Ask Dr. Dave: Please explain tack, peel adhesion and shear strength in regards to pressure-sensitive adhesives.
PSAs can be formulated with a range of adhesive and cohesive properties.
Question: We are using a pressure-sensitive adhesive for making tapes, but are somewhat confused about the different properties such as tack, peel adhesion and shear strength. Can you provide an overview of these adhesives?
Answer: Pressure-sensitive adhesives (PSAs) are defined as adhesives that, when dried or fully cured, are permanently tacky and will adhere to surfaces by applying pressure. They can be supplied as liquids or in finished form as labels, tapes, and films. The tack, or “stickiness,” of a PSA enables it to adhere to a surface under light pressure with minimum contact time.
PSAs are not liquids; rather, they are soft thermoplastics that flow and wet a surface at room temperature. The applied pressure ensures good wetting of the surfaces. PSAs, in contrast to adhesives that cure to solids, continue to flow after application, and their bond strengths tend to increase over time. The balance between adhesive strength and cohesive strength of the adhesive determines the suitability of a PSA for a particular application. Thus, a PSA for a permanent label only needs good adhesive strength under the conditions of use, while a packaging tape, which can be under stress and exposed to high temperatures, needs both high adhesive and cohesive strength. A PSA for a protective film needs good cohesive strength, but the adhesive strength should remain low to enable the film to be removed without leaving adhesive residue on the surface.
PSAs are manufactured using solvent-based, hot-melt, UV and emulsion processes. However, manufacturers have been moving rapidly away from solvent-based systems toward more environmentally acceptable technologies. UV-cured and hot-melt PSAs can be processed very quickly because no solvents or water need to be removed.
PSAs can be formulated with a range of adhesive and cohesive properties. For example, tackified rubber-based adhesives show excellent peel adhesion to many substrates, but they perform poorly at elevated temperatures. Acrylic-based adhesives, in contrast, either have outstanding peel adhesion and poor elevated temperature performance or low peel adhesion and outstanding elevated temperature performance.
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