[Advanced Materials] A look at aramid: intro, process, and applications
Aramid fiber production processes.
Synthesis of para-aramid fiber. The main raw materials for para-aramid are terephthaloyl chloride (TPC) and p-phenylenediamine (PPD). Para-aramid must undergo polycondensation under anhydrous conditions. The preparation methods include: (1) Interfacial polycondensation: dissolve the dicarboxylic acid chloride in a water-immiscible organic solvent such as benzene or carbon tetrachloride, then dissolve the diamine in water (with a small amount of NaCO3 or NaOH added to absorb the HCl produced by the reaction), then mix the two solutions. The instant they are combined, polycondensation occurs at the interface between the two liquids, forming a polymer film. Because the reaction takes place at the interface, it is called interfacial polycondensation. (2) Low-temperature solution polycondensation: currently the most mature process for synthesizing aramid fiber. The industrialized Kevlar and Technora fibers are both made this way. In a glass polymerization reactor equipped with a stainless steel stirrer and flushed with dry N2, add an NMP solution containing a certain amount of anhydrous LiCl and pyridine, add powdered p-phenylenediamine at room temperature, and after it dissolves, cool the solution to a set temperature with an ice-water bath, then add a stoichiometric amount of powdered terephthaloyl chloride while speeding up stirring. As the reaction proceeds, the solution viscosity increases and the liquid surface rises; after a few minutes, rod climbing occurs and gelation appears. Keep stirring for a few more minutes, crush the yellow gel mass, then let the product stand for more than 6 h. Add a small amount of water to the resulting polymer, crush and filter it, then wash it repeatedly with cold and hot water to remove residual solvent, LiCl, HCl, and pyridine until the wash liquid is neutral. Dry the polymer at 100°C for more than 5 h to obtain a dry polymer. Then mix the polymer in cold concentrated sulfuric acid and heat to 75°C to form a nematic liquid crystal solution, which is then spun.
Synthesis of meta-aramid fiber. Meta-aramid is made by polycondensation of IPC and MPD, and there are low-temperature polymerization, interfacial polycondensation, emulsion polymerization, and gas-phase polymerization methods. Among these, low-temperature polymerization and interfacial polycondensation are more common. DuPont in the US uses low-temperature polymerization for the polycondensation and dry spinning; Teijin in Japan uses interfacial polymerization for the polycondensation and wet spinning; two domestic companies, Tayho Advanced Materials in Yantai, Shandong, and Caiyan in Guangdong, both use low-temperature solution polymerization and wet spinning to produce meta-aramid. (1) Low-temperature polymerization: under stirring, dissolve MPD in N,N-dimethylacetamide (DMAc) solvent, cool to about 0°C, then under stirring add IPC and raise the temperature to 50–70°C for the reaction. HCl is produced during the reaction, so Ca(OH)2 should be added to neutralize it, turning the solution into a DMAc-CaCl2 system; adjusting its concentration makes it ready for wet spinning. This method uses less solvent, has simple operating steps, and gives high production efficiency, so low-temperature polycondensation is widely used. (2) Interfacial polymerization: dissolve IPC in tetrahydrofuran (THF) to form the organic phase; dissolve MPD in aqueous sodium carbonate solution to form the aqueous phase; then under vigorous stirring add the organic phase to the aqueous phase so that polycondensation occurs rapidly at the interface between the two phases. The resulting polymer precipitates out and is filtered, washed, and dried to give a solid product. Interfacial polymerization reacts fast and yields a polymer of high relative molecular mass, and can be used to prepare high-quality spinning dope, but the process is complex and demands high equipment requirements, which leads to higher investment.
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