An adhesive is a substance capable of usefully holding two surfaces together by surface attachment. For instance, two glass plates can be held together by a thin layer of water between them. When an attempt is made to pull the two glass plates apart it is found that the glass will probably break before they can be separated. However, the two glass plates can be sheared quite easily with very little force, therefore, the water does not perform adequately as an adhesive.
An adhesive must have two fundamental characteristics. First it must posses the properties of a fluid in order to thoroughly wet out the two surfaces to be bounded and second, it must have the properties of a solid to resist the tensile and shear forces which can be applied to separate the two surfaces.
With conventional adhesives, this is done in several ways. One is to apply the adhesive in a solution form for application, the solvent then being evaporated to leave a firm, hard joint. Another method is to apply the adhesive at elevated temperatures at which time the adhesive is a fluid. When the joint has been formed. the adhesive has cooled and becomes a solid at room temperature. Yet another method, is to use polymers which are liquid at room temperature but can be cured by cross-linking agents to form a three-dimensional solid network. Many commercial adhesives are combinations of these three simple designs.
However, pressure sensitive adhesives do not fall into any of the above categories. The basic design of the adhesive is such that it possesses, at one time, both the properties of a liquid and solid. Because of this, they call it “viscoelastic” A similarity may be seen in road tar. It resists the wear and tear of passing vehicles, therefore, behaves like a solid. However, if a ball of tar were allowed to stand, it would soon be seen to collapse and flow over the surface on which it stood. “silly putty” is another example. When a high force is applied, it will bounce like a rubber ball. But when a low force is applied, it stretches like chewing gum. And so it is with pressure sensitive adhesives. Because of this characteristic, pressure sensitive adhesives can be applied without the need for heat, solvents, unusual pressures or other special techniques to form a useful bond.
Another unique characteristic build into pressure sensitive adhesives is that they not only can form a useful bond but can be unbonded quite easily. When this unbonding takes place, it does not do so trough the adhesive layer which would be expected, but at the boundary between the adhesive and the applied surface, leaving the surface as clean as it was originally.
Note: This material is an excerpt from Shuford Literature.