Nanomechanical properties evolution of thermally aged water-borne coatings

Abstract

Water-borne coatings are increasingly used in industrial and consumer applications because of their low volatile organic compound (VOC) content and reduced environmental impact compared with solvent-borne systems. However, their mechanical integrity and durability remain key challenges, particularly under cyclic thermal or environmental loading. Therefore, understanding how these viscoelastic properties evolve with thermal aging is essential for predicting the durability of these water-borne coatings. The effect of thermal aging on the mechanical and nanomechanical behavior of water-borne coatings was investigated using a combination of bulk and nanoscale characterization techniques. Based on industry standard tests, two accelerated aging protocols were performed, the cold check test and the water uptake and freeze stability test, to simulate cyclic thermal stress. The progression of viscoelastic material properties over multiple aging cycles has been investigated based on industry standards using Dynamic Mechanical Analysis (DMA), revealing an increase in the storage modulus with progressive aging. Two Atomic Force Microscopy (AFM) methods were employed to assess local mechanical changes using both on- and off-resonance AFM techniques. The AFM measurements showed a corresponding increase in stiffness at the nanoscale, along with the development of surface cracks and other topographical features. Off-resonance AFM measurements on these features demonstrated that the local storage modulus, a measure of stiffness, in and around the surface cracks remained comparable to that of the surrounding matrix, whereas local cylindrical rods showed an increased storage modulus. These findings contribute to a better understanding of the mechanical and structural evolution of water-borne coatings under thermal cycling, linking stiffening to nanoscale defect formation, surfactant removal, additional crosslinking, and oxidation, all of which reduce polymer chain mobility to increase stiffness. The observed changes underscore the coupled chemical, mechanical, and morphological nature of the aging process in water-borne coatings. This thesis provides insights into thermally induced coating failure and coating degradation mechanisms, thereby potentially accelerating the development cycle of water-borne coatings.