Protein structure and wood adhesives

Interest in biobased adhesives has led to many studies on protein adhesives. Unfortunately, many assume that denatured proteins adopt a permanently extended shape. Neither the properties of the applied soy protein dispersion, properties of the cured adhesive, nor the general protein literature are consistent with proteins being very unfolded or extended. In the presence of water, a compact state (hydrocolloid) is more favorable since it minimizes the number of exposed hydrophobic groups. Each protein chain minimizes contact with water by burying most hydrophobic domains, which is achieved through folding into a compact globular structure and by aggregating with other protein globules to help shield remaining surface hydrophobic domains. Proteins can unfold, but the energy required to mix hydrophobic proteins with water causes them to quickly collapse and aggregate. These globules aggregate with other proteins to form pre-gels (physical bonds) that when heated form a gel during bond formation. Under hot bonding conditions the water is driven from the gel; however, when immersed in water, protein gels can swell again, which weakens the cohesive strength of the adhesive. For plant proteins like soy, addition of chemical reactants to reinforce these gel-forming structures are usually necessary to make bonded wood products that remain strong with water exposure.