Adhesive is a substance that bonds surfaces together. Adhesives include such materials as cement, epoxy, paste, polyurethane, white glue, and cyanoacrylate (super glue). Adhesives are used in homes, offices, and schools. They also play an important role in the manufacture of many products, including aircraft, automobiles, books, furniture, and toys. In addition, they are used in the construction of buildings and roads. Many people use adhesives to make simple household repairs.
Since ancient times, people have made adhesives from such natural materials as beeswax, egg yolks, tree sap, and proteins from animal hides, hooves, and blood. Today, most adhesives are made from plastics and other synthetic materials.
Adhesives vary in the way they are applied and in the manner in which they form a bond. For example, adhesives made from the synthetic materials polyethylene and polyvinyl acetate, called hot melt adhesives, are heated to a liquid state before being applied. The bond forms as the adhesive cools and hardens. Epoxy adhesives are usually sold as two substances in separate tubes. The two substances must be mixed before being applied to the surfaces to be joined. The molecules of epoxy adhesives cross-link (interconnect) to form a thermoset material, which cannot be deformed by heat or pressure. Cyanoacrylate reacts with water on the joined surfaces to create a strong, but brittle, adhesive bond.
