Microencapsulation by a Solvent-Free Supercritical Fluid Process: Use of Density, Calorimetric, and Size Analysis to Quantify and Qualify the Coating

Coating of microparticles or microencapsulation is a widely used operation in which a thin layer of a coating agent is deposited onto a solid particle. Currently, the technique faces two challenges: being solvent-free and being applicable for coating particles smaller than 80 microns. In this work, several techniques are used to test the feasibility of a new solvent-free coating process using supercritical fluid technology. Some model microparticles, glass beads, and an active compound (AC) are coated with Precirol®, which is a mixture of fats. The process involves two steps: first supercritical CO₂ is dissolved in molten Precirol®, then the melt solution is sprayed onto the host particles by a rapid expansion. The particles coated in this way are examined by particle size analysis, environmental scanning electron microscopy (ESEM), and IR spectrophotometry. Also mass ratios of host particle to coating are evaluated using both helium pycnometry to determine sample density and calorimetry to measure the heat of fusion of the coating agent. The results show that these analytical techniques can be used for qualitative analysis as well as quantification of the coating. Thus they give relevant information on reproducibility and feasibility of the new process.