An adhesive bond is formed when two materials are attached to each other through the use of an adhesive, such as glue or epoxy. The bond can be produced through various methods depending on the type of substance used. Adhesive bonding has a wide variety of applications and offers several advantages over other methods of fastening. It also has certain disadvantages, including irreversibility and the need for drying time under optimal conditions.
Several different microscale forces interact during adhesion. When surfaces physically interlock with one another, the adhesive bond is mechanical. If a chemical bond between two substances forms, then chemical adhesion is at work. Van der Waals forces, which are attractions between molecules with areas of opposite charge, are responsible for dispersive adhesion — a car decal can adhere to smooth glass without chemical adhesives due to this interaction.
Most adhesive substances used today are synthetic compounds made from polymers, which are organic molecules that consist of long chains of carbon and hydrogen units. Epoxy consists of two different polymers, one a resin and the other a hardener. When these two polymers interact, a strong chemical bond forms between them. The attraction creates an adhesive bond through chemical adhesion.
Other types of glue, such as white glue and rubber cement, are solvent-based adhesives. These substances consist of polymers dissolved in a solvent. The solvent dries and evaporates in air, leaving behind the polymers, which then solidify. A layer of hardened glue binds the two materials together in an adhesive bond.
The development of glues and epoxies capable of forming strong adhesive bonds has led to widespread use of adhesives in the construction industry. Unlike nails and screws, adhesives spread across the joined area, creating an even distribution of stress. They are also lighter than metal fixtures and join materials together invisibly, forming a seal that helps prevent corrosion. Furthermore, adhesives are versatile: they can be used for irregularly-shaped joints and can successfully join almost any two materials.
Despite these advantages, adhesives have their limitations. One drawback is the curing, or hardening time: substrates must be prepared and supported while the adhesive bond is forming. During this time, temperature and moisture must be controlled. If the bond comes into contact with excess heat or chemical substances that can dissolve it, even after it has hardened, it may structurally fail. It is also difficult to diagnose problems with glued joints because they cannot be disassembled without being broken apart. The permanence of certain types of adhesive bonding is therefore problematic for construction projects.