Machine Design


Available both as thermoplastic and thermoset resins, polyimides (Pls) are a family of some of the most heat and fire-resistant polymers known. Moldings and laminates are generally based on thermoset resins, although some are made from thermoplastic grades. Unlike most plastics, Pls are available as laminates and shapes, molded parts, and stock shapes from some materials producers. Thin-film products enamel, adhesives, and coatings - are usually derived from thermoplastic polyimide resins.

Laminates are based on continuous reinforcements including woven glass and quartz fabrics, or fibers of graphite, boron, quartz, or organic materials. Molding compounds, on the other hand, contain discrete fibers such as chopped glass or asbestos, or particulate fillers such as graphite powders, MOS2, or PTFE.

Polyimide films and wire enamels are generally unfilled. Coatings may be pigmented or filled with particles such as PTFE for lubricity. Most adhesives contain aluminum powder to provide a closer match to the thermal-expansion characteristics of metal substrates and to improve heat dissipation.

PI parts are fabricated by techniques that range from powder- metallurgy methods to conventional injection, transfer, and compression molding, and extrusion methods. Porous polyimide parts are also available. Generally, those compounds that are the most difficult to fabricate have the highest heat resistance.

Properties:Polyimide parts and laminates can serve continously in air at 500°F; service temperature for interrmittent exposure can range from cryogenic to as high as 900° F. Glass-fiber reinforced versions retain over 70% of their flexural strength and modulus at 480° F. Creep is almost nonexistent, even at high temperatures and deformation at under load (4,000 psi) is less than 0.05% at room temperature for 24hr.

These materials have good wear resistance and low coefficients of friction, both of which are further improved by PTFE fillers. Self-lubricating parts containing graphite powders have flexural strengths above 10,000 psi, which is considerably higher than those of typical thermoplastic bearing compounds.

Electrical properties of PI moldings are outstanding over a wide range of temperature and humidity conditions.

Polymide parts are unaffected by exposure to dilute acids, aromatic and aliphatic hydrocarbons, hydraulic fluids, JP-4 fuel, and kerosene. They are attacked, however, by dilute alkalies and concentrated inorganic acids.

Polymide abrasives maintain useful properties for over 12,000 hr at 500°F, 9,000 hr at 575°F, 500hr at 650°F, and 100 hr at 700°F. Resistance of these abrasives to combined heat (to 575°F) and saltwater exposure is excellent.

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