Wood adhesives are substances capable of binding wood to itself or other materials. Many different substances have been used historically for bonding wood, but these adhesives were all made of natural materials until the late 1940s. Synthetic wood adhesive was introduced just after World War II, and has surpassed natural glue for wood binding in modern times.
Each specific wood adhesive can be placed into one of two distinct categories – natural and synthetic. Natural wood adhesives include animal, vegetable, casein, soybean and blood glues. Synthetic wood adhesives include glues derived from petroleum, gas, coal and other synthetic resins.
Animal glue, sometimes referred to as hot glue, is made from the hide and bones of cattle, sheep and horses. It must be applied to wood while hot, and forms a strong bond as it cools and loses moisture. The wood must also be warm during application, or the glue may cool too rapidly to allow a strong bond. Animal glue has a very low moisture resistance, and the bond will weaken considerably when exposed to high humidity.
Vegetable glue, also known as starch-based glue, is made from starch obtained from corn, rice, potatoes or cassava. Vegetable glue can be applied to wood when hot or cold, and a bond is formed as the glue loses its moisture. The curing process can be very slow, often taking at least 24 hours. Once cured, the bond can be weakened by high moisture content in the air.
Casein glue is made from milk or buttermilk curd, which is dissolved in a chemical solvent. Casein wood adhesive is most commonly sold in powder form, and must be combined with water to form a paste. The paste is applied to the wood, and like other natural glues, a bond is formed as the moisture evaporates. Casein has moderate moisture resistance, but can also stain the wood slightly when applied.
Soybean and blood glue are similar to other natural wood adhesives. They are used primarily for gluing wood veneer and plywood. Blood glues are some of the few natural wood glues which have a strong resistance to moisture.
Synthetic wood adhesives, also known as resins, are man-made polymers which resemble natural resins, but are created to meet specific woodworking needs. They have superior moisture resistance and create very strong bonds. Synthetic glues can be placed into one of two categories – thermosetting and thermoplastic.
Thermosetting adhesives include urea-formaldehyde, phenol, resorcinol, melamine and epoxy. Urea is the most popular wood adhesive, as it provides moderate moisture resistance and can cure in minutes at high temperatures. Phenol and resorcinol glues are expensive but versatile, and provide extremely durable bonds. Melamine glue is rarely used alone, but must be combined with urea to increase moisture resistance. Epoxy is the most expensive thermosetting wood adhesive, and is typically impractical for large scale woodworking projects.
Thermoplastic adhesives include polyvinyl acetates and thermal hot melts. Polyvinyl acetate adhesives are the common white glues used in the woodworking industry. Sometimes referred to as simply “wood glue,” polyvinyl has poor heat and moisture resistance, but is the easiest of the synthetic adhesives to use. Thermal hot melt adhesives are solid glues which must be heated and applied as drops or beads, and form a bond as the glue cools. The main advantages of thermal wood adhesives are their ease of handling and fast curing time.