Polyphenylene sulfide (PPS) is a crystalline, high-performance engineering thermoplastic characterized by outstanding high-temperature stability, inherent flame resistance, and broad chemical resistance. PPS resins and compounds manufactured in the U.S. are tradenamed Ryton by their producer, Phillips Chemical Co. PPS compounds are also marketed in the U.S. by several other companies that buy the resin from foreign sources.

A wide range of injection-molding grades of PPS is available. The series having various glass-fiber levels (designated R3, R-4, and R-5) is recommended for mechanical electronic applications requiring high mechanical strength, impact resistance, and insulating characteristics. All other compounds contain various mineral fillers in addition to glass-fiber reinforcement. Ryton R-7 and R-8 are suitable for electrical applications requiring high arc resistance and low arc tracking. The R-10 series of pigmented compounds includes several grades suitable for support of current-carrying parts in electrical components. PPS is essentially transparent to microwave radiation, so the R-11 series is suited specifically for microwave ovenware and appliance components.

Unreinforced PPS resins are also available as powders for slurry coating and electrostatic spraying. The resin coatings are suitable for food-contact applications as well as for chemical-processing equipment.

The injection-moldable PPS compounds require processing temperature of 600 to 650°F. Mold temperatures can range from 100 to 275° F to control the crystallinity. Cold-molding parts deliver optimum mechanical strength, and hot-molded highlY crystalline parts provide optimum dimensional stability at high temperatures.

PPS is also available in Iong-fiber-reinforced forms. One type, called starnpable sheet, contains fiber mat reinforcement for processing by compression molding. The other form contains reinforcement, and is designed for laminating and thermoforming. Reinforcement in both forms can be glass or carbon fiber.

Properties: Most PPS compounds are used for their combination of high-temperature stability, chemical resistance, dimensional reliability, and flame retardance. The compounds all have excellent stability at very high temperatures. UL temperature index is 200 to 240° C, depending on compound, thickness, and end use. In short-term excursions, the compounds have heat-deflection temperatures of 500°17 or higher, depending on the crystallinity of the molding. Mechanical strength of the compounds remains high at high temperatures. For example, the flexural modulus of Ryton R-4 is as high at 500° F as that of ABS at room temperature.

PPS has excellent resistance to a broad variety of chemicals, even at high temperatures. In fact, the resin has no known solvent below 400°F. The resins are flame-retardant without additives (UL 94V-0/5). The oxygen index of the resin is 44, and indexes of the compounds range from 47 to 53. Because flame retardance is inherent, regrind is as flame resistant as virgin material.

Mechanical properties of the various PPS compounds are tailored for target applications. The balance of properties can be controlled by the degree of crystallinity of a molded part. Amorphous moldings have optimum mechanical strength at room temperature, and crystalline moldings deliver optimum dimensional stability at high temperatures.