The article is thus quickly cleansed by being continuously bathed in clean solvent at its boiling point. The oil and grease is not re-vaporized with the solvent. No solvent is consumed but care must be taken that it is not waste by being drawn out on degreased articles or by disturbance of the vapor, for it is quite expensive. Chlorinated hydrocarbon solvent vapors are dangerously narcotic if inhaled, and they also decompose to poisonous fumes if drawn through a flame. Solvent vapor degreasing is therefore controlled by statutory regulations, but these are not irksome in a well-conducted plant.
Solvent vapor degreasing alone does not produce a water wet table surface and it must be followed by a wet degreasing process. This process works through a combination of saponification, emulsification and change of surface tension, all based on the use of alkaline solutions and all in fact operative in domestic washing. Whereas acids, as a class, are characterized by a high concentration of hydroxyl ions, i.e. by a low pH number, alkalis are characterized a high concentration of hydroxyl ions, i.e. by a low concentration of hydrogen ions, and hence a high pH number. It is high concentration of hydroxyl ions which facilitates metal cleaning. Sodium and potassium hydroxides (soda and potash) are the strongest and harshest alkalis; sodium metasilicate (waterglass), sodium cyanide and sodium borate (borax) are milder alkalis.
Alkaline solutions attack and combine with the polar groups of greasy compounds to form soaps which are soluble in water. This action proceeds best in hot solutions, and is called ‘saponification’. Soaps, and also a large number of synthetic detergens, have ability to break up oil and grease into small droplets and to hold them suspended in aqueous solution, without much tendency to recoalesce or to deposit on the solid surface. They do this by coating the exterior of each oil drop with a monomolecular film of the detergent molecules. These molecules are of elongated shape, one end favor contact with water but the other end prefer to be in oil. Thus the molecules arrange themselves in the surface of the oil drops with the water favoring end outwards and the oil-favoring end inwards. This stabilizes the oil droplets. Soaps and other specially formulated synthetic detergents also have the property of changing both the surface tension and the interfacial tension of liquid.