Crosslinking Reactions in High Impact Polystyrene Rubber Particles

14 Crosslinking Reactions in High Impact Polystyrene Rubber Particles

Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0142.ch014

DIETER J O S E F S T E I N , GERHARD FAHRBACH, a n d H A N S J Ö R G ADLER Kunststofflaboratorium, B A S F Aktiengesellschaft, Ludwigshafen/Rhine, West Germany

The crosslinking of rubber in the rubber particles of high impact polystyrene is investigated using suspension polymerization of rubber solutions in styrene as a model. Polybutadiene and butadiene copolymers with isoprene and styrene were used. The preferred crosslinking sites in butadiene and isoprene polymers are the 1,2- positions; 1,4-, and 3,4- are also involved in crosslinking reactions, but their reaction rate is significantly lower. The 1,2- reactivity is about the same in butadiene-styrene copolymers as in the homopolymers; in isoprene polymers it is less. For all investigated rubbers, the crosslinking reaction rate accelerates with increasing conversion of styrene; this is most striking in the conversion range above 98%. This phenomenon can be explained by copolymerization of the styrene monomer with the 1,2- units of the rubber backbone.

I

n general, high impact polystyrenes are multiphase systems consisting of a continuous rigid polystyrene phase and discrete rubber particles 0 . 5 - 1 0 μ in diameter. The incorporated rubber particles are crosslinked and contain grafted polystyrene, and their inner structure is determined by the manufac turing process and can vary considerably. The principle polystyrene structures have been described i n detail ( 1, 2). A c c o r d i n g to more recent theories, the toughness of high impact poly styrene is caused by flow and energy dissipation processes i n the continuous polystyrene phase. The rubber particles act as initiating elements. Considerable differences in the thermal expansion coefficients and i n the moduli of the polystyrene phase on the one hand and of the rubber particles on the other lead to an inhomogeneous stress distribution i n impact polystyrene. Stress maxima create zones of lower density, called crazes ( 3 ) , i n w h i c h the poly styrene molecules are extended parallel to the direction of stress. Macroscopically craze formation appears as whitening; the flow processes result i n irre versible deformation (cold flow). T o trigger craze formation, rubber particles must be crosslinked (4). Qualitatively it is w e l l known that crosslink density considerably influences the 148

In Copolymers, Polyblends, and Composites; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

14.

STEIN E T A L .

149

Polystyrene Rubber

toughness of high impact polystyrene ( I , 2, 5, β, 7). However, little is known about the crosslink density of rubber particles because of the difficulty i n determining it. This paper is an attempt to demonstrate the effect of rubber structure on its reactivity towards crosslinking to obtain a better understanding of this reaction during the manufacturing process.


Experimental Rubbers. T h e rubbers investigated—polybutadiene, butadiene-styrene copolymers, and butadiene-isoprene copolymers—were prepared b y anionic solution polymerization (initiator, n-butyllithium) i n n-hexane on a laboratory scale. Rubber configurations were varied b y adding tetrahydrofuran during the polymerization process. Polymerization was terminated b y adding small amounts of methanol; precipitation of the rubber followed, then washing with large amounts of methanol containing 0 . 1 % phenolic antioxidant. T h e rubbers were dried in vacuo at 50°C for 1 or 2 days (voltaile matter < 0 . 5 % ). The molecular weights of these polymers, M , were 200,000^300,000 as estimated b y the viscosity number 77 ( 0 . 5 % i n toluene at 2 5 ° C ) a n d the [ 7 7 ] - M correlation for 1,4-polybutadiene (8) and 1,2-polybutadiene ( 9 ) . T h e only exception, Taktene 1202 containing 2 % 1,2-vinyl groups, h a d a viscosity number of 272 c m / g which corresponds to an M of approximately 200,000. Rubber compositions and configurations were determined b y I R spectros copy using the extinction coefficients of Sirnak and Fahrbach (10). T h e data are presented i n Table I. Preparation of H i g h Impact Polystyrene. H i g h impact polystyrenes were prepared according to the Stein a n d Walter method (11) except for the fol lowing alterations: 0.5 part paraffin o i l , 0.1 part phenolic antioxidant, a n d suspension stabilization b y 0.3 part of an organic suspension agent. T o reach higher conversions, some of the experiments were run i n suspension at 145 °C two hours longer. v

SN/O

3

v

Table I.

Composition and Configuration of the Rubbers

Butadiene, wt % No.

Total

1,2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

100 100 100 100 100 100 90 90 90 80 80 0 20 21 20 40 40 40 43 65 82 90

2 9 17 33 52 82 30 40 45 34 45 0 2 2° 2

α

a

4.

4° 3 9α α 8 9 α

7

e

α

Isoprene, wt %

1,4cis

1,4trans

Total

SA

ι,4cis

98 35 30 23 16 6 22 19 16 17 13 0 8 7 7 14 14 14 8 22 28 33

0 56 53 44 32 12 38 31 29 29 22 0 10 12 11 22 22 23 26 36 46 48

0 0 0 0 0 0 0 0 0 0 0 100 80 79 80 60 60 60 57 35 18 10

0 0 0 0 0 0 0 0 0 0 0 5 4 3 4 2 2 2 13 2 1 1

0 0 0 0 0 0 0 0 0 0 0 81 56 57 56 43 44 44 26 26 12 7

1,4- Styrene, trans wt %

0 0 0 0 0 0 0 0 0 0 0 14 20 19 20 15 14 14 18 7 6 3

0 0 0 0 0 0 10 10 10 20 20 0 0 0 0 0 0 0 0 0 0 0

T h e 1,2-isoprene content, included in this figure, is negligible.


V sp/c, cm /g 3

272 267 329 299 307 257 —330 —340 —330 —330 —290 —300 —290 —350 —350 —300 —310 357 368 —270 —270 —280

150

COPOLYMERS,

POLYBLENDS,

AND

COMPOSITES

Determination of the Swelling Index (SI). A solution of high impact polystyrene i n toluene (5:100) was centrifuged 1 hr at 25000 g. The super natant l i q u i d was decanted from the gel. Traces of solution remaining on the lip of the centrifuge cup were removed by filter paper. The gel was weighed before and after drying to constant weight, and from these two weights we calculated the swelling index. gj. _ wet weight of gel dry weight of gel


Determination of Residual Styrene Monomer Content. Residual styrene monomer content was determined by method II of Noffz and Pfab (12, 13). Results It is assumed that the crosslinking and chain propagation reactions competitive.

are

Reaction paths l a , l b , and l c illustrate the possible competitive reactions of polystyrene radicals and initiator radicals R * . In the rubber phase, the sub script Ρ indicates that the functional group is incorporated into a polymer chain. Double bonds and allylic hydrogen atoms w i l l be the sites of attack favored by the radicals. T h e rubber radicals produced by Reactions l b and l c can cause crosslinking of the rubber by recombination or propagation steps; i n the propa gation reactions, copolymerization of the rubber double bonds w i t h styrene must be considered. Since Reactions l b and l c precede the crosslinking and compete w i t h the propagation reaction (Reaction l a ) , one can expect that crosslinking of the rubber particles depends on conversion of the styrene monomer. W e tested this hypothesis w i t h a polybutadiene containing 9 % 1,2-vinyl units. A t styrene conversions below 9 5 % , the rubber crosslink density was too small for determining the SI. Therefore, measurements were limited to the range of very high styrene conversion. In Figure 1, the SI of the rubber particles is plotted against the amount of residual styrene monomer, i.e., n


14.

STEIN E T AL.

13

151

Polystyrene Rubber

swelling index

12 • 11 -


10 9 θ

.%] residual styrene content

7-I 0,50 Figure 1.

1.00

—ι

r

1.50

Swelling index vs. styrene conversion in polybutadiene

styrene conversion (wt % ) . The expected correlation was observed: the SI decreases w i t h increasing styrene conversion, i.e., low monomer concentration. It was therefore decided to include in the investigations only those experiments in w h i c h the styrene conversion scatter lay within the limits 99.50 0.15%. W i t h the aid of Figure 1, the effect of scatter was eliminated by correcting the Si's to the average styrene conversion, assuming that the curve i n Figure 1 is applicable to the other systems. Experiments i n which styrene conversion was outside the range of scatter were ignored. T o avoid misunderstandings about the importance and meaning of the swelling index i n the context of this study, a brief discussion of the morphology of high impact polystyrene is necessary. Figure 2 shows electron micrographs of typical high impact polystyrenes; the dark lamellae represent the rubber. As was described by other authors ( I , 2 ) , multiple oil-in-oil emulsions are ob tained; the rubber particles, discretely distributed in the continuous poly styrene phase, contain discrete areas of occluded polystyrene. The SI of these polystyrene-filled rubber particles can be measured after they are separated from the soluble continuous polystyrene phase. Under selected experimental conditions, it is not possible to extract the polystyrene occluded by the rubber lamellae. Consequently, one does not measure the SI of a uniformly crosslinked rubber but, indirectly, the osmotic pressure of the occluded polystyrene. This is evidenced by measuring the SI in methyl ethyl ketone (non-solvent for polybutadiene): for uncrosslinked polybutadiene, SI — 1.9; for the crosslinked rubber particles i n high impact polystyrene on the other hand, SI values range between 4 and 8. This indicates that it is not possible to calculate network densities from swelling indexes by the usual methods, for example by the Flory-Rehner equation. ±



152

COPOLYMERS,

Figure 2.

POLYBLENDS,

A N D COMPOSITES

Morphology of high impact polystyrene pre pared from various rubbers X5476

Stained with OsOi,: dark areas, polybutadiene; and light areas, polystyrene, a, Polybutadiene with 9% 1,2-; b, polybutadiene with 82% 1,2-; and c, copolymer of butadiene (43%) and iso prene (57%) with 9% 1,2-

The osmotic pressure of the polystyrene solution is balanced b y the elastic retractive forces of the more or less densely crosslinked rubber lamellae; osmotic pressures i n the swollen rubber particles are rather high because of the high positive virial coefficient of polystyrene i n toluene ( A = 5 X 10~ mole c m g" ) (14). F o r SI = 10 (corresponding to c = 0.1 g X c m ) and M = 5 Χ 1 0 , one 4

2

3

x - c

n

· R · Τ (J- + 4 c ) f

2

3

2

4

(2)

can estimate from Equation 2 that the osmotic pressure is 0.2 atm. Actual values are probably even higher since higher virial coefficients were not taken into account i n the calculation. Despite these limitations, one can qualitatively conclude from the results depicted i n Figure 1 that crosslinking increases w i t h conversion. Furthermore, the relative rate of crosslinking, i.e., formation of crosslink sites as a function of styrene conversion, probably also increases w i t h conversion. However, the second statement has yet to be proved i n special experiments i n w h i c h the formation of network sites is measured directly. According to reaction schemes l a , l b , and l c , crosslinking is influenced by the type and number of double bonds or allylic hydrogen atoms i n the rubber. Polybutadiene microstructure can be controlled easily b y suitable anionic polymerization conditions i n which parameters like double bond n u m -



14.

STEIN E T A L .

Polystyrene Rubber

153

ber and molecular weight distribution remain constant. Figure 3 illustrates the effect on the crosslinking reaction of various concentrations of 1,2-vinyl units in the polybutadiene. T h e decrease i n swelling indexes indicates clearly that crosslinking strongly increases with increasing 1,2- content. Since 1,2-polybutadiene is completely different from the 1,4- polymer, one might conclude that a change i n phase inversion mechanism (15) occurs i n going from 1,4- to 1,2-polybutadiene. Figure 2 illustrates that this is not the case; the differences in rubber particle size and internal structure i n the various rubbers are very small. Therefore the large differences i n swelling index must result primarily from different reactivity of the diene configurations towards crosslinking. T h e data depicted i n Figure 3 were supplemented b y investigations of butadiene-styrene copolymers w i t h styrene contents of 10 and 20 w t % . It is evident from Figure 4 that crosslinking of these rubbers is also essentially determined by the number of 1,2-vinyl units. T h e data for styrene copolymers coincide w i t h those of the polybutadiene curve. Measurements of butadiene-isoprene copolymers are summarized i n a plot of swelling index vs. 1,2- content (Figure 5 ) . These data were obtained at 9 9 . 8 5 % styrene conversion rather than 9 9 . 5 0 % (Figures 3 and 4 ) , and this difference must be kept in m i n d . Crosslinking also increases w i t h increasing 1,2- content i n butadiene-isoprene copolymers although a l l points lie above the polybutadiene curve; this applies especially to copolymers w i t h l o w 1,2contents. T h e curvature i n Figures 3 and 4 is not apparent i n Figure 5 because of the limited range of the 1,2- content i n the butadiene-isoprene copolymers.


154

COPOLYMERS,

POLYBLENDS,

A N D COMPOSITES

swelling index 12

conversion of styrene 99,50 i 0,15 %

10


curve for homopolybutadiene

6 •

12 double bonds[%J ι

ι

20 Figure 4.

ι

1

ι

40

ι

1

60

ι

1

'

80

Swelling index of butadiene-styrene copolymers vs. 1,2 con tent in the rubber

Δ, 90/10 wt % butadiene/styrene, and O, 80/20 wt % butadiene/styrene

Discussion If one applies reaction schemes l a , l b , a n d l c to polybutadiene, then the data i n Figure 3 indicate that the double bonds and/or the allylic hydrogen atom of the 1,2- configuration must be primarily responsible for the crosslinking reaction. V a n der Hoff (16) and recently Hergenrother (17, 18) were able to demonstrate that crosslinking of pure polybutadiene (i.e., i n the absence of polymerizing monomer) with dicumyl peroxide proceeds via a propagation step b y Reactions 3 and 4. Hergenrother i n particular demonstrated that the RO*

C=C

+

+

ROH

I CH CH I C=C

C=C

+

C=C I C* CH

I I c—c—c*

(3)

etc.

(4)

1,2-vinyl configuration is more reactive than the 1,4- configuration. These findings were confirmed b y Reichenbach (19). Analogous results were ob tained b y Heusinger and co-workers (20, 21) b y radiation crosslinking of polybutadienes and polyisoprenes of different configurations. T h e y found that the crosslinking reaction of the 1,2- configuration proceeds via a chain reaction


14.

STEIN E T AL.

155

Polystyrene Rubber

w i t h kinetic chain lengths of 2 - 5 , whereas 3,4- a n d 1,4- configurations partici pate i n the crosslinking reaction b y recombination. Fischer (22) on the other h a n d showed that the ratio of the rate constants from reaction paths l a and l b for an attack b y a polystyrene radical on the 1,2- double bond is


^

= 1.5 X ΙΟ-a

(T = 110°C)

In contrast to the grafting reactions ( 2 2 ) , the crosslinking reaction during preparation of high impact polystyrene becomes effective only at very high styrene conversions ( > 9 5 % ) at w h i c h the rubber double b o n d concentration in the total reaction mixture approaches the monomer concentration. In rubber lamellae, this shift i n concentration i n favor of the rubber double bonds is even more pronounced because of the incompatibility of polybutadiene and poly styrene. Styrene is distributed i n moderately concentrated solutions (up to 30 w t % polymer) between the polystyrene and the polybutadiene phases according to E q u a t i o n 5 (23): ( = ) . - . * ( = >

»)

where (m/p) is the weight ratio of styrene monomer to polymer i n the indi vidual phases a n d subscripts Β a n d S indicate the rubber a n d polystyrene phases respectively. F r o m Equation 5 and the relative rate constant k /k , the concentration ratios ζ ( = rubber double bonds/styrene double bonds ) i

V2

2 Figure 5.

4

6

1,2 Configuration vs. crosslinking of copolymers

8 butadiene-isoprene

The numbers indicate wt % isoprene in the rubber


u

156

COPOLYMERS,

Table II.

POLYBLENDS, AND COMPOSITES

M o l a r Concentration Ratio of Double Bonds (z) and Relative Rates ( ϋ / υ ) 1 2

π

Rubber concentration, 5 wt % based on total charge ; configuration, 100% 1,2 units


a 6

Styrene Conversion, %

_ z

z

0 50 80 95 99

0.1 0.2 0.5 2 10

0.1 1 5 22 115

a

b

Vi/v

b

u

10" 0.0002 0.008 0.03 0.17 4

Initial value. In the rubber phase.

and the relative rates t) /t>ii of competing reactions l a and l b i n the rubber phase can be estimated. The values for various styrene conversions are pre sented i n Table II. Despite the small r values for copolymerization of styrene and the 1,2-vinyl units of polybutadiene, it is expected at styrene conversions over 9 5 % that crosslinking copolymerization of styrene and rubber w i l l occur. I n addition, the allylic radicals formed i n initiation Reaction 3 start graft copolymers of styrene monomer and polybutadiene double bonds. The existence of allylic radicals i n our system is corroborated by the work of Fischer (22). In graft copolymerization, the 1,2- configuration is preferentially incorporated into the copolymer (16, 17, 18, 19) as i n Structure 6. Networks are apparently formed 12

(6)

w h i c h contain the structural element shown i n Figure 6. If the 1,4- configura tions participated significantly i n the crosslinking reaction, then the styrenebutadiene copolymer data w o u l d not lie on the curve shown i n Figure 3. For example, rubber # 1 0 contains only two-thirds the 1,4- configuration of rubber # 4 because of the equal amount of 1,2- configuration i n both rubbers.

S - styrene monomer units x.y.z >1 Figure 6.

Schematic of a network of polybutadiene and polystyrene



14.

STEIN E T A L .

157

Polystyrene Rubber

This model of crosslinking polymerization cannot be applied directly to butadiene-isoprene copolymers. It is known that the polyisoprene crosslink mechanism differs from that of polybutadiene (16, 17, 20, 21). Crosslinking of polyisoprene occurs b y transfer of a hydrogen atom from rubber to a free radical and ensuing recombination of two rubber radicals. As one might expect, introduction of 1,2- configurations b y copolymerization w i t h polybutadiene increases the reactivity towards crosslinking, although to a smaller extent. This effect is also observed when the 1,2-vinyl content is increased at a constant butadiene-isoprene ratio i n the copolymer (compare rubber # 1 9 w i t h rubbers 16, 17, and 1 8 ) . Steric blocking of the 1,2- double bonds b y the methyl groups of the isoprene units could be implicated; however this argument is not sup ported b y the fact that no decrease i n reactivity is observed i n butadiene and styrene copolymers although the bulky phenyl groups should already be effec tive, even at lower concentrations.

CH=CH2 (1) Rn-CH -CH-

, R -CH-CH -CH -CH-

»

n

2

2

(2) R -CH-CH pCH -CH^CH-CH CHn

2

2

Rn-CH-CH3 • CH transfer -CH "CHÇ-CH -CH -CH-

intromolecukir

2

2

2

2

2

CHo -:

(3) 2

•

Ç=CH -CH -CH-

2

2

Figure 7.

..

..

kombination

z

>

CH

•

ÇH=CH -CH -CH2

CH

2

•

z

2

2

Δ

ÇH=CH -CH-CH 2

2

Intermolecular transfer reaction in butadiene-isoprene copolymers

W e explain the apparently lower reactivity of the 1,2- units i n isoprene copolymers b y an intramolecular chain transfer reaction. Figure 7 shows the probable reaction sequence. In the first step, the radical of a growing chain adds to the 1,2- double bond of a butadiene unit. I n the second step, an intra molecular chain transfer reaction of the 1,2-vinyl group w i l l compete w i t h the addition of styrene or rubber double bonds because of the high local concentra tion of allylic hydrogen atoms i n the neighboring isoprene units. This transfer reaction corresponds to the attack by peroxide radicals on allylic hydrogen atoms of polyisoprene already postulated b y van der Hoff (16). Rubber crosslinking probably also occurs by recombination (third step), but this path appears to be less efficient than crosslinking via the chain mechanism of the 1,2- double bonds. Acknowledgments W e thank our colleagues E . G . Kastning, P . Simâk, H . Hendus, and A . Echte for assistance and many valuable discussions.


158

COPOLYMERS,

Literature


1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

POLYBLENDS,

AND COMPOSITES

Cited

Willersinn, H . , Makromol Chem. (1967) 101, 296. Keskulla, H . , Appl. Polym. Symp. (1970) 15, 51. Bucknall, C . B., Smith, R. R., Polymer (1965) 6, 437. Bohn, L . , Angew. Makromol Chem. (1971) 20, 129. Zitek, P., Mysik, S., Zelinger, J., Angew. Makromol Chem. (1969) 6, 116. Dow Chemical Co., U.S. Patent 3,243,481 (1968). Wagner, E . R., Robeson, L . M . , Rubber Chem. Technol. (1970) 43, 1129. Kraus, G., Gruver, J. T., J. Polym. Sci. Part A (1965) 3, 105. Anderson, J. N., Barzan, M . L . , Adams, H . E . , Rubber Chem. Technol (1972) 45, 1270. Šimák, P., Fahrbach, G., Angew. Makromol Chem. (1971) 16/17, 309. Monsanto, U.S. Patent 2,862,906 (1956) example 1. Noffz, D., Pfab, W., Z. Anal. Chem. (1967) 228, 188. Noffz, D., Pfab, W., "Polystyrol," in "Kunststoff-Handbuch," R. Vieweg, G. Dau miller, Eds., Vol. 5, p. 156, Carl Hanser, München, 1969. Brandrup, J., Immergut, Ε . H . , "Polymer Handbook," 4, 126, Interscience, New York-London-Sidney, 1965. Bender, B. W., J. Appl Polym. Sci. Part A (1965) 3, 2887. van der Hoff, Β. M . E., Appl. Polym. Symp. (1968) 7, 21. Hergenrother, W . L . , J. Appl. Polym. Sci. (1972) 16, 2611. Hergenrother, W . L . , J. Polym. Sci. Part A-1 (1973)11, 1721. Reichenbach, D., Kaut. Gummi Kunstst. (1965) 18, 9. Heusinger, H., Kaufmann, R., von Raven, Α., Katzer, H . , IUPAC Internat. Symp. Macromol, Aberdeen, Sept., 1973. Heusinger, H . , Kaufmann, R., von Raven, Α., Katzer, H., Makromol. Chem. (1973) 163, 195. Fischer, J. P., Angew. Makromol. Chem. (1973) 33, 35. Stein, D. J., unpublished data.

RECEIVED May 10,

1974.


Crosslinking Reactions in High Impact Polystyrene Rubber Particles

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