Enzyme modified soy flour adhesives

Soy protein based adhesives have not been used extensively in wood products since the 1960's because of inferior performance, stability, and water resistance issues relative to petroleum based adhesives. The early soy protein adhesives were made from defatted flours and were dispersed in alkaline solutions to denature proteins and make more polar groups available for adhesion. Recent research has focused on soy flour (SF) and soy protein isolate (SPI) adhesives due to increased phenol prices and concerns over the use and exposure to formaldehyde. In the present work enzyme hydrolysates of SF and SPI were evaluated in phenol formaldehyde (PF) and polyamide-epichlorohydrin (PAE) adhesive formulations. In soy/PF blends the degree of hydrolysis (DH) was an integral factor in both strength and durability. DH >18% was detrimental to the internal bond and thickness swell of medium density fiberboard (MDF) specimens with PF blends. MDF made with blends of hydrolyzed SF/PF with up to 20% soy solids did not significantly differ in strength or durability compared to pure PF resin. Addition of hydrolyzed SF to the adhesive matrix increased the modulus of elasticity of MDF.\nSimilar results were observed when hydrolyzed soy flour was incorporated with PAE. Hydrolysis of SF led to lower viscosity resins, but DH >10% decreased shear strength of plywood specimens. PAE is known commercially for its wet strength attributes, but when PAE was <10% of the resin formulation, lap shear samples delaminated during wet strength testing. In both plywood and MDF systems, the addition of urea to soy flour hydrolysates before combining with PAE increased the wet strength and dimensional stability of finished products.