Adhesion
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adhesion
Adhesion
Adhesion
the attachment of surfaces of two unlike solids or liquids to each other. An example of adhesion is the attachment of water drops to glass. Adhesion and absorption have the same causes. In quantitative terms, adhesion is characterized by the specific work done in separating the adhering bodies. This work is calculated per unit area of the surfaces in contact, and it depends on the way in which the separation is carried out: by shear along the interface or by peeling off in a direction perpendicular to the surface. Adhesion is sometimes greater than cohesion, which characterizes the cohesive forces joining particles within a body. In that case the reparation of the two bodies results in the rupture of the weaker one.
Adhesion between solids with uneven surfaces is usually not great, since they are actually in contact only over isolated protruding areas of their surfaces. Adhesion of a liquid and a solid or of two immiscible liquids may attain maximum values because of the complete contact over the entire contact area. When a solid is coated by a liquid polymer, the polymer penetrates into recesses and pores in the solid. After the polymer has cured, a bonding sometimes known as mechanical adhesion takes place. In that case the cohesion in the cured polymer must be overcome in order to peel off the polymeric film. In order to achieve the maximum adhesion, solids are joined in a plastic or elastic state under pressure—for example, with rubber cement or in cold welding of metals. Firm adhesion is also achieved when a new solid phase forms on the interface—for example, in electroplating or in the case of surface-active chemical compounds (oxide films, sulfide films, and so forth).
Adhesion of polymers is favored when the macromolecules are polar molecules having a large number of chemically active functional groups. Active additives whose molecules make a firm bond with the film on one end and a firm bond with the substrate on the other end, thereby forming an oriented absorptive layer, are introduced into the composition of an adhesive or of a film-forming polymer in order to improve adhesion. Autoadhesion may occur when two volumes of the same polymer come into contact, when the fusion of macromolecules or portions of the polymer occur from one volume into the other. The strength of this bond increases with time, tending toward a limit known as cohesive strength. The phenomenon of adhesion occurs in welding, soldering, tin-plating, adhesive bonding, fabrication of photographic materials, and application of polymeric paints, coats, and varnishes to protect metal parts from corrosion. The reasons for failure of the adhesive joint in the last case are the stresses generated through shrinkage of the film and the difference between the thermal expansion coefficients of the film and the metal.
Adhesion is not only a precondition for the formation of a high-quality coating bonding a welded or adhesive joint; it is also responsible for the enhanced wear on rubbing parts. A layer of lubricant is introduced to hinder contact between the surfaces and thereby eliminate adhesion.
REFERENCES
Krotova, N. A. O skleivanii i prilipanii. Moscow, 1956.Voiutskii, S. S. Autogeziia i adgeziia vysokopolimerov. Moscow, 1960.
Deriagin, B. V., and N. A. Krotova. Adgeziia. Moscow-Leningrad, 1949.
V. I. SHIMULIS
Adhesion
a fibrous structure by which organs of the serous and synovial cavities adhere. Adhesions usually develop as a result of inflammatory processes. A body part, for example, a lung, the heart, or a joint, may become limited in its mobility and dysfunctional. Adhesions in the abdominal cavity can lead to the development of intestinal obstruction. They are often accompanied by pain. Adhesions are treated with physical therapy and sometimes surgery.