Properties of Plywood Bonded with Formaldehyde-Free Adhesive Based on Poly(vinyl alcohol)-Tannin-Hexamine at Different Formulations and Cold-Pressing Times

The plywood industry's sustainability, performance, and production costs depend on wood adhesives and the hot pressing technique. In this investigation, a cold-setting plywood adhesive based on polyvinyl alcohol (P), tannin (T), and hexamine (H) was produced. The physical and mechanical properties of plywood were examined at different formulations such as tannin concentration (10% and 20%), hexamine content (5%, 10%, and 15%), and cold-pressing time (3, 6, 12, and 24 h). This study showed that high tannin and hexamine content also increased the solids content, but decreased the average viscosity of the adhesive. Markedly, the cohesion strength of PTH-based adhesives increased from 5.57 Pa at 1/s to 1411.6 Pa at 400/s shear rate, regardless of the adhesive formulation. The shear modulus subsequently decreased as a function of the shear rate and increased with a higher tannin and hexamine content. This study revealed that the higher tannin and hexamine content and longer cold-pressing times could produce plywood with the tested adhesive that met the Japanese standard strength requirements. A combination of PTH-based adhesive prepared with formula 2 and 24 h cold-pressing resulted in the highest TSS value of 1.42 MPa, MOR values of 88.7 MPa, MOE values of 14,025.6 MPa, and wood failure of 47.2%. This study showed the possibility of fabricating eco-friendly plywood panels bonded with PTH-based adhesive using the cold-pressing process as an alternative to conventional plywood.