Investigation of Capillary Suspension Drying Behavior with Simultaneous Stress and Weight Measurements

Abstract

Coatings and ceramic green bodies usually contain polymeric additives to prevent cracking caused by drying induced stresses, which requires subsequent removal steps. A new coating formulation using capillary suspensions was found to reduce cracking, while being absent of polymeric additives. This dissertation investigates the drying behavior of capillary suspensions. The cantilever deflection method, commonly used for investigation of drying stresses, is significantly improved. The simultaneous stress and weight measurement technique of the same coating in a temperature and humidity controlled chamber developed in this dissertation is unprecedented. Enhanced by visual observation and coating profiling, we propose liquid flow considerations to explain the observed prolonged constant drying period. Stress measurements performed at different drying conditions and suspension formulations show a decrease in peak stress by up to 40\% for the capillary suspension over the regular suspension without polymeric additives. A complex interplay between yield stress, capillary suspension formulation, and humidity that is capable of reducing the peak stress of a regular suspension is revealed. The role of water, an integral part of most capillary suspensions, was further investigated. We find that capillary suspensions can be formed from partially miscible liquid-liquid systems at low added fluid concentrations ($\mathrm{\lesssim \, } $4 vol%), just outside the miscibility gap, by adding particles followed by mixing. Even when stored in humid conditions, capillary suspension formation is observed. These findings have an effect on undesired flocculation of formulations when stored in a humid environment, but also impact stress reduction during drying.