A new formaldehyde-free wood adhesive from renewable materials

A formaldehyde-free adhesive that consists of soy flour (SF) and a new curing agent (CA) was developed and evaluated for making interior plywood. Three types of plywood panels (seven-ply maple/white fir/pine/white fir/pine/white fir/maple, five-ply yellow poplar, and five-ply aspen) were prepared with the SF–CA adhesives and evaluated for their water resistance. The CA was derived from the reaction of epichlorohydrin (ECH) and ammonium hydroxide in water. Effects of the reaction time, reaction temperature, NaOH usage, heat treatment of CA, addition order of reactants in the preparation of the CA, and storage time of the CA on the water resistance of plywood panels bonded with SF–CA adhesives were investigated. The reaction time required for the completion of the reaction significantly decreased as reaction temperatures increased. The addition of NaOH to the SF–CA adhesive improved the water resistance and dry shear strength of the five-ply aspen panels. All plywood panels met the requirements for interior plywood when the CAs were prepared at 45–60 °C no matter whether the CA was heat-treated or not. Mixing ECH and ammonium hydroxide all at once resulted in better water resistance of the resulting plywood panels than adding either of ECH or ammonium hydroxide to the other dropwise. The viscosity of heat-treated CA was comparable to that of untreated CA when the CA was prepared at 50 °C. Both heat-treated and untreated CAs could be stored at room temperature for at least two months without compromising the water resistance of the resulting plywood panels.