Cross-Linking efficiency of styrene-butadiene rubber with dicumyl peroxide and evaluation of interaction parameter

Die Makromolekulare Chernie 109 (1967) 217-230 (Nr. 2527)

From the Department of Applied Chemistry, Indian Institute of Technology, Kharagpur, India

Cross-linking Efficiency of Styrene-Butadiene Rubber with Dicumyl Peroxide and Evaluation of Interaction Parameter

By S. K. BHATNAGAR* and S. BANERJEE

(Eingegangen am 14. April 1967)**

SUMMARY : The cross-linking efficiency of dicumyl peroxide with styrene-butadiene rubber (SBR-

1507) has been measured by the swelling method. The values are found to be 2.5 and 5.0 for unpurified and purified SBR, respectively. The lower value in the former case has been ascribed to the destruction of peroxy radicals by the antioxidants present in the rubber. The physical cross-links due to chain entanglement for purified SBR are found to be 0.14. mole/g of rubber (0.07. cross-links due to chain entanglement/g of rubber). The solubility parameter for purified SBR as determined from both swelling and viscosity measurements is found to be 8.45 (kcal/ml)lla or cohesive energy density as 71.403 kcal/ml. Using this value of the solubility parameter, the solvent-polymer interaction parameter is calculated and is further verified by HUMMEL’S method. The average value is found to be 0.37.

ZUSAMMENFASSUNG: Die Vernetzerwirksamkeit von Dicumylperoxid bei Styrol-Butadiengummi (SBR-1507)

wurde mit Hilfe von Quellungsmessungen erfal3t. Die erhaltenen Werte sind 2,5 bzw. 5 fiir ungereinigten bzw. gereinigten SBR. Der niedrigere Wert im ersten Fall wurde der Ver- nichtung von Peroxyradikalen durch Antioxydantien im Gummi zugeschrieben. Die physi- kalischen Vernetzungen, die beim gereinigten SBR auf Kettenverknauelungen zuriickzu- fiihren sind, betragen OJ4. Mol/g Gummi (oder 0,07. Vernetzungsstellen der Kettenverknauelungen/g Gummi). Die Loslichkeitsparameter fiir gereinigten SBR aus Quellungs- und Viskositgtsmessungen betragen 8,45 (kcal/ml)*/2 bzw. als Kohasions- energiedichte 71,403 kcal/ml. Mit diesem Wert des Loslichkeitsparameters wird der Lo- sungsmittel-Polymer-Wechselwirkungsparameter berechnet und durch die Methode von HUMMEL bestatigt. Der Durchschnittswert betriigt 0,37.

Introduction

The vulcanization of natural rubber by organic peroxides was first introduced by OSTROMISLENSKY l) in 1915. Since then peroxides have gained considerable importance in both natural and synthetic rubbers. They are especially useful when sulfur vulcanization is undesirable.

*f Rubber Laboratory, Bata Shoe Company Private Ltd., Batanagar, India. **) Revidiertes Manuskript vom 1. Juli 1967.

217

S. K. BHATNAGAR and S. BANERJEE

Amongst the peroxides available, dicumyl peroxide (DICUP) is excep- tionally suitable because of its simple chemistry of vulcanization. It is, in recent years, of much theoretical interest since it gives a quantitative estimate of cross-links. In natural rubber, it produces one -C-C- cross- link per molecule of peroxide decomposed2). In many synthetic rubbers, however, more than one cross-link is produced per molecule of peroxide decomposed.

In this paper, the cross-linking efficiency of dicumyl peroxide with styrene-butadiene rubber (SBR-1507), both purified and unpurified, is reported. The value of the FLORY-HUGGINS solvent-polymer interaction parameter has also been evaluated by HUMMEL'S methods) and from cohesive energy density measurements.

Experimental Materials

DICUP was obtained from Hercules Powder Co. (U.S.A.) and was purified before use. SBR was Goodyear's Plioflex, nonpigmented type, containing 23.5 yo bound styrene. Pale crepe (PLC-1X) was procured from Bata Shoe Company (India). The rubbers were purified by acetone extraction for two days to remove the antioxidants, etc. No significant change in the composition of rubber was observed. All the solvents were of AnalR grade and were fractionally distilled and the fractions distilling a t their respective boiling points were used.

Mixing and vulcanization Peroxide was mixed into the rubber on a laboratory size 2 roll miU a t 30 =k 5 O C with ut-

most care. The concentrations of peroxide were varied from 0.125 to 4.00 phr (partsper 100 parts of rubber). The vulcanization was carried out for 70 min a t 155 f 0.5OC in an electri- cally heated press. The vulcanizates were immediately cooled in distilled water and stored in cold and dark place.

Determination of cross-link density

.

For the estimation of the cross-link density, samples of approximately 1.0 cm diameter, 0.20 cm thickness and 0.30 g weight were punched out from the central portion of the vulcanizates and allowed to swell in benzene containing 0.5% phenyl-P-naphthyl amine a t 35 & 0.01 O C . Swollen samples taken out after 8, 16, 24, 32, 40, and 48 hrs intervals respectively were surface dried with filter paper and quickly weighed in tared, stoppered weighing bottles. The 48 hrs period was found to be sufficient for the attainment of swelling equilibrium. Samples were dried in an air oven for 24 hrs a t 7OoC then in vacuum and finally were weighed. The volume fraction of rubber (Vr) was calculated and the number of physically effective cross-links were determined using the FLORY-REENER equation4)

.

l/(M;) = -[v, + xV: + In (1 - Vr)]/@VgV1/' (1)

where V,, x, @ and V, are the volume fraction of rubber, the FLORY-HUGGINS interaction parameter, density of rubber and molar volume of solvent respectively. The values used are given below.

218

Styrene-Butadiene Rubber with Dicumyl Peroxide and Evaluation of Interaction Parameter

SBR Natural ~ rubber 0.37 0.94 88:4 88.64

The primary molecular weight (M) of the rubbers were obtained from the determination of intrinsic viscosities in benzene and converted to molecular weights by the following published relations5.6)

NR: (q) = 6.03.10-4 M0.67

SBR: (q) = 2.29*10-7M1.3S (2)

(3 )

Determination of cohesive energy density ( C E D ) For the determination of CED (ie., square of solubility parameter) of SBR, solvents

of nearly the same polarity and structure as the rubber and having solubility parameters, 8, in the range of 7 to 10 (k~al/ml)'/~, were chosen. The values of the solubility parameters of the solvents were taken from the existing literature7"). The following methods were used to determine the swelling coefficient (Q), ke . , ml of solvent imbibed per unit volume of rubber, and intrinsic viscosity (q) of the rubber in these solvents.

Table 1. Swelling and viscosity data for the measurement of cohesive energy density a t 35 * 0.01"C -

No.

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18 19 20 21 22

Note:

Solvents

n-Hexane n-Hep tane n-Octane n-Decane Methyl cyclohexane Cyclohexane Decalin Tetralin Isopropyl benzene Carbontetrachloride rn-Xylene Mesit ylene Ethyl benzene Toluene Bromo benzene o-Xylene Benzene Chloroform Tetrahydrofuran Chloro benzene Dioxane Carbondisulfide

7.30a) 7.45b) 7.608) 7.7Ic) 7.80") 8.20 b,

8.30C) 8.40C) 8.40 b, 8.60 b,

8.70C) 8.80a) 8.80a) 8.90b) 8.90 9.00C) 9.15b) 9.30 b,

9.50b) 10.008) 10.00b) 10.00b)

ref. 6) . b, ref. s), and C) ref. ').

130.0 147.5 162.4 194.9 128.4 108.0 157.0 136.8 140.0 97.0

123.0 138.8 123.0 107.0 106.2 120.0 89.0 81.0 81.5 92.5 86.0 61.0

0.60 0.60 0.58 0.84 1.30 1.42 1.68 2.12 2.00 2.00 1.78 1.30 1.50 1.58 1.56 1.58 2.46 0.90 0.80 1.56 0.14 0.30

( Q ) ml /d of rubber

0.8305 1.0070 0.9018 0.8277 1.7390 2.0360 2.0890 3.4050 4.8316 4.9280 - -

3.6780 2.5950

2.6800 2.9160

1.4260

1.0270

-

-

-

-

219


A vulcanizate for the swelling measurement was prepared from SBR, mixed with 0.5 phr DICUP, by vulcanizing for 70 min. a t 155 f 5OC. The swelling coefficient Q is found a t equilibrium swelling by the method described above. The values obtained are recorded in Table 1.

For the viscosity determination, 0.50, 0.375, 0.250, and 0.125 g/100 ml solutions were prepared by dissolving the required amount and filtering them in standard volumetric flasks. The solutions were made up to the mark and allowed to equilibrate for 1 day a t room temperature in a dark place. A usual suspended level viscometer was used to record the flow times for pure solvent and solution. The measurements were done a t 35 f 0.01 "C and an average of 3 readings was taken for the calculations. The intrinsic viscosities (q) computed from the usual method of extrapolation of qap/c against c (Le., concentration in g/100 ml) plots to zero concentrations are recorded in Table 1.

Determination of free D I C U P 2 g samples of vulcanizate, prepared with 4.0 phr DICUP were extracted with a mixture

of glacial acetic acid: benzene= 5:l in stoppered bottles for 7-10 days with frequent shakings. 25 ml of the mixture was used. Excess of saturated K I solution was added and the mixture was heated to incipient boiling for nearly 15 min. with occasional shakings. Without cooling, the solution is titrated with 0.1N sodium thiosulfate solution to the dis- appearance of the yellow colorlo).

Time of vulcanization (mid, Fig. la

I ! I I I

20 40 60 80 100 0 Time of vulcanization (min.)

Fig. Ib

Fig. la. Change in dicumyl peroxide (DICUP) concentration with time of vulcanization

Fig. lb. Plot of log (free DICUP) against time of vulcanization. Starting concentration at 150 (curve 1) and 160°C (curve 2), for SBR

of DICUP: 4 phr; curve 1: 160OC; curve 2 : 15OOC

Results and Discussion

It is evident from Fig. 1 that DICUP decomposes according to first order law since log (free DICUP) against time of vulcanization plot gives a straight line (Fig. 1 b). It may be seen that at 70 min almost the whole

220


peroxide has decomposed. This is further demonstrated by the maximum cross-linking attained which remains practically constant afterwards. This is graphically represented in Fig. 2. The studies of thermal decomposition of DICUP in solvents and in rubbers have also shown that the rate of decomposition of DICUP remains relatively unaffected by the nature of the m e d i ~ m ~ t ~ ~ - ~ ~ ) .

{me of vulcanization (min)

Fig. 2

Dicumyl peroxide (phr)

Fig. 3

Fig. 2. Increase of physical cross-links a t different concentrations of DICUP with time of vulcanization for purified SBR a t 155 O C . The concentrations are indicated on the curves.

No. of curves denotes amount of DICUP in phr

Fig. 3. Variation of physically effective cross-links with concentration of DICUP for natural rubber (NR, curve 1) purified SBR (curve 2) and unpurified SBR (curve 3)

a t 155OC

Cross-linking efficiency of SBR Cross-linking efficiency is the ratio of the number of chemical cross-

links, 1/(2Mc), to the number of molecules of DICUP decomposed. I n this study, the cross-linking efficiencies for purified and unpurified SBR are found from the swelling analysis. For natural rubber, the cross- linking efficiency as determined from the slope of the plot between physically effective cross-links, corrected for chain ends (Le. 1/(2 Mi) + 1/M) and concentration of DICUP is found to be unity and is in very good agreement with others2). This also ascertains the purity of DICUP. The value of 1/M for NR, purified SBR and unpurified SBR are found t o be 0.06.10-4, 0.07 - 10-4 and 0.072.10-4, respectively, and are computed from Eqs. 2 and 3 after determining the intrinsic viscosities. For purified

221


and unpurified SBR, the cross-linking efficiencies are found as 5.0 and 2.5, respectively. The lower value for the latter is ascribed to the destruction of the peroxy radicals by the antioxidants present in the rubber. The value for purified SBR has been reported by LOAN13) as 12.5. This shows that the cross-linking efficiency does not remain the same for all types of SBR. Nevertheless, the value is much higher than observed for natural rubber. The following types of reaction are believed to take place.

CH, CH, CH, I I I

1 I I CH, CH, CH,

C,H,-C-0-0-C-C,H, d 2 C,H,-C-0. resp. (ROO)

CH3 CH3 I I

I CH,

C,H,-C-O- & C,H,-C-0 + .CH, resp. (R.)

Radicals R. and RO. formed by the decomposition of DICUP abstracts hydrogen atom from natural rubber as follows.

R. 01 RO. + --cH~-c=cH-cH~- --+ RH or ROH + --cH~-c=cH-~H- I I (6) CH, CH3

-CHz-C=CH-CH- c-) -CHz-C-CH=CH- resp. (Pa) I I (7) CH, CH,

P. + P. d P-P

The other hydrogen atoms can also be abstracted but the one shown in above reaction is most abundant and resonance stabilized. Obviously, the recombination is the only pos- sible way despite the high concentration of the double bonds.

In SBR, the cross-linking efficiency is quite high and may be either due to the presence of pendant vinyl groups and styrene residues or the polymerization of the polymer. The latter is more probable since the cross-linking efficiency of polybutadiene is also of the order of 109. The polybutadienyl radical formed is capable of attacking the double bonds in the polymer and grafts to them. The following explanation given by LOAN'3) on the basis of the analysis of the product is advanced here.

RO. + -CHz-CH=CH-CHa- -CHz-CH-$H-CH2- resp. (R,) I (8)

RO

222


--CH,- CH- CH-CHZ-

CHZ-CH-CH-CHZ- + -CHz-CH=CH-CHZ- + R O 1 I

RO d -CHz-CH-$H-CHZ-

-CH2-CH-CHz-CH2- + -$H-CH=CH-CH2- (9)

< R 2 > I

RO

Termination is brought about by chain transfer. Similarly, CH, radicals take part.

The cross-linking data from swelling measurement are shown in Fig. 3. It is noteworthy that the curve for unpurified SBR does not show line- arity beyond 2.5 phr DICUP. This shows the departure from the theoretical FLORY’S equation.

It is also interesting to note that the linear portions of the curves in Fig. 3, when extrapolated to zero concentration of DICUP, do not pass through the origin. The values from the intercept on the ordinate give a measure of physical cross-links due t o the chain entanglement and must be taken into account when calculating the chemical cross-links. The values for natural rubber, purified SBR and unpurified SBR are found to be 0.20. 0.14 * 10-4 and 0.10 * 10-4 mole/g of rubber, respectively. From the knowledge of chain entanglement and primary molecular weight, the physically effective cross-links can be easily converted into chemical ones by the equation proposed by KRAUS et al.l4).

Determination of interaction parameter (x) The nature of interaction parameter between polymer molecules and

solvents has been a subject of extensive theoretical and experimental investigations. The earlier reported values for x for SBR vary consider- ably and can give erraneous results. For SBR, this parameter has been evaluated by HUMMEL’S method3) and from the measurement of CED as described later. These methods are found to confirm further the value of 0.37 for SBR reported by BHATNAGAR and BANERJEE‘) from osmotic pressure measurements.

223


x from HUMMEL'S method

Very recently H U M M E L ~ ) proposed a method for determining x from swelling measurements of DICUP cured vulcanizates of natural rubber. The same method has been extended t o evaluate x for SBR. The concentration of cross-links can be determined by swelling measurements using Eq. (1). The determination of x by this method assumes that the physically effective cross-links are equal to the chemical ones (ie., l/(MA) = l/(Mc)). The assumption is justified if vulcanizates are prepared which are not lightly cross-linked. In highly cross-linked vulcanizates, chain entanglement and chain end corrections can be neglected. This is particularly true with SBR owing to higher cross-linking efficiency. The concentrations of DICUP used were, therefore, relatively high t o comply with the assumption.

DICUP gives quantitative cross-links. Since the vulcanization time given is sufficient to decompose the peroxide completely, the vulcanizates prepared with different concentration should satisfy the relation

,

If one DICUP concentration is twice the other, then (MA)l/(Mi)2 = 2. This has been demonstrated by HUMMEL~) and LOAN^^) for natural rubber and SBR, respectively. From the swelling analysis, the volume fractions for the 2 vulcanizates can be determined. These values are recorded in Table 2 along with the concentration of DICUP used. Unlike for sulfur cured vulcanizates, the value of x for DICUP vulcanizates has been shown to be independent of the peroxide concentration by BLANCHARD and WooTTON 15). Obviously, for 2 vulcanizates having cross-linking density and ( M Z ~ ' ) ~ and volume fractions V,, and Vr2, the value of x will remain the same and can be calculated since

or dividing the 2 equations,

If (DICUP),/(DICUP), = 2, then (Mf)l/(Mf)2 = 2 and if volume fractions are determined experimentally, the value of x can be calculated.

224


Here, the graphical method has been used t o evaluate x. A family of curves, as shown in Fig. 4 has been drawn between (M;l)‘ and V, for different values of x using Eq. (1). The perpendiculars A and B are drawn at Vrl and Vr, (determined experimently for 2 stocks containing DICUP

Fig. 4. Plot of physical cross-links against volume fraction of rubber for the evaluation of x. For significance of symbols, cf. text

in the ratio 2: l ) . The values of (M;l)’ corresponding to different values of x are read off from the intersection of A with the curves (from x =

0.30 to 0.44). The curve C is then drawn satisfying the relation ( M L ~ ) ~ / (M;l)l’ = 2 since the concentration of DICUP in one stock is twice the other. Obviously, the value of x will be the interaction of curves B and C, and shall satisfy both the curves A and B. Table 2 records the values of x obtained using different sets of concentrations of DICUP.

225


Table 2. Results showing x values from HUXMEL'S method

(1) (I11 (111)

2.0 1.0 0.378 0.281 1.0 0.5 0.281 0.196 0.5 0.25 0.196 0.139

x from CED The term cohesive energy density, CED, first proposed

X

0.395 0.370 0.370

by HILDE- BRAND^^) and applied to mixing of two liquids was extended and success- fully applied to polymer systems by GEE^'). CED (i.e., square of the solubility parameter, gS) of polymer is related to x by the following

(12) equation

where 8, and 8, are the solubility parameters (i.e., 4CED) for solvent and polymer, respectively, R and T are the molar gas constant and temperature in "K, x8 is a constant which includes all non-regular contributions to x. SCOTT and MAGATW ascribed to it a value of 0.30 for hydrocarbon rubbers, and evaluating 8, by swelling or viscosity the value of x can be determined.

x = % + Vs/RT(G, - &)* -_

25

2.c

- e 9 1.5 - Y c=

1.c

a:

C

5.c

4.c - L a3 53 's 3.c .. - 5 E

0 - - -

2.c

1s

0

Fig. 5. Variation of intrinsic viscosity (5a) and swelling coefficient (5b) with solubility parameters of the solvents. Curves 1: SBR; curve 2: polystyrene

226


The values of intrinsic viscosities (q) and swelling coefficients Q, determined experimentally in different solvents have been recorded in Table 1. The plots of (q) and (Q) against the solubility parameter of the solvents, as, are shown in Fig. 5 a and b for purified SBR. The curves are nearly of the same nature except that 2 peaks corresponding to as =

8.45 and 9.15 (kcal/ml)l/2 are observed in the former case. From swelling method, only one peak a t as = 8.45 (kcal/ml)l/2 is observed. The value 8.45 gives a measure of the solubility parameter of SBR or polybutadiene while 9.15 corresponds to the solubility parameter of polystyrene. In earlier communication by MANGARAJ, BHATNAGAR and RATH 19) this value has been evaluated by a viscometric study. It is interesting that the same solvents as observed in earlier communication19) give the second peak. For the absence of a second peak from swelling measurements cannot be given a conclusive explanation but the authors feel that it may be because of the peaks being very close which would make them indistinguish- able. Moreover, polystyrene may go into solution instead of swelling. However, to come to a definite conclusion, studies with a few more solvent are warranted. The following explanation can be advanced in sup- port of the appearance of two peaks by the viscometric study.

GEE1') studied the relation of swelling of copolymer with solubility parameter and assumed that copolymers are mutual solutions of 2 com- ponents and shall behave as a uniform liquid, i.e., the copolymer should have a solubility parameter which is an average of the homopolymers. However, be indicated that the theory of swelling is much more satis- factory for natural rubber than for synthetic rubbers, especially for those prepared from copolymers. The appearance of two peaks has been reported by BEERBOWER et ~ 2 . 8 ) for nitrile rubber from swelling measurements. According to them acrylonitrile forms both blocks of alternating copolymer and blocks of homopolymer with butadiene. However, YERRIC and BECK~O) made a similar study on silicone rubbers. They showed that polydimethyl siloxane and polyphenylmethylsiloxane homopolymers give peaks at 7.5 and 9.0 (kcal/ml)1l2. Random copolymers prepared, having composition of 44.5 mole-% dimethylsiloxane and 55.5 mole-% phenylmethylsiloxane, and blends of polydimethylsiloxane (70 mole-%) and polyphenylmethylsiloxane (30 mole-%) were also studied and again 2 peaks corresponding t o 7.5 and 9.0 were obtained. In the present study the values of 8.45 and 9.15 probably correspond to polybutadiene21) or SBR and poly~tyrenel~). It is therefore clear that the two peaks appear for the 2 homopolymers represented by the polymer or copolymer and homopolymer with styrene.

227


From Rubber intercept

(Qmax) (Q) (?)

SBR 8.45 8.45 8.45 8.45

The solubility parameter can be further confimed by the intercept method’). FLORY and REHNER4) deduced Eq. (1) by taking into account the free energy of mixing of polymer in solvent and free energy due to elastic deformation for a perfect network. For liquids without specific interaction with polymer ASm (i.e., change in entropy of mixing) is independent of the nature of the liquid, but AH, (heat of mixing) depends on the nature of liquid. Using the SCATCHARD-HILDEBRAND 22) equation

Calculated X Sr

8.45 0.3696

and assuming K to be constant, GEEz3) showed that (Q), i.e., l/Vr, is a function of V’i2 (8, - 6,) and it has a maximum a t 6, = 8,. He showed that the (Q) us. V, (8, - 8r) curve is GAussian and

so that plot of [l/Vsln (Qmax)/(Q)I1/2 against 6, of the solvents gives a straight line having an intercept equal to $. GEE^^) also showed that solvents having the same CED as the polymer correspond t o maximum value of (q), and thus pointed out the analogy between swelling coefficient and intrinsic viscosity. Obviously, replacement of (Q) by (3) should show similar behaviour. The complete theory has been discussed by MANGARAJ et aZ.79l9) in details.

The values of (qmax) and (Qmax) are estimated from the maximum value attained from Figs. 5a and 5 b and

is plotted against 8, as shown in Figs. 6 a and 6b. The points lying on the curve shown by full line in Fig. 5a have been used to draw Figs. 6 a and 6b. Almost all points lie on a straight line and the value of 8, which is equal t o Sr at [1/Vs In (Qmax)/(Q)]1/2 or [l/Vs In ( q m a x ) / ( ~ j ) ] ~ / ~ = 0, can be read off from the intercept on the abscissa. The values of 6, (or im are given in Table 3 and are found t o be again 8.45.

228


The value of x obtained by substituting 8, = 8.45 in Eq. (4) is 0.3696 and is in good agreement with those found from HUMMEL’S method and the osmometric method reported earlier 5).

I I 1 1 I

8 9 10 6, (cal/ml)”Z

Fig. 6. Computation of 8, from viscosity (6a) and swelling measurement (6b) by the intercept method for SBR

In conclusion, it may be observed that the method of estimating CED from viscosity is sufficiently accurate and easy. 2 values of Sr were obtained which probably correspond to a blend of homopolymers and to the copolymers.

The authors are thankful to the authorities of the INDIAN INSTITUTE OF TECHNOLOGY, Kharagpur for providing facilities for carrying out the work. Thanks are also due to HERCULES POWDER COMPANY, U.S.A. and BATA SHOE COMPANY, India for the free gifts of dicumyl peroxide and rubbers, respectively.

229


'1 I. I. OSTOWSLENSKY, Shurnal Fisitschesskoi Chimii 47 (1915) 1887. a) B. M. E. VAN DER HOFF, Ind. Engng. Chem. 2 (1963) 273. *) K. H-L, Rubber Chem. Technol. 37 (1964) 894. 4, P. J. FLORY and J. REHNER, Jr., J. chem. Physics 11 (1943) 521. 6, S. K. BHATNAGAR and S. BANERJEE, Rubber Chem. Technol. 38 (1965) 961.

'1 D. MANGARAJ, Makromolekulare Chem. 65 (1963) 29. *) A. BEERBOWER, D. A. PATTISON, and G. D. STAFFIN, ASLE Trans. 6 (1963) 80. 8 , C. J. SHEEHAN and A. L. BISIO, Rubber Chem. Technol. 39 (1966) 149.

lo) K. HUMB~EL and G. KAISER, Kautschuk u. Gummi 14 (1961) 171; C. KOKUTNUR, J.

n, M. S. KHARASCH, A. FONO, and W. NUDENBERG, J. org. Chemistry 16 (1951) 105. la) €I. C. BAILEY and G. W. GODIN, Trans. Faraday SOC. 52 (1956) 68. lS) L. D. LOAN, J. appl. Polymer Sci. 7 (1963) 2259. 14) G. KRAUS and G. A. MOEZVGEMBA, J. Polymer Sci. 2 (1964) 227. 16) A. F. BLANCHARD and P. M. WOOTTON, J. Polymer Sci. 34 (1959) 627. 16) H. HILDEBRAND, J. Amer. chem. SOC. 51 (1929) 66. 17) G. GEE, Trans. Instn. Rubber Ind. 18 (1943) 266. lS) R. L. SCOTT and M. MAGAT, J. Polymer Sci. 4 (1949) 455. lg) D. MANGARAJ, S. K. BHATNAGAR, and S. RATH, Makromolekulare Chem. 67 (1963) 75. m) K. B. YERRICK and H. N. BECK, Rubber Chem. Technol. 37 (1964) 261.

L. MULLINS and W. F. WATSON, J. appl. Polymer Sci. 1 (1959) 245.

Amer. chem. SOC. 63 (1941) 1432.

D. F. WILCOCK, J. Amer. chem. SOC. 68 (1946) 691. G. SCATCHARD, Chem. Reviews 8 (1931) 321; S. E. WOOD and J. H. HILDEBRAND, J. chem. Physics 1 (1933) 817.

G. GEE, Annu. Rep. Progr. chem. 39 (1942) 7. as) G. GEE, Trans. Faraday SOC. 38 (1942) 418.

230

Documents

Cross-Linking efficiency of styrene-butadiene rubber with dicumyl peroxide and evaluation of interaction parameter