Distribution of latex during drying in confined geometries, 2010 TAPPI Advanced Coating Fundamentals Symposium

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Latex is the most expensive component in a coating formulation. Therefore, the efficient use of latex is critical to reduce costs, improve the “green” aspects of paper production, and reduce cost fluctuations. Dilute model latex suspensions with up to 7.5 weight percent latex were placed in confined geometries and dried by evaporation. The geometries were comprised of spherical or cylindrical glass lenses placed on a glass cover slip forming a point or line contact. Drops of la-tex suspension were placed in the contact region. Images during drying and after drying were obtained with an optical microscope and video camera. The influence of latex type, surface con-tact angles, latex concentration, and glycerin additives were characterized. A mechanism is pro-posed to explain the characteristic patterns that form after drying. Instead of latex being pulled into the contact as the meniscus retreats, most of the latex is left behind in distinct “finger” pat-terns, far from the contact point. Little or no latex reaches the contact even for the highest con-centrations studied. Some images of the fingers during evaporation reveal that they are wet, pos-sibly increasing the local evaporation rate, thus increasing latex transport to the growing fingers. Increasing contact angles of the glass surfaces inhibited finger formation and resulted in more latex transport to the contact point. High contact angles cause reduced evaporation near the three phase contact line and reduce transport of latex to the contact line. Addition of small amounts of glycerin initially improved ordering of the fingers but, at higher concentrations, ultimately stopped their formation, possibly by accumulating in the fingers, stopping the enhanced evapora-tion. Implications for real paper coatings are discussed.