Flame Resistance and Bonding Performance of Plywood Fabricated by Guanidine Phosphate-Impregnated Veneers

Abstract

In this study, fire-retardant plywood was fabricated using a simple guanidine phosphate (GP) impregnation treatment of the veneers, and the influence of the treatment on the flame resistance and bonding strength of the plywood was fully investigated. The results showed that GP modification could effectively endow the plywood with excellent fire resistance and smoke-suppression effect. When 10% GP solution was applied, the limiting oxygen index (LOI) of the impregnated wood was up to 37%, which was almost twice of unmodified plywood. The heat-release rate (HRR) and total smoke production (TSP) were also greatly decreased from the pristine 94.14 MJ/m2 and 0.77 m2 to that of modified 43.94 MJ/m2 and 0.08 m2, respectively. The excellent fireproof performance was mainly due to the thermal decomposition of GP to phosphoric acid and guanidine during combustion, which could promote the catalytic carbonization of wood and release of incombustible CO2 and NH3 to dilute and decrease the combustible gases, thus collectively preventing the wood form burning. However, the guanidine phosphate modification could seriously damage the bond performance of plywood, especially the UF resin adhesive-bonded plywood. When 10% guanidine phosphate was applied, the dry and wet bonding strength of the UF resin adhesive-bonded plywood were decreased to only 0.7 MPa and 0.12 MPa, respectively, which may be due to the blocking effect of GP in wood pores and the hygroscopic and soluble properties of GP itself in water, thus decreasing the effective bonding between wood veneers. What’s worse, the poor water resistance of the UF resin adhesive was also adverse to the bonding strength of plywood. Surprisingly, the PF resin adhesive was proved to be suitable for gluing the GP-modified wood without obviously decreasing the bonding strength, which could be used to prepare plywood with both high bonding strength and flame resistance.