Wood glue
Wood glue is an adhesive used to tightly bond pieces of wood together. Many substances have been used as glues. Traditionally animal proteins like casein from milk or collagen from animal hides and bones were boiled down to make early glues. They worked by solidifying as they cooled or dried. Later, glues were made from plant starches like flour or potato starch. When combined with water and heated, the starch 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 and epoxy resins. Some resins 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
, especially hoof glue and 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 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, 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. 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 for use in aircraft construction.The greenhouse gas emissions are 2.04 kg CO2 equivalent/kg of Urea-formaldehyde adhesive.
Resorcinol-formaldehyde
resin glue is very strong and durable. Historically, it was the dominant glue in exterior grade plywood manufacture and the production of wooden aircraft. It must be mixed before use, is toxic, and has a dark purple cured color, which may not be acceptable in some uses. 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 waterproof plywood. It is cured at elevated temperature and pressure and also available as a dry film to be sandwiched in between layers of veneer. The greenhouse gas emissions are 2.88 kg CO2 equivalent/kg of PF adhesive.The energy consumption and greenhouse gas emissions associated with the production of Urea-formaldehyde are lower than those of Phenol formaldehyde adhesives.
But Urea-formaldehyde adhesive is judged to have a nearly 50% higher life cycle impact than Phenol formaldehyde 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 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, formaldehyde-protected lignin 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 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
glue 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 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. 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 ."Recently, some new polyurethane-based resins have been used to glue even green wood having high moisture content.