Strategy Construction to Improve the Thermal Resistance of Polyimide-Matrix Composites Based on Fiber–Resin Compatibility

Carbon-fiber-reinforced thermoset polyimide composites have found wide applications in various aerospace areas. In this paper, the influence of carbon-fiber sizing on the thermal properties of polyimide composites was studied. Nonlinear detriment of the epoxy sizing was found to affect the composite's thermal resistance. Furtherly, the mechanism, possibly responsible for the nonlinear detrimental effect of the epoxy sizing, was investigated through curing kinetics analysis and chemical structure characterization. Thermal curing activation energy change was found, possibly arising from the insertion of a flexible segment into the polyimide chain by epoxy-amine reaction. Based on the proposed mechanism, a strategy to manipulate the thermal resistance was established and verified. By the precuring of the carbon-fiber sizing, the polyimide composite exhibited obvious thermal resistance improvement, indicated by an enhancement of the glass transition temperature by 20 °C, and a weight-loss reduction under thermal aging at 400 °C by 25%. Simultaneously, a good fiber-matrix interface was maintained. This strategy provides a new route to enhance the thermal properties of polyimide composites from the viewpoint of carbon-fiber-matrix compatibility.