Adhesive

Adhesives bond two or more materials at their surface, and may be classified as structural or nonstructural. Structural adhesives can support heavy loads, while nonstructural adhesives cannot. Most adhesives exist in liquid, paste, or granular form, although film and fabric-backed tape varieties are also commercially available.

Adhesives have been used since ancient times. The first adhesives were probably made from boiled-down animal products such as hides or bones. Organic, i.e., carbon-based, adhesives have also been derived from plant products for use with paper products. While many of these organic glues have proven effective in the adhesion of furniture and other indoor products, they have not been effective in outdoor use where they are exposed to harsher environmental conditions.

Although inorganic adhesives, which are based on materials not containing carbon , such as the sodium silicates (water glasses) for bonding paper board, are sold commercially, most adhesives in common use are made of synthetic, organic materials. By far, the most widely used adhesives today are synthetic, polymer-based adhesives.

Synthetic adhesives may be made of amorphous thermoplastics above their glass transition temperatures; thermosetting monomers as in the case of epoxy glues and cyanoacrylates; low molecular weight reactive species as in the case of urethane adhesives; or block copolymers, suspensions, or latexes.

Types of adhesive bonding

Adhesive bonding may originate in a variety of ways. It may be the result of mechanical interlocking of the adhesive with the bonded surface, covalent bonding between bonded surfaces, or secondary electronic interactions between the bonded materials.

In mechanical adhesion, the adhesive flows around the substrate surface roughness so that interlocking of the two materials takes place. The adhesive may penetrate the substrate surface. Surface interpenetration often involves polymer diffusion ; this type of bonding depends on the ability of the polymer adhesive to diffuse into the bonded surface.

Secondary electronic bonding may result from hydrogen bonds between the adhesive and substrate, from the interactions of overlapping polymer chains, or from such nonspecific forces as Van der Waals interactions.

In the case of covalent bonding, actual primary chemical bonds are formed between the bonded materials. For example, graft or block copolymers may bond different phases of a multicomponent polymeric material together.

Bonding applications

Adhesives are characterized by their shelf life, which is defined as the time that an adhesive can be stored after manufacture and still remain usable, and by their working life, defined as the time between mixing or making the adhesive and when the adhesive is no longer usable. The best choice of adhesive depends on the materials to be bonded.

Bonding metals

Epoxy resin adhesives perform well in the structural bonding of metal parts to each other. Nonstructural adhesives such as polysulfides, neoprene, or rubber-based adhesives are also available for bonding metal foils. Ethylene cellulose cements are used for filling recesses in metal surfaces.

When bonding metals to non-metals, the choices of adhesives are more extensive. In the case of structural bonding, for example, polyester-based adhesives may be used to bond plastic laminates to metal surfaces; lowdensity epoxy adhesives may be used to adhere light plastics such as polyurethane foam to various metals; and liquid adhesives made of neoprene and synthetic resins may be used to bond metals to wood. General purpose rubber, cellulose, and vinyl adhesives may be used to nonstructurally bond metals to other materials such as glass and leather.

Bonding plastics

Thermoplastic materials including nylon, polyethylene, acetal, polycarbonate, polyvinyl chloride, cellulose nitrate, and cellulose acetate are easily dissolved by solvents and softened by heat . These limitations restrict the use of adhesives with such materials, and solvent or heat welding may prove better bonding alternatives for adhering these materials.

Solvent cements can frequently be used to bond thermoplastics together. These cements combine a solvent with a base material that is the same as the thermo-plastic to be adhered. In view of environmental considerations, however, many adhesives manufacturers are now reformulating their solvent-based adhesives. General purpose adhesives such as cellulosics, vinyls, rubber cements, and epoxies have also been used successfully with thermoplastics.

Thermosetting plastics, including phenolics, epoxies, and alkyds, are easily bonded with epoxy-based adhesives, neoprene, nitrile rubber, and polyester-based cements. These adhesives have been used to bond both thermosets and thermoplastics to other materials, including ceramics , fabric, wood, and metal.

Bonding wood

Animal glues, available in liquid and powder form, are frequently used in wood bonding. But animal glues are very sensitive to variations in temperature and moisture. Casein-type adhesives offer moderate resistance to moisture and high temperature, as do urea resin adhesives, which can be used to bond wood to wood, or wood to plastic.

Vinyl-acetate emulsions are excellent for bonding wood to materials that are especially porous, such as metal and some plastic laminates, but these adhesives also tend to be sensitive to temperature and moisture. Rubber, acrylic, and epoxy general-purpose adhesives also perform well with wood and other materials.

Fabric and paper bonding

General purpose adhesives including rubber cements and epoxies are capable of bonding fabrics together, as well as fabrics to other materials. When coated fabrics must be joined, the base adhesive material must be the same as the fabric coating. Rubber cements, gum mucilages, wheat pastes, and wood rosin adhesives can be used to join paper or fabric assemblies.

Resources

books

Green, Robert E. Machinery's Handbook. 24th ed. New York: Industrial Press, 1992.

Petrie, Edward M. Handbook of Adhesives & Sealants. New York: McGraw-Hill, 1999.

Pocius, A.V. Adhesion and Adhesives Technology. Cincinnati, OH: Hanser Gardner Publications, 2002.

Sperling, L. H. Introduction to Physical Polymer Science. New York: John Wiley & Sons, 1992.

Veselovskii, R.A., Vladimir N. Kestelman, Roman A. Veselovsky. Adhesion of Polymers. 1st ed. New York: Mc-Graw-Hill, 2001.

Wu, S. Polymer Interfaces and Adhesion. New York: Marcel Dekker, 1982.

periodicals

Amis, E.J. "Combinatorial Investigations of Polymer Adhesion." Polymer Preprints, American Chemical Society, Division 42, no. 2 (2001): 645-646.

McCafferty, E. "Acid-base Effects in Polymer Adhesion at Metal Surfaces." Journal of Adhesion Science and Technology 16, no. 3 (2002): 239-256.

Randall Frost

KEY TERMS

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Composite

—A mixture or mechanical combination (on a macroscopic level) of materials that are solid in their finished state, that are mutually insoluble, and that have different chemistries.

Inorganic

—Not containing compounds of carbon.

Monomer

—A substance composed of molecules that are capable of reacting together to form a polymer.

Organic

—Containing carbon atoms, when used in the conventional chemical sense.

Polymer

—A substance, usually organic, composed of very large molecular chains that consist of recurring structural units.

Synthetic

—Referring to a substance that either reproduces a natural product or that produces a unique material not found in nature, and that is produced by means of chemical reactions.

Thermoplastic

—A high molecular weight polymer that softens when heated and that returns to its original condition when cooled to ordinary temperatures.

Thermoset

—A high molecular weight polymer that solidifies irreversibly when heated.