Decomposition temperature, aramids

It was, however, observed that such systems under appropriate conditions of concentration, solvent, molecular weight, temperature, etc. form a liquid crystalline solution. Perhaps a little digression is in order here to say a few words about liquid crystals. A liquid crystal has a structure intermediate between a three-dimensionally ordered crystal and a disordered isotropic liquid. There are two main classes of liquid crystals lyotropic and thermotropic. Lyotropic liquid crystals are obtained from low viscosity polymer solutions in a critical concentration range while thermotropic liquid crystals are obtained from polymer melts where a low viscosity phase forms over a certain temperature range. Aromatic polyamides and aramid type fibers are lyotropic liquid crystal polymers. These polymers have a melting point that is high and close to their decomposition temperature. One must therefore spin these from a solution in an appropriate solvent such as sulfuric acid. Aromatic polyesters, on the other hand, are thermotropic liquid crystal polymers. These can be injection molded, extruded or melt spun. 

A copolymeric derivative of the para-aramid fibres was introduced by Teijin in 1985 under the tradename Technora . This fibre is based on the copolymeric structure copoly-(paraphenylene/3,4 -oxydiphenylene terephthalamide) and is claimed to have a much higher chemical resistance than PPTA as well as increased abrasion and steam resistance, which are useful properties in many protective applications. Technora has a decomposition temperature of about 500 °C, and other properties are comparable with PPTA although its LOI value is slightly lower at 25 vol%. 

A promising approach toward reducing flammability of polymer systems is to alter the condensed-phase chemistry at elevated temperatures. Structure modification can alter the decomposition chemistry to favour transformation of the polymer to a char residue. Such an increase occurs when thermally induced stable crosslinking structures and/or aromatic rings are produced. Work by the Brooklyn Polytechnic in New York has examined systems such as polystyrene, aramids, PF resins and nitrile polymers such as SAN. 

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