Polymer International 42 (1997) 85È89

Elastic Behaviour of Ternary Rubber-filledVulcanizates

M. S. Sobhy

Department of Physics, Faculty of Science, Cairo University, Beni-Suef-branch, Beni-Suef, Egypt

(Received 2 July 1996 ; accepted 26 July 1996)

Abstract : Near-equilibrium stressÈstrain measurements have been carried out onternary rubber vulcanizates. The e†ect of variation of the butyl rubber contenton the elastic behaviour of the ternary rubber vulcanizates has been studied. Ithas been found that butyl rubber (IIR) is less sensitive to the vulcanizationsystem used than either natural rubber (NR) or styreneÈbutadiene rubber (SBR).One can obtain a partially crosslinked system with an IIR phase embedded inthe crosslinked matrix of NR and SBR. The role played by carbon black duringmixing of the ternary blend has been investigated. The MooneyÈRivlin relation-ship was used to describe the behaviour of the ternary rubber matrix. The con-stants and have been calculated by use of the strain-ampliÐcation factor2C1 2C2and the total crosslink density of the ternary rubberÈcarbon black systems hasbeen investigated. The data have been evaluated in terms of the molecular theo-ries of rubber elasticity. The elastic behaviour was found to be intermediatebetween the affine and phantom limits of the theory.

Key words : ternary rubber vulcanizates, butyl rubber, natural rubber, styreneÈbutadiene rubber, stressÈstrain measurements, MooneyÈRivlin equation.

INTRODUCTION

Elastomers are generally crosslinked in a randommanner and therefore it is difficult to identify the prin-cipal e†ects of modiÐcation through mixing of certaincomponents on the mechanical properties. Manystudies have been published describing the mechanismby which carbon black reinforces elastomers,1,2 thedeformation mechanisms of Ðlled elastomers due to thestate of cure and the particle size of a given Ðller,3 thedependence of hysteresis on carbon black loading4 andthe elastic behaviour of Ðlled elastomer vulcanizates.5

An important aspect of the structure of a rubberblend is the nature of the interphase bonding.6 Duringblending of natural rubber (NR) and styreneÈbutadienerubber (SBR) Ðlled vulcanizate, the bond between NRand carbon black is quite weak and it has been foundthat carbon continues to migrate in the SBR phase7.Transfer of a portion of the carbon black from one

phase to another would lower its modulus proportion-ately more than the increase in modulus of the phasewith higher carbon black concentration.8

Since butyl rubber (IIR) is less sensitive to sulphurvulcanization than either NR or SBR, one can obtain apartially crosslinked system of IIR phase embedded inthe crosslinked matrix of NR and SBR, depending onthe concentration of each.9 The role played by carbonblack during mixing with rubber reÑects the elasticbehaviour of the ternary system used.8

In most experiments, the Ðrst stressing cycle has beenperformed in uniaxial extension at equilibrium toobtain the quantitative data required to study suchsystems. Another indication of the reinforcing potentialin relation to the di†erent degree of crosslinking of vul-canizates is the softening that occurs upon stretching,relaxation and subsequent extensions. This process,called the Mullins e†ect,10 has been observed duringthis study, where the second stressing cycle has been

85Polymer International 0959-8103/97/$09.00 1997 SCI. Printed in Great Britain(

86 M. S. Sobhy

attributed to the breakage of both agglomerates, orweak bonds between the rubber molecules and carbonblack particles.8

Smallwood3 showed that the elastic behaviour ofrubber containing lightly reinforcing Ðllers could bedescribed by the following theoretical expression rela-ting the elastic modulus E of the Ðlled rubber to themodulus of the matrix :E0

E\ E0(1 ] 2É5c) (1)

where c is the volume concentration of Ðller. Seriousdepartures occurred for systems with high concentra-tions of reinforcing Ðllers.

Kontou and Spathis5 proposed the relation :

E\ E0(1 ] 0É67 f c ] 1É62 f 2c2) (2)

where f is a factor describing the asymmetric nature ofthe aggregated clusters as expressed by the ratio of theirlength to width. Unambiguous Ðtting of these theoreti-cal relations is difficult owing to the fact that strain con-tinues to change after stress is applied. It is thereforeimportant to perform equilibrium stressÈstrain mea-surements in order to obtain molecular information.

In this study, the second stressing cycle has beenemployed in order to obtain nearÈequilibrium stressÈstrain measurements. The e†ect of IIR variation on theelastic behaviour of the ternary rubber-Ðlled vulca-nizates has been studied. The MooneyÈRivlin param-eters and have been evaluated with the use of2C1 2C2the strain-ampliÐcation factor

EXPERIMENTAL

StyreneÈbutadiene rubber (SBR-1502), natural rubber(NR) and butyl rubber (IIR) were blended together indi†erent ratios using mechanical mixing. All materialsused were supplied by The Transport and EngineeringCompany (TRENCO, Alexandria, Egypt). All blends

were loaded with the same concentration of carbonblack (40 phr of HAF-N330). The rubber mixtures wereprepared on a two-roll mill with diameter 170 mm,working distance 300 mm, speed of slow roll 24 rpm, nipgap less than 3 mm, gear ratio 1É4 and time of mixing30 min. Table 1 lists all ingredients of the blend com-position according to the sequence of their addition.Curing time was deduced using a Monsanto Rheometerat 150 ^ 2¡C.

StressÈstrain measurements of the second stressingcycle were made in simple extension on specimens withan average cross-sectional area of 2 mm] 6 mm. Refer-ence marks were made on each specimen about 36 mmapart. The vulcanizates were tested after an initialstressing of 17 ] 105N m~2, and then relaxed. Thesamples were allowed to relax for 30 min and then theywere stretched for a second time. The measurementswere carried out with an Instron type 1121 tester atroom temperature and at a crosshead speed of5 mm min~1. The force was measured after 15 min foreach application of a load, in order to obtain near-equilibrium measurements. The distance between thereference marks was measured with a cathetometer(precision ^0É001 cm).

RESULTS AND DISCUSSION

Near-equilibrium stressÈstrain measurements werecarried out for all vulcanizate samples. The data arepresented in Fig. 1, according to the MooneyÈRivlin11equation :

f [A0(j [ j~2)]~1\ 2C1] 2C2 j~1 (3)

where f is the force required to maintain the sample atan elongation ratio j which is deÐned as 1] e, with ethe strain produced by stress p, is the unstrainedA0cross-sectional area of the sample, is a term per-2C1taining to ideal elastic behaviour, and is a term2C2which expresses departures from ideal elastic behaviour.

TABLE 1. Composition of the rubber samples (S1 to S9)

Ingredients (phra) S1 S2 S3 S4 S5 S6 S7 S8 S9

SBR(1502) 10 30 60 10 60 30 100 — —

NR 30 10 30 60 10 60 — 100 —

IIR 60 60 10 30 30 10 — — 100

Stearic acid 2 2 2 2 2 2 2 2 2

HAF(N-330) 40 40 40 40 40 40 40 40 40

Processing oil 10 10 10 10 10 10 10 10 10

MBTSb 2 2 2 2 2 2 2 2 2

PbNc 1 1 1 1 1 1 1 1 1

Zinc oxide 5 5 5 5 5 5 5 5 5

Sulphur 2 2 2 2 2 2 2 2 2

a Parts per hundred parts by weight of rubber.

b Dibenthiazyl disulphide.

c Phenyl-b-naphthylamine.

POLYMER INTERNATIONAL VOL. 42, NO. 1, 1997

Elastic behaviour of vulcanizates 87

Fig. 1. Stress-strain curves in the second stressing cycle of allÐlled rubber vulcanizates samples reinforced with di†erent

amounts of IIR.

The behaviour of blend vulcanizates at large defor-mations can be conveniently described by the MooneyÈRivlin equation. Each of the curves shows a linearrelation over a range of low to moderate extensions.The straight line obtained by Ðtting the experimentalpoints with a least-squares analysis can be extrapolatedto inÐnite deformation (j~1\ 0) and the value of thereduced force is then i.e. phantom limit. In a2C1,similar manner, extrapolation to zero deformation(j~1\ 1) leads to which is not far from(2C1] 2C2),the modulus recorded by measurements at small defor-mations, i.e. the affine limit. The constant is the2C2slope of the line.12,13

The estimation of the constants and2C1 2C2becomes difficult at higher elongations, where all theexperimental stressÈstrain curves show pronouncedupturn, as is clear from Fig. 1.

In Fig. 2, the constants and have been2C1 2C2obtained with the use of the strain-ampliÐcation factor

X, which takes into account both the disturbance ofstrain distribution and the absence of the deformationof Ðllers.5 The tendency of carbon blacks to agglomer-ate into chain-like clusters is also taken into consider-ation, and this factor X for the three individualmixtures (S7, S8 and S9) is given by :

X \ p/eE0\ E/E0\ 1 ] 0É67 fc] 1É62 f 2c2 (4)

where e is the strain produced by stress p and is theE0modulus of each matrix, implying that the local strainsare, on average, X times greater than the overall strain.The value of the shape factor f in eqn (4) was adjustedto obtain the best Ðtting to the experimental data. Theextension ratio in eqn (3) should be recalculated fromthe measured overall strain by putting

"\ 1 ] Xe (5)

Now, the expected value of of each blend is givenE0by

E0(ternary blend)\ ;i

E0iRi\ E0(SBR) RSBR

] E0(NR)RNR] E0(IIR)RIIR (6)

where is the fraction of rubber in the ternary blendRimatrix. Also, the value of the strain-ampliÐcation factor

of each blend can be obtained from:XiX \ ;

iXiRi\ XSBRRSBR ] XNR RNR] XIIRRIIR (7)

where has the same meaning as above.XiIn Table 2, values of the shape factor f, the strain-

ampliÐcation factor X, the expected modulus of theternary blend matrix the Mooney parametersE0 , 2C1and and the calculated values of the modulus of2C2 Eithe ternary blends are presented. Values for the purecomponents were evaluated for the purpose of compari-son with the blends.

The results thus obtained are quite satisfactory inmost cases, supporting the use of the variable strain-ampliÐcation factor X.

TABLE 2. Values of constants and obtained from stress–strain curves by use of the strain-2C1

2C2

amplification factor and other calculations

Sample IIR f X E0

2C1a 2C

2C

2/C

1G

expb E (eq. 4)

Code (phr) (105 N mÉ2) (105 NmÉ2) (105 N mÉ2) (105 N mÉ2) (105 NmÉ2)

S1 60 4·77 4·01 1·15 4·72 1·99 0·42 6·71 4·61

S2 60 4·48 3·78 1·21 6·41 1·83 0·29 8·24 4·57

S3 10 4·37 3·66 1·45 4·80 2·67 0·56 7·47 5·31

S4 30 4·96 4·33 1·24 3·50 2·40 0·68 5·90 5·37

S5 30 4·23 3·52 1·39 4·38 2·21 0·51 6·59 4·89

S6 10 4·81 4·15 1·36 4·84 4·54 0·94 9·38 5·64

S7 — 3·79 3·09 1·60 4·89 4·97 1·02 9·86 4·94

S8 — 5·28 4·71 1·30 3·61 3·87 1·07 7·48 6·12

S9 100 5·48 3·97 1·00 — — — — 3·97

a Represents the Young’s modulus E.

b Represents the shear modulus at low strains.

POLYMER INTERNATIONAL VOL. 42, NO. 1, 1997

88 M. S. Sobhy

Fig. 2. StressÈstrain curves of Fig. 1 replotted with the use of the variable strain-ampliÐcation factor.

POLYMER INTERNATIONAL VOL. 42, NO. 1, 1997

Elastic behaviour of vulcanizates 89

As observed from Table 2 for the series of blends inwhich IIR was varied, there is an increase in the con-stant as the IIR content increases. As discussed2C1before, describes the behaviour of the rubber blend2C1and is a direct reÑection of the number of networkchains per unit volume, according to theories of rubberelasticity.14 This could suggest formation of homoge-neous blends with high crosslink density with the lowerpercentage (10 phr) of IIR. Also, it would conÐrm thesuggestion7 that the IIR phase is more miscible in theNR phase. On the other hand, carbon black prefers tomigrate into the SBR phase in the blend. The non-uniform distribution of carbon black can inÑuencevarious properties such as the shape of the stressÈstraincurves, (as clearly observed in Fig. 2) and YoungÏsmodulus E. However, during mechanical mixing ofcarbon black with elastomers having a low degree ofunsaturation, e.g. butyl rubber, sufficient interaction,primarily chemisorption, tends to prevent any sub-sequent transfer of the carbon black.9,15

Values of the constant generally appear to be2C2slightly lower than those of as would be expected in2C1the case of the second stressing cycle. This is mainlybrought about during the Ðrst deformation of therubberÈcarbon black system, i.e. decrease of the total(physical and chemical) crosslink density.5,16

Finally, the variation of the calculated Mooney con-stants and upon IIR addition is represented by2C1 2C2the ratio presented in Table 2, owing to the factC2/C1that the attachment between rubber molecules andcarbon black particles makes a contribution to thenature of the interphase bonding of the rubbernetwork,6 acting as a physical crosslink.

CONCLUSIONS

The e†ect of IIR variation on the elastic behaviour ofthe ternary rubberÈcarbon black system vulcanizates isunderstandable in terms of the ratio parameter C2/C1.The higher value of for the crosslinked networkC2/C1is explained by a spatially non-random distribution ofrubbers of high saturation (NR and SBR). The resultsobtained show that ternary blends having up to 30 phrof IIR have acceptable compatibility, i.e. they are homo-geneous blends. On the other, hand, the results forblends having 60 phr of IIR could indicate the forma-

tion of a partially crosslinked system embedded in theNRÈSBR continuous phase.

The procedure employed to Ðt the data obtainedfrom near-equilibrium stressÈstrain measurements bythe semiempirical Mooney equation may be modiÐed totake into account values of the modulus of the ternaryrubber-Ðlled vulcanizates by the use of a variable strain-ampliÐcation factor X.

The parameters and obtained by using the2C1 2C2strain-ampliÐcation factor are quite satisfactory in mostcases, supporting the assumptions about the nature ofthe interphase bonding between the ternary blends Ðlledwith carbon black.

ACKNOWLEDGEMENTS

It is a pleasure to express my special gratitude to Prof.Dr G. Hinrichsen for his support during this work inTU-Berlin. Also, many thanks to both Dr G. M. Nasr,Cairo University, Giza, Egypt, for his interesting sug-gestions, and Dr M. A. Sharaf, Cairo University, Beni-Suef, Egypt, for discussions on rubber elasticity.

REFERENCES

1 Rigbi, Z., Rubber Chem. T echnol., 55 (1982) 1180.2 Kraus, G., Angew. Makromol. Chem., 60/61 (1977) 215.3 Payne, A. R., in Reinforcement of Elastomers, ed. G. Kraus, Wiley-

Interscience, New York, 1965, p. 76.4 Ulmer, J. D., Hess, W. M. & Chirico, V. E., Rubber Chem.

T echnol., 47 (1974) 729.5 Kontou, E. & Spathis, G., J. Appl. Polym. Sci., 39 (1990) 649.6 Kausch, H. H., Makromol. Chem. Macromol. Symp., 48/49 (1991)

155.7 Cotton, R. G. & Murphy, L. J., Rubber Chem. T echnol., 61 (1988)

609.8 Medalia, A. I., Rubber Chem. T echnol., 51 (1978) 437 ; 59 (1986)

432 ; 64 (1991) 481.9 Sircar, A. K., Rubber Chem. T echnol., 54 (1981) 820.

10 Mullins, L. & Tobin, N. R., J. Appl. Polym. Sci., 9 (1965) 2993.11 Mark, J. E. & Erman, B., Rubberlike Elasticity, A Molecular

Primer. Wiley-Interscience, New York, 1988.12 Mark, J. E., Rubber Chem. T echnol., 48 (1975) 495 ; 55 (1982) 762.13 Sharaf, M. A. & Mark, J. E., J. Polym. Sci. : Part B: Polym. Phys.,

33 (1995) 1151.14 Porter, M., Skinner, T. D. & Wheelans, M. A., J. Appl. Polym. Sci.,

11 (1967) 2271.15 Roland, C. M., Rubber Chem. T echnol., 62 (1989) 456.16 Treloar, L. R. G., T he Physics of Rubber Elasticity, 3rd edn. Clar-

endon Press, Oxford, 1975.

POLYMER INTERNATIONAL VOL. 42, NO. 1, 1997