Recycling of aramid fiber using subcritical and supercritical water

Poly(p-phenylene terephthalamide) (Kevlar (R)) 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 (R) 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 (R) fibers, with yields of approximately 95% for each monomer obtained at 250 degrees C, 4 MPa, and an initial molar ratio of sodium hydroxide to the Kevlar (R) 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 (R)-fiber hydrolysis was examined assuming a cylindrical-surface reaction model, and temporal changes in the decomposition efficiencies of Kevlar (R) fibers were found to be reproduced well using this model. (C) 2019 Elsevier Ltd. All rights reserved.