Effect of Glycol Lignin Addition to in-situ Polymerizable Thermoplastic Epoxy / Carbon Fiber Reinforced Composite

Glycol lignin is a type of technical lignin extracted from lignin with polyethylene glycol and is a polyphenolic polymer. In addition to its high heat resistance, polyphenolic polymers have a structure that approximates the graphite structure of carbon fiber, so high adhesion with carbon fiber is expected. In this study, we investigated the effect of adding glycol lignin to the matrix resin to improve the mechanical and thermo-mechanical properties of CFRTP. CFRTP with glycol lignin was prepared by adding glycol lignin to the base resin of an in-situ polymerizable thermoplastic epoxy resin, then adding a catalyst and applying the resin to carbon fiber fabrics, followed by heat press molding to promote polymerization while impregnating the resin into the carbon fiber bundles. The mechanical properties of the fabricated CFRTP were evaluated, and the following findings were obtained regarding the effect of glycol lignin addition. The interfacial shear strength between carbon fiber and the resin increased as the amount of glycol lignin increased. On the other hand, for resins with more than 20 wt% glycol lignin, the fracture strain was lower than the fracture strain of the carbon fiber. As a result, the maximum flexural strength of CFRTP was obtained at a glycol lignin addition of 10 wt% due to the balance between the increase in interfacial shear strength and the decrease in the fracture strain of the resin. This flexural strength was about 9% higher than that of CFRTP without glycol lignin. Furthermore, as the amount of glycol lignin added increased, the CFRTP tended to deform more easily at low temperatures, while maintaining a certain storage modulus in the high temperature range above 140 °C.