Abstract
Over a period of years, inrreasing demand for softwoods in the Eastern United States has led to an increase in the growth of hardwoods on cut-over softwood sites. Unfortunately these hardwood trees are often of a size and shape unsuitable for the production of high-grade lumber and veneer. They do, however, represent a viable, economic soures of raw material for plywood, fiberboard, particleboard, and oriented strandboard (or flakeboards), all products that require the successful use of adhesives in their manufacture. The current status of gluing eastern hardwoods is reviewed in this report, with emphasis on hardwoods growing on southern pine sites. The subjects covered include adhesives, wood and wood-surface properties and their interactions with the adhesive, and the quality of the bonds produced when these hardwoods are used in the manufacture of end joints, laminates, plywood, and other composite panels. A variety of adhesives are available that equal or exceed the strength of the hardwoods being bonded. The choice of a particular adhesive is dictated in large measure by the adhesive price and the end-use criteria for the finished product. In discussing the gluing of eastern hardwoods, the approach taken is that the fundamentals that determine the quality ofan adhesive bond should remain the same whether the substrate is a softwood or a low-, medium-, or high-density hardwood. To illustrate the differences encountered in gluing the various hardwood species and the best approach for dealing with them in terms of bonding fimdamentals, in this report we will concentrate on: ·The quality and character of the surface as affected by wood structure. ·Bond strength, dimensional change, porosity, and compaction of composites as affscted by species' density. ·Ability of the resin to wet the surface and penetrate the fine structure of the cell wall. Gross penetration as affected by wood structure, resin viscosity; and resin flow. ·The interaction between pH of tannins, or other extractives, and the curing mechanism of resins. Adhesives are available to provide the necessary structural integrity for plywoods, particle- boards, flakeboards, and fiberboards with hardwood substrates; however, in many cases the adhesive cost may be considered excessive in terms of current commercial practice. Development opportunities lie in providing a family of adhesives that will provide exterior bonds at a competitive price over the whole range of southern hardwoods, including those at the high end of the density scale.
Citation
Sellers, Terry, Jr.; McSween, James R.; Nearn, William T. 1988. Gluing of Eastern Hardwoods: A Review. Gen. Tech. Rep. SO-71. New Orleans, LA: U.S. Dept of Agriculture, Forest Service, Southern Forest Experiment Station. 30 p.
