Adhesives in various forms have been in use for centuries, and have evolved over this time to fit many new applications. There are many types of adhesives available today, each with unique characteristics. In general, adhesives can be grouped into the following categories:
A) Water Based – These are adhesives that use water as a carrier or diluting medium, and set by allowing the water to evaporate or be absorbed by the substrate. There are several types of water-based adhesives.
1) Vegetable Glues – These are adhesives based on starch. They are usually amber to brown in color, commonly know as dextrine adhesives. These can also be made in a high viscosity, high tack version called jelly gum. Relatively low cost adhesive commonly used in paper bonding, packaging and labeling. Low moisture resistance. Bond lines tend to be brittle.
2) Resin Cements – These are adhesives based on an emulsion of EVA (Ethylene Vinyl Acetate) or PVA (Poly Vinyl Acetate) polymers blended to an emulsion with water as a carrier. Capable of bonding to wood, paper, some plastics and foams and many other substrates. White in color. Bonds have higher degree of moisture resistance than dextrine, but cost of resin cement is higher. Certain resins may be blended with dextrines to form a hybrid product. Bond lines have some flexibility, and are relatively clear when dry.
3) Animal/Protein Glues – The two major types of adhesive in this category are hot animal glue (which is made from processed animal parts) and casein glue (derived from milk). Hot animal glue is amber to brown in color, and is applied at approximately 140°F. It can be thinned with water. When first applied it has very high tack, but dries to a non tacky film. Commonly used in situations where the high tack will hold the parts together while setting, but which will not be exposed to high temperatures or high humidity. Casein glue is applied at room temperature, but forms a bond with a high degree of moisture resistance. Commonly used for labeling beer, champagne and some types of wine bottles. Casein is light to tan in color.
4) Latex Cements – These adhesives are a blend of latex or other elastomers in a water base emulsion. In most cases they are applied to parts, allowed to dry, and form a layer, which serves as a contact cement (two way cements). Some types can also be applied to one surface and will form bonds as they dry (one way cements). Can be formulated to remain tacky or become dry to the touch (contact types). Generally white in color. Wide variety of uses such as self-stick envelopes, fabric bonding, and leather goods.
B) Thermal Adhesives – Thermal adhesives are those adhesives that are brought to a liquid state by heating, and are applied to the product hot – either as liquid or as a high viscosity paste. The most common types are hotmelt adhesives and waxes.
Hotmelt adhesives have seen tremendous development over the past thirty years. These adhesives are blends of various polymers, but most are based on a high percentage of EVA (ethylene vinyl acetate). To obtain the desired characteristics, other polymers may be blended into the mix, as well as waxes, oils, various types of rubber, and tackifying resins.
Hotmelt adhesives can be used to bond many types of materials, and are available in three general types categorized by how fast they set up after application. The general categories are fast set, delayed set, and pressure sensitive.
Fast setting hotmelts are types that form a bond very quickly as they cool. These are used for situations where fast setting is important such as the sealing of flaps on cartons or certain labeling applications. Delayed set hotmelts are also known as adhesives having a “long open time”. These adhesives remain tacky for some period of time after application, but eventually set to form a bondline that has very little residual tack. They are useful in applications where the parts must be positioned after application of the adhesive, or in situation where the parts connot be assembled immediately after the adhesive is applied. Shoe and leather goods assembly is an example of one area where delayed set adhesive are used.
Pressure sensitive hotmelts remain tacky indefinitely after application. This allows the adhesive to be applied to a part that may not be assembled to a substrate for a long period of time. It also allows the bonding of parts that are difficult to bond (example would be polyethylene foam). In some cases pressure sensitive hotmelts are applied to a part and put on silicone coated release paper. The release paper is peeled off to expose the pressure sensitive layer, which is then bonded to the substrate.
Pressure sensitive hotmelts are available in many degrees of tackiness, from adhesives that form a temporary bond that can be easily broken (fugitive bond adhesives) to very aggressive pressure sensitives that will tear fiber from the substrates if removal is attempted.
Specialty hotmelts are also available which have characteristics tailored to specific types of applications or temperature ranges. A few of these are as follows:
a) Remoistenable hotmelts – This is a type of fast-set hotmelt that is formulated with a component of the blend being sensitive to moisture. If moistened, it forms a tacky layer that can be used for envelope sealing or labels.
b) Polyamid hotmelts – These are high temperature hotmelts with high performance characteristics. Generally the application temperature is around 400° F, and structural strength is higher than the more common types of hotmelt.
c) Reactive hotmelts – These are hotmelts that are formulated with a chemistry similar to polyurethane polymers. After application, an isocyanate component of the blend reacts with moisture in the air or substrate to form a polyurethane compound. Once cured, the material is no longer thermo plastic in nature but has excellent flexibility, high bond strength, high moisture and heat resistance, and resistance to most chemicals. The disadvantage of reactive hotmelts is that they require specially designed application equipment, since any adhesive that is exposed to atmospheric moisture will react with it and form an inert polymer. Applications for the reactive hotmelts are found in bookbinding, footwear construction, and recreational vehicle assembly.
Waxes are the oldest form of thermal adhesive, having been used for sealing documents for centuries. In today’s world they see use as laminating adhesives for foils and films. Bonds are formed to the substrates when hot, but the strength is sufficient to keep the materials bonded at lower temperatures. One special form of adhesive wax is paste-up wax. This is a blend of sticky waxes and tackifying agents that is used to form a temporary bond that allows parts to be removed and repositioned after bonding. It is used by newspapers and printers during page layout (paste-up) process since it allows photos and columns of type to be moved around as the page layout is being developed.
C) Two part Adhesives – These are adhesives that are made by mixing two or more components that react chemically to form a chemically crosslinked adhesive. In general, they are higher cost than other types of adhesives but also provide very high strength bonds and outstanding performance characteristics. The most common two part adhesives are epoxies, polyurethane’s, acrylics, and silicones.
Two part adhesives are able to cure in the absence of air or moisture, and are often used to form structural bonds to metal, wood and plastic components.
Epoxies consist of a base resin and a hardener. In most cases the base resin is a high viscosity paste, and the hardener (catalyst) a lower viscosity, but mixes can be formulated to differing viscosities and mix ratios. Most types will set at room temperature, but some require a heat cure to trigger the crosslinking reaction. Heat will accelerate the cure rate of most epoxies, and will often help the epoxy form better bonds and attain higher strength levels. Some types of epoxies are available as single component pastes which are kept cold to inhibit the reaction, but which will form bonds and crosslink when exposed to heat.
Polyurethane adhesives are available as two part formulas or as one-part components which are pre-mixed but mixed with a carrier material such as solvent. Polyurethane’s generally form bonds that are more flexible than epoxies but are quite tough. Urethanes form strong bonds to most materials, and can form strong bonds to rubber, plastics, metal, wood, paper, ceramic, and fabrics. Most types are limited to service temperatures below 250° F.
Polyurethane adhesive are available in a wide range of viscosities and mix ratios. They must be very well mixed to obtain top quality bonds. Some types contain isocanates or heavy metal catalysts than can pose health risks to workers, and require extra handling precautions.
Acrylic adhesives are available either as two part adhesives or as versions that are cured by exposure to ultraviolet light. Acrylic adhesives produce bonds with excellent peel strengths as well as high shear and impact strengths. They are generally more tolerant of dirty or poorly prepared surfaces then other adhesives. Acrylics
Acrylics are available in a wide range of viscosities – from quite watery to thick pastes. Most types (with the exception of the U.V. cured types) are prepared by mixing the two components, but some types are available that allow the one component to be applied to one substrate, and the second component to the other. When the two substrates are brought together, the reaction occurs to bond the parts. Acrylics are generally limited to temperatures below 300° F.
Silicone adhesives are available as both one part and two part adhesives. The one-part versions are known as RTV silicones (room temperature vulcanizing) and cure by reacting with moisture in the atmosphere. These are used most often as caulking and gasketing materials. Two part silicones offer higher performance, and can be used for bonding metal glass and ceramic components. These types of silicone adhesives find use in the electronics industry.
The primary advantage of silicone adhesives and sealants are their temperature resistance. Silicones can be formulated to withstand temperatures as high as 500° F, but provide flexible bond lines or sealing throughout their service range.
D) Moisture Cure Adhesives – Moisture cure adhesives are formulated to react with the moisture in the air or in the substrates to form a cured polymer layer with high strength. They are actually two component adhesives with one component being moisture. The two best-known types are silicone and polyurethane. The silicones are known as RTV silicones (room temperature vulcanizing), and are most commonly used as caulking compounds, gasket compounds, and sealants.
Polyurethane moisture cure adhesives are available in liquid form. In most cases the urethane monomer is dissolved in a solvent carrier, and reaction with moisture occurs as the solvent evaporates. Some types of water-borne urethanes are also available, but the newest types of moisture cure urethanes are made in the form of hotmelt adhesives. These are called reactive hotmelts, and exhibit a dual property. They are applied like regular hotmelts, but after application begin to crosslink with moisture to form a tough adhesive layer with high resistance to heat, moisture, and impact.
E) Ultraviolet Cure Adhesives – These are adhesives which contain monomers that will crosslink upon exposure to ultraviolet light to form a polymer. The crosslinking (or cure) can happen in less than a second at proper energy levels, so these adhesive can be used in high speed situations. Acrylic adhesives lend themselves to U.V. curing quite well, but U.V. cure versions of silicones, urethanes/acrylic blends and cyanoacrylates are also used.
Ultraviolet cures adhesives can form high strength bond lines on materials which will pass the U.V. light. The primary advantage of U.V. cure adhesives is the fast cure speed.
F) Cyanoacrylate Adhesives – These are fast setting one component adhesives that are popularly known as “crazy glue”. Cyanoacrylates are solvent free and react with the moisture on the surfaces of the substrate materials to form a rigid plastic adhesive layer that has high strength characteristics. The cured adhesive is very high in tensile and shear strength, but low in peel strength.
Cyanoacrylates are expensive compared to other adhesives, but only a very small amount is needed to cover the area to be bonded, since this material works best when spread into a very thin bond line. The material is available in a range of viscosities from water thin to thickened versions that are in the form of thixotropic pastes or gels.
G) Anaerobic Adhesives – These adhesives cure to a solid polymer in the absence of oxygen. They are commonly used as thread – locking compounds and retaining compounds for metal parts such as bearings and shafts. Anaerobic adhesives remain liquid as long as they are exposed to the atmosphere, but cure rapidly once confined. They are packaged in special containers that can “breathe” to prevent the materials from setting up in the containers. They are easy to use and are available in a range of viscosities and bond strengths. Some versions are available for making structural bonds between substrates that need to be laminated.
H) Film Adhesives – These are adhesives that are made in the form of sheets. In most cased they are carried on release paper, but some types are carried on release paper, but some types are heat activated and do not require release paper. Film adhesives are made from water base, solvent base, or hotmelt adhesives which are cast into a thin film leaving only the adhesive. They find use in situations where the release paper can be left in place and peel off prior to application to the substrate. They are popular for mounting of plastic components such as warning stickers, die cut parts such as letters and numbers, and a multitude of other parts. This form of adhesive also finds use for cold laminating of paper, plastics and films. The heat reactivated versions find use in fabric bonding and industrial applications where heat can be applied to the substrates to melt the adhesive.
Some types of film adhesives are cast onto a supporting material such as a scrim cloth or nonwoven fabric. These prevent stretching of the adhesive in use and simplify handling. Double back carpet tape is made in this manner. Film adhesives tend to be expensive relative to other adhesives because the cost of the release paper carrier must be included in the price of the adhesive. For many applications, the release paper stays with the product until it is applied, so the cost premium is justified. Film adhesives are also useful in cases where liquid adhesives might distort the substrates to be bonded. This is the case with some types of thin papers, films and foils, especially in low volume applications where ease of handling is of primary importance.