Dynamic covalent dual-network strategy for acorn starch: A biopolymer with integrated bonding performance, flame retardancy, and recyclability.

The molecular engineering of polysaccharide networks that integrate high mechanical strength, flame retardancy, and recyclability is a central challenge in sustainable biopolymer science. In this study, a fully bio-based acorn starch network was constructed through a dual dynamic-covalent strategy. Reversible imine bonds (Schiff base, CN) between oxidized starch and arginine impart chemical recyclability, while permanent polyamide-epichlorohydrin (PAE) cross-links reinforce the network and suppress water uptake. Aluminium phytate and guanylurea phosphate form a synergistic P-N-Al intumescent system that promotes char formation and quenches radicals in both the condensed and gas phases. The optimized Bio-OSAHP-Al adhesive exhibited a dry shear strength of 1.84 MPa and a wet shear strength of 0.82 MPa on wood adherends. After surface impregnation with guanylurea phosphate (GUP), the resulting plywood (Bio-OSAHP-Al-GUP) achieved a limiting oxygen index of 33.6% and a UL-94 V-0 rating. The dry and wet shear strengths of the final GUP-treated plywood were 1.69 MPa and 0.75 MPa, respectively, still exceeding the interior-grade plywood standard. The dynamic imine linkages further enable mild-condition chemical recycling and soil degradation, while the phosphonate-aluminium chelates confer mold resistance. This work establishes a macromolecular design framework that transforms underutilized starch into a high-performance, multifunctional biopolymer material, offering a sustainable alternative to formaldehyde-based adhesives.