CHATPER Vlll

DEGRADATION BEHAVlOUR OF

NATURAL RUBBER- ALUMINIUM POWDER

COMPOSITES

Results of this chapter have been communicated to Polymer Degradation and Stability

N a t u r a l rubber (NR), unlike many other polymers, is highly susceptible

to degradation, due to the presence of active double bonds in the main chain.

Degradation of NI< i~s accelerated mainly by heat, humidity, light, ozone,

oxygen, radiation etc. In routine technological evaluation. rubber

vulcanizates are subjected to accelerated ageing tests to get information

about their service life. This problem was well recognized by Baker [ I ] who

examined the effect of temperature on the ageing behaviour of natural

rubber compound::. Effects of high-energy radiation, thermal and ozone

exposure in natural rubber composites containing short sisal fibre were

reported 121. Many of the chemically unsaturated rubbers are prone to attack

by ex-en the minute quantities of ozone present in the atmosphere. Such

attack afl'ccts the surface appearance of rubber products and it causes loss of

physical properties, especially in thin-walled articles. Natural rubber when

properly cornpounded either with suitable waxes or antiozonants. will get

ozone resistance as good as or even better than many synthetic rubbers [3].

Degradation of NR by radiation is also a serious problem, mainly from

gamlrla-ray irradiation. which is usually applied in sterilization process.

With gamma ray irradiation high molecular weight materials may

decompose. Ihe mechanism of characteristic changes in gamma ray

irradiated pol>mer. including degradation and crosslinking has been studied

by Shintani and Jakarnura 141. The effects of temperature and environmental

factors on the perfclnnance of polymers and various degradation reactions

including their mechanisms are available in the literature [j-101.

Table VIII. 1 Formulations of mixes

Ingredients GUM HAF GPF ACB CLY SIL lOAl 20AI 30 Al 40 Al HR Si-69 CON TDI

Natural rubber 100 100 100 100 100 100 100 100 100 100 100 100 100 100

Stearic acid 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5

Zinc oxide 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5 .0 5.0 5.0 5.0 5 .O

TDQ I .o I .o I .o 1.0 1.0 1.0 1.0 I .0 I .0 I .0 1 . 0 I .0 I .o I .o Aluminium powder 10 20 3 0 40 10 10 10 10

Acetylene black - 40 - 1 China clay 40 ! Precipitated silica - 40 - 1 I Hexa I .O - I Resorcinol - Si-69 - Cobalt naphthenate - TDI

! CBS 0.6 0.6 0.6 0.6 0.6 0.6 0.6 O h 0 .6 Oh 0.0 O.(, 0.0 O h :

7 - Sulphur 2.5 2.5 2.5 2.5 2.5 2.5 2.5 -. > 2.5 2.5 - . ., 1.3 7 5 7 5 ;

9 - 7

~~

-. . . ~~ - .~~~

TDQ - 2,2,4-trimethyl-] ,2-dihydroqui11oli1ie Hesa - hexamethylene tctranii~ie T1)I - toluene diisoc)aliate Si-69 - bis[3-(triethoxysilyl ) propyl] tetrasulphide CBS - N-cyclohexyl benzothiazyl sulphenamide

Introduction of filler into polymers leads to a wide range of

interactions arising at the polymer-filler interface. These dispersed fillers

considerably influence the properties of the polymer composites, including

their degradation and stability. The major factors that control these

properties are the surface chemistry of the filler, nature, shape and size of

particles, size distribution and specific surface area etc. [ l l ] . This chapter

contains the ageing properties of aluminium powder filled natural rubber

composites.

The formula1:ion of mixes used for the study was given in Table

VI1I.I. Resistance towards heat, ozone, gamma-irradiation and flame

resistance were studied. the procedures of which are described in section

11.4. Effects of loacling of aluminium powder and various bonding agents

have been investigated. For comparison of results. vulcanizates containing

conventional fillers are also included in the study.

MI/. 1. Effect of Heat Ageing

Figure VIII. I illustrates the percentage decrease in tensile strength

and modulus (300%,) due to thennal ageing for natural rubber composites

containing 40 phr. loading of various fillers. After keeping at 7 0 ' ~ for 14

days, the percentage decrease in tensile strength followed the order

clay<aluminium powder I G P F < acetylene black<HAF<silica. The results

showed that clay and aluminium powder incorporated vulcanizates have

better retention of mechanical properties after ageing compared to other

fillers. 7'he action of oxygen on natural rubber is activated by heat. For

assessing the long-tern1 serviceability of the vulcanizates, variation in

modulus along with the variation in tensile strength is helpful. From the

figure i t is clear that the percentage decrease in 300% modulus due to ageing

follows the order. aluminium powder<clay<acetylene black <GPF<HAF

<silica. This showed that aluminium powder filled natural rubber

composites have the minimum loss in its properties compared to other

fillers. The influence of temperature is not direct, as in normal chemical

reactions in promoting the reaction but it is mainly an indirect effect, which

finally leads to polymer degradation.

Variation in tensile strength of natural rubber vulcanizates containing

different loadings of aluminium powder after thermal ageing is shown in

Figure V111.2. An increasz in tensile strength is observed with aluminium

powder filled samples at all loadings.

300% Modulus 1

figure VIII. I . Percentage decrease in tensile strength and modulus (300%) of NR irulcanizates containing various fillers. at7er ageing at ;I@C for 14 days

/ 1 sO day07 daya14 days

1 5 --- GUM 10 At 20 Al

Figure Vll1.2. Variation in tensile strength of aluminium pmvder filled natuml rubber composites after ageing at 7@C

// EB (1 d a v 7 d a y I 14 days fl

GlJM 10 Al 20 Al 30 Al 40 Al

Figure M11.3. Variation of modulus (3000/0) of aluminium powder filled nahrml rubber composites nfter ageing at 7@C

The higher retention in tensile strength (>loo%), after ageing at 7 0 ' ~ for 7

days of the vulcanizates could be due to the continued cross-linking of the

elastomer. Ageing for 14 days at 7 0 ' ~ results in a decrease of tensile

strength at all loadings. However, the filled samples showed better retention

than the gum sample. Figure VIII.3 shows the variation in modulus (300%)

of the aluminium powder filled vulcanizates. From the figure it is clear that

as the loading of aluminium powder increased, the modulus also increased.

The increase in modulus is due to the higher extending of cross-links formed

in the composites, in presence of aluminium powder. Due to thermal ageing

the gum composite gradually loses the modulus due to polymer degradation.

In aluminium powder filled vulcanizates an increase in modulus is observed

after 7days ageing and a decrease after 14 days ageing at 70°c, the same

trend as in the case of the tensile strength values after ageing. In rubber

compounds. high temperature causes two competing reactions namely the

cross-link formation and scission of chains. The slight increase and then the

decrease in tensile strength and modulus values of these composites can be

explained on the b'asis of these processes. At lower periods of ageing the

extent of main chain scission is less and effect of cross-linking predominates

but at longer periods the reverse situation occurs resulting in lower retention

of tensile strength ancl modulus.

Figure VIII.4 shows the tensile strength of natural rubber-aluminium

powder composite!; containing different bonding agents, after ageing. The

bondin@coupling agents used were hexamethylene tetramine-resorcinol

system (1IK). his[3.-(triett~osysilyI)prop!l] tetrasulphide (Si-69), cobalt

naphthenate (CON) and toluene diisocyanate (TDI). The 300% modulus of

these composites and its variations due to thermal ageing are presented in

Figure VI11.5.

i3 z W K I- (I)

1 - (I) z W I-

/j; 0 day0 7 dayso 14 days 30

- 10 Al H R Si-69 CON TDI

Figure V111.4. Dependence of tensile sh.ength with ageing at 7OCfor NR- aluminium powder (10 Phr) composites containing different bonding agents

13 0 day= 7 days814 days

10 Al H R 3-69 CON TDI

Figure VII1.S. Dependence of 3 W h modulus with ageing at 7PCfor M- aluminium powder (1 0 Phr) composites mntaining different bonding agents

It is seen that both the tensile strength and modulus slightly increased in all

cases after ageing at 7 0 ' ~ for 7 days. Further ageing at the same temperature

caused more polymer degradation than crosslink formation resulting in a

decrease of tensile strength and modulus.

Table Mii.2. Variation in the elongation at b d vaiues ("h) of the composites after ageing at 70%. -

Sample - 0 day 7 days 14 days

800 788 760

HAF 3 74 371 362

CJPF 356 344 328

1 ACB 378 371 346

SIL 445 439 425

C L Y 490 479 368

790 785 779

20 Al 75 1 745 730

iii Al 660 642 620

10 Al 550 544 539

715 702 695

51-69 I

642 63 1 620 I CON 750 729 710

' ID1 L 733 719 7 00

'l'he elonga1:ion at break (%) values of the composites are presented in

Table V1II.Z. I t is seen that the elongation at break (EB) is maximum for

gum \ulcanizate and the loading of alurniniu~n powder decreased the

elongation at break. Presence of bonding agent further decreased the EB.

The irnpr(l\ed adhesion in presence of bonding agent restricts the mobility

of polymer segment., which tinally results a reduction in elongation. The

thermal ageing caused a slight reduction in the elongation at break in all

cases. Both the cross-link formation and main chain scission at high

temperature causes reduction in polymer elongation. We have seen that the

tensile strength and modulus are slightly increased by ageing due to

additional (7days at 7 0 ' ~ ) crosslinks formed during the ageing period. This

additional crosslinks reduced the elongation of the composites. Ageing at

7 0 ' ~ for 14 days further decreased the maximum elongation, but here the

main mechanism invalves the chain scission due to polymer degradation as

evident from the loss in tensile strength values.

Mll.2. E f f d of Gamma &diati0~

The effect of y-radiation on the tensile properties of various filler

incorporated vulcanizates is given in Figure VI11.6. Gamma irradiation

caused a decrease in tensile strength of all compounds, but the extent of

decrease is different. From the figure it is clear that the percentage decrease

is minimum for aluminium powder filled compounds, whereas in other

cases a sharp fall in tensile strength was obsen.ed. Though gamma

irradiation can cause crosslinking and polymer degradation, the reduction in

tensile value shows that chain scission leading to polymer degradation is the

main reaction hert:. Figure V111.7 shows the effect of gamma radiation on

tensile strength of natural rubber-aluminium powder composites at different

loadings of alurnir~ium powder. The tensile strength of the gum sample was

reduced sharply kern the initial value after a radiation dose of 18 M rads. At

all loadings. aluminium powder incorporated samples showed better

resistance towards y-radiation than the gum samples. The effect o f various

bonding agents on NR-aluminium powder composites towards gamma

irradiation is sho-wn in Figure VIII.8. Here also the irradiation caused a

reduction in the tenlsile strength, the same trend as observed in composites

having n o bonding agent.

RADIATION DOSE (M rad)

Figure M11.6. Variation in tensile strength with gamma irradiation for 'or- vul'mnizates containing different fiNers

14 0 6 12 18

RADIATION DOSE (M rad)

Figure Mil. 7. &Red of gamma radiation on natuml ~bber-aIuminium Powder Composites

RADIATION DOSE (M rad)

Figure M11.8. Yariatiun in tensile strength with gamma inndiar7on for MR-aluminiuminjum powder (IOPhr) composites containing different bonding agents

MZf.3. Ozone Resistance af Me-Aluminium Pmvder Composites

For assessing the ozone resistance of the aluminium powder filled

vulcanizates, test samples having 20% strain were exposed to ozonized air

having 50 pphm ozone concentration. Characteristic cracking in rubber is

not obsenrd unless 3 tensile strain is imposed during exposure, and once

the strain is exceeded. a crack will gro\\- at a constant rate independent of

additional strain. and for tnari! rubben in direct proportion to ozone

concentration [3].

Figure V111.9. Optical photomicrographs of ozone exposed ( 1 0 hrs) NR- vulc~niz~tes containing 40phr fillers (a) HHF (b) GPF (c) acetylene black (d) precipitated silico (e) china clay and ( f ) aluminium powder

In the case of NR-aluminium powder composites it is seen that

cracks were developed on the surface of all the samples immediately after

two hours of exposure to ozone. However, the nature and intensity of cracks

due to ozone ati.ack are different for various vulcanizates. Optical

photographs of the surfaces of vulcanizates after 10 h of ozone exposure are

presented in Figures VIII.9(a-f). In the case of aluminium powder

incorporated samples the cracks are small and discontinuous whereas a lot

of continuous, deep cracks are observed in the \ulcanizate having no

aluminium powder. It may due to the higher extent of cross-links formed in

these composites. due to the higher thennal conductivity of aluminium

powder. 'The presence of aluminium powder as an inert particle in the

elastomer prevents the gro\\th of cracks along the polymer. A growing crack

in the reactive phase must. sooner or later encounter an inert particle and

come to a halt, further propagation being possible only by the crack

circumventing the particle or jumping over it. Except for very small

panicles circum~:ention is unlikely. since it would invol\-e the crack

propagating parallel to the applied stress, ie.. in a direction involving a

minimuni release ol'elastic stored energy [12]. The length of cracks in the

reactke phase is g:ovemed h! the density and nature of the unreactive

particles present in the pol!mer. From the photograph it is clear that

incorporation of aluminiulrl powder will impart better ozone resistance

compared to other liller incorporated vulcanizates. The same trend is

obseried \vith vul'canirate containing bonding agents.

Vlll,4. Limiting Oxygen Index Values ofthe Vulcanizates

I'hc lirrliting ~.)x)-gcn itides (1,OI) values of natural rubber composites

containins aluminiu~n po\vdi.r arc given in Table V111.3. The LO1 is defined

as thc \ o lu~ne tiaction of o\!gi.r~ in an oxygen-nitrogen atmosphere that will

just \upport stead! candle l ~ h c burning of a material It has been nidely 188

applied as a measure of the polymer flammability. All fillers including

aluminium powder increased the LO1 value of the gum compound and LO1

value increased with aluminium powder loading. But the addition of

bonding agent was found to have less effect on the LO1 values.

Table YllI.3. LO/ Ynlues of the Campsites

/ Sample LOI (%) j 1 GUM 16.4 1 / GPF 18.2 1 ACB

SIL

CLY

10 Al

20 Al

30 Al

40 Al

HR

Si-69

CON

'ruI - --

VIIl.3. ~nclusions

Compared to other fillers. ,~Iurninium powder tilled natural rubber

con>posirt.s retained the mechanical properties after thennal aseing. Apeing 0 . , at 70 C lor 7daq.s slightly increased the tensile strength due to the continued

crosslinkins in the composites. but degradation predominates on prolonged

ageing. resulting ;3 decreased tensile value. The elongation at break is found

to decrea.;e on prolonged ageing. Gamma irradiation caused a decrease in

tensile strength for all compounds and is minimum for aluminium powder

incorporated samples. The trend remains same irrespective of aluminium

powder loading and presence of various bonding agent. Optical photographs

of ozone exposed :samples showed that cracks were developed on the surface

after t\vo hours of' orone exposure. In the case of aluminium powder filled

vulcanizarrs, the cracks developed are small and discontinuous, whereas in

other samples the cracks are deep, wide and continuous. There is no

adverse effect due to the presence of bonding agents like hexamethylene

tetra~nine-rcsorcinc~l system: bis[3-(triethoxy silyl) propyl] tetrasulphide,

cobalt naphthenate and toluene diisocyanate. on the ageing performance of

natural rubber-aluminium powder composites. However HR system shows a

better rcrrrition of properties after the ageing period due to the continued

resin tilr the ageing process. The LO1 values are increased by the

incorporar~on of \ arious fillers including aluminium po\\der.

R. Raker. NK Technoloa. 19, 28. 1998.

S . Varghese, B. Kuriakose and S. Thomas, Polvm. Degradation and Stability, 44, 55, 1994.

P. M. Lewis, NR Technology, 3, 1, 1972.

11. Shintani and A. Jakamura, J. Appl. Polym. Sci., 42, 1991, 1979.

C. S. Kim, Rubber Chem. Technol., 42. 1095, 1969.

A. l'hiang Chan~ya, K. Mahxuchi and F. Yoshii, J. Appl. Polym. Sci., 54, 525, 1994..

D. Barnard, M. E. Cain, J. I . Cameen and 1'. H. Houseman, Rubber Chem. Technol.. 45,38 1. 1972.

J . R . Sheltin, Rubber Chem. Technol.. 45, 359, 1972.

N. I,. Hancox, I'lastics Rubber and Cornp. Proces. Appl., 27, 97. 1998.

M. S. Sambhi. Rubber Chem. Teclznol.. 55. 181, 1982.

M. 1'. B p k . degradation of Filled Pol!mers3. Ellis Honvood Ltd. England. 199 1 .

E. K. Andreus. .I. Appl. Po1j.n~. Sci.. 10. 17. 1966 .