sbrinfluence of hydrogenation and styrene content

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J O U R N A L O F M AT E R I A L S S C I E N C E 3 4 (1 99 9) 1741– 1747

Inuence of hydrogenation and styrene contenton the unaged and aged properties ofstyrene-butadiene copolymer

M. DE SARKAR, P. P. DE, A. K. BHOWMICK ∗

Rubber Technology Centre, Indian Institute of Technology, Kharagpur-721302, India

The inuence of hydrogenation and styrene content on the properties of styrene butadienecopolymer (SBR) has been studied. There is an increase in glass transition temperature ( T g )associated with the reduction of double bonds by hydrogenation. The hydrogenatedpolymers demonstrate higher tensile strength, elongation at break and modulus values.The superior mechanical properties of HSBRs are due to the generation of polyethylenecrystallites. The thermo-oxidative stability of saturated polymer is much higher than that of its unsaturated analogue. The samples with higher styrene content show higher modulus,but lower tensile strength and elongation at break, due to lower crystallinity. Thethermo-oxidative stability of HSBR increases with increase in styrene content. C 1999 Kluwer Academic Publishers

1. IntroductionThe hydrogenation of polymers containing olenicunits play an important role in the development of polymer science and is one of the oldest polymer mod-ication reactions [1, 2]. Recent advances in polymertechnology have prompted a re-investigation of hydro-

genation of unsaturated polymers. Hydrogenation of diene elastomers improves both chemical as well asphysical properties of the existing polymers. Hydro-genated styrene-butadiene rubber (HSBR) can be pre-pared not only in the latex form, but also is an uniquethermoplastic elastomer [3]. Properties of such elas-tomers would obviously depend considerably on de-gree of saturation and proportion of styrene units. Pre-liminary reports are available on the hydrogenation of styrene-butadiene rubber latex [3, 4]. But the literaturesurvey indicates a lack of a detailed investigation on theeffect of level of hydrogenation and styrene content onproperties of HSBR, particularly after aging.

The present investigation is principally aimed at asystematic study of the effect of level of hydrogenationand styrene content on the aged and unaged propertiesof unvulcanized SBR.

2. Experimental2.1. MaterialsSBR latex with 17% styrene content, used as a substratefor hydrogenation was obtained from Nippon Zeon Co.Ltd., Japan. SBR lattices with styrene content 23 and28% were procured from Apcotex Lattices Ltd., India.Hydrogenated styrene-butadiene rubber (HSBR) wasprepared by diimide reduction of SBR latex [4]. Thesolid rubber was dried in vacuum before any test.

∗Author to whom all correspondence should be addressed.

2.2. Moulding of the samplesThe HSBR and SBR samples were compressionmoulded in an electrically heated Moore Press betweentwo aluminium foils at 150 ◦ C for 4 min at a pressureof 4 MPa to get sheets of 1 .75 ± 0.05 mm thickness.

2.3. Dynamic mechanical thermal analysisThe dynamic mechanical spectrum of the aged and un-aged materials were obtained using Dynamic Mechan-ical Thermal Analyser (DMTA MKII) from PolymerLaboratories Ltd., UK. All the samples (43 .5 × 13 .3 ×∼1.8 mm) were analysed in a dual cantilever bendingmode with a strain of 64 µ m (peak to peak displace-ment) in the temperature range from − 50to70 ◦ C. Fre-quencies selected were 0.1, 1 and 10 Hz. The testingrate was 2 ◦ C/min.

2.4. Wide angle X-ray diffractionspectroscopy

The aged and unaged samples were subjected to a Ironltered Co- K α radiation generated from Philips PW1719 X-ray generator at an operating voltage and cur-rent of 40 kV and 20 mA respectively. The diffrac-tion pattern of the samples was recorded with PhilipsX-ray diffractometer (PW-1710). Samples of the samethickness and area were exposed. The diffractions wererecorded in an angular range from 10 ◦ 2θ to 50 ◦ 2θ ata scanning speed of 3 ◦ /min. The crystalline and amor-phous components were separated by curve tting con-sidering the relatively high noise level of the diffrac-tograms; the base line was taken as a straight line inthe 2 θ range from 10 ◦ to 40 ◦ and no further corrections

0022–2461 C 1999 Kluwer Academic Publishers 1741



Figure 1 Typical X-ray diffractograms of HSBR with 94% saturation.

were applied. Theamorphouspeak of thedifferentsam-ples was found to be centered at 2 θ = 22 .7 ± 0.1, thelower limit being for the HSBR with 94% saturationand upper one for the control SBR. The amorphous re-ection contributions were subsequently resolved withcurve tting by a non-linear least square method un-der the assumption that the intensity peak prole couldbe approximated by a Gaussian function [5]. An illus-tration of the separation of the diffraction trace into itscomponentsis shown inFig.1 for HSBRwith94%satu-ration. The degree of crystallinity was determined fromthe ratio of areas under the crystalline peak and amor-phous halo according to Equation 1. The error in thecrystallinity determinations, is estimated to be ± 0.5%.

Crystallinity (%) = I c

I c + I a× 100 (1)

where I c = area under crystalline peak and I a = areaunder amorphous halo.

2.5. Infra-red spectroscopyInfra-red spectral analyses were performed in a PerkinElmer 843 double beam recording spectrophotometer.The samples were analysed as thin lms. The spectrawere taken with 5.4 cm − 1 resolution in the range of

wave numbers from 4000 to 400 cm−

1 at a noise levelof 0.1% transmittance.

2.6. Measurement of physical propertiesThe tensile strength, elongation at break and modulusat 100 and 300% elongation were measured on dumb-bell specimens [(BS-E) type, cut with a hollow punchfrom the test slab] according to ASTM D412-93 inZwick-1445 Universal Testing Machine at a strain rateof 500 mm/min at 30 ± 2 ◦ C. The average of three datapoints was taken and the experimental error was ± 5%(deviation from the median value).

2.7. Aging testsOxidative aging of the samples was carried out in aTest Tube Aging Tester (Seisaku SHO Ltd., Toyaseiki,

Japan)forvarying lengths of temperature and time. Thetime of aging was varied from 2 to 48 h and the tem-perature from 75 to 150 ◦ C.

2.8. Gel contentPercent gel content of unaged and aged samples weremeasured gravimetrically by immersing the samples

in toluene at 30◦C for 72 h, the equilibrium swellingcondition.

3. Results and discussion3.1. Effect of hydrogenation on unaged

properties of SBR3.1.1. Dynamic mechanical properties Dynamic mechanical properties of the control and thehydrogenated samples are displayed in Fig. 2. The con-trol SBR show a strong transition at − 37 ◦ C, whilethe HSBR samples with 74 and 94% saturation reg-

ister peaks at − 28 and − 18◦C respectively (Fig. 2a).This peak temperature is ascribed to the glass transition

(a)

(b)

Figure 2 (a)Temperaturedependence of losstangent(tan δ)forSBRandHSBRs with 74 and 94% saturation at a frequency of 10 Hz, (b) Tem-perature dependence of storage modulus ( E ) for SBR and HSBRs with74 and 94% saturation at a frequency of 10 Hz.

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TABLE I Effect of degree of hydrogenation on dynamic mechanical properties of SBR and HSBR with 17% styrene content

E (Pa)Degree of Degree of

Samples hydrogenation (%) T g (◦ C) tan δ at T g at T g at 30 ◦ C crystallinity (%)

SBR 0 − 37 1.42 0 .44 × 109 0.31 × 108 —HSBR 74 − 28 0.94 0 .50 × 109 0.33 × 108 3.5HSBR 94 − 18 0.42 0 .89 × 109 1.9 × 108 8.4

temperature ( T g). It is clear that with increase in thesaturation level, the glass transition temperature of theamorphous rubber phase increases. There is an in-crease in T g of19 ◦ C at 94%hydrogenation ascomparedto the control sample. This may be due to the grad-ual replacement of amorphous segments by crystallineunits of HSBR. There is an increase of percent crys-tallinity from 3.5 to 8.4%, as the hydrogenation level isincreased from 74 to 94% (Table I). A similar shift in T gvalues towards high temperature, as observed by DSCexperimentswas reportedearlier byus [4]. It is also seen

from Table I that the peak tanδ

is decreased from 1.42for pure SBR to 0.42 for HSBRwith 94% saturation be-cause of the increase in crystallinity and appearance of more ordered structure. Thestorage modulus ( E ) valueof SBR at T g increases with increase in saturation dueto the same reason given above. The storage moduliof the various samples are plotted against temperature(Fig. 2b). The storage modulus at a temperature muchhigher than T g i.e. 30 ◦ C increases with increase in satu-rationdueto enhanced crystallinity. As shownin Fig. 2band Table I, there is an increase of storage modulus byapproximately 6 times with increasing the saturationlevel from 0 to 94%.

3.1.2. Physical properties The effect of hydrogenation on the physical proper-ties of HSBR samples with 74, 87 and 94% saturationhas been investigated. The stress-strain properties of HSBRs along with the control SBR are depicted inFig. 3. It is seen that the saturation of the butadieneunits changes the physical properties tremendously. Adrastic improvement in tensile strength and elongationat break is seen above 74% saturation. Below 74% sat-uration, the HSBR samples show comparable physical

properties as that of the control (not shown in gure forclarity). Table II records the values of tensile strength,elongation at break, work to break and modulus cal-culated for these samples. It is observed from the datathat tensile strength, modulus and elongation at break increase with saturation. Control SBR shows a low ten-sile strength (0.8 MPa) with low elongation at break

TA BLE I I Effect of degree of hydrogenation on physical properties of SBR and HSBR with 17% styrene content

Modulus (MPa)Degree of Tensile strength Elongation Work to break

Samples hydrogenation (%) 100% 300% (MPa) at break (%) (kJ/m 2 )

SBR 0 0.73 — 0.8 170 0.5HSBR 74 0.79 1.00 1.3 692 3.1HSBR 87 1.28 2.10 5.4 991 12.2HSBR 94 1.53 2.35 6.8 981 15.5

Figure 3 Stress-strain curves for (a) SBR and HSBRs with (b) 74%,(c) 87% and (d) 94% saturation.

(170%), whereas, its 94% saturated counterpart showsa strength of 6.8 MPa with remarkably high elongationat break (981%). The superior mechanical propertiesexhibited by the HSBR samples are due to appearanceof polyethylene crystallites and increased crystallinityas observed by X-ray diffraction method (Table I). Thestrain induced crystallisation of HSBRs having highsaturation level can not be ruled out, as there is an up-turn in the stress-strain curve around 250% elongation.It is reported that hydrogenated nitrile rubber samplesregister a high value of tensile strength due to the abovereason [6].

3.2. Effect of hydrogenation on theproperties of aged SBR

In order to understand the effects of hydrogenation on

the properties of rubbers after aging, the behaviour of SBR and its 94% saturated version (HSBR-94) with17% styrene content was examined. Table III comparestheproperties of SBRandHSBR-94. It is readily appar-ent from the data that the inherent oxidation resistanceof hydrogenated materials as compared to the controlis more pronounced. Upon aging of both the samples

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Figure 6 Effect of aging time on degree of crystallinity of HSBR-94 atdifferent temperatures.

Figure 7 X-ray diffraction patterns of HSBR-94 before and after aging.

Figure 8 Effect of agingtime on 100%modulus ofHSBR-94at differenttemperatures.

increase in aging time at 75 ◦ C. At 125 ◦ C, the ten-sile strength decreases rst, then registers an increasebefore a nal decrease. At 150 ◦ C, the tensile strengthdecreases gradually with time. It is interesting to no-tice that at all temperatures, the tensile strength drops

down at the initial stage of aging. The changes in ten-sile strength with temperature and time of aging are inline with changes in degree of crystallinity (Fig. 6). Thedegree of crystallinity decreases at higher temperatureand for prolonged time of aging. It is again clear from

Figure 9 Effect of aging time on elongation at break of HSBR-94 atdifferent temperatures.

the X-ray diffractograms of unaged and aged HSBR-94samples (Fig. 7). Unaged HSBR-94 shows a major re-ection at 24.6 ◦ along (110) plane which may be due tothe presenceof crystallinepolyethylene segments.Withincrease in temperature or time of aging, the intensityof this peak decreases.

The modulus values, however, increase with increasein aging temperature as shown in Fig. 8 inspite of thedecrease in crystallinity. The increase in modulus withincrease in aging time or temperature is due to the ap-pearanceof cross-linked or gelledstructure(Scheme1).Increase in temperature or prolonged time of aging as-sists gel formation. HSBR-94 forms 12% gel, while thecontrol SBR aged for 48 h at 150 ◦ C forms 92% gel un-der similar condition due to presence of unsaturation(Table III). At very high aging time or temperature,

the gelled network disintegrates, lowering the values of modulus.

Theelongationat breakvaluedecreases with increasein aging time at all temperatures (Fig. 9) due to theformation of the cross-linked structure and subsequentdisintegration.

The rate of change in tensile strength with aging timewas found for HSBR-94 from the initial linear regionof the plot of percent retention in tensile strength ver-sus time of aging at various temperatures (Fig. 10).

Figure 10 Plot showing retention of tensile strength against aging time.Inset shows Arrhenius plot.

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Figure 11 Stress-strain curves for HSBRs having (a) 17% (b) 23% and(c) 38% styrene content and with same saturation level ( ∼94%).

(a)

(b)

Figure 12 (a)Plot showing retentionof tensile strength against aging time at 150 ◦ C for HSBRswith 17,23 and28% styrene content, (b) Plot showingretention of elongation at break against aging time at 150 ◦ C for HSBRs with 17, 23 and 28% styrene content.

The activation energy for the property deteriorationwas calculated using the Arrhenius equation. A typi-cal Arrhenius plot is shown as an inset in Fig. 10. Theactivation energy of deterioration of tensile strength isfound to be 40 kJ/mol. It may be mentioned that theactivation energy value for thedegradation of lled vul-canized EPDM rubber was reported to be 50 kJ/mol inthe temperature range of 125–175 ◦ C [9].

3.3. Effect of styrene content on the agedand unaged properties of HSBR

An investigation on the inuence of styrene content onthe physical properties of HSBR was undertaken. Thestress-strain properties of theHSBR samples with vary-ing amounts of styrene content are plotted in Fig. 11.The tensile strength of the samples decreases with

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TA BLE V Effect of styrene content on physical properties of HSBR

Level of hydrogenation (%) a 94 94 94Styrene content (%) b 17 23 28Degree of crystallinity (%) c 8.4 4.3 1.6Tensile strength (MPa) 6.8 5.5 4.2Elongation at break (%) 989 470 205Work to break (kJ/m 2) 15.5 7.4 2.7Modulus (MPa)

100% 1.53 2.18 2.77

300% 2.35 4.18 —aAs determined by infra-red spectroscopy.bAs determined by NMR spectroscopy.cAs determined by X-ray diffraction studies.

increase in styrene content. This may be due to de-crease in crystallinity. The HSBR sample with 28%styrene level contains only 1.6% crystallinity, whereasthe samples with 17 and 23% styrene level contain 8.4and 4.3% crystallinity respectively, though these sam-ples have almost same saturation level (Table V). Now,higher level of butadiene (lesser styrene level) allowslonger sequential runs of hydrogenated butadiene seg-ments (polyethylene segments) to form. In general, thelonger the runs, the more complete their saturation andthe higher the percent crystallinity. The crystallite sizeis ultimately limited by a styrene unit (whichis toolargeto be incorporated into the polyethylene crystallites).

Ageing behaviour of HSBR samples with differentstyrene contents is depicted in Fig. 12. The tensilestrength and elongation at break values decrease muchmore rapidly in HSBR with 17% styrene content com-pared to HSBRs with higher styrene content. HSBRwith 28% styrene content retains 95% of its tensile

strength and 50% of elongation after aging at 150 ◦ Cfor 48 h, whereas, HSBRs with 17% styrene contentretains 69% tensile strength and 30% of elongation un-der the same condition. The thermo-oxidative stabilityof HSBR samples increases with increase in styrenecontent while aged at the temperature range studied(75–150 ◦ C).

4. ConclusionsThe effect of hydrogenation and inuence of styrenecontent on the aged and unaged properties of styrene-

butadiene copolymer has been investigated in thispaper.

(1) There is an increase in glass transition temper-ature associated with the reduction of double bondsby hydrogenation as observed by Dynamic MechanicalThermal Analysis.

(2) The hydrogenated polymers are found to be su-perior in terms of mechanical properties. The tensilestrength, elongation at break and modulus values in-crease with increase in saturation level. The superior

mechanical properties of HSBRs are due to the appear-ance of polyethylene crystallites, as evidenced from theX-ray diffractograms.

(3) The thermo-oxidative stability of the hydro-genated polymer is higher than that of the control.

(4) The samples with higher styrene content showhigher modulus, but lower tensile strength and elonga-tion at break due to lower crystallinity.

(5) The thermo-oxidative stability of HSBR sampleswith same saturation level increases with increase instyrene content.

AcknowledgementOne of the authors (M. De Sarkar) is grateful to CSIR,New Delhi for providing research fellowship to carryout the work.

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nology,” Vol. 7 (Interscience, New York, 1967) p. 557.2. J . W I C K L AT Z , in “Chemical Reactions of Polymers,” edited by

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Rubber Chem. Technol. 67 (1994) 288.

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Polym. J. 10 (1978) 315.6. S . B H AT TA C H A R J E E , A . K . B H O W M I C K and B . N .

AVA S T H I , Ind. Eng. Chem. Res. 30 (1991) 10867. J . E . F I E L D , D . E . W O O D F O R D and S . D . G E H M A N ,

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bility 16 (1986) 221.

Received 29 Julyand accepted 15 October 1998

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sbrinfluence of hydrogenation and styrene content

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