Tannin formaldehyde wood adhesives synthesized by photocatalytic degradation larch tannins to avoid the harm of formaldehyde

Due to the high mean degree of polymerization (mDP) of condensed tannins, condensed tannin-based adhesives have high viscosity, poor water resistance, and low shear strength, and thus are limited in their application. Degradation of tannins to reduce its mDP is a feasible method to solve the above problems. In this study, photocatalytic degradation of larch tannins (LT) was carried out to obtain LT with different mDP. Larch tannin-formaldehyde (LT-F) adhesives were prepared with different mass ratios of F to LT, and then used to prepare plywood with poplar veneer. The effect of different mDP of LT and different mass ratios of F to LT on the properties of the LT-F adhesives were explored. The results showed that LT with different mDP could be obtained by photodegradation. When the photodegradation time was 6 h (LT6, with mDP 3.71), and the mass ratio of F to LT6 was 10 %, the LT6–10 %F adhesives had low viscosity, good water resistance, and high shear strength that exceeded the requirements of the Chinese national standard (GB/T9846–2015, ≥ 0.70 MPa). The formaldehyde emission was 0.15 mg/L, lower than poplar veneer (0.23 mg/L). Therefore, LT6–10 %F adhesives can still be safely used in wood industry although formaldehyde is used.