- Liquid crystal polymer
Liquid crystallinity in polymers may occur either by solving a polymer in a solvent (lyotropic liquid crystal polymers) or by heating a polymer above its glass or melting transition point (thermotropic liquid crystal polymers). The main example of lyotropic LCPs is the commercial aramid known as Kevlar. Chemical structure of this aramid consists of lineararly substituted aromatic rings linked by amide groups. In a similar way, several series of thermotropic LCPs have been commercially produced by several companies (e.g. Vectra). A high number of LCPs, produced in the 1980s, displayed order in the melt phase analogous to that exhibited by non-polymeric
liquid crystals. Processing of LCPs from liquid crystal phases (or mesophases) gives rise to fibers and injected materials having high mechanical properties as a consequence of the self-reinforcing properties derived from the macromolecular orientation in the mesophase. Today, LCPs can be melt-processed on conventional equipment at high speeds with excellent replication of mold details.
A unique class of partially crystalline aromatic polyesters based on
p-hydroxybenzoic acidand related monomers, liquid crystal polymers are capable of forming regions of highly ordered structure while in the liquid phase. However, the degree of order is somewhat less than that of a regular solid crystal. Typically LCPs have a high mechanical strength at high temperatures, extreme chemical resistance, inherent flame retardancy, and good weatherability. Liquid crystal polymers come in a variety of forms from sinterable high temperature to injection moldable compounds. LCP can be welded, though the lines created by welding are a weak point in the resulting product. LCP has a high Z-axis coefficient of thermal expansion.
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 causticmaterials.
Because of their various properties, LCPs are useful for electrical and mechanical parts, food containers, and any other applications requiring chemical inertness and high strength.
* [http://www.ides.com/generics/LCP.htm The Plastics Web]
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