These are materials capable of fastening two other materials together by means of surface attachment. The words glue, mucilage, mastic, and cement are synonymous with adhesive. In a generic sense, the word adhesive implies any material capable of fastening by surface attachment, and thus will include inorganic materials such as portland cement and solders such as Wood’s metal. In a practical sense, however, adhesive implies the broad set of materials composed of organic compounds, mainly polymeric, that can be used to fasten two materials together. The materials being fastened together by the adhesive are the adherents, and an adhesive joint or adhesive bond is the resulting assembly. Adhesion is the physical attraction of the surface of one material for the surface of another.
From an industrial manufacturing standpoint, the advent of the stealth aircraft and all the structural adhesive bonding it entails has drawn widespread attention to the real capabilities of adhesives. Structural bonding uses adhe-sives to join load-bearing assemblies. Most often, the assemblies are also subject to severe service conditions. Such adhesives, regardless of chemistry, generally have the following properties:
• Tensile strengths in the 1500 to 4500 psi range
• Very high impact and peel strength
• Service temperature ranges of about -65 to 3500°F
If these types of working conditions are expected, then one should give special consideration to proper adhesive selection and durability testing.
Theories
The phenomenon of adhesion has been described by many theories. The most widely accepted and investigated is the wet ability-adsorption theory. Basically, this theory states that for maximum adhesion the adhesive must come into complete intimate contact with the surface of the adherend. That is, the adhesive must completely wet the adherend. This wetting is considered to be maximized when the inter-molecular forces are the same forces as are normally considered in intermolecular interactions such as the van der Waals, dipole-dipole, dipole-induced dipole, and electrostatic interactions. Of these, the van der Waals force is considered the most important. The formation of chemical bonds at the interface is not considered to be of primary importance for achieving maximum wetting, but in many cases it is considered important in achieving durable adhesive bonds.
If the situation is such that the adhesive completely wets the adhered, the strength of the adhesive joint depends on the design of the joint, the physical properties of the adherents, and, most importantly, the physical properties of the adhesive.
Parameters
Innumerable adhesives and adhesive formulations are available today. The selection of the proper type for a specific application can only be made after a complete evaluation of the design, the service requirements, production feasibility, and cost considerations. Usually such selection is best left up to adhesive suppliers. Once they have been given the complete details of the application they are in the best position to select both the type and specific adhesive formulation.
Types and Forms
Adhesives have been in use since ancient times and are even mentioned in the Bible. The first adhesives were of natural origin; for example, bitumen, fish oil, and tree resins. In more modern times, adhesives were still derived from natural products but were processed before use. These modern natural adhesives include animal-derived (such as blood, gelatin, and casein), vegetable-derived (such as soybean oil and wheat flour), and forest-derived (pine resins and cellulose derivatives) products.
Forms include liquid, paste, powder, and dry film. The commercial adhesives include pastes, glues, pyroxylin cements, rubber cements, latex cement, special cements of chlorinated rubber, synthetic rubbers, or synthetic resins, and the natural mucilages.
Characteristics
Adhesives are characterized by degree of tack (or stickiness), by strength of bond after setting or drying, by rapidity of bonding, and by durability. The strength of bond is inherent in the character of the adhesive itself, particularly in its ability to adhere intimately to the surface to be bonded. Adhesives prepared from organic products are in general subject to disintegration on exposure. The life of an adhesive usually depends on the stability of the ingredient that gives the holding power, although otherwise good cements of synthetic materials may disintegrate by the oxidation of fillers or materials used to increase tack. Plasticizers usually reduce adhesion. Some fillers such as mineral fibers or walnut-shell flour increase the thixot-ropy and the strength, while some such as starch increase the tack but also increase the tendency to disintegrate.
Classification
Adhesives can be grouped into five classifications based on chemical composition.
Natural
These include vegetable- and animal-based adhesives and natural gums. They are inexpensive, easy to apply, and have a long shelf life. They develop tack quickly, but provide only low-strength joints. Most are water soluble. They are supplied as liquids or as dry powders to be mixed with water.
Adhesives Classified by Chemical Composition
|
Natural |
Thermoplastic |
Thermosetting |
Elastomeric |
Alloysa |
|
|
Types within group |
Casein, blood albumin, hide, bone, fish, starch (plain and modified); rosin, shellac, asphalt; inorganic (sodium silicate, litharge-glycerin) |
Polyvinyl acetate, polyvinyl alcohol, acrylic, cellulose nitrate, asphalt, oleo-resin |
Phenolic, resorcinol, phenol-resorcinol, epoxy, epoxy-phenolic, urea, melamine, alkyd |
Natural rubber, reclaim rubber, butadiene-styrene (GR-S), neoprene, acrylonitrile-butadiene (Buna-N), silicone |
Phenolic-polyvinyl butyral, phenolic-polyvinyl formal, phenolic-neoprene rubber, phenolic-nitrile rubber, modified epoxy |
|
Most used form Common further classifications |
Liquid, powder By vehicle (water emulsion is most common but many types are solvent dispersions) |
Liquid, some dry film By vehicle (most are solvent dispersions or water emulsions) |
Liquid, but all forms common By cure requirements (heat and/or pressure most common but some are catalyst types) |
Liquid, some film By cure requirements (all are common); also by vehicle (most are solvent dispersions or water emulsions) |
Liquid, paste, film By cure requirements (usually heat and pressure except some epoxy types); by vehicle (most are solvent dispersions or 100% solids); and by type of adherents or end-service conditions |
|
Bond characteristics |
Wide range, but generally low strength; good resistance to heat, chemicals; generally poor moisture resistance |
Good to 150-200°F; poor creep strength; fair peel strength |
Good to 200-500°F; good creep strength; fair peel strength |
Good to 150-400°F; never melt completely; low strength; high flexibility |
Balanced combination of properties of other chemical groups depending on formulation; generally higher strength over wider temp range |
|
Major type of useb |
Household, general purpose, quick set, long shelf life |
Unstressed joints; designs with caps, overlaps, stiffeners |
Stressed joints at slightly elevated temp |
Unstressed joints on lightweight materials; joints in flexure |
Where highest and strictest end-service conditions must be met; sometimes regardless of cost, as military uses |
|
Materials most commonly bonded |
Wood (furniture), paper, cork, liners, packaging (food), textiles, some metals and plastics; industrial uses giving way to other groups |
Formulation range covers all materials, but emphasis on nonmetallics—esp wood, leather, cork, paper, etc. |
Epoxy-phenolics for structural uses of most materials; others mainly for wood; alkyds for laminations; most epoxies are modified (alloys) |
Few used "straight" for rubber, fabric, foil, paper, leather, plastics, films; also as tapes; most modified with synthetic resins |
Metals, ceramics, glass, thermosetting plastics; nature of adherends often not as vital as design or end-service conditions (i.e., high strength, temp) |
a "Alloy," as used here, refers to formulations containing resins from two or more different chemical groups. There are also formulations that benefit from compounding two resin types from the same chemical group (e.g., epoxy-phenolic).
b Although some uses of the "nonalloyed" adhesives absorb a large percentage of the quantity of adhesives sold, the uses are narrow in scope; from the standpoint of diversified applications, by far the most important use of any group is the forming of adhesive alloys.
Casein-latex type is an exception. It consists of combinations of casein with either natural or synthetic rubber latex. It is used to bond metal to wood for panel construction and to join laminated plastics and linoleum to wood and metal. Except for this type, most natural adhesives are used for bonding paper, cardboard, foil, and light wood.
Synthetic Polymer. The greatest growth in the development and use of organic compound-based adhesives came with the application of synthetically derived organic polymers. Broadly, these materials can be divided into two types: thermoplastics and thermosets. Thermoplastic adhesives become soft or liquid upon heating and are also soluble. Thermoset adhesives cure upon heating and then become solid and insoluble. Those adhesives that cure under ambient conditions by appropriate choice of chemistry are also considered thermosets.
An example of a thermoplastic adhesive is a hot-melt adhesive. A well-known hot-melt adhesive in use since the Middle Ages is sealing wax. Modern hot-melt adhesives are composed of polymers such as polyamides, polyesters, ethylene-vinyl acetate copolymers, and polyethylene. Modern hot melts are heavily compounded with wax and other materials. Another widely used thermoplastic adhesive is polyvinyl acetate, which is applied from an emulsion.
Thermoplastic Adhesives
They can be softened or melted by heating and hardened by cooling. They are based on thermoplastic resins (including asphalt and oleo-resin adhesives) dissolved in solvent or emulsified in water. Most of them become brittle at subzero temperatures and may not be used under stress at temperatures much above 150°F. As they are relatively soft materials, thermoplastic adhesives have poor creep strength. Although lower in strength than all but natural adhesives and suitable only for noncritical service, they are also lower in cost than most adhesives. They are also odorless and tasteless and can be made fungus resistant.
Pressure Sensitive. Pressure-sensitive adhesives are mostly thermoplastic in nature and exhibit an important property known as tack. That is, pressure-sensitive adhesives exhibit a measurable adhesive strength with only a mild applied pressure. Pressure-sensitive adhesives are derived from elastomeric materials, such as polybutadiene or polyisoprene.
Thermosetting Adhesives
Based on thermosetting resins, they soften with heat only long enough for the cure to initiate. Once cured, they become relatively infusible up to their decomposition temperature. Although most such adhesives do not decompose at temperatures below 500°F, some are useful only to about 150°F. Different chemical types have different curing requirements. Some are supplied as two-part adhesives and mixed before use at room temperature; some require heat or pressure to bond.
As a group, these adhesives provide stronger bonds than natural, thermoplastic, or elas-tomeric adhesives. Creep strength is good and peel strength is fair. Generally, bonds are brittle and have little resilience and low impact strength.
Elastomeric Adhesives
Based on natural and synthetic rubbers, elasto-meric adhesives are available as solvent dispersions, latexes, or water dispersions. They are primarily used as compounds that have been modified with resins to form some of the adhesive "alloys" discussed below. They are similar to thermoplastics in that they soften with heat, but never melt completely. They generally provide high flexibility and low strength, and without resin modifiers, are used to bond paper and similar materials.
Alloy Adhesives
This term refers to adhesives compounded from resins of two or more different chemical families, e.g., thermosetting and thermoplastic, or thermosetting and elastomeric. In such adhe-sives the performance benefits of two or more types of resins can be combined. For example, thermosetting resins are plasticized by a second resin resulting in improved toughness, flexibility, and impact resistance.
Structural Adhesives
Structural adhesives are, in general, of the alloy or thermosetting type and have the property of fastening adherends that are structural materials (such as metals and wood) for long periods of time even when the adhesive joint is under load. Phenolic-based structural adhesives were among the first structural adhesives to be developed and used.
The most widely used structural adhesives are based on epoxy resins. Epoxy resin structural adhesives will cure at ambient or elevated temperatures, depending on the type of curative. Urethanes, generated by isocyanate-diol reactions, are also used as structural adhesives. Acrylic monomers have also been utilized as structural adhesives. These acrylic adhesives use an ambient-temperature surface-activated free radical cure. A special type of acrylic adhesive, based on cyanoacrylates (so-called super-glue), is a structural adhesive that utilizes an anionic polymerization for its cure. Acrylic adhesives are known for their high strength and extremely rapid cure. Structural adhesives with resistance to high temperature (in excess of 390°F, or 200°C) for long times can be generated from ladder polymers such as polyimides and polyphenyl quinoxalines.
Three of the most commonly used adhesives are the modified epoxies, neoprene-phe-nolics, and vinyl formal-phenolics. Modified epoxy adhesives are thermosetting and may be of either the room-temperature-curing type, which cure by addition of a chemical activator, or the heat-curing type. They have high strength and resist temperature up to nearly 500°F (260°C).
A primary advantage of the epoxies is that they are 100% solids, and there is no problem of solvent evaporation after joining impervious surfaces. Other advantages include high shear strengths, rigidity, excellent self-filleting characteristics, and excellent wetting of metal and glass surfaces. Disadvantages include low peel strength, lack of flexibility, and inability to withstand high impact.
Neoprene-phenolic adhesives are alloys characterized by excellent peel strength, but lower shear strength than modified epoxies. They are moderately priced, offer good flexibility and vibration absorption, and have good adhesion to most metals and plastics.
Neoprene-phenolics are solvent types, but special two-part chemically curing types are sometimes used to obtain specific properties.
Vinyl formal-phenolic adhesives are alloys whose properties fall between those of modified epoxies and the thermoset-elastomer types. Vinyl formal-phenolics have good shear, peel, fatigue, and creep strengths and good resistance to heat, although they soften somewhat at elevated temperatures.
They are supplied as solvent dispersions in solution or in film form. In the film form the adhesive is coated on both sides of a reinforcing fabric. Sometimes it is prepared by mixing a liquid phenolic resin with vinyl formal powder just prior to use.
Other Adhesives/Cements
Paste adhesives are usually water solutions of starches or dextrins, sometimes mixed with gums, resins, or glue to add strength, and containing antioxidants. They are the cheapest of the adhesives, but deteriorate on exposure unless made with chemically altered starches. They are widely employed for the adhesion of paper and paperboard. Much of the so-called vegetable glue is tapioca paste. It is used for the cheaper plywoods, postage stamps, envelopes, and labeling. It has a quick tack, and is valued for pastes for automatic box-making machines. Latex pastes of the rub-off type are used for such purposes as photographic mounting, as they do not shrink the paper as do the starch pastes. Glues are usually water solutions of animal gelatin, and the only difference between animal glues and edible gelatin is in the degree of purity. Hide and bone glues are marketed as dry flake, but fish glue is liquid. Mucilages are light vegetable glues, generally from water-soluble gums.
Rubber cements for paper bonding are simple solutions of rubber in a chemical solvent. They are like the latex pastes in that the excess can be rubbed off the paper. Stronger rubber cements are usually compounded with resins, gums, or synthetics. An infinite variety of these cements is possible, and they are all waterproof with good initial bond, but they are subject to deterioration on exposure, as the rubber is uncured. This type of cement is also made from synthetic rubbers that are self-curing. Curing cements are rubber compounds to be cured by heat and pressure or by chemical curing agents. When cured, they are stronger, give better adhesion to metal surfaces, and have longer life. Latex cements are solvent solutions of rubber latex. They provide excellent tack and give strong bonds to paper, leather, and fabric, but they are subject to rapid disintegration unless cured.
In general, natural rubber has the highest cohesive strength of the rubbers, with rapid initial tack and high bond strength. It also is odorless. Neoprene has the highest cohesive strength of the synthetic rubbers, but it requires tackifi-ers. Graphite-sulfur rubber (styrene-butadiene) is high in specific adhesion for quick bonding, but has low strength. Reclaimed rubber may be used in cements, but it has low initial tack and needs tackifiers.
Pyroxylin cements may be merely solutions of nitrocellulose in chemical solvents, or they may be compounded with resins, or plasticized with gums or synthetics. They dry by the evaporation of the solvent and have little initial tack, but because of their ability to adhere to almost any type of surface they are called household cements. Cellulose acetate may also be used. These cements are used for bonding the soles of women’s shoes. The bonding strength is about 10 lb/in.2 (0.07 MPa), or equivalent to the adhesive strength of the outer fibers of the leather to be bonded. For hot-press lamination of wood the plastic cement is sometimes marketed in the form of thin sheet.
Epoxy resin cements give good adhesion to almost any material and are heat-resistant to about 400°F (204°C). An epoxy resin will give a steel-to-steel bond of 3100 lb/in.2 (22 MPa), and an aluminum-to-aluminum bond to 3800 lb/in.2 (26 MPa).
Some pressure-sensitive adhesives are mixtures of a phenolic resin and a nitrile rubber in a solvent, but adhesive tapes are made with a wide variety of rubber or resin compounds.
Furan cements, usually made with furfural-alcohol resins, are strong and highly resistant to chemicals. They are valued for bonding acid-resistant brick and tile.
Acrylic adhesives are solutions of rubber-based polymers in methacrylate monomers. They are two-component systems and have characteristics similar to those of epoxy and urethane adhesives. They bond rapidly at room temperature, and adhesion is not greatly affected by oily or poorly prepared surfaces. Other advantages are low shrinkage during cure, high peel and shear strength, excellent impact resistance, and good elevated temperature properties. They can be used to bond a great variety of materials, such as wood, glass, aluminum, brass, copper, steel, most plastics, and dissimilar metals.
Ultraviolet cure adhesives are anaerobic structural adhesives formulated specifically for glass bonding applications. The adhesive remains liquid after application until ultraviolet light triggers the curing mechanism.
A ceramic adhesive developed by the Air Force for bonding stainless steel to resist heat to 1500°F (816°C) is made with a porcelain enamel frit, iron oxide, and stainless steel powder. It is applied to both parts and fired at 1750°F (954°C), giving a shear strength of 1500 lb/in.2 (10 Mpa) in the bond. But ceramic cements that require firing are generally classed with ordinary adhesives. Wash-away adhesives are used for holding lenses, electronic crystal wafers, or other small parts for grinding and polishing operations. They are based on acrylic or other low-melting thermoplastic resins. They can be removed with a solvent or by heating.
Electrically conductive adhesives are made by adding metallic fillers, such as gold, silver, nickel, copper, or carbon powder. Most conductive adhesives are epoxy-based systems, because of their excellent adhesion to metallic and nonmetallic surfaces. Silicones and poly-imides are also frequently the base in adhesives used in bonding conductive gaskets to housings for electromagnetic and radio-frequency interference applications.
Properties
An important property for a structural adhesive is resistance to fracture (toughness). Thermoplastics, because they are not cured, can deform under load and exhibit resistance to fracture. As a class, thermosets are quite brittle, and ther-moset adhesives are modified by elastomers to increase their resistance to fracture.
Applications
Hot-melt adhesives are used for the manufacture of corrugated paper, in packaging, in carpeting, in bookbinding, and in shoe manufacture. Pressure-sensitive adhesives are most widely used in the form of coatings on tapes. These pressure-sensitive adhesive tapes have numerous applications, from electrical tape to surgical tape. Structural adhesives are applied in the form of liquids, pastes, or 100% adhesive films. Epoxy liquids and pastes are very widely used adhesive materials, having application in many assembly operations ranging from general industrial to automotive to aerospace vehicle construction. Solid-film structural adhe-sives are used widely in aircraft construction. Acrylic adhesives are used in thread-locking operations and in small-assembly operations such as electronics manufacture, which require rapid cure times. The largest-volume use of adhesives is in plywood and other timber products manufacture. Adhesives for wood bonding range from the natural products (such as blood or casein) to the very durable phenolic-based adhesives.
