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:
- Anaerobic
- Cyanoacrylates
- Heat Cure
- Moisture Cure
- Radiation Cure
- 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.