adhesive

Substance that sticks two surfaces together. Natural adhesives (glues) include gelatin in its crude industrial form (made from bones, hide fragments, and fish offal) and vegetable gums. Synthetic adhesives include thermoplastic and thermosetting resins, which are often stronger than the substances they join; mixtures of epoxy resin and hardener that set by chemical reaction; and elastomeric (stretching) adhesives for flexible joints. Superglues are fast-setting adhesives used in very small quantities.

Natural water-based adhesives Typical natural substances used for water-based adhesives are starch, casein, tree exudates, skin, and bones.

Starch, usually provided from maize, potato, tapioca, or sago, is extracted from the vegetable matter by disintegration and extraction with cold water. Dextrin is prepared from starch by roasting it in the presence of acid or by acid hydrolysis. Its adhesive properties vary widely with the source of the starch and the degree of hydrolysis.

Casein is prepared from milk by precipitating its protein with acid or rennet. After further purification the casein is dissolved in a solution of alkali or urea to form a strong adhesive.

Among tree exudates, natural rubber latex is obtained from incisions in the bark of Hevea brasiliensis. Gum arabic (acacia) and gum tragacanth are solid exudates from Acacia leguminosae and Astragalus leguminosae respectively. These natural substances (except latex) are brittle when dry and support mould growth when wet. Adhesives are formulated from them by the additions of water, plasticizers, fungicides, tack, and wetting agents. Their adhesive action is due to the formation of physico-chemical bonds and penetration to give mechanized keying. Setting and hardening of this type of adhesive depends on loss of water; therefore they are mainly used for joining porous substrates such as paper, board, and wood. As a group these natural adhesives are less waterproof and strong, but cheaper than the synthetic-resin-based adhesives.

Natural non-water-based adhesives Bitumen, derived from asphalt; shellac, produced by parasitic tree insects; and resin, an exudate from pine trees, are examples of naturally occurring resins used both as hot melt adhesives and as spirit-based cements.

The marine bacterium Shewanell colwellii secretes a natural glue, PAVE (polysaccharide adhesive viscous exopolymer), in large quantities. Because PAVE can be used in wet conditions and is resistant to sea water, it was being developed commercially in 1994 as a sealant for ships' hulls.

In 2000, US researchers showed that the intriguing adhesion of a gecko's foot, which allows the animal to walk on vertical walls and on ceilings, is based purely on the van der Waals forces between the extremely thin hairs (known as setae) on the foot and the surface. As the interaction of each individual hair is very weak by itself, the gecko can peel off a foot very rapidly, even if it has appeared to be glued to the surface. By 2007, several laboratories have succeeded in producing synthetic materials that come close to geckolike adhesive.

Synthetic adhesives Many modern adhesives are based on the increasing number of synthetic resins available. Thanks to the chemists' closer control over these polymers, a wider variety of type and nomenclature of adhesives has evolved. Definitions, common uses, and the names of some resins used in these adhesives are listed below.

Solution adhesives are resins dissolved in a volatile organic solvent, used for bonding porous materials. They include natural and synthetic rubbers, nitrocellulose, polyvinyl acetate, and polymethyl methacrylate.

Emulsion adhesives are resins dispersed in an aqueous base, used for bonding porous materials. Natural and synthetic rubbers, polyvinyl acetate, and polymethyl methacrylate are used.

Contact adhesives are emulsions or solutions formulated to bond impervious materials. Both faces are covered with adhesive and allowed to dry before being brought into contact. They are used for bonding plastics, sticking rubber shoe soles, and self-seal envelopes.

Pressure-sensitive adhesives are used on tapes or sheet material, sometimes with a nonstick backing paper. Applied pressure forms a bond. Some adhesives are formulated for ease and cleanliness of removal (permanently tacky adhesive), and some to give a permanent bond. The materials used include modified natural and synthetic rubbers and polyisobutylene.

Thermoplastic adhesives are solventless adhesives, softened by the application of heat before bonding. They may be remelted after bonding and are used for high-speed packaging, labelling, and unsewn bookbinding. Polyamides, polyvinyl acetate, and its copolymers are used.

Thermosetting adhesives are solventless, and are cured by heat to form a bond that, once cured, cannot be resoftened by heat. They are used for exterior plywood and for bonding brake linings to shoes. They are made from phenolic and epoxy groups of resins.

Two-part or chemical-cure adhesives consist of resin and hardener, which are mixed together shortly before use and set by chemical action without the necessity for the application of heat. They are used for bonding aluminium alloys in the aircraft industry and varied domestic applications. Resins used include epoxypolyamide and resorcinol.

Structural adhesives are adhesives of high strength, toughness, and creep resistance, used for bonding load-bearing members. They are mainly confined to chemical-cure and thermosetting adhesives.