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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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Precision Cleaning With Supercritical Fluid: A Case Study
John E. Giles, Jr. and Robert G. Woodwell, Jr.
1.0 INTRODUCTION
Many of the commonly used solvents for precision cleaning are being eliminated due to their suspected involvement in reduction of the earth’s ozone layer. Production of these chemicals, known as ozone depleting substances (ODS), is being eliminated by an international treaty known as the Montreal Protocol. This is an international agreement, first proposed in 1987 and entered into force in 1989, which limits production of chlorofluorocarbons (CFCs) and halons due to concerns that these substances were damaging the earth’s ozone layer. The Montreal Protocol was modified in 1990 and again in 1992 to completely eliminate the production of chlorofluorocarbons, carbon tetrachloride, methyl chloroform (1,1,1 -trichloroethane) and halons by 1996.
In conjunction with the Montreal Protocol, the Clean Air Act of 1990 further defined the production and use of these ozone depleting materials. This act also created a program to identify alternatives to Class I and Class II ozone-depleting substances and to publish a list of acceptable and unacceptable substitutes. This is known as the Significant New Alternatives Policy or SNAP. Under the SNAP rules, it is illegal to replace a Class I or Class II substance with an unacceptable substance or process. Supercritical fluids are acceptable alternatives for solvent applications under the SNAP rules.
CFC-113 is a Class I substance which has been used for many years in the precision cleaning industry. The area of precision cleaning encompasses those areas in which a high level of cleanliness is necessary to ensure proper operation of precise mechanisms. Several key factors which may define precision cleaning applications are: (i) critical cleanliness standards for particle levels and/or organic (molecular) contaminants, (ii) sensitive material compatibility, (Hi) physical characteristics such as geometry or porosity which makes dewatering difficult in the cleaning process and (iv) relatively high cost of components being cleaned. Examples of some precision components include precision mechanisms, gyroscopes, inertial guidance systems, precision bearing assemblies, space hardware, disk drive components, medical components and a wide range of similar items.
CFC-113 has many properties which make it an ideal solvent for these applications, including chemical stability, low toxicity, low viscosity, low surface tension, high density, low residues, fast evaporation rates and nonflammability. Chemical stability is important when cleaning precision instruments due to the wide variety of materials (metals, ceramics and polymers) which can be exposed to the solvent during cleaning operations. This is especially important in the field of precision inertial instruments due to the widespread use of beryllium and other active metals for lightweight, dimensionally stable components. Beryllium is extremely susceptible to corrosion from ionic materials such as chlorides, and CFC-113 provides a stable solvent for cleaning these components. Low toxicity and nonflammability are important since many cleaning operations are performed manually by the technicians in closed cleanroom environments. Low viscosity, low surface tension and high density are important for
removal of micron and submicron particles because of the high attractive forces between the particles and the substrates. High density also provides holding power for large amounts of dissolved organic compounds. The low residue of CFC-113, typically below 1 ppm for precision grades, ensures that there are no solvent residues left behind by the cleaning process which may inhibit bonding, affect system operation or cause future compatibility problems. The fast evaporation rate of CFC-113 prevents pockets of residual solvent being trapped in small porous surfaces or small crevices where time-consuming bake-outs would be required to remove the residual solvents.
Solubility properties of CFC-113 are also an important consideration. CFC-113 has the ability to dissolve a wide variety of halogenated and non-halogenated substances including hydrocarbon oils, fluorinated oils and greases, ester oils and silicone oils. Another important advantage of CFC-113 in the precision inertial instruments industry is in removal of damping fluids. These high viscosity fluids are expensive (greater than $ 15,000 per pound). Many are composed of materials such as polychlorotrifluoroethylene and polybromotrifluoroethylene which are not soluble or not compatible in common solvents. CFC-113 has a long history of usage in this application and is compatible with all damping fluids.
2.0 PRECISION INERTIAL INSTRUMENTS
Momentum control gyroscopes and precision guidance instruments are used to control and detect movement in satellites, space probes and platforms, airplanes and missiles. They are, composed of a number of extremely complex electromechanical systems with circuit card assemblies, wire windings, lamination stacks, ball, roller, and gas bearings, and potted components which contain small voids and crevices. Tolerances on these assemblies can be in the microinch range. These instruments can contain lubricants and the previously discussed damping fluids.
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