Polymer International Polym Int 49:216±222 (2000)

Some considerations concerning the dynamicmechanical properties of cured styrene–butadiene rubber/polybutadiene blendsAJ Marzocca,* S Cerveny and JM MendezUniversidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Departmento Fısica, Cuidad Universitaria, LPMPyMC, Pabellon1, Buenos Aires (1428), Argentina

(Rec

* CoUnivCont

# 2

Abstract: The dynamic mechanical response of several binary mixtures of a styrene±butadiene

copolymer and high cis-polybutadiene has been studied. The loss tangent and shear modulus were

measured with a free damping torsion pendulum at temperatures between 143 and 343K in argon

atmosphere. From the loss tangent data the glass transition temperature of each sample was evaluated.

The results can be represented by the Fox equation that relates the glass transition temperature of the

blend with that of constituent polymers. The in¯uence in the loss tangent data of the crystallization of

the high cis BR used in the blend is discussed. A study of the separation of the crystalline and

amorphous parts in the polybutadiene using the storage modulus data is presented. Finally, the loss of

crystallinity at different contents of SBR in the blend is analysed using the dynamic mechanical data.

# 2000 Society of Chemical Industry

Keywords: tand; blends; BR; SBR; crystallization; styrene±butadiene rubber; polybutadiens

INTRODUCTIONIn general, the main technical reason for blending two

elastomers is to achieve a compound with better

properties than those of the individual phases. Con-

sidering the structural behaviour, it is recognized that

while two rubbers may be virtually insoluble, elasto-

meric blends prepared by mechanical mixing may be

macroscopically homogenous. This condition is ob-

tained if mechanical mixing is intense enough and the

viscosities after mixing are suf®ciently high to prevent

gross phase separation.1

In an early work, Corish2 showed that blends of

rubbers having appreciable differences in their solubi-

lity parameters displayed two glass transition tempera-

tures (Tg) associated with each of the constituent

polymers. However, if the solubility parameters were

close, the blend could exhibit a unique value of Tg

intermediate between those of the constituent rubbers.

One interesting case to analyse is the compound of

polybutadiene (BR) and styrene butadiene rubber

(SBR). These blends are popular in the rubber

industry because of their uses in tread rubber

compounds in tyres.

Measurements of the loss tangent (tand) of uncured

samples of SBR/BR blends as a function of tempera-

ture3 show that two peaks appear related to the glass

transition of each phase. After the blend has been

vulcanized a unique peak is obtained, and it is stated

that the blend may function as a one-phase system.3,4

eived 14 April 1999; revised version received 28 July 1999; accepted

rrespondence to: AJ Marzocca, Universidad de Buenos Aires, Facuersitaria, LPMPyMc, Pabellon 1, Buenos Aires (1428), Argentinaract/grant sponsor: University of Buenos Aires; contract/grant number

000 Society of Chemical Industry. Polym Int 0959±8103/2000/$1

These facts suggest that the two-phase system is liable

to change to a homogeneous system when crosslinking

is produced.

By means of electron microscopy, Callan et al5

observed discrete zones of each phase, con®rming that

BR/SBR blends are not homogeneous at the molecular

level. Inoue et al6 studied the problem of covulcaniza-

tion of SBR/BR blends using light scattering analysis.

They concluded that there is a structural change in the

blend produced by the broadening of the polymer±

polymer interface promoted by the curing reaction.

The purpose of our paper is to analyse some

characteristics of the dynamic mechanical behaviour

of cured SBR/high cis BR blends ®lled with carbon

black. Firstly, the glass transition temperature ob-

tained from the location of the tand peak in the loss

tangent versus temperature diagram was analysed as a

function of the composition of the blend.

Secondly, the in¯uence of crystallization associated

with high cis BR on the loss tangent data of the blend

was studied, and a simple method is presented to

quantify this. The change in the loss modulus of high

cis BR when carbon black is added to the compound is

also discussed.

Finally, the covulcanization between the BR and

SBR phases, which would be responsible for the

presence of a unique tand peak associated with the

glass transition, was related to the rate constant per

allylic hydrogen in the compound.

27 October 1999)

ltad de Ciencias Exactas y Naturales, Departmento Fısica, Cuidad

: 01/TY05

7.50 216

Dynamic mechanical properties of SBR/BR blends

EXPERIMENTAL TECHNIQUESIn this work, cured compounds of polybutadiene (BR)

cis-1.4 96% and a copolymer of styrene±butadiene

(SBR-1502) ®lled with carbon black were prepared.

SBR-1502 is a commercial grade rubber produced by

a cold process that contains 23.5% bound styrene, ie a

molecular proportion in the chain of one styrene for

every six or seven butadienes. The chemical structure

of butadiene in the SBR copolymer consists of 55%

trans-1,4, 9.5% cis-1,4 and 12% 1,2-butadiene. The

molecular weights of both constituent elastomers were

measured by GPC, values of Mn of 176000g molÿ1

and 124800g molÿ1 being obtained for the SBR and

BR, respectively. The densities of the polymers were

0.935gcmÿ3 for SBR and 0.910gcmÿ3 for BR.

Six blends were prepared with different proportions

of each pure elastomer. The compositions of each

blend are given in Table 1. The compounds were

mixed in a laboratory mixer at 296K, with cold

running water at 77 rev minÿ1 and ram pressure of

700kPa. For samples A and F, the following cycle was

used: 0min, elastomer; 1min, half of black; 2min, rest

of black and rest of the ingredients; 3min, sweep and

dump at 478K.

For samples B, C, D and E a masterbatch with BR

and SBR was prepared previously, and then the rest of

the ingredients were added in the ®nal batch as in

samples A and F. Finally the compounds were banded

in a two-roll mill to give a ®nal sample thickness of

about 5mm.

Rheometer curves were measured using a Monsanto

MDR2000 instrument at 433K to obtain the cure

time, t100%, to achieve the maximum torque of the

curves.

Sample sheets of 150mm�150mm�2mm were

vulcanized at 433K up to time t100%, guaranteeing that

all the vulcanization reaction took place. The

measured densities and the t100% values of each cured

compound are given in Table 1.

Dynamic mechanical properties were measured on

strip 33mm�2.7mm�2mm samples cut from the

cured sheets using a die. Measurements of loss tangent

and shear modulus were performed with an automated

damped torsion pendulum in Ar atmosphere at

60torr.7,8 Tests were performed between 143K and

Table 1. Compound formulations (in phr), density and t100% (MDR2000, 433K)

Sample A B

SBR 1502 100 80

96% cis-1,4 BR ± 20

Carbon black HAF N-330 50 50

Zinc oxide 4 4

Stearic acid 2 2

Aromatic oil 15 15

Antioxidant 2 2

TBBS 1 1

Sulphur 2 2

t100% (min) 25.8 19.2

Density (gcmÿ3) 1.127�0.002 1.121�0.002 1

Polym Int 49:216±222 (2000)

343K using a temperature ramp of 1K minÿ1.

Oscillatory frequencies between 0.1sÿ1 and 20sÿ1

were achieved. The maximum shear strain in the

dynamic measurements was always less than 5�10ÿ3,

thus ensuring linear viscoelastic behaviour.

Application of the rigorous statistical model of ®lled

polymer9 networks to stress±strain tests allowed the

crosslink density to be estimated for each sample.

ASTM D412 samples for tensile tests were cut from

the cured sheets. Stress±strain curves were measured

with an Instron 4201 at room temperature at a

deformation rate of e=6�10ÿ4sÿ1. A load cell of

50N was used to obtain good sensitivity. Strain was

measured by a large deformation extensometer (In-

stron XL) with 20mm gauge length. A PC controlled

the tests with software made in Basic language. The

stress±strain curves were obtained for the three

different samples of a given sheet and the average

curve was calculated. The standard deviation in stress

of the three tests was less than 0.015MPa while for the

strain it was insigni®cant.

RESULTS AND DISCUSSIONFigures 1(a) and 1(b) show, respectively, tand and

shear storage modulus G' as a function of the

temperature for all cured blends that were studied. It

is known that the presence of carbon black in the

compound would affect the location in temperature of

the peak of tan d only slightly.10 The curves present a

single glass transition temperature, the value of which

drops between the glass transition of compounds made

only with one kind of elastomer (samples A and F).

This indicates that the blends behave as if they were

homogeneous from the point of view of their mech-

anical properties. When tan d is measured using a free

decay pendulum, depending on the type of elastomer,

a critical condition for the oscillation can be attained

near the glass transition temperature (Tg). In this

situation, it is very dif®cult to obtain a collection of

data in that temperature zone. This behaviour is

presented in samples A and B of our tests. Then, the

glass transition temperatures of each sample were

obtained from the maximum of a gaussian curve that

®tted the data of Fig 1(a). The values are given in

C D E F

60 50 40 ±

40 50 60 100

50 50 50 50

4 4 4 4

2 2 2 2

15 15 15 15

2 2 2 2

1 1 1 1

2 2 2 2

17.7 16.2 14.2 11.0

.123�0.002 1.120�0.002 1.114�0.002 1.102�0.002

217

Figure 1. (a) Loss tangent (tand) as a function of the temperature of SBR/BR blends. The dashed lines in samples A and B correspond to the fittinggaussians. (b) Storage modulus G' as a function of the temperature of SBR/BR blends.

Figure 2. Glass transition temperature Tg of SBR/BR blends as a functionof weight percentage of BR (oBR) —, Fox equation.

AJ Marzocca, S Cerveny, JM Mendez

Table 2. The frequency obtained at the glass transition

temperature was in the range 0.35±0.98Hz; the shift of

Tg due to this frequency variation is negligible.

Some features can be pointed out from Fig 1(a).

Firstly, at high presence of BR in the blend, there is a

decrease in the maximum value of tan d. Secondly, the

position of the maximum of each curve is related to the

proportion of both elastomers in the compound. When

a blend composed of two elastomers behaves as a

unique phase, the Fox law can be used to obtain the

glass transition temperature of the blend11

1

Tg

� !1

Tg1

� !2

Tg2

�1�

Table 2. Glass transitiontemperature (Tg) and areabelow the peak (O) of tandcurves of figure1.a

Sample Tg (K) O (K)

A 239�2 15.0

B 227�2 23.9

C 216�2 34.1

D 209�2 38.4

E 198�2 38.5

F 186�2 56.3

218

where oi is the weight fraction of the constituent

elastomer. Tg1 and Tg2 are the glass transitions of the

compound made only with the pure elastomers 1 and

2, respectively.

Figure 2 shows the result of the application of eqn

(1). From this ®gure it can be concluded that the Fox

law applies over the whole range of compositions

studied. However, some dispersion in the results is

observed mainly in the 40 SBR/60 BR blend. This

effect was also pointed out by Sircar and Lamond,12

and it would be consequence of the inversion of the

phases during the uncured stage.

Other conclusions can be established by analysing

the area below the loss tangent peak for each sample

measured. Compound F shows two relevant peaks in

Fig 1 associated with the BR glass transition and

crystallization, respectively.13 In Fig 3, the G' values

are shown as a function of the temperature of the

compound together with the curve of cured gum of the

same composition without carbon black.

In a polymer that crystallizes, such as high cis BR,

the crystalline and amorphous phases have separate

contributions to the modulus. Following Kundu and

Tripathy14 any relaxation property could be expressed

in the two limits of the Voigt and Reuss schemes.

Then, the storage modulus can be introduced as

G0v � naG0a � �1ÿ na�G0c �2�

1

G0R� na

G0a� �1ÿ na�

G0c�3�

where n is the volume fraction and the subscripts a and

c correspond to the amorphous and crystalline phases,

respectively.

In order to consider the in¯uence of the carbon

black fraction (f) in the storage modulus of the BR

compound, the Guth±Smallwood relationship is

used15

G0

G00� � � 1� 2:5�� 14:1�2 �4�

Polym Int 49:216±222 (2000)

Figure 3. Storage modulus G' as a function of the temperature BRcompound and BR gum. For details see text. Figure 4. Normalized loss tangent (tandn) of SBR/BR blends.

Dynamic mechanical properties of SBR/BR blends

where G' and G'0 are the shear modulus of the ®lled

and un®lled material, respectively.

Sirkar and Lamond12 analysed the in¯uence of the

carbon black in the crystallization of BR and con-

cluded that for 50phr of black in the compound, the

loss of crystallinity is around 10%. This fact can then

be considered in analysing our results of the dynamic

mechanical properties of the BR compound (sample

F).

In Fig 3 we have named the curve of the BR gum

vulcanizate I and that of the BR compound II.

Following the ideas mentioned previously, we could

consider the following relationships based on the Voigt

equation:

G02I � naG

0a � �1ÿ na�G0c �5�

G02II � G

01I ��na � �0� �G0c��1ÿ na ÿ �0� �6�

The superscripts 1 and 2 correspond to the storage

modulus at one temperature in the rubbery zone and

in the crystallisation zone, respectively (Fig 3). In eqn

(6)G'a can be changed to G01I . The factor b0 was

introduced in eqn (7) to consider the change in the

volume fraction of the crystalline phase due to the

presence of carbon black in the compound.

Then using eqns (6) and (7), it is easy to obtain

na � G02II ÿ �fGI

02 � �0�G01I ÿG

02I �g

G02II ÿ �G02

I

�7�

From the values of G' in ®g 3 and considering

b0=0.1,13 na=0.89 is obtained, and then

nc=(1ÿna)=0.11. Then replacing na in eqn (6),

Gc=7.8�108Pa is obtained. As was mentioned

previously, Gc is the shear modulus of the crystalline

phase of cis BR. It is interesting to note that this value is

as high as that obtained for this polymer in the glassy

state (Fig 3).

The presence of crystallization in BR affects the

width of the tand peaks in the SBR/BR blends. We

propose a simple methodology to estimate from this

mechanical dynamic data how much this in¯uences

the results. Firstly, all the curves of Fig 1(a) are

Polym Int 49:216±222 (2000)

normalized to the maximum tand values (Fig 4) that

correspond to the sample A (SBR 100phr). The

normalization to this curve is because SBR does not

crystallize, and our objective is to quantify the loss

crystallinity when SBR has been added to the blend.

The area below each tand peak is estimated by

subtracting the background of each curve and taking

the baseline into account. Particularly, sample F is

®tted by means of two gaussian curves to represent the

glass transition and the crystallization of the polymer,

and the area below each contribution is calculated (Fig

5). The area of each curve is given in Table 2. It can be

noticed that the values increase at higher content of

BR in the sample.

The contribution of the BR crystallization in the

area of each blended sample ci can then be calculated

as

ci � t

i ÿ !iA ÿ �1ÿ !i�0F �8�

where ti is the total area below the peak of sample i

(i =A,¼ F), OA the area below the peak in sample A

(100% SBR) and 0F the area below the peak of glass

transition in sample F obtained from the gaussian ®t

(Fig 5). Using the result of applying eqn (4), the loss of

crystallinity of each sample is calculated as

� ci =c

F, where cF is the area below the crystal-

lization peak of sample F. The loss of crystallinity is

shown in Fig 6 as function of the content of BR in the

blend. These results are similar to those obtained by

DSC measurements in SBR/BR carbon black ®lled

blends.12 It can be mentioned that the DSC measure-

ments require some special precautions in order to use

them in a proper way.12

The last point to be considered is the role of

crosslinking in the dynamic mechanical properties of

vulcanized blends. Callan et al5 assert than uncured

blends of SBR/BR are micro-heterogeneous with very

small domain sizes which are capable of covulcaniza-

tion, and thus give a single glass transition temperature

when cured. As a consequence, a broadening of the

thermal response given a diffuse T interval is

g

219

Figure 5. Normalized loss tangent of BR compound (sample F) withcrystallization. Two gaussian curves are shown using dotted lines.

Figure 7. Mooney stress sM (eqn (10)) vs deformation function f(l') forSBR, BR and 50SBR/50BR.

AJ Marzocca, S Cerveny, JM Mendez

obtained. The crosslink distribution in the blends

affects the mechanical properties of the material.

One way to estimate the crosslink density is by

means of the analysis of uniaxial stress±strain data of

the compounds. This is an alternative methodology to

obtain the crosslink density and it was demonstrated

that the results are similar to those achieved using

swelling techniques.16,17 Heinrich and Vilgis9 devel-

oped a rigorous molecular statistical model of ®lled

polymer networks with quenched structural disorder

coming from the chemical crosslinks between poly-

mers and from an ensemble of rigid and highly

dispersed multifunctional ®ller domains. This model

is an extension of a previous one that was successful in

explaining the mechanical behaviour of cured un®lled

rubbers.16±20

In the model, the relationship between uniaxial

applied stress s and the strain ε is given by

�M � �=��ÿ �ÿ2� � Gcr��� �Ge���f ��� �9�

where sM is called Mooney stress, l=1�e and

f ��� � 2

�

��=2 ÿ �ÿ��2 ÿ �ÿ1

�10�

Gcr��� � Gcr�1ÿ �� � kBT�f� �11�

Figure 6. Loss of crystallinity () as a function of weight percentage of BR.

220

Gcr is the contribution of the crosslinks to the shear

modulus and Ge that due to the entanglements; f is

the ®ller volume fraction, kB is the Boltzmann constant

and T the absolute temperature. b�1 in the case of

highly crosslinked systems,16,17 kf is the coupling

density of the contact polymer ®ller and is calculated

as9kf � AR�R=bPRwhere AR is the speci®c surface area

of the ®ller, rR the density of the ®ller and bPR is a

typical length scale of the coupling problem which can

be taken as the Kuhn's statistical segment length ls. In

the case of our compounds, the carbon black used was

N330 with AR=81 m2sÿ1 (ref 21) and �R=1.785

gcmÿ3. The value of ls can be taken as 1.06nm and

0.96nm for SBR and cis BR respectively.22 In the case

of blends, a mixture law can be used for kf.

The shear modulus Gcr=Gcr (f=0) of the corre-

sponding un®lled network is related to the polymer±

polymer junctions and is expressed by

Gcr � �RT=Mc �12�where r is the density of the polymer, R is the gas

constant and Mc the molecular weight between cross-

links.

In the case of ®lled rubber l should be replaced by

the intrinsic tension ratio l' in which the pure hydro-

dynamic effects of the ®ller particles are considered as

�0 � ��ÿ 1��eff � 1 �13�where weff is an ampli®cation factor9

�eff � 1� 2:5�eff � 14:1�2eff �14�

Polym Int 49:216±222 (2000)

Table 3. Crosslink densities (2Mc)ÿ1, crosslink modulus (Gcr) and kinetic parameters of SBR/BR blend at 423K

Sample Gcr (MPa) (2Mc)ÿ1�10ÿ5 (molgÿ1) n Rate constant, k (sÿ1)(103) k per allylic hydrogen (sÿ1)(103)

A 0.2713�2�10ÿ4 3.40�0.01 4.37 2.12 0.64

B 0.2790�2�10ÿ4 3.57�0.01 3.99 2.76 0.80

C 0.2896�2�10ÿ4 3.81�0.01 3.90 3.56 0.99

D 0.2915�2�10ÿ4 3.83�0.01 3.90 3.62 0.99

E 0.2813�2�10ÿ4 3.48�0.01 4.05 4.08 1.10

F 0.2933�2�10ÿ4 3.73�0.01 4.04 4.52 1.13

Figure 8. Normalized rheometer curves fitted to eqn (18) for SBR/BRblends at 423K: * experimental data; —, eqn (18).

Dynamic mechanical properties of SBR/BR blends

This relationship contains the effective ®ller volume

fraction feff, which is function of the speci®c proper-

ties of the carbon black ®llers. Medalia21 proposes a

relationship between feff and f

�eff � ��1� 0:5��1� 0:02139ÿDBP�0:685ÿ 1�� �15�

where ÿDBP is the dibutyl phthalate (DBP) adsorption

number in cm3 per 100g used as an empirical

measurement of the carbon black structure.

As an example, the generalized Mooney plot (sM vs

f(l')) of three compound (samples A, D and F) are

shown in Fig. 7. In order to calculate l', the eqns (15±

17) were used considering ÿDBP=102cm3 (100gÿ1)

for N330.21

Following eqn (10), the modulus Gcr (f) is obtained

from the linear part of the range of moderate

deformations in the plots of Fig. 7. When Gcr (f)

had been estimated, the crosslink density (2Mc)ÿ1 was

calculated for each sample using eqns (13) and (14) by

considering the density of the blends of polymer as

1=� � P2i�1

oi=�iwhere ri is the density of each polymer

phase. The values are given in Table 3. These results

indicate that the crosslink density increases slightly

when cis BR is added to the blend; however, it seems

that the crosslink density has higher values near the

composition 50phr SBR/50phr BR.

Normally, blends have a tendency to be an uneven

distribution of crosslinks between the phases. When

this distribution is nearly even, the properties generally

improve.23 The changes in the rheometer torque

curves re¯ect the variations of crosslink density in

cured samples. Isayev and Deng24 have discussed

isothermal curing kinetic models when analysing DSC

data of the vulcanization reaction in elastomers,

following which a kinetic equation for the state of

cure (a) of the form

� � fk�t ÿ t0�gn

1� fk�t ÿ t0�gn �16�

was proposed with interesting results. k is the rate

constant for the vulcanization, n is the order of the

kinetic equation, t0 is an induction time and t is the

reaction time. From the rheometer data, it can be

stated that

� �Mh ÿMt

Mh ÿMl

�17�

Polym Int 49:216±222 (2000)

where Mt is the torque at time t, and Mh and Ml are the

maximum and minimum torque, respectively. Using

eqns (18) and (19) the rheometer data of our blends

were ®tted with excellent agreement. Figure 8 shows a

comparison of the calculated and measured data,

pointing out the applicability of the concept. The

parameters obtained from eqn (18) are given in Table

3 for each blend.

Following Chough and Chang,25 hydrogens of allyl

and benzyl groups are much more reactive than those

of tertiary carbon, and are attacked by sulphur radicals

to make crosslinks. Considering that statistical repeat

units of BR and SBR have 4 and 3.3 allylic hydrogens,

respectively, the rate constants per allylic hydrogen are

given in Table 3 and the value diminishes at higher

contents of SBR in the blend. This result is consistent

with previous investigations.25 It is interesting to note

a fact that can be related to the homogeneous

behaviour of the cured SBR/BR regarding dynamic

mechanical properties compared with the heteroge-

neous systems such as, for example NR/BR or

221

AJ Marzocca, S Cerveny, JM Mendez

NR/SBR. In these cases, the rate constant per allylic

hydrogen decreases in the blend with respect to both

the pure cured elastomers,25 but this behaviour is not

present in cured SBR/BR where, as can be seen from

our results, a monotonous variation is observed. These

features could be associated with the co-vulcanization

(interphase crosslinking) between phases in highly

compatible polymers such as BR and SBR, or a more

even distribution of crosslinks in the phase. To

elucidate this problem more research must be done.

CONCLUSIONSThe dynamic mechanical properties of SBR/high cisBR blends have been analysed. The following points

were observed:

. The Fox equation can be used to obtain the glass

transition temperature of the blend over the whole

range of compositions.

. The storage modulus behaviour of the high cis BR

compound is explained by considering the crystal-

lization of the polymer and taking into account the

volume fractions of the amorphous and crystalline

phases. A simple way to obtain these quantities has

been discussed.

. The in¯uence of the crystallization of high cis BR in

the loss tangent of the SBR/BR blends was

measured by means of the area below the tand peak.

It is interesting to note that the results obtained for

loss of crystallinity are similar to those reported in

previous researches using DSC.

. The presence of a single peak in the loss tangent of

the blends can be related to the covulcanization

between the BR and SBR phases. The fact that the

rate constant per allylic hydrogen increases mono-

tonocally with the level of BR in the blend would

indicate a difference, regarding the vulcanization,

from other elastomeric blends.

222

ACKNOWLEDGEMENTSThis work was supported by the University of Buenos

Aires, Argentina. (01/TY05).

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Polym Int 49:216±222 (2000)