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

Abstract

AbstractThe 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 (POCl3) greatly improved the moisture resistance of soy flour adhesive. In the present study, we investigated the effects of inorganic oxidizing agents, such as NaClO2 and Ca(NO2)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(NO2)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(NO2)2 adhesive is non‐toxic and environmentally safe.