There are many sources and types of adhesive. Many early adhesives relied on animal products, and many adhesives manufactured today reply upon the output of rendering plants. Native Americans used buffalo hooves to make adhesives by melting them down.
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 less strong, 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 prior to application. An epoxy resin adhesive is a common example of this (commonly called super glue). 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, the adhesive literally forms chemical bonds with the material, and is distinct from sticking, the action of common glues.
This type of adhesives are incredibly strong, and are used for high stress applications, such as attaching wings to aircraft.
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 know as the substrate)
Cohesive failure is the failure of the adhesive to retain it's structural integrity. Adhesive will remain on both substrate surfaces, but the two items will fall apart. This often happens when you try to remove a price label.
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