Possibility of replacing urea formaldehyde resin by soy adhesive in production of plywood

Todays, environmental problems of synthetic adhesives such as formaldehyde emission danger for human health cause to consider bio adhesives for wood based composites production by researchers and industries. Human health problems and formaldehyde emission from wood-based composites are some of the major drawbacks of the traditional synthetic adhesives such as urea formaldehyde resins. There have been many attempts to decrease formaldehyde emission and replace urea formaldehyde resins with bio-based adhesives for wood-based composites. Recently, researchers were developed bio based adhesives for wood based product manufacturing with acceptable physical and mechanical properties. These adhesives are totally environment-friendly and they have no gas emission. In this research, feasibility of replacing soy based adhesives by urea formaldehyde resin for plywood manufacturing was investigated.

Soy flour and soy protein combined with two types of different hydrolysable tannin and condensed tannin namely chestnut tannin and mimosa tannin by 5 wt% (as cross-linker agents) to produce different soy adhesives. Some properties of soy adhesives were evaluated by thermo-mechanical analyzer and differential scanning calorimetry. The adhesives were tested dynamically by thermomechanical analysis (TMA) on a Mettler 40 apparatus. Thermal transition properties of modified and unmodified soy flour samples were measured with an INNUO DSC 500-B instrument. Also, three ply plywood specimens with Populus deltoides plies were manufactured and their mechanical strength (shear strength) and delamination properties were investigated.

The results obtained from thermo mechanical analysis showed that addition of tannins to soy based adhesives cause to decrease in their gelation time and temperature. Also, results of differential scanning calorimetry analysis showed a decrease in the denaturation temperature of soy protein by tannins addition. The results of plywood mechanical test showed an increase in the shear strength of specimens which prepared with improved-soy based adhesive by tannins as crosslinking agents. Their shear strength were evaluated higher than European standards, EN-314-2, and manufactured plywood by urea formaldehyde resin. Delamination test based on ANSI HPV- HP1 standard, also, showed the lower cyclic water resistance of all types of plywood which manufactured by soy based adhesives without crosslinking agents and, urea formaldehyde resin even indoor condition. But, addition of tannins as crosslinking agent to soy based adhesives, successfully, increased their delamination resistance for indoor and outdoor applications

Based on obtained results, soy adhesives because of acceptable physical and mechanical properties according to EN-314-2, have good potential to replace urea formaldehyde resin for plywood manufacturing.