Formaldehyde-free wood adhesives from soybean protein and lignin : development and characterization Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3484zk46q

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  • Presently, the production of wood composites relies on the formaldehyde-based wood adhesives such as phenol-formaldehyde (PF) and urea-formaldehyde (UF). However, their dependence on exhaustible fossil fuels and the emission of carcinogenic formaldehyde prompt to develop an environmentally friendly adhesive from renewable natural resources. This work focuses on development and characterization of forma1dehydefree wood adhesives from renewable soybean protein and lignin. Soybean protein is an abundant, inexpensive, and readily available natural product. Soybean protein-based adhesives were widely used as wood adhesives in 1930s-1960s and are completely replaced by synthetic formaldehyde-based adhesives today because wood composites bonded with soybean protein-based adhesives have relatively lower strength and lower water resistance than those bonded with formaldehyde-based adhesives. Marine adhesive protein from mussels is a strong and water-resistant adhesive. However, the production of marine adhesive protein is difficult and costly. The marine adhesive protein contains three key functional groups: a catechol moiety, a primary amino group, and a mercapto group. In this research, soybean protein was modified using marine adhesive protein as a model. We found that imparting soybean protein with one of the three key functional groups found in the marine adhesive protein converted soybean protein into a strong and water-resistant wood adhesive. Another formaldehyde-free new wood adhesive was also developed through modification of soybean protein with maleic anhydride followed by mixing with polyethylenimine (PEI). Wood composites bonded with this new adhesive were very strong and very water-resistant. The reaction mechanisms in the modification of soybean protein with maleic anhydride and the curing mechanisms of the adhesive were investigated in detail. Demethylated kraft lignin (DKL) has a high amount of the same key functional group, catechol moiety, as the marine adhesive protein. We found that a combination of DKL and PEI (a polyamine with abundant primary amino groups) was a strong and water-resistant wood adhesive. It was found that the curing mechanism of the DKL-PEI adhesive is similar to that of marine adhesive protein. The effects of DKL/PEI weight ratio, hot-press conditions and the molecular weight of PEI on adhesive performance were also investigated in detail.
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