Toughness and Brittleness of Plastics

19 Super High Impact Polystyrene Based on Polystyrene and Butadiene-Styrene Block Polymer Blends Downloaded by UNIV OF ARIZONA on December 4, 2012 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0154.ch019

R. R. DURST, R. M. GRIFFITH, A. J. URBANIC, and W. J. VAN ESSEN General Tire and Rubber Co., Research and Development Division, Akron, Ohio 44329 A blend of a rubbery styrene-butadiene block polymer and commercial polystyrene has been prepared which can be injection molded into articles having high impact strength and most of the other desirable properties of high quality ABS. The impact strength of the molded articles depends on the particle size of the rubbery block polymer component, its composition, and molecular weight. Impact strength reaches a maximum when the average particle size of the rubbery compound is near 1 μm and when the styrene content of the block polymer is preferably in the range 50 ± 10%. A relatively high molecular weight, preferably ca. 250,000 M , is required to process successfully the blend while retaining the impact strength. When molecular weight is below 150,000 M , impact strength decreases with increased processing. Articles molded from this polymer blend have an impact strength, flexural modulus, heat distortion, hardness, and appearance comparable with high quality ABS. n

n

Attempts to improve the impact strength or to toughen polystyrene are not new. Some early attempts to improve the impact strength by blending butadiene rubber with polystyrene failed because of the poor bond between the separate phases (1,2,3,4). Commercial high impact polystyrene (HIPS), having rubber present during the polymerization of the styrene, will reach an Izod impact of 2.2 ft-lbs/inch compared with 0.2 ft-lbs/inch for unmodified polystyrene. Styrene grafts on the rubber were sufficient to improve the bond between phases; this resulted in the higher impact (5,6,7,8,9). The commercial 239 In Toughness and Brittleness of Plastics; Deanin, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

TOUGHNESS A N D BRITTLENESS OF PLASTICS

Downloaded by UNIV OF ARIZONA on December 4, 2012 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0154.ch019

240

HIPS products however do not have sufficient impact strength for many uses. Skilled researchers agree that HIPS with impact strength approaching 7.5 ft-lbs/inch notch can be prepared by adding more rubber. However the hardness, flexural modulus, tensile, and heat distortion temperatures are then drastically reduced. Van Henten, at the Shell Plastic Laboratories (II), showed that styrene-butadiene block polymers can be blended with commercial HIPS to upgrade its impact strength to 5.8 ft-lbs/inch. Childers, at Phillips Petroleum (12), blended commercial polystyrene with block polymers in a Brabender plastograph. To control rubber particle size he added a peroxide during the blending operation, thereby creating crosslinks. With this technique he achieved an impact strength of 5.9 ft-lbs/inch. Here we describe work which led to a high impact polystyrene blend, designated as PS/SBS, with a notched Izod impact of ca. 7.5 ft-lbs/inch. This product was obtained by mechanically or solution blending crystal polystyrene with a proprietary, unbranched graded styrene-butadiene-styrene block polymer. The blend has impact strength, flexural modulus, heat distortion, hardness, and molded appearance more equal to a high quality ABS resin than other polystyrene blends reported. The morphology, processing, and properties of the PS/SBS are briefly discussed and compared with those of commercial HIPS and ABS resin. Development of PS/SBS Blend Initially various rubbery butadiene and styrene-butadiene block polymers were screened as impact-modifying agents for polystyrene. Commercial polystyrene and various rubbers were blended by dissolving the polymers in benzene and by subsequently precipitating them with isopropyl alcohol. The solid polymer blends were dried and molded into test bars. Laboratory and commercial polybutadiene and polystyrene were used in several combinations with the block polymer prepared in our laboratory. A preliminary screening indicated that excellent impact could be obtained using 15-20 wt % butadiene based on the total polymer blend. At #20 wt % butadiene, several block polymers were screened for optimum impact and overall balance of properties. Two-component systems (block polymer-polystyrene) and three-component systems (block polymer—polybutadiene—polystyrene ) were tried. The impact varied with the styrene content of the block polymer in both two- and three-component systems as shown in Figure 1. Subsequent work showed that the best overall balance of impact, flexural modulus, and heat distortion was obtained at 15% butadiene.

In Toughness and Brittleness of Plastics; Deanin, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

19.

DURST E T A L .

Super High Impact Polystyrene


O* I ' I /

\ 1 1

\

V \

241

O-BLOCK, POLYSTY.PBD #-BLOCK, POLYSTY

10 20 30 40 50 60 70 80 90 PERCENT STYRENE IN BLOCK

Figure 1.

POLYMER

Impact strength vs. composition

For economic reasons we achieved the same quality blends by mechanically mixing commercial polystyrene with the block polymers. Good quality products were obtained from two-roll mills, the Brabender plastograph, the Banbury mixer, and continuous high intensity mixers. With high shear mixing equipment such as the Banbury or continuous mixer, the use of block polymers with a number average molecular weight (M„) below 150,000 did not give the expected improvement in impact over polystyrene. This anomalous behavior occurred because the particle size of the discrete rubber phase averaged less than 0.2 ton compared with an average of 1 fim for blends with optimum impact obtained in the solution blends. We attempted to prevent excessive dispersal of rubber particles by blending at a higher temperature which decreases the viscosity of the mix and reduces the shear experienced by the rubber. This technique slightly improved the impact strength, but the product still failed to match the solution blended polymers. Block polymers of a higher molecular weight, around 250,000 M », dispersed into particles in the 1 ton range even in the intensive mixing units. The use of higher molecular weight block polymers resulted in polymer blends with properties equal to the best achieved by the solution blending technique. These blends retained their high impact during fabrication and in reprocessing operations. After the molecular weight and composition of the block polymer were fixed at appropriate levels, mechanical blending was scaled-up to both larger and continuous dispersion mixers. An acceptable and reproducible product was made at all levels up to the highest output tried, e.g., 1000 lbs/hr.


TOUGHNESS A N D BRITTLENESS O F PLASTICS


242

Figure 2. Phase contrast photomi crographs of HIPS, PS/SBS, and ABS

Figure 3. Transmission electron micrographs of HIPS, PS/SBS, and ABS


19.

DURST E T A L .

Super High Impact Polystyrene

243


O

"D" PARTICLE DIAMETER IN M Figure 4.

Size distribution of rubber particles in PS/SBS by thin section microscopy

Figure 5. Effect of molecular weight of the block polymer on its particle size and blend impact strength in mechanical dispersions in polystyrene Photograph MolWt Av Particle Diam Notched Izod Impact i(Mn) (fim) (ft-lb/inch) J/m (a) 250,000 1 7.5 400 (b) 165,000 0.2 1.0 53 Blends contain constant composition, styrene-butadiene block copolymer


244

TOUGHNESS A N D BRITTLENESS O F PLASTICS


Morphology of PS/SBS Blend Rubber particle size has been recognized as an important structural parameter in HIPS and ABS polymers (JO). The dispersed phase in PS/SBS has been examined and compared with that in HIPS and ABS. Figure 2 shows thin section phase contrast photomicrographs of samples of PS/SBS, HIPS, and ABS resin. Figure 3 shows the transmission electron micrographs of the same materials. In HIPS onefindspredominantly 1-10/xm rubber particles (8, JO). In PS/SBS the rubber particles resemble those in ABS, but the average particle size is near 1 ton in diameter in contrast to ABS with an 0.3-0.5 /*m average particle size. The ABS particle size range agrees with observations in the literature (JO). The weight fractions of various particle sizes in PS/SBS were estimated from phase contrast and electron micrographs. The broad distribution (see Figure 4) was expected from mechanical blending. The effect of the molecular weight of the block polymer on the size of the rubber particle and on the impact strength of the PS/SBS samples made under the same high shear mixing conditions is shown in Figure 5. The higher molecular weight results in a larger average particle size for the rubbery phase and in a higher impact strength for the PS/SBS. Properties of PS/SBS Blend PS/SBS can be injection molded into test specimens and commercial articles on both laboratory- and commercial-size equipment. Processing conditions and cycles recommended for HIPS and ABS were acceptable for PS/SBS. The properties of PS/SBS are compared with HIPS and ABS in Tables I and II. PS/SBS is similar to HIP in color, hardness, tensile strength andflexuralstrength and to ABS in impact strength, stiffness, and heat resistance. PS/SBS is intermediate between ABS and HIPS in tensile creep, gloss, and chemical resistance. Normal finishing methods Table I.

Physical Properties of PS/SBS, HIPS, and ABS Polymer

Property

Color Gloss, 60° Melt flow Rockwell " R " Gardner impact

Units —

g/10 min (condition g) —

J/mm in.-lbs/mil

PS/SBS

ABS

HIPS

white 70-86 1.0

white 70 3.5

yellow 90 1.7

103 9.9 2.2

103 0.9 0.2

105 11.8 2.6


19.

DURST E T A L . Table II.

245

Super High Impact Polystyrene Physical Properties of PS/SBS, HIPS, and ABS Polymer


Property

Unnotched, Izod impact Notched, Izod impact Tensile yield strength Elongation Flexural modulus x 10Flexural strength 5

Deflection temp (Unannealed 1.82 MPa (264 psi) Annealed 2 hr at 73°C)

U nits

J/m 1 ft-lbs/in. S J/m I ft-lbs/in. < MPa I psi S

%

< MPa / psi

j/ psi MPa

PS/SBS

1185 22 400 7.5 29.4 4200 28 0.021 3.0 46 6600

HIPS

747 14 117 2.2 19.6 2800 38 0.019 2.7 30 4300

ABS

1334 25 374 7.0 41.2 5880 30 0.020 2.8 62 8800

87

75

88

93

82

94

can be used with PS/SBS. It can be painted, solvent welded, vacuum metallized, and hot stamped. Summary Commercial polystyrene can be efficiently and reproducibly toughened by mechanical blending with a styrene-butadiene rubbery block polymer of specific composition. It is not necessary for this rubber to be crosslinked, but it must have sufficiently high molecular weight to resist particle size breakdown during processing of the blend into finished articles. The particle size distribution may be broad, but the diameter of most rubber particles must average 1 /mi for optimum impact strength and other physical properties. The end result of this work has been a high impact, rubber-modified polystyrene blend which has processibility and end use properties much closer to ABS than any polystyrene blend previously reported. Acknowledgment This paper is published with permission of the General Tire and Rubber Co. The authors gratefully acknowledge the contributions made by R. T . Giuffria, V. M . Bauer, and H . F. Oswald for the phase contrast photomicrographs and transmission electron micrographs and S. L . Aggarwal, R. A. Livigni, and R. E . Bingham for valuable advice on polymer preparation and characterization.


246

TOUGHNESS AND BRITTLENESS OF PLASTICS

Literature Cited


1. 2. 3. 4. 5. 6. 7. 8. 9.

Turley, S. G., J. Polym. Sci. (1963) C1, 101. Willersinn, H., Makromol. Chem. (1967) 101, 297. Simmons, P., Rubber Plast. Age (1967) 48, 442. Rosen, S. L., Polym. Eng. Sci. (1967) 7, 115. Wagner, E. R., Robeson, L. M., Rubber Chem.Technol.(1970) 43, 1129. Keskkula, H., Appl. Polym. Symp. (1970) 15, 51. Fettes, E. M., Maclay, W. N., Appl. Polym. Symp. (1968) 7, 3. Skeist, I., Rubber World (May 1967) 89. Deland, D. L., Purdon, J. R., Schoneman, D. P., Chem. Eng. Prog. (1967) 63, (No. 7), 118. 10. Bucknall, C. B., Drinkwater, I. C., J. Mater. Sci. (1973) 8, 1800. 11. van Henten, J., Plastica (1972) 25, 4, 144. 12. Childers, C. W., U.S. Patent 3,429,951 (1969). RECEIVED October 18, 1976.


Toughness and Brittleness of Plastics

Recommend Documents