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Formaldehyde-based adhesives such as urea-formaldehyde (UF) and phenol-formaldehyde (PF) adhesives dominate the current wood adhesive market. However, these adhesives are derived from petrochemicals and carcinogenic formaldehyde. In this study, we developed two formaldehyde-free adhesives based on abundant, renewable, and inexpensive soy flour (SF).
The first adhesive was mainly composed of SF, polyethylenimine (PEI), and maleic anhydride (MA). The optimum formulation of this adhesive and the optimum hot-press conditions for making plywood were investigated. A three-cycle soak test and a boiling water test (BWT) were employed to evaluate the strength and water-resistance of plywood bonded with the soy flour-based adhesive.
Results showed that SF, PEI, MA and NaOH were all essential components for the adhesive and the SF/PEI/MA weight ratio of 7/1.0/0.32 resulted in the highest water-resistance. When the hot-press temperature was in the range of 140-170 ºC, both the water-resistance and the shear strengths of plywood bonded with the adhesive remained statistically the same, except that the dry shear strength of plywood at 170 ºC was statistically lower than that at 160 ºC. When the hot-press time ranged from 2 to 6 min at 160 ºC, the plywood panels made at the hot-press time of 5 min led to the highest boiling water test/wet (BWT/w) shear strength among all samples, and the plywood panel made at the hot-press time of 5 min had a higher dry shear strength than that at 3 min. Plywood panels bonded with this SF/PEI/MA adhesive exceeded the water-resistance requirements for interior applications.
The second adhesive consisted of SF, trimethylolpropane triglycidyl ether (TMTE) and a catalyst. The following catalysts were investigated: calcium hydroxide, sodium carbonate, triethylamine, tetra-n-octylammonium bromide, phosphoric acid and ammonium sulfate. The effects of these catalysts on the water-resistance of plywood bonded with the SF-TMTE adhesive under different hot-press conditions were investigated. It was found that all catalysts improved the water-resistance of the resulting plywood to some extent. Among all catalysts
investigated, calcium hydroxide and triethylamine were better than other catalysts in terms of improving the water-resistance of the resulting plywood panels. These preliminary results also revealed that increasing hot-press temperature and increasing hot-press time enhanced the water-resistance. The water-resistance of plywood panels bonded with these SF-TMTE adhesives still could not meet water-resistance requirements for interior applications. |
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