Adhesives

An adhesive is a substance that sticks to the surface of an object such that two surfaces become bonded. A typical home improvement store carries many different adhesives for many different applications. Why are there so many adhesives? The answer is found in examining how an adhesive works and, in particular, what happens at the molecular level. The interaction of molecules is known as intermolecular bonding, or secondary bonding. Primary bonding, also known as intramolecular bonding, is the interaction of atoms within a molecule and includes covalent and polar covalent bonding. Secondary bonding includes dipole–dipole bonding (the interaction of molecules that have a permanent net dipole moment) and hydrogen bonding (an interaction that occurs when a hydrogen atom is bonded to an N, O, or F atom in a molecule).

Adhesives cure when the small resin molecules (mers) join together to form extremely large molecules known as polymers. For example, one of

the simplest polymers is polyethylene. The mer (basic building block of the polymer) is ethylene, H₂C=CH₂. The addition of an initiator (R·) causes the formation of the radical RCH₂CH₂·. A radical is a species that has an unpaired electron and is very reactive because it seeks the source of electrons. This radical will attach the ethylene mer (the double bond in ethylene is rich in electrons) to start a chain reaction that continues until very large polymer molecules form. This and other forms of polymerization processes are the basis for the formulation of polymers. This process is known as curing when dealing with adhesives.

Two criteria must be met in order for a molecule to possess a permanent net dipole moment: (1) an unequal sharing of electrons within the molecule such that one or more intramolecular bonds has a partial positive end and a partial negative end, and (2) a geometry such that the vector sum of the individual dipole moments does not equal zero. The ability of an atom within a molecule to attract electrons is known as electronegativity, a concept proposed by Linus Pauling who established a table of relative electronegativities. In Pauling's table, fluorine is the most electronegative element and is given the value of 4.0. The greater the difference in electronegativity between two atoms within a molecule, the larger is the dipole moment in that bond. Because the bond between two atoms having unequal electronegativities has a partial positive end and a partial negative end, it is said to be a polar bond. If the geometry of the molecule is such that the vector sum of all of the dipole moments does not equal zero, then the molecule is polar. The electronegativities for carbon and oxygen are 2.5 and 3.5, respectively; therefore, the carbon–oxygen bond is a polar bond. A carbon dioxide molecule has two carbon–oxygen bonds; however, its geometry is such that the vector sum of the two dipole moments equals zero, and thus carbon dioxide is a nonpolar molecule. The electronegativity of hydrogen is 2.1, thus a hydrogen–oxygen bond would be polar. A water molecule has two hydrogen–oxygen bonds. The geometry of a water molecule (the H–O–H bond angle is 104.5°) is nonsymmetrical, hence the vector sum of the dipole moments is not equal to zero and water is a polar molecule.

Polar molecules will attract other polar molecules because of their net dipole moments. Water molecules, however, have an additional attraction for one another, based on hydrogen bonding. This attraction is so strong that, although water is a small molecule and small molecules tend to be gases, water is a liquid at room temperature. This aspect of the chemistry of water demonstrates that hydrogen bonding is a relatively strong force that can hold molecules together.

So how does this relate to the many types of adhesive products that exist? In order for an adhesive to bond (hold together) two surfaces (substrates), there must be several types of interaction between the adhesive and both substrates. The first type of interaction is that the adhesive must wet the substrate, meaning that the adhesive must spread itself out into a film that covers the substrate surface. In order for this to happen, the adhesive must have a low enough viscosity so that it will flow. Viscosity is the resistance of a liquid to flow. Water has a low viscosity whereas honey has a high viscosity. Because viscosity is temperature dependent, the application of a cold adhesive to a substrate, or the application of an adhesive to a cold substrate, may result in poor wetting. Another factor that affects wetting is the relative strengths of cohesive forces (between like molecules, such as two adhesive molecules) and those of adhesive forces (between unlike molecules, such as an adhesive molecule and a substrate molecule). If the cohesive forces among adhesive molecules are weaker than the adhesive forces between the adhesive molecules and the substrate surface, then the adhesive molecules will spread out over the substrate and wet its surface. An adhesive that has a relatively low viscosity and is able to wet the substrate surface will flow into any tiny cracks or pores on the substrate surface, thus promoting what is known as mechanical bonding. Mechanical bonding increases the strength of an adhesive bond and, as a result, a forced separation of the two substrate surfaces is more apt to tear the substrate surfaces.

Mechanical bonding is one of several ways that an adhesive bonds substrates. All surfaces, except those that are highly polished, have pores. If the adhesive flows into these pores and then polymerizes, a mechanical bond is formed. It is similar to placing a wick into liquid candle wax. Once the wax solidifies the wick can not be easily removed. A mechanical bond has formed.

Because the interactions of adhesive molecules with substrates are so critical, it makes sense that some adhesives would be more appropriate for a specific substrate than others. Adhesives are designed for specific applications. For example, adhesives known as "super glues" (cyanoacrylates) are useful around the home in the bonding of common substrates (e.g., dishes, toys, etc.), which can take place in a matter of seconds. Yet their usefulness is limited when bonding wood because the cure time (the time it takes for an adhesive to undergo polymerization and become capable of holding the two substrates together) in this instance is much longer. Cyanocrylates tend to be brittle thus they are vulnerable to impact and dramatic changes in temperature. To reduce these shortcomings, small amounts of finely ground rubber has been used as filler. The rubber introduces flexibility thus reducing brittleness. In addition, cyanocrylates are attacked by polar solvents. Polar solvents will weaken cured cyanocrylate bonds over time. Therefore, applications involving water, alcohols, or other polar solvents should be avoided. Polymerization is the reaction of small molecules combining to form very

large molecules. The reason for the longer cure time in the bonding of wood has to do with chemical reactions (between adhesive and substrate) that affect the rate at which the cyanoacrylate molecules polymerize. A typical cyanoacrylate product is a low viscosity liquid that readily flows over a substrate's surface. When the cyanoacrylate molecules encounter a basic environment, the small molecules within the adhesive polymerize. Many common surfaces tend to be basic, so a cyanoacrylate adhesive works fine in these instances; wood, however, tends to have an acidic surface, and because an acidic environment will inhibit polymerization, cure time is increased.

Cyanoacrylates are not appropriate for the bonding of the steel parts of an automobile, because of the environments that the car will be exposed to. Those environments include such things as rain, variations in temperature, exposure to solvents (such as gasoline, oil, and windshield washer solution), ozone, acid rain , salt spray, and ultraviolet light from the Sun. (A more appropriate adhesive for car parts would be an epoxy-based adhesive.) Another example of a "special" adhesive would be the one used to attach a new rearview mirror in an automobile. Because the cured adhesive in this case will be exposed to wide variations in temperature and to an extremely large amount of ultraviolet light from the Sun for prolonged periods of time, an adhesive formulated specifically for these conditions should be used.

Finally, the strength and permanence of the bond formed between adhesive and substrate must be considered when one is selecting an adhesive. Most of the time it is desirable to have maximum strength and permanence; the very common Post-it note, however, is a counterexample. Its adhesive is neither strong nor permanent. The adhesive formulation that eventually led to the Post-it was initially considered a failure because of the adhesive's weakness. It was while singing in his church choir that Arthur Fry of the Minnesota Mining & Manufacturing Company (3M) envisioned a potential use for what seemed to be a useless material.

Table 1 lists some common types of adhesives and their uses. Because of the different possible substrates and combinations of substrates, and because adhesives are subject to such a range of environmental conditions, it is no wonder that there are so many types of adhesives on the market. However, if one has some knowledge of how adhesives bond to substrates and the types of substrates being bonded, the task of selecting adhesives will not be overwhelming.

see also Bonding; Pauling, Linus.

Daryl J. Doyle

Bibliography

Doyle, Daryl J. (1989). "A Review of Ultraviolet (UV) Radiation as an Adhesive Curing Agent." Society of Manufacturing Engineers Technical Paper AD89–534, Adhesives '89, September 12–14, 1989, Atlanta, Georgia.

Doyle, Daryl J. (1990). "Criteria for Proper Adhesive Selection: From Application to Viscosity." Society of Manufacturing Engineers Technical Paper AD90–450, Adhesives '90, October 1–4, 1990, Schaumburg, Illinois.

Doyle, Daryl J. (1990). "Viscosity and Its Importance to Adhesive Dispensing." Society of Manufacturing Engineers Technical Paper AD90–710, Adhesive Technology for Automotive Engineering Applications, November 1–2, 1990, Dearborn, Michigan.

Fry, Arthur L. (1989). "The Choir Singer's Bookmark." Guideposts.

Plummer, Christine (1993). "The Story of Post-it™ Notes." Chem Matters.

Adhesive

Types of adhesive bonding

Bonding applications

Bonding metals

Bonding plastics

Bonding wood

Fabric and paper bonding

Resources

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

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

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.

Adhesives

Adhesives bond two or more materials at their surfaces. Any time you reattach a handle that has broken off a cup, paste a photograph into an album, or tape a message to a friend's door, you use an adhesive.

Natural adhesives such as beeswax, resin (tree sap), and bitumen (asphalt) have been used since earliest times. Ancient Egyptians used flour paste in the making of papyrus (reed paper) and glue made from animal skin and bones for woodworking. Monks of the Middle Ages (400–1450) used egg white to glue gold leaf to their illuminated (decorated) manuscripts.

Flour paste, animal glue, and egg white are examples of natural adhesives. Natural adhesives are still widely used but have been replaced for most applications by synthetic adhesives. Synthetic adhesives are compounds invented by chemists with special properties that make them useful for various kinds of bonding. For example, two metals can be bonded to each other using an epoxy resin, a type of plastic. Or metal can be bonded to wood using a rubber like material known as neoprene.

New adhesives and new applications

Researchers are constantly looking for new kinds of adhesives and new ways to use them. For example, engineers at European Airbus, one of the world's largest manufacturers of airplanes, has found that plasticlike adhesives can be used to hold airplane sections together. The adhesives are cheaper and easier to use than traditional welds and rivets.

A better adhesive is not always a stronger adhesive. The material used on Post-it™ notes is an example. This adhesive is just strong enough

to hold a Post-it™ note to another piece of paper, wood, glass, or plastic. But it comes loose easily and can be reattached many times. Post-it™ notes were created in 1974 by Arthur Fry, a chemist at the Minnesota Mining and Manufacturing Company (3M). Fry made use of a "failed" discovery by a colleague who was trying to make a very strong adhesive, instead producing a very weak one.

One of the most widely used forms of adhesives is adhesive tape. Adhesive tape consists of an adhesive that has been attached to paper, cloth, rubber, plastic, or some other material. The familiar Scotch tape™ is perhaps the best known of all adhesive tapes. Scotch tape™ was invented in the 1920s by Richard Drew, another chemist at the 3M company. Drew found a way to coat Du Pont's newly invented cellophane tape with a thin layer of adhesive to make it the all-purpose aid present in most households today. Variations of the original Scotch tape™ in use today are carpet tape, masking tape, duct tape, and sealing tape.

ad·he·sion / adˈhēzhən/ • n. 1. the action or process of adhering to a surface or object: the adhesion of the Scotch tape to the paper. ∎  the frictional grip of wheels, shoes, etc., on a road or other surface: the front tires were struggling for adhesion. ∎  Physics the sticking together of particles of different substances. ∎  allegiance or faithfulness to a particular person, party, or set of ideas: his adhesion to the old bureaucracy.2. Med. an abnormal union of membranous surfaces due to inflammation or injury.