Abstract
The aim of this study was to effectively improve the water resistance of a defatted soybean flour (DSF)-based adhesive by subjecting DSF to thermo-chemical treatment in the presence of sodium dodecyl sulfate (SDS), and then the crosslinking with epichlorohydrin-modified polyamide (EMPA). The effect of thermo-chemical treatment on the structures and properties of the DSF and DSF-based adhesive were investigated by plywood evaluation, boiling-water-insoluble content, and acetaldehyde value measurements, as well as FTIR, X-ray photoelectron spectroscopic (XPS), X-ray diffraction spectroscopy (XRD), thermogravimetric analysis (TGA), and rheology analyses. The test results revealed that the water resistance of the DSF-based adhesive was significantly improved, attributed to the formation of a solid three-dimensional crosslinked network structure resulted from the repolymerization of DSF, the Maillard reaction between the protein and carbohydrate, and chemical crosslinking between the crosslinker and DSF. Moreover, SDS destroyed the hydrophobic interactions within protein and inhibited macromolecular aggregations during the thermal treatment. Therefore, more reactive groups buried within the globular structure of the soybean protein component of DSF could be released, which supported the repolymerization, Maillard reaction, and chemical crosslinking of DSF, thereby leading to an improved crosslinking density of the cured DSF-based adhesive. In addition, the adhesive composed of thermo-chemically treated DSF and EMPA exhibited preferable viscosity and viscosity stability suitable for the production of wood composites.
Highlights
Formaldehyde-based adhesives are widely used in the timber industry due to their acceptable water resistance, durable adhesion performance, and low price [1]
epichlorohydrin-modified polyamide (EMPA) aqueous solution was synthesized in our lab using diethylenetriamine, adipic acid, and epichlorohydrin, which had a solid content of 13.8%, pH of 2.6, and a viscosity of 96.4 mPa.s (25 ◦ C)
The plywood bonded with the “defatted soybean flour (DSF)+EMPA” adhesive could withstand a 28 h boiling-dry-boiling hydrothermal test; the aged wet bond strength could not meet the required value (0.98 MPa) for structural use according to the commercial standard JIS K6806-2003
Summary
Formaldehyde-based adhesives are widely used in the timber industry due to their acceptable water resistance, durable adhesion performance, and low price [1]. These adhesives are mainly derived from non-renewable petroleum resources and contain a formaldehyde component that is considered to be a potential human carcinogen [2,3]. Defatted soybean flour (DSF), a byproduct of the soybean oil industry, is one of the most promising raw material alternatives for the development of wood adhesives. The widespread application of traditional DSF-based adhesives is limited due to their poor water resistance and high viscosity [4]
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