Wood structure and adhesive bond strength

Abstract

Much of the literature on the bonding of wood and other lignocellulosic materials has concentrated on traditional adhesion theories. This has led to misconceptions because wood is a porous material on both the macroscopic and microscopic levels. A better understanding of wood bonding can be developed by investigating the theories of adhesion and bond strength, taking into consideration the unusual structure of wood. Wood is not uniform in the millimeter, micrometer, and nanometer scales. The interaction of adhesive with wood needs to be considered on these different spatial scales. In addition, emphasis needs to be placed on the stress concentration and dissipation mechanisms that are active in bonded wood. Because most adhesives bond wood sufficiently to give wood failure under dry conditions, the emphasis is on durable bonds, especially those exposed to moisture and/or heat variations. The new hypothesis emphasizes that for durable bonds, the adhesive needs to give during wood expansion or to restrict wood expansion to lower stress in the interphase regions. Among the experiments that support this hypothesis, one study involves the failure mechanism of epoxy wood bonds. Available information indicates that the fracture occurs near the surface within the epoxy layer. A second study is the bonding of acetylated wood with epoxy adhesives. Under wet conditions, acetylated wood expands less than does untreated wood and less stress thus occurs at the interface. In addition, this hypothesis proposes that the primer, hydroxymethylated resorcinol, is not a coupling agent but stabilizes the wood surface.

Citation

Frihart, Charles R. 2006. Wood structure and adhesive bond strength. Characterization of the cellulosic cell wall. Ames, Iowa : Blackwell Pub., 2006: ISBN: 2005016442: pages 241-253.