Adhesives and glues are substances that are capable of bonding two solid materials together at their surfaces. While they have been used by humans for centuries, it is only in the twentieth century that synthetic adhesives were developed. Archeological evidence suggests that ancient Egyptians used resinous adhesives at least 6,000 years ago. Other adhesives including starch, sugar, casein, bitumen, shellac, pitch and glues made from animals and fish were used around 1500 B.C.. These first adhesives were of natural origin and are still used today in a form that is not substantially different.
The first synthetic adhesive was produced in 1869. This material was incorrectly termed nitrocellulose and was created by a reaction between nitric acid, sulfuric acid, and cellulose. Today, this product is known as cellulose nitrate. In 1912, Leo Baekeland produced phenol-formaldehyde resins, a basic material for many of today=s adhesives. High strength, elastomeric adhesives were available in 1928 when a reaction that produced polychloroprene was developed. Later in the 1930s, pressure sensitive tapes were developed.
The first metal bonding adhesive was developed by Nicholas de Bruyne in 1941. This material was used in the construction of aircraft. Later in the decade, epoxy resin adhesives were introduced. During the 1960s the extremely strong cyanoacrylate adhesives were developed. These products, called super glues, became adhesive when exposed to moisture in the air. Other adhesives that were developed during this time include silicones and anaerobic adhesives. Since that time, most of the advances in adhesive technology have been the result of formulation modifications using varied polymers. In the late 1980s Post-It™ notes were introduced using a microstructured adhesive.
A variety of theories about how adhesives work have been proposed. While none of these satisfactorily describes all aspects of adhesion, they do attempt to explain observed phenomena. The leading theories of adhesion include the diffusion, electrostatic, surface energetics, and mechanical theories.
The diffusion theory states that adhesion is a result of the solubility of the adhesive to the substrate. When the adhesive is applied to the substrate, it is suggested that a solution of the substrate and adhesive is formed if the solubility characteristics are equal. This creates a stable phase that bonds the two surfaces together. While this theory provides some insights, it is mostly applicable to situations in which polymers adhere to each other. It is not applicable to systems where the substrate and adhesive are radically dissimilar.
The electrostatic theory suggests that adhesion is the result of differences in the electronegativities of adhering materials. According to the theory, when two materials are brought into contact there is an amount of electron transfer that occurs. This results in a charge layer being formed which causes the materials to stay together. An example of this theory can be seen when using static electricity to make a balloon stick to a wall.
The surface energetics and wettability theory describes adhesion in terms of intermolecular and interatomic forces. For these forces to have an effect, the adhesive must come in close contact with the substrate. This is only achieved through Awetting@ of the surface. According to this theory adhesion is a result of bonding across the interface. An example of this theory is chemical adhesion that occurs when the chemical groups from the adhesive covalently bond with those of the substrate. Secondary adhesion occurs similarly through hydrogen bonding.
The mechanical theory describes adhesion in terms of a physical interlocking of the adhesive with the substrate. Mechanical adhesion occurs when the adhesive material flows into and on the microscopically rough substrate surface. This creates a lock and key effect similar to velcro. The viscosity of the adhesive and the contact time with the substrate are important parameters for mechanical adhesion. Viscosity adhesion occurs by restricted movement due to the viscous nature of the substrates.
For a material to be a good adhesive it must has a variety of characteristics. It should have a liquid surface tension that is lower than the wetting tension of the substrate. It should be applied to a surface that is significantly rough to improve adhesion. For polymeric substrates, the adhesive should be somewhat mutually soluble allowing diffusion between the two to occur.
Adhesives can be classified by the type of delivery of the adhesive or by the polymer used in the adhesive. In general, there are five categories of adhesives including structural, natural, pressure-sensitive, hot melt, and solvent-based adhesives.
Structural adhesives are some of the strongest adhesive materials available. They are based on resin systems, typically thermosets, and are meant to serve as permanent bonds. They are supplied as low-molecular-weight polymers that solidify when polymerized. They are sold in a variety of forms including two-part systems, pastes, and films. The most common examples include epoxy resins, acrylic adhesives, phenolic resins, elastomeric adhesives, high-temperature-resistant adhesives, and urethane adhesives.
Epoxy resins are typically based on the polymerization reaction of bisphenol A with epichlorohydrin. The product is sold as a two-part system in which the user applies the epoxy resin, then an amine hardener that causes the resin to cure. Epoxy resins are used in the construction of aircraft and automobiles. They are also a component of plastic cement.
Phenolic resins have been used as adhesives since the early 1900s. They are produced by the polymerization reaction of phenol and formaldehyde. To cure, heat is typically required to drive off excess solvent. This type of adhesive is used in the production of plywood. They also are noted as the most adhesive material to aluminum.
A variety of acrylic adhesives are available. Anaerobic adhesives are unique reactive adhesives. They consist of a mixture of hydroperoxides and dimethacrylates that polymerize in the absence of oxygen. This type of material is useful in anchoring screws and bolts. One of the most famous and strongest acrylic adhesives is Super Glue™, or Krazy Glue™. This adhesive is based on cyanoacrylates which spontaneously polymerizes in moist air.
Many structural adhesives that are used for aerospace purposes are lightweight and high-temperature resistant. These are typically resins that have high glass-transition temperatures. An example of this type of adhesive is polyimide that is formed by the polymerization reaction of an aromatic amine with an aromatic anhydride. Since many of the structural adhesives become brittle after they cure, adhesives formulators developed elastomeric adhesives. These materials contain compounds that reduce brittleness without significantly reducing adhesion. This property is useful in applications where vibrational forces can stress adhesives. The final class of structural adhesives is the urethanes. These materials are based on the polymerization reaction of a diol with a diisocyanate. They are primarily sold as two-part structural adhesives. An important application of these kinds of adhesives is in the automotive industry.Polyurethanes are used to bond polyester cords in rubber tires.
Many natural-based adhesives are available. In general, these adhesives are not as strong as the synthetic adhesives. Natural rubber has been used as an adhesive for over 100 years. In 1825, the Macintosh raincoat, which consisted of two layers of cotton bonded by a layer of natural rubber, was introduced.Protein adhesives are used as structural adhesives. They are based on polyamino acids obtained from various animal and plant sources. Curing is typically dependent on heat. Starch-based adhesives are derived from plants. They are primarily used for binding paper and as envelope adhesives. Cellulose is another natural material that provides adequate adhesion. Cellulose nitrate was one of the first modified adhesives produced. It is a general all purpose adhesive which is waterproof and flexible. Modified methylcellulose is used to make wallpaper paste. Other natural adhesives are takifying resins that are derived from coal, petroleum or wood tar.
Pressure sensitive adhesives are polymeric-based adhesives that melt at room temperature. When pressure is applied to the adhesive, they become flowable thereby covering the substrate. As the pressure is removed adhesion takes place. Many tapes use this kind of adhesive material. Pressure-sensitive tape uses a blend of glycerol and abietic acid esters with natural rubber on cellophane. In addition to pressure-sensitive adhesives, hot melt adhesives have been developed. Since thermoplastics melt when heated and reform when cooled, they make good adhesives. This is the principle behind glue guns.Nylon polymers are often used for their formulation.
Solvent-based adhesives work through the action of the substrate or adhesive. The material is put on the substrate and when the solvent evaporates bonding occurs. Adhesion is aided if the solvent interacts with the substrate. An example is model airplane glue that tends to dissolve some of the plastic. This helps to create a solid weld. Latex adhesives are another type of solvent-based adhesive. They are polymeric materials that provide adhesion after their aqueous solvent evaporates. These are the same types of polymers that are used for latex paints. Latex adhesives are used for bonding pile to carpets.
While natural resins dominated the adhesive market years ago, in 1999 they account for less than 10% of the market. Instead elastomers, thermoplastics, and thermosets have the bulk of market share in this three billion-dollar plus market.
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