With single component adhesives, the adhesive components are premixed in their final proportions. They are however 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:

1. Anaerobic
2. Cyanoacrylates
3. Heat Cure
4. Moisture Cure
5. Radiation Cure
6. Silicones

1. Cyanoacrylates

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 themosets and the resulting bonds have high strength and high resistance to heat. These bonded joints are, however, very brittle and are 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, anaerobically curing adhesives are often simultaneously used for their sealing properties because they are very resistant 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, e.g. in bearings and electric motors.

Screws on a motor housing are secured against self-loosening using an adhesive. By exceeding a certain breakaway torque, the screw can be loosened again.

2. Cyanoacrylates

Cyanoacrylates (CA’s) are known for their “instant” bonding to most surfaces. They are used for a wide range of industrial applications as well as the ever-popular Super Glue and Krazy Glue® consumer products. Most manufacturers warn that caution should be used in handling CA’s 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. CA’s are suitable for bonding many combinations of materials and are in general used for bonding small components. CA’s 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, e.g. as a textile adhesive and for spray-on bandages.

3. Heat Cure

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 are used to formulate single component heat curing adhesives:

1. Epoxy Resin

Epoxy resin is the most widely used raw material for formulating heat curing one component adhesives. They are encountered everywhere – 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.

2. Phenol-formaldehyde Resins

Phenol-formaldehyde adhesives (usually called phenolic resins for short) cure at temperatures between 212 and 285oF (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 212oF (100°C), the liberated water is present in gaseous form. In order 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 (boat adhesives) and for bonding wood in furniture manufacture. In addition they are used in the bonding of brake and clutch linings in vehicles.

3.Polyurethane

Heat cured urethane adhesives are typically cured between 200 and 390oF (100 and 200 oC). 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 Cure

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 urethanes 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 melts have been developed that combine the initial strength of hot melts with the improved heat resistance of moisture cured adhesives.

5. Radiation Cure

Radiation curing adhesives require 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 (optics, glass design). 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.