Enhancing physical-mechanical properties and flame retardancy of fir with lignin-based aqueous modifier

A lignin-based wood modification strategy was proposed by impregnating a composite aqueous modifier consisting of allylated lignin (AL), unsaturated phytic acid amine (TEPA-GMA-PA), and N-hydroxymethyl acrylamide (NMA). The system was designed to enhance the dimensional stability, mechanical properties and flame retardancy of fast-growing fir. AL was synthesized through etherification of industrial alkali lignin with allyl glycidyl ether, endowing the lignin with reactive unsaturated sites for crosslinking. Meanwhile, TEPA-GMA-PA was prepared via a ring-opening addition reaction between glycidyl methacrylate and tetraethylenepentamine, followed by neutralization with phytic acid. The successful synthesis of AL and TEPA-GMA-PA was demonstrated by Fourier Transform Infrared Spectroscopy and Nuclear magnetic resonance spectroscopy. The resulting compound modifier was blended with NMA to form an aqueous polymerizable system. Leaching test demonstrated that the modifier underwent in situ polymerization within the wood structure to form water-insoluble macromolecules, reducing the leaching rate to 34.9%. Modified wood exhibited significant mechanical enhancement with 30.8% and 19.6% increases in modulus of rupture and modulus of elasticity, respectively, while maintaining impact strength to natural wood. Flame retardancy improvements were confirmed by cone calorimetry showing 35.6% reduction in total heat release and 7.2% decrease in total smoke production. Furthermore, the limiting oxygen index increased from 22.6 to 34.7%. The proposed strategy provided a new approach for improving of high-value utilization of wood with lignin-based modifier.