A Formaldehyde-Free Water-Resistant Soy Flour-Based Adhesive for Plywood

The phasing out of the use of urea–formaldehyde adhesive in the fabrication of interior-used hardwood plywood requires development of environmentally friendly bio-based wood adhesives. We recently reported that phosphorylation of soy flour (SF) using phosphoryl chloride (POCl 3 ) greatly improved the moisture resistance of soy flour adhesive. In the present study, we investigated the effects of inorganic oxidizing agents, such as NaClO 2 and Ca(NO 2 ) 2 , to further improve the wet bonding strength of phosphorylated SF (PSF) wood adhesive. We report that addition of 1.8 % (wet weight basis) Ca(NO 2 ) 2 to phosphorylated SF (PSF) adhesive formulation containing 25 % soy flour solids increased the wet bonding strength to greater than 3 MPa at 140 °C hot-press temperature. The water resistance testing of the glued three-ply hardwood plywood panels passed the three-cycle soak/dry test recommended by the American National Standard for Hardwood and Decorative Plywood/Hardwood Plywood and Veneer Association protocol (ANSI/HPVA HP-1-2004). Since the process involves only inorganic chemistry and no petroleum-based chemicals such as formaldehyde or polyamidoamine–epichlorohydrin are used, the PSF + Ca(NO 2 ) 2 adhesive is non-toxic and environmentally safe.