High temperature performance of soy-based adhesives

摘要

We studied the high temperature performance of soy meal processed to different protein concentrations (flour, concentrate, and isolate), as well as formulated soy-based adhesives, and commercial nonsoy adhesives for comparison. No thermal transitions were seen in phe-nol-resorcinol-formaldehyde (PRF) or soy-phenol-formaldehyde (SoyPF) or in as-received soy flour adhesive during differential scanning calorimetry scans heating at 10°C/min between 35 and 235 °C. Heat flow rates decreased in the order soy flour (as received) > SoyPF > PRF > emulsion polymer isocyanate (EPI). In thermogravimetric analysis (TGA) scans from 110 to 300 °C at 2°C/min, total weight loss decreased in the order soy flour (as-received) > SoyPF > PRF > casein > maple > EPI. For bio-based materials, the total weight loss (TGA) decreased in the order soy flour (as-received) > concentrate, casein > isolate. Dynamic mechanical analysis from 35 to 235 °C at 5 cC/min of two veneers bonded by cured adhesive showed 30-40% decline in storage modulus for maple compared to 45-55% for the adhesive made from soy flour in water (Soy Flour) and 70-80% for a commercial poly(vinyl acetate) modified for heat resistance. DMA on glass fiber mats showed thermal softening temperatures increasing in the order Soy Flour < casein < isolate < concentrate. We suggest that the low molecular weight carbohydrates plasticize the flour product. When soy-based adhesives were tested in real bondlines in DMA and creep tests in shear, they showed less decrease in storage modulus than the glass fiber-supported specimens. This suggests that interaction with the wood substrate improved the heat resistance property of the adhesive. Average hot shear strengths (ASTM D7247) were 4.6 and 3.1 MPa for SoyPF and Soy Flour compared to 4.7 and 0.8 MPa for PRF and EPI and 4.7 for solid maple. As a whole, these data suggest that despite indications of heat sensitivity when tested neat, soy-based adhesives are likely to pass the heat resistance criterion required for structural adhesives.