Industrial & Engineering Chemistry Research, Vol.43, No.2, 515-524, 2004
Deposition of thin polymeric films from liquid carbon dioxide using a high-pressure free-meniscus coating process
Free-meniscus coating processes can be used to deposit a wide variety of coatings. However, the physical properties of the coating solutions often lead to the deposition of nonuniform films. Recently, it has been recognized that compressed carbon dioxide can be used as an environmentally benign solvent for industrial processes. We investigate the use of liquid carbon dioxide as the solvent in free-meniscus coating processes because its physical properties are much different from standard coating solvents. The surface tension and viscosity of liquid carbon dioxide are an order of magnitude smaller than those of typical solvents. Additionally, the density of liquid carbon dioxide is strongly dependent on temperature and pressure. The Tallmadge four-force inertial theory is used to demonstrate that these unique physical properties will result in the formation of thinner films at the same withdrawal velocities as those used with conventional solvents. We then demonstrate experimentally that process variables can be controlled in a high-pressure coating chamber to deposit films of a perfluoropolyether lubricant with controlled thickness in the range of 25-350 Angstrom on a silicon surface with a native oxide. It is shown that the withdrawal velocity, polymer solution concentration, and evaporation rate can be used to control the coating process to produce submicron films on a surface.