The performance of tannin-based resins in comparison with melamine-formaldehyde resin for improving the physical and mechanical properties of wood

The performance of formaldehyde-free tannin and tannin-resorcinol-formaldehyde resins was investigated in comparison with melamine-formaldehyde resin to improve the physical and mechanical characteristics of oak wood. Melamine-formaldehyde resin was diluted to a concentration of 25% and then 5% carbamide was added to reduce its viscosity. For the synthesis of formaldehyde-free tannin resin, condensed tannin powder with a concentration of 30% in distilled water was used, and the pH was adjusted to the range of 10 using a 33% solution of sodium hydroxide in water. Tannin-resorcinol-formaldehyde resin was also synthesized at pH 8. After impregnation of the wood samples using the empty cell process and drying them with a moisture-based schedule, the resins were cured in an oven at a temperature of 103 ± 2°C for 48 hours and then at a temperature of 120°C for 4 hours. The results showed that modifying wood with all three types of resin leads to a decrease in equilibrium moisture content, improvement in dimensional stability, and surface water repellency. However, it does not have a significant effect on the hardness and modulus of elasticity of the wood. The weight percent gain after modification with melamine-formaldehyde resin (23.8%) was almost twice its value for wood modified with tannin-based resins. Melamine-formaldehyde resin had a better performance in comparison with tannin-based resins, while the intensity of wood color change after modification was also less. The modification process with tannin-resorcinol-formaldehyde resin was more successful in improving the properties compared to formaldehyde-free tannin resin.