The ability of water to dramatically weaken many types of adhesive bonds has been widely studied. One surprising result is the existence of a critical moisture level in the bond. Above this level the strength drops to very low values. Numerous studies have demonstrated that this is an interface effect, but they do not explain what determines the value of the critical concentration. Several papers have correlated a change in water sorption behavior for bulk adhesive with the critical concentration; however, we know that the water concentration near the interface can be quite different than that in the bulk adhesive. Consequently, it is important to compare bulk water content, interface water concentration, and adhesive strength. The work here uses neutron reflectivity to characterize the distribution of moisture near the adhesive-substrate interface at a resolution of tenths of nm. This is combined with sorption measurements on bulk adhesive and adhesive fracture measurements using the shaft loaded blister test. The reflectivity data show that there is a very thin region in the adhesive next to the interface where the water concentration is much higher than that in the bulk adhesive. Below the critical relative humidity for loss of adhesion strength, the average thickness of this water rich layer is one to two water molecules. For samples that are susceptible to moisture attack, this layer thickness significantly at the critical relative humidity. This thickening is not observed for material systems that do not show a sudden drop in adhesion strength with increasing water content.
Proceedings Title: Procedings Adhesion Society
Conference Dates: February 20-24, 2010
Conference Location: Daytona, FL
Conference Title: Adhesion Society Annual Meeting
Pub Type: Conferences
adhesion, durability, fracture, humidity, interface, reflectivity, sorption, water