Mesoscopic fibrillation properties of pressure sensitive adhesives based on latex films

Slow pulling experiments with atomic force microscope (AFM) tips were performed on industrial acrylic latices, where the Brownian motion of the tip was monitored in parallel to the static force. From the noise power spectra of the tip's thermal motion, one can infer the effective spring constant and the drag coefficient of the tip-sample system. The results from AFM pulling experiments correlate well with macroscopic tack tests performed on the same materials at comparable stress levels. The fraction of "successful pulls", meaning pulls where an extended loop of adhesion hysteresis forms, decreases with aging. Presumably, the internal cohesion of the material increases as film formation proceeds, such that the formation of cusps or tip-induced deformations of the surface becomes less likely. For the most tacky material, the formation of a continuous film was incomplete even after 6 months of storage. The successful pulls are characterized by a discrete number of steps in both static force and the spring constant. The steps are attributed to the rupture events occurring inside the film. Internal heterogeneity has an influence on the tack.