Noted for its sparkling clarity, hardness, ease of processing, and excellent colorability, polystyrene is a low-cost amorphous thermoplastic that often competes favorably with higher-priced resins. PS is available in a wide range of grades for injection molding, extrusion, and foam molding. Modifications to the basic, general-purpose PS resin include grades for high heat, and for various degrees of impact resistance. Clarity and gloss are reduced, however, in the impact grades.
Polymers in the styrene family include ABS and SAN, described in another chapter, and the styrene-maleic anhydride (SMA) copolymers. Structural characteristics of these resins are similar, but the SMA resins have the highest heat resistance.
Expandable polystyrene (EPS) is a specialized form of the resin used to make lowdensity (0.75 to 10.0 lb/ft) foam shapes and blocks. These materials are suited for thermal insulation and energy absorption. Copolymers of polystyrene formulated into EPS-type foamable resins provide improved beat tolerance, solvent resistant, or cushioning.
Expandable PS foams are produced in a two-stage process. The resin particles are first expanded to the density required in the finished item while remaining as discrete particles. In the second step, the foamed particles are placed in a vented mold of the final shape. Steam is forced into the mold, heating the particles by direct contact and causing them to expand and fuse together to form a void-free, dry foam part. Pressures in the process are usually below 50 psi, allowing the use of relatively low-cost aluminum tooling.
Properties: Unmodified polystyrene is rigid and brittle, has moderate strength, and is crystal clear. Impact strength is increased significantly by blending the polymer with rubbers such as polybutadiene.
Heat resistance is low compared to that of most thermoplastics; maximum recommended continuous-service temperature is well under 200°F. Electrical properties are good at room temperature and are affected only slightly by higher temperatures and varying humidity conditions.
Polystyrene is soluble in most aromatic and chlorinated solvents. It is insoluble in alcohols such as methanol, ethanol, normal beptane, and acetone. Most foods, drinks, and household fluids that have no effect on polystyrenes, but the resins are attacked by citrus-fruit-rind oil, gasoline, turpentine, and lacquer thinner.
Styrene copolymers: A higher heat resistance distinguishes the styrene-maleic anhydride copolymers from their base styrenic and ABS families. The SMA copolymers have high molecular weights (200,000 to 250,000 average), and are processed by injection molding, extrusion, injection-blow molding, and extrusion-blow molding.
These resins have excellent melt rheology and flow characteristics at melt temperatures of 400 to 550'F and with mold temperatures of 120 to 150°F. Styrene copolymers set up rapidly and provide fast molding cycles. Processibility of both crystal-clear and impact-modified copolymers is excellent. Heat properties, processing ease, and costs provide an excellent balance for cost-effective applications.
Crystal-clear SMA resins can be pigmented to produce a wide range of transparent, translucent, or opaque colors. Best results are obtained with master batches or concentrates using SMA or polystyrene as the carrier resin.
Properties: Heat resistance of SMA resins is typically 20 to 50°F greater than that of polystyrene, SAN, and ABS. Glassfiber reinforcement further improves thermal properties, rigidity, flexural properties, dimensional stability, and impact strength. Typical glass content in these materials is 10 to 20%, depending on end-use requirements.
Both dry blends and precompounded glass-reinforced materials are available. These reinforced resins provide a cost-effective balance of thermal and structural properties and processibility. Both the crystal-clear and impact-modified grades meet FDA and DoA requirements for food-contact use. Flammability rating of the copolymers is UL94 HB.
SNIA-PC blends: Blends of styrene maleic anhydride copolymers and polycarbonate provide an excellent balance of processibility and heat resistance, impact strength, and ductility. Large parts can be molded with fewer gates and lower tonnage machines because of their processibility, and small parts can be molded with pinpoint gates without loss of impact strength.
Properties: Heat resistance and impact strength at room and low temperatures are the primary attributes of SMA-PC blends. Deflection temperature under load (1/8-in. bars) ranges from 226 to 242°F. Gardner and notched Izod impact-strength values are 480 in-lb and 10 to 12 ft-lb. These blends are also compounded with glassfiber reinforcement for improved flexural modulus and heat resistance.