Adhesives are materials used for joining objects. They include binders, cements, glues, mucilages, gummed tapes, and pressure-sensitive tapes. The process of joining consists of applying the adhesive to the component materials, assembling the joint, and then allowing the adhesive to harden under pressure. Adhesive bonding differs from other joining techniques because no structural contrivances, such as bolts, rivets, screws, or staples, are used to hold the components together.
Adhesives hold components, commonly called adherends or substrates, by both mechanical and chemical adhesion. Mechanical adhesion is the portion of the adhesive strength given by mechanical interlocking of the adhesive in the pores of the adherend. Chemical adhesion is the remaining portion of the adhesive strength. It is given by a chemical reaction between the adhesive and the adherend or by intermolecular electrical forces that cause mutual attraction between the adherend and the adhesive.
Types of Adhesives
Adhesives are compounded from one of two basic types of materials: thermoplastic and thermosetting. Thermoplastic materials soften under heat and are soluble in selected solvents. Natural adhesives are predominantly thermoplastic. Thermosetting materials harden (cure) under the action of heat or catalysts. Once hardened, they do not melt, and they are insoluble. Thermoplastic and thermosetting adhesives are supplied as air-drying material, fusible material, pressure-sensitive tape, and chemically reactive material.
Air-drying adhesives usually are thermoplastics. They are converted to the solid state by evaporation of the solvent. They bond by mechanical adhesion and mutual attraction. The adhesive is applied to the cleaned substrates and, if necessary, is baked or air dried for a short time to remove residual solvent before assembling the joint. After assembly, pressure is applied on the components.
Fusible adhesives are mainly thermoplastics, such as animal glue or sulfur. They are converted to the solid state by cooling previously melted adhesive material. They bond by mechanical adhesion and mutual attraction. The adhesive is melted and applied to the cleaned adherends. Then the joint is assembled, and the adhesive is allowed to solidify under pressure.
Pressure-sensitive adhesives are predominantly thermoplastics. They are supplied in tape form on a paper, plastic, or similar backing. Some tapes require no moisture, heat, or other treatment before or after application. Other tapes are activated with water or solvent before application. Pressure-sensitive tapes bond by mechanical adhesion and mutual attraction.
Chemically reactive adhesives are predominantly thermosetting materials. They are converted to the solid state by catalysts or heat. They bond primarily by chemical reaction and secondarily by mechanical adhesion and mutual attraction. The adhesive is activated and applied to the cleaned substrates. Then the joint is assembled and cured under pressure at room or high temperature.
Uses for Adhesives
Adhesives have been used for thousands of years for joining paper and, to a lesser extent, wood. Hydraulic-setting cements and natural asphalt have been used as structural adhesives since ancient times. It was not until the late 1930's, when synthetic resins were introduced, that adhesives were developed for joining a great variety of materials. These synthetic resins, unlike natural counterparts, such as animal glue or starch, have high strength and give long service. Adhesives now have many applications. They are used in dentistry, medicine, electrical applications, metal joining, optics, and packaging and bonding paper. Adhesives also are used to bond plastics, rubber, and wood.
Thermoplastic plastics, because of their sensitivity to heat and solvents, can be joined by heat-welding or solvent-welding techniques as well as by conventional adhesive-bonding techniques. Thermosetting plastics can be joined only by adhesive bonding.
Adhesive bonding of thermoplastics is best achieved with adhesives that have a composition similar to that of the adherend. Thus, cellulose nitrate is bonded with cellulose nitrate base adhesives, acrylics with acrylic-base cements, and so on. Thermoset plastics can be bonded with adhesives that do not have a composition similar to that of the adherend. Phenolic, resorcinol, melamine, furane, epoxy, synthetic rubber, and isocyanate base adhesives are used.
Rubber materials generally are bonded with rubber-base adhesives in order to retain elasticity. The best bonds are achieved with adhesives formulated from the same rubber as the adherend. Available adhesives include those based on natural, butyl, polysulfide, urethane, neoprene, silicone, and nitrile rubbers.
Wood can be bonded with interior, semidurable, and weatherproof adhesives. Interior adhesives are used for assemblies subject to normal indoor temperature conditions where relative humidities are not high and do not fluctuate widely. Animal glue and vinyl are suitable for these applications.
Semidurable adhesives are used for assemblies subject to severe conditions for limited periods. These bonding agents deteriorate slowly and ultimately fail completely. Urea and casein are suitable for these conditions.
Weatherproof adhesives are used for marine service and other applications where there are severe conditions for protracted periods. These adhesives are more durable than wood and are virtually indestructible by weather, mold, and heat. Phenolic, resorcinol, melamine, furane, and epoxy adhesives are commonly used.