Stress and microstructure development in particle-based coatings

Abstract

Particle-based coatings have a wide range of uses and applications in everyday life. Stress development during the drying process has the potential to impact the performance of the coating. Stress development can be monitored in-situ using a cantilever deflection technique with a laser-photodiode combination. Stress development in the film is directly related to the development of the coating microstructure during drying. Cryogenic scanning electron microscopy (cryoSEM) is a powerful characterization method capable of visualizing the microstructure of the coating during the intermediate stages of drying. Using this method, the coating is frozen to arrest microstructure development and solidify the sample so that it can survive the high-vacuum environment of the SEM. This thesis explores the connections between stress and microstructure development in particle-based coatings during drying. Characterization is often complicated by lateral drying, a common phenomenon in particle-based coatings. To avoid these complications, walled substrates were developed which are used to suppress lateral drying and promote drying uniformity. CryoSEM revealed that latex coatings dried on substrates (with photoresist walls) exhibit a greater degree of drying uniformity. Silicon cantilevers with poly(dimethyl siloxane) (PDMS) walls along the perimeter were used to suppress the effects of lateral drying during stress measurement. The walled cantilevers were used to characterize stress development in ceramic particle coatings and latex films. For the ceramic particle coatings, stress measurements were combined with cryoSEM revealing the origins of stress development in hard particle coatings. Stress development was correlated with the extent of drying and the degree of saturation in the coating. Stress development in latex particle coatings was influenced by the composition and morphology of the latex particles. Additionally, the influence of coalescing aids on stress development was also investigated. The film formation behavior was studied using a variety of techniques including AFM, cryoSEM, and minimum film formation temperature (MFFT) measurements.