ADHESIVE

substance used to join separate materials through surface attachment. Egyptians used glue in woodworking 3,300 years ago and also used flour paste in the production of papyrus. Several other naturally occurring organic materials, including beeswax and vegetable resins, were important as adhesives in ancient and medieval industries in Europe, and the development of glues and sealants (protective coatings) derived from animal tissue greatly improved the efficiency of wood and paper manufacture. During the 19th century the introduction of rubber-based products was an especially significant development. It has been during the 20th century, however, that the most revolutionary changes in the industry have occurred, owing primarily to the invention of synthetic resins and compounds. Modern research indicates that adhesion occurs on a minute and intimate molecular level. This explanation, called the adsorption theory, holds that adhesion depends on the same molecular attraction as that which binds all solid matter; any two materials can thus theoretically adhere if they are placed in close enough molecular proximity. Other factors, however, such as tensile strength, evaporative properties, and flexibility, determine a material's practicality as an adhesive. Most adhesives, both natural and synthetic, are based on polymers. Polymers consist of giant, complex molecules formed by the union of many simpler molecules, called monomers. Polymeric substances offer a great degree of strength and flexibility, two necessary attributes of any sophisticated adhesive. The most important types of polymers employed in modern adhesives are thermoplastic resins, thermosetting resins, and elastomers. The thermoplastics provide strong, durable adhesion at normal temperature but can be softened by heating for manipulation and reconfiguration without undergoing chemical deterioration. Thermosetting resins form permanent bonds that cannot be heated to the softening point without undergoing a chemical change. They are extremely useful in industry, especially aircraft and aerospace technology. Elastomers are rubbery polymers that provide important properties of flexibility and tack (stickiness). In various combinations, these polymers are used to make adhesives of many types, including contact cements, usually applied to both bonding surfaces; hot-melt adhesives, applied exclusively in a molten state; pressure-sensitive adhesives, carried on sheets or tapes; and remoistenable adhesives, in which a solution is applied to a backing, allowed to dry, and then reactivated by water. Natural adhesives are made from both animal and vegetable materials. Animal glues are generally a preparation of collagen, the primary component protein of mammalian bone and skin. These substances are still used in the production of wood and paper materials. Other important mammalian glues are refined from casein, a milk protein, and from blood albumin. Vegetable adhesives are those adhesives extracted from plants that also are soluble or dispersible in water. Starches, obtained from a number of grains and vegetables, form the base material for many such adhesives. Another group, the natural gums, include agar, a colloid (minute particle suspension) of marine plants; algin, a derivative of seaweed; and gum arabic, extracted from the acacia tree. Although natural adhesives are less expensive to produce, most important modern adhesives are synthetic. Based on complex polymeric compounds, synthetic adhesives are strong, heat-resistant, and durable. They are easily modified to suit specific functions and can be manufactured in a constant and uniform manner. any substance that is capable of holding materials together in a functional manner by surface attachment that resists separation. "Adhesive" as a general term includes cement, mucilage, glue, and paste-terms that are often used interchangeably for any organic material that forms an adhesive bond. Inorganic substances such as portland cement also can be considered adhesives, in the sense that they hold objects such as bricks and beams together through surface attachment, but this article is limited to a discussion of organic adhesives, both natural and synthetic. Natural adhesives have been known since antiquity. Egyptian carvings dating back 3,300 years depict the gluing of a thin piece of veneer to what appears to be a plank of sycamore. Papyrus, an early nonwoven fabric, contained fibres of reedlike plants bonded together with flour paste. Bitumen, tree pitches, and beeswax were used as sealants (protective coatings) and adhesives in ancient and medieval times. The gold leaf of illuminated manuscripts was bonded to paper by egg white, and wooden objects were bonded with glues from fish, horn, and cheese. The technology of animal and fish glues advanced during the 18th century, and in the 19th century rubber- and nitrocellulose-based cements were introduced. Decisive advances in adhesives technology, however, awaited the 20th century, during which time natural adhesives were improved and many synthetics came out of the laboratory to replace natural adhesives in the marketplace. The rapid growth of the aircraft and aerospace industries during the second half of the 20th century had a profound impact on adhesives technology. The demand for adhesives that had a high degree of structural strength and were resistant to both fatigue and severe environmental conditions led to the development of high-performance materials, which eventually found their way into many industrial and domestic applications. This article begins with a brief explanation of the principles of adhesion and then proceeds to a review of the major classes of natural and synthetic adhesives. Additional reading Jacqueline I. Kroschwitz (ed.), Encyclopedia of Polymer Science and Engineering, 2nd ed., 17 vol. (1985-90), is the most comprehensive source of information on polymer science and includes articles on the major topics treated in this article; it is also available in a condensed, 1-vol. edition, Concise Encyclopedia of Polymer Science and Engineering (1990). Two additional reference works are Geoffrey Allen and John C. Bevington (eds.), Comprehensive Polymer Science: The Synthesis, Characterization, Reactions & Applications of Polymers, 7 vol. (1989); and Joseph C. Salamone (ed.), Polymeric Materials Encyclopedia, 12 vol. (1996). Books on polymer science for the nonscientific reader are Hans-Georg Elias, Mega Molecules (1987; originally published in German, 1985); and Raymond B. Seymour and Charles E. Carraher, Giant Molecules: Essential Materials for Everyday Living and Problem Solving (1990).Hal F. Brinson and Cyril A. Dostal (eds.), Adhesives and Sealants (1990), describes important adhesive systems and technologies and discusses application areas. S.R. Hartshorn (ed.), Structural Adhesives: Chemistry and Technology (1986), provides detailed descriptions of thermoset adhesives and application areas. A.J. Kinloch, Adhesion and Adhesives: Science and Technology (1987), treats the technology and testing of adhesives. Irving Skeist (ed.), Handbook of Adhesives, 3rd ed. (1990), describes both natural and synthetic adhesives and includes formulations. Lieng-Huang Lee (ed.), Adhesive Bonding (1991), thoroughly addresses fundamental aspects of wetting, spreading, and the mechanics of adhesion science. Roscoe A. Pike