Adhesives

An adhesive is a material that, by means of surface attachment, can hold together solid materials. Adhesives have been used for most of recorded history. They are mentioned in Egyptian hieroglyphics, in the Bible, and in the writings of the early natural philosophers. The physical strength of an assembly made by the use of adhesives, known as an adhesive joint, is due partly to the forces of adhesion, but primarily to the cohesive strength of the polymeric materials used to formulate the adhesive. Thus, the range of strengths available in adhesive joints is limited to the strengths of the polymers useful in the formulation of adhesives. Indeed, the technology of adhesives tracks well with the technology of polymers. As new polymers were synthesized, new adhesives were developed that used those polymers.

Adhesives are typically classified by their use or application. Thus structural adhesives are those materials used to join engineering materials such as metals, wood, and composites. Usually, it is expected that an adhesive joint made with a structural adhesive is capable of sustaining a stress load of 1,000 psi (6.9 MPa) for extended periods of time. Hot melt adhesives are those adhesives that are applied from the melt and whose properties are attained when the adhesive solidifies. Pressure-sensitive adhesives provide adherence and strength with only finger pressure during application. Adhesive tapes are manufactured by applying a pressure-sensitive adhesive to a backing. Rubber-based adhesives are, as the name implies, based on elastomers and are usually applied as a mastic or spray applied from solvent or water. Pressure-sensitive adhesives can be considered to be a subset of rubber-based adhesives.

The ease of application of pressure-sensitive adhesives is superior to all other types of adhesives except possibly hot melt adhesives. Responsivity to finger pressure alone forming a bond is a desirable property, and pressure-sensitive adhesives of sufficient strength to perform structural tasks have been developed recently. One of the major uses of these double-coated foam tapes is to fasten most of the exterior and interior decorative and semistructural materials to the body of an automobile. The use of these foam tapes allows faster assembly and eliminates mechanical fasteners, which are a source of corrosion.

Each of the major classes of adhesives described above can be further classified by its chemistry. Thus, the majority of structural adhesives are based on one or more of the following chemistries: phenolic, epoxy, acrylic, bismaleimide, imide, and protein (derived from blood, soybean, casein, and so on). The majority of hot melt adhesives are based on one or more of the following chemistries: waxy hydrocarbons, polyethylene, polypropylene, ethylene-vinyl acetate, polyamides, and polyesters. Rubber-based adhesives are, for the most part, formulated using neoprene, nitrile, and natural rubbers. Pressure-sensitive adhesives are based on natural rubber, vinyl ethers, acrylics, silicones, and isoprene-styrene block co-polymers. Many paper-binding adhesives are based on dextrin or other



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