Recycling of aramid fiber using subcritical and supercritical water

Poly(p-phenylene terephthalamide) (Kevlar®) fibers, with good chemical and mechanical strengths, were hydrolyzed into their constituent p-phenylenediamine and terephthalic acid monomers using supercritical water, subcritical water, and subcritical water with sodium hydroxide. The two monomers recovered from the Kevlar® fibers were purified using simple extraction and precipitation methods, and the purity of each monomer exceeded 99%. Treatment with subcritical water and sodium hydroxide for 6 h completely decomposed the Kevlar® fibers, with yields of approximately 95% for each monomer obtained at 250 °C, 4 MPa, and an initial molar ratio of sodium hydroxide to the Kevlar® unit of five. On the other hand, treatment with subcritical and supercritical water resulted in lower decomposition efficiencies and monomer yields. In addition, the kinetics of Kevlar®-fiber hydrolysis was examined assuming a cylindrical-surface reaction model, and temporal changes in the decomposition efficiencies of Kevlar® fibers were found to be reproduced well using this model.