The adhesive force generated by a small short-term pressure, called tack, is measured by a probe tack test on pressure-sensitive adhesives (PSAs); the maximum force is evaluated by cavity growth at the interface between the PSA layer and the probe surface. As the PSA layer becomes thinner, it is more difficult to measure the tack with a cylindrical probe because of the uneven contact resulting from misalignment. A spherical probe is preferable to obtain reproducible contact on the PSA layer, but the contact area should be taken into account if the contact pressure affects the tack performance. Tack was measured on PSAs with various thicknesses in different contact areas to clarify their effect. The results showed that a larger contact area on a thinner PSA generated higher adhesive stress with larger strain. It was found that the maximum adhesive stress was not affected by the contact pressure, but it was strongly correlated to the contact radius divided by the PSA thickness. In addition, a video microscope observation showed that, in all of the experimental cases, the adhesive stress always reached the maximum when cavities were generated at the interface between the PSA and probe surface. Therefore, the criterion of cavity growth was introduced for the evaluation of the maximum adhesive stress. As a result, the experimental results, even at different release rates, were in good agreement with the estimation by considering the effect of confining a thin layer. Furthermore, the theoretical estimation indicated the ultimate value, which was not dependent upon the PSA thickness or contact area. It was defined as a material property, referred to as the "ultimate tack strength" of PSAs.

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