Metal Finishing Guide Book


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cleaning, pretreatment & surface preparation VAPOR DEGREASING WITH CHLORINATED SOLVENTS BY JAMES A. MERTENS THE DOW CHEMICAL CO., MIDLAND, MICH. Degreasing is an essential part of the modern production process, particularly in industries fabricating or assembling metal parts including aircraft, appliance, automotive, electronics, and railroad manufacturers. The process is widely used to remove oils and oil-borne soils, such as chips, metal fines, and fluxes, from objects that have been stamped, machined, welded, soldered, molded, or diecast. Parts ranging from tiny printed-circuit assemblies to diesel motors can all be safely, completely, and quickly cleaned with modern vapor degreasing techniques, in most cases more effectively than with aqueous or semiaqueous processes, particularly when a high level of cleanliness is required. Vapor degreasing is ideal for reaching into small crevices in parts with convoluted shapes to remove stubborn soils. Parts degreased in chlorinated solvent vapors come out of the process dry; there is no need for an additional drying stage, as in aqueous and semiaqueous processes. The chlorinated solvents trichloroethylene, perchloroethylene, and methylene chloride are the solvents most commonly recommended for the vapor degreasing process for the following reasons: • The chlorinated solvents have high solvency for organic materials, as well as good chemical compatibility with a wide range of materials (e.g., metals, glass, plastics, elastomers, etc.), so that parts consisting of several materials can be effectively cleaned in these solvents. • They are virtually nonflammable in most end-use conditions (consult MSDS) because they have no flash point as determined by standard test methods. • They have low latent heat of vaporization, resulting in relatively low energy requirements. • They have relatively high stability and are noncorrosive. Further, vapor degreasing grades of the chlorinated solvents contain stabilizing additives that prevent the buildup of corrosive material and inhibit the formation of oxidation products. • Because the vapors of the chlorinated solvents are heavier than air they can be contained relatively easily in degreasing equipment. • The chlorinated solvents can be readily recycled, thus reducing waste and permitting high solvent "mileage" in a continuous degreasing operation. • The latest generation of vapor degreasing equipment permits fully enclosed operation, thus virtually eliminating the loss of solvent vapors to the environment. PRINCIPLES OF VAPOR DEGREASING The traditional vapor degreasing process is carried out in either a batch or an in-line degreaser. The standard batch degreaser is an open-top tank into which the dirty parts (the "work") are lowered (see Fig. 1). Solvent in the bottom of 106

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