Adhesive

There are many sources and types of adhesive. Many early adhesives were made by rendering animal products, and these are still used today. Native Americans melted down buffalo hooves to make glue.

Modern adhesives fall into two broad categories:

Drying adhesives

These adhesives are either applied hot, and simply allowed to harden as they cool, or are a mixture of ingredients (typically polymers) dissolved in a solvent. As the solvent evaporates, the adhesive hardens. These adhesives are typically weaker, and are used for household applications.

Reactive adhesives

These most commonly come in two separate containers, and the two ingredients of the adhesive must be mixed in certain proportions immediately before application. Some epoxy resin adhesives are supplied in this form. Generally one ingredient is a monomer, or resin, and the second is a reaction initialiser. When the two are mixed together, a polymerisation reaction occurs which solidifies the adhesive.

Reaction adhesives may also react with the surface of the materials to be stuck together. This is bonding, in which the adhesive forms chemical bonds with the material, and is distinct from sticking, the action of common glues.

A special case of this kind of adhesive is so-called "super glue", which reacts with moisture in the air, and therefore does not need any mixing before application.

This type of adhesives are very strong, and are used for high-stress applications, such as attaching wings to aircraft.

Temporary adhesives

Another kind of specialist adhesives are the temporary adhesives designed to stick and unstick, such as those used in Blu-tack and Post-It Notes[?].

Adhesive failure

Adhesives may fail in one of two ways:

Adhesive failure is the failure of the adhesive to stick or bond with the material to be adhered (also known as the substrate).

Cohesive failure is structural failure of the adhesive. Adhesive remains on both substrate surfaces, but the two items separate. This often happens when you try to remove a price label.