Chapter 4

STUDIES ON NATURAL RUBBER/STYRENE-BUTADIENE

RUBBER BLEND

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STUDIES ON NATURAL RUBBER/STYRENE -BUTADIENE RUBBER BLEND

A common composite article such as a tyre is amixture of wire (metal), textile cord (organic fibre) and rubbercompounds. The rubber compound itself is a mixture of elastomer,filler and curatives. Moreover, the elastomer part may be amixture of two or more rubbers. Fig.4.1 shows the properties of4 different types of rubber related to tyre properties, expressed

in terms of their glass transition temperature (Tg).1 It may beseen that cis-polybutadiene (BR) has the lowest wet grip and airimpermeability but has the best low temperature and heat buildup performance. Chlorobutyl (Cl IIR), however, has very goodwet grip and air impermeability properties but poor (high) heatbuild up and poor low temperature performance.

Natural rubber (NR) and styrene-butadiene rubber (SBR)are the world's leading general purpose rubbers and about two­thirds of their total output goes into tyres but they also findwidespread use in many other areas of rubber goods productionsuch as belting, flooring and mats, hose, shoe soles and heelsand many other industrial and domestic applications. In India, alarger amount of NR is used than SBR because NR is readilyavailable while in countries like USA a larger amount of SBRis used. While passenger car tyres may be made entirely fromSBR, in commercial vehicle tyres NR/SBR blends are used and

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the natural rubber content generally increases with the sizeof the tyre. In these applications the requirements of maximumwear resistance, good resistance to tread groove cracking andrib tearing plus minimum heat generation can be met only bynatural rubber.2 The tread wear and wet skid resistance of SBR

elastomers and blends is dealt with by Kienle.3 Information onthe practical aspects of NR/SBR blends were reported bySpringer,4 including flex cracking, processing and vulcanizing.Fatigue characteristics of mixtures of SBR and NR have beeninvestigated by Fugimoto.5 Because of their allround importancethis study was undertaken on NR/SBR blends.

Like NR, SBR is an unsaturated hydrocarbon polymer.Hence unvulcanized compounds will dissolve in most hydrocarbon

solvents and Other liquids of similar solubility parameterswhile cured stocks will swell extensively.6 Both materials willundergo many olefinic type reactions such as oxidation, ozoneattack, halogenation, hydrohalogenation and so on, although theactivity and detailed reactions differ because of the presenceof the adjacent methyl group in the natural rubber molecule. Bothrubbers may be reinforced by carbon black and neither can beclassed as a heat-resistant rubber.

Compared with natural rubber, raw SBR is more uniform

in many cases. It is more uniform in quality and compounds are

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more consistent in both processing and product properties. Itis more uniform in the sense it usually contains fewer undesiredcontaminants. SBR does not breakdown to any great extent on

mastication. The synthetic material is usually supplied at aviscosity considered to provide the best balance between dis­persion of ingredients and ease of flow in extrusion, calender­ing and moulding. This provides savings in both energy consumpt­ion and time, and hence on costs. However, mill mixing of NRis usually easier and the synthetic rubber lacks tack and greenstrength and this is of consequence in tyre building, parti­cularly with radial tyres.

While natural rubber is crystalline with a Tm ofabout 50°C, SBR is amorphous. Although crystallinity in naturalrubber is reduced by the presence of crosslinks and fillers andother additives, it still crystallizes on extension giving arubber of good tensile strength, even with gum stocks. On theother hand gum vulcanizates of the amorphous SBR are weak and

it is essential to use reinforcing fillers such as fine carbonblacks to obtain products of high strength. Black-reinforcedSBR vulcanizates do, however, exhibit very good abrasion resist­ance, generally being superior or comparable to natural rubbercompounds above 14°C. On the other hand they have lowerresilience and resistance to tearing and cut growth. The ageing

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behaviour of SBR is quite different from that of naturalrubber: the synthetic material tends to crosslink rather thanexhibit chain scission on oxidation.7

In the 1960s, considerable work was undertaken by

the Malaysian Rubber Producers Research Association to studythe nature of sulphur crosslinks in natural rubber vulcanizatesIt was found that whereas conventional accelerator-sulphursystems led to substantial amounts of disulphidic and poly­sulphidic linkages, there were very few monosulphidic cross­links. On the other hand, if the sulphur content was reducedand the accelerator content boosted, the vulcanizates had amuch higher proportion of monosulphidic crosslinks. In thecase of natural rubber, the higher monosulphidic content ledto a distinct improvement in resistance to thermal and oxida­tive ageing but at the expense of a much reduced flex life.8Such higher accelerator-low sulphur systems are frequentlyreferred to as efficient vulcanization systems (EV systems).Subsequent studies using SBR9 have shown several interestingfeatures which may briefly be summarised as follows.

1. Conventional SBR vulcanizates have a monosulphidic cross­link content similar to that of an EV natural rubber

system.

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2. The replacement of a conventional cure by an EV curingsystem also increases the monosulphidic content with SBR,in this case to a value about twice that for a naturalEV system.

3. Whereas the crosslink density of conventionally cured SBRvulcanizates increases on ageing at elevated temperatures(e.g. 110°C) the EV-cured material has a stable crosslinkdensity at the same temperature.

4. The reduction in fatigue life shown with natural rubberEV systems is not duplicated with SBR. Indeed EV SBR

compounds after ageing show much better fatigue resistancethan conventional compounds after ageing.

In general it would appear that use of EV systemsin SBR leads to a lower aged modulus and hardness,better reten­tion of elongation at break and a general overall improvementin compression set and heat build-up. In practice, because ofthe high cost of accelerators, true EV systems are not oftenused with SBR. Semi-EVs (that is of intermediate accelerator­

sulphur ratios) that provide a useful compromise between costand performance are commonly employed.

From the foregoing discussion it is clear that thenetwork structure of vulcanizates formed and its effect on the

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physical properties varies considerably with the nature ofelastomers.1O While some amount of literature is available on

the influence of network structure on the physical propertiesof NR vulcanizates, similar information on synthetic rubbervulcanizates is meagre. Thus in designing suitable formulationsfor blends of elastomers like NR and SBR, careful considerationhas to be given to prepare optimum network structures suitablefor particular applications. However, no definite rules areavailable for designing compounds of NR/SBR blends for achiev­

ing a set of vulcanizate properties. SBR is slower curing thanNR and higher accelerator levels are necessary in order toobtain equivalent cure times.l1’12 Based on this simple state­ment, compounds for NR/SBR blends are now designed by giving

sulphur and accelerators at intermediate levels between thoserequired for NR and SBR. This is far from satisfactory sincethe curatives may be unproportionally distributed between NRand SBR and the network structures formed in the two elastomers

may be different. In this study different formulations areemployed for a 50/50 NR/SBR blend by varying the amounts of

sulphur and accelerator and the cure characteristics and physi­cal properties of the vulcanizates are compared. The networkstructure of the vulcanizates is also deciphered by chemicalprobes. During service the degree of crosslinking may increaseor decrease depending upon the elastomer, temperature and otherfactors with a possible rearrangement in the nature of

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crosslinks.1O To understand the behaviour of NR/SBR blends

in service the vulcanizates were subjected to acceleratedthermal ageing for a specified period of time and then thephysical properties again evaluated. The network structurewas further examined by chemical probes to observe the changesoccurring during ageing.

I. Studies on 50/50 blends of NR and SBR without fillers

Experimental

In this study fillers were not used to avoid theeffect of filler distribution between the two rubbers. Bothconventional and semi-EV curing systems were employed for

study. The formulations used are shown in Tables 4.1 and4.2. The compounds were prepared on a laboratory mixing mill.The cure curves of the mixes were taken at 150°C on a MonsantoRheometer model R 100.

The compounds were vulcanized upto the respective

optimum cure times on a steam heated laboratory hydraulicpress. The tensile properties of the vulcanizates were deter­mined according to ASTM D 412 (1980) on a Zwick universal

testing machine. The compression set and hardness of thevulcanizates were determined according to ASTM D 395 (1969)

and ASTM D 2240 (1968) respectively. The ageing resistance

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of the vulcanizates was determined by keeping them at100°C for 48 hours in an air oven and then measuring theretention in physical properties.

The total crosslink density and the percentageof polysulphidic, disulphidic and monosulphidic linkageswere determined as per the procedures outlined in Chapter 2from the swelling data of the vulcanizates in benzene. Freesulphur was determined iodometrically by converting it tosodium thiosulphate according to ASTM D 297-72A. Zinc

sulphide sulphur was determined iodometrically from theformation of cadmium sulphide as described in BS 902, pt1310 (1958).

Results and Discussion

The cure curves of the compounds are shown in

Figs.4.2-4.5. The cure characteristics calculated from thecure curves are also shown in Tables 4.1 and 4.2. Increas­ing the amount of CBS at constant sulphur level reduces thescorch safety and speeds up vulcanization both in the conven­tional and semi-EV systems as expected. Variation of sulphurat constant accelerator level also produces similar behaviourbut with less pronounced effects. The maximum torque, ameasure of the crosslink density, increases steadily withincrease in sulphur or CBS both in the conventional and

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semi-EV systems. The curing behaviour of the blend is thussimilar to that of a single elastomer. Unlike the curing ofNR no compound shows any tendency for reversion. However,

the curing behaviour of the blends seems to be more tiltedtowards an NR behaviour. This might be because of aslightly larger share of curatives going to the NR phase.This preferential migration might leave the NR phase slightlyover crosslinked and the SBR phase slightly under crosslinked.

Table 4.3 shows the results of the chemical

characterisation of the vulcanizates employing conventionalsystems. The crosslink density is found to increase withaddition of sulphur or CBS as expected. It is found thatpolysulphidic crosslinks constitute the major part of cross­links in this case. The concentration of monosulphidic anddisulphidic crosslinks before ageing could not be estimatedsince the vulcanizate sample partly dissolved in benzeneafter the cleavage of poly-and disulphidic linkages, obviouslydue to the very low concentration of monosulphidic linkages.However, it is observed that there is a significant conversionof polysulphidic linkages to di- and monosulphidic linkagesduring ageing. The overall crosslink density is also foundto increase with ageing. This behaviour could be explainedbased on the outline reaction scheme originally proposed forvulcanization of NR. In the transformation of the initial

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polysulphidic networks into the final network in NR twoprincipal reactions were identified.l3_16 The first reactionis desulphuration giving rise to crosslink shortening andleading eventually to monosulphidic crosslinks. The secondreaction is thermal decomposition forming cyclic mono- anddisulphides and conjugated dienes and trienes in the rubberbackbone and zinc sulphide. It appears that the first react­ion is more important in NR/SBR blends under moderate tempera­

tures. This might be the reason for the improved reversionresistance of the blends too.

Free sulphur content and the amount of sulphurexisting as zinc sulphide increase with higher concentrationsof sulphur but decrease with higher concentrations of CBS asexpected. while a part of the free sulphur gets used up infurther reactions,probably additional crosslinking,the amountof zinc sulphide sulphur remains more or less the same duringthermal ageing. This further shows that thermal decompositionof polysulphidic network via the second route mentionedearlier is not significant.

Table 4.4 shows the results of the chemical chara­

cterisation of the vulcanizates of the semi-EV system. As inthe case of the conventional system, the crosslink densityincreases with increase in the amount of sulphur of CBS.

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However, the amount of polysulphidic linkages is much lessin this case compared to the conventional system. The con­centration of monosulphidic linkages was found to be higherbut its exact determination was found to be difficult as

before. As in the case of the conventional system there isa significant conversion of polysulphidic linkages to di- andmonosulphidic linkages at the time of ageing. The variationof free sulphur and zinc sulphide sulphur is found to besimilar to that of the conventional system. It appears thatthe changes in the network structure occurring during ageingin this case are similar to those of the conventional system.

I

The physical properties of the vulcanizates areshown in Figs.4.6-4.15. Tensile strength of the vulcanizates(Figs.4.6 and 4.7) is found to improve steadily when theamount of sulphur or CBS is increased both in the conventionaland semi-EV systems which might be due to a higher degree ofcrosslinking induced by them. Vulcanizates of the convent­ional system are generally found to display higher strength.This might be due to a higher concentration of polysulphidiccrosslinks17—l9 in the conventional vulcanizates than thatof the semi-EV vulcanizates. The deterioration in tensilestrength of the vulcanizates with ageing may be principallydue to main chain scission.20-23 The chain shortening of alarge fraction of polysulphidic crosslinks may also be

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138

contributing to this effect. There seems to be an optimumnetwork structure which gives maximum retention in tensilestrength. Other mechanical properties like elongation atbreak (Figs.4.8 and 4.9), modulus (Figs.4.1O and 4.11),compression set (Figs.4.12 and 4.13) and hardness (Figs.4.14 and 4.15) are found to depend more directly on thedegree of crosslinking and less on the nature of crosslinks.Elongation at break decreases with increase in the degreeof crosslinking whereas modulus, compression set and hard­ness improve vdth higher degree of crosslinking. The varia­tion in these properties with thermal ageing also followsthe variation of crosslink density. The elongation at breakdecreases with ageing, whereas modulus, compression set andhardness improve both in the conventional and semi-EVsystems.

11. Studies qn filled so/so blends pf NRP and_SBR

Experimental

Since all industrially important NR/SBR blends

th|-I­}._|}....|

(D

contain fillers, this study was done on d 50/50 blendsof NR and SBR. The formulations employed for study are givenin Table 4.5. The compounds were prepared on a laboratorymixing mill. The cure curves of the mixes were then takenat 150°C on a Monsanto Rheometer model R 100.

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140

The compounds were then vulcanized upto theiroptimum cure times at 150°C on a steam heated laboratoryhydraulic press. The tensile properties of the vulcanizateswere determined as per ASTM D 412 (1980) on a Zwick universal

testing machine. The hardness of the vulcanizates wasdetermined according to ASTM 2240 (1968) and expressed in

shore A units. The ageing resistance of the vulcanizateswas determined by keeping the sample at 100°C for 48 hoursin an air oven and then measuring the retention in theseproperties.

The overall crosslink density and the percentageof polysulphidic, disulphidic and monosulphidic linkages ofthe vulcanizates before and after ageing were determinedfrom the swelling data in benzene by the procedures outlinedin Chapter 2. The amount of free sulphur was determinediodometrically by converting it to sodium thiosulphate accord­ing to ASTM D 297-72 A. Zinc sulphide sulphur was determined

iodometrically from the formation of cadmium sulphide asdescribed in BS 902 pt. 1310 (1958).

Results and Discussion

The cure characteristics of the compounds cal­culated from the cure curves are shown in Table 4.5. when

the amount of CBS is increased at constant sulphur level,

141

the vulcanization becomes faster as evidencedlnr the decreasein the cure times. Increasing the amount of sulphur, keepingthe amount of CBS constant also speeds up vulcanization.However, the increase in cure rate in this case is not aspronounced as in the previous case. The maximum torque, ameasure of the crosslink density, increases steadily withincrease in sulphur and/or CBS. The curing of the blends

shows more of an NR behaviour than that of SBR. Thismight be because the conventioanl curatives used in thisstudy are more soluble in the NR phase and hence a largershare of the curatives is going to the NR phase. This meansthat the state of cure in the SBR phase is not optimum. How­ever, the presence of SBR seems to give very good reversionresistance to the blend since none of the mixes shows anytendency for reversion. This is in conformity with the beha­viour of the gum compounds.

Table 4.6 shows the results of the swellingstudies done to examine the network structure of the vulcani­zates. The total crosslink density increases with increase insulphur and/or accelerator. Vulcanizates with comparativelyhigher amount of sulphur and lower amounts of accelerator arefound to have a larger percentage of polysulphidic crosslinks.The total crosslink density is found to be slightly higherthan that of the corresponding gum compound employed in

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143

section I. This might be due to the mild accelerator actionof carbon black. The slightly lower percentage of poly­sulphidic crosslinks in the vulcanizates also confirm thisreasoning. However, the percentage of disulphidic and mono­sulphidic linkages could not be estimated in the vulcanizatesbefore ageing as in the case of the gum vulcanizates sincethe samples got dissolved in the solvent partly after treat­ment with the reagent to cleave polysulphidic and disulphidiclinkages, obviously due to the low concentration of mono­sulphidic linkages. The crosslink density of the vulcanizatesis found to increase with ageing. An examination of thepercentage of poly-, di- and monosulphidic linkage of thevulcanizates before and after ageing indicates that theincrease in crosslink density might be due to a fraction ofthe polysulphidic linkages getting shortened with additionalcrosslinking. This is also similar to the behaviour of thecorresponding gum vulcanizates. The variation in the cross­link density with ageing increases with higher amounts ofsulphur or accelerator in the original mix. This might bedue to a larger proportion of polysulphidic crosslinks beingavailable for shortening in the former case, whereas in the

latter case it might be due to a higher amount of accelerator.

It is observed that the amounts of free sulphur andzinc sulphide sulphur increase with increasing amounts ofsulphur but decrease with increasing amounts of accelerator‘

144

in the original mix. whereas the amount of zinc sulphidesulphur remains more or less constant with ageing, part offree sulphur gets used up in further crosslinking at thetime of ageing. This is again the behaviour shown by thegum compounds.

Except tensile strength (Figs.4.16 and 4.17) 'other physical properties show a definite relationship withcrosslink density. Elongation at break (Figs.4.18 and 4.19)decreases with increase in crosslink density whereas modulus(Figs.4.2O and 4.21) and hardness (Figs.4.22 and 4.23) showa direct proportionality with crosslink density as expected.The variation of elongation at break, modulus and hardness

with ageing also could be explained by the increase in cross­link density with ageing.

The tensile strengths of the vulcanizates arefound to depend on the proportions of poly-, di- and mono­sulphidic linkages in addition to the crosslink density.A higher percentage of polysulphidic crosslinks is found toresult in higher tensile strength as expected. The tensilestrengths of the vulcanizates are found to decrease withageing. This might be due to the decrease in the concentra­tion of polysulphidic crosslinks and due to other factorssuch as main chain scission occurring during ageing. The

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153

vulcanizates with less amount of sulphur and more acceleratorare generally found to display higher ageing resistance whichindicates that monosulphidic linkages are more resistant toageing than polysulphidic linkages as expected.24

Conclusions

The study helps to draw the following conclusionsabout the cure characteristics and vulcanizate properties of50/50 NR/SBR blend.

1. The total crosslink density and the proportions of mono-,di-, and polysulphidic linkages in both filled and gumvulcanizates could be estimated by the simplified chemi­cal probes described in Chapter 2.

2. Formulations employing conventional and semi-EV curingsystems show good resistance to reversion. There areonly marginal improvements in thermal ageing resistanceof the vulcanizates employing a semi-EV system.

3. The NR phase takes a larger share of the curativesleaving the SBR phase slightly under-crosslinked. How­ever, this does not seem to affect the physical propertiesmuch. The distribution of carbon black between the

rubbers also does not lead to serious problems in thisblend. .

154

The overall crosslink density increases at the time ofthermal ageing accompanied by chain shortening of the

polysulphidic linkages with additional crosslinking.

The dependence of physical properties on the networkstructure is more or less similar to that observed forSBR.

The amount of combined sulphur in the vulcanizateslightly decreases with increase in sulphur and/ordecrease in accelerator contents as seen from the

amounts of free sulphur and zinc sulphide sulphur inthe vulcanizates. Part of the free sulphur is foundto take part in additional crosslinking at the time ofageing whereas the amount of zinc sulphide sulphurremains more or less the same.

Varying the amounts of sulphur and accelerator producessmooth variations in the curing behaviour and vulcani­zate properties. Hence designing suitable formulationsto achieve specific vulcanizate properties is notdifficult.

REFERE N9 E §

P.J.Corish in "Polymer Blends and Mixtures", D.J.Walsh,

J.S.Higgins and A.Maconnachie (Eds.), Martinus NijhoffPublishers, Dordrecht (1985), p.245.

T.S.Stephen and C.S.Fah in "Rubber Handbook", R.O.Babbit (Ed.)

R.T.Vanderbilt Company, CT (1978).

R.N.Kienle, E.S.Dizon, T.J.Brett and C.F.Eckert, Rubber Chem.Technol., 44, 996 (1971).

A.Springer, RAPRA translation, 1170, April 1964.

K.Fugimoto and co-workers, J.Soc.Rub.Ind. Japan, 46, 216(1973).

J.A.Brydson, "Rubber Chemistry", Applied Science Publishers,London (1978).

J.A.Brydson in "Developments in Rubber Technology”, A.Whelan

and K.S.Lee (Eds.), Applied Science Publishers, London (1981).

. D.J.Elliot in "Development in Rubber Technology", A.Whelan

and K.S.Lee (Eds.), Applied Science Publishers, London (1979).

E.R.Rodger in "Development in Rubber Technology-1“, A.whelan

and K.S.Lee (Eds.), Applied Science Publishers, London (1979).

N.J.Morrison and M.P0rter, Rubber Chem.Techno1., 57, 63(1984).

155

156

S.H.Morell in "Rubber Technology and Manufacture", C.M.Blow

and C.Hepburn (Eds.), Butterworths, London (1982), p.172.

D.C.Blackley, "Synthetic Rubbers", Applied Science Publishers,London (1983), p.119.

L.Bateman, C.G.Moore, M.Porter and B.Saville in "The Chemistryand Physics of Rubber like Substances", L.Bateman (Ed.),Maclaren and Sons Ltd., London (1963), Chap.15.

A.Y.Coran in "Science and Technology of Rubber”, F.R.Eirich

(Ed.), Academic Press, New York (1978), Chap.7.

D.S.Campbell, J.Appl.Polym.Sci., 14, 1409 (1970).

C.R.Parkes, D.K.Parker and D.A.Chapman, Rubber Chem.Technol.,

45, 467 (1972). .G.M.Bristow and R.Til1er, Koutschuk Gummi Kunststoffe, 23,

55 (1970).

E.D.Farlie, J.App1.Polym.Sci., 14, 1127 (1970).

F.P.Baldwin and G.Verstrate, Rubber Chem.Technol., 45, 709(1972).

T.Colelough, J.I.Cunneen and G.M.C.Higgins, J.Appl.Polym.Sci.,12, 295 (1968).

A.G.Veith, J.Polym.Sci., 25, 355 (1957).

A.V.Tobolsky, J.Appl.Polym.Sci., 27, 173 (1956).

157

E.J.Blackman and E.B.McCall, Rubber Chem.Technol., 43, 651

(1970).

M.Porter in "Organic Chemistry of Sulphur", S.Oea (Ed.)

Plenum Press, New York (1977), Chap.3.