Advanced thermoforming techniques of cured carbon fiber reinforced epoxy polymers based on a reshapable vitrimeric matrix

Abstract

A carbon fiber-reinforced polymer (CFRP) with thermoforming capabilities has been developed by introducing an epoxy resin with dynamic disulfide bonds as a matrix. The influence of critical parameters on the efficiency of thermoforming processes, including the number of plies, dynamic hardener content, and the curvature of geometry, have been deeply studied. Flat cured composites were placed inside curved molds to replicate their form. The thermoforming efficiency showed a high dependency on the amine-to-epoxy ratio, increasing the shape fixity rate (Rf) from 40 to 50 % to 70–80 % with the excess of pending amines in the matrix. Moreover, an increased number of plies resulted in a reduction in thermoforming efficiency due to the accumulation of shearing stress, which ultimately led to the formation of delamination defects. Indeed, the welding of two different specimens was achieved while simultaneously being thermoformed with the most severe curvature, exhibiting an almost 100 % thermoforming efficiency. In addition, the material showed superior shape memory properties, recovering more than 70 % of its original flat shape in all tested conditions. Finally, a proof-of-concept was developed by obtaining a T-shape stringer from flat laminates through thermoforming and welding to prove the potential use of the developed materials in industry.