Effect of waterborne epoxy resin sizing on the surface and properties of high‐strength‐high‐modulus polyimide fibers

To improve the interface adhesion between high‐strength‐high‐modulus polyimide (PI) fibers and epoxy resin (EP) matrix, waterborne epoxy resin (WEP) sizing agents are used to modify the surface of PI fibers. The sizing treatment of PI fibers was carried out by the self‐made sizing machine, and the effect of WEP sizing concentrations on the structure and properties of PI fibers were discussed in detail. The surface roughness, activated carbon percentage, and surface energy of WEP‐sized PI fibers increased with increasing WEP sizing agent contents. While, the mechanical and thermal properties of WEP‐sized PI fibers decreased slightly with the increment of WEP sizing agent concentrations. The optimum WEP sizing concentration was recognized to be 2 wt%. The interfacial shear strength and the interlinear shear strength values of PI fibers and epoxy resin (PI/EP) composites were improved by 175.22 and 25.31%, respectively. Moreover, the mechanism of surface characteristic improvement with WEP sizing treatment were proposed, contributing to the enhancement of mechanical interlocking and chemical interaction between fibers and matrix.