Whether some type of glue, tape or other bonding agent, an adhesive is something that has the ability to join objects together. Glue consists of a chemical mixture in liquid, solid or semi-solid form. Tape is a combination of a solid backing material such as aluminum, paper or plastic coated with adhesive, so it can rightly be said to provide adhesive strength rather than to be purely an adhesive. Adhesives are categorizable according to certain properties. Categories include nonreactive, reactive, synthetic and natural adhesives. Chemical makeup, curing method and application vary largely among the different categories of adhesive, making it a very diverse substance.
The primary difference between nonreactive and reactive adhesives is that the latter requires a chemical reaction in order to cure or harden. Nonreactive adhesives, on the contrary, form their bonds by the application of pressure, on contact, by means of a thermoplastic or through drying. Reactive adhesives are either one-part or multi-part glues that require the mixture of certain components to cause a chemical reaction or harden with the help of a source of external energy like water, high heat or ultraviolet light. Synthetic adhesives include elastic polymers, thermoplastics, thermosetting glues such as epoxy, and emulsion bonding agents. Natural or bio-adhesives were and are derived from plant or animal matter. Papier-mache adhesive paste is one such example.
Nonreactive, pressure-sensitive adhesives include spray adhesives, common white glue, wood glue and most types of tape. Contact adhesives are in many cases synthetic and include cyanoacrylate, polyurethane "yellow" glues, high-strength epoxies and acrylics. Solvent- and water-based glues are examples of drying adhesives, as are the very common PVAs. Hot-melt adhesives include all manner of hot glue sticks, powders, pellets and strips. They are commonly used in craft making as well as for packaging. Epoxies of various kinds are reactive adhesives, requiring high temperatures or ultraviolet light to cure. Many reactive adhesives are thermosets, that is, they form an irreversible bond once cured.
The properties of glue vary, largely dependent on the type of adhesive in question. Adhesives are engineered to form bonds of many strengths. Some, such as aerosol sprays, allow for repositioning and even removing. Most adhesives, however, are intended to form permanent bonds. The strongest bonds are formed by epoxies, urethanes, acrylics and other thermosets, although certain adhesives allowed to cure under high pressure will form strong bonds as well, such as with wood glue. Adhesives can be fast-drying or slow-setting, depending on the application. Fast-drying or instant adhesives include cyanoacrylate. Better known as superglue, this adhesive forms an aggressive bond with moist, nonporous substrates or surfaces, even skin. Many glues can be stained, sanded, painted or otherwise molded once cured. Adhesives commonly dry clear, but yellowish or fully opaque-drying adhesives are quite common.
Other properties determine how resistant an adhesive is to moisture, extreme temperatures, solvents and other chemicals. Superglue, for instance, is often used for underwater applications to join coral pieces in aquariums. Epoxies are resistant to exposure to high heat, chemicals, water and active force, making them among the strongest available. The weaker the bond, the less resistance an adhesive will have. Depending on the function of the glue, high resistance to various elements and conditions may or may not be important.
Many adhesives include solvents of various kinds in their pre-applied state. Solvents help to keep elastic polymers and other semi-solid components from hardening. They have the disadvantage, however, of emitting a noxious or harmful odor, and they can also be quite flammable. The application of industrial-strength adhesives requires specific safety protocols. Even when using adhesives in the home, keep in mind that suitable ventilation is necessary when working with some adhesives. With some glues, contact with the skin should be entirely avoided.