16.7 WOOD AND WOOD PRODUCTS

Wood is an important structural material consisting of a cellulosic composition with a highly porous nature. Adhesives are commonly employed to bond wood in the furniture industry. They are increasingly being used in laminating and veneering of wood-based products onto composite panels. Sealants are commonly applied to wood framing members in the construction industry. Several properties are unique to these materials that will affect their ability to be joined.

Wood is an anisotropic material. That is, wood and other cellulosic materials have different properties when measured along different axes. Tensile and shear strengths of wood are greater along the longitudinal direction, parallel with the wood fibers.

Wood is also a hygroscopic material because of its cellulosic nature. Generally, dimensional stability as a result of changing moisture content is highest in the longitudinal direction, somewhat less in the tangential direction, and least in the radial direction. Maximum performance is achieved if the wood has a moisture content during bonding that is close to the average moisture content anticipated during service. In this way, the internal stresses induced in the joint as a result of moisture change in service will be at a minimum. Therefore, wood substrates are generally preconditioned to a known moisture content before bonding. Wood changes little in dimension as a result of temperature changes, unless such changes also affect the internal moisture content.

Wood of different species will vary considerably in density or specific gravity. Higher-density wood of any species is usually stronger...