The second components are principally amines or acid anhydrides. Generally the epoxides give bonds of considerable strength and durability, but until recently they tended to be too brittle for many purposes. Developments beginning in the 1970s have enhanced the toughness of these and other structural adhesives.

Urethanes

These involve the reaction of an isocyanate with an organic compound containing a hydroxyl group. Like the epoxides, variation of the properties of the final polymer can readily be controlled with two ingredients to give a product that may be an elastomer, a foam, or one that is stiff and bristle-like. Urethanes are increasingly used in a wide variety of situations.

Phenolics

The phenolics group of adhesives includes two that are somewhat different in their uses. The first, urea/formaldehyde formulations, were developed in the 1920s and 1930s and are mainly significant in the manufacture of plywood and similar products. The second group is phenol/ polyvinyl formal formulations mainly used in aircraft construction for bonding aluminum and developed during World War II. Phenolics all involve curing under considerable pressure at an elevated temperature, typically 1500_C for 30 minutes at a pressure of 10 atmospheres for an aircraft adhesive. The bonds are of considerable strength and durability, suitable for primary aircraft structures.

Silicones

Silicones, generally silicone (or siloxane) rubbers, are largely used as sealants that combine adhesion with their gap-filling characteristics. Commonly used for sealing around baths and similar fittings, they cure by reaction with moisture from the environment. Industrially, particularly in automobile construction, there are many situations where providing a bond of moderate strength together with filling a gap between parts, which may amount to several millimeters, is required.

Acrylates

The acrylates include four types of adhesives.

1. Anaerobic adhesives (c. 1950) are formulations in which polymerization is prevented by the presence of oxygen. If oxygen is removed and ferrous ions are present, the liquid very quickly polymerizes to a hard, rather brittle solid. The main use for this is in thread locking in machinery and in the securing of coaxial joints.
2. Cyanoacrylates, or ‘’super glues,” were developed in 1957. They are colorless, very mobile liquids derived from cyanoacrylic acid. They readily polymerize, particularly in conjunction with the imperceptible film of moisture that is invariably present on surfaces. The bonds are very susceptible to attack by water and are only stable below about 80_C. Nevertheless, they are extensively used in product assembly in the electronics industry where they are likely to be exposed to only benign conditions.
3. Reactive acrylics (sometimes called ‘’second generation” acrylates, developed in 1975) depend upon a polymerization reaction that follows a free radical path. This means that the ratio of the two components is relatively unimportant, so careful control of quantities is unnecessary. In parallel with the development of this system, a technique was perfected for increasing the toughness of the cured adhesive by incorporating minute particles of rubber. The adhesive is in two parts: a viscous gel and a mobile liquid. These two are spread one on each side of the joint. When the two are brought together, they react quickly to give a strong bond, which is handleable in 2 to 3 minutes, with full working strength in 1 hour and ultimate strength in 24 hours. These adhesives not only give a strong bond of high toughness very quickly, they are also able to contend with oily surfaces. They provide an exceedingly satisfactory product that meets a number of requirements in advanced assembly, especially within the automobile industry.
4. A series of acrylic adhesives has been produced which are cured by irradiation with ultraviolet light. Clearly they can only be used where the radiation can reach the adhesive; for example, where one component is transparent to the UV wavelength. While a considerable range of these products has been developed, very little information has been released about their composition.