Structural and Pressure Sensitive Adhesives
Adhesives may be broadly divided in two classes: structural and pressure sensitive. To form a permanent bond, structural adhesives harden via processes such as evaporation of solvent (for example, white glue), reaction with UV radiation (as in dental adhesives), chemical reaction (such as two part epoxy), or cooling (as in hot melt). In contrast, pressure sensitive adhesives (PSAs) form a bond simply by the application of light pressure to marry the adhesive with the adherend. Pressure sensitive adhesives are designed with a balance between flow and resistance to flow. The bond forms because the adhesive is soft enough to flow, or wet, the adherend. The bond has strength because the adhesive is hard enough to resist flow when stress is applied to the bond. Once the adhesive and the adherend are in proximity, there are also molecular interactions such as van der Waals forces involved in the bond, which contribute significantly to the ultimate bond strength. PSAs exhibit viscoelastic (viscous and elastic) properties, both of which are used for proper bonding.
In contrast with structural adhesives, whose strength is evaluated as lap shear strength, pressure sensitive adhesives are characterized by their shear holding. The ability to hold in a shear mode is related to the formulation, coating thickness, rub-down, temperature, etc.
Read more about this topic: Pressure-sensitive Adhesive
Famous quotes containing the words structural, pressure and/or sensitive:
“The reader uses his eyes as well as or instead of his ears and is in every way encouraged to take a more abstract view of the language he sees. The written or printed sentence lends itself to structural analysis as the spoken does not because the reader’s eye can play back and forth over the words, giving him time to divide the sentence into visually appreciated parts and to reflect on the grammatical function.”
—J. David Bolter (b. 1951)
“Much of the pressure contemporary parents feel with respect to dressing children in designer clothes, teaching young children academics, and giving them instruction in sports derives directly from our need to use our children to impress others with our economic surplus. We find “good” rather than real reasons for letting our children go along with the crowd.”
—David Elkind (20th century)
“Compared to other parents, remarried parents seem more desirous of their child’s approval, more alert to the child’s emotional state, and more sensitive in their parent-child relations. Perhaps this is the result of heightened empathy for the child’s suffering, perhaps it is a guilt reaction; in either case, it gives the child a potent weapon—the power to disrupt the new household and come between parent and the new spouse.”
—Letty Cottin Pogrebin (20th century)