Wood glue is an adhesive

Adhesive

An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...

 used to tightly bond pieces of wood

Wood

Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

 together. Many substances have been used as glues.

Types of wood glue

• For many centuries animal glue
  Animal glue
  An animal glue is an adhesive that is created by prolonged boiling of animal connective tissue.These protein colloid glues are formed through hydrolysis of the collagen from skins, bones, tendons, and other tissues, similar to gelatin. The word "collagen" itself derives from Greek κόλλα kolla, glue...
  
  , especially hide glue was the traditional glue for all kinds of woodwork. It is still used today, mainly for musical instruments (lutherie) and in making sympathetic repairs to antique furniture. You can now find a liquid version of it, Old Brown Glue. Hide glue does not creep. Hide glue joints are easy to repair, by just heating and adding more hide glue.
• Urea-formaldehyde
  Urea-formaldehyde
  Urea-formaldehyde, also known as urea-methanal, named so for its common synthesis pathway and overall structure, is a non-transparent thermosetting resin or plastic, made from urea and formaldehyde heated in the presence of a mild base such as ammonia or pyridine...
  
   resin adhesives feature low cost, low cure temperatures, resistance to microorganisms and abrasion, and light color. It does not creep, and can be repaired with epoxy. Can rapidly deteriorate in hot, moist environments, releasing formaldehyde (a carcinogen);
• Resorcinol
  Resorcinol glue
  Resorcinol glue is a glue that is high in both wet and dry strength and resistant to high temperatures. It is used for gluing lumber or assembly joints that must withstand severe service conditions. Resorcinol withstands outdoor exposure, tropical or sub zero temperatures, as well as salt or...
  
  -formaldehyde resin glue is very strong and durable (resisting immersion in boiling water, mild acids, salt water, solvents, mold, fungus, ultraviolet light, etc.). It must be mixed before use (liquid resin and powdered catalyst). It has a dark purple color which may be objectionable in some uses. Toxic. For many years, the FAA has stated that "Resorcinol is the only known adhesive recommended and approved for use in wooden aircraft structure and fully meets necessary strength and durability requirements" for certificated aircraft, however in fact the vast majority of wooden aircraft built in recent decades (mostly amateur-built aircraft) instead use other types of adhesives (primarily epoxy resin systems) which offer greater strength and even more importantly, much less criticality in perfect application technique. Most newer adhesives are much more tolerant to typical construction mistakes (such as small gaps or misalignments between parts) than resourcinol, which offers virtually no tolerance for such everyday construction situations. This can pose major difficulties, especially in complex assemblies. However, resourcinol is still used by some builders/restorers, and is commonly seen in vintage aircraft.;
• Phenol formaldehyde resin
  Phenol formaldehyde resin
  Phenol formaldehyde resins include synthetic thermosetting resins such as obtained by the reaction of phenols with formaldehyde. Sometimes the precursors include other aldehydes or other phenol. Phenolic resins are mainly used in the production of circuit boards...
  
   is commonly used for making plywood. It is cured at elevated temperature and pressure.;
• Casein
  Casein
  Casein is the name for a family of related phosphoprotein proteins . These proteins are commonly found in mammalian milk, making up 80% of the proteins in cow milk and between 60% and 65% of the proteins in human milk....
  
   glue was used to make strong and robust joints in early aviation, but fell out of favor due to its susceptibility to attack by bacteria.
• Hobbyists commonly use polyvinyl acetate
  Polyvinyl acetate
  Polyvinyl acetate, PVA, PVAc, poly, is a rubbery synthetic polymer with the formula n. It belongs to the polyvinyl esters family with the general formula -[RCOOCHCH2]-...
  
   (PVA), also known as "white glue" or "hobby and craft", and aliphatic resin emulsion, commonly referred to as "carpenter's glue" or "Yellow glue", which has similar relative ultimate strength. The two have different grip characteristics before initial set, with PVAs exhibiting more slip during assembly and yellow glue having more initial grip. PVAs are non-toxic and very easy to use, but hard to repair since nothing else sticks well to the hardened glue. PVAs will creep under constant load.
• Polyurethane
  Polyurethane
  A polyurethane is any polymer composed of a chain of organic units joined by carbamate links. Polyurethane polymers are formed through step-growth polymerization, by reacting a monomer with another monomer in the presence of a catalyst.Polyurethanes are...
  
   glue (trade names include Gorilla Glue and Excel) is becoming increasingly popular, especially where water resistance is required. They bond to textile fibers, metals, plastics, glass, sand, ceramics, and rubber, in addition to wood. However, in water-saturating tests, polyurethane bonds "were much less durable than the resorcinol bonds on both [Douglas-fir and yellow birch]."
• Epoxy
  Epoxy
  Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....
  
   cures under a wider range of temperatures and moisture content than other glues, does not require pressure while curing, and has good gap-filling properties. Bonds to cured glue of most types (except PVA). Sensitive to ultraviolet light and salt water, can lose strength at temperatures above 101 °F (38.3 °C) (some types are much-more heat-resistant), can be more prone to fatigue than the wood it bonds (though is often much stronger than the wood), can trigger long-term sensitivity (allergies) from overexposure, and is often expensive.;
• Cyanoacrylate
  Cyanoacrylate
  Cyanoacrylate is the generic name for cyanoacrylate based fast-acting adhesives such as methyl 2-cyanoacrylate, ethyl-2-cyanoacrylate , and n-butyl cyanoacrylate...
  
   (Crazy glue, Superglue, CA or CyA) is used mainly for small repairs, especially by woodturners
  Woodturning
  Woodturning is a form of woodworking that is used to create wooden objects on a lathe . Woodturning differs from most other forms of woodworking in that the wood is moving while a stationary tool is used to cut and shape it...
  
  . Bonds instantly, including to skin. Cured CA is essentially a plastic material. Versions are available that tend towards being able to wick into tight joints and bond instantly (at a reduced strength) or thicker formulations which can fill gaps and bond more slowly, usually with greater strength. Versions are also available that are foam safe (regular CA dissolves most plastic foams) which are usually also marketed as low odor. CA is stiff enough that normal wood movement can often break the bond, at least in some applications. Often, too much adhesive is applied which leads to a much weaker bond. CA has quickly become the dominant adhesive used by for builders of balsa wood models, where its strength far exceeds the base materials.;
• Contact cement for veneers
  Wood veneer
  In woodworking, veneer refers to thin slices of wood, usually thinner than 3 mm , that are typically glued onto core panels to produce flat panels such as doors, tops and panels for cabinets, parquet floors and parts of furniture. They are also used in marquetry...
  
  ;
• Hot glue
  Hot glue
  Hot melt adhesive , also known as hot glue, is a form of thermoplastic adhesive that is commonly supplied in solid cylindrical sticks of various diameters, designed to be melted in an electric hot glue gun...
  
   for temporary uses;
• Homemade glue
  Glue
  This is a list of various types of glue. Historically, the term "glue" only referred to protein colloids prepared from animal flesh. The meaning has been extended to refer to any fluid adhesive....
  
   for paper, wood, and internal uses.

Usage

Several wood glues have poor "gap-filling" ability (they bond tightly to wood, but not to itself). Therefore, woodworkers commonly use tight-fitting joints that need surprisingly little glue to hold large pieces of wood. Most wood glues need to be clamped

Clamp (tool)

A clamp is a fastening device to hold or secure objects tightly together to prevent movement or separation through the application of inward pressure...

while the glue sets. Epoxy resins and some other glues can be thickened with structural fillers (or with thicker formulations of the resin) to help fill gaps, however it's obviously preferable to try to minimize gaps in the first place.

External links

• DIYinfo.org's Wood Glues — A comprehensive wiki section on wood glues and their applications.