3D visualization of cavitation in a probe tack experiment on pressure-sensitive-adhesives

Pressure-Sensitive-Adhesives(PSA) have viscoelasticity and are able to bond two surfaces only by contact. The adhesion energies measured in experiments are much larger than thermodynamic work of adhesion, which is derived from interfacial free energy, because the rupture occurs through the nucleation and growth of cavities and the formation of fibrils. It is known that the cavitation process is largely-concerned with performance of PSA, but observations of cavitation have been done only from beneath glass substrate and 3D view of cavitation hasn't been captured yet. We invented new experimental system to observe the cavitation in PSA as shown on Fig.l. In our experiments, we prepared acrylic PSA cross-linked at different densities and applied them to attain 190 μm thick. Then, the probe was brought into contact with the PSA and removed at 1μm/s. By these experiments, we succeeded in obtaining the pictures of cavities as shown on Fig. 2 and Fig. 3. The latter was more cross-linked than former. In less cross-linked case, cavities became spherical and grew large to get into touch with glass substrate. In contrast, in more cross-linked case, many smaller and flatter cavities appeared and coalesced before long. Based on these results, we concluded that the stress concentration at the edges of cavities was attributed to the nucleation of new cavities and to the coalescence of adj acent cavities.