Soybean-based adhesive resins for composite products

There is a growing interest in developing wood adhesives from renewable resources to reduce the dependency on petroleum-based chemicals and to expand nonfood industrial uses of agricultural commodities. Proteins possess properties common to many polymeric substances for adhesives. In addition, protein molecules contain a variety of functional groups for the formation of cross linkages. A soybean-based adhesive resin was formulated by crosslinking soy flour with 30 percent (w/w) of a phenol-formaldehyde (PF) prepolymer. The resultant soybean-based resin was evaluated in laboratory production of medium density fiberboard (MDF) and flakeboard. MDF boards were made from mixtures of hybrid aspen whole-tree fiber and cornstalk fiber, and flake-boards were made from juvenile hybrid aspen flakes. For fiberboard, PF-bonded boards had higher modulus of rupture and modulus of elasticity than both urea-formaldehyde (UF)- and soy resin-bonded boards. The internal bond strengths of the PF- and UF-bonded boards were higher than that of the soy resin-bonded boards. The soy-resin-bonded boards had the same percent thickness swelling after 24-hr. cold water soak as those bonded with UF. Soy resin-bonded boards withstood 2-hr. boil tests and had a thickness swell about 10 percent higher than that of PF-bonded boards. The juvenile aspen flakeboards bonded with PF resin were superior to boards bonded with PF-crosslinked soy resin, but the latter met commercial standard requirements. Results of this study suggest that the soy resins may be suitable for some specific uses and that the juvenile hybrid aspens and cornstalks can be used as supplemental furnishes for composite products.