Preparation of carbon fibers coated with epoxy sizing agents containing degradable acetal linkages and synthesis of carbon fiber-reinforced plastics (CFRPs) for chemical recycling

New epoxy resins containing degradable acetal linkages were synthesized by the addition reaction of bisphenol-A (BA) and epoxy-functionalized vinyl ethers containing hydrophilic oxyethylene chains, 2-(vinyloxy)ethyl glycidyl ether (VEGE) and 2-[2-(vinyloxy)ethoxy]ethyl glycidyl ether (VEEGE). Carbon fibers were applied with the obtained degradable epoxy resin-based sizing agents (designated as BA-VEGE and BA-VEEGE) in ordinary (1.4 similar to 2.2 wt%) or excess (6.4 similar to 13.4 wt%) amounts. Interfacial adhesion between the carbon fibers applied with the degradable epoxy resin-based sizing agents and matrix resins (bisphenol-A-type epoxy resin) was evaluated by the microdroplet method. Carbon fibers with both degradable epoxy resins as a sizing agent showed improved adhesive properties compared with the desized carbon fibers. Using the degradable sizing agent-applied carbon fibers, carbon fiber-reinforced plastics (CFRPs) were prepared by laminating prepreg sheets and heating them under pressure. The tensile properties of the CFRPs with [0](50) lay-up did not depend on the structure of the sizing agents, but the tensile strength decreased as the amount of sizing agent used increased. On the other hand, the tensile properties of the CFRPs with [0/90](12S) lay-up were not dependent on the structure or volume of sizing agents used. The impact toughness of the CFRPs was evaluated by the charpy impact test. When an ordinary volume of sizing agent was applied, the CFRPs with degradable epoxy resin-based sizing agents exhibited higher levels of impact strength than the commercial sizing agent-based CFRPs. However, applying an excessive volume of sizing agent to carbon fibers led to a decline in impact strength. The degradation reaction was conducted under acidic conditions by the treatment of HCl at room temperature or 70 degrees C. CFRPs with degradable epoxy resins as sizing agents in ordinary volumes were barely decomposed due to insufficient degradable regions in the CFRP components. However, the CFRPs applied with excess degradable sizing agents decomposed and carbon fibers were recovered.