Hemicelluloses and other Polysaccharides for Wood Adhesive Applications
Sammanfattning: The growing environmental awareness has led to an increased interest in bio-based polymers as replacement for fossil-based materials. The purpose of the work described in this thesis was to investigate the possibility of using hemicelluloses and other polysaccharides as replacement for fossil-based polymers in wood adhesives. Together with cellulose and lignin, hemicellulose is the main constituent of wood. In the pulp industry, significant amounts of hemicelluloses are obtained as by-products and combusted for energy recovery, but there is a growing interest in the biorefinery concept where all side-streams are utilized. If valuable applications, such as adhesives, of hemicelluloses and other by-products are found, large amounts can be obtained from the pulp industry. Water dispersions of hemicelluloses and other polysaccharides have been prepared and evaluated as adhesives for bonding different wood substrates together. The dry bond strength, water resistance, and heat resistance were investigated by exposing the bonded wood specimens to different conditioning methods and thereafter measuring the tensile shear strengths. As a replacement, the bio-based wood adhesive must possess similar or even better properties than the fossil-based adhesives. A commercial poly(vinyl acetate) (PVAc) wood adhesive used for indoor applications has been used as a reference benchmark. Wood hemicelluloses themselves do not have sufficient bonding performance probably because their low molecular weight does not provide adequate strength and makes the adhesive too brittle. The addition of dispersing agents and crosslinkers to the hemicellulose dispersions can significantly improve the bonding performance, and hemicellulose in combination with poly(vinyl amine) showed promising results superior those of PVAc. A fully bio-based adhesive comprising of hemicellulose and chitosan, another bio-based polysaccharide, obtain surprisingly good bonding performance especially with regard to water resistance. Gums, polysaccharides with similar structures to those of hemicelluloses but with higher molecular weights, have also been studied and locust bean gum dispersions without any modification showed a very good bonding performance with high dry bond strength and water resistance on a par with those of PVAc and a heat resistance superior to that of PVAc. Chitosan has very good adhesive properties especially with regard to water resistance, but the high viscosity of the chitosan dispersion makes it difficult to apply. Chitosan-grafted-PVAc dispersions were therefore prepared and an adhesive very similar in appearance to PVAc was obtained with a good bonding performance as well as good applicability.
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