Preparation of Polyurethane‐Based Sizing and Its Effect on the Mechanical Properties of Basalt Fiber/Polyamide 6 Composites

ABSTRACT Basalt fiber (BF), an eco‐friendly high‐performance fiber, exhibits inherent limitations such as surface inertness and poor interfacial compatibility with thermoplastic matrices including polyamide 6 (PA‐6), which hinder its application in composites. To address this issue, this study developed a novel waterborne polyurethane (WPU)‐based sizing for BF. This approach leverages the structural similarity between the urethane bonds (–NHCOO–) in WPU and the amide groups (–NHCO–) in PA‐6 to enhance fiber–matrix adhesion. Using a Taguchi orthogonal design, the optimized sizing composition was determined to comprise 2 wt% film former, 0.25 wt% coupling agent, 0.3 wt% lubricant, and 0.1 wt% antistatic agent. The study further investigated the effect of sizing on the mechanical performance of BF and BF/PA‐6 composites. Results showed that the concentration of the coupling agent (KH‐550) in the sizing formulation significantly influenced the tensile strength of BF. Moreover, sizing markedly improved the composite properties by promoting both physical entanglement and chemical bonding at the fiber–matrix interface. The interfacial shear strength of the composites treated with the optimized sizing reached 17.05 MPa, which is four times higher than that of the unsized composites. These findings provide an effective strategy for tailoring sizing chemistries to protect the fiber surface and overcome interfacial challenges in BF/PA‐6 composites.