PERFORMANCE OF SOY PROTEIN ADHESIVE ENHANCED BY ESTERIFICATION

Abstract

Soy protein has shown great adhesion performance to cellulosics and other materials for the manufacture of particleboard, plywood, and various composites. The weakness of a protein-based adhesive is its relatively poor water resistance for outdoor uses. Various chemical and enzymatic methods have been employed to improve the adhesive performance of soy protein. This research studied the effects of esterification on adhesive strength, thermal properties, solubility, and conformational change of soy protein. Free-carboxyl groups of soy protein were esterified with ethanol and different concentrations of hydrochloric acid as catalysts. Esterification rendered protein molecules more hydrophobic by adding the ester groups. With moderate esterification, soy protein showed the maximum increase in adhesiveness and water resistance. The optimum esterification conditions were 10 h with no catalyst. At the optimum condition, esterification increased tensile strength by 20.6%, 61.6%, and 48.1% for dry, wet, and soak strengths, respectively. Hydrochloric acid induced protein denaturation and unfolding by hydrolyzation, and resulted in more ester groups attached on the unfolded structure of protein. The hydrolysis and excess attachment of hydrophobic groups might be detrimental to adhesive performance, especially to water resistance.