Although there are approximately 1,500 adhesive products manufactured in the United States, less than a dozen are suitable for woodworking.
Before getting into the individual types of adhesives, it might be helpful to know how glue bonds wooden parts together. It is helpful to understand a little about the chemical makeup of wood and how an adhesive interacts with these components during the bonding process. Wood is a complex mixture of organic chemicals and water.
About 95 percent of a board consists of cellulose, hemicelluloses, and lignin, which form the structural matrix of wood and give it its rigidity, strength, and elasticity. The remaining five percent contained in dry wood is composed of tannins, essential oils, resins, gums, coloring agents, and sugars. This chemical mixture of extractives is responsible for wood’s smell, color, and decay resistance. Unfortunately, extractives in some resinous woods, such as teak and rosewood can interfere with the gluing process.
Once an adhesive is applied to adjacent wood surfaces and the pieces are clamped up, the structural elements of the wood are linked together by the bonding process. First, the liquid adhesive is absorbed into the wood and its polymer molecules intermingle with the structural fibers of the wood. Then, the adhesive’s polymer molecules coalesce or come together, surround the structural fibers, and harden, mechanically interlocking the fibers.
Thermosetting glues such as epoxy, urea-formaldehyde, and resorcinol are cured by a chemical reaction, usually after two components have been mixed, while thermoplastic adhesives, such as yellow and white glues cure by evaporations. Once either type of glue is dry, the thin layer of cured adhesive between the two wood surfaces acts like a bridge holding the boards together.
Yellow and white glues are probably the most often and most popular glues used in woodworking today. Both are polyvinyl acetates (PVA) adhesives that come in three main varieties: yellow aliphatic resin, white or craft glue, and cross-linking PVA emulsion. All of these have a balanced set of properties, which make them
ideal fro gluing wood. They are easy to use, have quick grab, set rapidly clean up with water, are non-toxic, and work in most wood-gluing situations. In addition, the liquid adhesives will spoil if frozen. However, PVA adhesives have poor creep resistance and they should never be used in structural assemblies, like load-bearing beams, without some form of mechanical fastenings such as nails or screws.
Resorcinol and urea formaldehyde
Urea formaldehyde and resorcinol formaldehyde adhesives are most frequently used for bonding wood when strong, creep- and water-resistant bonds are required. Urea-formaldehyde (UF) adhesive sometimes called plastic resin glue comes as a one-part powder. The powder is a mixture of dry resins and hardeners that if kept dry will remain storable indefinitely. Water is added to dissolve the chemicals and activate the adhesive. The pot life after mixing is relatively long, but the viscosity of the activated glue slowly increases until after about an hour, the adhesive is too thick to work with. Once cured, UF adhesives produce structural bonds and the tan glue line is hardly noticeable even on light-colored woods. Interior load-bearing beams and hardwood plywood panels are often glued with UF adhesives. However, it is not 100 percent waterproof.
Resorcinol formaldehyde or RF adhesives have high strength, and exceptional solvent resistance and when properly cured will withstand prolonged immersion in water, making them perfect for marine applications. RF glues come as two-part kits: part one is the resorcinol resin dissolved in ethyl alcohol; the other part contains powdered paraformaldehyde. The premeasured components are stirred together to activate the adhesive, but careful mixing is necessary to avoid lumps.
Working with RF and UF adhesives can cause health issues, so work in a well-ventilated area, wear a mask and take breaks whenever possible. This is because they both give off a formaldehyde gas.
With their high strength, great gap-filling capacity, ability to structurally join difficult-to-bond materials, and waterproof nature, epoxies are surely the high-performance adhesives of the woodworking world. Epoxy consists of an epoxy resin and an amine hardener. Typically, equal parts of resin and hardener are mixed to activate the adhesive and start the curing process, which works by chemical reaction rather than solvent evaporation. The exact mixing proportions are fairly critical; too much of either component will adversely affect bonding strength. Because of the lack of solvent, epoxy has exceptional gap-filling ability.