Silicone Resin Coating of Micro-Sized Ferrite Particles Using Supercritical Carbon Dioxide

An environmentally friendly and efficient polymer coating method for micro-sized particles was developed using supercritical CO₂. Because this method used supercritical CO₂ as the solvent to dissolve the coating material, we avoided environmental pollution from organic solvents, saved the energy required to evaporate/remove organic solvents, realized a uniform coating film on the fine particles, and prevented agglomeration of the coating particles. The solubilities of the five silicone resins used as coating materials were measured using the flow method, and the data were well correlated by Chrastil's equation with an average deviation of 5.7%. Resins comprising numerous methyl-group side chains exhibited high solubilities and were suitable coating materials. A new semi-flow-type coating method using supercritical CO₂ was also developed, which deposited a film with a uniform thickness of 0.2-1.3 μm on whole fine particles. Notably, in this method, the film thickness was easily controlled. A simple and rapid technique was developed for measuring the coating thickness using X-ray fluorescence analysis. The model for calculating the coating film thickness was based on the material balance of the coating material. This model satisfactorily predicted the thickness with an average error of 0.085 μm by measuring the solubility of the coating material in supercritical CO₂, integrated flow volume of supercritical CO₂, particle diameter, density and charged weight of the fine particle, and coating material density.