Measuring and predicting sealant adhesion

Abstract

Current sealant test methods do not lend themselves well to the measurement and prediction of sealant performance. T he objective of this work was to demonstrate that measurement of sealant material properties as a function of temperature and relative humidity for a specific sealant and substrate can be used to predict the long term performance of sealant joints independent of the joint geometry.

The material properties of a silicone sealant were characterized as a function of temperature, relative humidity and test rate. The fracture energy of the silicone sealant bonded to glass, aluminum or stainless steel was measured with the 180° peel, the 45° peel and the pure shear butt joint test methods. Scanning electron microscopy, electron spectroscopy for chemical analysis and atomic force microscopy were used to analyze the failure surfaces.

The failure mode for silicone sealant on aluminum changes from near the aluminum oxide layer to cohesive in the sealant when the relative humidity is less than 35%. The temperature shift factor was not related to the WLF theory but worked well with an Arrhenius theory. The activation energy for the fracture process was 31.2 kcal/mol.

Crack growth data for an expansion joint was calculated for the weather conditions in Miami, Florida and Wittman, Arizona using the climate data obtained for the year 1994. For both climates, the nearly all the crack growth occurred during the winter months. The predicted annual crack growth data for Miami was 0.7 mm and the predicted annual crack growth for Wittman was 3.7 mm.

The fracture energy of the silicone/stainless steel joint is proportional to the amount of PDMS left on the surface. Carbonaceous contamination is not displaced by the sealant. Contamination inhibits primary bonding (ionic or covalent) and results in a lower fracture energy. Roughness increases the fracture energy; and this effect is more pronounced when the surfaces are cleaner. Water lowers the fracture energy especially for the dirtier surfaces. Roughness reduces the effect of the water damage but doesn’t eliminate it.