Green Kraft lignin functionalization for advanced UF adhesives enabling Low-Emission and Fire-Resistant particleboards

Abstract

Lignin, the second most abundant biopolymer after cellulose, offers a highly functional aromatic backbone that can be engineered to act as a multifunctional component in thermosetting resins. In this work, we report a one-pot in situ functionalization strategy that transforms Kraft lignin into a reactive phosphorus/nitrogen (P/N)-functionalized additive, eliminating multi-step pre-treatment routes. Here, we introduce a green, one-pot functionalization strategy in which Kraft lignin is chemically modified directly within a urea-based system using diammonium phosphate as a dual phosphorus-nitrogen source. This integrated design approach enables the simultaneous introduction of reactive P/N functionalities that enhance lignin-UF compatibility, promote char-forming pathways during combustion, and limit formaldehyde release through increased interfacial reactivity. Structural and elemental analyses (FTIR, solid-state 31P NMR, XPS, ICP-OES, SEM, and TGA) confirm the successful incorporation of phosphorus and nitrogen moieties and their distribution within the lignin matrix. When incorporated at 10% into UF adhesive formulations for particleboards, the functionalized lignin modifies curing behaviour and translates molecular-level changes into stable mechanical performance, reduced formaldehyde emissions, and enhanced fire resistance of elaborated panels. Specifically, the optimized formulation achieved a 35% reduction in formaldehyde emission and a 14.1% decrease in peak heat release rate while maintaining full compliance with ANSI A208.1 mechanical requirements. These results demonstrate that rational lignin functionalization within UF-compatible environments is an effective route to transform lignin from a passive filler into an active, multifunctional additive, establishing a practical strategy to simultaneously address emission control and fire safety in UF-bonded wood composites through a single bio-based modification step.