MODEL FOR UNDERSTANDING THE DURABILITY PERFORMANCE OF WOOD ADHESIVES

Wood has been bonded for thousands of years, but it has been only for the past 100 years that structural wood bonds have been obtainable. Furthermore, structural wood bonds with exterior durability have been available for about 60 years. The development of new or improved adhesives for these applications has been hindered by having only limited wood bonding models that encompass both chemical and mechanical aspects. Developing a general wood bond model has been difficult because of the structural and chemical complexity of wood across the spatial scales from nanometers to meters and the wide variety of adhesive chemistries. Most adhesives readily form strong wood bonds, but few adhesive bonds are able to withstand accelerated aging tests involving water exposure. The specific tests depend upon the product; however, water saturation of the bonded assembly has been generally considered important to determining bond integrity over the useful life of the product. Some tests involve placing the assembly in room-temperature or boiling water, followed by oven drying to look for delamination in the bondline. Others involve testing of wet samples for shear strength and area percentage failure in the wood or measuring internal bond strength. These tests were developed because the swelling and shrinking of wood with changes in moisture content lead to strong forces on the bondline. These forces are usually a shear mode in the bondline plane but can also involve normal forces (perpendicular to the bondline) due to warping of wood as it dries (Figure 1).