Self-extinguishing and hydrophobic epoxy composites containing hydrothermal liquefaction-derived biochar and whisker-like particles based on tailored PVP-coated silica fibers

The hydrothermal liquefaction (HTL) of waste biomass produces bio-oil along with solid, aqueous, and gaseous co-products. The utilization of solid residue (biochar) is a crucial step in achieving the sustainability and circularity of the entire HTL process. Here, we propose the valorization of biochar derived from HTL of municipal sewage sludge as a functional additive for epoxy resins to enhance their flame retardancy. Biochar samples from HTL, obtained under different operative conditions, were characterized and incorporated into an epoxy resin cured with a cycloaliphatic amine. Biochar was used in combination with whisker-like particles, made of silica coated with electrospun poly(vinylpyrrolidone) (PVP) and functionalized to enhance compatibility with the polymer matrix. The synergy of these fillers with ammonium polyphosphate and urea enabled the preparation of no-drip self-extinguishing composites (V0 rating at UL-94 vertical flame spread tests), showing excellent fire performance, as assessed by cone calorimetry and pyrolysis combustion flow calorimetry, with a limited effect on the viscoelastic behavior and some impact on the flexural properties. Notably, a strong flame retardant action in the condensed phase, with a slight effect in the gas phase, was responsible for the formation of a ceramic continuous char, which decreased the peak of the heat release rate (∼36%) as well as the total smoke release (∼10%) during the burning process. Besides, the tailored whisker-like particles were able to migrate at the surface of composites, providing water contact angles of ∼120°, suggesting a potential use of the designed materials as water-proof protective coatings or components for multifunctional infrastructures.