Wood glue is an

adhesive Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation. The use of adhesives offers certain advantage ...

used to tightly bond pieces of

wood Wood is a structural tissue/material found as xylem in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulosic fibers that are strong in tension and embedded in a matrix of lignin t ...

together. Many substances have been used as glues. Traditionally animal proteins like

casein Casein ( , from Latin ''caseus'' "cheese") is a family of related phosphoproteins (CSN1S1, αS1, aS2, CSN2, β, K-casein, κ) that are commonly found in mammalian milk, comprising about 80% of the proteins in cow's milk and between 20% and 60% of ...

from milk or

collagen Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...

from animal hides and bones were boiled down to make early glues. They worked by solidifying as they dried. Later, glues were made from plant starches like flour or potato starch. When combined with water and heated, the

starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diet ...

gelatinizes and forms a sticky paste as it dries. Plant-based glues were common for books and paper products, though they can break down more easily over time compared to animal-based glues. Examples of modern wood glues include

polyvinyl acetate Polyvinyl acetate (PVA, PVAc, poly(ethenyl ethanoate)), commonly known as wood glue (a term that may also refer to other types of glues), PVA glue, white glue, carpenter's glue, school glue, or Elmer's Glue in the US, is a widely available adh ...

(PVA) and

epoxy Epoxy is the family of basic components or Curing (chemistry), cured end products of epoxy Resin, resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide fun ...

resins. Some resins (i.e., glues) used in producing composite wood products may contain formaldehyde. As of 2021, “the wood panel industry uses almost 95% of synthetic petroleum-derived thermosetting adhesives, mainly based on urea, phenol, and melamine, among others”.

Types

Animal glue

Animal glue Animal glue is an adhesive that is created by prolonged boiling of animal connective tissue in a process called Rendering (animal products), rendering. In addition to being used as an adhesive, it is used for coating and sizing, in decorative co ...

, especially hoof glue and

hide glue Animal glue is an adhesive that is created by prolonged boiling of animal connective tissue in a process called rendering. In addition to being used as an adhesive, it is used for coating and sizing, in decorative composition ornaments, and as ...

, 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) or hooves 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 A musical instrument is a device created or adapted to make musical sounds. In principle, any object that produces sound can be considered a musical instrument—it is through purpose that the object becomes a musical instrument. A person who pl ...

(

lutherie A luthier ( ; ) is a craftsperson who builds or repairs string instruments. Etymology The word ' is originally French and comes from ''luth'', the French word for "lute". The term was originally used for makers of lutes, but it came to be ...

), 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 Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...

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

Urea-formaldehyde Urea-formaldehyde (UF), also known as urea-methanal, so named for its common synthesis pathway and overall structure, is a nontransparent thermosetting resin or polymer. It is produced from urea and formaldehyde. These resins are used in adhesive ...

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 Formaldehyde ( , ) (systematic name methanal) is an organic compound with the chemical formula and structure , more precisely . The compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde. It is stored as ...

carcinogen A carcinogen () is any agent that promotes the development of cancer. Carcinogens can include synthetic chemicals, naturally occurring substances, physical agents such as ionizing and non-ionizing radiation, and biologic agents such as viruse ...

). Supplied as a fine white powder which is mixed with half its weight of cold water for use. Mixed adhesive remains usable for around three hours, depending on temperature. Providing it is kept dry, unused powder has a shelf life of up to a year. The adhesive has the ability to fill gaps between ill-fitting components. It has been produced since the 1930s as ''Aerolite'' to bond

plywood Plywood is a composite material manufactured from thin layers, or "plies", of wood veneer that have been stacked and glued together. It is an engineered wood from the family of manufactured boards, which include plywood, medium-density fibreboa ...

for use in aircraft construction. The greenhouse gas emissions is 2.04 kg CO2-eq./kg of

adhesive. - available in th
Wikipedia Library

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.). Historically, it was the dominant glue in exterior grade plywood manufacture and the production of wooden aircraft. It must be mixed before use (liquid resin and powdered catalyst), is toxic, and has a dark purple cured color, which may not be acceptable in some uses. 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 Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also co ...

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 Phenol formaldehyde resins (PF), also called phenolic resins or phenoplasts, are synthetic polymers obtained by the reaction of phenol or substituted phenol with formaldehyde. Used as the basis for Bakelite, PFs were the first commercial syntheti ...

is commonly used for making waterproof

. It is cured at elevated temperature and pressure and also available as a dry film to be sandwiched in between layers of veneer (

Tego film Tego film is an adhesive sheet used in the manufacture of waterproof plywood. It is applied dry and cured by heat, which allows for high-quality laminates that are free from internal voids and warping. Tego film plywood products were used in aircraf ...

). The greenhouse gas emissions is 2.88 kg CO2-eq./kg of PF adhesive. The energy consumption and greenhouse gas emissions associated with the production of

are lower than those of

Phenol formaldehyde Phenol formaldehyde resins (PF), also called phenolic resins or phenoplasts, are synthetic polymers obtained by the reaction of phenol or substituted phenol with formaldehyde. Used as the basis for Bakelite, PFs were the first commercial synthetic ...

adhesives. But

adhesive is judged to have a nearly 50% higher life cycle impact than

mainly because of acid based emissions during its production process.

Lignin–phenol–formaldehyde

Lignin–phenol–formaldehyde resin adhesives are generally synthesized by reacting a mixture of isolated

lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidit ...

(for example, kraft, soda or biorefinery lignin) and phenol with formaldehyde under alkaline conditions. Lignin–phenol–formaldehyde resin adhesives have higher viscosity, are more deeply coloured and require more severe curing conditions than urea–formaldehyde and phenol–formaldehyde resin adhesives. In lignocellulosic biomass, lignin acts as a glue that provides strength to cell walls by effectively binding cellulose and hemicelluloses together. Milled wood lignin(MWL), formaldehyde-protected lignin (FPL) and acetone-protected lignin adhesives that were prepared with lignins separated under either mild conditions or protection by aldehyde or ketone demonstrated reasonable bonding strengths after hot pressing at 190 °C and 1.5 MPa for 8 min; both the dry and wet adhesion strengths met the minimum requirement of 0.7 MPa. From the results, the slightly condensed or protected lignins from different sources could be directly used as wood adhesives without additional physical or chemical treatments. Adhesion performance of these adhesives improved with reduced condensation degrees and increased with higher hot-pressing temperatures. Multilayer plywood products using lignin adhesives met the mechanical requirements for applications in various fields. Lignin adhesives prepared from lignins protected with other aldehydes (for example, acetaldehyde, propionaldehyde, and furfural) showed qualified adhesion performances >0.7 MPa. Preparing lignin-based wood adhesives from lignocellulosic biomass promotes the use of green adhesives and contributes to the development of profitable biorefining schemes. It is a significant advancement in the field of sustainable adhesive technology and has the potential to impact the plywood manufacturing industry positively.

Polyurethane

Polyurethane Polyurethane (; often abbreviated PUR and PU) is a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane term ...

glue (trade names include ''Gorilla Glue'' and ''Excel'') is becoming increasingly popular in the US after being used for years in other countries. It bonds to textile fibers, metals, plastics, glass, sand, ceramics and rubber, in addition to wood. Polyurethane wood adhesives are normally

prepolymer In polymer chemistry, the term prepolymer or pre-polymer, refers to a monomer or system of monomers that have been reacted to an intermediate-molecular mass state. This material is capable of further polymerization by reactive groups to a fully c ...

s terminated with

isocyanate group In organic chemistry, isocyanate is the functional group with the formula . Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyan ...

s. 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. Polyurethane glues expand when they cure, improving adhesion where the fit is not tight. Unlike PVA glues, they can be used to glue end grains. However, in water-saturating tests, polyurethane bonds "were much less durable than the resorcinol bonds on both ouglas-fir and yellow birch"

Epoxy

Epoxy resin Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also co ...

, 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 Powder metallurgy (PM) is a term covering a wide range of ways in which materials or components are made from metal powders. PM processes are sometimes used to reduce or eliminate the need for subtractive processes in manufacturing, lowering mat ...

and rubber or

plasticizer A plasticizer ( UK: plasticiser) is a substance that is added to a material to make it softer and more flexible, to increase its plasticity, to decrease its viscosity, and/or to decrease friction during its handling in manufacture. Plasticizer ...

s are very tough and shock resistant. The most common epoxy resins are based on reacting

epichlorohydrin Epichlorohydrin (abbreviated ECH) is an organochlorine compound and an epoxide. Despite its name, it is not a halohydrin. It is a colorless liquid with a pungent, garlic-like odor, moderately soluble in water, but miscibility, miscible with most p ...

(ECH) with

bisphenol A Bisphenol A (BPA) is a chemical compound primarily used in the manufacturing of various plastics. It is a colourless solid which is Solubility, soluble in most common organic solvents, but has very poor solubility in water. BPA is produced on a ...

, resulting in a different chemical substance known as bisphenol A diglycidyl ether (commonly known as BADGE or DGEBA). Bisphenol A-based resins are the most widely commercialised resins but also other

bisphenol The bisphenols () are a group of industrial chemical compounds related to diphenylmethane; commonly used in the creation of plastics and epoxy resins. Most are based on two phenols, hydroxyphenyl functional groups linked by a methylene bridge. Ex ...

s are analogously reacted with epichlorohydrin, for example

Bisphenol F Bisphenol F (BPF; 4,4′-dihydroxydiphenylmethane) is an organic compound with the chemical formula . It is structurally related to bisphenol A (BPA), a popular precursor for forming plastics, as both belong to the category of molecules known as b ...

. Epoxy can trigger long-term sensitivity (

allergies Allergies, also known as allergic diseases, are various conditions caused by hypersensitivity of the immune system to typically harmless substances in the environment. These diseases include Allergic rhinitis, hay fever, Food allergy, food al ...

) from overexposure, and is often expensive.

Cyanoacrylate

Cyanoacrylate Cyanoacrylates are a family of strong fast-acting adhesives with industrial, medical, and household uses. They are derived from ethyl cyanoacrylate and related esters. The cyanoacrylate group in the monomer rapidly polymerizes in the presence ...

(''Crazy glue'', ''Superglue'', ''CA'' or ''CyA'') is used mainly for small repairs, especially by

woodturner Woodturning is the craft of using a wood lathe with hand-held tools to cut a shape that is symmetrical around the axis of rotation. Like the potter's wheel, the wood lathe is a mechanism that can generate a variety of forms. The operator is kno ...

s. 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 is made from milk proteins. It was used to make strong and robust joints in early aviation, and was ubiquitous in the form of "white glue" such as Elmer's Glue-All, but fell out of favor due to its susceptibility to attack by bacteria.

Polyvinyl acetate (PVA)

Polyvinyl acetate Polyvinyl acetate (PVA, PVAc, poly(ethenyl ethanoate)), commonly known as wood glue (a term that may also refer to other types of glues), PVA glue, white glue, carpenter's glue, school glue, or Elmer's Glue in the US, is a widely available adh ...

(PVA), also known as "white glue", "hobby and craft" or “school glue” is non toxic, PH neutral, inexpensive, and easy to use, and is therefore the most commonly used type of wood glue. Joints should be tight fitting and clamped during curing for maximum strength. PVAs remain flexible after they have cured, however, and will creep under constant load. Joints that were previously glued with PVA may be hard to repair since most glues (including PVA itself) do not adhere well to cured PVA glue. PVA glues are not waterproof, however type 2 PVAs are water resistant.

Aliphatic resin

''Aliphatic resin'', also known as "carpenter's glue" and "yellow glue," is a synthetic adhesive (in this case, an

aliphatic compound In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated (in which all ...

) 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 A combustible material is a material that can burn (i.e., sustain a flame) in air under certain conditions. A material is flammable if it ignites easily at ambient temperatures. In other words, a combustible material ignites with some effort a ...

, good bonding strength, and moderate moisture resistance. It is more heat- and water-resistant than

"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 stain Wood stain is a type of paint used to colour wood comprising colourants dissolved and/or suspended in a vehicle or solvent. Pigments and/or dyes are largely used as colourants in most stains. The initial application of any paint or varnish is ...

s 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 veneer Veneer refers to thin slices of wood and sometimes bark that typically are glued onto core panels (typically, wood, particle board or medium-density fiberboard) to produce flat panels such as doors, tops and panels for cabinets, parquet fl ...

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: The type I PVA glue was Titebond III, a waterproof glue. The epoxy was from System Three. The PVA glue was Elmer's Carpenter's glue. The liquid hide glue was from Old Brown Glue. The hot hide glue was J.E. Moser's. The polyurethane was Gorilla brand.