Tannin-hexamethylenetetramine wood adhesives with high performance and environmentally friendly characteristics: based on photocatalytic degradation of larch tannins

To address the problem of formaldehyde hazard and low bonding strength (caused by high mean degree of polymerization (mDP) of tannin itself) of traditional tannin adhesives, this paper used the hexamethylenetetramine (H) and larch tannins (LT) with different mDP (degraded by ultraviolet photocatalysis) to prepare larch tannins-hexamethylenetetramine (LT-H) adhesives at different mass ratios of H to LT (5%, 10%, 15%), and the poplar veneer plywood was prepared using LT-H adhesives. In this study, the effects of different mDP of LT and different mass ratios of H to LT on the properties of adhesives were explored. The results indicated that adjusting the photocatalytic degradation time could obtain a different mDP of LT. When the degradation time was 6 h (LT6) and the mass ratio of H to LT was 10%, the dry shear strength, 3 h hot water (63 ℃) shear strength, and 3 h boiling water shear strength of the LT6-10%H adhesive were 2.40, 1.31, and 1.20 MPa, respectively, all of which exceeded the requirements of the Chinese national standard (GB/T9846-2015, ≥ 0.70 MPa). The formaldehyde emission of the LT6-10%H adhesive was 0.16 mg/L, which was lower than the formaldehyde emission of poplar veneer (0.23 mg/L). The LT6-10%H adhesives could replace current formaldehyde-based options, achieve eco-friendly characteristics while maintaining high-performance advantages.