Graduate Thesis Or Dissertation
 

Development of formaldehyde-free wood adhesives

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  • Wood adhesives are essential components of wood composites. At present, the commonly used wood adhesives, phenol-formaldehyde (PF) and urea-formaldehyde (UF) resins, are petroleum-based synthetic resins that may result in emission of formaldehyde in the production and use of wood composites. Because the world's oil reserves are naturally limited and formaldehyde is hazardous to human health, wood composite industries would greatly benefit by development of formaldehyde-free wood adhesives from renewable resources. Marine organisms, such as mussels, produce adhesive proteins for binding to wet surfaces in seawater. Marine adhesive is a protein rich in 3,4-dihydroxy-phenylalanine (DOPA) and lysine residues. By means of a phenol-oxidative enzyme, the marine adhesive protein cross-links to form a tough and water-resistant polymer. Inspired by the strong adhesive properties of the marine adhesive protein, we investigated the possibility to develop a formaldehyde-free adhesive for wood composites using the marine adhesive protein as a model. Because the extracted and synthetic marine adhesive proteins are expensive and not readily available, we focused our efforts on surrogates of the protein. In this study, we investigated whether a polymer with a phenolic hydroxyl group or an amino group, the two key functional groups of the marine adhesive protein, could be converted to a wood adhesive system. First, we investigated poly(4-vinylphenol) (PVP), a polymer containing a phenolic hydroxyl group, as a key material for a wood adhesive. The aqueous suspension of PVP could be used as a wood adhesive to produce wood composites with the shear strength of up to 3 MPa. Addition of compounds with amino groups, such as I ,6-hexanediamine or diethylenetriamine, to the PVP suspension, resulted in a significant increase of the shear strength. When the molar ratio of the phenolic hydroxyl group in PVP vs. I ,6-hexanediamine was 3:1, the shear strength of wood composites bonded with PVP/1 ,6-hexanediamine could be twice as high as that with the aqueous PVP suspension alone. Then, we investigated whether chitosan, a natura' polymer with an amino group, could have a potential as a wood adhesive. We found that a chitosan-based wood adhesive system could bond wood veneers strongly when chitosan, a phenolic compound, and laccase were all present. Ten phenolic compounds with different chemical structures were studied. The adhesive strength of a chitosan-phenolics-laccase system was highly dependent upon the chemical structure of the phenolic compound used in the system. The shear strengths of wood composites made with the systems containing a phenolic compound with only one hydroxyl group were related to the system's viscosity and to the oxidation rates of the phenolic compounds by laccase. However, for the adhesive systems containing a phenolic compound with two or three phenolic hydroxyl groups adjacent to each other, no correlations among shear strengths, relative oxidation rates of the phenolic compounds by laccase, and the viscosities were observed. Results from this study reveal that a strong, water-resistant and formaldehyde-free wood adhesive could be developed through mimicking of marine adhesive proteins. Although the new adhesive systems worked on the same principles as marine adhesives, materials in the new adhesive systems did not have to be protein or polypeptides.
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