Adhesives offer many advantages over binding techniques such as sewing, mechanical fastening, thermal bonding, etc. These include the ability to bind different materials together, to distribute stress more efficiently across the joint, the cost effectiveness of an easily mechanized process, an improvement in aesthetic design, and increased design flexibility. Disadvantages of adhesive use include decreased stability at high temperatures, relative weakness in bonding large objects with a small bonding surface area, and greater difficulty in separating objects during testing. Adhesives are typically organized by the method of adhesion. These are then organized into reactive and non-reactive adhesives, which refers to whether the adhesive chemically reacts in order to harden. Alternatively they can be organized by whether the raw stock is of natural or synthetic origin, or by their starting physical phase.
Adhesives are often used as an alternative to mechanical fasteners. When using adhesives, however, the entire joint must be given even more consideration than when using mechanical fasteners. Unlike a bolt or rivet, an adhesive's properties may change depending on where it is used.
Functions: The main, though not the only, function of adhesives is to fasten parts together. Adhesives transmit stresses from one member of a joint to another with a more uniform distribution than conventional mechanical fasteners give. Consequently, adhesives often allow structures that are mechanically equivalent to, or stronger than, conventional assemblies to be built at lower cost and weight. For example, epoxy adhesives may reach tensile adhesion values of 3,000 psi, comparable to a …-in.-diameter spot weld every square inch in low-carbon steel.
Adhesively joined structures and products are inherently smooth. Exposed surfaces are not defaced, and contours are not disturbed as with other types of fastening systems. This is important both to function and appearance.
Light-gauge materials are often good candidates for adhesive bonding, because the uniform stress distribution permits full use of the strength and rigidity of the adherends, without the distortion induced by other fastening methods.
Many adhesives easily join dissimilar materials if proper surface treatments are used. Metal adherends that would ordinarily corrode because of their electromotive series relationship can be protected from corrosion by a layer of nonconductive adhesive that both joins and isolates them.
Adhesives also have an advantage where temperature variations are expected in the service of a product containing dissimilar materials. A flexible adhesive of adequate thickness can accommodate differences in thermal expansion among the adherends and prevent damage that might occur if a stiff fastening system were used.
Thermoset adhesives, such as epoxies and anaerobics, can be formulated to retain much of their strength over a broad temperature range, up to 300°F. Selecting a curing temperature close to the operating temperature can reduce the effect of thermal-expansion differences between materials.
An adhesive's continuous bond also seals against liquids or gases, provided they do not attack the adhesive. Some adhesives are used in place of solid or cellular gaskets.
Some adhesive formulations can impart mechanical damping to a structure. A related characteristic, fatigue resistance, can be improved by the ability of such adhesives to withstand cyclic strains and shock loads without cracking. In a properly designed joint, the adherends generally fail in fatigue before the adhesive fails.
Adhesives are sometimes used with mechanical fasteners for sealing flange joints or holding the parts together while the bond forms.
Thin or fragile parts can be bonded. Adhesive joints do not usually impose heavy loads on the adherends, as in riveting, or localized heating, as in welding. The adherends will also be relatively free from heat distortion.
However, adhesives are not a joining cure-all. The very fact that so many adhesive formulations are available, for example, makes selecting the best adhesive for an application more difficult than choosing a mechanical fastening system. These variations also complicate control procedures on incoming materials, assembly processing, and testing of the finished product. Also, even though adhesive-bonding operations can be automated, they may require more highly skilled personnel than do other methods.
Structural or performance adhesives are load-bearing adhesives. That is, they add strength to the products being bonded. These adhesives are used to build things as varied as office furniture and automobiles. The seven most commonly used structural adhesives are:
Commonly Used Structural Adhesives
Epoxy adhesives are the oldest, most widely used structural adhesive. Offering the greatest shear strength -- up to 10,000 psi -- they can be modified to meet a variety of bonding needs. Generally, epoxy bonds are rigid; they fill gaps well with little shrinkage.
Epoxies, however, cure slowly unless oven heated, operate well at temperatures only to 450°F, demand precise mixing equipment, and are relatively brittle.
Acrylic adhesive formulations tolerate dirtier and less-prepared surfaces than other structural adhesives. They challenge epoxies in shear strength -- up to 6,000 psi -- and offer flexible bonds and superior peel and impact resistance.
Although acrylics are two-part adhesives, the resin is applied to one surface and an accelerator or primer to the other. The parts can be set aside for weeks with no detrimental effects. Once they are mated, handling strength is typically achieved in a few minutes. Curing can be completed at room temperature.
Acrylics, however, have a strong odor, are flammable, and have suffered from a lack of reproducible bonds in mass production.
Urethane adhesives bond permeable and impermeable materials. They are known for toughness (up to 2,200 psi) and flexibility even at low temperatures. Their shear strength can approach that of epoxies. They have excellent water and humidity resistance, but uncured urethanes are sensitive to moisture.
Anaerobics are single-component adhesives that are easily applied by hand or with automation. They are available in machinery and structural grades. Machinery grades offer high cohesive strength for cylindrical assemblies and a range of strength for threaded assemblies. Structural grades offer high tensile/shear strength for flat surface assemblies.
The basic cure mechanism is the deprivation of oxygen (hence the name "anaerobic" -- "without air"). However, this curing system can be combined with other cure mechanisms such as heat, primer, accelerator, and ultraviolet light for customized assembly-line treatments.
Anaerobic adhesives fill gaps up to 0.040 in. when properly engineered for viscosity and curing mechanism and are usually limited to service applications in the range of 300°F.
Cyanoacrylates vie with anaerobics for ease of curing and adaptability to assembly-line production. Some have low viscosity, exhibit poor impact and heat resistance, and are vulnerable to moisture and solvents. However, modern elastomer-modified cyanoacrylates offer greater toughness, viscosity (up to gel grades), and resistance to moisture, solvents, and heat. The modified types bond virtually any surfaces and have better handling characteristics due to slower cure times.
The original materials are still among the most widely used bonding materials. They cure in seconds at room temperature due to their reaction to trace levels of moisture on most surfaces. These adhesives have variable viscosities and excellent tensile strength.
Hot melts have moved into areas of low-stress product assemblies -- even of metals. They form flexible and rigid bonds, achieve 80% of bond strength within seconds, bond permeable and impermeable materials and usually require no elaborate surface preparation. Hot melts are insensitive to moisture and most solvents, but they soften at high temperatures.
Silicone adhesives have one component and cure at room temperature. They are readily dispensed by hand or automation. Their operating temperature range is essentially unequaled and is particularly unmatched on the high end. Service life for these adhesives is long, and they resist UV and ozone attack. Silicones' gap-filling capabilities make them ideal sealants in many applications.
Silicones cure when they are dispensed and contact moisture in the air. Drawbacks include some rather long cure times and relatively high material costs. Tensile and shear strengths tend to be rather low. However, peel strength and impact resistance are good.
More on Adhesives
Selecting the proper adhesive involves consideration of
substrates to be bonded,
end-use environment, and
Manufacturing Conditions include machinery, material-handling methods, and plant conditions.
The design engineer should determine what machinery is already in place at the manufacturing facility. The engineer may recommend the appropriate applicators for the type of adhesive required. Spray and extrusion applicators are excellent for applying low-viscosity fluid adhesive products. Roller applicators normally apply medium-viscosity adhesives. Pot applicators are suited to a high viscosity.
Machinery is not limited to applicators. The manufacturer's materials handling also is important. For example, how much time does the manufacturing process allow for bonding? Does product handling require the initial bond strength to be greater than normally needed? Is there time for a product which bonds more slowly?
Manufacturing conditions involve more than the machinery and material-handling methods in the plant. Plant conditions such as the condition of the equipment and the skill of the product personnel should also be considered.
Substrates to Be Bonded
The openness, or porosity, of the substrate to be bonded can place additional demands on an adhesive system. Excessive penetration, hardness or impenetrability can make some adhesives unsuitable.
Adhesion to a coated surface such as painted or plated steel must take into account the surface coating, not only the base substrate. The coating has a profound effect on whether or not certain adhesive systems will be suitable for use in that application.
For example, many molded plastics have residual mold-release agents on their surface, and most attempts to bond these plastics will fail unless the surface is cleaned. A solvent wiping usually is adequate to render the surface bondable with most appropriate adhesives.
Likewise, some base metals quickly oxidize, so the surface they provide is not really a metal, but a metal oxide. However, many surfaces can be treated to achieve more suitable levels of adhesion.
It is essential to know what the coating is. Substrate characteristics place requirements on the selection of the proper handling system.
The end-use environment includes all conditions to which the adhesive bond will be subjected during the useful life of the product. Considerations vary with the application and include stress, the kind of joint being used, temperatures, exposure to moisture, flexibility, age, stability, and aesthetics.
These include stress placed on the glue line during construction as well as end-use stress -- the conditions the construction will be expected to encounter.
There are four types of stress:
Shear, when the adherends move in parallel planes.
Tension, when the adherends are pulled apart in the same plane.
Cleavage, when two adherends are pried apart at the end of a lap joint.
Peel, an exaggerated form of cleavage that occurs when a flexible adherend is bent away from the bond line.
There are three bonding failure types:
Adhesive: the adhesive peels off the substrate surface.
Cohesive: the adhesive sticks to the substrate surface but rips itself apart.
Delamination: the adhesive sticks together, and to the substrate surface, but pulls the coating off the metal.
There are many different kinds of joints including butt, lap, beveled lap, scarf lap, and invert-T.
Generally, adhesive-bonded joints in load-bearing structures should be loaded essentially in shear, minimizing the stress induced by peel, cleavage, and impact forces.
Joints must be designed specifically for adhesive bonding; seldom can an assembly designed for another method of fastening be successfully bonded without being modified. Adhesively bonded joints must be stressed in their strongest directions -- in tension, shear, and compression -- and load must be minimized in the peel and cleavage directions.
The adhesive and the substrate can become brittle due to low temperatures or may melt or decompose under conditions of extreme heat.
Heat sensitivity of adhesives is an important consideration in their selection and can be one of their most severe limitations. While some can withstand temperatures as high as 700°F, most are limited to service under 200°F. Most high-temperature adhesives require an oven cure, although some cure at room temperature. If an adhesive is not formulated for high-temperature service, its strength drops considerably in such environments. Low temperatures, on the other hand, make many adhesives brittle and stress joints internally.
Consider water and humidity, as well as exposure to solvents and such.
Other items include flexibility, aging stability, and aesthetic questions such as what are the joint's flexibility requirements? And its life span? What color is required and what level of gloss?
Cost factors relate the cost of adhesive bonding to other values in the manufacturing process.
If several adhesive systems meet the requirements for an application but significantly differ in price, more detailed analysis could determine an actual bonding cost per unit. Criteria involved include waste, process speed, rejects/failures, packaging, reliability, availability, and service.
Waste increases the cost per bond and the impact on the machinery. Cleaner application characteristics lead to less downtime and increased use of production equipment.
If one adhesive provides faster production speeds than the others, that value should be included in the cost/value ratio.
Rejects and Field Failures
Rejects are a fact of life, and usually an acceptable level of rejection has been established. If an adhesive system offers performance properties that reduce reject rates, higher initial costs can be overcome, and profits may increase.
By figuring the costs surrounding packaging considerations -- product consistency, batch-to-batch reliability, product availability and the strength of the supplier's backup service -- the total cost of each contending adhesive presents itself.
Adhesives can be applied with any type of liquid-handling tool such as a brush, spatula, trowel, dip, spray, curtain, flow gun, or flow brush. However, production-line conditions generally require use of automatic or semiautomatic dispensing equipment which can apply dots, bands, or beads. Most serious designers will not consider an adhesive without also considering a suitable application method.
Dip coating and spraying can also be used for flat parts, but are especially suitable for contoured parts. Brushing is widely used to apply liquid and thin-paste adhesives: Equipment is simple, waste is minimal, and limited areas of contoured shapes can be coated without masking. However, high production rates and uniform adhesive thicknesses are difficult to achieve.
Proper surface preparation is essential for achieving good bond strength with any adhesive. The extent of preparation depends on the type of substrate, the chemical nature of the adhesive, and the bond strength required. In general, the higher the desired strength, the more complex the prebond treatment.
Prebond treatments include cleaning, abrading, and chemically altering the surfaces. At the minimum, all surfaces must be clean. This means removing oil, grease, rust, scale, and mold releases by solvent, chemical, or abrasive means. For parts subjected to mild environmental conditions and moderate loads, cleaning may be all that is required. However, for critical applications, it is usually necessary to chemically alter the substrate surface to form an intermediate molecular layer with a higher chemical affinity for the adhesive being used. Bond strength and reliability are significantly increased, especially in abusive environments.
A solvent wipe or vapor degreasing is adequate for many noncritical applications. However, for highest strengths and long-term reliability, a chemical treatment is required. The treatment differs for each metal, but all require strong oxidizing solutions that chemically create a tightly bound surface layer of metallic oxide. Most adhesives will bond to such layers with much higher strengths than to pure metal surfaces.
Ceramics and Glasses
Chemical surface treatments are not normally required for these materials. A solvent or detergent cleaning is usually sufficient.
Most thermoplastics can be successfully bonded with adhesives. However, the type of adhesive chosen depends on the chemical nature of the plastic. For example, polystyrene is easily bonded with solvent adhesives, polyethylene can be joined with hot melts, and flexible PVC bonds well with cyanoacrylates.
But with most thermoplastics, the highest bond strengths are achieved only if the surfaces are first made more chemically polar. Polar surfaces develop very strong chemical bonds to many adhesives. Even fluorocarbon polymers, to which almost nothing will stick, can be easily bonded with epoxies after treating their surfaces with an acid solution of metallic sodium. Various surface treatments are used for different plastics, including oxidizing flames, electrical corona discharge, and oxidizing acid baths.
All thermoplastic materials first require a thorough cleaning and light abrading of the surface. Mold-release agents, lubricants, and plasticizers are generally present on plastic surfaces and will prevent proper wetting by the adhesive if not removed.
Thermoset materials: Epoxy, phenolic, polyester, silicone, and diallyl phthalate usually need only a solvent cleaning and light abrasion to remove fingerprints, mold releases, and other normal contaminants. Careful selection of the proper adhesive for each substrate is essential, however, for successful bonds.
Mechanical surface abrasion, such as sanding or sandblasting, is sometimes the best way to prepare a surface. This method not only cleans the surface, but roughens it, which often improves the bond. Anaerobics, for example, when used in threaded assemblies, rely on the interlocking of microscopic surface roughness for their bonding action.
Surface preparation may be totally avoided with acrylic adhesives based on reactive-fluid technology. These adhesives have components that cut through surface film and bond to the metal directly. A primer is first applied to the parts to be joined, then the adhesive is added and the parts are pressed together.
Pressure-Sensitive Adhesive Tapes
Rubber and acrylic-based pressure-sensitive adhesive tapes are common for industrial fastening. Silicone tapes are also used in special applications. Adhesive-tape selection requires knowledge of what types of surfaces will be fastened and the forces and environments to be encountered.
Rubber- based tapes are used where no temperature extremes are encountered. They build up to final bond strength in a short time. Rubber-based tapes are suitable for temporary holding applications such as mounting rubber and photopolymer plates that are removed after a production run, or they may be used for permanent applications such as mounting hooks and appliance trim. Rubber-based tapes offer better initial adhesion than most acrylics and have better long-term adhesion to materials with low surface energy, such as polyethylene and polypropylene.
Acrylic adhesives retain adhesion over a wide temperature range, resist moisture and ultraviolet light better than rubber-based types, and are easier to apply. Acrylic adhesives come in two basic types. In one, a foam carrier is coated with acrylic adhesive on both sides; a protective layer of liner material separates the tape layers and is peeled off in use. The second type has no carrier, just a thin film of pressure-sensitive acrylic adhesive between one or two siliconized release liners.
Foam tapes are used where there may be relative movement between the surfaces, or where irregular surfaces must be joined. The foam fills in the irregularities, improving adhesive contact. The most common types of foam carriers are polyurethane, polyethylene and polyvinyl chloride. Polyurethane and polyethylene foams are important in mounting applications. Vinyl foams are for general-purpose use. Care must be used in selecting an adhesive that resists the plasticizer migration of some soft vinyl materials. Each type of carrier is available in a variety of thicknesses, typically 0.030 to 0.125 in.
Transfer adhesive tapes, those without carriers, are used where one or both of two surfaces are flexible and both surfaces are relatively smooth. These products provide a thin adhesive profile where the finished appearance is important. Acrylic adhesives in these products typically have high shear strength and resist solvents and moisture. Transfer tapes usually have a 0.001 to 0.002-in. thickness. After the tape has been applied to one surface, the protective liner is peeled away, exposing the adhesive for attachment to the second surface.
Silicone Pressure-Sensitive Adhesives
Silicone pressure-sensitive adhesives resist temperature extremes from -100 to 500°F exceptionally well, without becoming brittle or soft. This range is nearly double that of conventional acrylic adhesives (-50 to 250°F). Like rubber-based adhesives, silicones adhere well to a variety of both low and high-energy surfaces and are chemically inert.
Silicone adhesive products are usually provided in two forms. Coated on a polymeric film, silicones are used for splicing, masking, or sealing. Silicones also act as a transfer adhesive for applications where an acrylic transfer film cannot be used.