Wood glue

Wood glue is an adhesive used to tightly bond pieces of wood together. Many substances have been used as glues.
Types.
Animal glue.
Animal glue, especially hide glue, was the primary adhesive of choice for many types of woodworking, including furniture and lutherie, for many centuries. It is manufactured from rendered collagen from the skins (hides) of animals. It is chemically similar to edible gelatin and is non-toxic if ingested. Hide glue is still used today in specialized applications: musical instruments (lutherie), for replica furniture, and for conservation-grade repairs to antique woodwork. Hide glue is measured on the basis of its gel strength, a measure of how many grams of force it requires to depress a plunger into a 12.5% protein solution of the glue at . Glue is manufactured in standard grades from . strength is the most commonly used for woodworking; is the highest normally used for instrument building; is the lowest used for general woodwork. Glue above strength requires excessive dilution and so leaves too little glue in joints for effective adhesion, so it is not commonly used. Liquid versions of hide glue are now available; typically they have urea added to keep the glue liquid at room temperature and to extend drying time. Examples of liquid hide glue are Old Brownie Glue or Titebond Liquid Hide. Hide glue does not creep. Hide glue joints are easy to repair, by just heating and adding more hide glue.
Urea-formaldehyde.
Urea-formaldehyde resin adhesives feature a low effective cost, low cure temperatures, resistance to microorganisms and abrasion, and light color. It does not creep, and can be repaired with epoxy. It can rapidly deteriorate in hot, moist environments, releasing formaldehyde (a carcinogen).
Resorcinol-formaldehyde.
Resorcinol-formaldehyde resin glue is very strong and durable (resisting immersion in boiling water, mild acids, salt water, solvents, mold, fungus, ultraviolet, 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 Federal Aviation Administration (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 resorcinol, which offers virtually no tolerance for such everyday construction situations. This can pose major difficulties, especially in complex assemblies. Resorcinol is, however, still used by some builders/restorers, and is commonly seen in vintage aircraft.
Phenol formaldehyde.
Phenol formaldehyde resin is commonly used for making plywood. It is cured at elevated temperature and pressure.
Polyurethane.
Polyurethane glue (trade names include "Gorilla Glue" and "Excel") is becoming increasingly popular. It bonds to textile fibers, metals, plastics, glass, sand, ceramics, and rubber, in addition to wood. Polyurethane wood adhesives are normally prepolymers terminated with isocyanate groups. When exposed to moisture, isocyanates react with water and thus cure the adhesives. Therefore, one component polyurethane adhesives are also named as moisture-cure polyurethanes. In addition, interactions between polyurethanes and wood polymers can significantly influence bond performance. However, in water-saturating tests, polyurethane bonds "were much less durable than the resorcinol bonds on both [Douglas-fir and yellow birch]."
Epoxy.
Epoxy resin, usually as a two part mix system, 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: near-perfect joints with very small gaps actually produce weaker bonds. Use of epoxy requires careful attention to the mixing ratio of the two parts. It bonds to most cured wood glues (except PVA). Two part epoxy adhesive is very resistant to salt water, most epoxy is heat resistant up to , the formulations containing powdered metal and rubber or plasticizers are very tough and shock resistant. Epoxy can trigger long-term sensitivity (allergies) from overexposure, and is often expensive.
Cyanoacrylate.
Cyanoacrylate ("Crazy glue", "Superglue", "CA" or "CyA") is used mainly for small repairs, especially by woodturners. It bonds instantly, including to skin. Cured CA is essentially a plastic material. Versions are available that are able to wick into tight joints but bond with reduced strength (because much drips out and much soaks into the wood leaving very little on the surface for the bond), or thicker formulations (gel) which can fill very small gaps, do not flow out of the joint, and do not soak so quickly into wood. Thinner cyanoacrylate glue does not bond more quickly nor form shorter polymer chains than the gel version when used on wood. The chemical nature of wood significantly delays polymerization of cyanoacrylate. When it finally polymerizes in a wood joint, enough gel remains for a better bond than with the thin version. When using the gel, too much glue will weaken the resulting bond. Likewise, applying too little of the thin super glue will result in almost no glue at all remaining in a wood joint, causing a weak bond or no bond at all. Versions are also available that are "foam safe" (regular CA dissolves most plastic foams) which are usually also marketed as "low odor". Cyanoacrylate is stiff but has a low shear strength (brittle) thus normal wood bending can break the bond 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 builders of balsa wood models, where its strength far exceeds the base materials.
Casein.
Casein 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.
Polyvinyl acetate.
Polyvinyl acetate (PVA), also known as "white glue" or "hobby and craft", is non-toxic and very easy to use, but hard to repair since most glues (including PVA itself) do not adhere well to hardened PVA glue. PVAs will creep under constant load. "Elmer's Glue-All" is an example of PVA adhesive.
Aliphatic resin.
"Aliphatic resin", also known as "carpenter's glue" and "yellow glue," is a synthetic adhesive (in this case, an aliphatic compound) with a light yellow color and creamy texture used most frequently to bond together pieces of wood. Compared to other adhesives, it has low odor and flammability, moderate bonding strength, and moderate moisture resistance. It is more heat- and water-resistant than polyvinyl acetate "white" glues, has a heavier consistency that results in fewer drips, and sets at temperatures above and up to , though it is considered unsuitable for outdoor use. Its faster set-time than white glues can make its use on complex projects difficult. It cures in approximately 24 hours, and results in a glue line that is either translucent, pale tan, or amber. Before it cures, it can be cleaned up with tap water (like white glue). Unlike white glue, its heat resistance and hardness when cured means it can be sanded, though it will not absorb wood stains applied on top of it. Excess resin must be sanded off or otherwise removed before staining. It has less tendency to "creep" (slide during clamping) than white glue. Aliphatic resin has a similar use profile and relative ultimate strength as PVA. The two glues differ in grip characteristics before initial set, with PVAs exhibiting more slip during assembly and yellow glue having more initial grip. Brands include "Titebond" and "Lepage".
Contact cement.
Contact cement for wood veneers.
Hot glue.
Hot glue for temporary uses.
Usage.
Several wood glues have poor "gap-filling" ability, meaning they either soak into the wood and leave the gap empty, or remain to fill the gap but have little structural integrity. 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 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 is preferable to try to minimize gaps in the first place so the problem is not faced.
Mechanical resistance.
"Fine Woodworking" magazine ran a number of tests to evaluate the mechanical resistance of wood joints with different glues: