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Durable wood bonding with epoxy adhesivesAuthor(s): Charles R. Frihart
Source: Proceedings : 26th Annual Meeting of the Adhesion Society, Inc. : Adhesion Fundamentals from Molecules to Mechanisms and Modeling : February 23-26, 2003, Myrtle Beach, SC. Blacksburg, VA : The Adhesion Society, c2003: pages 476-478.
Publication Series: Miscellaneous Publication
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DescriptionAlthough wood was one of the earliest materials to be adhesively bonded, the factors that contribute to strong wood bonds are still not well understood. Wood is a very complex substrate in that it is non-uniform in most aspects. On the macro scale, it is a porous structure with different sized and shaped voids for fluid flow. The structural cells contain four different wall layers, and there is a middle lamella region between the cells. Each cell wall layer is made up of different amounts of three structural components: cellulose, mostly present as a crystalline and rigid polymer; hemicellulose, a mixture of branched carbohydrate polymers; and a matrix of lignin, a crosslinked aromatic polymer. Wood is easy to bond, probably because of its porosity and polar surface. However, few adhesives make a bond that can withstand exterior conditions. With wood, generally the most severe condition involves either stress under wet conditions or cycles of water soaking followed by rapid drying. During a wetting and drying cycle, the wood expands and contracts, whereas most adhesives do not change significantly in volume. Thus, there is a large stress-strain gradient at the wood/adhesive interphase region. Most books and review articles on wood adhesion cover the normal fundamentals of adhesion, but not bond strength. The emphasis is on what takes place at the interface, with discussion of the typical list of chemical interactions that can take place between adhesive and substrate. For bond strength, the interphase regions are also important. Emphasis has been on the preparation of wood surfaces for bonding (1), because it is easy to damage the wood surface so that the surface cells are weak (2). For example, sanding a surface makes it smooth, but pressure from mechanical sanders crushes the surface cells. Planing and knife cutting were found to be the most effective for producing a good surface, with blade angle and sharpness being important factors. Although some adhesives give very durable bonds to wood, epoxy adhesives surprisingly do not. The epoxies are known normally for their good durability. They bond well to plastics, which are less polar than wood, and to metals, which are more polar than wood. The reason that epoxies do not form excellent bonds to wood for exterior applications is not clear. This study provides better insight to factors important to wood bonding and evaluates several models for explaining failure mechanisms of wood bonds.
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CitationFrihart, Charles R. 2003. Durable wood bonding with epoxy adhesives. Proceedings : 26th Annual Meeting of the Adhesion Society, Inc. : Adhesion Fundamentals from Molecules to Mechanisms and Modeling : February 23-26, 2003, Myrtle Beach, SC. Blacksburg, VA : The Adhesion Society, c2003: pages 476-478.
KeywordsEpoxy resins, adhesives, testing, wood surfaces, surfaces, durability, bonding
- Why do some wood-adhesive bonds respond poorly to accelerated moisture-resistant tests?
- Wood structure and adhesive bond strength
- Model for understanding the durability performance of wood adhesives
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