Impact of Solvolysis Process on Both Depolymerization Kinetics of Nylon 6 and Recycling Carbon Fibers from Waste Composite

Carbon fibers reinforced polymers (CFRP) are extensively used. Regarding the high cost of production, the carbon fibers reuse is an important challenge. Chemical recycling using sub and supercritical fluids shows good prospects for recycling carbon fibers. Thus, solvolysis of CFRP waste was investigated to determine suitable operating conditions for preserving mechanical properties of carbon fibers, and to evaluate the kinetics parameters as well as the main reaction scheme. The experiments of decomposition of nylon 6 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. The microstructure of the recovered carbon fibers was observed using environmental scanning electron microscopy (ESEM) and Raman spectroscopy. The amount of carbon, measured by Total organic carbon (TOC), in the liquid phase highlights that the depolymerization was enhanced compared to a complete degradation into CO2. In subcritical region, the resin removal reached 98.95 wt% at 280 °C in 30 min while, in supercritical region, it has already reached 97.18 wt% at 400 °C in only 15 min. The monomer of the resin and other molecules were identified by gas chromatography coupled to mass spectroscopy (GC-MS) in liquid phases recovered from both regions. Assuming a pseudo-first order reaction, the activation energy was evaluated to be 77.79 and 78.51 kJ/mol in the subcritical and supercritical regions respectively. These kinetics parameters highlight that the reaction mechanism pathways are similar in both regions. ESEM results revealed clean carbon fibers. The tensile strength of recovered carbon fibers was close to that of virgin fibers. However, Weibull moduli of recovered carbon fibers showed a large distribution of defects which was confirmed by Raman analysis that displayed a slight modification in carbon structure.