Polymer Blends and Composites in Multiphase Systems - American

14 Model Studies of Rubber Additives in High-Impact Plastics MAURICE MORTON, M. CIZMECIOGLU, and R. LHILA 1

2

Institute of Polymer Science, The University of Akron, Akron, OH 44325

Downloaded by UNIV OF SYDNEY on October 11, 2015 | http://pubs.acs.org Publication Date: May 1, 1984 | doi: 10.1021/ba-1984-0206.ch014

Polymer latex blends were used to prepare dispersions of rub ber, ranging from 5 % to 20 % by weight, in rigid thermoplastics. The three thermoplastics used were polystyrene (PS), poly(vinyl chloride) (PVC), and styrene-acrylonitrile copolymer (SAN), and the three types of rubber used were polybutadiene (PBD), styrene-butadiene rubber (SBR), and polychloroprene (PCP). We studied the effect of particle size and rubber-plastic bonding on the impact resistance of the thermoplastic. With PS, which fractures by a crazing mecha nism, no optimum was found in the rubber particle size be low 1.2 μm, which represented the largest particle size possi ble by emulsion polymerization; the impact strength increased monotonically with particle size. With PVC, which fractures by a shear yielding mechanism, an optimum was found at a particle size of about 200 nm. For the SAN, a broad plateau maximum was found, ranging from 300 to 800 nm. This plateau was presumably a result of the presence of both types of fracture mechanisms. The presence of chemical bonds between the rubber and the plastic greatly enhanced the impact strength in all cases. THE

EFFECT O F F I N E L Y DIVIDED RUBBER I N C L U S I O N S on the impact resistance

of plastics has been known for some time and represents a well-developed technology (I). However, the morphology of these heterophase materials is usually quite complex, and has made unequivocal establishment of any re lationship between the physical properties and the morphology difficult. Thus, for example, high-impact polystyrene (HIPS) is usually prepared by polymerization of a styrene monomer containing about 5-10 % of dissolved rubber. As the polystyrene forms, phase separation of the rubber occurs as 1

Current address: Jet Propulsion Laboratory, Pasadena, CA 91103 Current address: Tuck Tapes, Inc., New Rochelle, NY 10801

2

0065-2393/84/0206-0221$06.00/0 © 1984 American Chemical Society

In Polymer Blends and Composites in Multiphase Systems; Han, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.


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POLYMER BLENDS AND COMPOSITES IN MULTIPHASE SYSTEMS

a dispersion i n the polystyrene solution. The dispersed rubber particles are swollen with styrene monomer, however, that continues to polymerize and forms a dispersion of polystyrene within the dispersed rubber particles. Hence this system ultimately has a three-phase morphology. As an added complication, during the polymerization of the styrene, considerable crosslinking of and grafting to the rubber occurs. Some consensus has been reached that an optimum i n the particle size of the rubber (2-4) exists and depends on the mechanism of fracture. Thus, where failure occurs through craze formation, this optimum i n rubber par ticle size is higher than when a shear yielding mechanism is operative. Some studies have also been carried out on the influence of cross-linking of and grafting to the rubber. The conclusions concerning the effect of rubber cross-linking are rather ambiguous (6-8), but a small extent of such crosslinking should give optimum properties (5-9). Similarly the improved properties resulting from chemical bonding at the rubber-plastic interface have also been noted (J, 4, 10-12). Some time ago we thought that it might be possible to throw more light on these systems by using as a model blends of polymer latex. In these blends the particle size of the rubber inclusions could be established a priori, and cross-linking of the rubber and/or rubber-plastic bonding could be deliberately introduced and measured. This model involved the prepa ration, by emulsion polymerization, of various synthetic rubber latices, the blending of these latices w i t h the latex of the desired plastic, the coagula tion of the blended latex (in isopropyl alcohol to remove emulsifier), and the molding of the dry polymer blend. Three different plastics were used: polystyrene (PS), styrene-acrylonitrile copolymer (SAN), and poly (vinyl chloride) ( P V C ) . Three different rubbers were also used: polybutadiene (PBD), styrene-butadiene rubber (SBR), and polychloroprene (PCP). W h e n cross-linking of the rubber was desired, the rubber latex was treated w i t h a peroxide at elevated temperature, prior to blending. W h e n bonding between the rubber and plastic was desired, a peroxide was incorporated i n the latex blend, thus carrying out the grafting (and the unavoidable con comitant rubber cross-linking) during the molding step. Polystyrene (PS) The PS used i n this work was D o w 788 latex. For this study a series of P B D latices was prepared, w i t h particle sizes i n the range of 50 to 400 n m , by using a "seeded" emulsion polymerization technique. Because this method is limited i n attaining larger particles, a commercial P B D of particle size 700 nm was also obtained. The latices we prepared had relatively uniform particle sizes. The commercial latex, however, showed a rather wide distri-



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Rubber Additives in High-Impact Plastics

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bution of sizes. Similarly, a series of SBR latices was prepared w i t h particle sizes ranging from 80 to 450 n m , and a commercial sample w i t h a particle size of 865 nm was also used. The fact that these rubber latex particles be came incorporated into the PS without distortion or coalescence was clearly evidenced by transmission electron microscopy of a thin microtomed sample of the molded composite (see Figure 1). The effect of these rubber inclusions on the Izod impact test for PS is shown in Figures 2 and 3, which show clearly that the larger particle sizes of the rubber have a greater effect on impact resistance. A n optimum had been suggested i n the rubber particle size of about 1-2 μπι for high-impact PS, but the range of sizes shown i n Figures 2 and 3 is not high enough to corroborate this suggestion. Some other work here w i t h P C P latex has

Figure 1. Transmission electron microphoto graph of an ultra-thin film of PS (-50 nm), stained with 0 S0 and containing 10% SBR (865 nm). 2

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Figure 2. Effect of polybutadiene particle size on impact strength of PS (Impact strength of PS = 0.25 ± 0.01 ft-lhs/in notch). Key: Ώ, PS/PBD 90/ 10; and O, PS/PBD 95/5.

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Figure 3. Effect of PBD and SBR particle size on impact strength of PS (Impact strength of PS = 0.25 ± 0.01 ft-lhs/in notch). Key: O, PS/PBD 95/ 5; and Δ , PS/SBR 95/5.


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shown that the impact strength of PS is still rising even when the rubber particle size reaches 1.2 μπι. Figure 3 also suggests another conclusion: that a rubber having a lower glass transition temperature (T ), such as P B D , has a slightly greater effect than the SBR. Figure 4 shows that these rubber-toughened PS samples, prepared by latex blending, also exhibit the expected decrease i n stiffness. The softer P B D apparently exerts a greater softening effect than the stiffer SBR. Figure 5 shows the effect of both grafting and cross-linking of the rub ber on the impact strength of PS. Chemical bonding of the rubber to the plastic apparently has a marked effect i n improving the impact resistance; thus, at the highest grafting level shown (a "grafting index" of 22, i.e., 22 % by weight of PS attached to the rubber), a 50 % increase i n impact strength is found. O n the other hand, cross-linking of the rubber has only a minor effect i n improving impact resistance, and excessive cross-linking actually decreases it drastically. Bucknall's conclusions (5, 9) about the desirability of light cross-linking may be correct.


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Polyvinyl chloride) (PVC) PS fractures by a crazing mechanism: formation of microvoids precedes the actual development of cracks (2, 14). Hence the role of the rubber inclu sions is both to initiate and to terminate such crazes; these effects are ac-

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Effect of rubber content onflexural modulus of PS. Key: •, PS; O , PS/SBR; and Δ , PS/PBD.


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Figure 5. Effect of grafting and cross-linking on impact strength of PS (PS/ SBR 90/10). Graf ting index (G.I.) = (wt of grafted PS/wt of rubber) χ 100. Key: O , G.I. = 22; G.I. = 15.5; A, G.I. = 8; and O, G.I. = 0. complished most efficiently when the size of the rubber particles is in the range of 2 to 4 μπ\ (2-4, 11). A polymer like P V C , w h i c h is more ductile than PS, fails by an en tirely different mechanism, known as shear yielding, (i.e., "cold drawing") as first proposed by N e w m a n and Strella (15). The optimum particle size for impact resistance should decrease w i t h increasing ductility of the plastic (I, 26) and this optimum for P V C should be i n the range of 0.1 to 0.2 μπι (5-17). To investigate the factors affecting the rubber-toughening of P V d by using our model systems, a series of P B D latices was prepared w i t h particle sizes i n the range of 70 to 450 n m . In addition, a commercial SBR latex of particle size 940 nm was also used, because such a large particle size could not be produced i n our laboratory. The P V C latex was Geon 150X20 made by the Goodrich Chemical C o . The effect of rubber particle size on the Izod impact test is shown i n Figure 6. As expected, a maximum i n impact resistance was obtained i n the rubber particle size range of 150 to 200 n m . A hint of another possible maximum occurs at much higher particle size, possibly a result of the existence of a crazing mechanism i n this system, but the data are too limited for any definite conclusion. The effect of rubber-plastic bonding for this system is shown i n Figure 7 by using 10% P B D i n the optimum range of 250 n m . Again, a grafting index of 20 led to an increase of almost 50 % i n impact strength. The effect



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Figure 6.

Effect of PBD particle size on impact strength of PVC (10 parts).

of bonding was also examined w i t h a lower particle size w i t h a P C P at 77 nm. In this case, the effect of grafting was only minor, presumably because these small rubber particles had only a limited effect i n raising the impact strength from the value of 0.4 ft-lb/in. notch of the unmodified P V C . Styrene-Acrylonitrile Copolymer (SAN) The S A N latex used was an experimental lot of a 70/30 copolymer obtained from The Goodyear Tire & Rubber C o . , Chemical Division. The unmodi fied polymer had an Izod impact strength of 0.28 ft-lb/in. notch. For this work a series of P C P latices was prepared, w i t h particle sizes i n the range of 77 to 1290 n m , and these latices were used as the rubber modifiers for the plastic matrix. The effect of rubber particle size on the impact strength of this system is shown i n Figure 8. A very broad maximum of impact strength in the particle size range of 300 to 800 nm is obvious. These results agree with those of other investigators (18, J9), who studied a very similar sys tem, that is, acrylonitrile-butadiene-styrene graft copolymers (ABS). A p parently the S A N copolymers can fail by both mechanisms, crazing and shear yielding (9, 16), so that both the smaller and larger rubber particles can be effective i n preventing fracture. The effect of rubber-plastic bonding on the impact properties of the S A N copolymer is shown i n Figure 9, when 10 % P C P w i t h a particle size of


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Figure 7. Effect of rubber-plastic bonding (grafting) on impact strength of PVC (lOpartsPCP). Key: PBD (249 nm); Θ, PCP (152 nm); and A, PCP (77 nm).

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MORTON ET AL.

229

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Effect of rubber-plastic bonding (grafting) on impact strength of SAN (10 parts PCP, 332 nm).

332 n m was used as the rubber dispersion. Here the effect is quite dramatic; a grafting index of 25 resulted i n doubling the impact strength of the nongrafted material and more than tripling the impact resistance of the un modified S A N . Acknowledgment This work was supported i n part by a grant from The B F G o o d r i c h C o . Literature Cited 1. Bucknall, C. B. "Toughened Plastics"; Applied Science Publishers, Ltd.: Lon don, 1977. 2. Merz, E. H . ; Claver, G. C.; Baer, M. J. Polym. Sci. 1956, 22, 325. 3. Rose, S. L. Polym. Eng. Sci. 1967, 7, 115. 4. Newman, S. In "Polymer Blends"; Paul, D. R.; Newman, S., Eds.; Academic Press: New York, 1978; Vol. 2, Chap. 13. 5. Ref. 1, Chap. 7. 6. Wagner, E. R.; Robeson, L. M. Rubber Chem. Technol. 1970, 43, 1129. 7. Keskkula, H . ; Turley, S. G. Polymer 1978, 19, 797. 8. Saam, J. C.; Mettler, C. M . ; Falender, J. R.; Dill, T. J. J. Appl. Polym. Sci. 1979, 24, 187. 9. Bucknall, C. B. In "Polymer Blends"; Paul, D. R.; Newman, S., Eds.; Aca demic Press: New York, 1978; Chap. 14.


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10. Keskkula, H. Appl. Polym. Symp. 1970, 15, 51. 11. Chang, E. P.; Takahashi, A. Polym. Eng. Sci. 1978, 18, 350. 12. Paul, D. R. "Polymer Blends"; Paul, D. R.; Newman, S., Eds.; Academic Press: New York, 1978; Chap. 12. 13. Boyer, R. F.; Keskkula, H. "Encycl. Polym. Sci. Technol"; John Wiley and Sons: New York, 1970; p. 392. 14. Bucknall, C. B.; Smith, R. B. Polymer 1965, 6, 437. 15. Newman, S.; Strella, S. J. Appl Polym. Sci. 1965, 9, 2297. 16. Bucknall, C. B.; Street, D. G. SCI Monog. 1967, 26, 272. 17. Purcell, T. O. Polymer Prepr. Am. Chem. Soc. 1972, 13(1), 699. 18. Parsons, C. F.; Suck, E. L. In "Multicomponent Polymer Systems"; Platzer, Norbert A. J., Ed.; ADVANCES IN CHEMISTRY No. 99; American Chemical Society: Washington, D.C., 1971; p. 340. 19. Schott, N. R.; Dhabalia, D. Org. Coat. Plast. Chem. 1974, 34(2), 380. RECEIVED

for review January 20, 1983.

ACCEPTED

August 29, 1983.


Polymer Blends and Composites in Multiphase Systems - American

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