Research On Recycling Mechanism And Process Of Carbon Fiber/Epoxy Resin Composites By Supercritical Fluids

The recycling and reuse of the carbon fiber reinforced resin matrix composites (CFRP), which is the key measure to sustainable application of the advanced composites, fits the desire of green to the sustainable development of national economy. The recycling of the high performance carbon fibers from the waste CFRP has the great economic and environmental significance. The surface performance, mechanic performance and recycling efficiency of the carbon fibers are the key factor of impact on recycling value of CFRP, so developing a high efficiency, high value and environmentally friendly recycling method, technology and process is a necessary precondition for improving comprehensive effect of recycling process. This article proposed and investigated the recycling method, technology and process of CF/EP composites based on theoretical and experimental researches.The main research contents include the following six aspects:(1) The high performance carbon fibers can be recycled by supercritical fluids. The mass transfer of supercritical fluid during degradation of CF/EP composites was analyzed from two aspects:mass diffusion of supercritical fluid in CF/EP composites, and mass diffusion of degradation products in supercritical fluid. Mass transfer model and equation of supercritical fluid was established based on a proposed concentric cylindrical representative volume element. The reaction kinetic parameters were incorporated into the mass transfer equation, and the concentration distribution and mass transfer rate calculation method of supercritical fluid considering Fick and non-Fick effect was proposed.(2) The process test system of recycling CF/EP composites by supercritical fluids was built up. When the methanol, ethanol, n-propanol and n-butanol was used as cosolvents, promoting effects of the different cosolvents on supercritical CO2 degradation of CF/EP composites was investigated based on analysis of degradation rate of epoxy resin; the effects of reaction temperature and time on degradation rate of epoxy resin was analyzed when supercritical acetone, methanol, ethanol, n-propanol, n-butanol and isopropanol was used for degradation of CF/EP composites; degradation capability of supercritical fluids on CF/EP composites was investigated from aspects of dipole moment, dielectric coefficient and Hildebrand parameter; and the selection mechanism of supercritical fluids used for degradation of CF/EP composites was proposed.(3) The reaction kinetics models of CF/EP composites degradation in supercritical fluids were established by analyzing the chain scission reaction of a cross-linked network in CF/EP composites. The reaction order of CF/EP composites degradation can be obtained by the Levenberg-Marquardt and universal global optimization algorithm, combining with principle of correlation coefficient approximation. The reaction rate constant k was calculated at different reaction temperatures, and the activation energy E and the pre-exponential factor ko of CF/EP composites degradation reaction can be calculated by performing linear regression on change relation of Ink with 1/T. Finally, reaction kinetics equations of CF/EP composites non-catalytic degradation in different supercritical fluids were also proposed.(4) The effects of reaction temperature and time on the components content of main liquid phase products was investigated based on analysis of the components of liquid phase products form non-catalytic and catalytic degradation at different temperatures and times. The components of supercritical n-butanol blank sample was analyzed, then non-catalytic and catalytic degradation process of CF/EP composites in supercritical n-butanol was inferred from the aspects of chain scission reaction based on molecular collision and free radicals reaction theory. Finally, degradation mechanism of CF/EP composites was proposed.(5) The effects of the process parameters, including temperature, time, solvent contents, mass of CF/EP composites, catalyst and its concentration, on degradation of epoxy resin was investigated by single-factor experiment, and general rule of degradation of epoxy resin in supercritical n-butanol was revealed. Influential rules of the process parameters on surface morphology, surface performances and mechanical performances of the recycled carbon fibers was revealed by monofilament tensile, microdroplet method, SEM, AFM, XPS and Roman spectrum. The optimum ranges of process parameters were investigated by response surface methodology, and quantitative relation between degradation of epoxy resin and the process parameters was proposed. Finally, the optimum process parameters can be obtained, and re-resource performance of the recycled carbon fibers was investigated under condition of optimum process parameters.(6) According to change degradation rate of epoxy resin with reaction time, the reaction order n and reaction rate constant k of catalytic degradation can be obtained by the Levenberg-Marquardt and universal global optimization algorithm, combining with principle of correlation coefficient approximation. The activation energy E and the pre-exponential factor ko of CF/EP composites catalytic degradation reaction can be calculated by performing linear regression on change relation of Ink with 1/T. Finally, catalytic degradation kinetics equation of CF/EP composites in supercritical n-butanol was also proposed, and kinetics equation was verified by degradation test.