Dispersion adhesives from soy flour and phenol formaldehyde

Abstract

Higher petroleum prices and greater interest in bio-based adhesives have stimulated a considerable amount of research on incorporating soybean flour into wood adhesives in recent years. In some cases, soy was used at low levels as an extender for phenol-formaldehyde (PF) adhesives; in other cases, highly hydrolyzed soy flour was used. Although progress was made in getting soy to react with PF, incorporation of the soy was still fairly low. Addition of the flour to hot aqueous caustic, followed by stabilization of the alkaline soy mixture by reaction with formaldehyde, was a recent key step toward higher incorporation of soy flour in PF adhesives (1). These conditions provided an ideal protein structure to allow the reaction of phenol and formaldehyde with functional groups on the protein while minimizing undesired hydrolysis of the protein backbone. Good performance was obtained using nearly equal quantities of soy flour and phenol-formaldehyde resin for the face adhesive in producing strandboard (1). A ratio of 40% soy to 60% PF resulted in an adhesive with performance equal to that of a commercial PF face adhesive in both cure speed and final properties of the resultant strandboard (2). A limitation on soy flour incorporation in these adhesives may be caused by the slow curing reactions of the hydroxymethylamine groups on the protein under basic conditions. Amino resins cure slowly under alkaline conditions but work well under acid cure conditions (3). Therefore, the question was whether an acidic soy-PF resin can be made and whether this would allow for higher soy contents in the adhesive. Soy flour consists mainly of proteins (41–48%) and soluble and insoluble carbohydrates (30–40%), along with some other minor components. The protein in its native state is only moderately soluble, but treatment with caustic is known to increase the solubility of soy protein by increasing the net charge of the proteins. Acidification to pH 4–6, near the isoelectric point of many soy proteins, generally leads to precipitation of much of the protein. Likewise, the phenolic resins are usually soluble in aqueous caustic and precipitate out upon acidification to pH 4–6. This research was aimed at trying to make stable acidic mixtures of soy-PF resins with high soy contents that could be readily applied to and bond wood flakes to make strandboard.

Citation

Frihart, Charles R.; Wescott, James M.; Traska, Amy E. 2007. Dispersion adhesives from soy flour and phenol formaldehyde. Proceedings : 30th annual meeting of the Adhesion Society, Inc. : February 18-21, 2007, Tampa Bay, FL. [S.l. : s.n.], c2007: pages 150-152.