Machine Design

Liquid-Crystal Polymers

Liquid-crystal polymers are a unique class of wholly aromatic polyester poly mers that provide previously unavailable high-performance properties. Particularly outstanding is their heat-deflection temperature at 264 psi of 460 to 606°F. Structure of the LCPs consists of densely packed fibrous polymer "chains" that provide selfreinforcement almost to the melting point.

Before the commercial introduction of LCP resins in 1984, liquid-crystal polymers could not be injection molded. Today's resin can be melt processed on conventional equipment into thin-wall as well as heavywall components at fast speeds with excellent replication of mold details and efficient use of regrind.

Commercial melt-processible resins now available are Xydar formulations (biphenol based) from Amoco Performance Products and Vectra resins (naphthaline based) from Hoechst Celanese Corp. Like most thermoplastics, molding these high -temperature resins requires heated tools and equipment capable of producing melt temperatures of 545 to 640° F for Vectra resins and 700 to 850°F for Xydar materials.

Properties: LCP resins are characterized by outstanding strength at extreme temperatures; excellent mechanical-property retention after exposure to weathering and radiation; good dielectric strength, arc resistance, and dimensional stability; low coefficient of thermal expansion; excellent flame retardance; and easy processibility. UL continuous-use rating for electrical properties is as high as 240° C and, for mechanical properties, 220° C. The high heatdeflection value of biphenol-based resins permits molded parts to be exposed to intermittent temperatures as high as 315° C without affecting properties. Resistance to high temperature flexural creep is excellent, as are fracture-toughness characteristics.

LCPs are exceptionally inert. They resist stress cracking in the presence of most chemicals at elevated temperatures, including aromatic or halogenated hydrocarbons, strong acids, bases, ketones, and other aggressive industrial substances. Hydrolytic stability in boiling water is excellent. Environments that deteriorate the polymers are high-temperature steam, concentrated sulfuric acid, and boiling caustic materials.

Oxygen index of LCP resins ranges from 35 to 50%. When exposed to open flame, the material forms an inturnescent char that prevents dripping and results in extremely low generation of smoke containing no toxic by-products. The resin has UL 94V-0 and 5V flammability ratings at' 1/16 in. and an NBS smoke-chamber rating (NBS-Ds,-4) of 3 to 5. Compounds are unaffected by high doses of ionizing or cobalt 60 radiation (up to 10 billion rads), withstand high levels of ultraviolet radiation, and are transparent to microwaves and other radiation of similar wavelength.

Easy processibility of the resins is attributed to its liquid-crystal molecular structure, which provides high melt flow and fast setup in molded parts. However, molded parts are highly anisotropic, and knit lines are much weaker than other areas. Properties are not affected by minor variations in processing conditions, and no postcuring is required.

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