Recent research attention has focused on soybean-based adhesives because of increased phenolic resin prices and adhesive demand. This study was conducted to investigate the adhesive properties of crosslinked soy flour and/or soy hydrolyzate with phenol-formaldehyde (PF) resins for use in structural panels. Neutral phenolic soy (NPS) and alkaline phenolic soy (APS) resins were prepared and used to bond southern pine plywood. Adhesive bond quality of the soybean-based phenolic resins was evaluated by glueline shear test following the procedure detailed in Product Standard PS 1-95 for construction and industrial plywood. Within the range of variable levels investigated, the following conditions produced higher wet wood failure than a control glue mix. For NPS resins best results were obtained with a 160 cps PF, a 30 minute assembly time, and no extender. For APS resins best results were obtained with a 50 cps PF, a 200°C press temperature, 19% corn-cob powder, and a 60 minute assembly time. Comparable results were obtained with either a 40% or 50% PF level in the resins. Under these conditions, the wet wood failure of plywood bonded with both resins approached the requirement of PS 1-95 for construction and industrial plywood. The APS resins were also used to fabricate homogeneous hybrid poplar flakeboards with different resin solid levels (5%, 7%, and 9%), press temperatures (175 and 200°C), and press times of 8 and 10 minutes. Internal bond strength, wet modulus of rupture, and dimensional stabilities of flakeboard improved with increased press time, press temperature, and PF level in the resins. In particular, increased press time can be used to offset the poor internal bonding associated with high resin content and the excessive moisture present in the mat. However, APS resin-bonded flakeboards provided higher mechanical and better dimensional stability properties compared to the Canadian Standard Association O437 standard, except for modulus of elasticity, which could be easily improved by flake alignment. Optimum production condition for flakeboard bonded with the APS resins are a 5% resin level, a 50% PF level, a 200°C press temperature, and an 8 minute press time. Although NPS and APS resins can be competitive with other conventional adhesives, further improvements are required to reduce press times for industrial applications.
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