A large number of adhesives for wood bonding are on the market today which are based on several chemistries.
Polyvinyl acetate or PVA glues are the workhorse of the wood working industry. These adhesives are inexpensive, non-toxic, can be cleaned with water, are easy to use and, ones fully cured, are very strong. Both, indoor-use only and water proof PVA glues are commercially available. They are used both in the wood processing industry and by hobbyists to bond a multitude of wood products such as doors, windows, furniture and numerous other wood articles. Besides wood bonding, PVA glues are extensively employed in the packaging industry to bond and seal high energy surfaces such as paper and corrugated cartons. They are often the preferred adhesive in the food packaging industry because they are cheap, non-toxic, water clear and do not yellow when exposed to heat and light.
Urea-formaldehyde adhesives (UF resins) are probably the most widely used thermosetting resins
in the wood bonding industry. They are obtained from polycondensation of formaldehyde with urea. UF resins are noted for their high strength, cost effectiveness and fast cure. At elevated temperatures,
they can be cured in as little as two seconds. A major drawback of urea- formaldehyde resins compared with other thermosetting wood bonding resins such as phenoplasts and polymeric diisocyanates,
is the lack of moisture resistance especially at elevated temperatures. Furthermore, the reversal bond-forming reactions can lead to the release of formaldehyde which is harmful.
UF resins are extensively used for the manufacture of particle boards and plywood for interior paneling and furniture.
Melamine-formaldehyde resins (short melamine) are obtained from polycondensation of melamine (2,4,6-triamino-1,3,5-triazine) with formaldehyde. These thermosetting resins have better moisture and chemical resistance and shorter cure schedules than UF resins but are more expensive. They are extensively used for bonding low- and high- pressure laminates and overlays. They are also used in the production of many structural wood products, particularly when superior moisture resistance and durability is needed. To lower cost, melamine is sometimes copolymerized with urea and formaldehyde (UMF resins). The properties of these resins are typically between those of UF- and MF resins. MUF resins are used in the manufacture of oriented strand boards and fiberboards that have to be moisture and water resistant and more durable.
Phenolics are an important class of thermosetting adhesives that are produced by polycondensation of formaldehyde with phenol. Since they cure at elevated temperatures, they are predominantly used in industrial wood joining processes. They possess high strength and durability and are often an excellent choice for bonding load-bearing joints such as finger-jointed studs and I-joints. They are also used for bonding plywood and oriented strand boards. However, phenolics are more expensive than urea-formaldehydes resins (UF), and are, therefore, only used for exterior wood applications that need to be water resistant.
Resorcinol-Formaldehyde (RF) are obtained from polycondensation of formaldehyde with resorcinol. Like PF resins, resorcinol resins form very durable and strong bonds with excellent humidity, water and corrosion resistance. However, unlike phenolic resins, RF resins can be cured at room temperature because these resins cure about 10 times faster than phenolic resins. A major drawback is the high price of resorcinol. To lower cost, but to maintain ambient cure, resorcinol are sometimes copolymerized with phenol and formaldehyde. Both resins are used in wood bonding applications where high bond strength and water resistance is required. For example, they are used in the production of outdoor furniture, boat building and for laminated beams in load-bearing wooden structures.
Cyanoacrylate adhesives, also known as ‘‘superglues", are single component liquids that have fast cure, and excellent shear strength. This class of adhesives is available only as low-viscose liquids that cure in seconds at room temperature and bond well to many substrates including wood. They typically have poor heat and moisture resistance and have relative low peel and impact strengths, also newer developements resulted in adhesives that exibit some toughness. Cyanoacrylates are usually used where there is minimal environmental stress and when fast setting times are required.
Emulsion polymer isocyanates (EPI) are water-based two-part emulsion adhesives which are made up of a high molecular weight polyhydroxyl functionalized polymer latex (first part) and a water-emulsifiable polyisocyanate (second part) which are mixed prior application. These adhesives cure rapidly at ambient temperature to water clear or light-colored bond lines. They are often an excellent choice for most types of wood since the isocyanates form strong and durable bonds with the cellulose of wood. These resins also bond plastics and other non-wood substrates. Typical applications include parquets, furniture, windows, and load bearing wooden structures such as I-beams. A major drawback is there higher cost.
Epoxy adhesives are known for their outstanding corrosion and weathering resistance and high performance. They are generally two part adhesives which are made up of an amine or mercaptan hardener and an epoxy resin that are mixed prior use. These adhesives are often an excellent choice for structural bonds but are more expensive than most other wood adhesives. However, not all epoxy adhesives will provide durable bonds on wood unless they are specifically designed for this application. A major advantage of epoxy adhesives is ambient cure and good gap filling ability, while most other wood adhesives are not gap filling. They can also be used in bonding wood to many other substrates.