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

, 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 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, 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 Brown 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

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.

Resorcinol-formaldehyde

-formaldehyde resin glue is very strong and durable. It must be mixed before use. It has a dark purple color, which may be objectionable in some uses. Toxic. For many years, the Federal Aviation Administration 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 instead use other types of adhesives, 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 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

is commonly used for making plywood. It is cured at elevated temperature and pressure.

Polyurethane

glue is becoming increasingly popular. They bond 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 ."

Epoxy

, 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. 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 from overexposure, and is often expensive.

Cyanoacrylate

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, or thicker formulations 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 which are usually also marketed as low odor. Cyanoacrylate is stiff but has a low shear strength 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

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

, also known as "white glue" or "hobby and craft", is non-toxic and very easy to use, but hard to repair since most glues 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 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. 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" 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

for wood veneers.

Hot glue

for temporary uses.

Homemade

Homemade glue for paper, wood, and internal 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 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:
GlueJoint strength as a percentage of Type III PVA glue
Type III PVA glue 100%
Slow-set epoxy99%
PVA glue95%
Liquid hide glue79%
Hot hide glue76%
Polyurethane58%