Chemically curing adhesives are reactive materials that require chemical reaction to convert them from liquid (or thermoplastic) to solid. Once cured, these adhesives generally provide high strength, flexible to rigid bond lines that resist temperature, humidity, and many chemicals. They may be classified into two groups, namely single component and two component.
Single Component Adhesives
With single component adhesives, the adhesive components are premixed in their final proportions. However, they are chemically blocked. As long as they are not subjected to the specific conditions which activate the hardener, they will not bond. They require either high temperature or substances or media (light, humidity) from the surroundings to initiate the curing mechanism. The containers in which this type of adhesive are transported and stored must be carefully chosen to prevent any undesired reactions. These adhesives are usually 100% solid systems. The six major sub-classes are anaerobic adhesives, cyanoacrylates, heat curing adhesives, moisture cursing adhesives, radiation curing adhesives, and silicones.
1. Anaerobic adhesives: Anaerobic adhesives cure under the absence of oxygen. Oxygen inhibits the reaction thus preventing cure. When the adhesive is placed in a bond line and the accessibility of oxygen is restricted, cure proceeds quite rapidly. So that the adhesive does not cure prematurely, the adhesive in its container must remain in contact with oxygen up until the time it is used. This is achieved using air-permeable plastic bottles which are only half filled and which, prior to filling, are flushed with oxygen. Anaerobic adhesives are thermosets and the resulting bonds have high strength and high resistance to heat. These bonded joints are very brittle and not suitable for flexible substrates. Curing occurs exclusively in the joined area and only relatively small gap widths can be bridged (maximum gap: about 0.1 mm).
Besides their bonding function, anaerobic adhesives are often simultaneously used for their sealing properties because of their resistance to oils, solvents and moisture. All these properties make this type of adhesive suitable for mounting engines in the automotive industry. Other typical areas of application are for securing screws and for bonding concentric parts.
2. Cyanoacrylates: Cyanoacrylates are known for their “instant” bonding to most surfaces. They are used for a wide range of industrial applications. Most manufacturers warn that caution should be used in handling cyanoacrylates because of the danger of bonding skin to itself. Some users have found that cyanoacrylates are subject to degradation when exposed to moisture. They are thermoplastic when cured and consequently are limited in temperature capability and chemical resistance. Cyanoacrylates are suitable for bonding many combinations of materials and are in general used for bonding small components. Cyanoacrylates are popular for bonding all types of glass, most plastics, and metal. In addition to many applications in optics, microelectronics and transportation, there is a growing and diverse spectrum of applications for special cyanoacrylates in the area of medical technology.
3. Heat curing adhesives: Single component heat curing adhesives require high temperatures for a specified period of time to achieve cure. Because they are heat cured, cured adhesives generally provide the highest strength, heat resistance and chemical resistance. Although adhesion to plastic substrates is generally good, they can only be used to bond plastics that have softening points sufficiently above the cure temperature of the adhesive. These adhesives are supplied as liquid, paste, and film. Three main types of materials, namely epoxy resin, phenol-formaldehyde adhesives, and polyurethane, are used to formulate single component heat curing adhesives.
a. Epoxy resin: the most widely used raw material for formulating heat curing one component adhesives. They are used in the automotive industry, aerospace industry, and metal fabrication sector. In microelectronics, they are used with silver powder added as electrically conducting adhesives. Other additives can be used to formulate adhesives with heat-conducting properties. Cured adhesives are hard and rigid but tough with excellent chemical and heat resistance.
b. Phenol-formaldehyde resins: Phenol-formaldehyde adhesives (usually called phenolic resins for short) cure at temperatures between 100 and 140°C depending on the composition of the adhesive. During the cure, water is liberated from the adhesive. As the curing process requires temperatures above 100°C, the liberated water is present in gaseous form. To avoid foaming, phenolic resins are cured under pressure. Pure phenolic resins are very brittle and sensitive to peel stress. That is why they usually contain additives to increase the elasticity. Modified phenolic resin adhesives generally give high bond stability and bonds with good mechanical properties. They also have good temperature stability. Phenolic resin adhesives are mostly used for wood structures that require resistance to water and weathering and for bonding wood in furniture manufacture. Additionally, they are used in the bonding of brake and clutch linings in vehicles.
c. Polyurethane: Heat cured urethane adhesives are typically cured between 100 and 200°C. Some systems contain small amounts of a component that becomes volatile at the cure temperature. These systems can produce some foaming in the bond line. Bonds formed with heat cured polyurethane adhesives are generally tough and hard with high strength but are still elastic. Major applications are in the automotive industry for bonding body components and structures.
4. Moisture curing adhesives: Moisture curing single component systems are viscous adhesives that typically consist of non-volatile urethane prepolymers. These systems require moisture to trigger the curing reaction. Cured adhesives range from hard and rigid to soft and flexible depending on formulation. A major application for moisture curing adhesives is the installation of windshields in automobiles. Another is the bonding of plastic (polycarbonate) window panes to an aluminum ship structure. Recently, single component moisture curing polyurethane hot melt adhesives that combine the initial strength of hot melt adhesives with the improved heat resistance of moisture curing adhesives have been developed.
5. Radiation curing adhesives: This kind of adhesives requires no high temperatures, no solvents and no particularly complex equipment to be cured. All that is needed are light waves of defined wavelength. Curing times range from as little as 1 second up to several minutes. Typically, radiation curing adhesives only cure during the time they are exposed to radiation. As a result, they must be irradiated after the substrates are joined. This requires that at least one of the substrates must be permeable to the specific wavelengths of light that initiate the cure of the adhesive.
The curing process for these adhesive does not merely depend on the wavelength of the light. Optimum cure also depends on the dose of radiation used and thickness of the bond line. The choice of raw materials determines the elasticity and the deformability of the cured adhesive. Radiation curing adhesives are often used for bonding glass. These adhesives are also used for joining transparent plastics and as a liquid seal for metal/plastic casings. They are also being increasingly used in dental technology.
6. Silicones: Single component silicones cure by exposure to moisture. They are well known as adhesive/caulks sold in a many colors for household use. There are a variety of formulations available for industrial applications for bonding and sealing glass and metal.
Two Component Adhesives
Two component adhesives are 100% solids systems that obtain their storage stability by separating the reactive components. They are supplied as “resin” and “hardener” in separate containers. It is important to maintain the prescribed ratio of the resin and hardener to obtain the desired cure and physical properties of the adhesive. The two components are only mixed together to form the adhesive a short time before application with cure occurring at room temperature. Since the reaction typically begins immediately upon mixing the two components, the viscosity of the mixed adhesive increases with time until the adhesive can no longer be applied to the substrate or bond strength is decreased due to diminished wetting of the substrate. Formulations are available with a variety of cure speeds providing various working times after mixing and rates of strength build-up after bonding. Final strength is reached in minutes to weeks after bonding depending on the formulation. Adhesive must be cleaned from mixing and application equipment before cure has progressed to the point where the adhesive is no longer soluble. Depending on work life, two component adhesives can be applied by trowel, bead or ribbon, spray, or roller. Assemblies are usually fixed until sufficient strength is obtained to allow further processing. If faster rate of cure (strength build-up) is desired, heat can be used to accelerate the cure. This is particularly useful when parts need to be processed more quickly after bonding or additional work life is needed but a slower rate of strength build-up cannot be accommodated. When cured, two component adhesives are typically tough and rigid with good temperature and chemical resistance.
Two component adhesives can be mixed and applied by hand for small applications. However, this requires considerable care to insure proper ratio of the components and sufficient mixing to insure proper cure and performance. There is usually considerable waste involved in hand mixing as well. As a result, adhesive suppliers have developed packaging that allows the components to remain separate for storage and also provides a means for dispensing mixed adhesive, e.g. side-by-side syringes, concentric cartridges. The package is typically inserted into an applicator handle and the adhesive is dispensed through a disposable mixing nozzle. The proper ratio of components is maintained by virtue of the design of the package and proper mixing is insured by use of the mixing nozzle. Adhesive can be dispensed from these packages multiple times provided the time between uses does not exceed the work life of the adhesive. If the work life is exceeded, a new mixing nozzle must be used. For larger applications, meter-mix equipment is available to meter, mix, and dispense adhesive packaged in containers ranging from quarts to drums. Four major types of two component adhesives are epoxies, methyl methacrylates, silicone adhesives, and urethanes.
1. Epoxies: Two component epoxy resin based systems are the most widely used structural adhesives. They can be formulated into fast curing systems with 2 to 5 minute work life that give rigid bond lines which are somewhat brittle. Longer work life systems usually contain additives to provide flexible or tough bond lines. Two component epoxy adhesives are used to bond metal, plastic, fiber reinforced plastic, glass, and some rubbers.
2. Methyl methacrylates: Methyl methacrylate adhesives can provide faster strength build-up than epoxy adhesives and are more tolerant of oil on the substrate. They are used for bonding plastics to each other and bonding metals to plastics. Classic applications for this type of adhesive are in the automotive industry and in rail car manufacturing.
3. Silicone adhesives: Two component silicones are generally used for production line assembly, e.g. in electronics and the electrical industry as well as in the production of household appliances, in the automotive industry, and for window manufacture. Cured properties are similar to single component moisture curing silicones. Two component silicones are used instead of single component silicones when adhesive film thicknesses of over 6 mm are required or for large bonding areas. This type of silicone adhesive is also used when the available humidity in the air does not suffice for the complete curing of single component silicone.
4. Urethanes: Two component urethane adhesives can be formulated with a wide range of cured properties ranging from soft and flexible, to tough and elastic, to hard and rigid. They are used to bond materials with different flexibility or different thermal coefficients of expansion including glass to metal, fiber reinforced plastic to metal, and aluminum to steel.
This article is reproduced from Adhesives.org/Sealants.org.