An adhesive is a substance that bonds surfaces to each other by mechanical adhesion (the adhesive filling the pores of the substance) or in some cases by chemical reaction. Natural adhesives, usually called glues, are made by boiling down animal skin, bones, horns, and hooves. They consist of a jelly of hydrolyzed collagen (fibrous protein) – mainly gelatin – mixed with many other animal substances. It dries to form a hard, tough skin or film. There are also vegetable glues made from starch (flour and water), rubber, soybeans, and other sources.
Synthetic adhesives fall into two main categories: thermoplastic and thermosetting. Thermoplastic adhesives (including most animal and vegetable glues) set on cooling or evaporation of the solvent. Thermosetting adhesives (including the epoxy resins) set on heating or when mixed with a catalyst. There are now many strong, long-lasting adhesives designed for use in such varied fields as electronics, medicine, house-building, and book-binding, and for bonding plastics, wood, and rubber.