Impact of subcritical and supercritical water on both depolymerization kinetics of nylon 6 and recycling carbon fibers from waste composite

Carbon fibers reinforced Polymers (CFRP) are extensively used. Their residues are being discarded every year, which creates serious ecological and social problems. Many efforts have been launched in their further utilization after use. Chemical recycling using sub and supercritical fluids shows good prospects for recycling carbon fibers. Thus, solvolysis of waste CFRP was investigated. The experiments were carried out at temperatures in subcritical (280-350°C) and in supercritical (400-600°C) regions under the estimated pressure of 25 MPa for reaction times of 1-120 min. Carbon rate recovered in liquid phase was measured by Total Organic Carbon (TOC). The microstructure of the recovered carbon fibers was observed using scanning electron microscopy (SEM). In subcritical region, the rate of decomposition efficiency reached 98.95 wt.% at 280°C in 30 min while , in supercritical region, the resin removal has already reached 97.18 wt.% at 400°C in only 15 min. Carbon rate recovered in liquid phase was in agreement with decomposition rate for both regions. The results revealed clean carbon fibers without physical damages and present tensile strength close to one of virgin fibers. In subcritical region, the identification of the recovered organic products indicated that monomer of the resin and other molecules were obtained. Assuming a pseudo-first order reaction, the degradation kinetics was studied and the activation energy was evaluated to be 77.79 kJ/mol and 45.81 kJ/mol in the subcritical and supercritical regions respectively. This difference in overall kinetics parameters clearly highlights that the reaction mechanism pathways are different in both regions.