A Formaldehyde-Free, Soy Protein-Based Adhesive with P-N-Si Synergistic Flame Retardancy for Plywood Application

To overcome the flammability and formaldehyde-emission problems associated with plywood, a novel soy protein-based intumescent flame-retardant adhesive with phosphorus-nitrogen-silicon (P-N-Si) synergism (C@Si@APP) was developed in this study. C@Si@APP was initially prepared by coating silane-modified ammonium polyphosphate with collagen. Subsequently, C@Si@APP was combined with lignin and adenine and incorporated into a soy protein, collagen, and isocyanate (MDI)-based adhesive matrix to fabricate a flame-retardant adhesive for eucalyptus plywood. Cone calorimeter (CONE) analysis demonstrated that the flame-retardant plywood bonded with SPI/C/MDI/C@SiAPP18 achieved the most effective flame-retardant and smoke-suppression performance, and the peak heat release rate, total heat release, and total smoke production were reduced by 23.5%, 20.7%, and 43.9%, respectively, and the release rates of CO and CO₂ were also effectively suppressed. Mechanistic analysis indicated that the enhanced flame retardancy was mainly governed by a condensed-phase mechanism, in which phosphorylation and siloxane crosslinking promoted the formation of a compact and thermally stable char layer that protected the underlying material, whereas a gas-phase contribution involving the dilution of flammable volatiles by non-combustible gases (NH₃, H₂O, and CO₂) and possible radical scavenging by phosphorus-containing species is proposed as a plausible complementary pathway inferred from the TGA-FTIR results and the previous literature.