11 - A Practical Guide to Supercritical Fluid Cleaning

Abstract

This chapter briefly describes supercritical fluid (SCF) cleaning, focusing on carbon dioxide (CO2) as a fluid. It is mentioned that SCFs offer a combination of liquid-like solubilities and gas-like transport properties which are superior to many liquids. The low cost and ease of recyclability of SCF CO2 make it an excellent choice for cleaning applications. A summary of common contaminants and their cleanability from various substrates using CO2 is given in the chapter by Spall and Laintz.It is mentioned that one of the key difficulties in parts cleaning of high technology components using conventional solvents is drying. Drying is also often a very energy intensive operation. Supercritical carbon dioxide has two advantages over liquid solvents for parts drying. First, a supercritical fluid has no surface tension, so no bubble or meniscus is formed during the drying operation. The concepts applied for understanding SCF cleaning are the same as that used to describe much of liquid chemical processing. The difficulties in working with SCF cleaning revolve around the fact that pressure vessels have to be used. Simple things like insuring that stirring is effective or knowing when the cleaning process is over become more difficult in this environment. However much higher pressure vessels are used in industry for supercritical carbon dioxide cleaning for over 60 years and equipment is available from several vendors for SFC cleaning. If given the choice of equipment today and its cost relative to conventional solvent based cleaning, the advantages of SCF cleaning make it very attractive compared to aqueous or organic based systems. New cosolvents, surfactants and microemulsion techniques, are supplementing capabilities of solvent tenability. Finally drying, one of the most difficult aspects of cleaning parts for critical applications is easily achieved with SCF.