Adhesive
Adhesive
Adhesives bond two or more materials at their surface, and may be classified as structural or non-structural. 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 were probably made from boiled-down animal products such as hides or bones. Organic (i.e., carbon-based) adhesives were later derived from plant products for use with paper. While many organic glues are effective in adhering furniture and other indoor products, they have not been effective in outdoor use where they are exposed to harsher environmental conditions.
Although inorganic adhesives (based on materials not containing carbon) such as sodium silicates (water glasses) are sold commercially, most adhesives in common use are made of organic materials and are synthetic. By far the most widely used adhesives are polymer based.
Synthetic adhesives can be made of amorphous thermoplastics; thermosetting monomers (as in the case of epoxy glues and cyanoacrylates); low-molecular-weight reactive species (such as urethane adhesives); or block copolymers, suspensions, and latexes.
Types of adhesive bonding
Adhesive bonding can 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 the two materials interlock. The adhesive may penetrate the substrate surface, often via 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 covalent bonding, actual primary chemical bonds form 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— the time that an adhesive can be stored after manufacture and still remain usable—and their working life— 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. 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 nonmetals, adhesive choices are more extensive. For structural bonding, for example, polyester-based adhesives may be used to bond plastic laminates to metal surfaces; low-density 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 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 provide better bonding alternatives for adhering these materials.
Solvent cements can be used to bond thermoplastics. They combine a solvent with a base material that is the same as the thermoplastic 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.
KEY TERMS
Composite— A mixture or mechanical combination (on a macroscopic level) of materials that are solid in their finished state, mutually insoluble, and have different chemistries.
Inorganic— Not containing 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— 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 returns to its original condition when cooled to ordinary temperatures.
Thermoset— A high-molecular-weight polymer that solidifies irreversibly when heated.
Bonding
wood Animal glues, available in liquid and powder form, are frequently used in wood bonding, but they 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. General-purpose rubber, acrylic, and epoxy adhesives also perform well with wood and other materials.
Fabric and paper bonding
General-purpose adhesives including rubber cements and epoxies can bond 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
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.
Roman A. Veselovsky (Veselovskii, R. A.), and Vladimir N. Kestelman. Adhesion of Polymers. 1st ed. New York: McGraw-Hill, 2001.
PERIODICALS
Crosby, Alfred J., Alamgir Karim, and Eric J. Amis. “Combinatorial Investigations of Polymer Adhesion.” Polymer Preprints, Division of Polymer Chemistry, American Chemical Society Publication, 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