Processing bulk natural bamboo into a strong and flame-retardant composite material

We report a simple and effective approach to process natural bamboo directly into a high-performance composite material with light weight, high tensile strength, high modulus, and low flammability. Our process involves delignification of natural bamboo followed by surface treatment of highly aligned cellulose fibers in the delignified bamboo and finally infiltration of epoxy resin into the porous structure of the surface-treated delignified bamboo. Surface treatment of the highly aligned cellulose fibers by boric acid (BA) under alkaline conditions resulted in the formation of cross-linked structures between the hydroxyl groups of BA and the diol or carboxylic groups of the cellulose backbone. Because the aligned bamboo fibers were well-preserved, the BA-bamboo/epoxy composite showed increases of 234% and 177% in tensile strength and impact strength, respectively, compared to the pristine epoxy. The BA-bamboo/epoxy composite exhibited comparable mechanical properties but significantly improved flame retardancy compared to the delignified bamboo/epoxy composite. Specifically, the limiting oxygen index of the BA-delignified bamboo/epoxy composite was 26.5% larger and the peak heat release rate was 63% smaller than those of the pristine epoxy resin in cone calorimeter measurements. Thermogravimetric analysis indicated that BA promoted charring of the bamboo/epoxy composite, which was responsible for the significantly improved flame retardancy. The simultaneous enhancement of the mechanical properties and flame retardancy makes such bamboo-based composites highly suitable for use as structural materials.