Browsing by Subject "Drying"

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    Flow and Drying Dynamics in Gravity- and Capillary-Driven Coating Processes
    (2017-06) Lade, Robert
    Liquid-applied coatings are ubiquitous. Buildings, bridges, soda cans, compact discs, and newspapers make up a small fraction of everyday objects whose surfaces are enhanced by coatings. Typical processing steps for a liquid-applied coating include coating formulation, application, post-deposition flow, and solidification. This thesis focuses on the balance between the last two steps of this process and how this balance influences coating behavior and the ultimate quality of the final film. Specifically, post-deposition coating flows driven by gravity or capillarity are investigated in liquid systems that undergo evaporation-induced drying. In Chapter 2, coating defects caused by excessive gravity-driven flow (‘sag’) are studied. A novel particle tracking method is first developed to monitor sag in a model aqueous polymer system. A computational model is developed concurrently to validate the measurements made using particle tracking. This model is then used to generate a novel framework for predicting sag in liquid-applied coatings. Chapters 3–5 focus on capillary-driven flows in open microchannels. First, in Chapter 3, capillary flow dynamics of non-evaporating liquids are studied and compared against existing theoretical models. In Chapter 4, this work is extended to open microchannels fabricated using several three-dimensional (3D) printing technologies. 3D printed microchannels are found to confer unique flow dynamics to the capillary flow, including a distinct start–stop motion caused by surface roughness introduced by the 3D printing process. Finally, in Chapter 5, the influence of drying on capillary flow dynamics is investigated, again using a model aqueous polymer coating system. Drying is found to permanently pin the advancing contact line partway down the channel; three mechanisms of pinning are identified and characterized. Post-pinning flows induced by the coffee ring effect are found to lead to highly non-uniform dry film morphologies. The influence of surfactant, drying rate, and channel width are investigated. Throughout all of this work, the goal is to better understand the balance between flow and drying to facilitate prediction and control of coating behavior during relevant coating processes. As part of this goal, case studies are conducted throughout this thesis, investigating flow and drying behavior in real systems used in commercial coating processes, including latex paints and functional inks used in the manufacture of printed electronic devices.
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    Understanding particulate coating microstructure development.
    (2010-09) Roberts, Christine Cardinal
    How a dispersion of particulates suspended in a solvent dries into a solid coating often is more important to the final coating quality than even its composition. Essential properties like porosity, strength, gloss, particulate order, and concentration gradients are all determined by the way the particles come together as the coating dries. Cryogenic scanning electron microscopy (cryoSEM) is one of the most effective methods to directly visualize a drying coating during film formation. Using this method, the coating is frozen, arresting particulate motion and solidifying the sample so that it be imaged in an SEM. In this thesis, the microstructure development of particulate coatings was explored with several case studies. First, the effect of drying conditions was determined on the collapse of hollow latex particles, which are inexpensive whiteners for paint. Using cryoSEM, it was found that collapse occurs during the last stages of drying and is most likely to occur at high drying temperatures, humidity, and with low binder concentration. From these results, a theoretical model was proposed for the collapse of a hollow latex particle. CryoSEM was also used to verify a theoretical model for the particulate concentration gradients that may develop in a coating during drying for various evaporation, sedimentation and particulate diffusion rates. This work created a simple drying map that will allow others to predict the character of a drying coating based on easily calculable parameters. Finally, the effect of temperature on the coalescence and cracking of latex coatings was explored. A new drying regime for latex coatings was identified, where partial coalescence of particles does not prevent cracking. Silica was shown to be an environmentally friendly additive for preventing crack formation in this regime.