On the Origins of Sudden Adhesion Loss at Critical Relative Humidity

Adhesion loss due to environmental factors is a critical fundamental problem in the adhesive industry. Many adhesives exhibit catastrophic failure when the environment exceeds a critical relative humidity. The physical mechanisms of this adhesion loss at a critical humidity, which marks the onset of discontinuous decrease in adhesion properties, are still highly debated despite the wealth of data in the literature. In this work to elucidate the fundamental mechanisms of adhesion loss at a critical relative humidity, detailed characterization of the moisture distribution within the film using neutron reflectivity and the adhesive strength using a fracture mechanics approach are used in tandem. We reveal a change in the interfacial characteristics that correspond with the discontinuity in the bulk sorption near the critical relative humidity. A discontinuity in the bulk sorption occurs at the critical relative humidity, but a shift in the width of the interfacial moisture concentration is also observed at this same humidity. To explain these changes in the moisture distribution within the adhesive film, we propose a mechanism for the critical relative humidity based upon stress accumulation at the polymer/substrate interface.