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Journal of Scientific & Industrial ResearchVol. 62, August 2003, pp 820-826

Effect of Compounding Ingredients on Rheometric Characteristics and PhysicalProperties of a Rubber Based Shoe Sole

N Karak * and M Roy

Chemical Sciences Department, Tezpur University, Tezpur 784028

Received: 20 November 2002; accepted: 03 April 2003

The effects of base polymer and filler types like, NR, NRISBR, NRJSBRJBR blends, Hind clay and SRF N770 fillersand their doses on rheometric and physical properties of a shoe sole compound are studied. In addition to hardness, otherrelated important properties of the shoe sole like, tensile strength, modulus at 100 per cent strain, abrasion resistance, cutgrowth resistance up to 300 per cent, elongation at break are also measured for the vulcanizates. The curing characteristics ofthe vulcanizates are determined by using a Monsanto MDR 2000 rheometer. The economic validity of the differentformulations is also evaluated.

Keywords: Base polymer, Rubber based shoe sole, Rheometric properties, Shoe sole

Introduction

The performance of a shoe sole depends on thechoice of base polymer, other compoundingingredients, and their proper doses. The basicrequirements for a shoe sole are as follows: (i) Thehardness has an important role on the performance ofthe shoe sole. It should have optimum value, as higherhardness decreases the comfort of the user due tolowering of enveloping and shock absorbing capacityby decreasing flexibility. Also lower hardnessdecreases frictional coefficient and strength of thesole; (ii) The sole should have high flex resistance, asduring walking the material is under continuousflexing stress and therefore it undergoes fastercracking for a low flex resistance sole; (iii) Theabrasion resistance should be high, as it is undercontinuous friction with the road; (iv) Again it shouldhave high coefficient of friction otherwise it will slip;(v) Another critical requirement of a shoe sole is thatit should tightly adhere to the upper parts of the shoe,otherwise parts get separated; (vi) Especially the soleshould have dimensional stability under both dry andwet conditions even under the influence of load andtemperature of service; (vii) The specific gravity ofthe sole should be minimum so that it is light in

* Corresponding authorEsmail: nkarak@tezu.ernet:in

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weight which offers better comfort to the user; (viii)Above all the sole material should be economicallyviable by keeping its better performancecharacteristics. Considering all the above facts, a fewrecipes of a rubber based shoe sole are formulated inthe present study.

The rubber footwear industry has met diversityof quality depending on the end uses of the productand hence there is no typical formulation for it. Both,natural and synthetic rubbers are used extensively formanufacturing shoe sole. A polymer blend has someadvantages over their individual componentsl-4. There

I f 'i·I? hvsi h . Iare severa 0 reports - on p ysico-c errucainteraction of carbon black with elastomers and itseffect on the mechanical properties of thevulcanizates. In shoe sole, to meet the demand fornon-black compounds, siliceous, silicate, clay, etc.,are also used. The interaction of these fillers withelastomers on different properties of the vu\canizates. I dl~16IS a so reporte . .

In the present study the authors report the effectof base polymer and filler types like, NR. NRiSBR,NRlSBRlBR blends, Hind clay and SRF N770 fillersand their doses on rheometric characteristics andphysical properties of a shoe sole compound. Theeconomic validity of the different formulations is alsoevaluated.

KARAK & ROY : RUBBER BASED SHOE SOLE 821

Experimental Procedure

Materials

The following materials supplied by various firms were used . RMA 5 (Kerala Co-operative Marketing Federation), SBR 1958 (Usha Chemicals), SBR 1502 (Chemin Enterprise), Cis-rub (Western Chemicals ), HSL-accinox TQ (ICI Limited) , SRF N770 (Philips Carbon Black Limited), White Oil(New India Petrochemicals Industries), Cumarone Indene res in( Royal Chemicals), Sulphur (Uil as h Oil) , CBS­cyclohexyl benzthi azy l sulphenamide (Bayer India Limited), DPG-diphenyl guanidine (Cyanide and Chemical s), TMTD-tetramethylthiuram disu lphide (Merchem Limited), ZnO (Tribeni Industries), Stearic acid (Godrej Soaps), Salicylic acid (Aitra Laboratori es), and Red iron oxide (Aitra Laboratories)

Compounding

Mixing of the compounded rubber stocks was carried out on a two roll open mi II (0. 15 m X 0.32 m) at a fri ct ion ratio of I : 1.25 , driven by 7.5 HP motor, with drill type cooling arrangement on the ro lls, using conventional rubber mixing procedure in all the cases 17 The tota l mixing time was 30 min between 65 ± 5 °C, maintained in all the cases. However, during mixing of sulphur and accelerators the temperature was brought down to 40-45 °C. The compounded rubbers were then kept for I d at ambient temperature for proper maturation before vulcanization .

Rheometric Study

The rheometric studi es were carried out using a Monsanto rheo meter, MDR 2000 at 160°C for 15 min. From the rheometri c studies, minimum torque, maximum torque , scorch safety , time to achieve 90 per cent cure (t ,)()), etc., were obta ined directly from the instrument. The difference in torque, optimum cure time, and cure rate were ca lcul ated from the above data by using the following formula: Diffe rence in torque (de l-torque) = Maximum torque - Minimum torque, Cure rate = { (90 per cent of maxi mum torque) - (Minimum torque + 2)} I (Optimum cure time -Scorch safety) , and Optimum cure time = (t'!O + 5) min .

Vulcan i z.a rion

V~_;lcanization of the stock compounds was performed in a steam heated hydraulic press (0.4m x 0.38 m) at 160 °C for their res pect ive optimum cure

time under a line pressure o f 1.9 ± 0 .5 MPa. The vulcanized samples were then kept at room temperature in a dark place for further study .

Physical Test M ethods

For testing each physical property, at least three spec imens per test were tested and mean of these was taken as the va lue of the property. The hardness was measured by using a Shore A duromete r model SHR­MARK-III, as per the ASTM D 676-59 standard procedure. Tensil e strength , modulus at I 00 per cent strain and elongation at break we re determined, according to the ASTM D 412-51T, using dumbell specimens in a universal tens ile testing machine with a separation rate of grips 0 .5 mlmin. The abrasion index of the samples was measured by using a Dupont

. abrader, as per the BS 903 part A9: 1957 standard procedure. The cut growth res istance of the samples was measured by using a Ross Flex ing machine as per the BS 5131-2.1: 1991 standard procedure and the flex was continued up to 300 per cent cut growth of the samples. The specific grav ity of the samples was measured by conventional liquid displacement technique at 30 ± 2 °C. The cost of the co mpounds was calculated as Cost (Rs )lkg = Total cos t of the ingredient in RsiTota l rec ipe weight in kg. Then it was converted to cos t per unit vo lume (Rs I kg) = cost

in Rs per kg x specific gra vity.

Results and Discussion

The compounding rec ipes of the diffe rent vu lcanizates with varying base polymer and fille r types and their compositions are shown in Tabl e I .

The Effect of Base Polymer

The increase in cure rates (Tab le 2) for vu lcanizates I to 3 is due to the fact that SBR and BR hav ing lower cure efficiency than NR. Since NR contains some basic proteinous substances and also the meth yl group in NR en hance the curing process by activating the double bond in the repeating unit 17

.

Further, SBR contains less amount of unsaturati on in the main chain as only butadi ene unit having doubl e bond in the main chain and both SBR 1958 and 1502 grades con ta in re lati ve ly large amount of styrene moie ty while having ste rically hinclerable aromati c groups. As the cure rate Jecreases from vulcanizates I to 3 the -;corch safety and optimum cure time increase with changing 100 NR to 75 NR with 25 SBR to 50 NR with 25 SBR and 25 BR (a ll in phr) content in the

822 1 SCI IND RES VOL 62 AUGUST 2003

Table 1- Compounding recipes of the vulcanizates

Ingredien ts (phr) Stock number

2 3* 4 5

RM A5 100.0 75.0 50.0 100.0 70.0

SBR 1958 25.0 30.0

SBR 1502 25.0

Cis-rub 25 .0

Hind clay 87.6 87.6 2 1.7 10.0

Chi na clay 87.0

SRF N770 78.2 80.0

White oil 2.0 2.0 2.0 17.3 5.0

ZnO 5.0 5.0 5.0 5.0 .'i .O

Stearic acid 1.5 1.5 1.5 1.5 1.5

Salicylic acid 0.5 0.5 0.5 0.5 0.5

C-1 resin 2.0 2.0 2.0 2.0 2.0

HSL 1.0 1.0 1.0 1.0 1.0

Sulphur 2.2 2.2 2.2 2.4 2.2

CBS 1.4 1.4 1.4 1.5 1. 5

DPG 0.3 0.3 0.3 0.3 0.25

Red iron ox ide 1.0 1.0 1.0

*Another mod i tied formulation of 3* as 3 with 0.4 phr TMTD was used for actual study

Table 2- Cure characteristics and physical properties of the vulcanizates with variation of base polymer compos iti on

Properties Stock number

Cure characteristics 2 3* 3

Del-torque (N-m) 1.16 1.20 1.27 1.53

Scorch time (min) 3.3 3.7 6.48 2.91

Optimum cure time (min) 10.3 11 .63 15 .55 9.96

Cure rate (N-m I min) 0. 115 0.104 0.100 0.162

Physical properties

Tensile strength (Mpa) 11.77 9.17 10.17

Mod.at I 00 per cent (Mpa) 2.77 2.68 1.58

EB (per cent) 560 400 450

Hardness (shore A) 60 72 63

Abras ion index 120 130 150

Cut growth (kc) 57 50 60

Specific gravity 1.33 1.33 1.31

KARAK & ROY: RUBBER BASED SHOE SOLE 823

blends . The increase in difference of maximum to minimum torque, i.e., del-torque in rheometric curve, may be due to the fact that the stock viscosity of 75/25 NR/SBR and 50/25/-25 NR/SBR/BR blends is higher than I 00 NR. Thi s is due to the presence of bulky aromatic group in SBR and also initial viscosity of NR/BR blend is higher than thei r individual polymer18

. As in case 3* the optimum cure time is very high therefore a modified formulation is 3, with using 0.4 phr sulphur donor curing agent viz. TMTD which offers synergistic effect in curing reaction has been used in the study. Although TMTD is mainl y used as a very fast acce lerator, in combinati on with sulphur, accelerator CBS and secondary accelerator DPG, it acts as a sulphur-donor vulcanizing agent. Thus the cure rate is higher than the vulcanizates I to 3 without TMTD. Again , TMTD may also lower the ac ti vation energy of the curing reaction by complex formation with su lphur and rubber molecul es and thus acce lerate the acti on of sulphur19 and hence increases the cure rate. As cure rate increases so the scorch safety and optimum cure time of the vulcanizate decrease compared to first three formulation s. The increase of de l-torque is due to the fact that the tota l amount of curing agents is higher and thus the ex tent of curing is a lso hi gh.

The variation of phys ical properties (Table 2) of the vulcani zates with different base polymer

compositions is supported by the earlier reported results 1

• 2

• The resu lts can be exp lained by the fact that

NR vulcanizate has hi gher strength because of strain­induced crystallization, but the affinit y of BR and

SBR towards fill er is high 1' . Again, as NR, SBR, and

BR all are non-polar rubbers, their blends are

compatible up to certain extent g1vtng less heterogeneity in the structure than that of the incompatible blends. The indi vidual po lymer properties are, therefore, affected by the properties of

the blends. As NR is a se lf-rei nforcing rubber and level of curing is higher than SBR or BR', so tensile

strength , modulus and e longation at break values of the vulcanizate I are higher than vulcani zates 2 and 3,

where NR content is less . The hi gher tensile strength and elongation at break of the vulcanizate 3 compared to 2 is due to higher levei of curing, supported by

higher del-toque as more amount of curing agent i .·

used. But lower modulus at I 00 per cent strain is not much clear though it may be due to the presence or

BR in the blend.

Again , as BR has the highest abras ion res istance due to the highest chain fl ex ibility and the lowest g lass transi tion te mperature (T g = -I 08 °C), hence the vulcanizate 3 has the hi ghest abrasion res istance. The higher abrasion index of the vulcani zate 2 than the vulcani zate I is due to the fact that abras ion resistance of SBR is higher than NR above 14 °C (ref. 21 ). On the othe r hand, cut growth res istance of SBR is the lowes t and for NR it is the highest, thus the cut growth up to 300 per cent is hi gher for the vulcanizate 1 than 2. However, as the level of curing in the vulcani zate 3 is hi gher than vulcani zate I so the cut growth is a lso hi gher than I , though the difference is not significant .

As all the three formulation s contai n almost same amount of ingredients and there is almost no difference in specific grav ity of the base po lymers so all the vulcanizates have a lmost same specific grav ity .

Hardness

As SBR has sterically hindered phenyl groups in each repeating unit and the amount of SBR is the highest in the vulcanizate 2 so the hard ness of th is vu lcani zate is maximum, followed by the vulcani zate 3. The vulcanizate 1 has the minimum hardness, as it conta ins only NR . Also BR and SB R has hi gher affinity towards fill er which further e nhances the hardness of the vulcanizates 2 and 3.

The Effect of Filler Types and Their Doses

The increase of cure rate (Table 3) of the vulcanizate 4 is due to the fact that a lthough the total amount of filler used is higher th an the vuca ni zate I (to maintain the proper hardness) (Tabl e I) but the tota l amount of curatives is a lso littl e hi gher th an the vulcanizate I . As it is expected that the higher amount of filler in vulcanizates 4 will adsorb more amount of curatives so :1 little hi gher amount of curat ives (0 .2 phr sulphur and 0. 1 ph r CBS) was used . Again, to incorporate higher amou nt of fi lle r. especial ly SRF black higher dose of process oi I was also used. Thi s higher amount of o il not only he lps in better d ispers ion of the fillers but also causes uniform dispers ion of the curatives for which the cure rate of the vulcanizate 4 is highe r than the vulcanizate I. T he increase of cure rate in the vulcanizate 5 compared to 2 (Table 3) is due to the fac t that here a lso a higher amount of process oil was used to incorporate SRF black fille r. Althou gh the total amount of fill ers in both the formulat ion '. i·: almost same but this hi gher

824 J SC I !NO RES VOL 62 AUGU ST 2003

Tab le 3- Cure characteri sti cs and physical properti es or the vulcanizates with variati on or fill er type and dme

Properties

Cure cham clnislics

Del-torque (N-mJ 1.1 6

Scorch time (min) 3.3

Optimum cure time (min) 10.3

Cu re rate (N-m / min ) 0. 11 5

Ph ys ical properti es

Tensile strength (Mpa) 11 .77

Mod .at JOOper cent (Mpa) 2.77

EB (per cent) 560

Hardness (shore A) 60

Abrasi on index 120

Cut growth (kc) 57

Specific grav it y 1.33

amount of oil causes better di spersion of the curatives a long with the fill e r, which increases the cure rate. As cure rate increases thus the scorch safety and the optimum cure time decrease for vulcani zates 4 and 5 compared to I and 2, respec ti vely. The increase of del -torgue is due to the presence of highe r amount of fi ller in the vulcanizate 4 , which increases the stock viscos ity and better dispersion of curat ives in both vulcani zates 4 and 5, which increases the ex tent of cur ing. Further, in the vulcanizate 4 , although amount of the filler is about I 0 phr hi gher than the vul cani zate I but amount of the process o il is a lso high ( 15 phr), as a result the increment of de l-torque is not so hi gh.

The increase of tensile strength , abrasion index and cut growth resistance (Table 3) for the vulcani zate 4 compared to I may be due to the presence of hi gher amount of semi-reinforcing SRF N770 fill e r. As the amount of semi-reinforc ing fill e r increases the inte rac ti on between filler and po lymer a lso increases. Thi s phys ico-chemical inte raction increases the hysteres is behaviour, which results in storing of a part of input energy in the matri x and thereby requiring la rge amount of energy to failure 1 ~.

Stock number

4 2 ."

1.30 1.20 1.4)

1.65 3.7 1.76

lU I 11 .63 8.'2 1

0. 143 0.104 0. 165

i 2.85 9. 17 14.7 1

1.95 2.68 2.99

445 400 500

62 72 70

170 130 170

65 50 80

1.2 1 1.33 1.24

Thi s is due to the fact that during app licat ion of stress on the matrix the rate of recovery of input energy depends on the number of persistent bonds be tween the network of polymer and filler su rface and as the number increases the hysteres is also increases. Thus the tens ile strength, cut growth, and abras ion res istance improved . The lower value of modu lu s for the vulcanizate 4 may be due to the presence of large amount of white oil, which reduces the initial stra in value.

The tensile strengt h, modulus, e longation at break, ab ras ion res istance and cu t growth res istance of the vulcani zate 5 increases compared to 2 (Tab le 3) which may be due to higher re inforc ing effect of SRF compared to clay filler and also be tter affinity of SBR towards SRF black fill e r.

The specific gravity values of vulcani za tes 4 and 5 are somewhat lower compared to their non-black fill ed vulcanizates is due to the absence of heavy weight red iron ox ide (hi gh spec ific grav ity) pigment in their recipes (Table I ). The use of thi s pigment in carbon black filled vulcani zates is irrelevant , as it w ill not abl e to impart its co lour by superseding the black colour of the SRF fill e r.

KARAK & ROY: RUBBER BASED SHOE SOLE 825

Hardness

The hardness of the vulcanizate 4 is higher compared to the vulcanizate I and is due to the presence of higher dose of filler. But the increase is not so high due to application of higher dose of white oil. The negligible decrease of hardness of the vulcanizate 5 compared to 2 is due to application of higher dose of process oil, though the amount of filler is almost same.

The Economic Validity of the Different Formulations

The rates of the ingredients were obtained from their respective supplier (Table 4). These were utilized for evaluating the economic validity of the different formulations for commercialization of the product. The cost calculation resulted in the following order for the different formulations: vulcanizate 5 > 4 > 2 > 3 >I. Although the formulati on 5 has the highest cost but the vulcanizate also gives the bes t performance characteristics with respect to cure characteristics and physical properties. However, as the product with higher cost may not be acceptable to

Table 4- Cost of the different ingredients and thei r fu nctions in the used formulations

Ingredients Cost Function (Rs/kg)

RMA 5 38.80 Self reinforci ng base polymer

SBR 1958 80.50 High grip, rigid base pol ymer

SBR 1502 64.60 Same as above

Cis-rub 52.00 High abras ion index base polymer

Hind clay 1.65 Semi-reinforcing non-black tiller

China clay 1.00 Same as above

SRF N770 36.05 Semi-reinforcing black til ler

White oil 27. 18 Non-staining process oil

ZnO 78.85 Co-activator for sul fur vulcanizati on

Stearic acid 34.80 Same as above plus process aid

Salicylic acid 88.55 Retarder for vulcani zat ion

C-1 resin 50.50 Adhesion promoter

HSL 126.70 Antioxidant

Sulphur 9.30 Main curing agent

CBS 222.50 Delayed action ultra fast accelerator

DPG 3 10.00 Delayed acti on slow sec. accelerator

TMTD 320.00 Sulphur donor curing agent

Red iron oxide 4.35 Pigment

the users thus by appl ying the general rul e of optimum performance with minimum cost the best choice is the vulcani zate 3.

Conclusions

The present study shows that the type and dose of the base polymer and the filler affec t the hardness of vulcanizates. The hardness is immensely affected by incorporation of res in mod ified SBR 1958 base polymer. Not only the hardness but a lso the type and dose of base pol ymer and fi ller have significant effects on both cure characteristics and phys ical properties of vulcanizates. To get optimum performance of a shoe sole, the refore, one has to carefully select the amount and type of the base polymer and fill er.

The cost per unit volume calculation indicates that one has to pay the highest cos t for the best performance, though the optimum performance with mtmmum cost result in the bes t acceptabl e vulcanizate. The present study indi cates that the vucani zate 3 with TMTD may be chosen for commercialization of the product.

Acknowledgements

The financial support for this study by Tezpur University, Tezpur and Bata India Ltd , Kolkata is gratefully acknowledged . The authors wish to thank DrS Sinharoy of Bata Indi a Ltd for hi s gu idance in the work.

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