Soybean resins, as protein-based wood adhesives, have been extensively examined in this study. Hydrolysis and modification of soy flour to produce soybean-based resins were evaluated in terms of desired resin attributes and bonding performance as wood adhesives. End use performance was determined in selected wood composite products. Two methods were applied to modify soy flour for improving the curing speed, bond strength and water resistance. One method was to directly mix a hydrolyzed soy solution with a synthetic resin. Another method was to first hydrolyze soy flour, followed by reacting with other chemicals.
Soybean-based resins produced from low hydrolyzed soy flour modified with melamine resins were successfully applied to plywood production. There is potential for these resins to be used for interior grade plywood and possibly for exterior grade plywood products provided cost effective ways to improve the applicability and stability of the target resins during plywood manufacturing can be found. A pilot plant study would be required to determine the relationship between performance and costs.
Hydrolysis was an effective method under alkaline conditions for reducing the viscosity of soy resins. However, hydrolysis also reduced the bond strength of soy resins. Soybean-based resins produced from highly hydrolyzed soy flour modified with various synthetic resins showed comparable bond strength to the control phenol-formaldehyde (PF) resin in the lap shear test. However, plywood products bonded with these resins showed low percentage wood failure even under dry conditions, and also low water resistance. The particleboard product bonded with one of these resins showed very low formaldehyde emissions, but the bond performance was poor compared to the control urea-formaldehyde (UF) resin. Therefore, soybean-based resins from highly hydrolyzed soy flour are not recommended at this time.
Unhydrolyzed soy flour was combined with a PF resin in the manufacturing of medium density fibreboard (MDF) product with ultra low formaldehyde emissions. The soy flour showed excellent compatibility with the PF resin, indicating that there is potential to partially replace PF resin with soy flour in the production of MDF to lower adhesive cost.
Copolymerized SoyM resins showed high bond strength and high curing speed in the lap shear test. Formic acid was found to be an effective catalyst to accelerate the curing speed for these resins. Quantification of the benefits of using these SoyM resins with a formic acid catalyst to replace urea-formaldehyde adhesives in MDF and particleboard products is recommended.