Eco-friendly synthesis of ultrahigh strength and modulus phenolic fiber-reinforced polybenzoxazine aerogels with thermal insulation, flame resistance, and machinability

Polybenzoxazine aerogels have emerged as promising candidates for applications in thermal insulation and adsorption owing to their porous structure, low density, and outstanding thermal stability. However, current preparation methods often rely on toxic or highly polar solvents, and the resulting products exhibit inadequate mechanical properties, hindering practical application. Herein, a novel and eco-friendly strategy of constructing high-strength composite aerogels with eco-friendly water-soluble benzoxazine monomers and phenolic fibers is presented. The resulting aerogels, exhibiting a nanoporous framework at a bulk density of 0.387 g cm⁻³ and demonstrate outstanding mechanical performance. Specifically, the aerogels achieve a compressive strength of 6.11 MPa at 5 % strain, a compressive modulus of 126 MPa, and a flexural modulus of 136 MPa, all of which collectively confer excellent machinability. Meanwhile, with a low thermal conductivity of 0.0552 W m⁻¹ K⁻¹ and a limiting oxygen index of 30.7 %, PF/PBz composite aerogels can maintain the overall structure under the butane torch (∼1200 °C) and quickly self-extinguish after the flame is removed, showing superior thermal insulation and flame retardancy. This research provides valuable scientific insights for the design of advanced composite aerogels that possess superior mechanical strength and thermal insulation with environmental sustainability.