Densification and polyelectrolyte assisted fabricating laminated wood composites for outstanding flame retardancy and mechanical properties

Laminated wood composites have widely used in construction, furniture and decoration; however, it remains a great challenge to address the trade-off between flame retardancy and mechanical strength in applications. In this work, we reported a strategy for fabricating a flame-retardancy-improved and mechanically strengthened laminated wood composites via delignification, densification and self-assembly of polyelectrolyte. The delignification significantly increased the porosity of wood cell walls, facilitating the impregnation and accumulation of an intermediate adhesion molecule of polyethyleneimine (PEI) within wood cell walls. It provided a sticky bridge to enhance crosslinking network with ammonium polyphosphate (APP) and wood cell walls in process of densification. Hence, a densely-laminated wood coordination with PEI/APP flame-retardant layer can enable excellent flame-retardant properties with an increase of 77.30% in limited oxygen index and a significant decrease of 24.22% in peak heat release rate (120 s) compared to control laminated wood composite. Meanwhile, the strains transferred more uniformly in the interior of wood composites across the stable crosslinking network via digital image correlation (DIC), resulting in an enhanced bonding strength. Besides, the permeation and distribution of PEI within wood cell walls can be successfully observed with the fluorescence imaging by chemical grafted the fluorescent molecule probe. This work offered a strategy for designing wood composite with high mechanical properties and flame-retardant for construction applications.