Polyamide-imides are engineering thermoplastics characterized by excellent dimensional stability, high strength at high temperature, and good impact resistance. Molded parts can maintain structural integrity in continuous use at temperatures to 500°F.
Polyamide-imide, produced by Amoco Performance Products Inc. and sold under the Torlon tradename, is available in several grades including: a general-purpose, injection-molding grade; three PTFE/graphite wear-resistant compounds; a 30% graphite - fiberreinforced grade; and a 30% glass -fiber-reinforced grade. Additional grades are developed to meet special requirements.
Torlon resins are moldable on screw-injection-molding machines. Molds must be heated to 425° F, and the barrel and nozzle should be capable of being heated to about 700° F. High injection speed and pressure (20,000 psi or greater) are desirable. Developing optimum physical properties in injectionmolded or extruded parts requires postcuring through an extended, closely controlled temperature program gradually reaching 500 ° F. The specific time/temperature program depends on part configuration and thickness.
Properties: Room temperature tensile strength of unfilled polyamide-imide is about 28,000 psi and compressive strength is 32,000 psi. At 450° F, tensile strength is about 9,500 psi - as strong as many engineering plastics at room temperature - and continued exposure at 500°F for up to 8,000 hr produces no significant decline in tensile properties.
Flexural modulus of 730,000 psi of the unfilled grade is increased, with graphitefiber reinforcement, to 2.9 million psi. Retention of modulus at temperatures to 500°F is on the order of 80% for the reinforced grade. Creep resistance, even at high temperature and under load, is among the best of the thermoplastics; dimensional stability is extremely good.
Polyamide-imide is extremely resistant to flame and has very low smoke generation. The unfilled grade is rated 94V-0 at thicknesses as low as 0.008 in. and has an oxygen index of 45%. Reinforced grades have surpassed FAA requirements for flammability, smoke density, and toxic gas emission.
Radiation resistance of polyarnide-imide is good; tensile strength drops only about 5% after exposure to 109 rads of gamma radiation. Chemical resistance is good, the resin being virtually unaffected by aliphatic and aromatic hydrocarbons, halogenated solvents, and most acid and base solutions. It is attacked, however, by some acids at high temperature, steam at high pressure and high temperature, and strong bases.
Torlon moldings absorb moisture in humid environments or when immersed in water, but the rate is low, and the process is reversible. For example, at 50% RH and 70°F, they absorb about 1% moisture (by weight) in 1,000 hr. Parts can be restored to original dimensions by drying.