Bulk and Interfacial Contributions to the Adhesion of Acrylic Emulsion-Based Pressure-Sensitive Adhesives

The performance of pressure-sensitive adhesives (PSAs) depends on the bulk viscoelastic properties of the adhesive material itself and on the surface with which it is placed into contact. In this work, a probe technique was used to quantify the adhesion and to develop a protocol to interrogate these bulk and interfacial effects. Model acrylic emulsion-based PSAs with different acid contents were used. A rate- dependent work of adhesion was obtained from a simple probe-tack test, where the indenter was retracted from the adhesive layer at a fixed rate. For a given adhesive formulation, a universal relationship was obtained between the average stress under the indenter and an effective strain rate. Complementary information was obtained from a set of programed oscillatory tests designed to probe the strain-dependent properties of the adhesive. The results suggest that an adhesive failure criterion based on the stored elastic energy can be utilized to determine the failure strain, defining a critical strain energy that depends on both the adhesive and on the surface with which it is in contact. The adhesive performance is determined by this critical strain energy and by the stress-strain rate relationship for the adhesive. Application of these concepts to adhesive design was demonstrated by the construction of an optimized two-layer adhesive.