High performance and eco-friendly tannins glyoxal wood adhesives: Based on UV light-induced photocatalytic degradation of larch tannins.

The development of tannins adhesives with high performance and eco-friendly properties is crucial for addressing the dependence of traditional formaldehyde-based wood adhesives on petrochemical resources and the harm of formaldehyde emission. However, conventional methods for preparing tannins adhesives generally involve the use of tannins with high mean degree of polymerization (mDP) and formaldehyde, resulting in poor bonding properties and formaldehyde emission, hindering their industrial application. Herein, the larch tannins (LT) were degraded by photocatalytic degradation to obtain tannins with different mDP, and the tannins glyoxal (G) adhesives were synthesized with different mass ratios of G to LT, then used to prepare plywood with poplar veneer. The effect of different mDP and different mass ratios of G to LT on the properties of adhesives such as viscosity, curing time, shear strength, formaldehyde emission, water insoluble rate, humidity absorption rate and thermal properties were explored. The findings indicated that photocatalysis could lead to regular degradation of LT, and the different mDP of the LT could be obtained by adjusting the photocatalytic degradation time in a certain time range (1-6 h). When the mass ratio of G to LT6 (the LT with the maximum formaldehyde reactivity and mDP 3.71after 6 h of photocatalytic degradation) was 10 %, the LT6-10 %G adhesives imparted to the plywood with excellent bonding strength and water resistance under the hot-pressing temperature 140 °C. The dry shear strength, 3 h hot water shear strength (63 °C) and 3 h boiling water shear strength were as high as 1.83, 1.12, and 0.83 MPa. Its superior bonding performance exceeded the requirements of the Chinese national standard (GB/T9846-2015, ≥0.70 MPa). The formaldehyde emission was 0.12 mg/L, lower than poplar veneer (0.23 mg/L). LT6-10 %G adhesives sufficiently to be considered as a substitute for the formaldehyde-based wood adhesives currently in use.