Coating of Fine Particles with Stearic acid Using Rapid Expansion of Supercritical Carbon Dioxide

Abstract

Objective: The rapid expansion of supercritical solution (RESS)using carbon dioxide without cosolvent was applied for coating fineparticles of corn starch with stearic acid. Methods: The coatingprocess was performed by introducing supercritical carbon dioxide(ScCO2) into a vessel containing mixture of corn starch and stearicacid for 40 min, then depressurizing the fluid after 20 min ofequilibration. The products were characterized using scanningelectron microscopy (SEM), particle size analysis, vapour sorptionanalysis, differential scanning calorimetry (DSC) and Fouriertransform infrared (FT-IR) spectroscopy. Results: The resultsshowed that coating of corn starch particles with stearic acid couldbe obtained by this process using ScCO2 at temperature of 60 °Cand pressure of 3000 psi. Deposition of the coating material onto thecore particles occurred during depressurization of supercriticalsolution. The coating was evidenced by an increase in size of coatedcorn starch particles with increasing proportions of stearic acid in themixture. DSC thermograms showed melting endotherm of stearicacid at 57 °C. FT-IR spectra exhibited combined absorption bands ofboth starting materials which implied the presence of unchangedstearic acid and corn starch in the coated products. Vapour sorptionresults showed lower percentage of mass increase of coated cornstarch, as compared to that of uncoated one, indicating the presenceof protective layer of stearic acid. SEM images confirmed coating ofcorn starch particles without agglomeration of the coated particles,which was achieved at corn starch to stearic acid weight ratio of2.95:0.05. Conclusion: Film formation of stearic acid on corn starchparticles could be accomplished via rapid expansion of supercriticalsolution (RESS) process without organic solvent.Keywords: fine particle coating, rapid expansion of supercriticalsolution, corn starch, stearic acid