Effect of alkali treatment on the wettability and tensile failure of twisted bamboo fiber/epoxy composites

The twisted bamboo fiber (TBF) was used as the reinforcement phase and the epoxy resin-anhydride system as the matrix phase to prepare the twisted bamboo fiber/epoxy resin (TBF/EP) composite. The effects of alkali treatment on the wettability and tensile failure of TBF/EP composites were investigated by varying the concentration of NaOH solution (1wt%-5wt%). Nano-scale and micro-scale experimental techniques, such as SEM, surface tension testing, and in-situ loading, were employed to analyze the fiber-resin interface, wetting properties, and tensile mechanical properties of the composites. The results show that alkali treatment reduces the surface energy and polarity of the fibers, resulting in a decrease in the wetting force between TBF and the matrix from 0.45 mN to 0.1 mN. The TBF/EP composite modified with 3wt%NaOH solution exhibits a tensile strength (TS) of 273.70 MPa, which is 178.64% higher than that of the untreated composite. In-situ analysis reveals that the failure process of TBF involves fiber fracture and fiber sliding, while the failure process of the TBF/EP composite includes matrix shear yielding and fiber fracture. Moreover, as the wetting properties improve, the inhibitory effect of the fibers on matrix yielding increase. Therefore, the strength of the TBF/EP composite is mainly derived from the reinforcement of the fibers and the interface, which is influenced by wetting properties and stress transfer effect between TBF and matrix.