Improved water resistance of bio-based adhesives for wood bonding

Synthetic resins, such as phenol-formaldehyde (PF), are dominant in wood bonding for exterior and semi-exterior applications because of their excellent water resistance. Replacement of petroleum-based resins with ones having high biomass content would further enhance the environmental preferability of reconstituted wood-based materials. Past studies on using soybean flour in these applications have not led to viable adhesives with hi-h biomass content. However, use of proper denaturation, stabilization, and crosslinking reactions leads to high-soybean-content adhesives that can replace PF as the face resin for strandboard. In the first step, the soybean flour is treated with hot caustic to denature the protein but with limited hydrolysis that can produce small peptide fragments. Once the denaturation has opened the protein structure, the exposed functional groups can be reacted with formaldehyde to produce a stabilized protein. This material can then be reacted with phenol and formaldehyde to produce a strong wood-bonding adhesive. The adhesive has been used to produce strandboard having strength and water-resistance values equal to those obtained using a commercial phenolformaldehyde adhesive. The soybean-PF adhesive is 40% to 66% by weight soybean flour, thus reducing dramatically the use of petroleum-based phenol and formaldehyde.