Process Limitations

The principal limitations of the vapor degreasing process are related to the materials it can clean without damaging effects and the soils it can remove effectively. Size and shape of workpieces, quantity of work, and degree of cleanness obtainable may also limit the applicability of vapor degreasing, but to a lesser extent. Normally, these variables merely determine the degreaser design selected.

Materials. All common industrial metals can safely be degreased with a minimum of difficulty, provided the chlorinated solvent is properly stabilized for vapor degreasing and the degreaser is properly operated. Iron parts are more susceptible to rusting after degreasing, especially in humid atmospheres.

Compatibility with Nonmetals. Some chlorinated solvents attack rubber, plastics, and organic dyes; this must be considered when degreasing assemblies with both metallic and nonmetallic components. Trichlorotrifluoroethane and 1,1,1-trichloroethane are less aggressive to many nonmetallic parts and have been the preferred solvents for these assemblies.

Solvent Stability. Vapor degreasing solvents can be decomposed, resulting in hydrogen chloride gas. This gas is very irritating, toxic, and corrosive to metals. Sources of solvent decomposition include:

• Exposure to surfaces hotter than about 175 °C (350 °F)

• Prolonged exposure to metal fines (particularly aluminum)

• Excessive soil accumulation in the boiling chamber

• Excessive and prolonged exposure to water

• Contamination with aluminum or iron chloride salts

• Exposure of the liquid or vapor to ultraviolet light

The vapor degreasing solvents have variable resistance to decomposition under the various conditions above. Trichlorotrifluoroethane is the most inherently stable of the group. Stabilizers or inhibitors are added to these solvents especially for this use. Solvent products made for other uses are likely to be insufficiently stabilized for the rigors of vapor degreasing. With proper stabilization of the degreasing solvent and good operating and maintenance practices, solvent stability is essentially secured.

Quantity of work to be processed is not a significant factor when considering the use of vapor degreasing, so long as the equipment was designed to mechanically handle the workload and has sufficient heat input. Available units range from those that are suitable for occasional cleaning of a few parts to completely automated installations geared to high-production operations.

Degree of Cleanness Obtainable. Under normal operating conditions, vapor degreasing provides a degree of cleanness that is suitable for subsequent polishing, passivating, assembly, phosphating, or painting. However, when parts are to be electroplated or subjected to other electrochemical treatments, vapor degreasing is seldom adequate and must be followed by another cleaning operation, such as electrolytic alkaline cleaning. Vapor degreasing is used immediately preceding the alkaline cleaners to remove most of the soil, thus prolonging the life of the final cleaners.

Radioactive and water-break testing techniques have indicated that a degree of cleanness between 0.1 and 1.0 monomolecular layers of soil is attainable in vapor degreasing. Under normal operating conditions, the degree of cleanness is usually near the upper level. Surface condition and section thickness may affect the degree of cleanness obtainable by vapor degreasing. For example, a polished surface is easier to clean than a grit-blasted surface. Thin sections receive less cleaning action than heavy sections, because the former equalize in temperature with the vapor zone in less time.

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