Liquid crystal polymers (LCPs) are thermoplastic aromatic polymers that exhibit a highly ordered structure in the melt or in solution as well as in the solid state. This highly ordered configuration leads to properties that are highly anisotropic. LCPs have structures with a succession of para-oriented ring structures to give a stiff rodlike chain with a high axial ratio (ratio of length of molecule to its width). This stiff rodlike structure results in high mechanical properties measured in the direction of flow, particularly on the surface, properties usually associated with reinforced thermoplastic resins. Hence, LCPs are also referred to as self-reinforcing polymers. The principal monomer used in all commercial thermotropic LCPs is 4-hydroxybenzoic acid (HBA). A homopolymer of HBA is liquid crystalline, but does not flow at temperatures below 500°C. Consequently, other monomers are used to provide a melt processable LCP. It is possible to manufacture a continuum of heat distortion temperature (HDT) LCPs by varying the comonomers and their molar ratio. Monomers are selected based on the resultant HDT, or their ability to reduce cost, improve processability, and yield desired physical properties. Given the wide range of LCPs that can be produced, the industry has adopted a system that classifies the LCPs by type based on HDT. There are three general families based on thermal performance:
- Type-I
- LCPs offer the highest melting points and thermal resistance and are, correspondingly, the most difficult to process. Type-I polymers also have the highest tensile strength and modulus of the three types. Examples of Type-I LCPs are Amoco's XYDAR and Sumitomo's EKONOL. Heat-deflection temperature upper limits range between 260 and 355°C.
- Type-II
- LCPs have a relatively high heat-deflection temperature upper limit (210-260°C) when compared to most engineering and performance polymers. Examples of such a product are VECTRA LCPs, many of which are based on a copolymer of hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid. Type II LCPs offer a good combination of properties and good processability using standard fabricating equipment such as injection molding machines. XYDAR II is another example of a Type-II LCP.
- Type-III
- LCPs have relatively low heat-distortion temperatures (up to 210°C). An example is a 60/40 molar ratio copolymer of p-hydroxybenzoic acid and poly(ethylene terephthalate). This product is manufactured by Unitika, and has a heat-deflection temperature of 68oC, unfilled. The 80/20 copolymer has a heat-deflection temperature of 110-163°C depending on fabrication conditions. Glass reinforcement increases the HDT of both copolymers.
The figure below graphically shows the cost and performance relationships among selected high temperature (performance) polymers. The cost data are based on current market and list prices, combined with density factors to convert cents per pound to cents per cubic inch. The performance ratings are based on weighted calculations combining mechanical, processing, thermal, and electrical property comparisons. RELATIVE COST/PERFORMANCE RELATIONSHIPS
OF PERFORMANCE POLYMERS
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