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

Abstract

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.