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Supercritical fluid cleaning - McHardy J.

McHardy J., Sawan P.S. Supercritical fluid cleaning - Noyes publications, 1998. - 304 p.
Download (direct link): spercrificalfluidcleaning1998.pdf
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Most instruments are composed of a wide variety of materials. Metallic components are fabricated of beryllium alloys, copper alloys, 300 and 400 Series stainless steels, steel alloys, aluminum alloys,
magnesium alloys, magnetic alloys and various superalloys. Polymeric components include thermoset polymers, thermoplastics, elastomers and inorganic and organic protective coatings. In addition to metallic and polymeric components, there are a significant number of ceramic materials such as glass terminal seals, optical components, and bearings.
These instruments require contamination-free surfaces during fabrication so they may operate reliably for extended periods in the harsh military and space environments. This places extreme burdens on the cleaning processes used both during their manufacture and prior to their deployment. Any cleaning process must be compatible with the wide variety of materials as well as being effective at removing a wide variety of contaminants such as hydrocarbon, ester, and fluorocarbon lubricants, handling debris, damping fluids, and process residuals. In addition, the effectiveness of cleaning processes must be confirmed as each process is performed. Compatibility testing is of utmost importance, since the assembly and test of instruments can require many months, and the life of the instruments can be several years.
In summary, the varied nature of substrates being cleaned and the contaminants present, and the required reliability of military and space hardware has led to the development and use of a wide variety of alternative materials and processes rather than a single ODS replacement solvent or process. Supercritical fluid is one of these alternative processes and the rest of this chapter discusses the trials and tribulations associated with implementation of supercritical fluid as a production cleaning process.
Supercritical fluids have long been known for their abilities to dissolve organic contaminants. Their ability to display a wide range of solvent characteristics and the ability to tune solubility with small changes in temperature and pressure were identified early on in our search for alternative cleaning methods. The gas-like diffusivity and low surface tension combined with liquid-like densities were important
since these qualities would enhance the cleaning effectiveness on parts which had small pores and crevices which could trap contaminants. Another important advantage was the ability to use the temperature and pressure solubility dependence of supercritical fluid extraction to capture and condense contaminants. One of the primary contaminants found on instrument parts is the damping fluid itself. During the instrument filling process, a large variety of tooling and fixturing can be exposed to these highly viscous, expensive fluids. Previously, these fluids would be removed using CFC-113 and then reclaimed from the solvent washing process. The original idea was that supercritical fluids could be used to remove damping fluid and that the damping fluid would be deposited in the separator of the system while the supercritical fluid would return to the gaseous state and exit the system. This would eliminate the added cost associated with reclaiming the damping fluid from the solvent rinsate.
Perfluoropolyether fluids (ex., Krytox™) are commonly used as lubricants, dielectric fluids or heat transfer fluids in a number of space and military products. These fluids and other perfluorolubricants are almost impossible to dissolve in solvents other than halocarbons. Additionally, other halogenated damping fluids have compatibility problems with some of the common solvents which can dissolve the fluid. Early experiments with supercritical carbon dioxide indicated that it would dissolve both halogenated damping fluids and perfluoropolyether lubricants and thus was a candidate as a replacement solvent cleaning process.
Another key area where supercritical fluid was identified as a possible solution was end housing cleaning. End housings are common components of many guidance instruments and are typically constructed of a number of laminated metal cores wound with wire and then impregnated and potted with epoxy into a metal shell. The construction of end housings creates a large number of microscopic voids and crevices which can trap small quantities of damping fluid that are very difficult to remove. The previous process for cleaning these parts consisted of numerous cycles of a process known as vacuum pressure cleaning using CFC-113. In this process, the end housing is alternatively exposed to pressure and vacuum while submerged in CFC-113. The process is repeated until damping fluid was
no longer visibly weeping from the voids. This was an extremely time-consuming process and it was believed that the high pressure and high diffusivity of the supercritical fluid would provide an effective method for removing the small quantities of fluid which were trapped in the end housings. Also, by controlling the density of the supercritical fluid through pressure changes, it was believed that the fluid could be pumped out of these small pores.
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