Polymer International Polym Int 53:1426–1430 (2004)DOI: 10.1002/pi.1393

Effect of heterogeneities on the physicalproperties of elastomers derived frombutadiene cured with dicumyl peroxideL Gonzalez,∗ A Rodriguez, A Del Campo and A Marcos-FernandezInstituto de Ciencia y Tecnologia de Polimeros (CSIC), Juan de la Cierva 3, 28006 Madrid, Spain

Abstract: The scope of this paper is to continue our earlier study of the peroxide curing of elastomersderived from butadiene. Experimental evidence indicates that abstraction of allylic hydrogen and additionto the double bonds can act as a mechanism for cross-linking. The addition reaction may give rise to apolymerization reaction between adjacent double bonds, thus generating a heterogeneous network, withnegative effects on the physical properties of the vulcanizates. 2004 Society of Chemical Industry

Keywords: rubber vulcanization; cross-linking; dicumyl peroxide; physical properties; networks

INTRODUCTIONIn general, vulcanization of rubbers with peroxides isof considerable practical interest, and a wide variety ofelastomers, principally saturated rubbers, can be curedin this way. The vulcanizates show low compressionset and creep behaviour. Nevertheless, this cure systemdoes have disadvantages for unsaturated rubbers,with the vulcanizates then presenting low tensileand tear strength properties. In addition as peroxidedecomposition is a first-order reaction, it is onlyaffected by temperature, and presents a lack of delayedaction during cure. These factors have restricted theiruse in diene rubbers.

At present, many peroxides are available, andit is reasonable to expect that further research inthis area will result in peroxides providing a moredesirable balance of properties for some uses thanthose currently available.

The mechanism of peroxide vulcanization has beenthe subject of important reviews by Loan,1 Elliot andTidd,2 Parks and Lorenz,3 Braden and Fletcher,4

Moore and Watson5 and Dluzneski.6

Many works ‘suggest’ that under suitable condi-tions, the efficiency should approach 100 %, whichmeans that one cross-link would be formed for everymolecule of peroxide decomposed. The cross-linkingreaction involves the homolytic decomposition of theperoxide, and all of the radicals thus formed reactby abstraction of hydrogen atoms with formation ofpolymeric radicals. The combination of two polymericradicals leads to the formation of one cross-link. Thesecross-links consist exclusively of carbon–carbon bondsbetween polymeric chains.7

The radical peroxide, in the presence of dienerubbers, in addition to abstraction of a hydrogenatom in the allylic position, can attach to adouble bond and initiate polymerization betweenadjacent double bonds, on different chains, thusgenerating a small but densely cross-linked polymercluster.8 The presence of such heterogeneities inthe cross-linked network, with regions densely cross-linked and regions weakly cross-linked, are of greatimportance to static properties, such as mechanicalstress–strain behaviour.9 As a continuation of ourearlier studies,10 in this present work we haveinvestigated the cross-linking of elastomers derivedfrom butadiene rubber (BR), styrene–butadienerubber (SBR) and nitrile–butadiene rubber (NBR)with dicumyl peroxide systems in which bothmechanisms are active and therefore a heterogeneousnetwork is obtained. The physical properties of theirvulcanizates are compared with those obtained witha saturated (hydrogenated) butyl rubber (HNBR),which does not present the addition mechanism,and therefore does not form heterogeneities in thenetwork. This different behaviour between thesetypes of rubbers allows us to evaluate the effect ofheterogeneities on the ultimate properties of rubbervulcanizates.

EXPERIMENTALMaterialsDicumyl peroxide (DCP) was obtained from Merck,and recrystallized from methanol and water before use.

The BR used was Intene 50, manufactured byEniChem, with the following isomeric distribution:

∗ Correspondence to: L Gonzalez, Instituto de Ciencia y Tecnologia de Polimeros (CSIC), Juan de la Cierva 3, 28006 Madrid, SpainE-mail: lgonzalez@ictp.csic.es(Received 24 January 2003; revised version received 5 June 2003; accepted 3 July 2003)Published online 22 July 2004

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Physical properties of elastomers derived of butadiene

1,4-cis, 35 wt%; 1,4-trans, 45 wt%; 1,2-vinyl, 20 wt%.The SBR used was Intol 1500, an emulsion-polymerized material, manufactured by EniChem, forwhich the microstructure of the butadiene monomerunits in the copolymer is as follows: 1,4-trans, 72 wt%,1–4-cis, 12 wt% and vinyl, 16 wt%, and with a styrenecontent of 23 wt%. The nitrile rubber used was Krynac825, a cold-polymerized material containing 61 wt%butadiene, manufactured by Bayer. The hydrogenatednitrile rubber used was Tornac A, manufactured byBayer, with 34 wt% acrylonitrile, and less than 1 %residual double bonds.

Blending and curingThe masterbatches were compounded on a two-rollmill, at a temperature of 40–50 ◦C. Measurements ofthe degree of curing were conducted in a MonsantoMoving Die Rheometer (Model MDR 2000E) and ina Rubber Process Analyser (RPA 2000), both fromAlpha-Technologies. All samples for physical testingwere cured in a thermofluid press.

Physical testingTensile strength tests were performed at roomtemperature in an Instron tensile tester (Model 4361)with a grip separation speed of 50 cm min−1. Thetest samples were cut out from the vulcanized sheetsby using a microtensile dumbell-type die. All tensileresults reported are the average values obtained fromfive tests.

Determination of network chain densityThe volume fraction of rubber in the swollen networkof the vulcanizates, Vr, was determined by means ofequilibrium swelling in toluene at 30 ◦C for BR, SBRand NBR, and in methyl ethyl ketone for HNBR.The equilibrium swelling ratio was used to calculatethe chemical network chain density, applying theFlory–Rehner equation.11

Infrared spectraA Perkin-Elmer spectrophotometer (Model SpectrumOne NTS) with a Perkin-Elmer Universal ATRsampling accessory was used to record spectra of thecured samples. The resolution was 4 cm−1, and thespectra were the average of four scans.

RESULTS AND DISCUSSIONThe formulations prepared were 100 parts by weightof rubber plus 3 parts by weight of DCP. The cross-linking process was followed with an oscillating dierheometer at 150, 160 and 170 ◦C. Figure 1 showsthe elastic modulus S′ for the different compoundscured at 170 ◦C. The rheogram of the BR materialdoes not yield a curve with the typical shape.12 Therheogram up to high conversion follows a straightline, and the increase of the torque seems to dependonly on the reaction time, which suggests a zero-order

reaction. The torque maximum of the BR is veryhigh with respect to the other rubbers. The rheogramsof SBR, NBR and HNBR present the typical curvesthat are expected. The rheograms were essentiallynon-revertible and with very short induction periods.

Figure 2 shows the loss modulus S′′ for the fourcompounds. The behaviour of S′′ for BR is surprising,being practically null during the first 20 min of curing,then increasing significantly before reaching a plateau.SBR also presents this increment in S′′, but far lessdramatically, while the increment is even lower forNBR. Finally, HNBR presents the usually expectedbehaviour during the whole cross-linking process.

0

40

80

0

20

40

60

BR

160

120

Tor

que,

S´

(dN

m)

Tor

que,

S´

(dN

m)

SBR

NBR

HNBR

0 400 60

Time (min)

20

Figure 1. MDR rheometer curves of the elastic shear modulus, S′, forelastomers cured at 170 ◦C, a frequency of 1.66 Hz and an angleof 0.5 ◦.

0 40 60

0

4

8

12

16

BR

20

Tor

que,

S ''

(dN

m)

Tor

que,

S ''

(dN

m)

Time (min)

HNBR

NBR

SBR

2.0

1.5

1.0

0.5

0.0

Figure 2. MDR rheometer curves of the viscous shear modulus, S′′,for elastomers cured at 170 ◦C, a frequency of 1.66 Hz and an angleof 0.5 ◦.

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L Gonzalez et al

The cure data obtained for the compoundscured at 170 ◦C are given in Table 1. The torqueincrement �T (Tmax − Tmin) depends on the butadienecontent in the rubber, with a maximum for BR,100 wt% of unsaturation, followed by SBR, 77 wt% ofunsaturation, then NBR, with 61 wt% of unsaturationand finally HNBR, which is almost fully saturated.

In Table 2, the �T values for BR and HNBR,cured at 150, 160 and 170 ◦C, are given. The �Tvalue reduction for BR is very dependent on increasingcuring temperature, while for HNBR the �T valueswere practically constant. These elevated values of�T for BR, decreasing with curing temperature,and associated with significant increments in theloss modulus S′′, lead us to think that crackswere developing in the rheometer die under sheardeformation. For this reason, we resorted to a RubberProcess Analyser instrument (RPA 2000) to study thecure state over a wide range of strain amplitudes.Figure 3 displays the rheograms for the polybutadieneIntene 50 cured at 170 ◦C and angles of 0.5, 1 and5 ◦, which correspond to 7, 14 and 70 % sheardeformations respectively. In the vulcanization of thesample at 0.5 ◦ (7 % strain), S′′ increases when S′ isat its maximum, with the peroxide being practicallyexhausted. In the sample cured at 1 ◦ (14 % strain),S′′ starts to grow when S′ is still increasing; here, theperoxide is not completely consumed yet and cross-linking proceeds, almost reaching a plateau. Finally,the sample cured at 5 ◦ (70 % strain) presents amaximum value of the loss modulus S′′, similar to thatobtained for the elastic modulus S′, which starts todevelop from the very beginning of the curing process.If we look at the behaviour of S′ in the sample subjectedto high deformation, cracks should be produced inthe vulcanizate within the rheometer chamber, witha correspondent decrease in the elastic modulus S′.However, because some peroxide is still available,after approximately 7 min curing a new increment inS′ is produced, due to new cross-links being formed.Effectively, the material extracted from the rheometerchamber is very stiff and brittle and the rheometercrushes the rubber while in the chamber.

Table 3 shows the variation in network chain densityfor all compounds vulcanized at 170 ◦C for theiroptimum times. The high network chain density foundin BR can be attributed to the ‘polymerization’ of

Table 1. Rheometer data obtained for the rubber compounds cured

at 170 ◦C

Parameter BR SBR NBR HNBR

t97 (min)a 13.2 13.4 11.8 8.6t�2 (min)b 0.38 0.57 0.71 1.07Tmax (dN m)c 114.5 25.5 16.7 10.1�T (dN m)d 113.2 24.6 16.2 9.3

a t97 = time to 97 % of maximum torque.b t�2 = time for 2dN m rise above minimum torque.c Maximum torque.d Torque increment.

Table 2. Variation of the torque increment, �T (dN m), for BR and

HNBR cured with 3 phr of DCP at different temperatures

Temperature (◦C) BR HNBR

150 147 8.9160 131 8.8170 113 9.3

Table 3. Network chain density of compounds cured at 170 ◦C for

their optimum time

Parameter BR SBR NBR HNBR

Specific gravity (g cm−3) 0.90 0.93 0.99 0.96Cross-link density

(×104 mol mm−3)a1.0 1.0 1.1 1.1

Cross-link density(×104 mol mm−3)b

54.0 4.8 3.1 1.8

a One cross-link formed per DCP molecule.b Calculated using the Flory–Rehner method.

0 10 20 30 40

0

50

100

150

200

0

50

150

1°

0.5°

5°

1° 5°

Time (min)

Tor

que,

S '

(dN

m)

200

100

Tor

que,

S "

(dN

m)

0.5°

Figure 3. RPA rheometer curves of the elastic and viscous shearmoduli for the BR elastomer cured at 170 ◦C, a frequency of 1.66 Hzand angles of 0.5, 1 and 5 ◦.

the double bonds of the unsaturated polymer. Thisreaction route can explain that the material was fairlystiff and brittle and that the rheometer rotor crushesthe rubber while in the chamber. This ‘polymerization’reaction decreases when the polymer unsaturation isreduced. In NBR, presumably the addition reaction ismore difficult because of the high content of electronwithdrawing nitrile groups. For HNBR, the networkchain density obtained is similar to theoretical values;in this case, the dicumyl peroxide has an efficiencyclose to unity, and only acts through the allylichydrogen abstraction mechanism.

If the peroxide radical gives rise to the ‘polymer-ization’ reaction, the number of double bonds in thepolymer should decrease, and this fact could be appro-priately checked by IR spectroscopy.10 The bands at

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Physical properties of elastomers derived of butadiene

966, 728 and 911 cm−1, which correspond to the out-of-plane deformations γ (=C–H) trans, γ (=C–H) cisand ω (=CH2) vinyl, respectively, show a consider-able decrease in the cured sample, for the cis and inparticular bands, while the trans band practically doesnot change.

Table 4 is a compilation of the physical properties ofthe compounds cured at 170 ◦C for their optimum curetimes. The tensile strength values for BR, SBR andNBR are very low, whereas HNBR presents a very highvalue. The Shore A hardness shows a decrease withpolymer unsaturation for the different compounds,and must be attributed to a decrease in the networkchain density for these vulcanizates.

Generally, it is accepted that the failure propertiesof some vulcanized elastomers reflect the presenceof ‘intrinsic’ flaws.12–17 These flaws concentratestress, which then promotes fracture and fatiguein the materials. In his review, Bueche18 attributedthe great difference in tensile strength between aperfect and an experimental network to a series offactors that obviously negatively affect the tensilestrength of rubbers, and has suggested that an oftenignored effect, which nevertheless is very important,is the presence of flaws in the cured sample. Norelationship is drawn between flaws and any parameteror characteristic of the network. It is possible to thinkthat a polymerization reaction, involving adjacentdouble bonds, generates a small but densely cross-linked polymer, or cluster,19 and obviously, when thisheterogeneous network is subjected to strain, the non-uniform stress distribution may induce flaw formationand then the anticipated rupture of the material.

We think that the presence of flaws is not an‘intrinsic’ phenomenon in vulcanized rubbers andtherefore are not present in the material from thebeginning; we suggest that they are the product ofthe cure process, and that they are formed whenthe material is subjected to strain. For unsaturatedrubbers, where a gradient in heterogeneity (or‘polymerization’) is produced, failure takes place alsoat different strains. The vulcanized HNBR materialpresents a very elevated tensile strength, and weassume that in this rubber, which is practicallysaturated, the peroxide radical acts only through thehydrogen abstraction mechanism, and therefore wedo not observe the formation of inhomogeneities inthe polymer network, which are generated when afairly high concentration of double bonds exists inthe polymer, and therefore the addition mechanismtakes place. HNBR cured with DCP generates a morehomogeneous network.

Table 4. Physical properties of compounds cured at 170 ◦C for their

optimum cure time

Parameter BR SBR NBR HNBR

Tensile strength (MPa) 2.5 1.2 3.1 18.2Elongation at break (%) 30 60 140 700Hardness (Shore A) 89 64 57 47

In general, the effect of strain amplitude onthe elastic modulus (E′) or loss factor (tan δ)in a ‘pure gum’ rubber compound is negligible,showing practically constant values for both properties.However, BR cured at 170 ◦C and subjected to strainsweep analysis, shows considerable changes. Figure 4shows the strain sweep data obtained at a temperatureof 30 ◦C, a frequency of 5 Hz and a strain amplitudein the range 0.2–12 %. In this case, the elasticmodulus decreases, while the loss factor increases.Such unexpected behaviour could be explained by theformation of flaws during the strain process as a resultof the presence of inhomogeneities in the network.

CONCLUSIONSResults obtained on DCP-crosslinked BR, SBR,and NBR elastomers provide evidence that theperoxide radical basically acts through allylic hydrogenabstraction and addition mechanisms. The latter causesadjacent polymeric chains with double bonds topolymerize, thus forming extremely densely cross-linked zones or clusters, this effect being moreimportant when the unsaturation content increases. IRspectroscopy reveals a significant decrease in doublebond content for these polymers. The existence ofdensely cross-linked zones has a negative effect on thephysical properties of the vulcanizates, and promotesearly failure of the materials, which we believe isnot due to flaws present in the vulcanizates in theabsence of any strain imposed. On the contrary, for theDCP-cured HNBR vulcanizate, the tensile strengthis significantly elevated with respect to those of theunsaturated butadiene rubbers, due to the fact that inthis practically saturated rubber the ‘polymerisation’reaction is not possible.

0 5 10 15

1.5 × 107

1.3 × 107

1.1 × 107

E '

(MP

a)

Strain (%)

0.08

0.10

0.12

0.14

0.16

Tan

δ

Figure 4. E′ and tan δ as a function of strain for the BR elastomercured at 170 ◦C.

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ACKNOWLEDGEMENTThe authors wish to thank the Comision Asesora dePolıtica Cientıfica for partial financial support (MAT-0905/1998).

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(1955).5 Moore CR and Watson WF, J Polym Sci 19:237 (1956).6 Dluzneski PR, Rubber Chem Technol 74:451 (2001).7 Bateman L, Moore CG, Porter M and Saville B, Chemistry of

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11 Flory PJ, Principles of Polymer Chemistry, Cornell UniversityPress, Ithaca, New York (1953).

12 Braden M and Gent AN, J Appl Polym Sci 3:100 (1960).13 Gent AN, Lindley PB and Thomas AG, J Appl Polym Sci 8:455

(1964).14 Greensmith W, J Appl Polym Sci 8:1113 (1964).15 Roland CM and Smith CR, Rubber Chem Technol 58:806

(1985).16 Roland CM and Sobieswki JW, Rubber Chem Technol 62:683

(1989).17 Roland CM, Rubber World 208(3):15 (1993).18 Bueche F, Rubber Chem Technol 32:1269 (1959).19 Gonzalez L, Rodrıguez A and Marcos-Fernandez A, Peroxide

crosslinking of diene rubbers, in Recent Research Developmentsin Polymer Science, Part II, ed by Pandalai SG, MacLaren andSons Ltd, London, pp 485–508 (1998).

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