Lignin-reinforced phosphorus–boron synergy enabling adhesive-free bamboo biocomposites with coupled mechanical, flame-retardant, and UV-shielding performance

Abstract Replacing petroleum-derived feedstocks with renewable biomass is key to low-carbon material development, yet simultaneously achieving mechanical reinforcement, flame retardancy, water resistance, and ultraviolet (UV) shielding remains a central challenge for biocomposites. Here, formic-acid-pretreated Phyllostachys edulis (moso bamboo) powder served as the matrix, and adhesive-free multifunctional biocomposites were fabricated by co-impregnation with ammonium polyphosphate (APP) and boric acid, blending with lignin, and hot pressing (170 °C, 10–20 MPa, 60 min). Formic acid pretreatment (85 wt% formic acid, 100 °C, 120 min) afforded a 68.2% solid-phase yield, enriching cellulose from 50.72% to 68.85% while removing 61.9% of hemicellulose and 56.6% of lignin, thereby exposing reactive surface hydroxyls for hot-press self-bonding. Lignin (purity 96.43%, T g ≈ 157.4 °C) softened at the pressing temperature, infiltrated interparticle voids, and reinforced interfacial bonding. Structural analyses confirmed that hot pressing preserved the cellulose I framework, with XRD peaks at 2θ ≈ 15.9°, 22.2°, and 34.8°. Additional peaks at 16.6°, 23.7°, and 29.1° verified inorganic-phase incorporation. XPS detected B and P, while lignin reduced the O/C ratio to 0.671. FTIR further confirmed P–O, P–O–C, B–O, and lignin aromatic structures. At 6 wt% lignin and 20 MPa, the composite reached tensile and flexural strengths of 21.02 and 40.06 MPa, 2.6- and 4.3-fold higher than the lignin-free counterpart. Across the formulation series, the limiting oxygen index rose from 20.1% to 37.7%, the residual char at 800 °C reached 44.8%, and the material self-extinguished upon flame removal. The water contact angle exceeded 100°, and aromatic chromophores afforded broadband UV shielding across 200–400 nm, together with enhanced broadband absorption extending into the visible range (400–800 nm). The multifunctionality arises from a phosphorus–boron–lignin ternary synergy, offering a coupled processing–structure–property framework for valorizing formic-acid pretreatment products in construction and packaging.