S)
Fig. 2.24. Variation of volume swelling of IIR vulcanisates with different types of accelerators (a, b, c) cured at 1210C, 1490C, 1770C and 205% with respect to time. (Swollen in Cyclohexane at 25%. ). (144)
121%
TIMMminutes) 1000
Tetra Methyl Thiumm, Disulphide accelerator. (T MT D).
10
64
2.2.2 Halogenated'Butyl'Rubber
2.2.2.1 ' Air, 'ttoaM'and'6ý Ageing yg6n'c
An outstanding characteristic- of halogenated butyl vulcanisates is
the ability to withstand prolonged exposure to high temperatures with
relatively little degradation of physical properties. The heat aged
properties (145,146)
of a 60 phr carbon black loaded, thiuram-thiazole
curedjCIIR compound have been compared with NR sulphur cured at 121OC;
results are shown in Figure 2.25 where it is clear that the CIIR retains almost all its original tensile strength and over 40% of its elonga- tion after 18 days ageing. at 1210C, whereas the NR compound becomes
very weak and brittle. Figure 2.26 shows the level of heat resistance of CIIR compared with a conventional sulphur IIR vulcanisation system.
The effect of various cure systems on the heat resistance of 50 phr HAF loaded compound CUR exposed for 16 hours at 1930C are shown in
Figure 2.27. A zinc oxide cure is used for maximum heat resistance. Compound formula, shown in Table 2.4, have been used to manufacture CIIR tyre inner tubes which were then aged one day in hot air at
126) 1620C(
Several chemical additives for the ZnO cure were evaluated to deter-
mine whether further improvements in heat resistance could be achieved. These are 2-mercaptoimidazoline (MIA) and tetrachloro-p-benzoquinone (TCBQ). Of the two materials, TCBQ appears to be of the most interest for heat resistant inner tubes, Figures 2.28 and 2.29032).
In TCBQ-ZnO modified cures, satisfactory cures were obtained with 10 phr ZnO which appears in Figure 2.30 to be desirable for best heat
ageing compounds (132).
65
2
TENSILE STRENGTI!
(M Pall
fensHe strength
elongation. I --
n DAYS AT 1210C 036 18 036 18 ELASTOMER CIIR NR CURE SYSTEM (TMTD-MBTS) (Sulphur)
Fig. 2.25. CII R -Sulphur donor curing and NR- Sulphur
cured hecit resistant vulcanisates , aýed cit '121'C for different periods of time. ( 45)
66
1
STRESS (MPG)
AGEING CONDITION
Fig. 2.26. CIIR- ZnO cured and IIR- Sulphur cured heat
resistant compound cured 6 minutes at 165"C
and aged at Q48 hours at 1370C under 5
atmosphere air pressure 16 hours cit 1760C in air oven. (145)
properties properties Iv ____j ý V-
IIR CIIR Sulphur cure ZnO cure
67
18
-16
14
LID z 12 ix
10
8
6
4
2
0
tensile strength elongation
cu GJ
.0
z
C;
-60
-50
-40
-3C
-2C
10
CURE ZnC1 TMTD TMTD- Sulphur NA-22 Resin Resin ZDC SYS7TEII MBTS TMTD Sulphur
Donor
CURE TIME 30
AT 153% 40 40 45 60 40 60 60
(minutes)
% RETAINED
TENSILE 49 STRENGTH 29 39 28 54 40 43 38
ELONGATION 79 66 61 80 95 69 ill 93
Fig. 2.27. Comparative heat resistance of HAF black fitted CUR compounds as sh own by tensile strength and elongation retention after air ageing 16hours at 1930C. (145)
0
0
0
0
68
x-
0-2
STRESS (MPCL) O'l
- .1 rý-2 ZnO Chemically modified
TS M300
MIA TCBQ
A Li n ]IILB
]"- B
(U (U
> C11 Cj HZ> 0 L- >
0
Fig. 2.28. Modified CIIR-ZnO cure heat resistant vulcanisates cured 8 minutes at 1650C, aged at different conditions A and B.
A= Ageing 48 hours at 1380C under 5 atmosphere air pressure B= Ageing 8-96 hours at 1760C in circulating hot air (132)
3- 3-
AGED 2-
TENSILE 2- STRENGTH 1.
(M P a) 10
0.
5- -so
0- 4C
0. 30 5-
-2( 0-
0 0
0
0 AGED
ELONGATION
10 M)
0
0 0.5 1.0 1.5 2-0 TCBQ (Phr)
Fig. 2.29. Effect of TCBG level on CUR heat resistance compound cured 8 minutes at 165"C and age 96 hours at 1760C. ( 132)
69
ZINC OXIDE LEVEL *. 10
15 12
STRESS (M Pa) 9
6
3 0
S21, Tensile Strength
300% Modulus
C WA NO CURE
CM-- Wn c
(U (U (U :3
. &7;
> 0-00
Fig. 2.30. Effect of ZnO on CUR vulcanisate heat ageing. (132)
Reci pe: Rubber = 100, GPF-bl ack=60, Oi I= 18, Paraf fi ni c wax =I, TCBQ = 1. Cure: 8 minues at 1650C ý C= Aged (8-96 hour's at 1760C in a hot circulating air oven)
Parts by Weight
CUR HT 1068 100
GPF 60 Flexon 840 Oil 20 Stearic acid 2 ZnO 10.0
original physicaZ pr6perties cured 8 minutes at 1650C Hardness, Shore A 43 300% Modulus (MPa) 3-6 Tensile Strength (MPa) 8.5 Elongation at break % 4.0
Air aged 24 hours'at 1620C Hardness, Shore A 63 Tensile Strength (MPaj 2.9 Elongation at break % 370
Table. 2.4. CIIR heat resistant inner- tube. (126)
70
A compounding study was undertaken for the purpose of maximizing heat resistance in BUR compounds and showing how they compare with
optimized compounds of other low-cost elastomersC1473 . Tables 2.5
and 2.6 summarise the observations on the reference compounds.
A comparison of the BUR curing systems shown in Table 2.7 indicates
that at higher temperatures TMTD/MgO and ZDMC/MgO curing systems were found to give a higher degree of physical property retention than the
other curing systems. ' ,
Antioxidants in BUR were evaluated in a sulphur donor cured compound, Table 2.8. The MBI/AD , PA 86/MgO combination provides the best protec-
.. e tion against thermal degradation especially at the elevated tempera-
tures of 175OC( 147).
2.2.2.2 Stress'ýelaxation
Berger (125) studied the degradation of CIIR by continuous stress relaxa-
tion and compared it with IIR vulcanisates at different temperatures. Figure 2.31 shows that the vulcanisates of CUR and IIR at identical
crosslink densities have similar properties at low deformations at
room temperature. Figure 2.32 is an excellent illustration of the high temperature stability of CIIR. Tobolsky et al(140
) explained
that stress relaxation at 1000C occurred principally because of oxi- dative scission.
2.2.2.3 Swelling in solvent
Figure 2.3 '3
shows the work done-by Berger(125) in Studying the variation of the average molecular weight of CUR and IIR chains between cross- links with different curing ti. mes, by swelling the vulcanisates in cyclo- hexane. Figure 2.33 shows the excellent and well known cure charac- teristics of CUR, i. e. the vulcanisate system reaches its equilibrium at an early cure time, promoting rapid curing and then stabilizes at that value.
71
TABLE 2.5: FORMULATIONS, REFERENCE COMPOUNDS(147)
BIIR-A(Curing System Study Control BIIR (Polysar Bromobutyl X 23 100 Stearic acid 1 N-550 FEF Black 50 Paraffinic oil 10 ZnO 3
BIIR-B (Antioxidant Study Control
BIIR (Polysar Bromobutyl X2 100 Stearic acid I mgO 0.1 N-550, FEF Black 55 Naphthenic oil 15 ZnO 3 TETD 0.3
EPDM - Reference COMDound
EPDM 200-- Stearic acid 1 ZnO 5 MBTS 1 TeDEC 0.8 N-550, FEF Black 110 DPTTS 1 TMTD 1 Sulphur 0.75
NR - Reference Comoound
NR 100 Stearic acid 2 DNPD 0.5 MBT 1 MBTS 1 N-550, FEF Black 50 Paraffinic oil 10 ZnO 0.2 TMTD 1.25 TETD 1.25
/Continued
72
TABLE 2.5 ... continued
SBR - Reference Compound
SBR 100 Stearic acid 1 DNPD I ZMBI 2 MBTS 1 N-550, FEF Black 50 Paraffinic oil 10 ZnO 5 Sulphur 0.2 TMTD 1.0 TETD 1.0
IIR - Reference'Compound
IIR 100 N-550, FEF Black 50 Paraffinic oil 10 PF Cure 7 Stannous chloride dispersion 4
TABLE 2.6: HEAT RESISTANCE OF COMMON RUBBERSC147) (a)
Ageing BIIR-A BIIR-B EPDM NR SBR IIR/ Resin Cure
10 weeks @ 1000C 74 67 52 2 74 76 10 weeks @ 1250C 46 40 25 0 0 50 70 hours @ 1500C 41 47 22 0 57 69
168 hours @ 1500C 26 26 22 0 9 58 22 hours @ 1750C 21 26 27 8 7 53 70 hours @ 1750C 1 0 4 <1 <1 27 22 hours @ 2000C 4 0 <1 6 1 42
a) Tensile product retained
73
uj I-- CC V)
V) 1-4
cn LLJ
LLJ
ca
X: LU F- V)
V)
CD
CC
CD
CD 1-4 F- C: c
LLJ
c'J LLJ
-j ca
(1) C) CD
x 11 11 00 : Lo I CIJ I ý I I I cn I I 0 6-4 LC) 0i I kD I m I S- C3. x I W C-) im
(a. M 0- - :
C)
Cý
(L) CQ C%j rý- I rý% I ILD 1 4.0 I tn (A S- to i cv) I cn I CIJ I CIJ I t r - o) = I I I I I I
cc: (-) I I I I I I I
LL-
LC) Ln Cý C"! C) CD , 1 1 11 11 co Lo 1.0 10 1 00 1 cli 1 ID
ca C') i 1 t t 1
U) C'j U-) I CD I CM
LLJ I U-) I oi I ci Iv I
Lc) LC) Cý Cý C) r- 11 11 to U) I LC) I cn I r, *, U Co q*" Ln I X: CD t I M 0) tý4
S- Cr = ca CL (3, co
'I C'i
'I CD
U Lo I'll Cli 1 F- 1
r- -T - 0
00- 4-) q * " : r - r- 1 C", 1-4 r- f l . c , , 11 co U
I I I I I I I
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co
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-Nd S- 1 S- 1 S- 1 I-
1 0 1 0 1 0 10 1 0
(u cm CD I CD I C) I co I C%j I (D I C"i
i ý2 1 01 i Pl% i 04
"a
4-)
4-) u =2
-0 0 S- CL
o-% to
74
LLI
V)
V)
LLJ
LLJ
P-4 1.. 4 cm
0-4
tn F-
P-4 X C)
LL.
C) *--q F-
LLJ
ca c'j
LLJ
-i co
00 C) 1. -0 < r-ý 1 r- 1 CD I Kr I Pl% I co oll
C7) co a-
mx r*4% I
1 CY) 1
1 00 1
1 t. 0 1
1 00 1
1 qzr CY)
C3
00 1 1 1 1 C) I I I I I I
< 0) r. % I ko I LO I qzr I m I tD I CY) im" = %, 0 1 m I LO I zr I Lo I C*. j I im 1. +
CL 1.0 C%J I r- I rý, 1 00 1 CO O's I It*" m OD LO I C? ) I
- q*l I CY) I -*, :
r- I r-
I CD I I M I (D co tD I LO I kD I LC) I LO I cli I m I I I I I I
L) , I cn I r., I r, I c) I c) 1 00 CO 10 1 Cli I LO I CY, I LO I I
cm CY) to 1 (31 1 LO I C%i I km I LO C36 LO (Y) I -cr I (Y) I I** I C\i I CL
CL I co I cli I C)
C3 I M I O's I LO I M C\J
Ln I m I W-T I m cli :
C) m( D clq I co I Kr I to I LO I LO I LO
. m F- q: r I C%j I co I " I m I I
U
ca (0 S--a CL X rlý I C) I r-. I to I t. 0 I CD I CD
0 ko I qd" 1 -1*1 cli C\j I
4-3 1 1
C-) I C-) I C-) L) 0 1 0 1 0 1 0 1 0 1 0 0
C) I LO I C) I C) I LC) I LO I C) C) 1 C14 1 Lf) LO rl% I r" I 0ý
I 0i
4-3 1 4-) 4-) 4.3 1 4-) 1 4-) :
4-) (o I to I (a I (ts I to I (a I cc
I I I I (A 1 0 1 (A I (A W I (A s- I S-
0 1 0 1 0 1 0 1 0 1 0 1 0
CD CD C) co I C%j I C) I C"i 4) co co r-. LO I CIA rl%
0)
(0
75
10-1 4.
3
STRESS (M Pa)
2
I
CII-R
0LII 10 100 1000
TIME (minutes)
Fig. 2.31. Continuous stress retaxation of IIR and CHR
extended 50% at 250C (125)
. 16-1 -
2-0-
STRESS (M Pa)
1-5-
1.0 -
0.5-
0
r TTD
10 100 1000 TIME (minutes)
Fig. 2.32. Continuous stress relaxation in IIR and CIIR extended 50% CLt 100oC. (125)
76
3
3
3
Mc X10 -3 3
(molecular weight 3 between crosslink t ).
2
2
2
2
10 20 30 40 50 60 TIME OF CURP. (minutes)
Fig. 2.33. Average molecular weight of chain between (125)
crosslinks (Mc) against cure time for CHR and 1IR325.
Recipe: 1. IIR 325 = 200, St-acid = 2, ZnO = 10, TDEDC = 1.5, Sulphur = 3.0
2. CIIR = 200, St. acid = 2, ZnO = 10, TMTD = 1.0
77
CHAPTER 3
COMPOUND PREPARATION AND TESTING
3.1 INTRODUCTION
This chapter deals with the preparation and testing of'specimens for
mixing, moulding and testing of physical properties and are described in the following sections.
3.2 MATERIALS
3.2.1 Rubber
Three types of rubber were used in this work, IIR Polysar 301, CIIR HT1066 (Esso) and BIIR X2 (Polysar). They had the following
specified properties shown in Table 3.1. ,
3.2.2 Compounding Ingredients
3.2.2.1 Curatives
The curatives used as essential additives in the compounding of rubber: ZnO (Anchor), stearic acid (Anchor), sulphur (Anchor), accelerators (MBTS, TMTD) (Vulnax).
3.2.2.2 Carbon Black
Sterling V (GPF N-660).
3.2.2.3 frocessing Oil
Flexon 845 (Esso) and paraffinic oil (Iraq). They had the following specified properties (Table 3.2).
78
cv,
LLI -i Co
q: c ce b-4
LL. CD
vi Lia
LU c2- c> CY- Ci-
u b--4 LL.
LLJ
>1 4-J 0 M C\j = LLI 00 ON J
-in ý U-) Lc) cr) (3)
ý U
S- kD c = Lý c; r
08
C14 X
to 00 LO CY) 00 LO cn LO C, 4 Pý% rl% (A 00 r- r- a C) C) C) 0
S-
04 m to C) C"i W ko I- C) m LO 0*1
VI I I LO CD C) C)
4-) 0 LO C)
Cý
x
4J
4. )
>) 4J
(A 0 W a) 4. ) u L) 4-) CL > C W0 0 4-) a C7) Wz 4-)
.ý Ln C) r. > cm C) C7) 4-A 4-3 > 4-)
+3 4 ) 3c 0 . - f
4. ) r- Q)
>b
ol 00 + S- 4-) 4-1
r- r 0
o to Cl) u r- r- 4 4-3 0)
-j L. Ca C )
a 4-) V
0 0 ' .. ) V )
79
TABLE 3.2: TYPICAL PROPERTIES OF THE PROCESSING OILS USED
*
Iraqi Paraffinic Oil
Esso Flexon Oil 845
Specific gravity 15.8*C/15.8*C 0.863-0.879 0.870 (60OF/600F)
Flash point OC (OF) 200 (392) 235 (455) maximum
Pour point OC (OF) 7 (45) 10 5, M) Viscosity at 600C centistokes 15.3 16.0 (Cst) at 900C 6.25 6.85 Aniline point OC (OF) 104 (219) 98.6 (209) Refractive Index at 20OC 1.49 1.479
maximum Volatile matter:
3 hours at 162.70C (3250F) - 0.2 15 minutes at 1200C (2500F) 1.0 -
maximum Viscosity gravity constant 0.796-0.814 0.808 at 200C Ash content, % 0.05 0.01
* Petrolurn Institute test --methods
3.2.2.4 Othei-'Compounding'Ingredients
mgO (Maglite D) (Croxton+Garry) Antioxidant ADPA7RODS (Anchor)
MBI (Bayer) Copper Inhibitor ZDC (Anchor)
80
3.3 PREPARATION OF TEST SPECIMENS
3.3.1 Mixing of Rubber Compound
All compounds formulations-were mixed in two stages in an internal
mixer with fill factor of 0.85. The basic compound formulations used in this work
(129,145) on heat resistant inner tubes are listed in
Table 3.3.
TABLE 3.3: BASIC COMPOUND FORMULATION USED FOR HEAT RESISTANT TYRE INNER TUBES
Formulation Parts by Weight. IIR 301 CIIR HT1066 BUR X 2
Rubber 100 100 100 GPF-N660 Black 60 65 67.5 Stearic Acid 1 1 1 ZnO 5 5 3 MgO (Maglite D) - 0.3 0.75 MBTS 2 2 0.4 TMTD 1 1 0.75 Sulphur 1 - - Paraffinic Oil Variable over Variable over Variable over
the range the range the range (Flexon 845 and Iraqi Oil) (0-25) (0-20) (0-22.5)
3.3.2 Mixing Cycle
The following order of adding the ingredients was used:
0 81
Butyl Rubber (IIR):
a) Rubber b) ZnO, a portion of the carbon black
C) Stearic acid, remaining ingredients and oil d) Maximum dump temperature (1600C)
e) Curatives (MBTS, TMTD, Sulphur), were added in a second mixing stage.
The dump temperature of this should not exceed 1100C.
2. Halogenated Rubber (CIIR and BIIR):
a) Rubber(i) b)- Stearic acid, a portion of black, MgO, MBTS
C) Remaining ingredients, except curative (ZnO, TMTD) Maximum dump temperature 140*C
d) Curative in second mixing stage, the dump temperature should not exceed 950C.
3.4 TESTING OF UNVULCANISED AND VULCANISED'COMPOUNDS
Testing of the unvulcanised and vulcanised rubber mixes was mostly carried out using the following standard procedures (further details
are given in Appendix 1).
Number Type'of Test
BS 1673: Part 3: 1969 Mooney viscosity of unvulcanised compound
BS 903: Part A2: 1971 Stress-strain properties of vulcanised compounds
BS 903: Part A26: 1969 Hardness measurements of vulcanised compounds
BS 903: Part A6: 1969 Determination of compression set at constant strain (25%)'
BS 903: Part A3: 1982 Tear strength of vulcanised compounds ASTM D2084-71 T Rubber vulcanisation characteristics
as measured by ODR
82
3.5 Experimental
The aim of the present work (in this chapter and Chapter 4) was to investigate the effect of the processing oil level on the physical properties of IIR, CIIR and BIIR vulcanisates, and its effect on the heat resistance of these polymers judging this by the change in the physical properties retained after'ageing at such elevated temperatures.
Mixing of the rubber compounds of different oil levels and polymers, as shownAn Table3-3 , was carried out in a Banbury of .
1500 cc capacity. Using the optimum cure time (T95) obtained from the rheometer trace, mixes were cured in an electrically heated press at 171±10C under a pressure of 0.5 ton per square inch.
Physical testing of specimens was carried out on a JJ tensile tester in accordance with BS 903 procedures.
3.6 Results and Discussion
3.6.1 Vulcanisati6n Characteristics of Unvulcanised'Compound
This section illustrates the comparison of rheometer curves of IIR 301, CIIR HT1066 and BIIR X2 compounded with different levels of two processing oils of differing origins.
Control of vulcanisate properties is usually brought about by changing the levels of different types of plasticisers. It can be seen that such changes are effective in changing the ODR torque of the compound and hence the scorch and curing time (see Table 3.4).
Gýaphs 3.1 to 3.9 show the diffe , rence in curing characteristics of IIR,
CIIR and BIIR, in formulations based on'0-25 phr of Esso and Iraqi oil. It is apparent that the low curing functionality of IIR leads to very
83
slow curing rates compared--to the halogenated butyl rubber, which contains the, type of dual functionality that enables it to be
vulcanised with ZnO alone or a ZnO/sulphur donor system and to thus have a faster cure than IIR. CIIR shows faster curing rate, shorter scorch time, and a lower ODR torque than BIIR at the 0-15 phr oil level and slightly higher ODR torque than BIIR at the 20 phr oil level.
The following observations are made with respect to the oil origins in IIR and BIIR: cure rate with Esso oil at high oil levels (15-25
phr), is faster than with Iraqi oil; whereas low oil levels (5-10 phr) exhibited the same cure rate with both oils. In CIIR curing rate was faster with Iraqi oil at the levels 5-15 phr, and Esso oil gave a little faster cure rate than the Iraqi oil at the 20 phr oil level. In other words Esso oil showed faster curing*rates with IIR, CIIR and BIIR at the higher levels (20-25 phr) of oil.
Halogenated butyl compounds showed little tendency towards reversion even during extended curing cycles. Sulphur cured IIR vulcanisates exhibited a higher torque value and longer scorch time than that of the ZnO/TMTD halogenated analogues. The optimum cure of the ZnO/TMTD
cured halogenated compounds are approximately 7 minutes in CIIR, 11
minutes in BIIR at 171OC; the sulphur cured IIR is observed to be
satisfactorily cured at 17 minutes at 1710C. Additionally the ZnO/ TMTD cure can generally be considered as giving fast cure rates and reversion resistant vulcanisates (see all ODR figures). The faster
cure rate of the halogenated butyl rubber is observed to frequently
result in a reduced scorch time which, in some cases, results in
unacceptable factory processing properties.
3.6.2' Stress-Strain Properties
This section illustrates the effect of the following parameters on the variation of the physical properties:
84
1. The variation in the oil level 2.. The origin of the oil 3. The type of the polymer.
The results of this study are listed in Tables 3.5 and 3.6 as well as graphically in Figures 3.10 to 3.12. In general it can be seen that such changes are effective in changing the modulus of the compound and hence the properties related to modulus. Increasing the oil level tends to decrease hardness, tensile strength, modulus and to increase
elongation at break and tear strength.
Iraqi oil.
Table 3.5 illustrates the effect of the pol * ymer type and the variation
of the Iraqi oil level on the physical properties, it can be seen that the IN compounds appear to give higher hardness, tensile strength, modulus at 100% elongation and compression set (except tear strength and 300% modulus) than CIIR and BUR at all oil levels.
In addition the following observations were made:
i) CIIR compounds offered higher properties than BIIR except for tensile strength and 300% modulus where the BIIR showed better
val ues.
ii) CUR compounds exhibited higher tear strength than both BIIR and IIR vulcanisates.
iii) BIIR compounds gave lower compression set values at all levels, than those of CIIR and IIR.
Esso Oit:
Table 3.6 illustrates the properties of the above compounds with Esso oil,., the behaviour of Esso oil shows similar trends to those observed
1 85
with Iraqi oil except that the BIIR vulcanisates containing Esso oil bxhibited higher tensile strength and 100% modulus than both IIR and CIIR vulcanisates.
Figures 3.10 to 3.12 compare the behaviour of the different levels
of Iraqi and Esso oil on the physical properties of MR, CIIR and BUR vulcanisates. From these figures, it is clear that there is no significant differences obtained from changing the source of the oil with the exception of the following points:
Figure 3.10 presents data for the IIR vulcanisates with Iraqi and Esso oils; it is clear that all physical properties (except hardness and tear strength) of the rubber containing the Iraqi oil are slightly hi§her than the equivalent compounds with Esso oil.
ii) Figure 3.11 illustrates data for compounds based on CUR which indicate that hardness, tensile strength, elongation, with Esso oil containing vulcanisates have higher values than those with Iraqi oil except that the Iraqi oil CIIR -gave higher 100% and 300% modulus values. In addition Iraqi oil gave higher tear strength and lower compression set at 15-20 phr oil level vulca- nisates than with Esso oil
iii) Figure 3.12 illustrates data for compounds based on BIIR. Physical properties with Iraqi oil formulations exhibited lower values of hardness, tensile strength, modulus, elongation and compression set than compounds with Esso oil. Also higher tear strength values were obtained fromBIIR vulcanisates containing low Iraqi oil level (5-10 phr)".
From all these results it can be concluded that both oils gave inner tube compounds with satisfactory physical properties.
86
cm cm ZD w
q: r in LLJ
C4 V)
UJ _j cn U
LL- CD
V) LU
LU 0- CD ac CL
C)
LO qc: r 00
c 0 x
0
LU
4. )
1-:
cli X
co
Flo
Cl!
r-
CD m
S- ro
CL
w
0-ý Lr) LC) C)
CD LO
cli
Ln C) 04 Ln a rlý CV)
Mr- C) cn Ln C: ) CD CIQ
C) CIP) CD W cli LC) cr)
4-3 LO 4-3 :3 1- CM
0 (D r-
Mr- LO CV) CY)
C%j CM cli
Mr-
Ln r- C) C*lj C%j C%j r- cli
Ul) CD r- co C) 0i
LO CL r- CIP) C) to ; lj
4-3 C\j C%j r- 4-) :3 .ý cli co 0 C)
co r*, % CV) CV")
LO C) Cli
CY) C) CD
LO 14M Ln CD
LO LO
m CY)
r-
a. r. CD
C) Ln ko CY)
LO
CD CY) ON LO Oj
4-) 4-3 :3 Lr) -0 C) : 2C - co C%j
4J LO 4-3 . r- 4-) cd ý- 4-b to 0 LO 4-) &- of
(U cli 4J (D ulmu C) (1) ci (L) U= 06 S- C) o U C: 0 c S- a
o 0 Ln 0 C) 00
V) 12 -j C) US 4-) V % ý . ) - Co.
cii
0 L)
87
,a
LLJ -. i c2
4-
cr tu S-
4J . I- :c
(. 'J
0:: 8--1 e. -8 ca
0
r-
r-
C) CY) S-
>1 0 CL
S- CL
Ul) LO Lr) lc: r 4 C CV") C\j U-) (Y) 04
r-Lr- LO Lr) 0 . r- 0)
Ln C) C) to cr)
cxý LO r- r- CC) C%j
CV)
C) Q.. r- CV) C) cn C) (31 r- Ul)
CL r- LC) LO S4 C) Lý CY) C14 C%j W-r r-
Q. r- Ul) LO . r-
- CA 43) U ) C) C*4
r- Cl-i U') r- 4-)
Pl% C)
04 wr UD
r- r-
r- 0i
S- -a LO LO ca.. ý r- C) C4
LO ;j
rlý r-
S-
ca. , b Lr) CY)
Lo C) Ul) ýn
CL LO LO
CD CD C%j LO
43) 4-)
C3. LO
Ln (D
C) Co C%j
C: ) C) to CV)
C3.. " Lo ON C) C%i Ln
co .0 0 C) c -P 0 C) u ., - 0 (A X: r-- S- S-
r- 43) ou :3 tu r, % 4-) 4-) U 0--% 0 L) Ln
. r- r- = 4J tV V) 4A Ln C) 4J 4J c *V- % CU OJ (a L. cu
4.3 'r- E
, W 4--) r- S- - U (o = o Cl) :3 it Cl) u CL I- - -0. u r. - r_ r_ C5.0 0 0 CC In 0 .ý E r- S- - Cl
u Ca Lo (n C)
ES 4j CL-r- 9 r-ý . I to C)
88
IA C
LLI
C Gr CD
70 -
60- BUR
50-
CIIR 40-
30
20 ZERO OIL
10-
0 05 10 is 20 25 30 35 40 45 so
TIME (minutes)
Fig. 3.1. Vu[canisation characteristics (ODR trace) of HR, CUR and BIIR compounded without oil, cured af 1710C, 30Arc.
89
so
70
60
50
40 co I cc a 30
20
10
A
'D
LLJ
cr 0
5 Phr ESSO OIL
I. lR
BUR
CUR
L -L 05 10 15 20 25 30 35 40 45 50 TIME (minutes)
Fig. 3.2. Vulcanisation characteristics (ODRL trace) of IIR, 'CIIR and, BlIR compounded with 5 Phr Esso oit, cured at 1710C, 3Arc.
5 Phr IRAQI OIL
IIR
10
0
BUR
1U 15 20 25 - 30 35 40 45 50 TIME (minutes)
Fig. 3.3. Vu[canisation characteristics (ODR trace) of IIR, CIIR and BUR compounded with 5Phr Iraqi oil, cured at 171 0 C, 3Arc.
90
LU
r= a
Z
LLA
CD 1--
TIME (minutes)
Fig. 3.4. Vulcani sation characteristics (ODR trace) of IIR, CHR and BUR compounded with 10 Phr Esso oil, cured at 1710C, 30Arc.
80 10 Phr IRAM OIL
70-
60- IIR
50-
40- BIIR
CHR 30-
20
'10-
n 05 10 is. 20 25 , 30 35 40 45
. 50 55
TIME (minutes)
Fig. 3.5. Vulcanisation characteristics (ODR trace) of IIR, CUR
and BUR compounded with 1OPhr Iraqi oil, curedlat 1710C, 3Arc.
.
91
70
60 Z;
so
40 c3 £Z CD "- 30
20
ISPhr ESSO OIL
IIR
10 A<, /
0 'j 05 10 15 20 25 30 35 40 45 50 TIME (minutes)
Fig. 3.6. Vulcanisation characteristics (ODR trace) of IIR, CIIR and BIIR compounded with 1SPhr Esso oil, cured at 1710C, 3Arc.
80
BUR CIIR
ISPhr 1RAQI OIL «'3 70
60
LU =) 50 ci cý CD F- 40
30
20
10
0
IIR
BII R
CIIR
05 10 is 20 25 30 35 40 4S 50 ss TIME (minutes)
Fig. 3.7. Vulcanisation characteristics (0 DR trace) of IIR, CIIR and BUR compounded with 15Phr Iraqi oil, cured at 1710C, 3Arc.
92
E
V
LLJ m
c3 cz C: )
70 20-25Phr ESSO OIL.
60 -
50 - IIR 20Phr OIL
IIR 25Phr OIL 40 -
30 CUR 20Phr OIL - BUR 22.5Phr OIL
20 -
10
TIME (minutes)
Fig. 3.8. Vutcanisation characteristics (ODR trace) of IIR, CHR and BUR compounded with Esso oil, levels cis indicated, cured at 1710C, 3Arc.
In
uj 'I ci
20-25Phr ! RAO. I OIL 60 -
50 - IIR 20PhrOlL
40 - IIR 2SPhr OIL
3o - CUR 20Phr OIL
BUR 22-5Phr OIL 20 -
10
0 05 10 is 20 25 30 35 40 45 so 55
TIME (minutes)
Fig. 3.9. Vu[canisation characteristics (0 DR trace) of IIR, CUR and- BUR compounded with Iraqi oil, levels as indicated, cured at 1710C, 3Arc.
93
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98
rl4APTFP d
HOT AIR AGEING PERFORMANCE OF BUTYL AND HALOGENATED BUTYL VULCANISATES
4.1 INTRODUCTION
The ageing of rubber and rubber compounds has been the subject of study for many years. It is generally agreed that oxygen attack is the chief cause of the degradation of rubber by the scission of the long chains of the molecular structure and also by causing varia- tions in the crosslinking. The rate of ageing or degradation does however vary very widely depending upon the conditions and circumstan- ces prevailing and also the compounding of the stock.
It has been ascertained that both the network destroying and network forming reactions carry on side by side during vulcanisation and also subsequefttly. The overall balance is the "resultant" of these two
reactions at any one time.
Various methods have been developed to provide information on the
stability of rubber vulcanisates against oxidative degradation. One
of these methods, which has a good fundamental basis, is the oxidative hot air oven ageing which is well documented for specification and evaluation purposes. It is supported by several technological methods e. g. stress relaxation, swelling in solvent, etc. In the present research both these methods were utilized in evaluating the resistance of butyl and halogenated butyl vulcanisates to oxidative degradation.
99
4.2 HOT AIR-OVEN AGEING
4.2.1 Significance
Hot air-oven ageing has been used extensively in the rubber industry
as an accelerated process of evaluation of rubber products which are exposed to the conditions existing in service.
Losses in physical properties by hot air oven ageing was used to
measure the degree of butyl and halogenated butyl rubber deterioration.
4.2.2 Experimental
Test pieces in the form of dumb-bell type (2) of BS 903: Part A2 were used and they aged by means of a Wallace-Cell Ageing Block kept at 1000C, 1250C, 1500C with a constant rate of new air flow over the
specimens. Further details concerning this method are given in BS 903: Part A19: 1975 and described in the first chapter. Test
results were expressed as a percentage of change in the properties against ageing time.
4.2.3 Results and Discussion
A comparison of different oil levels with respect to air ageing at various temperatures are shown in Tables 4.1-4.6. These illustrate
stress-strain data obtained after hot air oven ageing at temperatures
ranging from IOOOC to 1500C for various periods of time, 3,7 and 14 days.
100
1. ButyZ'Rubber. -
Tables 4.1 and 4.2 illustrate the percentage change in physical properties of compounds based on IIR (Polysar 301), with different levels of Esso oil and Iraqi oil. The following observations were obtained: there was an increase in hardness, 100% and 300% modulus at the beginning of the 1000C ageing period, and then this was followed by a decrease after ageing at 1250C and 1500C. This gain could be expected to occur as the vulcanisation process would continue at the beginning of the ageing cycles. It was found that a rapid decrease in tensile strength with temperature and time occurred, and also there was a progressive decrease in elongation at break at the temperature of 1000C. It was also observed that elongation started to increase when the ageing temperature increased towards 1500C.
All the samples were observed to become more tacky and soft with ageing.
Checking the weight of the samples showed that there were undesirable losses in weight due to evaporation of volatile materials and these losses increased with increasing temperature and ageing time (Tables 4.1 and 4.2). Compounds containing the Iraqi paraffinic oil exhibited higher percentage of deterioration than the compounds containing Esso
oil. Also this deterioration increased with increasing oil levels of both types of oil. IIR compounds containing low and high Iraqi and Esso oil levels exhibited significant breakdown and became soft after 7 days ageing at 1500C.
2. ChZorobutyl Rubber
Tables 4.3 and 4.4 illustrate the percentage of physical properties change with compounds based on CUR HT1066 rubber with both oils under the same condittons as previously used for IIR. The following observations were obtaineds
101
i) Esso Oil: On heat ageing CUR with Esso oil showed an increase in compound hardness at 1000C and 1250C; however the hardness began to decrease when an increased ageing temperature of 1500C
was used. Continuous decrease in tensile strength and elongation at break
occurred with increasing ageing time and temperature. The MIOO% and M300% moduli increased throughout the ageing period at 1000C, 1250C and then this was followed by a decrease after ageing at 1500C resulting in values higher than the original unaged values. Weight losses increased with increase of ageing time and temperature.
ii) Iraqi Oil: Similar behaviour to the Esso oil was obtained with
compounds containing Iraqi oil excepi that the hardness showed
a continuous increase with increase in temperature, and the losses in weight from the compounds with Iraqi oil were greater than with the Esso oil. CUR compounds containing Iraqi oil exhibited better resistance to ageing than compounds containing the Esso oil.
3. Bromo ButyZ Rubber
Tables 4.. 5 and 4.6 record ageing data for BIIR vulcanisates with both
oils.
i) Esso oil: It can be seen that there is a continuous increase in hardness, and decrease in tensile strength and elongation with increasing temperature. The 100% and 300% moduli increased throughout the ageing periods at 1000C and 1250C; and also for the period of 3 days at 150OC; however at 1500C for 7 and 14 days a decrease in value occurred for 100% and 300% moduli resulting in values less than the original unaged values. Losses in weight again increased with increase of ageing time and temperature.
102
Iraqi oil: Similar results were obtained from compounds contai- ning Iraqi oil except that an increase in tensile strength occurred at 1000C, probably due to an insufficient state of
0 cure, while at temperatures of 125 C and 1500C the reverse was true. Losses in weight from the compounds with Iraqi oil were greater than with Esso oil. Percentage deterioration of BUR with both oils was nearly equal. t
General observation of the halogenated butyl vulcanisates showed them to retain their original appearance as well as a good percentage of their physical properties.
4.2.4 Conclusions
All three polymers (IIR, CUR, BIIR) exhibited continuous reduction in tensile properties with increasing ageing time and temperature. Judging reversion by tensile strength the compound containing the highest amount of oil showed the greatest rate of degradation. So to obtain better properties retention, the lower oil level com- pounds are preferred.
Halogenated butyl rubber offers an appreciably higher level of heat
resistance than butyl cured with conventional sulphur vulcanisation systems. At the temperature of 1500C the butyl compound became soft and sticky and lost all its physical properties, whereas the haloge-
nated butyl compound retained its original appearance by remaining rubbery and tack-free,. as well as keeping a good percentage of its
original physical properties. Sulphur donor accelerator (ZnO/TMTD)
sulphurless cured halogenated butyl exhibited better retention of properties than a sulphur cured butyl vulcanisate. Thus sulphur cured butyl confirmed the poor high temperature thermal stability of alkyl- alkenyl polysulphide bonds which formed during vulcanisation.
103
Effect of the Oil Type on the Ageing Properties:
I IIR: butyl compounds containing Esso oil exhibited better heat
resistance than those with Iraqi oil.
2. CIIR: chlorobutyl compounds containing Iraqi oil showed better heat ageing properties than compounds containing Esso
oil.
3. BIIR: compounds containing either Iraqi or Esso oil showed equivalent ageing resistance.
Note:
It is additionally noted that an ageing temperature of 1000C was not the most suitable temperature at which to measure and compare the degree of properties degradation because unexpectedly the
vulcanisation of some of the samples continued to occur at this temperature.
2. There are some fluctuations in the ageing results due to the
complications introduced by the existence of this incomplete
vulcanisation and the instability of some of the samples; it is
considered that this has resulted in an experimental error which is nearly 10%.
104
TABLE 4.1: HEAT AGEING PROPERTIES OF BUTYL POLYSAR 301 RUBBER WITH ESSO FLEXON 845 OIL*
Without 5 phr 10 phr 15 phr 20 phr 25 phr Oil Oil Oil Oil Oil Oil
Aged at 1000C for 3 days Hardness +1.3 +1.4 +1.6, +1.7 +3.6 +7.7 Tensile
Strength -3.2 -3.7 -5.2 -11 -11.5 -24 Elongation
at break % -15 -18 -18.4 -19.2 -22 -22.8 Modulus at
100% Elong. +51 +34.3 +34 +30 +25.4 +24 Modulus at
300% Elong. +24.3 +20 +20 +19.1 +17.6 +18 Loss in
Weight % -3.9 -2.6 -1.3 -1.2 -1.2 -1.0 7 days @ 1000C Hardness +0 -1.4 +0 +3.5 +7.4 +11.5 Tensile
Strength -4.7 -9.1 -15.7 -15.6 -18.8 -33 Elongation
at break % -20 -22 -22.4 -23 -24.4 -25 Modulus at
100% Elong. +46 +31.4 +34 +24.6 +16.4 +13.7 Modulus at
300% Elong. +20 +18.6 +18.1 +18 +13.7 +11 Loss in
Weight % -0.3 -0.4 -0.3 -0.5 -0.3 -0.4
14 days@ 1000C Hardness -1.3 -5.8 -3.2 +0 +0 +1.9 Tensile
Strength -9.1 -9.7 -10.1 -11.9 -14 -24.9 Elongation
at break % -18 -18 -20 -20 -20 -21.4 Modulus at
100% Elong. +46 +20.2 +46.3 +40.7 +35 +31 Modulus at
300% Elong. +20.5 +22.3 +26 +27.5 +27.2 +28.2 Loss in
Weight % -0.5 I -0.6 I -0.5 I -0.6 1. -0 4 1 *- -0.7 I
-j * as % chanqe
Continued..
105
TABLE 4.1 ... continued
Without Oil
5 phr Oil
10 phr Oil
15 phr Oil
20 phr Oil
25 phr Oil
Aged at 1250C for 3 days
Hardness -6.8 -9.4 -9.5 -9.6 -10.9 -11.5 Tensile +0 -1.13 -1.1 -3.3 -4.4 -2 7
Strength Elongation
-5 9 -10.4 -11.5 -14 -14.3 at break % Modulus at +34 +18.5 +18.1 +16.2 +13.9 +13.7 100% Elong. Modulus at +16.2 +19.3 +20.2 +22.6 +23.4 +25.3 300% Elong. Loss in
Weight % -0.4 -0.5 -0.3 -0.26 -0.3 -0.4
7 days @1250C
Hardness -17.8 -23.2 -24.6 -25.4 -25.5 -25.9 Tensile
Strength -2.2 -9.8 -13.3 -18.2 -24.1 -37.7 Elongation +15 +9 +4 +0 -3.7 -7.1 at break % Modulus at
100% Elong. -10.4 -11.8 -21 -21.5 -22 -26.3 Modulus at
300% Elong. -7.5 -7.7 -12.2 -12.4 -12.6 -18.2 Loss in
Weight % -0.5 -0.4 -0.6 -0.5 -0.6 -0.8
14 days @1250C
Hardness -27.4 -37.7 -40.4 -40.4 -41.8 -42.3 Tensile
Strength -24.7 -29.6 -36.6 -40.6 -44.3 -57.5 Elongation +20 +18 +4 +0 +0 -7.1 at break % Modulus at
100% Elong. -23 -27 -27.6 -32 -36.8 -42.7 Modulus at
300% Elong. -28 -32.3 -38.5 -32.6 -35.3 -45.1 Loss in
Weight % -0.6 -0.5 -0.9 -1.0 -1.1 -1.4
106
TABLE 4.1 ... continued
Without Oil
5 phr Oil
10 phr Oil
15 phr Oil
20 phr Oil
25 phr Oil
Aged at 1500C - f or 3 Tý s-
Hardness -37.6 -49.2 -54.7 - - - Tensile
Strength -62.9 -66.8 -78 -79.3 -79.3 -84.8 Elongation
at break % +35 +41 +24 +23 +18.5 +17.8 Modulus at
1OOZ Elong. -53.5 -61.2 -61.6 -63.7 -64 -67.5 Modulus at
30OZ Elong. -67.8 -72 -73.2 -73.4 -73.7 -78 Loss in
Weight Z -1.1 -1.5 -1.8 -2.8 -3.2 -5.2
7 days @1500C
Hardness - - - - - - Tensile
-82 7 -87 -90 -90 7 Heavily* Heavily Strength . . Degraded Degraded
Elongation at break % +40 +45.4 +44 +35.7
Modulus at 10OZ Elong. -62.8 -67.4 -67.5 -71
Modulus at 300% Elong. -80.7 -85.2 -87.2 -87.3
Loss in Weight Z -0.9 -2.0 -3.3 -4.3 -5.3 -6.6
14 days@ 1500C
Hardness Heavi ly Degraded Degraded Degraded Degraded Degraded Degraded
Tensile if 11 11 11 If Strength Modulus at If If it 11 100% Elong. Modulus at
30OZ Elong. Loss in
Weight % -1.6 -3 -4.61 -6.1 6.9 -9
* Soft and non-elastic
107
TABLE 4.2: HEAT AGEING PROPERTIES OF BUTYL POLYSAR 301 RUBBER WITH IRAQI PARAFFINIC OIL *
Without Oil
5 phr Oil
10 phr Oil
15 phr Oil
20 phr Oil
25 phr Oil
Aged at 1000C for 3 days
Hardness +1.3 +1.5 +1.6 +1.7 +1.8 +4 Tensile
Strength -3.2 +0 -2.8 -6.4 -10.5 -3.6 Elongation
at break % -15 -12 -15.4 -15.4 -18.5 -17.8 Modulus at
100% Elong. +51 +12.8 +23 +26 +29.8 +41 Modulus at
300% Elong. +24.3 +13.3 +18 +22.1 +23.1 +31.1 Loss in
Weight % -3.9 +0 +0.3 +0.2 +1.3 +0.3
7 days@ IOOOC
Hardness +0 +2.3 +2.5 +2.5 +2.7 +6.1 Tensile
Strength -46.8 -11.5 -16 -17.9 -22.7 -17.7 Elongation
at break % -20 -20 -19.2 -20 -22.2 -21.4 Modulus at
100% Elong. +46 +16 +28.7 +29.2 +31.2 +48 Modulus at
300% Elong. +20 +8 +14.7 +17.4 +17.5 +19.6 Loss in
Weight % -0.4 -0.6 -0.6 -0.6 -0.4 -0.1
14 da
Hardness -1.3 +2.3 +3.3 +3.3 +3.5 +7.1 Tensile
Strength -9.1 -18.5 -20.7 -23.6 -31.8 -24.6 Elongation
at break % -18 -24 -23.1 -23 -29.6 -25 Modulus at
100% Elong. +46 +11.3 +24.5 +23.7 +20.1 +18 Modulus at
300% Elong. +20.5 +9.7 +18.3 +19.4 +20.9 +22.8 Loss in
Weight % -0.5 I
-0.2 I
-1.1 I
-1.13 I
-1.24 I
-1.3 II
* As % change
108
TABLE 4.2 ... continued
Without Oil
5 phr Oil
10 phr Oil
15 phr Oil
20 phr Oil,
25 phr Oil'
Aged at 1250C- for 3 days
Hardness -6.8 -9.4 -13.3 -14.4 -15.1 -15.3 Tensile Strength _ýO -22.8 -26.8 -35 -40.7 -42.7
Elongation at break % -5 -12 -7.7 -7.7 -7.4 -7
Modulus at 100% Elong. +34 -4.6 -7.2 -20.1 -20.8 -23
Modulus at 300% Elong. +16.2 -6.2 -18 -20.5 -24.9 -25.6
Loss in -Weight % -0.4 -0.5 -0.57 -0.58 -0.6 0.62
7 days @ 125c)C
Hardness -17.8 -23.4 -26.7 -28 -28.6 -28.6 Tensile Strength -2.2 -28.7 -36.3 -40.7 -43.2 -43.8
Elongation at break % +15 +0 +3.8 +3.8 +3.7 +4.3
Modulus at 100% Elong. -10.4 -27.3 -29
- .3 -31.7 -36.4 -43
Modulus at 300% Elong. -7.5 -27 -30.3 -31.3 -32 -33.3
Loss in Weight % -0.5 -0.6 -1.1 -1.2 -1.4 -1.5
14 days@ 1250C
Hardness -27.4 -32.8 -33.3 -35.6 -38.4 -40.8 Tensile Strength -24.7 -47.7 -56.5 -57.4 -58.7 -59.8 Elongation at break % +20 +4 +46 +46 +5.1 +7.7
Modulus at 100% Elong. -23 -40.7 -42.5 -47.5 . -51.9 -52
Modulus at 300% Elong. -28 -48.6 -50.2 -51.1 -51.1 -52.3
Loss in Weight % -0.6 -1.0 -1.5 -1.8 -1.9 -2.4
109
TABLE 4.2 ... continued
Without Oil
5 phr Oil
10 phr Oil
15 phr Oil
20 phr Oil
25 phr Oil
Aged at 1500C for 3 days
Hardness -37.6 -50 -50 - - - Tensile
Strength -62.9 -82.8 -85.3 -87.4 -88.4 -89 Elongation
at break % +35 +12 +15.3 +19.2 +22.2 +25 Modulus at
100% Elong. -53.5 -72.9 -77.3 -77.4 -77.5 -77.7 Modulus at
300% Elong. -67.8 -82 -84 -85.5 -86.8 -87.2 Loss in
Weight % -1.1 -1.8 -3.0 -3.5 -4.2 -5
7 days@ 1500C
Hardness - - - - - - Tensile
Strength -82.7 -92.33 -95.5 --96.3 Heavi ly* Degraded Degraded
Elongation +40 +20 +23 +38.5 at break %
Modulus at 100% Elong. -62.8 -82.1 -83.5 -83.6 If
Modulus at 300% Elong. -80.8 -91.8 -92.1 -93.6
Loss in Weight % -0.9 -2.2' -3.5 -4.5 -4.9 -5.7
14 days@ 1500C
Hardness Heavi ly Degraded Degraded Degraded Degraded Degraded Degraded
Tensile Strength
Elongation at break %
Modulus at 100% Elong.
Modulus at If of it 300% Elong. Loss in
Weight % -1.6 -2.5 -4.2 -5.4 -6.3 -6.8
* Soft and non-elastic
110
TABLE 4.3: HEAT AGEING PROPERTIES OF CHLORORUTYL. HT1066 RUBBER WITH ESSO FLEXON 845 OIL*
. ............ Without
. oil 5 phr Oil
10 phr Oil
15 phr Oil
20 phr Oil
Aged at 1000C for 3 days
Hardness +0.9 +2.6 +2.7 +3.2 +3.4 Tensile Strength -4.2 -10.2 -0.1 -0.4 -1.5 Elongation at break % +5 +0 -4.1 -11.1 -14.7 Modulus at 100% Elong. 0 +11.2 +15.4 +15.7 +17.8 Modulus at 300% Elong. -6.8 +12.3 +13.5 +15.5 +19.8 Loss in Weight % -0.3 -0.2 -1.2 -1.1 -0.4
7 days at 1000C
Hardness +1.7 +1.7 +0 +0 +2.2 Tensile Strength -0.8 -1.1 -1.8 -2.2 -4.6 Elongation at break % +0 -4.5 -8.3 -11.1 -17.8 Modulus at 100% Elong. +3.8 +13.9 +15.4 +18.7 +24.4 Modulus at 300% Elong. +7.4 +14.3 +15 +16.7 +21.5 Loss in Weight % -0.24 -0.33 -0.36 -0.4 -0.44
14 days at 1000C
Hardness +0 +0 +1.8 +2.1 +2.2 Tensile Strength -4.6 -5.3 -5.4 -6.1 -6.2 Elongation at break % -10 -13.6 -16.6 -18.5 -21.4 Modulus at 100% Elong. +9.3 +18.3 +19.9 +25.8 +25.2 Modulus at 300% Elong. +14 +19.5 +20.6 +20.8 +29.9 Loss in Weight % -0.38 -0.39 -0.46 -0.48 -0.53
* as % change
ill
TABLE 4.3 ... continued
Without Ofl
5 phr Oil
10 phr Oil
15 phr Oil
20 phr Oil
Aged at 1250C for 3 days
Hardness +3.4 +3.5 +3.7 +4.3 +7.9 Tensile Strength -1.5 -1.6 -1.6 -2.9 -3 Elongation at break % -10 -13.6 -20.8 -22.2 -25 Modulus at 100% Elong. +3.2 +34.8 +37.2 +37.2 +43.7 Modulus at 300% Elong. +1.9 +29.7 +30.1. ' +30.7 +34.7 Loss in Weight % -0.25 -0.54 -0.65 -0.66 -0.66
7 days at 1250C
Hardness +2.5 +0 +0.9 +2.1 +2.3 Tensile Strength -9.4 -9.6 -7 -3.2 -11.9 Elongation at break % -20 -22.7 -25 -29.6 -35.7 Modulus at 100% Elong. +3.7 +45 +45.4 +51.2 +52 Modulus at 300% Elong. +8.3' +34 +34.3, +42 +42.7 Loss in Weight % -0.59 -0.62 -0.7 -0.74 -0.85
14 days at 1250C
Hardness +0 +0 +1.8 +3.2 +3.4 Tensile Strength -9.8 -9.7 -10.3 -14.6 -17.4 Elongation at break % -25 -31.8 -33.3 -33.3 -35.7 Modulus at 100% Elong. +29.2 +49.5 +52.8 +53.8 +60.5 Modulus at 300% Elong. +24.5 +52.6 +42.5 +42.5 +44.7 Loss in Weight % -0.6 -0.7 -1.0 -1.3 -1.3
112
TABLE 4.3 ... continued
Without Oil..
5 phr Oil
10 phr Oil
15 phr Oil
20 phr Oil
Aged at 1500 c for 3 days
Hardness -3.3 -3.5 -5.5 -6.4 -7.9 Tensile Strength -18.2 -19.2 -19.2 -27 -27.5 Elongation at break % -25 -27.2 -25 -33.3 -32.1 Modulus at 100% Elong. +7.3 +22.6 +27.6 +31.4 +35.4 Modulus at 300% Elong. +16.1 +20.3 +16.4 +25.5 +26 Loss in Weight % -0.6 -1.0 -1.3 -1.4 -1.5
7 days at 1500C
Hardness -6.7 -5.3. -8.3 -8.5 -10.2. Tensile Strength. -41.7 -41.7 -45.6 -49.1 -53.4 Elongation at break % -35 -36.4 -37.5 -40.7 -42.8 Modulus at 100% Elong. +7.9 +28.3 +30.5 +32.8 +35.6- Modulus at 300% Elong. -17.5 -1.6 -7 -7 -5.7 Loss in Weight % -0.8 -0.9 -1.0 -1.2 -1.9
14 days at 1500C
Hardness -8.4 -10.7 -11.1 -11.7 -12.5 Tensile Strength -51.9 -55.5 -55.7 -58.1 -62.2 Elongation at break % -45 -50 -50 -51.8. -53.5 Modulus at 100% Elong. +12.1 +32.6 +32.4 +38.1 +37.3 Modulus at 300% Elong. - - -19 -18.3 -16.7 Loss in Weight % -1.7 -2.0' -3.3 -4.5 -5.3
113
TABLE 4.4
HEAT AGEING PROPERTIES OF CHLOROBUTYL HT1066 RUBBER WITH IRAQI PARAFFINIC OIL*
Without Oil
5 phr Oil
10 phr Oil
15 phr Oil
20 phr- Oil
Aged at 1000C for 3 days
Hardness +0.9 +3.5 +1.9 +1.1 +1.2 Tensile Strength -4.2 -0.3 -2.2 -3.7 -5.1 Elongation at break % +5 +0 -4.3 -4.2 -7.4 Modulus at 100% Elong. -0.04 +75.6 +13 -15 +9.2 Modulus at 300% Elong. -6.8 -3.5 -4.2 -2.3 -1 Loss in Weight % -0.3 -1.7. -0.6 -0.5. -0.6,
7 days at 100 0c
Hardness- +1.7 +10.5 +4.8 +4.3 +3.6 Tensile Strength -0.8 +6.3 +6.9 +3.1 +7.9 Elongation at break % +0 +0 -4.3 -8.3 -11.1 Modulus at 100% Elong. +3.8 +8.0 +24.7 +4.3 +23 Modulus at 300% Elong. +7.4 +6.9 +14 +13.6 +19.4 Loss in Weight % -0.2 -0.6 -0.8 -1.0 -1.1
14 days at 1000C
Hardness +0 +12.3 +7.7 +7.6 +7.3 Tensile Strength -4.6 +15.2 +10.4 +4.1 +8.8 Elongation at break % -10 -14.3 -14.8 -16.6 -18.5 Modulus at 100% Elong. +9.3 +12.5 +33 +14.3 +14.3 Modulus at 300% Elong. +14 +37.6 +34.2 +26.7 +33.7 Loss in Weight % -0.9 _-0.7 -0.8 -1.1 -1.3
* as % change
114
TABLE 4.4 ... continued
Without Oil
5 ''phr Oil
10 phr Oil
15 phr Oil
20 phr Oil
Aged at 1250C for 3 days
Hardness -3.4 +18.5 +2.8 +2.2 +1.2 Tensile Strength -1.5 -0.1 -0.5 -2.6 -4 Elongation at break % -10 -9.5 -13 -16.6 -22.2 Modulus at 100% Elong. +3.2 +8.7 +12 +9 +8.3 Modulus at 300% Elong. +1.9 +7.4 +14.3 +23.1 +23.3 Loss in Weight % -0.25 -0.43 -0.5 -0.54 -0.57
7 days at 125 oc
Hardness -2.5 +25.9 +3.8 +3.2 +2.4 Tensile Strength -9.4 -1.4 -3 -6 -7 Elongation at break % -20 -19 -21.7 -25 -26 Modulus at 100% Elong. , +3.7 +12.7 +24.7 +15.6 +11.7 Modulus at 300% Elong. +8.4 +22 +27 +28.7 +33 Loss in Weight % -0.59 -0.68 -0.69 -0.76 -0.84
14 days at 1250C
Hardness +0 +27.7 +4.8 +4.3 +3.6 Tensile Strength -9.8 -2.2 -6.8 -11.5 - 11.9 Elongation at break % -25 -23.8 -26 -29.1 -29.6 , Modulus at 100% Elong. +29.2 +31.5 +31.8 +27.7 +20 Modulus at 300% Elong. +24.6 +24.4 +28 +29.4 +37.9 Loss in Weight % -0.6 -0.9 -1.3 -1.4 -1.7
115
TABLE 4.4 ... continued
Without . oil
5 phr ... Oil
10 phr Oil
15 phr Oil
20 phr Oil
Aged at 1500C for 3 days
Hardness -3.3 +24.5 +7.7 +6.5 +6 Tensile Strength -18.2 -3.5 -9.2 -1 5 -20.9 Elongation at break % -25 -19 -21.7 -25 -29.6 Modulus at 100% Elong. +7.4 + 11.9 +38.5 +25.2 +23 Modulus at 300% Elong. +16.1 +29.2 +20.2 +22.7 +22 Loss in Weight % -0.6 -1.9 -2.5 -3.3 -4.1
7 days at 1500C
Hardness -6.7 +26.3 +9.6 +8.6 +7.3 Tensile Strength -41.7 -22.8 -39.1 -46.0 -46.7 Elongation at break % -35 -42.8 -39.1 -41.6 -44.4 Modulus at 100% Elong. +7.9 + 48.8 +40.3 +26.3 +25.6 Modulus at 300% Elong. -17.55 +26.7 +0 -4.7 -6.2 Loss in Weight % -0.8 -2.2 -3.0 -3.7 -5.5
14 days at 1500C
Hardness -8.4 +26.3 +10.6 +9.8 +9.7 Tensile Strength -51.9 -48.6 -56.1 -60.6 -61.6 Elongation at break % -45 -47.6 -47.8 -50 -51.8 Modulus at 100% Elong. +12.2 + 21.9 +40.3 +24.6 +23.7 Modulus at 300% Elong. - - - - - Loss in Weight % -1.7 -2.8 -3.7 -4.8 -6.0
116
TABLE 4.5:
HEAT AGEING PROPERTIES OF BROMO BUTYL X2 RUBBER WITH ESSO FLEXON 845 OIL*
Without oil
5 phr Oil
10 phr Oil
15 phr Oil
22.5 phr Oil
Aged at 100 0c for 3 days
Hardness +10 +11.5 +12 +12 +12.5 Tensile Strength -6.5 -4 -0.8 -0.3 -0.5 Elongation at break % -11.1 -10.5 -10 -12.4 -13 Modulus at 100% Elong. +33 +29.3 +18.8 +13.8 +13.7 Modulus at 300% Elong. +4.2 +24.6 +24.8 +26 +28.4 Loss in Weight % -0.29 -0.17 -0.25 -0.07 -0.1
7 days at 100 0c
Hardness +12.7 +13.4 +14 +18.4 +15 Tensile Strength -9 -3.9 +2.5 +4.4 -6.2 Elongation at break % -16.6 -17.8 -18 -19 -21.7 Modulus at 100% Elong. +44 +35 +25.3 +22.1 +20.7 Modulus at 300% Elong. +18.4 +27.2 +37.8 +43 +48.7 Loss in Weight % -0.22 -0.15 -0.26 -0.43 -0.22
14 days at 100 oc
Hardness - +12.7 +15.3 +18 +18.5 +18.7 Tensile Strength -rlO. 5 -4 --6.2 +4.2 -0.6 Elongation at break % +16.6 +18.9 -20 -20.9 -21.7 Modulus at 100% Elong. +72.8 +51.4 +38.4 +38.3 +35.3 Modulus at 300% Elong. +18.2 +47.9 +35.3 +48.5 +49.2 Loss in Weight % -0.23 -0.05 -0.14 -0.3 -0.33
* as % chanqe
117
TABLE 4.5 ... continued
Without 5 phr Oil
10 phr Oil
15 phr Oil
22.5 phr Oil
Aged at 125 0c for 3 days
Hardness +14.5 +15.3 +15.3 +17.4 +17.5 Tensile Strength -1.3 -2.9 -6.8 -9.7 -8.4 Elongation at break % -22.2 -21 -20 -19 -17.4 Modulus at 100% Elong. +60.8 +50.8 +33.3 +21 +31.9 Modulus at 300% Elong. +19.2 +39.5 +39.7 +32 +41.9 Loss in Weight % -0.36 -0.37 -0.39 -0.49 -0.5
7 days at 125 0c
Hardness +15.4 +17.3 +18 +19.5 +20 Tensile Strength -4.8 -5.3 -8.9 -9.2 -11.2 Elongation at break % -27.8 -26.3 -25 -23.8 -17.4 Modulus at 100% Elong. +67.9 +55.5 +46.5 +29.3 +26.7 Modulus at 300% Elong. +23.7 +39.8 +41.7 +42.6 +34.9 Loss in Weight % -0.5 -0.6 -0.6 -0.7 -1
14 days at 1250C
Hardness . +16.4 +20.2 +21 +21.7 +22.5
Tensile Strength -11.4 -12.4 -16.9 -17.6 -16 Elongation at break % -27.8 -26.3 -25 -25 -21.7 Modulus at 100% Elong. +79.9 +59 +47.7 +41.3 +37.5 Modulus at 300% Elong. +20.6 +27.9 +31.4 +34.4 +34.4 Loss in Weight % -0.9 -1.0 -1.06 -1.16 -1.2,
118
TABLE 4.5 ... continued
Without 5 phr Oil
10 phr Oil
15 phr Oil
22.5 phr Oil
Aged at 1500C for 3 days
Hardness +1.8 +4.8 +5 +12 +21.3 Tensile Strength -24.6 -19.3 -19.2 -18.8 -17.3 Elongation at break % -27.8 -28.4 -29 -29.5 -30.4 Modulus at 100% Elong. +27.7 +32.7 +33.3 +32.6 +54.3 Modulus at 300% Elong. +5.4 +25.3 +26.8 +38.4 +44.9 Loss in Weight % -0.5 -1.8 -2.1 -2.7 -3.9
7 days at 150 0c
Hardness +0 +0.9 +1 +1.1 +6.3 Tensile-Strength -46 -43.3 -39.4 -37.6 -35.8 Elongation at break % -33.3 -34.7 -35 -35.2 -39.1 Modulus at 100% Elong. +24.4 +27.3 +28.1 +29.9 +49.7 Modulus at 300% Elong. - - +8.1 +15.1 +22 Loss in Weight % -0.7 -2.9 -3 -3.2 -5.8
14 days at 1500C
Hardness +3.6 +15.3 +16 +20.6 +35 Tensile Strength -62.7 -59.8 -61.2 -58.7 -56 Elongation at break % -38.9 -38.9 -40 -42.8 -43.5 Modulus at 100% Elong. +18.9 +25.1 +21.3 +22 +44 Modulus at 300% Elong. - - -31.2 -21.9 -6.3 Loss in Weight % -1.0 -3.3 -3.8 -4.6 -6.4
119
TABLE 4.6:
HEAT AGEING PROPERTIES OF BROMO BUTYL X2 RUBBER WITH IRAQI PARAFFINIC OIL*
Without . oil
5 phr Oil
10 phr Oil
15 phr Oil
22.5 phr Oil
Aged at 100 0c for 3 days
Hardness +10 +8.3 +13 +13.3 +13.5 Tensile Strength -6.5 +6.7 +8.3 +10.6 +19.3 Elongation at break % -11.1 -10.5 -10 -10 -9.1 Modulus at 100% Elong. +33 +34.5 +37.5 +38.8 +43 Modulus at 300% Elong. +4.2 +22.9 +34 +36.4 +30.6 Loss in Weight % -0.29 -0.45 -0.53 -0.55 -0.58
7 days at 100 0c -
Hardness +12.7 +10 +15 +18.8 +18.8 Tensile Strength -9 +7 +9.6 +13.9 +15.4 Elongation at break % -16.6 -21 -18 -15 -14.5 Modulus at 100% Elong. +44 +61.6 +57.7 +49.6 +49 Modulus at 300% Elong. +18.4 +44.4 +44.2 +59 +60.9 Loss in Weight % -0.2 -0.6 -0.7 -1.1 -1.3
14 days at 100 0c
Hardness +12.7 +16.6 +20 +20 +21.2 Tensile Strength -10.5 +10.7 +0.8 +16.4 +16.6 Elongation at break % +16.6 -21 -20 -18 -18 Modulus at 100% Elong. +72.8 +62.7 73.2 +75.5 +82.7 Modulus at 300% Elong. +18.2 +52.5 +16 +63 +78.9 Loss in Weight % -0.2 -0.6 -0.7 -0.9 -1.2
* as % change
120
TABLE 4.6 ... continued
Without 5 phr Oil
10 phr Oil
15 phr Oil,
22.5 phr Oil
Aged at 125 0c for-3 days
Hardness +14.5 +11.1 +14 +17.7 +18.8 Tensile Strength -1.3 -18.4 -12.4 -8.9 -0.8 Elongation at break % -22.2 -21 -20 -20 -9 Modulus at 100% Elong. +60.8 +24.7 +38.3 +46.7 +46.2 Modulus at 300% Elong. +19.2 +0.7 +11.5 +23.2 +41.4 Loss in Veight. % -0.4 -0.6 -0.8 -1 -1.2
7 days at'125 0c
Hardness +15.4 +12 +15 +24.4 +27.5 Tensile Strength -4.7 -26 -15.8 -9.1 -11.9 Elongation at break % -27.8 -26.3 -25 -20 -13.6 Modulus at 100% Elong. +67.9 +27.6 +53 +52.5 +72.6 Modulus at 300% Elong. +23.7 +3.3 +17.6 +26.4. +49.3 Loss in Weight % -0.5 -0.6 -1.0 -1.2 -1.6
14 days at 1250C
Hardness +16.3 +14.8 +21 +27.7 +28.7 Tensile Strength -11.4 -28.3 -23.1 -19.1 -14.9 Elongation at break % -27.8 -26.3 -25 -25 -22.7 Modulus at 100% Elong. +79.9 +33.3 +53.7 +58.2 +97.1 Modulus at 300% Elong. +20.6 +3.5 +19.5 +28 +50.6 Loss in Weight % -0.9 -0.7 -1.3 -1.7 -2.6
121
TABLE 4.6 ... continued
Without .. Oil
5 phr Oil
10 phr Oil
15 phr Oil
22.5 phr Oil
Aged at 150 0c - for 3 days_
Hardness +1.8 +9.3 +12 +15.6 +16.3 Tensile Strength -24.6 -33.3 -29.2 -28.6 -19 Elongation at break % -27.8 -21 -25 -25 -27.3 Modulus at 100% Elong. +27.7 +18.1 +31.5 +40.8 +44.9 Modulus at 300% Elong. +5.4 -6.4 +2.7 +11.7 +30.8 Loss in Weight % -0.6 -0.9 -1.2 -1.5 -1.8
7 days at 1500C
Hardness 1.0 +4.6 +9 +13.3 +15 Tensile Strength -46 -46 -44.2 -41.6 -36.6 Elongation at break % -33.3 -31.5 -35 -35 -36.4 Modulus at 100% Elong. +24.5 +17 +26.2 +28.8 +43.4 Modulus at 300% Elong. - -16.2 -4.2 -3.1 -13.2 Loss in Weight % -0.7 -1.2 -1.6 -2.1 -2.7
14 days'at 150 0c
Hardness +3.6 +0 +4 +4.4 +12.5 Tensile Strength -62.7 -66.3 -64.4 -63.6 -62.7 Elongation at break % -38.9 -36.8 -35 -35 -36.4 Modulus at 100% Elong. +18.9 +0.5 +14 +5.9 +14 Modulus at 300% Elong. - -49 -47.4 -46.4 -23.4 Loss in Weight % -1.0 -1.5 -2.0 -2.8 -3.6
122
4.3 STRESS'RELAXATION
4.3.1 Basic Principles
A second method has been developed to study the stability of rubber
vulcanisates against oxidative degradation i. e. the stress-relaxation method. Presently this is used to obtain fundamental data about the
chemical changes (ageing) of elastomers and the relationships between
the physical and structural chemical characteristics of butyl and halo-
genated butyl rubber. The extent of network scission during ageing
can be determined from continuous stress relaxation measurements.
Changes of total crosslink density are represented by the intermittent
stress relaxation curve. From the difference between the intermittent
and the continuous curves the extent of new crosslink formation can be determined.
4.3.2 Experimental
Stress relaxation in a butyl and a halogenated butyl was investigated
using a Wallace Extension Stress Relaxometer (see Figure 4.1). Essen- tially the equipment consists of a thermostatically controlled cast aluminium block heating unit having a series of circular chambers. Each
chamber may be used in conjunction with a single stress relaxation unit. Samples, in the form of strips. 4 mm wide, were die cut from sheets approximately 0.5-0.8 mm thick. Operation of the stress relaxation unit depends on the balancing of a metal cantilever beam (Figure 4.2). A sample held at constant elongation is connected to one end of this beam and located in a heating chamber. The beam is pivotted at the
other extreme and the downward movement applied by the sample under stress is balanced by a spring arranged centrally above the beam. Any change in stress of the sample is automatically compensated by
a change in the extension of the spring. A marker pencil attached directly to the spring is placed in contact with a drum chart rotating at a constant rate of one revolution every 12 hours. -
123
The stress relaxometer may be operated in "continuous" mode or "intermittent" mode.
In the first case, the sample is maintained at the desired elongation and the decrease in stress with time is monitored. In the inter-
mittent mode, the sample is extended to the desired elongation for
only a brief fixed period (30 seconds) during each'30 minutes. - 00eration of the "intermittent" mode is controlled by a time switch connected to a pneumatic piston arrangement on each sample grip. Decrease in stress with time is once again monitored, but in this
case, only the irreversible contribution to stress relaxation is
recorded.
Under normal operating conditions air is circulated at a constant rate through each sample chamber. A flowmeter monitors this circu- lation. By connecting a nitrogen supply to the gas inlet valve, measurements may be made in an Mert atmosphere. In the present work, continuous and intermittent stress-relaxation of the butyl and halogenated butyl vulcanisates in air and nitrogen, at elevated temperatures of 1250C and 1500C were measured in order to determine the stability of the networks investigated, and to know the separate effect of oxidative processes and purely thermal ageing processes. Both scission and crosslink formation can be obtained from a comparison of the stress relaxation results in air with those in nitrogen. The
change of total crosslink density in air during ageing can be consi- dered as the net result of crosslink density change in nitrogen, oxidative scission reactions and oxidative crosslinking:
Crosslink Density +
Oxidative +
Oxidative Change in nitrogen Scission Crosslinking
Crosslink density change in air,
Crosslinking formation = Difference between intermittent and continuous stress relaxation
124
FIGURE 4.1: WALLACE EXTENSION STRESS-RELAXOMLIER USED FOR STRESS RELAXATION STUDIES
125
5
F
L_... _... J
KEY..
A. Sample under elongation. B. Thermostatted cast aluminium block heating unit.
Cantilever beam. Compensating spring.
E. Rotating drum chart. F. Cantilever beam electronic balancing unit. G. Pneumatic piston.
Fig. 4.2. Block diagram of stress - relaxation apparatus.
$
126
In every experiment, the strip was extended to approximately 100%
extension.
4.3.3 Results and Discussion
Stress relaxation was recorded graphically on the Wallace stress relaxometer as a decrease in force with time.
For comparison of data for a series of samples it is more meaningful . '* ft to consider the variation in relative force (i. e. Tý with time. Ir
this case f(t) and f(o) represent the force applied 0 by the sample
at time (t) and at the start of the relaxation (i. e. immediately
after extension to 100%) respectively.
, ft Results are presented graphically here as (log T; ý) versus time (hour).
0
The following figures show the continuous and intermittent stress relaxation at temperatures of 1250C and 1500C in air and,, for onlyl,. ', selected oil levels, nitrogen atmospheres and the calculated cross- link formation for all previous compounds (Table 3.3).
Results of measurements made in air are compared with measurements under nitrogen atmosphere to separate the effect of oxidative processes and purely thermal ageing processes to scission and crosslink formation.
.. Continuous Mode:
Continuous stress relaxation at 1250C in air and nitrogen, of IIR, CIIR and BUR is illustrated in graphs 4'. 3-4.14. In general it can be seen that:
127
Initially, continuous stress decays rapidly in both air and nitrogen, this decay is attributed to thermal interchange
reactions between labile linkages, which are probably poly- sulphides., and this is not affected by oxygen since it follows
the same path in nitrogen as well as in air.
2. After the initiation, the scissions were considered to occur simultaneously and rapidly causing chemical stress relaxation.
3. Stress decay increases with increasing ageing time; it also increases with increase in oil levels in a rubber mix.
4. Decay in air is greater than in nitrogen, that means oxidative degradation as well as thermal degradation has taken place.
In air:
i) IN compounds showed a higher stress decay than halogenated butyl compounds in both Iraqi and Esso oils.
ii) CIIR compounds containing Iraqi oil showed better retention of stress than BIIR compounds contained for the same oil type.
iii) BIIR vulcanisates containing Esso oil exhibited a little improvement in stress retention when compared with CIIR
vulcanisates compounded with Iraqi and Esso oil.
Stress relaxation behaviour in a nitrogen atmosphere changed according to the oil level and it can be summarised as follows:
Stress decay (scission) at 1250C:
''Esso'oil
Low oil level: ý'IIR > CUR >BIIR (0-5 phr)
128
High oil level: (20-25 phr)
2. Iraqi oil:
Low oil level:
. (0-5 phr)
High oil level: (20-25 phr)
Intermittent'Moder.,
IIR > CIIR > BIIR
IIR > BIIR > CIIR
fý, IIR KIIR > BUR
Graphs 4.15-4.26 concern intermittent stress relaxation and it can be
seen from each graph:
Curves had a different pattern compared with that of continuous relaxation; the increase of the rate of intermittent relaxation on curves indicates new network formation caused by continued crosslinking by the vulcanising reagents being not completely used up in the vulcanising process, or it may be that cross- linking occurs as part of the oxidative chain scission reaction.
2. Intermittent curves in nitrogen show higher force value than in
air due to the absence of an oxidation process.
3. The network formation in compounds at a low oil level is higher than the network formation at a higher oil level.
4. The slowest-relaxation rate is shown by the most highly cross- linked compounds which are those containing the least amount of oil.
5. Crosslink formation in a halogenated polymer is greater than in a conventional butyl polymer in both air and nitrogen atmospheres.
129
Determination of Crosslink Formation Through Ageing
From the difference between the intermittent and continuous stress relaxation curves, the extent of new crosslink formation can be
calculated. These results are illustrated in graphs 4.27-4.38 and can be summarised as follows:
Crosslink*forwtion in air:
I. Esso oil :
"'IIR > CIIR >BIIR
Iraqi oil :
CIIR > BIIR : 5- IIR
In Nitrogen:
l. - Esso oil:
a)*Low oil level: (0-5 phr)
b) High oil level: (20-25 phr)
Iraqi oil :
a) Low oil level: (0-5 phr)
b) High oil level: (20-25 phr)
BIN > ., IIR KIIR
CIIR > IIR > BIIR
CUR > IIR > BIIR
CIIR > BIIR > IIR
At the temperature of 1500C, all these polymers in both stress relaxation modes showed a rapid increase in stress relaxation, and decayed to zero faster than at 1250C.. These results are illustrated in Figures 4.39-4.74 and summarised'as follows:
130
Stress Decay (scission) at 150OC:,
1. In air for Esso and Iraqi oils:
IIR > CIIR > BIIR
2. In nitrogen for Esso and Iraqi oils:
a) Low oil level: IIR > CIIR > BIN (0-5 phr)
b) High oil level: IIR > BIIR > CIIR (20-25 phr)
4.3.4 'Conclusions
Stress relaxation for IIR, halogenated IN at elevated temperatures has shown it to be an oxidative and thermal process. It is clear from the results of the test in air and nitrogen that two reactions, scission and crosslinking have taken place in both butyl and haloge-
nated butyl compounds, but the scission process is more predominant than the crosslinking process.
In general the following can beconcluded from the graphs 4.27-4.38
at 1250C and 4.63-4.74 at 1500C.
Scission takes place in both nitrogen and air atmospheres; in
nitrogen scission is considered to be possibly due to traces of oxygen being present or due to heat alone.
2. Crosslinking formation (at oil levels >5 phr) are low for the first hour of ageing due to the insufficient time having lapsed to enable crosslinking to predominate and thus the decay reac- tion predominates.
131
3. Crosslinking is considered to occur in both air and nitrogen, but in nitrogen it is greater than in air due to the absence of the, oxidative process.
4. All compounds in nitrogen have longer relaxation periods than in air.
Effect'of'Polymer Type, 'Curing'SyStem, Oil'LeVel'and'Temperature dn Crosslinking Formation:
Sulphurless cured halogenated butyl rubber showed better resis- tance to thermo-oxidative degradation for a longer time and also gave higher crosslinking formation than that obtained with sulphur cured butyl rubber under the same conditions. This is
attributed to:
i) Thermal stability of the ZnO/TMTD. sulphurless curing system in the halogenated butyl compounds.
Existence of the halogen group which increases the stability of the unsaturated group.
2. Increasing oil levels in both butyl and halogenated butyl
vulcanisates results in an increase in the rate of degradation, therefore compounds containing the highest amounts of oil showed the greatest rate of scission and lowest overall cross- link formation.
Effect of the Type of Oil'on the Crosslink Formation
ButyZ Rubber:
Esso oil in both air and nitrogen atmospheres with conventional butyl rubber, exhibits less scission and higher crosslinking forma- tion'than with Iraqi oil at temperatures of 1250C and'1500C.
132
ChZorobutyl Rubber:
Iraqi oil in chlorobutyl rubber showed better resistance to ageing than did the Esso oil in both air and nitrogen atmospheres and at temperatures of 1250C and 1500C.
BromobutyZ Rubber:
Bromobutyl with Esso oil, at all levels, at 1250C in an air atmos- phere and at just a low oil level'in nitrogen, exhibited higher
crosslinking tendencies than did the Iraqi oil. At 1500C, Iraqi oil at all levels and also at a low oil level, in nitrogen, showed higher
crosslinking formation than was demonstrated by the Esso oil.
4.4* DETERMINATION OF'DEVELOPMENT OFVULCANISATE ACIDITY DUE TO HEAT AGEING An Acetone'Extraction'Technique was Used*(see'below):
4-4.1' Introduction
Acetone extraction was used to extract the organic acid (acidity)
of both the unaged and aged butyl type compounds as a chemical method to measure the degree of rubber degradation through means of the acidity changes.
4.4.2' Experimental
A reflux type extraction product fitted with a condenser was placed immediately above the cup whichhel d the rubber. Approximately 6 grams of each sample, cut into small pieces was placed in the cup, and then extracted for 16 hours with acetone. (BS 1973: Part 5/5.5: 1969). This expresses the organic acid in the rubber as a percentage of the original mass of the test portion.
(148)
133
TIME (hours) TIME (hours) 5 10 15 20 25
. 30 35 40 45 50 5 10 15 20 25 30 35 40 45 50
'161 0
2 1111 _1 1-WITH 5 Phr OIL A 04
0
-2 11AHOdT OIL 2- WITH 5 Phr OIL 2-WITH 10 Phr OIL
3-WITH 15 Phr OIL 3 -WITH 10 Phr OIL *
-4 4-WITH 20 Phr OIL -4 4-WITH 15 Phr OIL 5-WITH 20Phr OIL
5-WITH 25 Phr OIL -6
6 -WITH 25 Phr OIL
1 -8 3
4 10 - 2 5 3
c: n - o 12- r_n
-12 C) - _j
-14- -14- 6
-16- -16 -
_18- -18 -
-20- -20
-22- 4 -22 -
-24- -24-
Fig. 4. 3. - Continuous stress relaxation Fig. 4. I*.. - Continuous stress relaxation of IIR with Iraqi oil at of II R with Esso oiL at 1250C in air 1250C in air
TIME (hours) TIME (hours) 0 5 10 15 20 25 30 35 40 45 50 5 10 15 20 25 30 35 40 45 50
A 0-1 0 111 11
X1cF1 1111
2 1 -WITH 5 Phr OIL -2 1- WITHOUT OIL 0 r 2- WITH 25 Phr OIL 2-WITH 5PhrO1L
-4 -4- 0 3 WITH 25 Phr 001L
-6 - -6- 2
-6- 3
-10 -10 Z9 2 Z, ? -12- -12 -
_j - 14 -14-
-16- -16
-18 -18
_20- -20-
-22- -22
- 24L -241 Fig. 4.5ý- Continuous stress relaxation Fig. 4.6. - Continuous stress relaxation
of IIR with Iraqi oil at of IIR with Esso oil at 1250C in nitrogen 125"C in nitrogen.
134
TIME (hours) TIME (hours) 0 0 S 10 15 20 25 30 35 40 45 50 0 0 5 10 15 20 25 30 35 40 45 50
101 . 10-1 X 2 -2
-4
-6 - 2 -6- 2
3 4
3
- _ . ; 45 -f _ 1 E P 1
-12 -12- 5
_j _j -14-
1- WITH 5 Phr OIL -14-
1- WITHOUT OIL 2- WITH 10 Phr OIL 2- WITH 5 Phr OIL
-16 3- WITH 15 Phr OIL -16 - 3- WITH 10 Phr OIL
-18 4- WITH 20 Phr OIL
-18 4- WITH 15Phr OIL 5- WITH 20 Phr OIL
-20-, -20-
-22 -22-
-241 -24- Fig. 4.7. - Continuous stress relmation Fig. 4 . 8: - Continuous stress relaxation
of CIIR with Iraqi oil at of CIIR with Esso oil at 1250C in 'air 1250C in air
TIME (hours) TIME (hours)
00 5 10 15 20 2S '30 35 40 45 50 _T 00 S 10 15 20 25 30 35 40 45 50
X161 X10 2
-4- 1
-6 - 6- 1
2 2 3
-10 10
-12 - -12-
-14 1-WITH 5 Phr OIL -14- 1- WITHOUT OIL 2 -WITH 20 Phr OIL 2- WITH 5 Phr OIL
-16 - -16- 3- WITH 20 Phr OIL
-16- _18-
-20 -20
-22-
-24- -24- Fig, 4.9. - Continuous stress relaxation Fig. 4.10. - Continuous stress relaxation
of'CIIR with Iraqi oil at of CIIR with Esso oil at 1250C in nitrogen 1250C in nitrogen.
135
TIME (hours) TIME (hours) S 10 15 20 25 30 35 40 45 50
- 5
-- - 0 0 10 15 '20 25 30 35.40 45 50 -- -- --
.X 16
1 10 1 X
T T ir 1-1 r
-2 - 2
-4 4 12 2
-6 -6 - 34
2 5 3 -8-
4 -10-
-12- --12-
-14- 1- WITH 5 Phr OIL
-A
-14- 1- WITHOUT OIL
2- WITH 10 Phr OIL 2- WITH 5 Phr OIL
-16- 3- WITH 15 Phr OIL -16- 3- WITH 10 Phr OIL
4- WITH 22-5 Phr OIL 4- WITH 15 Phr OIL
-18 - -18- 5- WITH 22,5 Phr OIL
-20 - -20-
-22 - -22 -
-24
L -24 -
Fig. 4.11: - Continuous stress relaxation Fig. 4.12. -Continuous stress relaxation of BIIR with Iraq! oil at of BIIR with Esso oil at 125'C in air 1250C in air
TIME(hours) TIME (hours)
0 5 10 15 20 25 30 35 40 45 50 05
0 10 15 20 25 30 35 40 45 50
X1 -I 9 2 2
-4 - -4- 1 2 -6 - 2 -6 3
-8-
12 - m -12 -
-14 - 1- WITH 5 Phr OIL -14- 1- WITHOUT OIL 2- WITH 22-5 Phr OIL 2- WITH 5 Phr OIL
-16 - -16 - 3- WITH 22-5 Phr OIL
-18 - -18 -
-20- -20
-22- -22-
-24 - -24 - Fig. 4.13. - Continuous stress relaxation Fig. 4.14
.- Continuous stress relaxation of BIIR \odth Iraqi oil at of BIIR with Esso oil at 1250C in nitrogen. 1250C in nitrogen.
136
X10-1 X10- 6- 6
4- 4-
2- TIME (hours) 2 25 30 35 40 45 4 0
10 '2
0
Cn 0 25 3b is ýO 45 -2- 3 -2 - TIME (hours)
-4- 6
-6 - 5 -6 -
-a- -8-
_10- 1-WITH 5Phr OIL _10- 1- WITHOUT OIL 2- WITH 1OPhr OIL 2- WITH 5Phr OIL 3- WITH 15Phr OIL 3- WITH 1OPhr OIL
-14- 4- WITH 20Phr OIL -14 4- WITH 15Phr OIL
-16- 5- WITH 25Phr OIL
-16 - 5- WITH 20Phr OIL 6- WITH 2SPhr OIL
-2oL - 2oL
Fig. 4.1 S . Intermittent stress relaxation Fig. 4'. 1 6. Intermittent st ress relaxation -of IIR with Iraqi oil at of IIR with Esso oi I at 125'C in air. 1250C in air.
. 10-1 6-
4-
2
0 cn
-2-
-4-
6
-8
-10-
-12-
-14
-16[
-20 Fig-4.17. Intermittent stress - relaxation
of IIR with Iraqi oil at 1250C in nitrogen.
X10- 6
4
2 15 20 25 30 35 40 45
0 TIME (hours)' 2
-2 -
-4-
6
-8-
1- WITH 5Phr OIL -10- 2- WITH 2SPhr OIL
-12 -
-14-
-16-
-18r-
a
TIME (hours) 2 5 10 15 20 25 30 35 40 45
3
1- WITHOUT OIL 2- WITH 5Phr OIL 3-WITH 2SPhr OIL
-LV-
Fig. 4.18. Intermittent stress relaxation of IIR with Esso A at 12S"C in nitrogen.
137
X10-1 X10-1 TIME (hours) 6- 6 _r - -r- - r-, - --r- - -r- I-, - -1 5 10 15 20 25 30 35 40 45 4- 4-
2 2 2- 10 11 a TIME (hoursý 6 25 30 35 40 45 1 -2
0 " 2
3 4.
-2- 5
-6 - -6-
_10- 1-WITH 5Phr OIL _10- 1- WITHOUT OIL
-12- 2- WITH 1OPhr OIL
-12- 2- WITH 5Phr OIL
3- WITH 15Phr OIL 3- WITH 1OPhr OIL -14- 4-WITH 20PhrOIL -14- 4-WITH 15PhrOIL
-16 - -16- 5-WITH 20PhrOIL
-18- -18
-20- -201
Fig. 4.19. Intermittent stress relaxation Fig. 4.20. Intermittent stress relaxation of CIIR with Iraqi oil at of CIIR with Esso oil at 1250C in air 1250C in air
X10-1 '10-1 6- 6-
4- 4
2 2
2 1,. p 2 3 0 0
cn 0 5 10 15 20 25 30 35 40 45 cn -2 TIME (hours) -2 TIME (hours)
-4- -4-
-6- -6-
_10- 1-WITH 5Phr OIL _10- 1- WITHOUT OIL
-12- 2 -WITH 20Phr OIL
-12 2- WITH 5Phr OIL 3- WITH 20Phr OIL
-14 -14-
-16- -16
- 18ý- -181
-2oL Fig. 4.21. Intermittent stress retaxation
of CIIR with Iraqi oil at 125ýC in nitrogen.
-zvý
Fig. 4.22. Intermittent stress relaxation of CIIR with Esso oil at 1250C in nitrogen.
138
- I TIME (hours) TIME(hours)
X10 5 10 15 20 25 30 35 40 45 - X1 1 5 10 15 20 25 30 35 40 45 6_ -T - -r - -T--r - -T- .1 -r- r -1 - --= *1 ____T * _T____r_ - r___r_1 -I
4- 4- 1
2- 1
2 2
0 -4 0EF; __ tM1 0 q3 St 4
cn 0
-2-
iibz _ _ _cn
.4 -4 -4-
5
-6- -6
-10 1- WITH 5Phr OIL _10- 1 -WITHOUT OIL
-12- 2- WITH 1OPhr OIL
-12 - 2- WITH 5Phr OIL
3 -WITH 1SPhr OIL 3 -WITH 1OPhr OIL -14- 4- WITH 22-5Phr OIL -14- 4- WITH 15Pýr OIL
-16- -16- 5 -WITH 22-5Phr OIL
_18- _18-
-20- -201- Fig. 4.23. Intermittent stress relaxation Fig. 4.24. Intermittent stress relaxation
of BIIR with Iraqi oil at of BIIR with Esso oil at 125"C in air. 125% in air.
X10- 1 . 10 -1 6 6
4-
2- 2 TIME(hours) 2 5 10 15 20 25 30 35 40 45 0
Ch 0 lb 1'5 210 215 3b 95 40 45 cn A
-3 - 2- TIME (hours) -2-
-4 -4-
-6- -6
-8-
_10- 1- WITH 5Phr OIL _10- 1 -WITHOUT OIL
-12- 2 -WITH 22-5Phr OIL -12-
2 -WITH SPhr OIL 3- WITH 22-5Phr OIL
-14- -14 - -16. -16.
-18 -18
-201 -20[ Fig. 4.2 5. Intermittent stress relaxation Fig. 4. 26. Intermittent stress relaxation
of BIIR. with Iraqi oil at of BUR with Esso oilat 125c'C in nitrogen. 1250C. in nitrogen.
139
x1O 5
4
3
2-
I TIME (hours)
5 10 15 20 25 30 35 40 0, i1*1111 --1 Cm
-2
-3
r
-4
-5 f 5Phr OIL ýý L l
-6- " " r 2 -WITH 1OPhr OtL 5 3- WITH 15Phr OIL 4 -WITH 20 Phr OIL
-8 5 -WITH 25Phr OIL -9-
Fig. 4.2 7 Calculated crosslink formation of IIR with Iraqi oil at 1250C in air.
OF 5
4 31
2 1 e2
3
cm 0 _j
; 30 35 ýO /5 10 15 20 2S
2 TIME (hours) 3
-2 4 3 6
1 -WITHOUT OIL 2- WITH 5Phr OIL
5- 3-WITH 1OPhr OIL
-6 - 4- WITH 15Phr OIL
-7 5- WITH 20Phr OIL 6- WITH 25Phr OIL
-8 -
_9L Fig. 4.28. Calculated crosstink
formation of IIR with Esso oil at 125% in air.
X1 0-1 5
4-
3-
2
10 15 25 30 35 40 TIME(hours)
22
-3
-4
-5-
-6 - 1- WITH 5Phr OIL
-7- 2-WITH 25Phr OIL
-8-
- 9L
Fig. 4.29. Calculated crosslink formation of IIR with Iraqi oil at 1250C in nitrogen.
X10-1 1
41
3
CA 0 -i -1
6 I
-3
2
5 10 15 20 25 30 35 40 TI ME (hours)
3
1- WITHOUT OIL 2-WITH 5PhrOIL 3 -WITH 2SPhr OIL
-9L Fig. 4.30. Calculated crosslink
formation of IIR with Esso oil at 1250C in nitrogen.
140
'10-1 x 10- 1 TIME (hours),
- 5 10 15 20 25 30 35 40
-- -* - - - -, -- -- r r T r T ii
4- 4-
3- 3-
2- 1 2-
2
0
1 10 15 20 25 30 35 40 M
-01 3 E (hours)
-2 -2 4
43 -3 ý 3 5
-4 -4
-5 1- WITH 5 Phr OIL -5 1- WITHOUT OIL
-6 2- WITH 10 Phr OIL 6 2- WITH SPhr OIL 3-WITH 15 Phr OIL 3 -WITH 1OPhr OIL 4-WITH 20Phr OIL -7 4- WITH 15Phr OIL
-8 - 5- WITH 20Phr OIL
-9- -9- Fig. 4.31. Calculated crosslink Fig. 4.32. -Calculated crosslink
formatidn of CIIR with formation of CUR with Iraqi oil at 1250C in air, Esso oil at 125"C in air.
X10-1 X10-1
4-
3- 3
2- ý 2 E (h ours ho ' 1`
ý urs) TIME (hours) 1ý 1 0 ý 15 20 25 30 ý5 40 5 10 15 20 25 30 35 40
0
1 2 0
_j 2 3
-2 - -2
-3 -3
F
-4
-5 1- WITH 5Phr OIL ' I I W ' T H -5 1 -WITHOUT OIL
-6 2- W I T H2 0Phr OIL -6 2- WITH 5Phr OIL
-7 3 -WITH 20Phr OIL
-8 -8-
_9L -9- Fig-4.33. Calculated crosslink Fig. 4.34. Calculated crosslink
formation of CIIR with formation of CIIR with Iraqi oil at 125% in nitrogen. Esso oil at 1250C in nitrogen.
141
X10-1 TIME (hours)
5 10 15 20 25 30 35 40 5- 5 _T_ _1_1
4- 4-
3- 3-
2- 2 1 1-
I ,1 2
8 ýI Ip 1 - M0 0
-4 lb fS 2*0 A30 Y5 Cn 0
j
43 4 4 - TIME hours) -1 - 2
-2 -2
-3 4 -3
-4
-5. 5
1- WITH 5Phr OIL 1- WITHOUT OIL -6 WI lop 2- WITH 1OPhr OIL 6 2-WITH 5Phr OIL -7 - 3- WITH 15 Phr OIL -7- 3 -WITH 1OPhr OIL
4- WITH 22.5Phr OIL -8
4 -WITH 15Phr OIL 5- WITH 22-513hr OIL
- 9L-
Fig. 4.35. -Calculated
crosslink Fig. 4 . 36. Calculated croýslink formation of BIIR with formation of BIIR with Iraqi oil at 125'C in air. Esso oil at 1250C in air.
TIME (hours) X10-1 5 10 15 20 25 30 35 40 X10-1 5 5 -
4- 4 2
3- 3
2- 2
ý, 1- 14-:? 1ý0 ý_ý 0
Cn 0
-J
0 CA 0 5 10 15 20 2S 30 35 40
TIME (hours) ýA 2
-2 -2 -3
r
-3 3
-4 -4 -
-5 -5 - l -WITH SPhr OIL 1- WITHOUT OIL -6 2- WITH 22-5Phr OIL -6 -2- WITH 5Phr OIL -7- -7 3- WITH 22-5Phr OIL
-8 1
-8
-9 -9 - Fig. 4.37 . Calculated crosstink Fig. 4.38. Calculated crosstink
formation of BUR with formation of BIIR with Iraqi oil at 1250C in nitrogen Esso oil at 1250C in nitrogen.
142
TIME (hours) TIME (hours) 48 12 16 20 24 0 48 12 16 20 24
0 1. III. --I *10 111111 -1 X10 A0
-2 I-WITH 5Phr OIL -2 1 -WITHOUT OIL
-4 2-WITH 10 Phr OIL
-4 2- WITH 5 Phr OIL
3-WITIi 15 Phr OIL 3- WITH 1OPhr OIL
-6 - 4 -WITH 20 Phr OIL -6 - 4- WITH 15 Phr OIL
-8- 5- WITH 25 Phr OIL
-8- 5- WITH 20 Phr OIL 6- WITH 25 PhrOIL
10-
CA -12- -12- cn 0
-14- 3
-16 - -16 -
-18 - 1
-18- 43
-20- 5 -20- 1
-22 -22
-241 -24 6
Fig. 4.39. Continuous stress relaxation Fig. 4 . 40. Continuous stress relaxation of II R with Iraqi oil at of IIR with Esso oil at 1500C in air 150'C in air
TIME(hours) TIME (hours 0 5- 10 15 20 25 30 35 40 45 50 5 10 15 2025 30 35 40 45 50
0 11 -1 0 . 10-1
1 111111 X10-1
2 -2
-4 -4 WITH 5 Phr OIL 1-WITHOUT OIL
-6 2-WITH 25Phr OIL -6 - 2- WITH 5 Phr OIL
-8 - -8 3 -WITH 25Phr OIL
_10-
-12 - 12 - Q
-14- -14- 3
-16 - -16 -
_1B - _1B - 2
-20 -20-
-22 - -22 1
-24 - -24 Fig. 4.41. Continuous'stresS relaxation Fig. 4.42. Continuous stress relaxation
of IIR with Iraqi oil at of IIR with Esso oil at 1500C in nitrogen. 150'OC in nitrogen.
143 TIME (hours) TIME (hours)
0 0
48 12 16 20 24 0. 48 '12 16 20 24
-- - - -" - 1 2
11111111111 1-WITH 5Phr OIL
A 0' 1
-2
111 T IIIiI r T 1 1 -WITHOUT OIL
2 -WITH 10 Phr OIL 2- WITH 5Phr OIL -4 3- WITH 15Phr OIL 4 3- WITH 1OPhr OIL
-6 4 -WITH 20Phr OIL
-6 4- WITH 15Phr OIL 5- WITH 20Phr OIL
-8 -
m -12- 0 12 -
_j _j 4 -14- -14- 5
-16 - -16 - 3
-18 - -18-
-20- 4
_20-
-22- -22
-24ý 3
-241 1
Fig. 4.43. Continuous stress relaxation Fig. 4.44. Continuous stress relaxation of CIIR with Iraqi oi I at of CIIR with Esso oil at 150"C in air. 1500 C in air.
TIME (hours) TIME (hours) 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 45 50
0 0 X10-1 '10" 2 1- WITH 5Phr OIL -2 1-WITHOUT OIL
2 -WITH 20Phr OIL 2-WITH 5Phr OIL -4 -4 3- WITH 20Phr OIL
-6 -6 -
-8
_10- ýýI 2
cn - 12 - -----12 - G _j
Im 0 _j -14- - 14-
-16 -16-
-18 - -18-
-20- -20-
-22- -22- L 3 -24 - -24
Fig. 4.45. Continuous stress relaxation Fig. 4. 46. Continuous stress relaxation of C. IIR with Iraqi oil at of CIIR with Esso oil at 150"C in nitrogen. 1500C in nitrogen.
144 TIME (hours) TIME (hours)
0 48 12 16 20 24
0 48 12 16 20 24
X10, 2 1- WITH 5 Phr OIL 2 1 -WITHOUT OIL
2-WITH 1OPhr OIL 2- WITH 5Phr OIL -4 3 -WITH 15 Phr OIL -4 3- WITH 10 Phr OIL
-6 4-WITH 22-SPhr OIL
-6 4- WITH 1SPhr OIL 5- WITH 22-SPhr OIL
2 3
cp -12 -12 0
-14- -14- 1
-16 - -16 - 2
4
-20- -20- 4
-22 - -22 - 5
-24L -24, 3
Fig. 4.47. Continuous stress relaxation Fig. 4 . 48. Continuous stress relaxation of BIIR with Iraqi oil at of BIIR with Esso oil at 1500C in air. 150'C in air.
TIME (hours) TIME (hours) 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 45 50
0 -T-1-7 II -1 0 11 1 -1 'icr, 2
I 1-WITH 5Phr OIL . 10-1
-2
11 1 -WITHOUT OIL
2-WITH 22-5Phr OIL 2- WITH 5Phr OIL -4 -4 3- WITH 22-5Phr OIL
-6 -6 -
-8 -
10 -
-12 - -'-12
-14 - 0
- 14- 2
-16 - 2 -16 -
-18 - -18 -
-20 - -20-
-22 - -22- 3
-? 4 - -24- Fig. 4.49. Continuous stress relaxation Fig. 4. 50. Continuous stress relaxation
of BIIR with Iraqi oil at of BIIR with Esso oil at 1500C in nitrogen 150"C in nitrogen.
145
TIME (hours) 5 10 15 20 25 30 35 40 45
1 -WITH 5Phr OIL 2- WITH 1OPhr OIL 3-WITH 15PhrOIL 4 -WITH 20Phr OIL 5- WITH 2SPhr OIL
IL 1 32
-20L 5
Fig. 4.51. Intermittent stress relaxation ofIIR with Iraqi oil at 1500C in air.
x 10- 6
4-
2-
0 15 20 25 30 35 40 45
-2 TIME (hours)
-4
-6 -
-10-
-12-
-14- 1-WITH 5Phr OIL -16 2-WITH 25Phr OIL
-181 21
Fig. 4.5 3.1 n te rm ittent 'stre ss " re laxati on of IIR with Iraqi oit at 150"C in nitrogen.
. 10-1 6-
4.
2- TIME (hours) 10 15 20 25 30 35 40 45
Cp 0
_J -2
-4-
-6 -
-8 -
_10-
-12- 11-WITHOUT OIL
-14- 2- WITH 5Phr OIL
'-2 3-WITH 1OPhr OIL -16- 3 4-WITH 15Phr OIL
-18 4 5-WITH 20Phr OIL
1 5 6 -WITH 25Phr OIL -20
Fig. 4.52. Intermittent stress relaxation of IIR with Esso oi I at 1500C in air.
X 10- 6-
4
2
0 20 2S 30 35 40 45
-2 TIME(hours)
-4
-6 -
-6- 31
-10
-12 -2
-14- 1 -WITHOUT OIL -16 - 2-WITH 5PhrOIL
_18- 3 WITH 25Phr OIL
-20- Fig. 4.54. Intermittent stress relaxation
of IIR with Esso oil at 150"C in nitrogen.
146
X10-1 6-
4-
2-
0 25 30 35 40 45
cn -2- TIME (hours) 2
-4- 3
-6 4
-8-
_10-
-12- 1- WITH 5Phr OIL
-14- 2 -WITH 1OPhr OIL 3"- WITH 1SPhr OIL 4- WITH 20Phr OIL
-20L Fig. 4.55. Intermittent stress relaxation
of CIIR with Iraqi oil. at 150"C in air
. 10-1
1 2r, TIME (hours)
5 10 15 20 25 30 35 40 45
0 2-
-6-
1-WITH SPhr OIL 2-WITH 20Phr OIL
-12
-14-
-16-
-18-
-20. Fig. 4.57. Intermittent stress relaxation
of CIIR with Iraqi oil at 150% in nitrogen.
xlo, l
6 -, 4-
2- TIME (hours) 15 20 25 30 35 40 45
0 10
-2-
-4- 2
-6- 3 4
5 _10-
-12 - 1- WITHOUT OIL
-14- 2- WITH SPhr OIL
-16- 3- WITH 1OPhr OIL 4- WITH 15Phr OIL
_18- 5 -WITH 20Phr OIL
-20- Fig. 4.56. Intermittent stress relaxation
of CIIR with Esso OR at 150"C in air
12
S)
_10- 1-WITHOUT OIL 2- WITH SPhr OIL
-12- 3 -WITH 20Phr OIL
-14-
-16
-is-
-201- Fig. 4.58. Intermittent stress relaxation
of CIIR with Esso oil at 1500C in nitrogen.
147
X10-1 3tjo- I
6- 6
4- 4-
2- TIME (hours) 2- TIME (hours) 15 20 25 30 35 40 45 30 35 40 45
01 1a114 -i 0 t -1
Cn 2- 2
CYI 2
I
32
-4- 43 -4. 4
-6 -6-
_10- I-WITH 5Phr OIL _10- 1 -WITHOUT OIL 2- WITH 1OPhr OIL 2- WITH 5 Phr OIL
-12- -12 , 3- WITH 15Phr OIL 3- WITH 1OPhr OIL
-14- 4 -WITH 22.5Phr OIL -14- 4- WITH 15Phr OIL
-16- -16 - 5- WITH 22-5PhrOlL
-18- -18-
-2oL -20L
Fig. 4.59. Intermittent stress relaxation Fig. 4.6 0. Intermittent stress relaxation of BIIR with Iraqi oil at of BIIR with Esso oil at 1500C in air. 1500C in air.
-10-1 6-
4-
-8 -
-10- 1- WITH 5 Phr OIL
-12- 2- WITH 22-SPhr OIL
-14-
-16 -
-21
Fig. 4.61. Intermittent stress relaxation of BIIR with Iraqi oil at 150"C in nitrogen.
X10-1 6
1 2
-4 -
-6
10 f5
3 .1 2
30 35 40 4 TIME (hours)
-10- 1-WITHOUT OIL
-12- 2- WITH SPhr OIL 3 -WITH 22-5Phr OIL
-16-
-18 -
-20-
Fig. 4.62. Intermittent stress 'relaxation of BI I- R with Esso oi I at 1500C in nitrogen.
148
X10-1 X10- 1 5-
4- 4-
3- 3
2- 2
W-61 0 0
_j 5 10 15 20 SO A 40 0
31 lb 1ý 2b is 3b 315 40 TIME (hours) TIME (hours)
-2 -2 4
-3 -3 32
1- WITH 5Phr OIL -4
1-WITHOUT OIL 2- WITH 10 Phr OIL 5 2-WITH 5Phr OIL 3-WITH 15Phr OIL 3- WITH ý 10 Phr OIL
-6 - 44
-WITH 20Phr OIL -6 - 4-WITH 15PhrOIL
-7 5- WITH 2SPhr OIL -7 6 5-WITH 20Phr OIL
2 6- WITH 25PhrOIL 1
5 -9 -9 .
Fig. 4.63. Calculated crosslink Fig. 4.64. Calculated crosslink formation of IIRwith formation of IIR with Iraqi oil at 150"C in air Esso oil at 1500C in air
X10-1 . 10-1 S 4-
3- 3-
2
t*: L!
0
10 15 20 25 30 35 40 an 0
5 20 25 30 35 40 TIME (hours) -I TIME (hours)
-2 -2
-3 -3 - -4 -4
-5- -5 1- WITH 5Phr OIL 1 -WITHOUT OIL 2-WITH 2SPhr OIL 2 -WITH SPhr OIL
-7- 2 -7 3- WITH 2SPhr OIL 3
-8 r -8 _9 _9L
Fig. 4.65. Calculated crosstink Fig. 4.66. Calculated crosslink formation of IIRwith formation of IIR with Iraqi oil at 150"C in nitrogen. Esso oil at 1500C in nitrogen. -
149 TIME (hours)
X10-1 5 10 15 20 25 30 35 40 5- T- I ---T -
4-
3- 21-
o 2 3
V
-2 4
-3 -
-4 -
1- WITH 5Phr OIL -6 - 2- WITH 1OPhr OIL
-7- 3-WITH 15Phr OIL
-B 4- WITH 20Phr OIL
-9 - Fig. 4.6 7. Calculated crosslink
formation of CUR with Iraqi oil at 1SOOC in air
X10-1 141
3
2
0
15 10 15 20 25 30 35 40
'FI ME 6ýo ur s-;
-2
-3 -4-
1- WITH 5Phr OIL 2- WITH 20Phr OIL
-7-
-gL- Fig. 4.69. Calculated crosslink
formation of CIIR with
Iraqi oil at 1500C in nitrogen.
X10-1 5
3 2
cn 0
-2
-3
-4
10 20 25 30 35 40 TIME (hours)
2
4
5 -5 - 1-WITHOUT OIL -6- 2- WITH 5Phr OIL
-7 -3- WITH 1OPhr OIL
-8 4- WITH 15PhrOlL
9ý 5- WITH 20Phr OIL
Fig. 4.68. 'Calcutated crosslink formation of CIIR with Esso oit at 1500C in air
TIME (hours) X10-1 5 10 15 20 25 30 35 fl-0
5[
4
3
2
0
-2
-3
r73
-4-
-5- 1 -WITHOUT OIL 2- WITH 5Phr OIL
-7 -3 -WITH 20Phr OIL
-8-
Fig. 4.70. Calculated crosslink formation of CIIR with Esso oil at 1500C in nitrogen.
150
. 10-1 X10' 5- 5
4-
3- 3
2 2
1 TIME (hours) S 10 15 20 25 30 35 40
0 1 1 0 15 20 ir5- 30 35 4
2 1 2 TIME (hours)
-2 3 4 -2 C 133
-3 -3 S4
-4 4 1 -WITH 5Phr OIL 1-WITHOUT OIL
-5- -5- 2 -WITH 1OPhr OIL 2 -WITH 5Phr OIL
-6- 3- WITH 1SPhr OIL 6- 3- WITH 1OPhr OIL
-7- 4- WITH 22-SPhr OIL -7- 4- WITH 15Phr OIL
-8 L -8 5- WITH 22-5Phr OIL
_9 Fig. 4.71. Calculated crosslink Fi9.4.7 2. Calculated crosslink
formation of BUR with formation of BUR with Iraqi oil at 1500C in air. Esso oil at 150"C in air.
xi 0' 5 r-
4-
3-
2
0
10 15 ý20
25 30 35 40 TIME (hours)
-2 v
-3
-4-
1 -WITH 5Phr OIL -6- 2-WITH 22-5PhrOIL
-7-
-8
-91 Fig. 4.73. Calculated crosslink
formation of BIIR with Iraqi oit at 1500C in nitrogen.
X10- 5
02
--- 3
5 10 15 20 25 30 35 40 TIME (hours )
-5 1-WITHOUT OIL
-6 -2 -WITH SPhr OIL
-7- 3 -WITH 22-SPhr OIL
-9- Fig. 4.74. Calculated crosslink
formation of BIIR with Esso oil at 150'C in nitrogen.
4
151
'/, Organic acid = (V 1- V2 ) X- NxFx2.5
(4.1) m
where Vi = titration ml of OAN NaOH for test portion V2 = titration ml of OAN NaOH for blank N= determined normality of NaOH M= 'mass of test portion in grams F= 28.4 which determines the acid as stearic acid.
Since the organic acid present in the'rubber is not a single chemical compound the value assigned to the factor (F) gives only an approximate figure for the organic acid content.
Table 4.7 shows the inner tube compound formulations of IIR, CUR and BUR with Esso and Iraqi oils which were extracted by hot acetone after ageing at 1250C for periods of time 3,7 and 14 days.
4.4.3 Results and'Discussion
Graph 4.75 illustrates the acidity of unaged and aged compounds after acetone extraction calculated by using equation'4.1. It can be seen that:
I. Acidity of the compounds increases with increasing ageing time. 2. Butyl compounds showed the development of higher acidity with
ageing than both the halogenated butyl compounds.
3. CIIR vulcanisates exhibited higher comparative acidity values than BIIR.
Halogenated butyl vulcanisates containing Esso oil showed higher acidity development than did the Iraqi oil mixes.
5. IIR compounds containing Iraqi oil had higher acidity values than those with Esso oil.
152
TABLE 4.7:
INNER TUBE COMPOUNDS FORMULATION USED IN ACETONE EXTRACTION
Parts by Weight IIR. (Polysar. 301). CIIR. (HT1066). BUR (X 2)
Rubber 100 100 100
GPF N-660 Black 60.0 65.0 67.5
Paraffinic Oil 25.0 20.0 22.5 1. Flexon 845 2.. Iraqi oil
tearic Acid l'. O 1.0 1.0
ZnO 5.0 '5.0 3.0
mgO - 0.3 0.75
TMTD 1.0 1.0 0.75
MBTS 2.0 2.0 0.4
Sulphur 1.0 - -
The acidity results thus confirmed the previous findings of the hot
air. oven ageing and stress relaxation experiments namely that the Iraqi oil caused higher degradation than the Esso oil with butyl rubber but less degradation when mixed with halogenated butyl rubber.
I
To account for this observed development of acidity in rubber, the following reaction schemes are discuss ed based on the various types
of crosslinking reactions known to occur. The following reactions are expected to occur during butyl type vulcanisations:
153
X10-2
t= ca
Fig. 4.75. Acidity %, of unaged and aged IIR, CIIR and BIIR with 'Esso and Iraqi oils, after aging, 3,7 and 14 days at 1250C.
0123456789 10 11 12 13 14 AGING TIME (days) at 125%
154
1. The Butyl Rubber/Sulphur Vulcanisation System
i) The initial step in vulcanisatiOn seems to be the reaction of sulphur with the zinc salt of the accelerator to give a zinc perthio-salt XSx Zn Sx X where X is a group derived from the
accelerator (e. g. thiocarbamate or benzthiazyl groups). This
salt reacts with the rubber hydrocarbon RH to give a rubber bound intermediate ((a) of equation'4.2)('149) and a perthio-accelerator group ((b) of equation'4.2):
X Sx Zn Sx X+ RH -. ). X Sx R+ ZnS + HSX_j X (4.2) (a) (b)
which, with further ZnO will form a zinc perthio-salt of lower
sulphur content; this may, nevertheless, again be an active sulphurating agent, forming intermediates, X Sx_l R. -In this way each molecule of accelerator gives rise to a series of inter-
mediates of varying "degrees of polysulphidity". The hydrogen atom which is removed is likely to be attached to a methylene group in theik-position to the double bond. The intermediate X Sx R then reacts with a molecule of rubber hydrocarbon RH to give a cross- link, and more accelerator is regenerated (see equation 4.3) (149)
x Sx R+ RH -* R Sx_l R+ XSH (4.3)
ii) On further heating, the degree of polysulphidity of the cross- links is known to decline. This process is catalysed by the x Sx Zn. Sx X and can result in additional crosslinks of the mono- or disulphidic types. It is also evident that the crosslinks which were initially at points 4 and 5 of equations 4.4 and 4.5 undergo an allylic shift, with the result that new configurations appear:
155"
CH3 (4)
--- CH -C CH CH-- 21 sx
R
CH3 I
--CH 2-C- CH = CH- -- I sx I R
R I sx I CH (5) CH2 2 1 11 -
--CH2 -C= CH - CHý---o- --CH 2C- CH CH2-- I sx I R
(4.4)
(4.5)
At the same time, disappearance of crosslinks of the disulphide
and polysulphide type occurs, with formation of conjugated trienes ((a) of equation 4.6)
CH3 CH3
CH2 C CH - CH - CH2 C= CH CH27-- I sx I CH CH R1313
CH 2-C= CH - CH CH -C= CH - CHf- (a)
RSXH (4.6)
iii) Other workers(150) results indicate that in polyisoprene sulphur can combine with the hydrocarbon, either by addition or as a bridge, between adjacent double bonds (see (a) and (b) of the
scheme in Figure'4.76). - Both of these reaction types would produce a ratio of 1 atom of sulphur per double bond lost, this
156
was found to occur in a simple rubber-sulphur compound. If this ratio is to be greater than 1, an additional. reaction must take place where sulphur can combine with the polyisoprene without loss of unsaturation. Such a possibility would be, for instance, a dehydrogenation reaction (see (c) of Figure 4.76). The observation that there is an excessive loss in unsaturation on overcure indicates that direct polymerisation without sulphur (see (d) of Figure'4.76), as well as reaction of oxygen at the double bonds, may also occur; this would be subsequent to the combination of most of the sulphur with the polyisoprene.
I CH,., I, -. ý S (a)
CH 3-C 00
CH2 CH2 -S- CH2 (b)
CH CH 3C+S3 C'ý -S-C,, ýj - CH3
CH 2 CH -S- CH + H2S (c) Rubber I1 11 CH -CH CH C 11ý 'I), C` - CH Polymerisa- 11212
(d) 33 tion CH V1, - C5 -C 31. H3
FIGURE 4.76: REACTION SCHEME FOR THE COMBINATION OF SULPHUR WITH POLYISOPRENE DURING VULCANISATION
2. The Halogenated Butyl Rubber, (ZnO/TMTD) Vulcanisation Reaction
Step 1.
A multistage process, probably involving both free radical and ionic mechanisms is believed to be involved in a TMTD/ZnO cure in which the first step is the attachment of a thiuram-containing residue with the
(151) rubber through a sulphur linkage . The pendant group ((d) of equation 4.7) subsequently breaks down under the influence of addi- tional ZnO to give a sulphur containing crosslink and the correspon- ding zinc dithiocarbamate(152,153). It should be noted, that on summation altogether three moles of thiuram disulphide enter into the overall reaction (see equation'4.7). Two of these are converted into dithiocarbamic acid ((a) of equation'4.7) and consequently into
157
zincdithiocarbamate ((b) of equation 4.7). One remains combined in
the rubber, two of its sulphur atoms forming the disulphide crosslink ((c) of equation 4.7) and the other two being part of the R2N-C(S)
radicals attached to a carbon atoms ((d) of equation'4.7).
CH 3 1 3 R2N-C(S)-S-S-C(S)-NR2
.+4 -- CH2 -C= CH - CH --- + 2ZnO 2
2R2N - C(S) - SH
I (a)
2Zn (S - C(S) - NR2) (b) (d)
CH 3 1 CH C= CH - CH2 --- I s (C) I S CH3 II CH -C= CH - CH2 -
Step 2:
ZnO also reacts with CUR or BIIR after an allylic shift of the
chlorine-or, bromine atoms. (see equations 4.8 and 4.9) (154-156)
CH2 -C --- + ZnO --ii. ---CH 2-c --- 11 11 CH CH II CH2 CH2 II or Cl(Br) OZnCl (OZnBr
CH3 I
2 -- CH -C CH - CH2
R2N-CS
(4.7)
(4.8)
Halogenated butyl polymer
158
--- CH2 -c+ cl 11 1 CH CH2 II CH CH 12 'll OZnCl- ----C -. -CH 2
or (OZnBr)
CH -C 2 CH
CH 2 1 0 1 CH2 I, CH 11
---CH2 -c ---
ZnCl, + MgO -* ZnO + MgC12
or (ZnBr2)
Step 3. -
+ ZnCl2
(or Zn Br2)
(4.9)
And/or halogenated butyl rubber reacts with ZnC12 (or ZnBr2), which is
initiated from the'thermal dissociation ofsomeof the allylic chlorine (or bromine) from the CUR (or BIIR) to yield hydrogen halide; sub- sequent reaction of this hydrogen halide with ZnO provides the catalyst
(157) ZnCI 2 (or ZnBr2) (see reaction scheme in Figure 4.77)
Step 4:
And/or halogenated butyl rubber reacts with ZnO in the presence of MgO; in which zinc atoms incorporate into the crosslink, and
1 . (156) summarised by the overall reaction
IH 12 2CH 2= CH -C- C1 (or Br) + ZnO + MgO
CH2 CH2 I I'
CH2 CH C-0- Zn. - 0-C- CH 'CH2
+ MgC'2 (4.10) (or Mg Br 2)
1,59
FIGURE 4.77: CIIR REACTION WITH ZnCl 2 Initiation
LK 3 CH 3
---CH =C- CH - Clig--'+ Zn C' 2 CH =C- CH - CH 2 I (+) Cl (ZnCl 3)
Propagation
CH3
I -CH =C- CH - CH2--
M (ZnC'3) (-
CH3 I C- CH - CH 2 I
cl
CH 3 1
--- CH = -C - CH --
CH
M
Termination
1 .1 CH -C- CH - CH 2---
cl (ZnCl
.. j) ")
CH 3 CH 3 II
---CH C- CH - CHf CH C- CH - CHr-
CH 3 CH 3
---CH -C- CH - CH 2 ---CH -C- CH - CH 2--- (+) II. I cl cl cl
(ZnCl. 3) CH 3+
ZnCl 2
1 ---Cti C- CH -, CHf---
CH 3 1
---C =C- CH - CH 2--- 1 cl
+ HCI + ZnCl 2
(4.12)
160
According to the above vulcanisation reactions of a butyl/sulphur
curing system and a halogenated butyl/ZnO-TMTD curing system various acid reaction products and salts are produced and these are extrac- ted by the hot acetone procedure; the acidity of the acetone used for the extraction has been measured. The products considered to
contribute to this acidity are as follows:
1. Butyl rubber vulcanisates:
a) Zinc perthio-salts b) Hydrogen sulphide c) Dithiocarbamic acid d) Compounds produced during oxidation of the butyl.
2. Halogenated butyl rubber vulcanisates:
a) Zinc dimethyl dithlocarbamate b) Hydrogen halide (HC1, HBr)
C) Dithiocarbamic acid d) Compounds produced during oxidation and dehalogenation of
the halogenated butyl.
It is observed that halogenated butyl rubber vulcanisates showed lower
acetone acidity values after extraction than those obtained from the butyl rubber vulcanisate. This is probably due, partly at least, to:
the presence of a high percentage of extractable zinc salts (ZMDC, zinc halide) in the halogenated butyl, and
ii) the partial liberation of HC1 (or HBr) to the atmosphere during the heat ageing process; this is in addition to
iii) the relatively low percentage of thiol groups present in the vulcanisation products of halogenated vulcanisates compared with the thiol groups content of the sulphur vulcanisates.
161
4.5 FACTORY TRIALS: TYRE INNER TUBES
4.5.1 Butyl Polysar 301_Inner Tubes
Introduction
The Iraqi tyre inner tubes factory at present uses conventional butyl
rubber in the manufacture of inner tubes and in order to improve the quality of these tubes a factory trial was undertaken to use the results obtained from this laboratory work. This work had shown that to obtain better physical property retention the lower oil level compounds are preferred. Therefore factory trials for inner tubes were based on IN Polysar 301 containing 15 phr of Iraqi oil. It was found that satisfactory processing was readily achieved by adjusting the carbon black level, to offer a balance of desirable tube properties and processing safety. Heat ageing and the accelerated wheel
(158) test which simulates rapid service of a finished tube was undertaken to try and confirm the laboratory results.
4.5.1.2 Compounding
An inner tube compound based on the following formula shown in Table 4.8 was mixed by using an internal mixer (Banbury No. 11) of the Iraqi tyre inner tube factory.
TABLE 4.8: IIR INNER TUBE FACTORY TRIAL COMPOUND FORMULATION
Materials Weights by Parts
IIR Polysar 301 100 GPF N-660 Black 50.0 Iraqi Paraffinic Oil 15.0 ZnO 5.0 Stearic Acid 1.0 TMTD 1.0 MBTS 2.0 SUlphur. 1.0
a
162
4.5.1.3 Results and Discussion
4.5.1.3.1 Unvulcanised Properties
Comparison of unvulcanised properties of the butyl inner tube compound prepared in two different ways was used; laboratory and factory. The
results are described in Table 4.9.
TABLE 4.9: UNVULCANISED PROPERTIES OF FACTORY AND LABORATORY INNER TUBE COMPOUNDS
Properties Factory Compound
Laboratory Compound
Mooney Viscosity, ML 1+4ý, 1000C 47 48.5 ODR Scorch Time t5 at 1710C 4.30 4.0 (minutes) Mooney Scorch at 1250C 32.0 30.0
. (minutes)
4.5.1.3.2''Vulcanisate*Characterittics
Figure 4.78 shows the ODR curves of these compounds cured at a temp- erature of 171OC; it can be seen that the laboratory compound showed a higher ODR torque and curing rate than that obtained with the equivalent factory compound.
4.5.1.3.3 Stress; -Stf-aih Properties
Table 4.10 presents the physical properties of the butyl inner tube compound (where formulation is described in Table 4.8) but prepared using different conditions:
1. Compound mixed and cured in the UK laboratory. 2. Compound mixed in Iraq and cured in UK laboratory. 3. Compound mixed and produced as an inner-tube in Iraq.
163
'D
0 C3. E 0 ILJ
0)
C- 0)
E
ci L- 0
m ci
1C2 ci
LLJ 7- ti
ci
ci
%4- 0
(A
.U 4- LA L- cu
cl
u
C= 0
Li
LL.
.1 C-
0
rn
0
Ul- NP (Sul'ql ) P0801
164
Results indicate that there is no significant differences obtained from the above compounds except. that the compounds mixed in Iraq (Nos 2 and 3) showed higher 100% and 300% modulus than those
obtained from the laboratory.
TABLE 4.10:
PHYSICAL PROPERTIES OF A FACTORY TRIAL IIR INNER TUBE COMPOUND COMPARED WITHLABORATORY PRODUCED INNER TUBE COMPOUND
Properties Laboratory Compound'.. "
Factory Compound
Finished Inner Tubes
Hardness (IRHD)' 47.5 53.5 47.5 Tensile Strength (MPa) 11.2 11.7 10.6. Elongation at break % 725 700 700 Modulus at 100% Elongation 1 0 1 2 1 3 (MPa) . . - . . Modulus at 300% Elongation 3 1 - 3.4 3.8 (MPa) .
4.5.1.3.4 Hot'Air-Oven Ageing
Table'4.11 shows the effect of the heat ageing on the tensile proper- ties retention of the above compounds after ageing 3,7 and-14 days
at 1250C. It is, clear that the rate of ageing or degradation does however vary very widely depending upon the conditions of the compound preparation. It was observed that there was a difference of 12-16% in tensile strength retention between the laboratory and factory compound results. This finding confirms that there is an effect of the following factors on the heat ageing properties of IIR inner tubes:
1. Inner tubes processing factors 2. - Conditions of mixing
165
3. Fillers and other ingredients dispersion efficiencies which are poorer in the factory mixes.
Comparing the heat ageing properties of the factory and laboratory
mixed butyl (with 15 phr Iraqi oil) with the laboratory mixed compound (containing 25 phr Iraqi oil) (see Table 4.2) the effect of the oil level on the rubber's heat resistance can be clearly seen.
4.5.1.3.5 Tube'Life'ý-'Wheel*Test
An indoor test (wheel test) was carried out on finished truck tyre inner tubes manufactured in Iraq to check their service performance. A comparison test was undertaken on two types of inner tubes, one produced with low oil levels (15 phr of Iraqi oil) and the second was the normal Iraqi inner tube product (21.5 phr of oil); the test
was run using a high speed indoor testing machine and under a standard (158)
service test condition
The results shown in Table 4.12, indicate that the normal truck inner tubes failed after 92 hours at a speed of 91 km/hour, while the low
oil level tubes failed after 104 hours at a speed of 100 km/hour. Failure was caused by thinning of the tube in the tyre shoulder area due to heat softening. Low oil level tubes therefore run for a longer time than high oil level tubes; this confirmed the laboratory
evaluation and conlusions of the effect of the oil level on the
promoting of the oxidative degradation of butyl inner tubes.
166
LLJ
LL. C)
Ln uj
cht: LLJ CL CD
0:: CL
:m C3 LLI
0: C: c CL M: C> u
LL.
LLJ
P-4
43 LLJ
cý 0-,
C: ) CD Co =
ci F-- 0
Lt) CM
LLJ «: c x b--4
N-4 LU cm
LL. CD
(n LU Lii cm
LLJ W CL LLI (Z) =
ime = c26 >-4
La
V)
= s--f Ki. LL-
Uj -. i cm . CC F-
LO 00 to
m CD m a C) CY) LO I q: r qr I
.0 q-t I I I I
(A r_ CY) >., P. - CIJ r- ý cc ý S-4 -0 Ln r- r ko r,: Cý r- (Y) I m CM 1
-00 1
(A >) m
LL. c; +
tA LO LO Lc) 00 co C%i
Lý Cý C4 Cý Cý r_ S- CIQ U-) I CV) ce) 1 :3 o -t: r 0 4- r- 06 E L) 0 0
LO CIQ CV
r- rl% CY) co C%j Oll (U (Z X 4-) a Cý C6 c) 06 C4 0 co
o (L) 4-) a) 4A U cc >% r*ý (0 ou
LL. -0 C4 Ln Cý 1, r,: I CIJ + + C? )
-0 >$ LO qdl Lr) 00 lwzt M c; oo c; ký C; C\;
0 M M I CIJ CL E
Qj (A X
Cj "o C: ) C) C'i =0 -0 C"i " r- I
>1 S - o
4. )
0 cr) CY) CV) .0 to
rý: C; cl; 1: ..: r- r- C: r C\J I CY) I I + +
0 . -
0 - r
4-3 I
4.3
. 1d 0 0 to r- r- ýR a) ui ui S- 4. )
4-1 cla JA JA Im C) C) C 4-) C) C) (1) to r- cn S- nc
4-3 C 4J 4-S V) 0 n3
4. ) cr) (U 4-) o (A S- c to = = (4 (U to tm r- r- (U CL = "0 CA C :3 = cn a L) f- c o -a -t3 tA L. #0 4) r- 0 0 C)
167
TABLE 4.12: BUTYL INNER TUBES WHEEL LIFE TESTING RESULT (TRUCK TYPE)
Inner Tubes with Low Normal Inner Tubes Oil Level Production
(15 phr oil) (21.5 phr oil)
Ma i speed x1mum 100 91 (km/hr).
Test for failure 104 92 time. (hours)
Temperature inside tyre during test 105 100 (110-
.... .... ......... ...
Condition of the Soft, stick to the Soft, stick to the tubes after tyre carcass at the tyre carcass at the failure shoulder shoulder
4.5.2 'Chlorobutyl Inner*Tubes
4.5.2.1 Introduction
Inner tubes made from regular IIR have been in service in most parts of the world, but in severe service conditions IIR tubes soften and sometimes stick to the interior wall of the carcass of-the tyre and can result in the loss of the tyre itself. CIIR tubes can be consi- dered as offering improved resistance to heat softening and growth. In this laboratory work CIIR (Grade HT 1066) was used, and because this CIIR produces compounds with inadequate given strength for processing into inner tubes, a higher green strength grade HT 1068 of higher Mooney viscosity (45-55 ML 1+8 at 1250C)replaced it in the factory trial to avoid processing problems.
168
4.5.2.2 Compounding
Inner tubes of the compound recipe given in Table 4.13 were mixed in
two stages in a factory internal mixer (Banbury No. 11).
Materials -Parts. by-Weight
CUR 1068 100.0 GPF N-660 Black 70.0 Iraqi Paraffinic Oil 28.0 ZnO 5.0 Stearic acid 1.0 MgO (Maglite D) 0.75 TMTD 1.0 MBTS 2.0.
. ...... . ....
TABLE 4.13: CIIR INNER TUBE COMPOUND FORMULATIONS USED IN THE FACTORY TRIAL
4.5.2.3 Results'and'Discussion
4.5.2.3.1 Hot-Air Oven Ageing
Table 4.14 illustrates the physical properties of unaged and aged CUR tubes; test specimens were cut from a factory cured tube. These specimens were aged under severe conditions of test (1001C, 125% and 1501C) for different intervals of time (3,7 and 14 days).
Ageing results indicate that the CIIR inner tube still had a tensile strength of 52% of its original value, after 14 days ageing at 150*C,
and remained rubbery thus confirming that CIIR vulcanisates will resist high'temperatures (up to 150*C) with relatively little degradation of physical properties.
169
(A LU CM
ce.
LU
ci
Z C: ) c2
LLJ
1. -1 u LU
LZ-
V)
v; LU an
dm LLI X
LL-
, im
LU
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170
4.2.3.2': Inner'Tubelift'Using a*High'Speed Wheel'Test
Using this test as applied to a passenger tube wheel test, chloro- butyl and conventional butyl inner tubes (normal Iraqi product) were tested under equivalent conditions(-'
58) . Results of this test,
together with a description of the conditions of the tubes after the test are given in Table 4.15. As expected, the IIR tube showed heat
softening in the shoulder area of the tyre. This softness and sticki- ness can make it difficult to remove the tube from the tyre, while CIIR tubes remained rubbery and tack-free for longer periods under the
same conditions, small cracking took place on the surface but without any visual evidence of heat softening,
It is apparent from the results of the wheel test and hot air ageing that the chlorobutyl inner tube offers an appreciably higher level of heat resistance than butyl inner tubes.
TABLE 4.15: CIIR INNER TUBES WHEEL LIFE TESTING RESULTS (PASSENGER TYPE)
CUR Tubes IIR Tubes
Maximum speed km/hour 85 75 Test for failure time 53 36 5 (hours) . Temperature inside the
OC d i t t t 104 98.5 ) ur ng es ( yre Temperature at tyre
O ld h 99 103 C) er ( s ou Temperature outside 36 38 tyre (OC) Condition of the tubes Rubbery, tack-free, Soft, sticks to after failure small cracks at the tyre carcass
surface
171
4.6 GENERAL CONCLUSIONS
All the degradation test methods combine to give the same conclusions
about the behaviour of butyl and halogenated butyl compounds at
elevated temperature and can be summarised as follows:
Losses of the physical properties increase with increasing
ageing time and temperature.
2. Compounds containing the highest amounts of oil showed the
greatest rate of degradation.
3. Halogenated butyl rubber offers better resistance to thermal
and oxidative degradation than the regular butyl rubber.
4. The IIR vulcanisates containing Iraqi type paraffinic oil exhibited poorer property retention than the equivalent contai- ning the Esso oil Flexon 845. However CUR with Iraqi oil demon-
strated superior heat resistance to IIR.
5. Ageing of both butyl and halogenated butyl rubber at elevated temperatures has been shown to be an oxidative and thermal
process.
6. Two reactions, scission and crosslinking, take place in both butyl and halogenated butyl compounds, but the scission process is more predominant than the crosslinking process.
7. At elevated temperatures, butyl compounds become soft and sticky and lose all their physical properties, whereas the halogenated butyl compounds retained their original appearance by remaining rubbery and tack-free as well as keeping a good percentage of their original physical properties.
172
.. CHAPTER 5
INVESTIGATION'OF*HEAT'RESISTANT'CROSSLINKING 'SYSTEMS'IN'CHLOROBUTYL'RUBBER
5.1 INTRODUCTION
The presence of both olefinic unsaturation and reactive chlorine in
chlorinated butyl rubber provides for a great variety of vulcanisa- tion techniques. Halogenation increases the reactivity of the double
bond and. also supplies active sites for crosslinking by the action of zinc oxide. Vulcanisation of CUR with ZnO is postulated to
proceed through a cationic polymerisation route giving rise to
stable carbon-carbon crosslinks.
When no organic accelerator is present in CUR, only partial vulcani- sation occurs with metallic oxides. The nature of the crosslinks
(. 155) which do form, however, is not known
Equations 4.8 and 4.9 represent a possible reaction but there is no evidence to prove the existence of ether bridges (150,152)
. Another
possible mechanism may involve the formation of ZnCl 21 first by
reaction of ZnO with HC1 released from the CUR by oxidation attack, followed by a Freidel Crafts type reaction involving the ZnCl 2 (see Figure 4.77). ZnCl 2 is probably the actual curative because the cure is inhibited by materials that delay the conversion of ZnO to ZnCl 2*
(155,157)
It is generally considered that when NO is used as a vulcanising agent in CIIR, 3 phr are recommended to ensure its ready availability as ZnO is difficult to disperse in rubber. Use of more than 5 phr does not appreciably increase rate of state of cure, but some excess of zinc oxide can be shown to be effective ip enhancing heat resistance charac-
(157) teristics
173
The cure rate of CUR with ZnO has been varioUsly described as fast
and slow. In many cases the ZnO cure system is fast but this depends to some extent on cure temperature and the method used to describe the cure rate.
Scorch and cure rate of the ZnO cure can be controlled by the level
of stearic acid. Stearic acid enhances cure rate and reduces scorch safety. One phr of stearic acid is suggested as a starting point. Certain formulations may require as much as two to three phr for
optimum cure rate, whereas others may necessitate substitution of paraffinic wax for the stearic acid for adequate scorch safety.
5.2 EXPERIMENTAL
Chlorobutyl rubber (Grade HT 10661 was used to study the effect of the vulcanisation system on the heat resistance of a CUR tyre inner tube.
A zinc oxide crosslinking system and the influence of increasing ZnO levels on the heat resistance of a CIIR inner tube compound were investigated.
Table 5.1 illustrates the formulations of CUR inner tube compounds which were used in the present work. This compound is cured with different levels of ZnO and comparedwith CUR compounds cured with a ZnO/TMTD (sulphurless) curing system.
174
.. Materials .... 1. ...... ZnO. Base ....... . ZnO/TMTD . .... ..... ...... 10-phir ..... ..... 20 phr
CUR 1066 100 100 100 GPF-N66o Black 65 65 65 Flexon 845 Oil 20 20 20 (Esso Oil) Stearic Acid 2.0 2.0 1.0 ZnO 10 20 5.0 Mgo - - 0.3 TMTD 1.0 MBTS 2.0
TABLE 5.1: CIIR HEAT RESISTANT TYRE INNER TUBE COMPOUND BASED ON DIFFERENT CURING SYSTEMS
5.2.1 Results and Discussion
5.2.1.1 Curing Characteristics of the Unvulcanised CompoundS
ODR curing characteristics of the compound in Table 5.1 are recorded in Figure 5.1; it can be seen that:
ZnO cured CIIR vulcanisates showed slower curing rates (t95 8.0 minutes) than that of ZnO/TMTD curing system compound (t95 : -, Q minutes).
2. ZnO curing base exhibited a somewhat higher ODR torque value (at 10 phr and 20 phr levels) than that of ZnO/TMTD curing system.
However the differences between the systems were not very large.
175
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176
5.2.1.2 . Cbmpouhd'Physical Properties
Table 5.2 shows the vulcanisate physical properties of CUR cured with different levels-of ZnO CIO phr and 20 phr) and compared with ZnO/TMTD (sulphurless) cured MR vulcanisates. The following
observations were obtained:
Compounds cured with ZnO gave lower hardnesses, elongation at break, 100% modulus and tear strength than those obtained from
a compound cured with ZnO/TMTD.
2. A ZnO curing system showed an improvement in compound compression set (lower value). This is believed to be due to the absence of polysulphidic crosslinks.
3. There is no significant differences in mechanical properties obtained by increasing the ZnO level from 10 phr to 20 phr.
TABLE 5.2: EFFECT OF CURING SYSTEM ON THE PHYSICAL PROPERTIES OF CIIR INNER TUBE COMPOUND
Physical Properties CIIR with 10 phr ZnO
CIIR with 20 phr ZnO
CUR with 5 phr ZnO/TMTD
Hardness (IRHD) 39.5 39 44 Tensile Strength (MPa) 8.7 8.5 8.2 Elongation at 600 600 700 Break % Modulus at 100% 0 8 0 8' 1 0 Elongation (MPa) . . . Modulus at 300% 3 5 3 3 3 5 Elongation (MPa) . . . . Tear Strength Nm-l 36 32.5 44.5 Compression Set 22 hours at 1000C % 15 14.5 38
177
5.2.1.3 'Hot4ft'Agolnq
The percentage change in physical properties of these compounds was
measured af ter agei ng in severe hot-air f or 3 days at 1500C and one day at 1650C. Results of heat ageing are as shown in Table 5.3.
The following observations were made:
At the higher temperatures of 1500C and 1650C, the ZnO cured CUR compounds exhibited severer deterioration than the ZnO/
sulphur donor curing system compounds.
2. Tensile strength losses in the compound cured by 10 phr ZnO were nearly three times (-82%1 higher than those obtained from the ZnO/TMTD compound (27.5%) at the end of 3 days ageing under 150 0 C.
3. The 10 phr ZnO level compound has lost 64.5% of its tensile
strength at the end of one day's ageing at 1650C, whereas TMTD/ ZnO cured CUR samples showed no losses at the end of the same time period and at the same temperature.
4. By increasing the ZnO level from 10 phr to 20 phr in ZnO curing system, little improvement (5% and 9.5%) resulted in tensile
strength retention after 3 days ageing at 1500C and one day
ageing at 1650C respectively.
5. Losses in weight from the CIIR compounds cured with-ZnO alone were somewhat greater than the crIR compound cured with ZnO/TMTD aged at 1500C.
178
TABLE 5.3: PERCENTAGE LOSSES IN PHYSICAL PROPERTIES OF ZnO AND ZnO/TMjD CURED CIIR VULCANISATES AFTER 3 DAYS OF AGEING AT 150 C AND ONE DAY AT 1650C
. ....... .......................
Physical Properties CUR with 10-phr. ZnO...
CUR with 20 phr ZnO
CUR with 5 phr ZnO/TMTD
% Cýaýne'dfter age-ing 3'days at 1500C Hardness +5.1 +5.1 -7.9 Tensile Strength -82 -76.9 -27.5 Elongation at Break % -41.7 -45.8 -32.2
Modulus at 100% Elongation +25.3 +19.8 +35.4
Modulus at 300% - -42 8 +26 Elongation .
Weight Losses % -3.5 -3.7 -1.5
% 'Ch=6ý, ý af Per ageing one day at 1650C
Hardness +3.5 +4.2 +2.5 Tensile Strength -64.5 -55 +2.2 Elongation at Break % -29.2 -29.2 -21
Modulus at 100% Elongation +3.3 +32.8 +49
Modulus at 300% Elongation -32.8 -15.4 +32.7
Weight Losses % -1.4 -1.6 -1.1
179
5.2.1.4 Swell{ngThSölvent
5.2.1.4.1''Ratit Pe-inciples
Swelling of rubber in the solvent is an effective technique to achieve the following objectives:
To determine whether or not any crosslinks were found or des- troyed between rubber chains after heating.
2. To determine the crosslink density.
The degree of swelling (the amount of solvent imbibed) is dependent
upon the crosslink density of the rubber networks; the greater the
crosslink density, the less is the degree of swelling. Percentage
swelling by volume, of the cured samples, was determined by using the following formula: (159)
Gain in weight x Gravity of specimen x 100 Gravity of-solvent Original weight of specimen
=% swelling by volume (5.1)
The Flory-Rehner(159 ) equation was used to calculate the Mc, the
number average molecular weight between crosslinks, from solvent swelling measurements. The equation is:
(1 - Vd + Vr +x Vr 2) =0V0m C_ 1v
r 1/3 (5.2)
where Vr is the volume fraction of rubber in swollen gum stocks deter-
mined from weight increase on swelling, and the densities of rubber and solvent
x is the polymer solvent interaction constant (0.415) for CUR in cyclohexane as found by Berger(160)
0
180
p is the density of rubber (0.92 gm/cm for CIIR) V0 is the molar volume of solvent (108.04 for cyclohexanej M is the number average molecular weight between crosslinks C ... I (crosslink density -20F)
c
The volume of the samples was determined by weighing the test speci- mens both in air and water. The difference between the two weights gave the volume of the samples.
From the base formulations, the amount of rubber present in the
weight in air of each specimen was calculated.
To obtain the value of Vr above, approximately 20 x 10 x2 mm of vulcanisate was weighed accurately and then immersed in cyclohexane at room temperature for six days. A swelling time of six days was chosen on the basis of the test results on several samples which showed no significant changes, after six days of immersion in cyclo- hexane. At the end of the immersion period the sample was removed, rapidly blotted with tissue and transferred to the weighing bottle to obtain the swollen weight of the sample. The crosslink density was calculated, based on the value of Vr obtained, using equation 5.2. Network density was expressed as moles of crosslink per gram of insoluble network.
5.2.1.4.2 Resultt and Discussion
Table 5.4 shows the crosslink density and swollen volume of unaged and aged, compounds which were used in Section 5.2.1.2-The results indicate that:
181
The swelling capacity Cor ability of the network to imbibe
solvent) decreases with increasi. ng the degree of crossIlInking.
2. Swollen volume of age4 samples was greater than the swollen volume of unaged samples; this means aged samples have lower
crosslink densities Cas Is clear from Table 5.41.
3. Crosslink density of unaged samples cured by ZnO alone exhibited higher value than the samples cured with ZnO/TMTD system.
4. Vulcanisates cured by the ZnO curing system showed a lower net- work density that those cured by ZnO/TMTD after 3 days ageing at 1500C. These results illustrated that the crosslinks formed during the vulcanisati-on of CUR with ZnO/TMTD through double bonds and the chlorine atoms of CUR (see Section 4.4.3) were stronger than those of the ZnO cures.
5. Crosslink density of unaged and aged vulcanisates, of the 20 phr ZnO cure system were found to be higher than those samples cured with the 10 phr ZnO level.
TABLE 5.4: CROSSLINK DENSITY AND SWOLLEN VOLUME OF UNAGED AND AGED SAMPLES OF CIIR CURED WITH DIFFERENT LEVELS OF ZnO, COMPARED WITH CIIR CURED WITH THE ZnO/TMTD (SULPHURLESS) CURING SYSTEM
% Volume Swollen Crossli mo
nk Density le/g
Unaged. Aged 3 days at 1500C Unaged Aged 3 days
at 1500C
CUR cured with 10 phr ZnO 238 370 4.8xlO-5 2.5xlO-5
CUR cured with 226 348 5 8XIO-5 2 8xlO-5 20 phr ZnO . .
CUR cured with 324 339 3 5xlO-s OXIO-5 3 5 pHr ZnO/TMTD . .
182
5.2.1.5'*Detorminatl6h'6f'VulCanisate_Acidiýy after Heat ! kgeing . (Usin§*th6*At6t6h6'Eýtratti6n'T6chnique)
Table 5.5 illustrates the acidity and pH values of the acetone. extract after 16 hours hot extraction of the unaged and aged samples used in Section 5.2.1.2(see Section 4.4 for more details about the,
acetone extraction techniquel. The following observations were made:
1. Unaged *ScVZes
i) Acidity decreases with increasing the ZnO level from 10 phr to 20 phr in ZnO cured CUR (pH of the acetone after extraction changed from 5.4 in the former to 7.4 in the latterl. This is
probably due to the presence of a large amount of ZnO which reacts with all HCI CHCI is produced-during vulcanisation; see Section 5.1. )to produce water and salts and other OR group con- taining compounds which were transferred into the acetone solution:
ZnO-+ 2HCI -,.. ZnCl, +H 20
ii) CUR vulcanisate cured with 10 phr ZnO showed higher acidity than that cured by ZnO/TMTD; this was probably due to the high
percentage of salts, which were extracted from the TMTD/ZnO
curing system compared to that of the system cured with ZnO
alone (see equations 4.7-4.12 in Section 4.4.3).
Aged'SampZes:
1. Acidity of all samples increased with increase of ageing time.
2. Acidity of aged samples which when cured with ZnO only Cat both levels), indicated higher acidity than those samples cured with ZnO/TMTD.
183
3. Aged samples cured with 10 ph-r ZnO level- showed higher
acidity than that of the samples cured with the 20 phr ZnO level.
TABLE 5.5: pH VALUE AND ACIDITY PERCENTAGE OF UNAGED AND AGED CIIR CURED WITH DIFFERENT LEVELS OF ZnO, COMPARED WITH CIIR CURED WITH ZnO/TMTD
Acidity %. - PH
Unaged Aged 3 days Unaged Aged 3 days ......... . -at., 1500C at 1500C
CIIR cured with 0 6 1 0 4 5 3 5 10 phr ZnO . . . .
CIIR cured with 0 0 5 7 4 4 5 20 phr ZnO . . .
CIIR cured with 5 phr ZnO/TMTD 0.3 0.6. 6.4 5.0
5.3 CONCLUSIONS
In general all test methods combined to give the same conclusion that the sulphur donor curing system (ZnO/TMTD) in CIIR was found to give a higher degree of physical property retention than the CUR compounds cured with zinc oxide alone. This is considered due to TMTD in the CUR sulphurless cure system being transformed, during vulcanisation, into zinc dimethyldithiocarbamate which is known to be a powerful antidegradant (see Section 7.4 for ZnO/TMTD vulcanisation reaction).
184
CHAPTER 6
INVESTIGATION OF USE OF ANTIOXIDANT IN PROTECTING HALOGENATED BUTYL RUBBER
6.1 LITERATURE REVIEW
There are not many experiments which have been done to investigate the use of antioxidants in butyl rubber types.
In recent work, various antioxidants were evaluated by Timar and Edwards( 123) in a sulphur donor
' BUR cured compound; Table 6.1
summarises their data by rating the various antioxidants in order of increasing effectiveness, relative to the respective control compounds. From Table 6.1 it has been found-that the beneficial effects of the
antioxidant are seen only at high temperatures higher than 1750C and the MBI/ADPA/MgO combination provides the best protection to the BUR against thermal degradation.
Baldwin-and Buckley et al (161)
studied the effect of the TMTD-MBTS
ratio, in the presence of the conventional antioxidant 2246, towards oxidative scission and high temperature resistance of CIIR vulcani- sates (shown in Table 6.2).
6.2 MECHANISM OF ANTIOXIDANT PROTECTION
Stabilization against thermal oxidation has been reviewed recently in
considerable detail by Shelton (162,163) , Ingold(164 )
and Deni sov(165). It was pointed out that the oxidative degradation and. crosslinking of the polymers can be inhibited by removing the active chain carrying (R02* ) radicals or by harmlessly decomposing the hydroperoxide (ROOH). Antioxidants are therefore classified into two types, Preventive
' Antioxidants (D) which decompose peroxide to n'on-radical products,
185
TABLE 6.1: ANTIOXIDANTS AND THEIR EVALUATION IN HEAT RESISTANT BIIR VULCANISATES (See also Appendix 11 (123))
Ageing Con- ditionsý': ',
1680 hr 1000C
1680 hr 1250C
70 hr 1500C
168 hr 1500C
22 hr 1750C
70 hr 1750C
22 hr 2000C
Control com- pound TETD 67 40 47 26 26 0 0 cureZJPiret RATING
PBNA 15 -
EMDHQ 5 - 5
DODPA ... .... .. - 5 5 5
BHT 5 10 10 -
2246P - 10 5 -
DCBDOTG - 10 5 15
APBN . ..... 5 5 15 5 20
DNPD 5 10 5 5
IPPD - - 20 25 10
NPP 5 15 10 10 10 -
ADPA 86 - 10 20 20 15
CPPDA 10 15 25 15
NBC 10 10 25 I
10
ADPA 86/MgO - 10 10 1
25 25 20
MBI I
10 I
15 25 30 25 - I MBI/ADPA 86/ MgO 10 35 40 60 50 40
Compound recipe: BIIR: 100; Stearic acid: 1; MgO: 0.1; FEF Black: 55, Naphthenic oil: 15; ZnO: 3; TETD: 0.3; Antioxidant: 2.
*7(T (Tensile strength'x*elongation at break)'aged x 100 p ret (Tensile strength x elongation at break) unaged
186
TABLE 6.2: EFFECT OF TMTD-MBTS RATIO ON HIGH TEMPERATURE RESISTANCE OF CIIR VULCANISATES IN THE PRESENCE OF THE ANTIOXIDANT 2246 (161)
Compound No. 1 2 3
TMTD/MBTS 1/1.0 1/2.0 1/3.0
Room temperature stress-strain properties, cured 40 minutes at 1350C:
Ultimate tensile strength (MPaj 17.9 17.3 17.1 Modulus at 100% elongation (MPa) 1.9 1.8 1.3 Modulus at 300% elongation (MPa) 11.0 9.6 8.2 Elongation at break % 470 525 625
Room temperature stress-strain after ageing for 16 hours at 1930C:
Ultimate tensile strength (MPa) 5.2 5.7 7.1 Modulus at 100% elongation (MPa) 2.8 2.6 2.4 Elongation at break % 170 210 295
Recipe* (phr): CIIR: 100; MgO: 2; Antioxidant 2246: 1.0; HAF black: 50; Stearic acid: 1; Necton 60: 5; Amberol ST-137X: 3; ZnO: 3; TMTDS: 1; MBTS: as indicated
187
and Chain Breaking Antiokidants CAH) which terminate the propagating chain but permit the chain initiatfng hydroperoxide to be regenerated. The mechanism of their action may be represented as follows:
ROOH +D -ý- Non radical products (Preventive Antioxidant)
RO 20 +, AH -)- ROOH + A* (Chain Breaking Antioxidant)
Scott(166 ) has pointed out that the Preventive Antioxidants represent the ideal case since they destroy hydroperoxides, which are the root cause of oxidative degradati'on, and many of them are believed to act via a catalytic mechanism. Nevertheless the majority of antioxidants used are of the chain breaking type.
The following reactions represent the kinetics of the chain-breaking antioxidants(167) :
R* +02 ROO* 1 (6.1)
I Chain reactions ROO* + RH ROOH + R* 1 (6.2)
ROO* + AH -, - ROOH + A* (6.3) Chain termination
Ae-i. -Inactive Product (Inhibition)
(6.4)
where RH is the substrate and AH is the antioxidant which is competing with the substrate for alkylperoxy radical. The reasons for this are as follows(166):
a) Kinetic chain-breaking antioxidants are palliative in that they deal with a situation In which deteriorati. onhas already set in, without dealing with the root cause.
188
b) Antioxidant activity is a composite function of the ability of the phenol or amine to release a hydrogen atom to take part in
the reaction of equation 6.3 and of the resulting aryloxy
radical to react further with the substrate RH (see equations 6.5a and 6.5bl:
A* + RH -+ AU + R* (. 6.5a)
or A* +02 -* AOO*
AOO* + RH ->. AOOR + R* (6.5b)
The rates of reaction of equations 6.3 and 6.5 will both be influenced by the steric environment of the aryloxy oxygen and in the same direction.
C) Similarly the ease of hydrogen abstraction by alkylperoxy (equation 6.3) is increased by electron releasing groups in the
aromatic ring, but the same electronic characteristic faVours the direct attack of oxygen on the phenolic hydrogen (equation 6.6) which is potential chain-initiating reaction:
AH + 02* -> A* + *OOH (6.6)
d) Finally, phenolic antioxidant contains the seeds of its own destruction. It has been shown that one of the oxidation products is dialkylperoxide formed by reaction of alkylperoxy with the phenoxy radical (equation 6.71
000
R1, R2 R1, R2
+ ROOO (6.7)
R3 R3QOOR (Phenoxy radical)(alkylperoxyl (Peroxydienone)
189
However phenolic antioxidants are relatively inefficient at high temperatures (above 1400CI, because the peroxydienone. decomposes
rapidly to give new initiating free radicals (equation 6.8)
0
Rl- R2
'hdat
OOR R3
6.3 EXPERIMENTAL
0
I +*OR
R3 0*
(6.8)
In this work a chlorobutyl tyre inner tube compound mixed with Esso
paraffinic oil was chosen to study and evaluate the beneficial effect of the antioxidant at high ageing temperatures. A carbon black filled
chlorobutyl compound formula was used to check the compound property retention:
i) after ageing at high temperatures in hot air (thermal oxidation) using accelerating ageing methods (i. e. the'hot-air oven and stress relaxation)
ii) after dehydrochlorination in a nitrogen atmosphere.
Also a white CUR compound was prepared to study the changes in the colour due to the oxidation and dehydrochlorination processes.
In all test methods comparison studies were made between the CUR
compound which contained antioxidant and the equivalent CUR compound containing no antioxidant.
190
6.3.1 Resultt and'Discussion
6.3.1.1 'UnVulcanised'ahd*Vulcanised Properties
Referring to Timar and Edwards(123) , antioxidant evaluation with BUR
compound, an ADPA/MBI/MgO combination was chosen to use in the sulphur donor cured CUR tyre inner tube compound mixed according to the
recipe shown in Table 6.3.
TABLE 6.3: ANTIOXIDANT CIIR BLACK COMPOUND FORMULATION
phr . ....... ...
phr
CUR 1066 100 100
GPF N-660 65 65
Carbon black Flexon 845 Oil 20 20
Stearic acid 1.0 ZnO 5.0 5.0
mgO 0.3 0.3
TMTD 1.0 1.0
MBTS 2.0 2.0
ADPA/MBI/MgO 0.8/0.5/0.7 -
Table 6.4 and Figure 6.1 illustrate the effect of using the antioxidant on the unvulcanised and vulcanisate characteristics of the CUR com- pound; it was observed that the CUR vulcanisates which contained antioxidant exhibited (i) higher Mooney viscosity, (ii) a longer scorch time, and (iii) a longer curing time than were obtained from the equi- valent compound in the absence of the antioxidant.
191
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192
TABLE 6.4: EFFECT OF THE ANTIOXIDANT ON UNVULCANISED PROPERTIES OF CIIR COMPOUND
Property ........... .
CIIR Without Antioxidant
CUR With Antioxidant
Mooney Viscosity ML (1+4 1000C 43 53 ODR scorch time at 1710C (mins) 21 314511 Optimum cure time, mins at 1710t 7' 11 Mooney scorch at 1250C (mins) 24 32
Table 6.5 records the physical properties of the CUR with anti- oxidant compared with the equivalent compound containing no anti- oxidant. Results of the CUR vulcanisate with antioxidant exhibited an improvement in all physical properties except the elongation at break which showed a decrease in the presence of the antioxidant.
6.3.1.2 Themal'Oxidation Process
6.3.1.2.1 *Hot-air oven ageing
Following the heat ageing results of the CIIR compound containing an antioxidant and comparing them with the control CUR compound con- taining no antioxidant after ageing in hot air for 1,3 days at 1750C and one day at 2000C, the decrease in tensile strength was used as a measure of the deterioration of the samples of vulcanised rubber; the following observations can be made from Table 6.6:
A compound containing the antioxidant retains about 45% of the original value of tensile strength at the end of 3 days ageing
-at 1750C and 31% at the end of one day's ageing at 2000C, whereas the compound that had no antioxidant lost 82% of its tensile strength at the end of 3 days at 1750C and 85% after one day's ageing at 2000C.
193
2. Samples without antioxidant became soft and tacky making the handling and processing difficult, while the CUR compound containing antioxidant eAlbited a rubbery appearance.
TABLE 6.5: PHYSICAL PROPERTIES OF CIIR VULCANISATES WITH AND WITHOUT ANTIOXIDANTS
Properties CUR Without Antioxidant
CUR With Antioxidant
Hardness (IRHD) 44 60
Tensile Strength (MPa) 8.2 9.3 Elongation at Break % 700 550
Modulus at 100% Elongation 1 0 1 5 (MPa) . . Modulus at 300% Elongation 3 5 5 0 (MPa) . .
6.3.1.2.2 Stress relaxation
The continuous stress relaxation mode was used to check the effect of the presence of the antioxidant on the rate of scission reaction at temperatures of 1500C, 1750C and 200OC; these results are recorded in Figure 6.2 The results show that the stress falls rapidly to zero in the case of the samples containing no antioxidant, as is clearly shown by the results at 8 hours at 1500C, one hour at 1750C,
and I ess than 30 minutes at a temperature of 200OC: while in the presence of the antioxidant samples in general show longer relaxation periods than those without antioxidant as can be seen by referring to the following results: more than 48 hours at 15CPC, 20 hours at 1750C
and 2 hours at 2000C. These show that the presence of the antioxidant results in a lower rate of decay.
194,
U 0 C) CD C-i
C3
0 Ln
ui CD dcr. ce Lij
ca 0-4 X
=C
LU CL C)
Q.
2
C-)
I',
w ix
L)
ta LLJ
-i co
CD CM
c9 4-) m C) cn to CD
+ to m m >b 1 8 +
ci
. f- LC) x
lý 19 1: ý r1 12 C : ! i
m Imt Ln CD cm C> LC) «c Ln M CY t4 + -41 -r-
LO
41 tu rz cý
+
"v
CZ) (Z cli
CM
' tlo cý uý In m Co CM CD cýi Ln to
10 4. -3
x 0 0
4-) c C)
Kc 4-J tio (D
4-) r- Co ler t 1
zc (n cr- b--4 L) b--4 0 C. i LC)
c ý
CYI m 8 U-) CM 1 m 1
c12 be ee cn C: ) (3 CZ) C: )
4J vi mo mo 0
m cu
c7) CY)
>% 0 -0 L Q 0 «
r_ « . a0 a0 0-
195
TIME (hours)
x1c
'01'.
without antioxidant at different temperatures in air. Fig. 6.2. Continuous stress relaxation of CIIR vulcanisates with and
196
The stress relaxation and hot air ageing results combined to give the same conclusion that the antioxidant ADPA/MBI/MgO combination enhanced the CUR compounds resistance against thermo-oxidative degradation at elevated temperatures.
6.3.1.2.3 Swel I Ing'In'sol vent
A 20 x 10 x2 mm of unaged and aged samples from both the CUR
compounds (with and without antioxidant) were immersed in cyclohexane for 6 days at room temperature Csee Section 5.2.1.4) for more details
about swelling method ). Network density, expressed as mole of crosslinks per gram of insoluble network, were determined from Flory's relationship (equation 5.21.
Swelling results are shown in Figure 6.3 and Table 6.7. The follo-
wing observations can be made from Figure 6.3.:
Crosslink density decreases with increasing ageing time for both compounds.
2. Samples with antioxidant are observed to keep a higher cross- link density than those without antioxidant, at both tempera- tures examined (1750C and 2000C).
3. After ageing at 2000C, the samples without antioxidant dissolved
completely in cyclohexane; this indicates that no crosslinks remained in the samples after ageing at this temperature.
Table 6.7 illustrates the swollen volume and crosslink density of these
samples, the swollen volume is expressed as percentage volume increase
and related to the crosslink density. It can be seen that:
197
-5 10 -ý. -
E
N w 0
C3
11: z
-j CA V)
Li
4-
1
3- with - antioxidant at 200%
-with antioxidant at 175%
2- without antioxidant at 1750C
1.
0 U234! 5
TIME (days)
Fig. 6.3. Crosslink density of CHR vulcanisates with and without antioxidant after ageing in air at temperature 175*C, 200'OC.
198
The samples of the higher level of crosslinks were observed to give the lowest percentage of swelling.
2. The samples without antioxidant showed greater swelling capa- city than those with antioxidant.
TABLE 6.7: EFFECT OF THE ANTIOXIDANT ON THE SWOLLEN VOLUME AND CROSSLINK DENSITY OF CIIR VULCANISATES AFTER AGEING IN HOT AIR AT 1750C AND 2000C
CUR Without Antioxidant
CUR With Antioxidant
Volume Crosslink Volume Crosslink Swollen % Density Swollen % Density
-mole/g mole/g
Unaged sample 324 3.5xlO-s 259 4.5xlO-5 Aged sample I day at 1750C 406 2.3xlO-5 328 3.410-5 3 days at 1750C 416 2. OXJO-5 366 2.6xlo-5 I day at 2000C - 339 3. OXIO-5
6.3.1.2.4 Determination of vulcanisate acidity development
after hot-air ageing using the acetone extraction technique
The acetone extraction technique is another method for obtaining fundamental data about the chemical changes of the CIIR vulcanisates during heat ageing and the affect of using an antioxidant on the
resistance of the compound to high temperatures.
The degree of the degradation, expressed as a percentage of increasing
acidity of the acetone extract, was determined from equation 4.1.
199
Hot-air oven ageing samples (1,3 days at 1750C and one day at 2000C)
were extracted by hot acetone for 16 hours; the acidity'values of the acetone extract are represented in Figure 6.4 and the aged - results compared with that of the unaged samples. It is observed that the acidity of the compound containing antioxidant exhibited lower values, than those without antioxidant. These results were confirmed by checking the pH value of the acetone after extraction
of the rubber compounds (see Table 6.8).
The antioxidant reaction products are considered to contribute to
this low acidity, i. e. other salts and a base compound containing NH group. See also the antioxidant reactions protective mechanism
of Section 6.4.
TABLE 6.8: pH VALUES OF UNAGED AND AGED (IN HOT AIR) OF CIIR VULCANISATES WITH AND WITHOUT ANTIOXIDANT AFTER ACETONE EXTRACTION
pH Value for CIIR Without Antioxidant
pH Value for CIIR With Antioxidant
Unaged samples 4.7 5.2 Aged I day at 1750C 3.42 3.48 Aged 3 days at 1750C 2.98 3.26 Aged I day at 2000C 2.25 3.18
1.
0
c2 G
Fig. 6.4. Acidity % antioxidant
200
AGEING TIME (days)
of CUR vulcanisates with and without after ageing in air at temperature
of 1750C, 200*C.
201
6.3.1.3 Dehydrochl6tihation'Process
6.3.1.3.1''Intr6dUttion
The techniques used for studying the dehydrochlorination fall into two groups
(168):
A: Measurement of the hydrogen chloride evolved. b: Observation of changes in colour.
The measurement of hydrogen chloride produced the most sensitive quantitative method used to study the dehydrochlorination; the
simplest techniques used to measure it are acid-base titration and pH measurement of the hydrogen chloride absorbed in water. Compared
with the titration technique the pH measurement is a procedure which (169) is much more sensitive and versatile
The Lovibond Flexible Optic Tintometer was used to measure the change in the colour of the white compound which, occurred after heat ageing.
A: Measurement of Hydrogen Chloride Evolved
A. 1 Experimental
The rate of dehydrochlorination of the compound samples was measured by titration of the hydrogen chloride evolved from the compound which was heated up in nitrogen for 1,3 days at 1750C and one day at 2000C,
using the simple apparatus shown in Figure 6.5. To avoid the undesi- rable effect of metals on the dehydrochlorination, it was constructed entirely from glass. The lower part of the flask was immersed in an electrothermal heater; it was carefully protected from heat loss by
a thick layer of fibre glass. The hydrogen chloride evolved was carried away by the gas stream and left the system at (A). Evolved HCI was absorbed into distilled water contained in a glass tube. The temperature was measured by a thermometer. All connections were tightened into Teflon sleeves.
202
N
WATEI
EL
Fig. 6.5. Apparatus used to measure dehydrochlorination by mecins of the Hct evolved from the CIIR compound with and without antioxidant after ýeat ageing in nitrogen at temperatures of 1750C and 2000C.
203
A. 1.1 Results and Dittussion
A. 1.1.1 Measurement Of'HCL'6v6lVed
The pH value and concentration of HCl evolved during dehydrochlori-
nation of the. CIIR vulcanisates (with and without antioxidant) are listed in Table 6.9. It can be seen that:
The samples containing antioxidant evolved more HCl than those without antioxidant, probably the existence of the antioxidants changes the kinetics and mechanisms of HCl evolution, hence
giving rise to a higher HCl concentration in the evolved gases, and/or the reaction mechanisms can change such that the frag-
ments of the reactions which are evolved (like SO and other 2 gases) will be more acidic than the comparable situation where antioxidant is absent (see Section 6.4).
2. By increasing the temperature to 2000C, the HC1 and other acidic gases evolved were increased in the presence of the antioxidant, whereas in the absence of the antioxidant the reverse was observed.
It can be concluded that the presence of the antioxidant had no positive influence on the CIIR dehydrochlorination process in the absence of oxygen.
204
TABLE 6.9: INFLUENCE OF ANTIOXIDANT IN CIIR VULCANISATE ON HCI EVOLVED AFTER DEHYDROCHLORINATION IN NITROGEN AT TEMPERATURES OF 1750C AND 2000C
Without Antioxidant With Antioxidant
I day at 3 days at 1 day at 1 day at 3 day8 at I day at 1750C 1750C 2000C 1750C 175 C 2000C
pH 6.95 6.93 7.4 6.4 6.2 6.12
Concen- tration 0.06x 0.06x Basic 0.075x 0.09X 0.105x of HCI 10-3 10-i-3 10-3 10-3 10-3
gm/ml*
* Water solution was titrated with 0401N NaOH solution as a base.
A. 1.1.2 Compound'S physical'properties'after dehydrochlorination
process
Table 6.10 illustrates the influence of the antioxidant on the mecha- nical properties of the CUR compound after performing the dehydro-
chlorination process in nitrogen at temperatures of 1750C and 2000C. It can be seen that:
The samples without antioxidant had lost 61% of their tensile
strength after 3 days ageing at 1750C, and 26.8% after one day's
ageing at 200OC; while the samples with antioxidant retained all their original tensile strength values.
2. Both samples (with and without antioxidant) showed an increase in the 100% and 300% modulus
205
Both samples exhibited a rubbery appearance and were a little bit harder after ageing than'before ageing (as can be seen from the small increase in the hardness value).
A. I. I. 3 Swelling'in solvdint
The CUR samples after heat ageing, in Section A. I. I. 2, were immersed in cyclohexane for 6 days at room temperature. Results of this test are illustrated in Figure 6.6 and Table 6.11; it was observed that:
A crosslink density of both compounds (with and without anti- oxidant) increased after ageing one day, at temperatures of 1750C and 2000C and decreased after 3 days at 1750C.
2. Network density, after dehydrochlorination at 1750C and 2000C
of the samples which contained antioxidant, was higher than those without antioxidant.
206
-5 xio
E
V)
C3
le
V) (A CD Li
1. Without antioxidant at 2000C
2. With antioxidant at 2000 C
3. With antioxidant at 175*C
4. Without antioxiddnt at 1750C
1
3
4
02345 AGEING TIME (days)
Fig. 6.6. Crosslink density of CIIR vulcanisates with and without antioxidant after dehydro ch to ri nation in nitrogen at temperature 175'C , 200"C.
'207
LAJ (5 CD ce. I-
0 C) C: ) 04
c; 0 Ln
CD
!2
LLJ
LLJ
LU
W-
b-0
LL C: )
tn LU
LU 0-
C) w
9c C-3
CD
uj
u 0 (D C) 04
cn qzr 0i 41 to cn (D C) CD Cý Uý
r- + LO 00 1 >w + r-
+
4-J r_
,a 0 . r- Lc) x 0
. I- L9 a! 4-J 4-) r_ fli rl% r- LO LO LO
+
. I- a
u 0 LO
tD cli CV) 00 4-) (a
9 cl-; 1.1ý r- LO
r- + cvo) cri r-. I + I + +
0 CD C) CNJ
co (VI) 4J tio (3N kc; C: ) r- r- g
LO cli C*4 1
+ + 4J c r-
-0 . r. - C-) X 0 0 LO
r_ C%j co %. D %0 4-A co cj cl; CD rý:
4-3 ko mr + m 0 1 1 + >1 (0
4-) -
-0 .r
ý-q C-) 0-4 0
C-) Ln 00
Ln m CY) I: r + rlý Ln a >1 + +
CA Jd
4. ) ccl bq tm (D C)
CL 4J (D C) LA 0 0) S- S-
41 c 4-3 c 4j c to 0 0
IA r- -i 4A (1) 4-) tn 4-3 u go 4-b
. r- M C 4A 4A r_ = r_ = r_ C71
S- c 0 . 1m (0 Q) r- 0 0 r- a) =
I I ul = uj = ui : 2C
208
TABLE 6.11: EFFECT OF THE ANTIOXIDANT ON THE SWOLLEN VOLUME AND CROSSLINK DENSITY OF CIIR VULCANISATE AFTER DEHYDRO- CHLORINATION IN NITROGEN AT 1750C AND 2000C
CUR Without CIIR With Antioxidant Antioxidant
Volume Crostlink Volume Crosslink Swollen % Density Swollen % Density
. mole/g mole/g
Unaged samples
324 3.5xlO-5 259 4.5xlO-5
Aged samples:
1 day @ 1750C 292.3 '4.3xlO-5 232 5. JXJO-5
3 days @1750C 332 3.2xlO-5 305 3' * 9X10-5 1 day @2000C 301 4. JXJO-5 235 5. OXIO-5
A. 1.1.4 * Detemination of vulcanisate acidity development after
'dehyqrochlorination'process . (nitrogen_atmosphere) using the acetone extraction technique
Approximately 6 grams of the samples after dehydrochlorination were extracted in acetone for 16 hours; pH value and acidity percentage of the acetone extract are represented in Figure 6.7 and Table 6.12. It was observed that the samples which had no antioxidant in their formula gave a higher acidity value compared with those containing antioxidant. These results are confirmed by checking the pH value of the acetone after extraction of the samples.
The following decomposition products, from the CUR vulcanisates which contained antioxidant, after heat ageing, are partially trans- ferred into the acetone solution which caused a reduction in the acidity. These are:
209
Most of the salts produced during the CUR vulcanisation reac- tion (e. g. ZMDQ (see Section 4.4.3 for the halogenated butyl
rubber ZnO/TMTD vulcanisation reactions).
2. Some of the other salts and a basAc compound containing NH
group formed due to the presence of the antioxidant (e. g. ZMBI, 172) ZBTS( , ZMBT and see also Section 6.4).
TABLE 6.12: pH VALUE OF CIIR VULCANISATES WITH AND WITHOUT ANTI- OXIDANT AFTER DERYDROCHLORINATION IN NITROGEN AT TEMPERATURES OF 1750C AND 2000C
-. pH, Value-of CIIR
Without Anti- With Anti-
...... oxidant oxidant
Unaged samples 4.70 5.20
Aged 1 day at 175 0c 2.80 3.20
Aged 3 days at 1750C 2.40 3.12
Aged 1 day at 2000C 2.60 2.95
B: Observation of the Colour Changes Which Occurred as a Result of the Dehydrochlorination Process
B. 1 Experimental
B. I. 1 Compound Preparation
White CIIR compound was prepared to examine the degree of dehydro-
chlorination taking place by means of colour changes during ageing of the samples for I and 3 days at 1750C and for one day at 2000C in
air and nitrogen atmospheres. A typical mill mixing procedure was used to prepare the white CUR compound (the compound formula is shown in Table 6.13). according to the following mixing cycle using a mill roll temperature of 1000c:
210
I
0
without antioxidant at 200% 0
I-
U
with antioxidant at 200'*C 0
0
C
C
O
thout antioxidant at 1750C
with antioxidant at 175"C
012345 AGEING TIME (days)
Fig. 6.7. Acidity % of CHR vulcanisates with and without antioxidant after dehydrochlorination in nitrogen at 175"C, 200"C.
211
CUR 2. Antioxidant, stearic acid, j filler 3. j CIIR 4. Oil, j filler 5. ZnO, MBTS, TMTD
TABLE 6.13: WHITE CIIR COMPOUND FORMULATION USED TO MEASURE THE CHANGES IN COLOUR DURING THE DERYDROCHLORINATION PROCESS
Materials Parts by Weight
. ..
With Antioxidant, and Acid Acceptor
.... .. System
Without Antioxidant, System
CUR 100 100 Flexon 845 oil 20 20 TO 2 3.0 3.0 MgO 0.3 0.3 Stearic acid 1.0 1.0 ZnO 5.0 5.0 TMTD 1.0 1.0 MBTS 2.0 2.0 Antioxidant MBI/ADPA/ 0.5/0.8/0.7 - MgO
B. 1.2 Measurement of Changing'the Colour
Samples of the white compound containing antioxidant and samples of the
control white CIIR compound contafning no antioxidant were aged in hot air and nitrogen atmospheres for 1,3 days at 1750C and one day at 2000C, then the colour change was measured by using the Lovibond Tintometer, which is in common use in industry for measuring compara- tive colours; the measurement technique required the matching of each sample with two out of three standard sets of coloured fields (blue,
yellow, red) with the brightness dial (e. g. Blue 2.5, Yellow, 3.0,
212
Brightness 67). The colour of the sample became darker when the
numbers of the colour and brightness were increased.
A: In Nitrogen
Table 6.14 records the colour changes which occur as a result of dehydrochlorination process in nitrogen at 1750C and 200OC; the following observations were made:
1. CIIR milcanigates with'Afti6xidant
In the comparison of the colour changes between the unaged and the
one day aged samples at 1750C, it was observed that the blue colour intensity changed from 1.6 Cfor unagedl to 1.8 (for aged). Simi- larly, the yellow colour changed from 1.0 (unagedl to 1.4 (aged) indicating that the aged sample was darker than the unaged sample. Also it was noticed that the colour of the sample after 3 days ageing at 1750C was much darker than those of one day's ageing, it was therefore required to adjust the brightness from 83 to 88 in order to match in with the standard colours.
The importance of the temperature effect on ageing was also demon-
strated as follows. Aged samples at 2009C were examined and it was observed that the colour changed dramatically from blue-yellow to
red-yellow as a result of ageing. To this end, the maximum brightness
capability of the instrument was employed (100) in order to carry out the colour measurement. Generally when the instrument brightness increased the samples colour darkened.
2. The CIIR VuZcanisation zjithout Antioxidant
As shown in Table 6.14, to match. the standard colours., the samples without antioxidant required more instrument brightness than those
with antioxidant, as they were darker in nature. This can be illus-
trated by the following results: the brightness changed from 83 to 88 for one day's ageing of the samples at 1750C, and 88 to 90 after 3 days ageing at 1750C.
213
CD
ix
LLJ cz
of LU
2t
X
CD =C
:: c
cn
3:
LLJ
1-4
U M LLJ
0 C)
Lli cli
C) 0 U')
0-1 rl% =: o r- 40
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LLJ CD C)
V) cr. LLJ ý- CD 1-4
I= L. ) 6-4
LLJ -i co
4A
S- =cc cn (U (U 0 *4 0 (L) 2 *r- S- cu S-
r- (1) -j a) ... ca .0 0 (A
03
x 0
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co co 00 C) + + + r-
+
M 0 0 3: 0 0 C):: U
>- c CD C%j
C\i
cu + (A + (A + 0) 0 ul + r_ (U (L) 4) cu (U Q)
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kn .ý (X) r- (3) r- (7) .ý S- . S- L.
Ca CD m ca
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cc r4ý co CYI C)
41 + + + =3 + 0 3. 3:
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0) C%J CY)
r- (D cn cm ca
0 0 0 U) LO C) CD Cl to
(U >0 >. $ >. $ M 'a fo to to (0 IM) "0 -0
214
cl LLJ CD,
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CC
U, I-
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W (A (A 4A + 1A 0) W (1) (D W (U (U 4) c = r_ = r- =C "a c
r- 4-) ý 4--) ý 4-) Q) 4J cna cn= M= W=
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SDLdwes a4l JO ssf3uýApp 6uýseaOuj
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x 0
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0. >- >- >- a
Cý 1-4 r- r- . C) S.. - + + +
W (1) W (L) (1) 0)
(ts r- 4J ý 4j P- 4-) r- = = m= I- . > S-
C; cn
4A 0 0 0 (1) U') Lc) C)
r- r.. 11% C) CIQ
,a 0) V) (n Q) r- >0 >1 >. # tm CL -
a to 10 to to E = (L) a a a a to =D V) CV)
215
In Hot Air
Table 6.15 shows the change in colour of these samples (with and without antioxidant) after ageing in hot air. In this case the
change in the sample colour was due to. the combined effects of dehydrochlorination (as a result of heating) together with thermal
oxidation (the, result of combined heat and oxygen) processes. The results showed that the colour change due to a combination of dehydrochlorination and thermal oxidation was slightly greater (i. e. the samples were a little darker) than those in which only dehydrochlorination alone had occurred.
Samples without antioxidant, after heat ageing one day at 2000C,
showed heavy degradation and became soft, the experiment was there- fore unable to proceed further due to melting of the rubber samples.
It can be concluded that the dehydrochlorination process and also the reaction due to oxygen and heat, in the samples containing no antioxidant was greater than that of the samples containing anti- oxi dant.
6.4 Conclusions
According to the thermo-oxidative and dehydrochlorination results of CIIR compounds, it was concluded that:
Losses in physical properties (based on tensile strengthl after hot-air ageing (-thermal oxidationj were found to be
greater than those after ageing in nitrogen Cdehydrochlorination)
see Tables 6.6 and 6.10. This is due to the presence of the Mgp, as acid scavenger, reducing the dehydrochlorination process due to its reaction with the HCl as follows:
MgO + 2HCl MgCl2 + H20
216
2. The crosslink density obtained (by means of the solvent swelling technique) in the samples subjected to the dehydro-
chlorination process were found. to be higher than those from the oxidation alone process Cin both CIIR compounds with and without antioxidant), see Figures 6.8 and 6.9.
3. In hot-air ageing both the thermo-oxidative and dehydrochlori-
nation processes were found to have taken place due to the
simultaneous presence of oxygen and the heat. Figure 6.10
shows the comparison between the acidity obtained after dehydrochlorination and oxidation at temperatures of 1750C
and 2000C of the CIIR samples containing antioxidant from
which it can'be seen that the acidity of the extracted solution is higher in the case of oxidation than that of dehydrochlor- ination. whereas with the sample containing no antioxidant, the reverse was observed (see Figure 6.11).
These results establish that the thermo-oxidative degradation process in CIIR is more dominant than dehydrochlorination degradation - Hence
the addition to get good physical property retention necessitated using the antioxidant combination MBI, mercapto-benzimidazole (peroxide decomposer type (I)) and ADPA, a condensate product of diphenylamine and acetone (chain breaking type (II)) together with the
acid acceptor MgO.
SH
MBI (I)
H aN
/c CH3 CH3
ADPA(II)
1ý3, N-C(S)-S-
NN Zn
S. S Zn
Ic
22 CH3
ZMDC (III) ZMBT(VI)
217
-5 10
1. Temperdture in air-1750C (thermal oxidation+ dehydrachlorination)
2. Temperature in nitrogen=2000C ( dehydrochlorination only)
3. Temperature in nitrogen-175"C ( dehydrochlorinction only)
0
uj M
ýd z
-i LA tA 0 cl: Li
2
3
1
0 012345
AGEING TIME (days)
Fig. 6.8. Crosslink density changes of CHR vutcanisates without antioxidant after ý
(a) ageing at 175*C, 200*C in air. b) dehydrochlorination in nitrogen
at 175'C, 200 0 C.
218
-5 XID
6
5
'i
IA z LU C3
%
Ln LA a ix Li
1. Temperature in air= 2000C (thernial oxidation and dehydrochlorination.
2. Temperature in nitrogen-2000C ( dehydrochlorination only)
3. Temperdture in nitrogen . 175% (dehydrochlorination only)
4. Temperature in air=175ýC. Mermat oxidation and dehydrochlorination)
3
1
2
23
1
0 AGEING TIME (days)
Fig. 6.9. Crosslink density of CIIR vulcanisates with antioxidant after-
(a) ageing at 1750C, 2000C in hot air. (b) dehydrochlorination in nitrogen at 1750C, 200'C.
4
219
x 0 >. - I-
�-I
Fig. 6.10. Acidity % of CUR vu[canisates with antioxiclant aftem- (a) ageing at 175PC, 200'C in hot air. (b) clehydrochlorination in nitrogen at 175%, 200'C.
0234 AGEING TIME (days)
220
01
C3
AGEING TIME (days)
Fig. 6.11. Acidity of CIIR vulcanisates without antioxidant after (a) ageing at 175'C, 200% in hot air. (b) clehydrochlorination in nitrogen at 1750C, 2000C.
01234
221
Protective'Mechanitm'of'Ahtiokidant ktion in a Sulphurless Curing System
In addition to the MBI/ADPA/MgO antioxidant combination, there are also another two additional supplementary antioxidant actions taking
place, whose protective mechanism is of the peroxide decomposer type,
as a result of the formation of by-products from vulcanisation in the
presence of TMTD and MBTS accelerators. These are:
i) ZMDC (zinc dimethyl dithio carbamate (III)) ii) ZMBT (zinc mercaptobenzthiazolate (VI))
The general protective mechanism of Antioxidant action in the ZnO/
sulphurless (TMTD) curing'system is known to occur by means of the following steps:
Protective mechanism'of'th6 peroxide deconposer typ_q'(the ABI,
ZMDC, ZABT Case)
(17017172), Shelton and other workers , reviewed the role of several classes'of organic sulphur compounds as preventive antioxidants. The activity of sulphides and disulphides appears to involve their
reaction with hydroperoxides to form sulphoxides and thiolsulphinates
which are the actual major contributors to the observed antioxidant effect. Bateman and his co-workers(171
) have more recently found that a variety of simple sulphoxides and thiosulphinates are also effective antioxidants and have a generally higher order of activity than the sulphide from which they are'derived.
MBI, ZMBT and ZMDC are considered to have a similar overall protective reaction mechanism, as that described above, in the presence of hydroperoxide.
222
Known protective reacti. oh'mdchahitm. of'ZMDC (See also Appendix III for ZMBI reaction)
Hol'dsworth and co-workers (167)
examined the decomposition of cumene hydroperoxide in the presence of a number of dith-iocarba-
mates, and concluded that the active peroxide decomposer is formed by an initial reaction between the metal complex and alkyl hydro-
peroxide, with quantitative precipitation of metal sulphate.
Copious evolution of SO 2 occurred when cumene hydroperoxide was reacted with ZMDC; it was attributed to the reaction of equation 6.9:
CH CH 0 3 ROOH k 11 N-C (S) Zn N-CCS)-S-0 Zn -']2 2 CH3
[CH
30
(unstable sulphonate)
0 CH3,11 CH 3,
.. _,, N-C(S)-S-OH N-C(S)-OH + SO
11 2 CH3 0 CH 3
I decomposes
CH 3N=C=S + CH30H Alcohol
Isothiocyanate
(6.9)
so 2 was proposed as the active peroxide decomposer by the following
catalytic reaction (173) (equation 6.10)
223
OH
ROOH + SO 2 [ROOS = 0]
OR/ RO* + *OS 4ýk-.
0 (Pro-oxidants)
ROH + SO 3
(Antioxidants)
(6.10)
Alternatively Brooks( 174)
has accounted for the formation of metal
sulphates as follows: (equation 6.11)
CH CH 0 CH 3 )-Sj ROOH 3 11 /3 **ýN-C(s Zn N-C(S)-S-Zn-S-C(S)-N
CH3 2 CH3 CH 3
CH 00 CH 3 ROOH_ N-C(S)-S-Zn-S-C(S)-N >ZnSO 4
CH 11 11 300 CH 3
Zinc sulphinates
CH ' CH ý'N-C(S) _S_C(S)_N/
3
CH 3 CH 3 (6.11)
Thiuram monosulphide
He proposed that the thiuram monosulphide would react With ROOH to form carbamyl thiocarbamyl disulphide from which dithiocarbamate
could be regenerated in the presence of zinc oxide and water.
As i first approximation, it can be assumed that a peroxide decomposer (ZMDC, MBI, ZMBT) does not enter significantly into the normal oxidation
224
chains, but reduces oxidation by holding the peroxide concentration at a very low level, thus reducing the rate of the initiation.
Chain breaking antioxidant'pr6tective action (ADPA)
At the same time another reaction can take place by means of the hydrogen donor antioxidant ADPA (chain breaking) to terminate kinetic chains by hydrogen transfer to form ROOH from R02* (which formed during the oxidation process) as follows (175) (see equation 6.12)
NH
+ RO 2*
c CH 3 CH3
ADPA
N' - op
+ ROOH
CH 3 CH 3
Relatively (stable) radical
I
RO
RO ,2 N
CH 3 CH 3
(6.12)
Hence CUR compounds are afforded protection both by dithiocarbamate,
225
and by the use of ZMBI in association with the antioxidant ADPA. It seems more likely that the explanation of many cases of syner- gism in rubber is due to the combined action of two antioxidants acting by distinct mechanisms Ce. g. -chain breaking and peroxide decomposing) with different activation energies which compensate for one another under varying conditions.
226
CHAPTFP' 7
INVESTIGATION OF'RUBBER'PROCESSING*OIL'S CONSTITUENTS
7.1 INTRODUCTION
It can be deduced from the investigations which have been carried out in the previous chapters that the type of oil and its level have an important effect on the accelerating rates at which a butyl
and halogenated butyl rubber degrade; therefore attention was directed to determine the constituents of the oil and to study the
changes in chemical composition of the oil that occurred as a result of heat ageing at high temperatures.
7.2 CHANGES IN PROCESSING'OIL COMPOSITION DUE TO HEAT AGEING
Table 7.1 records the analysis results of the unaged and aged Esso
and Iraqi processing oils*. Both oils were aged in a hot-air oven for 3,7 and 14 days at 1250C. It was observed that the Iraqi oil was found to be different in composition to the common European
rubber processing oils (e. g. Flexon 845); in the Iraqi oil was identified the presence of trace amounts of copper (3 ppm) and it
also contained higher amount of sulphur than the Esso oil. After heat ageing of both oils at a temperature of 1250C it was found that
an increase in the peroxide value-and unsaturation level occurred with an increase in ageing time. Increase of the unsaturation level,
after 14 days ageing, in Iraqi oil (at 66.6%) was greater than that
experienced with the Esso oil (14.3%). This confirmed that the Iraqi
oil has a greater trend to degradation than the Esso oil; also it
was observed that because of the poor ageing properties of the oil, this gave poor ageing properties of the IIR vulcanisates; at the same time there was the possibility that the existence of trace amounts of the copper present in the oil caused this acceleration of heat degradation.
227
It is known from the literature that the presence of trace amounts of copper have an adverse effect on the resistance of rubbers to
elevated temperatures. Therefore further work was undertaken to
study the action of the copper and its derivative on the ageing properties of the butyl and chlorobutyl type rubbers mixed with oil that was contaminated withL copper, and also to investigate the
effect of increasing the amount of copper on the compound's reten- tion of properties after heat ageing at high temperatures.
7.3 THE ACTION'OF THE'COPPER ON THE*HEAT AGEING PROPERTIES OF RUBBER
7.3.1 Literature Review
Certain metals, particularly copper, exert a powerful pro-oxidant effect on many kinds of rubbers. Most investigations which have been concerned with the action of copper and copper compounds on the
ageing of vulcanised rubber, have brought evidence that the deleterious (43)
effect is the result of some oxidation phenomenon
Kirchhof(l7Q , Esch( 177) and Lewin (178)
attributed a predominant role to fatty acids and resin acids, which they regarded as trans- forming copper derivatives into metal soaps. According to Bott and
(179) Gill , these are transferred into cupric oxide through the agency of zinc oxide, which is always present in rubber mixtures. On the other hand, Kawaska (180 ) has proposed a different theory, according to which
Note: I wish to thank Dunlop International Projects Limited for
carrying out the oil analysis in their laboratory (Tyre Chemical Laboratory, Fort Dunlop). In particular Mr J Sharrock (General Projects Manager of International Projects Ltd) and JJ Davies (Assistant General Projects Manager) and Mr RS Cursley (Coordi-
nator Special Projects Aircraft Tyre Division, Fort Dunlop).
'228
* 0) -
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i ca CA C)
V) C)
C)
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:* CL M to tA r- 1. L. r_ F- rl. >1 L. L. ) mc 4-b
0. C to m to 0) CD 0 to cn cc
229
the dissociated copper ion is responsible for the deleterious effect of copper. The differences in the behaviour of different copper com- pounds is explained by their greater. or less tendency to ionize.
Leyland et al (181 ) demonstrate the oxidation of raw rubber which was
strongly promoted by amounts of copper (30 to 100 ppm), especially when this is introduced as the salts of a weak organic acid. There is evidence that these salts act as pro-oxidants, increasing the
ratio of oxidation but not changing the nature of the oxidation
reaction.
Perdersen(182) studied the effect of copper in various accelerated vulcanisates, and he observed that the copper stearate is actually an "inverse catalyst" in sulphurless thiuram compounds. Investiga- tion of the inhibition of the metal-catalysed oxidation of raw rubber emphasises the effectiveness of zinc diethy1dithiocarbamate and tetramethyl thiuram disulphide against copper contamination. ,
7.3.2 Compound Preparation
This work was concerned with investigating the effect of copper on a butyl/sulphur curing system and chlorobutyl/sulphurless curing system compounded with:
1. Esso oil which is free of'copper. 2. Esso oil and added 3 ppm of CuSO 4 to the compound formula. 3. Iraqi oil which was contaminated with 3 ppm of copper.
Losses in physical properties by hot air-oven ageing and stress relaxa- tion were used to measure the degradation of IIR and CUR with and without the presence of the copper; and to identify the copper effect on the heat resistance of the butyl rubber types.
Table 7.2 illustrates the compounds whose formulations were used In this work.
230
TABLE 7.2: IIR AND CIIR TYRE INNER TUBE COMPOUNDS USED TO EVALUATE THE EFFECT OF TRACE AMOUNTS OF COPPER ON OXIDATIVE AGEING
Parts by Weight A B C D E F G
IIR Polysar 301 100 100 100
1-
CUR HT 1066 - - - 100 100* 100 100 GPF-M660 60 60 60 65 65 65 65 Paraffinic oil:
Esso oil 25 - 25 20 - 20 20 Iraqi oil - 25 - - 20 - -
ZnO 5.0 5.0 5.0 5.0 5.0 5.0 5.0 Stearic acid 1.0 1.0 1.0 1.0 1.0 1.0 1.0 TMTD 1.0 1.0 1.0 1.0 1.0 1.0 1.0 MBTS 2.0 2.0 2.0 2.0 2.0 2.0 2.0 Sulphur 1.0 1.0 1.0 - - - - mgO - - - 0.3 0.3 0.3 0.3 CUSO 4 (3ppm) - - (3ppm) (8ppm)
7.3.2.1 Butyl Rubber
Table 7.3 illustrates the influence of the copper content on the ageing properties of the following types of butyl inner tube compounds:
1. IIR mixed with Esso oil free of copper (Compound A). 2. IIR mixed with Iraqi oil contaminated with 3 ppm copper (Compound
B). 3. IIR mixed with Esso oil and added 3 PPm CuS04 to the rubber
compound C
It was clear from the results of heat ageing properties, after 3 and 7 days at 1500C, that the presence of trace amounts of copper (3 ppm) can cause an accelerated rate of oxidation and thus result in poor physical properties. The addition of 3 ppm CuSO 4 to the rubber compound containing the Esso oil (Compound C)
231
confirmed the effect of the presence of copper on the heat resistance of the butyl compound (as clearly shown from the increase in tensile
strength losses and increase of the compound elongation at the end of 3 days ageing at 1500C compared with the control). This result confi med the stress relaxation results shown in Figure 7.1 which also showed rubber degradation as due to the presence of copper. It would appear that the CuS04 may be changed to its oxides, possibly by the presence of ZnO in the mix, which act as oxidation catalysts in the following manner:
2Cu0*'''; '0* - Cu 20 +0 e-2Cu0
The oxygen is considered as being absorbed from the air and given up to the rubber in an active monatomic state, thus causing deterio-
ration.
It was concluded that the effect of copper metal contamination in
sulphur-vulcanised compounds indicated a need for protection, there- fore attehtion was drawn to the need to try to protect the compounds by using a copper inhibitor to reduce the effect of the presence of the copper on the heat ageing resistance of the butyl rubber inner tube compound.
7.3.2.1.1 The effect'on'the heat ageing propertiesof using a copper inhibitor in a butyl rubber compound
Two copper inhibitors were evaluated in an IIR inner tube to reduce the ageing effect of the copper metal, with which the Iraqi oil was contaminated, towards reversion:
1. EDTA (Diamino ethanetetra-acetic aci. d) 2. ZDC (Zinc diethyldithiocarbamatel.
X, fi
Log -
Fig. 7.1. Continuous stress relaxation of abutyl rubber tyre inner-tube compound contaminated with copper (3ppm) aged in hot air at 1500C and 100% extension.
232 TIME (hours) 23456789 10
233
TABLE7.3: INFLUENCE OF COPPER CONTENT ON THE AGEING OF A BUTYL RUBBER TYRE INNER TUBE COMPOUND AT 1500C
Butyl compound Butyl compound Butyl compound without copper with 3 ppm of +3 ppm CuSO 4
(Esso oil) copper ! oil) ( Ira % Chan e , .
q g Compound A Compound B Compound C
3. days . 7. days.. 3. days 7 days 3 days 7 days
Hardness Soft Soft Soft Tensile -84.8 Heavily -89 Heavily -91 Heavily Strength degra- degra- degra-
ded ded ded Elongation at +17 8 soft +25 soft +30 soft Break . and and and
non- non- non- Modulus at
-67 5 elastic -77 7 elastic
-79 elastic
100% Elongation . .
Modulus at 300% Elongation -78 -87.2 -88
Weight Loss % 5.2 -5.0 -5.4
1. EDTA's Copper Inhibition Action
1.1 Curing characteristics of unvulcanised compound
Figure 7.2 illustrates the vulcanisation characteristics (ODR) of IIR compouM (B) mixed with two levels of EDTA as a copper inhibitor (0.5 phr and 2.0 phr) compared with the equivalent compound containing no copper inhibitor.
It was noticed that:
234
11 Using EDTA in the mixi , ng formula caused an increase in curing
rate and decrease in scorch time. Optimum curing time of IIR
compound containing EDTA was 13 minutes at 1710C, and ODR scorch time was 3 minutes while the equivalent compound mixed without EDTA showed 17 minutes optimum curing time and 5j minutes of scorch time (by ODR).
ii) Increasing the EDTA level showed a slight increase in ODR torque
value.
1.2 Hot-air oven ageing
Table 7.4 shows the influence of EDTA on the heat ageing properties of the IIR vulcanisate after 3 days ageing at 1500C. As shown in Table 7.4 EDTA has no significant protective action against the oxidation of the copper contaminated compound. Therefore it was concluded that EDTA is not suitable as a copper inhibitor in butyl
rubber.
2. ZDCs Copper Inhibition Action
Zinc diethyldithiocarbamate, another copper inhibitor, was used to try to reduce the deleterious action of copper; IIR compoundB was mixed with different levels of ZDC (0.5-2 phr).
2.1 Unvulcanised and vulcanised'properties
Figure 7.3 and Table 7.5 illustrate the unvulcanised properties and vulcanised characteristics (ODR) of IIR compounds mixed with and without ZDC. The results showed:
An increase in curing rate of the compounds containing , ZDC (t95 13 minutes) compared with compound containing no ZDC (t95 17 minutes).
235
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CD
w-NP ( suioql) 3nodoi
236
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237
TABLE 7.4: INFLUENCE OF EDTA AS A COPPER INHIBITOR ON THE HEAT AGEING PROPERTIES OF IIR TYRE I. NNER TUBE COMPOUND AGED 3 DAYS AT 1500C
IIR Compound with IIR compound w ith Pro erties
0.5 phe EDTA phr EDTA p d Age 3 % Aged 3 % Unaged days at Change Unaged days at Change
.1 500C 1500C
Hardness (IRHD) 55 33 -40 59.5 40 -32.7
Tensile Strength (MPa) 9.7 1.9 -78 9.2 2.2 -76
Elongation at Break % 675 725 +7.4 675 725 +7.4
Modulus at 100% Elongation (MPa) 1.2 0.45 -63 1.3 0.5 -61
Modulus at 300% Elongation (MPa) 3.4 1.9 -42 3.5 1.12 -67.6
Weight Losses % - 5.2
ii) Addition of ZDC caused a reduction in scorch time (see Table 7.5)
compared with the compound with no ZDC (5j minutes) in the for-
mula.
Increasing the ZDC amount tends to increase curing rate and to decrease scorch tfme.
238
TABLE 7.5: INFLUENCE OF ZDC AS A COPPER IN14IBITOR ON THE CHARAC- TERISTICS OF UNVULCANISED IIR INNER TUBE COMPOUNDS
Butyl Compound With Butyl Unvulcanised Compound
Without Properties 0.5phr. ZDC lphr, ZDC 2phr ZDC ZDC
Mooney Viscosity (ML 1+4) at 37 37.5 40.5 40.5 1000C ........
Mooney Scorch at 1250C (mins) 32'30" 241 18130" 45
ODR Scorch time t5 at 1710C 3130" 31 214511 513011
ODR t? 5 at 1710C (mins 13 13 10 17
2.2 Stress-strain properties
Table 7.6 shows the effect of ZDC on the physical properties of IIR
compounds; it can be seen that:
i) Adding ZDC caused an increase in hardness and decrease in tensile strength compared with the same compound containing no ZDC (Com-
pound B).
Increasing the ZDC level tends to increase hardness and decrease tensile strength.
239
TABLE 7.6: INFLUENCE OF ZDC AS A COPPER INHIBITOR ON THE PHYSICAL PROPERTIES OF BUTYL TYRE INNER TUBE COMPOUND
Butyl compound with Butyl P rties Com ound rope p
0.5 phr I phr 2 phr Without ZDC.... ZDC ZDC ZDC
Hardness (IRHD) 41 54 55 49
Tensile Strength 11 3 10 5 9.5 10.8 (MPa) . .
Elongation at 700 750 700 700 Break %
Modulus at 100% 1 1 1 1 1 1 1 1 Elongation (MPa) . ........ . . .
Modulus at 300% Elongation (MPa) 3.2
I 3.4 3.4 3.6
I
2.3 Hot-air ageing
Table 7.7 illustrates the percentage change in physical properties of the IIR compound with different ZDC levels after heat ageing at 1500C for different periods of time. In the comparison of the results shown in Table 7.7 with those obtained from heat ageing of compound B con- taining no ZDC (Table 7.3) it was observed that the butyl compound containing ZDC can resist hot air for a longer time and retained more of its original properties than those obtainable when IIR is used in the absence of ZDC; the latter showed also completely degraded and liquified after 4 days ageing at the same temperature of 1500C (see Table 7.3).
Increasing the ZDC level showed an increase in heat , ageing property
retention; therefore the better protection against copper contamina- tion is afforded by the use of 2 phr of ZDC.
240
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LLJ :c
241
2.4 Stress-relaxation'test
The above heat ageing results were confirmed by the stress relaxation results, shown in Figure 7.4; continuous stress relaxation was made at the temperature of 1500C for IIR samples containing different levels of ZDC compared with the equivalent compound containing no. - ZDC; it was observed that the relaxation time increased from 5 hours in compounds which had no ZDC to 15 hours in compounds containing 2 phr ZDC. Therefore from the heat ageing and stress relaxation results it can be concluded that ZDC was a powerful copper inhibitor in MR.
7.3.2.2 Chlorobutyl Rubber
7.3.2.2.1 Heat ageing properties
Chlorobutyl rubber compounds (D, E, F, G) were aged in a hot air- oven at 1500C for 3,7 and 14 days. Table 7.8 shows the effect on the ageing-properties at 1500C of increasing the concentration of copper salt (CuS04) from 3 ppm (Compound F) to 8 ppm (Compound G); both were compared with a compound free of. copper (Compound D) and a compound which contained 3 ppm of copper contributed from the Iraqi oil (Compound E). The following observations were made:
i) Comparison of the CIIR compound free of copper (D) with the com- pound containing 3 ppm of copper (contributed from the Iraqi oil (E)); the results showed some little improvement in the physical properties retention; also the samples of compound (E) exhibited a more rubbery appearance than that of compound (D).
ii) Addition. of copper salt (CuSO4) in proportions of 3 ppm to. the rubber compound (Compound F), containing the Esso oil enhanced the rubber ageing properties of the CUR vulcanisates; whereas in the absence of copper (Compound D) ageing resistance was some- what inferior though the differences between (F) and (D) were not large.
242 TIME (hours)
456789 10 11 12 13 14 15
Logit
-1
-1
-1
0 112 -- -I
Ia- --- --
1
1 plot IIR vulcanisate Stress decay to
zero time (hours) 2 0 without ZDC 5
3- 13 with 0-5 PhrZDC a
x with 1-OPhrZDC 11 4.
with 2-OPhr ZDC
5-
6.
7-
8-
9-
0-
2.
3-
4-
6-
7-
91 1
20 1
Fig. 7.4 . The influence of ZDC, as a copper inhibitor on the heat ageing properties of abutyi, rubber tyre inner- tube compound aged at 150C and 100% extension.
plot IIR vulcanisate Stress decay to
zero time (hours)
0 without ZDC 5
13 with 0-5 PhrZDC a
x with 1-OPhrZDC 11
0 with 2-OPhr ZDC is
243
iii) Increasing the amount, of copper proportion (CuSO 41 from 3 ppm to 8 ppm (Compound G) showed a further improvement in the heat
ageing property retention wh-lch was much greater than that of the compound containing the 3 ppm of copper (Compound F).
7.3.2.2.2 Stress relaxation
Continuous stress relaxation measurements were undertaken on the above CUR compounds (Table 7.2) at a temperature of 1500C and 100% extension. These results are recorded in Figure 7.5 and the following observations were made:
Compound (D) which is free of copper possesses a very fast
stress decay to zero which takes about 9 hours.
2. Compound (E) which contained 3 ppm of copper, contributed from the Iraqi oil, showed a longer stress relaxation period (14 hours) than compound (D).
3. Compound (F) to which was added 3 ppm of CuSO 41 showed better retention of stress than compounds (D) and (E) (it took about 22 hours for the stress to decay to zero).
4. Compound CG) which contained 8 ppm of CuSO 4 gave the best improvement in stress retention and its zero stress relaxation time was 26 hours.
According to the above results it is clear that the beneficial effects of the copper metallic ions is shown more clearly by the stress relaxa- tion technique than by the measurement of the reduction in physical properties due to the hot-air ageing. Therefore it was concluded that the presence and the addi tion of trace quanti ties of copper were usef ul stabilizers in enhancing the heat ageing properties of chlorobutyl rubber; and, the addition of a particular copper salt CCuSO 41 in small proportions (3 ppm -8 ppm) as an antidegradant was found beneficial.
st C) L. )
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LAJ
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cc LLJ cn ca =D
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LLJ CL.
LLJ CY-
9-4
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Co
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4A
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.a LLJ :3 c"; cl; 44 0 LO lqr m I I r- cl a r%ý I I I +
:: c aE -= . r. 0
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I m (Y) cli + +
cn C) Cý C C) C) 0) (1) to r- CY) a)
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(A a (00 co 0 0
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I LU M X LLA 2: LAJ :c
244
245, -- TIME (hours)
X10-1 02468 10 12 14 16 18 20 22 24 26-
Log
-1
-1
Fig. 7.5. Continuous stress relaxation of Chlorobutyi rubber containing different percentages of Copper. (1SO*C and 100% extension. ).
Plot CUR Stress decay to vulcanisates zero time (hours)
2 without copper. 9
0 Esso oil . compound 0.
with 3ppm copper 14 4. 01 Iraqi oil
compound E.
x with 3ppm CuSO1. 22 compound F.
6 with 8ppm CuSOt, 26 compound G.
10-
12-
14-
16-
20-
Plot CUR Stress decay to vulcanisates zero time (hours)
without copper. 9 0 Esso oil .
compound 0.
with 3ppm copper 14 Iraqi oil
compound E.
x with 3ppm CuSO1. 22 I compound F. I
with 8ppm CuSOt, 26 I
compound G.
246
7.4 CONCLUSIONS
It was concluded, from the results of hot air ageing and stress relaxation at 1500C that:
In butyl rubber, the results established that the presence of trace amounts of copper promoted the oxidative scission of the IIR; this finding is analogous to that of other types of diene rubbers vulcanised with sulphur.
2. Zinc diethyldithiocarbamate was found to be a powerful copper inhibitor for butyl rubber.
3. In chlorobutyl rubber, the presence of small amounts of copper (3 ppm) was found to act as an antioxidant in CIIR; and increa-
sing the copper proportions to 8 ppm resulted in an increase in the vulcanisate's resistance to high temperatures.
4. A sulphur donor/accelerator (sulphurless) cured ch-lorobutyl vulcanisate showed better retention of mechanical properties than a butyl sulphur cured vulcanisate in the presence of small amounts of copper; this is probably due to the TMTD in the
sulphurless cure system being transformed during vulcanisation into zinc dimethy1dithiocarbamate (-ZMDC), and thus in the
presence of copper probably gives copper dimethyldithiocarba-
mate (CuMDC), which is known to possess a powerful antidegradant activity. Another reaction also probably takes place, the
chlorine atoms of CIIR may have some significance and will react with ZMDC in the presence of the copper to form the organic compound 4Cu(OH)Cl. 3[(CH3)2NC(: S3SI2 (43)
which enables CUR to resist ageing in the presence of copper metal.
247
A mechanism is given in Figure 7.6 to account for both the in-situ
production of ZMDC, which functfons as an antioxidant and also the mode of protection of chlorobutyl rubber is given utilising the..
presence of small quantitfes of a copper salt.
FIGURE 7.6:
A: In'Situ'Gýnerati6h'6f'the'Ahtid6gradaht'ZDC*from TMTD During VuIca -n i's affli 'on
Tetramethyl thiuram disulphide e. g. (CH 3)2 N-C(S)-S-S-C(S)-N(CH 3)2 during vulcanisation fs known to dissociate on heating into the dithiocarbamate radicals (CH 3)2 N-C(S)4-(Step 1)(152). In the presence of polyisoprene, dehydrogenation of the a-methylenic position and reaction at double-bond takes place (Step 2).
Step C152)
CH 3 CH CH , %. /33 ...,
N-C(S)-S-S-C(S)-N ..,
N-C(S) CH 3
""CH 3 CH 3
Tetramethyl thiuram disulphide
CH I "N-C(S)-S-ý,
CH 00, 3 Trithio radicals
and/or
CH3 CH 3 CH 3 "`N-C(S)-S-S-C(S)-N/ 2" N-C(S)-S
CH ol \ CH CH 1-1
333
248
Step 20 52)
Attack of trithio radicals on allyl units of polyisoprene leading to the formation of a mercaptan.
CH 3 %% a "I
N-C(S)-S-S-' CF[3
CH 3 1 2 -- CH 2-C = CH-CH2
CH 3 2
CH N-C(S)-S
3
CH 3 1 2- CH -C = CH-CH2 -- I
SH
mercaptan..
F,
This is followed by step 3 in which the reaction products of step 2 combine.
Step 3
In step 3 the reaction-ofthe resonance established dithiocarbamate
radicals of step 2 with the mercapto groups gives dithiocarbamic
acid, which is converted into zinc dithiocarbamate Cshown in step 4) and produces a disulphide crosslink:
249
cti 3
2 -- CH -C= CH - CH 2- I SH
Mercaptan
CH 3
2N C(S) - S,
CH 3 CH 3 1
CH C= C14 - CH 2 I s I s C14 3 11
-CH -C= CH - CH2
disulphide crosslinks
CH3
2N- C(S) SH
CH3
dithiocarbamic acid
and/or (153)
: reaction of dithiocarbamate radicals of step 2 with other allyl units of polyisoprene at their a-methyl group giving the forma- tion of -C-C- crosslinking as follows:
CH 3 CH 3 CH +2 ap 2C CH - CH 2N- C(S) -S
CH 3 CH 3 1 CH 2-C - CH - CHz--
II ss II C(S) c (S) II NN
CH 3 CH 3 CH3CH3
250
CH 3 1
CH2 c- CK - II ss II c (S) c (S) II NN
CH3 CH3 CH 3 CH 3
CH 2--' + ---CH 2-
CH 3
C= CH CH 2
19 CH 3 1
--- CH - CH C- CH2 CH 3
1. II
CH 2C- CH - CH 2 1 s I C(S) I N
CH 3 CH 3
CH 3 1
----CH2 -C=C- CH 2 CH3 II
-- CH -C- CH - CH --- 22 I s I c (S) I N
CH 3 CH3
CH 3 N- C(S) - SH
CH3
dithiocarbamic acid
251
and/or (153)
CH 3 I
-- CH 2-C= CH - CH
CH 3 1
CH -C CH - CH 2 1 C(S) I N
CH3 CH3
dehydrogenation at a-carbon atom
CH 3
N- C(S)
CH 3
CH 3
N- C(S) SH
ICH 3
dithiocarbamic acid
CH 3
N
CH3
Step 4
The powerful antidegradant (ZMDC) will be formed by the reaction of dithiocarbamic acid with zinc oxide according to the following reaction:
CH3 CH 3
2N- C(S) - SH + Zn 0 --a.. Zn (S - C(S) -N )2
CH3 CH 3
dithiocarbamic Zinc dimethyldithio- acid carbamate
H 20
252
OR the reaction of steps 1-4 alternatively can be a one-step process, explaining'that 3 moles of TMTD enter into the overall reaction; two moles are converted into dithiocarbamic acid and consequently into zinc dithlocarbamate, one mole combined in the rubber, two
of its sulphur atoms formfng the disulphide crosslink and the other two being part of the (CH 3)2 N-C(Sý radicals attached to the
(152) a-carbon atoms
CH3 CH 3 3N- C(S) -S -S C(S) N
CH 3 CH 3 Tetramethylthiuram disulphide
CH 3
2 Zn (S - C(S) -N )2 +
CH3
CH3 I
+2 -- CH -C CH - CH2 --- C (S) N
C H3
CH 3 1
4 --- CH 2-C= CH CH 2
Polyisoprene
2 ZnO
CH3 I
---CH -C= CH CH 2 I s I S CH 3 11 CH -C= CH - CH 2
disulphide crosslink
dehydrogenation at a-carbon atom
The summary of the mechanism has been given previously early in the thesis, page 157, but has been elaborated in fuller detail in this section for completeness.
253
FIGURE 7.6 - PART B:,
Protective'Actioh*of'the*C6pper'Ion'6n'the*Heat'Stability_of CIIR Vulcanisates
Tetramethylthiuram sidulphide is in part transformed, during vul- canisation, into ZMDC (as shown in Part A of Figure 7.6). and it
was therefore thought that, in the presence of copper,, copper dimethy1dithiocarbamate might be. formed, and/or:
ZMDC will react with the copper metallic ion, to form the power- ful antidegradant (CuMDC)
CH
Zn (S C(S) -N )2 + Cu 2+
CH3
CH 3ss CH N-C Cu C-N
\/ *-* e N, CH 3ss CH 3
(Copper dimethyldithiocarbamate)
2. Another reaction also probably takes place, the chlorine atoms of CUR may have some significance and will react with ZMDC in the presence of the copper to form the organic compound
CH3
4 CU(OH)Cl 3[ NN-
C(S S (43) which is considered
CH /] 3
responsible for the insensitivity towards copper of the chloro- butyl vulcanisates.
254
'CHAPTER' 8
GENERAL'CONCLUSIONS AND'RECOMMENDATIONSTOR
.. FURTHER*WORK
8.1 INTRODUCTION
Tyre inner tubes made of butyl rubber have been found to have poor heat resistance when used in the environment of Iraq. The present
research work was carried out with the objective of studying the
oxidative degradation of butyl and halogenated butyl rubber, tyre inner tubes.
Figure 8.1 illustrates by means of a line diagram a summary of the
research investigation and steps undertaken to establish the causes
of the Iraq problem of tyre inner tube degradation and the methods used to improve the resistance of the rubber compound to elevated temperatures.
8.2 CONCLUSIONS
All the experiments and tests combine to give same conclusions about the behaviour of the butyl and halogenated butyl rubber and can be
sumarized as follows:
8.2.1 Effect of Variation of Oil Level and Source on the Heat
Resistance of the Butyl Rubber Types
The results of Chapter 4 established that:
The greatest rate of degradation was shown with rubber compounds which contained the highest amounts of oil.
255
2. Halogenated butyl rubber offered better resistance to thermo-
oxidative degradation than regular butyl rubber.
3. In hot air ageing butyl rubber compounds showed a higher rate of stress decay than halogenated butyl compounds both of which contained and compared Iraq! and Esso processing oils. Chlorobutyl compounds containing Iraqi oil showed better
retention of stress than bromobutyl rubber compounds con- taining oil of the same type. Bromobutyl rubber vulcanisates containing Esso oil exhibited a little improvement in stress retention when compared with equivalent chlorobutyl rubber vulcanisates compounded with the same proportions of respec- tively Iraqi and Esso oil.
4. Ageing of butyl rubber types at elevated temperatures has been
shown to be an oxidative and thermal degradation process.
5. A combination of scission and crosslinking reactions was shown to take place in both butyl and halogenated butyl rubber com- pounds with-the scission process being more predominant than the crosslinking process (see Section 4.3.3).
,
6. At the elevated temperature of 1500C the butyl rubber compounds became soft and sticky and lost all their physical properties, whereas the halogenated butyl rubber (CIIR, BIIR) compounds retained their original appearance by remaining rubbery and tack-free as well as keeping a good percentage of their
original physical properties.
8.2.2 Enhancing the Compound's Heat Resistance
It is apparent from the results of the hot-air ageing and high-speed
256
tyre endurance wheel tests, with CIIR* and IIR inner tubes, that the CUR tyre inner tube offers an appreciably higher level of heat
resistance than IrR inner tubes. Therefore in the research atten- tion was directed to investigate the following parameters which tried
to enhance the heat resistance of the IIR and CUR inner tubes.
8.2.2.1 Effect'of'the vulcahisation system on the heat resistance of CIIR
The results of Chapter 5 showed that use of a sulphur donor curing system (ZnO/TMTD) in the CIIR was found to give a higher degree of physical property retention after heat ageing than the use of a metal oxide system based on ZnO alone.
8.2.2.2 The beneficial effects obtained by using anti- oxidants'in chlorobutyl rubber subjected to
ageing at high temperatures
The results of the hot-air ageing experiments in Chapter 6 esta- blished that both thermo-oxidative and dehydrochlorination processes were found to have taken place; however the thermo-oxidative degra- dation process was found to be more dominant than degradation by dehydrochlorination. Hence the addition of the antioxidant combi- nation MBI/ADPA together with the acid acceptor MgO was found
necessary to use to get adequate physical property retention.
Heat resistance of BIIR and CUR is considered approximately equal. CUR was chosen to carry out further investigations because of its faster cure rate, and better retention of stress- strain properties when mixed with Iraqi oil. Moreover CUR is more commonly used in the tyre inner tube manufacturing industry than BIIR.
257
8.2.2.3 Effoct'OftWrubber'processing oll's'composition . ahd'brf_jlh*on_'rUbber*ageing
It was deduced from the results of the experiments that the type of oil and its proportions had an important effect on the relative rate at which butyl and halogenated butyl rubber degraded. This
work established that the particular cause of rapid deterioration
of butyl rubber tyre inner tubes in Iraq was the presence of trace
quantities of copper fn the rubber processing oil produced by the local Iraqi refinery; the presence of this copper promoted oxida- tive scission of the IIR. It was considered that the effects of this copper metal contamination in the sulphur vulcanised IIR com- pounds could be overcome by protection of the rubber with the inhi- bitor ZDC. Zinc diethy1dithiocarbamate was found in this research to be a powerful inhibitor of copper promoted oxidation in butyl
rubber.
Replacement of the butyl rubber CIIR) by chlorobutyl rubber (CIIR)
overcame the problem of inner tube degradation and, unexpectedly, the presence of small amounts of copper in CIIR was, found to act as an antioxidant for that rubber provided that the accelerator system used for the CIIR contained a dithiocarbamate which was able to form, in situ, the copper dithiocarbamate salt.
8.3 RECOMMENDATION FOR FURTHER WORK 0
In view of the results of investigations carried out in the present
work, the following recommendations can be made:
It was found during the investigational work of Chapters 3 and 4 that use of the halogenated butyl rubber types improved the tyre inner tube life. Therefore it is proposed that it would be worth while to investigate the heat resistance of a blend of IIR and halogenated butyl compound.
258
2. In CIIR compounds it was found that the presence of the copper metallic ion in the proportion of 3-8 ppm was useful as a
stabiliser in enhancing the heat ageing properties. An
investigation which studied the minimum and maximum limits of copper ion addition which would give beneficial results is
recommended.
3. In the CUR compound it was established that the addition of CUSO 41 in the proportions of 3-8 ppm as an antidegradant was found beneficial; it is proposed that it would be worthwhile investigating the effect of other copper metals as inhibitors in halogenated butyl.
4. Results of the present work in Chapters 5,6 and 7 concern only CIIR. Further work could be done to investigate the action of
copper salts as agefng inhibitor for BIIR.
5. The presence of trace amounts of copper in CIIR, vulcanised with a sulphurless curing system, has resulted in the formation of CuMDC which is responsible for the insensitivity of CUR to
copper contamination, it is recommended as worthwhile to investi-
gate the addition of CuMDC directly to the copper free CUR
compound as a means of improving its heat resistance.
Ji
258/1
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260
Constant holding pressure
UPI
pli
Os
di
Low pla
Vulcanisation chamber (Volume = 7.3 cc)
FIGURE Al: Basic Principle of Monsanto Oscillating Disc Rheometer
+
4-)
0 u (A
o
cr s- 0
Key:
ML = Minimum torque MH = Maximum torque at a specified time or marching M modulus curve MHr = Maximum torque or reversion curve TS 5 L = Time for 5 (lb. in) in rise above M Tgo Time of , 90% maximum torque
PC Plateau curve Rc Reversion curve MMC
PC
RC Vulcanising Region T 90
Processing MH MH MHI safety r regioft-ý,
TS 5
16ý ML I 1 4 Time, Minutes -1-
FIGURE A2: Typical Type of Rheometer Traces of a Rubber Compound
,I
(Sinusoidal Oscillation at 3')
261
Vulcanisation characteristics of-rubber compounds are dependent
on curing systems employed. However, whatever system is employed, the vulcanisation process in a rubber compound occurs in three
stages (see Figure A2).
1. An induction period 2. A curing or crosslinking stage; and 3. a) A reversion or overcure stage (e. g. IIR)
b) A marching modulus Ce. g. SBR)
The induction period represents the time at vulcanisation temperature during which no measurable crosslinking has occurred. It is of prac- tical importance since its duration determines the safety of the
stock against "scorching" during the various processing steps which precede the final vulcanisation. Following the induction period, crosslinking proceeds at a rate which is dependent on the temperature
and the composition of the rubber compound. When crosslinking proceeds to full cure, continued heating produces an overcure or reversion which may result either in a further stiffening or softening of the
compound. In a development or production of rubber compounds, the
rubber technologist strives to arrive at a balance between a tendency to scorch and a vulcanisation rate which best fits the processing and cure requirements of the final product.
2. Vi scos i ty
Mooney viscometer instruments are widely used in the rubber industry (BS 1673: Part 3: 1969). It consists of a small cylinder in which a rotor is rotated in a mass of compound.
The torque required is measured by "Mooney" units. The code for the description of how the test operated is written:
ML 1+4"(1000C)
262
where: M= Mooney L= large rotor 1= warm-up time (minutes) 4(IOOOC)'= 4 minutes rotation at 1000C
Sample test should be 3 mm thickness and 49 mm diameter.
3. Mooney Scorch i
Measured with a Mooney viscometer and is the time before a compound starts vulcanising at 1250C temperature. The scorch time was deter-
mined from the trace of viscosity versus time (see Figure A3). The
scorch time is the time for a compound to rise 5 Mooney units above the minimum assigned as T5.
4. Vulcanisation
Having obtained optimum cure time for rubber mixes at a chosen temp-
erature (1710C), the mixes were then cured on an electrically heated
press. The mould pressure used was 0.5-1 ton/sq. in. Different types
of mould were used depending on types of test specimen prepared. For instance, for tensile and tear test specimens 152 x 152 x2 mm mould was used.
Cured test specimens were stored for 24 hours in polyethene bags prior to testing in accordance with British Standards BS 903.
CA 4-)
:3
0 0
-a& 263
- Minimum viscosity
FIGURE A3: Mooney Scorch Curve
.i.
Time, minutes
264
B: TESTING'OF'VULCANISATES
Physical'Tosting
For determining the stress-strain properties-and tear strength of rubber vulcanisates. ' British standard methods were employed.
The JJ tensile testing machine Model T5002 (JJ Lloyd Instruments Ltd)
was used in conjunction with an x-y plotter CPLIOO of JJ).
Both tests were carried out at a speed of 500 mm/min and load cell 500N.
1.1 Tensile Strength (BS 903 A2,19711
A dumb-bell shape cutter type (21 was used to prepare specimens for
tensile strength and elongation at break, Figure B4. Tensile strength is calculated as the applied force per unit'area of the original cross- section of the test length unit (MPa). "
The formula used to calculate the tensile strength was as follows:
Tensile Strength = Force to break (MPa) Cross-sectional areY
where the force to break is the stress applied so as to stretch the test piece.
265
25±1
-7 72
C%j
75
FIGURE B4: Dumb-bell Test Piece, Type (2) of BS Used in Stress- Strain Testing
1.2 Elongation at Break. (BS 903 Part A2,1971)
Dumb-beZ1 test piece method. In this technique elongation at break is defined as the tensile
strain in the test length of the break point (unit It was calculated by subtracting the initial distance between the reference lines on the dumb-bell test piece from the distance between the line at break point.
1.3 Modulus at a Given Tensile Strain (100% or 300%)
It is a tensile stress at a given strain and obtained as follows:
Modulus (at 100% or 300%) = Force at a given strain (MPa) Initial cross-sectional -area
266
1.4 Tear Strength
A crescent shaped cutter was used for the tear strength test (see
Figure B5). In this test the force measured is that required to
cause a nick, cut in a rubber test piece, to extend by tearing of the rubber; the force acts in a direction substantially normal to the plane of the cut. The following equation was used to calculate the tear strength (BS 903: Part A3: 1982).
Tear Strength =f = Nm-l t
where: F= maximum force in N t ='thickness of the test piece in mm
1.5 Hardness
A hardness measurement is a simple way of obtaining a measure of the
elastic modulus of a rubber or composite by determining its resistance to a rigid indentor to which is applied a, force. It is a measure of modulus at very small deformation. The measurement was carried out using the Wallace meter in accordance with BS 903: Part A26: 1969.
1.6 Compression Set at Constant Strain (25%) - Method A of BS 903: Part A6: 1969
This test meaLtftres the ability of rubbers to retain elastic properties after prolonged compression. The test piece used was a cylindrical disc of 29.0 t 0.5 mm diameter and was clamped and held at 25%
compression for a fixed length of time (22 hours) at a temperature
of 1000C. At the end of the compression period, the compression set at constant strain was calculated as follows:
267
Location of nic 45±2 mm
LO
I 68±1.5 mm
"Is - 110 nm -
FIGURE B5: Crescent Test Piece Used in Tear Strength Testing
Compression set at constant strain = to trx
100% t0tS
where: to original thickness of test piece tr thickness of test piece after recovery (30 minutes
recovery time) ts thickness of spacers (9.38 t 0.01 mm)
11,
268
APPENDIX II
NOMENCLATURE OF THE POLYMERS AND INGREDIENTS
Trade Mark , or Brand Name Description Supplier
ADPA 86 Reaction product of diphenyl Uniroyal (Aminox) amine and acetone Chemical
(Anchor
Antiox 2246 2,2'-methylene-bis (4-methyl-6- Anchor (CAO-5) t-butylphenol)
APBN Reaction product of PBNA and acetone Monsanto (Betanox SP) butylated hydroxytoluene
Amberol'ST-137 Reactive polymethylophenol resin Rohm and Haas Co
Amberol ST-137X Non-reactiVe polymethylol phenol resin BTMCS Benzothiazyl, monocyclosulphonamid
PIIR Bromobutyl rubber (brominated iso- Polysar butylene-isoprene)
BHT Butylated hydroxytoluene Aldrich Chemical Co.
CR Chloroprene rubber Dupont (Neoprene)
CIIR Chlorobutyl rubber (chlorinated Esso Isob_utylene-isoprene rubber)
CPPDA N-cyclohexyl-N'-Phenyl-para- Bayer (Antiox. 4010) phenylenediamine
Sigma London DETA Diethylene triamine Chemical Co.
Dibenzo GMF Quinone dioxime dibenzoate Naugatuck CheInical Div. US Rubb. Co.
DPTTS Dipentamethylene thiuram Akron' (Akrochem) hexasulphide Chemical
269
Trade Mark or Trade Name Description
.... .. Supplier
DCP 40 40% Dicumyl peroxide Hercules (Dicup 40C)
DCBDOTG Di-ortho-tolylguanidine salt Du Pont (Permalux) of dicatechol borate
DNPD ; Di-o-naphthyl-para-phenyl-enediamine Anchor
EPDM 404 Ethylene propylene rubber Esso (Vistalon 404) :. Chemicals
EPDM 2504 Ethylene-propylene diene Esso (Vistalon2504) monor rubber Chemicals
EMDHQ 6-ethoxy-2,2,4-trimethyl-1,2- Monsanto (Santoflex dihydroquinoline AW)
EDTA Diaminoethanetetra-acetic acid Sigma London Chemical*Co
FEF Black Fast extruding furnace black Cabot
GPF General purpose furnace black Cabot (Sterling V)
Hypalon 20 Chlorosulphonated polyethylene Du Pont
IIR Isobutylene-isoprene rubber Polysar (Butyl rubber)
IPPD N-isopropyl-N'-phenyl-para- Anchor (. Flexzone phenylenediamine (Uniroyal) 3C)
Kenmix GMF P-quinone dioxime Naugatuck Chemical Div. US Rubb. Co.
Mai 2,. mercaptobenzimidazole Bayer
Vulcafor 2-mercaptobenzothiazole Vulnax MBT
Vulcafor Benzothiazyl disulphide Vulnax MBTS
270
Trade Mark or Trade Name Description Supplier
MgO Magnesium oxide Croxton (Maglite D) Garry
MIA 2-mercaptoimidazoline Bayer
NBR Nitrile rubber (acrylonitrile buta- BF Good- diene isoprene rubber) rich,
NA-22 2-mercaptoimidazoline Du Pont
NPP Tri-(nonylated phenyl)-phosphite Uniroyal (Polygard) Chemicals
NBD Nickel dibutyldithiocarbamate Robinson Brothers
Necton 60 Non-staining extracted naphthenic Humble Oil oil and Refining
Co.
NR Natural rubber (Polyisoprene) MRPRA
PBNA Phenyl-e-naphthylamine Anchor
Permalux Di-ortho-tolylguanidine salt of Du Pont dicatechol borate
PDMI N, N'-metaphenylene-dimaleimide Du Pont (HVA-2)
PF Cure Cure active phenol-formaldehyde resin Schenectady (SP-1045)
SBR Styrene-butadiene rubber Shell Chemicals
SP-1055 Brominated polymethylolphenol resin Schenectady Varnish Co
TCBQ Tetrachlorobenzoquinone Uniroyal (. Vulklor) Chemicals
TDEDC Tellurium diethyldithlocarbamate Robinson Brothers
271
Trade Mark or Trade Name Description
... Supplier
TeDEC Tellurium diethyldithiocarbamate Robinson Brothers
TEDC Tellurium ethyldithiocarbamate
TETD Tetraethylthiuram disulphide
Vulcafor Tetramethylthiuram disulphide Vulnax TMTD
TMTM Tetramethylthiuram monosulphide Vulnax
ZnO Zinc oxide Anchor
ZDC ZincdiethylditKiocarbamate Anchor
ZDMC Zincdimethyldithiocarbamate Robinson Brothers
ZnMBT Zinc mercaptobenzthiazolate Monsanto
272
.. APPENDIX III
PROTECTIVE'MECHANISM OF MBTS AS A PEROXIDE
. DECOMPOSER TYPE ANTIOXIDANT
Baxter, Morgan and RoebuckC 1833 in their studies found that the
compound containing mercaptenzothiazole had the best ageing because
in it a higher proportion of the oxygen absorbed seems to be present in an "inactive" combination with the MBTS. It is well known that during the cure cycle most of the MBTS or MBT would be converted to
the zinc salt by reaction with zinc oxide (see equation 1), so any
good ageing characteristics possessed by the rubber should more
properly be attributed to the presence of this material.
N \\ N \ý\) [ C(
/c-s+ ZnO S,,, C - S] Zn s22
(MBTS) (ZMBT)
It has been postulated that metal salts of thiazoles, when present in a rubber compound, will slowly convert to sulphinates (ZBTS) by
reason of reaction with peroxy compounds such as hydropdroxides which form in hydrocarbons during normal ageing (see equation 2).
NN0 1 [10
S. -IC-S Zn +4 ROOH c-
IS -
lZn (2)
2s 0
ZMBT (ZBTS) (Zinc sulphinate + ROH
273
This would imply that the thiazole would eventually be exhausted as indicated by equation 2. in which each mole of normal thiazole
salt ideally inactivates or decomposes four moles of hydroperoxide.
There is also the possiblity that the thiazole will inactivate a total of six moles of hydroperoxides. Equation 3 shows two by-
products, normally zinc sulphate and sulphur dioxide, which can be further oxidized to zinc sulphate and sulphur trioxide respectively, thus accounting for the formation of two additional moles of hydro-
peroxide. The sulphur trioxide In the presence of excess of ZnO
and traces of a moisture would be converted to zinc sulphates.
0 %H0
c-s Zn 2 10-11
1/ý, / 11 2
(ZBTS) Zinc sulphinate
C-H + ZnS03 + S02 --(3)
s
(BT) Benzothiazole
The overall reaction would then be written as follows:
N
c-s Zn [0
/ -12 s
(ZMBT) Benzothiazole
The mechanisms of the protective action of mercaptobenzthiazole and its derivatives are recorded in considerable detail by BrooksCl 74)
Husbands and Scott(1843 and Al-Malaika et al C172)*
N 6 ROOH \\ CH+2 Zn S04
ZnO (H20) s/
(BT) Zinc sulphate
274
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