On the Wood-adhesives.

Wood adhesives are polymeric materials that are capable of interacting physically or chemically, or both, with the surface of wood in such a manner that stresses are transferred between bonded members, hopefully without rupture of the adhesive or detachment of the adhesive from the wood. Adhesives and the physicochemical phenomenon of adhesion play an important role in more than 70% of all woodbased materials in use today (Marra 1984). This includes not only bonded wood products but paper products, paints, and finishes. By far, the largest amount of adhesive is used to manufacture building materials such as plywood and other laminated veneer products, particleboard, oriented strandboard, fiberboard, laminated beams and timbers, edgeand endjointed products, windows and frames, architectural doors, and fiberglass insulation. Adhesives are used in lesser amounts to assemble building materials in residential and industrial construction, particularly in panelized floor and wall systems. Significant amounts are also used in nonstructural applications such as furniture, overlays, floor coverings, countertops, ceiling and wall tiles, trim, and accessories. The bonding of wood with adhesives is known to date back at least to the Pharaohs, and in all likelihood dates even further back in history (Keimel 1994). For example, the Egyptians used adhesives derived from animal sources to laminate wood for producing bows and furniture and for attaching decorative wood veneers. In addition, they are known to have used gum arabic, eggs, balsams, and resins from trees as sources of adhesives. Up to World War II the dominant adhesives for bonding wood were made from natural sources (Lambuth 1989). Natural adhesives that have been used by the forest products industry include adhesives derived from starch, soybeans, animal waste and by-products of the meat processing adhesives derived from starch, soybeans, animal waste and by-products of the meat processing and tanning industries, and casein from skim milk. Of these, protein-based adhesives (soy, blood, and casein) were most commonly used. However, adhesives derived from natural sources were limited to dry interior applications. During the past 50-60 years, the use of adhesives in the production of bonded-wood products has increased dramatically. For example, in 1997, approximately 1.6 × 109 of adhesive resin solids were used in North America (United States, Canada, and Mexico) to bond nearly 57 × 106m3 of glued wood products (Sellers 1998). This amount does not include adhesives used for the production of nearly US $30 million of furniture and other secondary products. The increased use of adhesives after World War II is largely due to the availability of low-cost, highly durable synthetic adhesives that could be produced from petrochemicals no longer needed to support the war effort (Lambuth 1989). Many of these adhesives, unlike their natural counterparts, could be used in exterior applications. Highly durable synthetic adhesives allow efficient and economical utilization of diverse and changing wood resources. Adhesives allow the manufacture of useful products from residues and waste wood; the manufacture of stronger, more efficient structures by removing or minimizing natural defects; the reassembly of smaller forms of wood such as veneer, flakes, and fibers into efficiently engineered shapes and products; and the manufacture of a variety of new composites by blending wood with nonwood materials such as plastics and cement. Large bonded panels, such as plywood and oriented strandboard, provide superior structural integrity and consume roughly 50% less wood than lumber sheathing. Fingerjointing provides usable lengths of lumber from otherwise unusable short pieces, and wood I joists place highstrength wood where it is needed and eliminates wood where it is not needed. In the future, the bonding of wood with adhesives will continue to be a major factor not only for the efficient utilization of forests as a source of raw materials but also for the efficient manufacture of wood products and the recycling of waste wood. Efficiency in converting trees and waste wood to useful products will help to ensure the continual conservation of forest resources.