(70ct) Coating of Ultra-Fine Particles Using Supercritical Fluids

Nanosized particles have a wide range of industrial applications. In certain applications, particle surfaces need to be modified or functionalized by coating them with another material to serve a specific purpose. As nanoparticles are extremely cohesive, it is very difficult to coat an individual particle by traditional methods. In this paper, nanoparticle coating is investigated through supercritical fluid-based methods. Agglomeration of particles is reduced by combining two techniques. The first approach uses a proprietary co-axial ultrasonic nozzle to spray the solution suspension into the SC CO2. Ultrasound is very effective in breaking agglomerates and the introduction of the co-axial flow enables CO2 to not only serve as an antisolvent, but also as mixing enhancer. The second approach uses a combination of solvents to tune the supersaturation of the polymer which serves as the coating material.

A PMMA film is coated on different kinds of silica and titanium dioxide particles; 20 nm, 120 nm and 600 nm sized silica and 20 nm sized titanium dioxide particles are first suspended into the polymeric solution. The polymer solution with suspended nanoparticles is then sprayed into supercritical CO2. The solvent is extracted into supercritical CO2 precipitating a PMMA film on the silica surface; 127 µm and 254 µm capillary tubes are used along with the ultrasonic nozzle and the results are compared.

Various imaging techniques such as FE-SEM (field emission scanning electron microscope), TEM, EELS (electron energy loss spectroscopy), and EDS (energy dispersive x-rays) are used to characterize the coating of particles. Photon correlation spectroscopy is used to measure the size distribution of particles before and after precipitation. TGA and DSC techniques are used to study the amount and thickness of coating.