Some Rheological Aspects of the Peel Strength of Rubbery Adhesives

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Abstract
The strength of adhesion between two types of SBS triblock elastomer and polyethylene terephthalate (Mylar) has been measured using a peeling geometry. Three aspects of the measured strength have been studied: its dependence upon thickness of the elastomeric adhesive layer, its dependence upon the speed of peeling, and the occurrence in some circumstances of tearing failure of the elastomer, rather than interfacial separation. The bond with the stiffer SBS elastomer (Kraton 1101) had a peel strength that was independent of thickness over the range studied: 0.1–6 mm. However, the peel strength with the softer SBS material (Kraton 3202) increased linearly with thickness before reaching a constant value at thicknesses greater than about 2 mm. This difference in the dependence of the peel strength upon thickness of the adhesive layer for the two elastomers is attributed to the formation of a relatively thin plastically-yielded layer at the peel front in the former case but not in the latter. The dependence of the peel strength of the Kraton 1101-Mylar bond upon the rate of peeling was strongly influenced by the degree to which equilibrium wetting had been achieved before testing. At low levels of interfacial interaction, the peel strength increased markedly with peeling rate, whereas, when the interaction was relatively strong, the strength did not vary much with changes in the rate of peeling. These features are again attributed to the tensile stress-strain behavior of this elastomeric adhesive, and the occurrence of plastic yielding under high local stresses. The adhesion between Kraton 3202 and Mylar underwent a transition in failure mode from rubber failure to interfacial detachment as the peeling rate was increased. The peeling rate at which the transition occurred was found to be proportional to the thickness of the adhesive layer, in accordance with a characteristic rate of straining. This transition is also attributed to ductile processes within the adhesive.

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