High Bonding Temperatures Greatly Improve Soy Adhesive Wet Strength

Soy wood adhesive bond strengths reported in different literature studies are difficult to compare because a variety of temperatures and other conditions have been used for the bonding and testing step. Some reports have indicated bond strengths are sensitive to bonding temperature, but the reason(s) for this has not been intensively investigated. Although these prior studies differ in other ways (such as type of soy, wood species, and test method), the effect of bonding temperature has not been clearly examined, which is important for focusing commercial applications. A tensile shear test using two-parallel-ply veneer specimens with smooth maple was used to measure both the dry and wet cohesive strength of soy adhesives. Although the soy adhesives gave very good strengths and dry wood failure, they often have low wood failure and shear strengths under wet conditions when bonded at 120 °C. However, wet strength greatly increased as the bonding temperature increased (120, 150 and 180 °C) for these two-ply tests with. This study examined the use of different types of soys (flours, concentrates and isolates) and different bonding temperatures and bonding conditions to evacuate several possible mechanisms for this temperature sensitivity, with coalescence being the most likely.