From shoes to space ships - adhesives perform the fundamental function of holding things together. But that's not as simple as it sounds. For a workable understanding of adhesion and its benefits and limitations, knowledge of the fundamentals is essential.
Adhesion vs. stress...
A fundamental of the technology
One of the common laboratory tests to measure adhesive performance is to "pull at" or stress the bond. The purpose is to see at what point stress overcomes adhesion and the bond breaks. With some adhesives, the materials stretch or break before the adhesive bond.
Adhesion vs. Stress — that's what a great deal of adhesive technology is all about.
Adhesion is the force that holds materials together. There are three basic types:
1)SPECIFIC ADHESION—is the molecular attraction between contacting surfaces.
2)MECHANICAL ADHESION—occurs when an adhesive is applied to rough or porous surfaces.
3)EFFECTIVE ADHESION—combines specific and mechanical adhesion for optimum strength.
Stress, on the other hand, is the force pulling materials apart. The basic types of stress in adhesive technology are:
1)TENSILE—"Pull" is exerted equally over the entire joint. "Pull direction" is straight and away from the adhesive bond. All adhesive contributes to bond strength.
2)SHEAR—"Pull direction" is across the adhesive bond. The bonded materials are being forced to slide over each other. All adhesive contributes to bond strength.
3)CLEAVAGE—"Pull" is concentrated at one edge of the joint and exerts a prying force on the bond. The other edge of the joint is theoretically under zero stress.
4)PEEL—One surface must be flexible. Stress is concentrated along a thin line at the edge of the bond. This line is the exact point where an adhesive would separate if the flexible surface were peeled away from its mating surface. Once peeling has begun, the "stress line" would stay out in front of the advancing bond separation.
Most adhesives perform better when the primary stress is tensile or shear. However, in most industrial applications, a combination of stresses are involved. For best performance the entire bond area should carry the bulk of the stress.