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Polymer Science and Engineering: The Shifting Research Frontiers
FIGURE 3.6 A lyotropic para-aramid liquid crystal polymer.
drawdown to orient the molecular chains, actually the liquid crystal domains, parallel to the fiber axis. An annealing step may be performed to improve structural perfection, resulting in an increase of fiber modulus. These fibers have very high modulus and tensile strengths as well as excellent thermal and environmental stability. Weaknesses include low compressive properties (endemic with all highly uniaxially oriented polymers) and a significant moisture regain. Worldwide fiber production capacity is about 70 million pounds (1991). Selling prices vary according to grade (i.e., modulus level) and market, ranging from as low as $8 per pound to over $50 per pound. Consumption worldwide in 1990 was about 50 million pounds, somewhat trailing capacity. Major markets include reinforcement for rubber and composites, protective apparel, ropes and cable, and asbestos replacement. The use of para-aramid fiber is projected to grow at greater than 10 percent per year worldwide over the next 5 years. The environmental issues involved in the handling and disposal of large quantities of sulfuric acid or other solvents may make thermotropic approaches more attractive in the future.
During the 1980s, thermotropic copolyesters were commercialized world-wide. More versatile than the lyotropic polymers, these nematic copolyesters (Figure 3.7) are amenable to uniaxial processing, such as fiber formation, and three-dimensional processing, such as injection molding, utilizing essentially conventional thermoplastic processing techniques. While fiber products exist, most of the commercial thermotropic copolyester is sold as glass-or mineral-filled molding resins, the majority into electrical and electronic markets. U.S. volume in 1991 was about 4.3 million pounds, at an average selling price of about $8 per pound. As in the case of the aramids, thermal and environmental stability is excellent. Advantages of these molding resins are the extremely low viscosity, allowing the filling of complex, thin-walled molds, excellent mold reproduction because of the low change in volume between liquid and solid, and fast cycle times. Weaknesses include property anisotropy and high cost.
The future growth of the main-chain nematogenic polymers will be dominated by two factors: