Engineering and non-engineering adhesives

Adhesives may be classified as either organic or inorganic materials in a number of different ways; for example by origin, by method of bonding, by end use or on a chemical basis (1). Table 2.1 gives a broad classification of the organic adhesives based upon origin under the general headings of animal, vegetable, mineral, elastomeric, thermoplastic and thermosetting adhesives.

Animal glues are generally based on protein either in the form of mammalian collagen, from fish or from milk. They tend to be ‘sticky’ which is useful for applications requiring an instant grab or bond.

The common vegetable glues are based on either starch or cellulose. Unmodified starch dispersed in water is used to form paper pastes but a large proportion is now used in modified forms such as dextrin. Cellulose-based glues are produced by reacting hydroxyl groups present in the polymer chain with different reagents to form a variety of adhesives.

Mineral adhesives include silicates and phosphates for high temperature use and naturally derived products such as bitumen and asphalt.

The elastomeric group of adhesives is based on natural rubber latex and its derivatives or totally synthetic rubber known as SBR (styrene butadiene rubber). There is now a wide range of synthetic rubber adhesives based upon SBR including nitrile and butyl rubber. Another elastomeric adhesive is the versatile polyurethane rubber group.

Thermoplastic adhesives are so called because they may be softened by heating and rehardened on cooling without undergoing chemical changes.