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The mechanisms of failure over time of a series of model acrylic pressure-sensitive adhesives under a moderate level of stress has been investigated with a probe method. Two competing mechanisms of failure have been observed: a progressive nucleation of cavities under stress and the propagation of existing cavities at the interface between the probe and the adhesive layer. Homogeneous creep of the adhesive was never observed as the only failure mechanism. In situations where the resistance to crack propagation was good relative to the resistance to cavitation, extensive nucleation of cavities was observed until a material-dependent and stable value of stress was achieved. On the other hand in situations were the resistance to crack propagation was weak, propagation led invariably to a complete failure of the adhesive bond. In addition to the stress relaxation, the energy dissipation was studied allowing to distinguish the different adhesives even further. This allowed determination of the optimal amount of a comonomer (acrylic acid) that had to be added to improve the long-term resistance of the adhesives under study. Further more we investigate the compliance of the confined adhesive layers and compare the obtained results to predictions from theoretical models.

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Abstract:The paper addresses techniques for checking the performance of rotational rheometers with coneplate, plate-plate, or concentric cylinder geometry. We focus on the determination of the viscosity as a function of the shear rate and of the magnitude of the complex viscosity as a function of the angular frequency. After summarizing the relevant definitions and test modes, we show examples of measurements in the linear viscoelastic range, and applications of the Cox-Merz relationship. Sources of reference fluids with defined viscosities are presented, and their use in tests for verification of accuracy is demonstrated. Relevant issues, predominantly for Newtonian reference liquids, are the exploration of measurement limits, related either to the shear rate range or to reliably accessible viscosity levels. Viscoelastic reference samples are also discussed. Prerequisites for sample preparation and loading are addressed. In particular, we present recommendations based on experience from various laboratories. Finally, we discuss the problem of temperature calibration, presenting techniques that allow the determination of the true sample temperature for a given set temperature of the rheometer. This paper summarizes contributions from various industrial and academic laboratories.