Adhesive, a substance used to bond two or more surfaces together. Most adhesives form a bond by filling in the minute pits and fissures normally present even in very smooth surfaces. Adhesive bonds are economical, distribute the stress at the bonding point, resist moisture and corrosion, and eliminate the need for rivets and bolts. The effectiveness of an adhesive depends on several factors, including resistance to slippage and shrinkage, malleability, cohesive strength, and surface tension, which determines how far the adhesive penetrates the tiny depressions in the bonding surfaces. Adhesives vary with the purpose for which they are intended. Such purposes now include the increasing use of adhesives in surgery.

Natural adhesives have been replaced in many uses by synthetics; but animal glues, starches, gums, cellulose, bitumens, and natural rubber cement continue to be used in large volumes.

Organic adhesives derived from animal protein include glues made from collagen, a constituent of the connective tissues and bones of mammals and fish; blood albumin glue, used in the plywood industry; and glue made from casein, a protein constituent of milk, employed in wood bonding and in the paint.

Vegetable adhesives include starches and dextrins derived from corn, wheat, potatoes, and rice, used for bonding paper, wood, and textiles; gums such as gum arabic, agar, and algin, which when moistened provide adhesion in such products as stamps and gummed envelopes; cellulose adhesives, used to bond leather, cloth, and paper; rubber cements; and resins such as tree pitch and mastic.

Synthetic adhesives, used either alone or as modifiers of natural adhesives, perform better and have a greater range of application than the natural products. Most of them form polymers, huge molecules incorporating large numbers of simple molecules to form strong chains and nets that link surfaces in a firm bond. Thermosetting adhesives, which are transformed into tough, heat-resistant solids by the addition of a catalyst or the application of heat, are used in such structural functions as bonding metallic parts of aircraft and space vehicles. Thermoplastic resins, which can be softened by heating, are used for bonding wood, glass, rubber, metal, and paper products. Elastomeric adhesives, such as synthetic or natural rubber cement, are used for bonding flexible materials to rigid materials.