Contamination and Substrate Material
Different contamination exhibits different chemical and physical properties and therefore has to be treated differently. Typical surface contamination includes oil- and aqueous-based cutting fluids, grease, wax, polishing paste, finger prints, fine metal particles and so forth.
Some of these soils show increased solubility in polar environments (water) while others in semi-polar organic solvents such as modified alcohols or in solvents with more unpolar characteristics like chlorinated or even hydrocarbon solvents.
For efficient dissolution and removal from the surface, it is necessary that the solubility parameters of the product are close to the solubility parameters of the contamination. The equal dissolves equal rule applies here. As a consequence, oil-based contaminations dissolve easier in oil-like (unpolar) solvents while polar contaminations like salts dissolve easier in polar (water-like) solvents.
Two characteristic properties are generally used to determine solvency power, the Kauri Butanol value and water solubility value. The former is used throughout the industry to determine the solvency power of unpolar, resin types of soil (higher values indicate higher solvency power). The latter is a measure of polarity and can be used to determine the ability of an organic solvent to dissolve polar contaminants.
The efficiency of the cleaning action is also influenced by the temperature at which the cleaning takes place. When cleaning certain contaminants such as polishing paste, high-melting waxes or zinc stearate, the hot solvent vapors may be able to soften or even melt the soils, making removal quicker.
After the contamination is removed, the solvent needs to be separated from the soil and recycled for re-use. This is done by distillation. It is important that the distillation is able to separate the soil and the solvent as effectively as possible since the quality of the recovered solvent will determine the final cleanliness of the part. This separation process is influenced by the boiling point of the solvent. Generally, the lower the boiling point of the solvent, the higher the degree of separation and subsequently, the better the cleanliness of the parts being treated.