Wood adhesives can be natural or synthetic. Natural adhesives include animal, vegetable, casein, soy and blood glues. Synthetic adhesives include thermosets and thermoplastics. Urea-formaldehyde is the most popular wood adhesive, while polyvinyl acetate is the easiest to use.
Wood adhesives are substances that bind wood to itself or to other materials. Historically, many different substances have been used to glue wood, but these adhesives were all made from natural materials until the late 1940s. Synthetic wood adhesive was introduced just after WWII and has superseded natural wood binding glue in modern times.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, soy and blood glues. Synthetic wood adhesives include glues derived from oil, gas, coal and other synthetic resins.
Animal glue, sometimes referred to as hot glue, is made from the skin and bones of cattle, sheep and horses. It must be applied to the wood while it is hot and forms a strong bond as it cools and loses moisture. The wood also needs to be warm during application, or the glue may cool too quickly to allow for a strong bond. Animal glue has very low moisture resistance and the bond will weaken significantly 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 it is hot or cold, and a bond forms when 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 the high moisture content in the air.
Casein glue is made from milk or buttermilk curds, 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 may also stain wood slightly when applied.
Soy glue and blood are similar to other natural wood adhesives. They are mainly used for gluing veneer and plywood. Blood glues are some of the few natural glues for wood that have a strong resistance to humidity.
Synthetic wood adhesives, also known as resins, are man-made polymers that 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 classified into one of two categories: thermosets and thermoplastics.
Thermosetting adhesives include urea-formaldehyde, phenol, resorcinol, melamine and epoxy. Urea is the most popular wood adhesive, as it provides moderate resistance to moisture and can cure within minutes at high temperatures. Phenol and resorcin 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 thermoset wood adhesive and is generally 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 simply as “wood glue,” polyvinyl has poor resistance to heat and moisture, but it’s the easiest synthetic adhesive to use. Thermal hot melt adhesives are solid glues that need to be heated and applied as beads 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.
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