Masking Film Composition and Method of Masking an Article Using Same

This application is related to co-pending application Ser. No. 641,437, filed Jan. 15, 1991, entitled "Apparatus and Method for Liquid Treatment of Article Surfaces," and to copending Ser. No. 07/708,148, filed May 31, 1991, now abandoned, entitled "Method of Masking a Bore Surface During Liquid Treatment of a Component Containing the Bore," both filed in the name of the present inventor, and both assigned to the assignee of the present application, and both such applications being incorporated herein by reference.

This invention relates to compositions and methods for masking portions of articles, especially metal articles, to be provided with surface coatings, such as paints, lacquers, enamels and other protective coatings, surface treating solutions and the like. Masking films prevent adherence of coating materials to the coated substrate and thereby provide a convenient means for obtaining desired functional treatment or visual design patterns.

The advantages of using a masking film in the coating industry to control unwanted application of coating media to selected metal and other surfaces have long been recognized.

Such advantages include, but are not limited to, coating cost reduction, for example, by using thin, low viscosity film-producing materials which are easily applied, e.g. by dipping or spraying, and which repel adhesion of applied coatings but which are readily removed after the coating operation has been completed.

A major problem which has been encountered with use of prior art masking films for dip coating processes is the tendency for highly viscous petroleum products-containing films to wash off during vigorous dip coating cycles, with consequent contamination of the coating product. Such wash-off effect can be eliminated by increasing the viscosity of the masking film; however, this also increases resistance of the film to removal after coating. Solvents which aid in removing low viscosity films also tend to remove the desired protective coatings. Examples of prior art masking films and their shortcomings include:

Therefore, it is an object of this invention to provide a masking film composition and method of using the same wherein the film has low viscosity that will resist undesirable the coating process and which can be removed easily in a mild solvent while having no effect on an applied dip coating.

In accordance with such object, a masking film composition of the present invention comprises only two readily available and relatively inexpensive ingredients, i.e. petrolatum and fumed silica powder.

Masking methods using this composition provide the advantages of conventional light masking films while providing ease of application and removal and reducing undesired wash-off characteistics of materials previously used for such purposes.

Petrolatum is a mixture of hydrocarbons refined from petroleum. For purposes of this invention, petrolatum of USP grade is useful. Such material is obtainable, for example, from Sonneborn Division, Witco Corp., Petrolia, Pa. 16050. This material is an odorless, opaque white semi-solid of 0.815-0.880 specific gravity, having a melting point of 100.4.degree.-140.0.degree. F. and flash point between 199.degree. and 390.degree. F. as determined by ASTM-D-92 test.

Fumed, or colloidal, silica is an amorphuous silica in the form of a fluffy white, odorless and non-flammable powder which has a specific gravity of 2.2 and is non-soluble in water. Such material can be obtained, for example, from Wacker Chemie GmbH, Munich, Germany or its U.S. affiliate, Wacher Chemicals (USA), Inc. (grade designation "Fumed Silica HDK H 20"), or from Cabot Corp., Billerica, Mass. 01821 ("CABOSIL," a trademark of Cabot).

The compositions of this invention have good shape retention (fixtropy) when applied as masking films to surfaces such as metal surfaces to be painted or treated with other protective coating or treated with other oil- or water-based materials. Such masking film products advantageously may have compositions within those ingredient range of the following Table showing good shape retention:

The Table 1 composition having the lowest silica range is smooth, but it does not have sufficient body or viscosity to retain its shape or to press into small holes or other voids, and it will "wash out" during the dip process, resulting in poor or no effective masking action.

Excessively high proportions of silica will cause the mixture to become too dry for easy application; the material is nhot smooth, must be forced into small holes and other voids, and requires more rigorous cleaning action during removal of the masking film.

On the other hand, the compositions showing "ideal range" provides good shape retention, smooth flow and easily flows into small holes and other voids in the surfaces being masked.

Accordingly, a volume ratio of silica to petrolatum of more than 1:12 and especially about 1:4 to about 1:2.5 is preferred.

When the two ingredients of this composition are mixed, the resulting product is of a thick grease-like consistency with high fixtropy. When applied to, e.g., metal surfaces, as a masking film, the composition retains its shape, coats evenly and repels various dip coatings.

After the dip coating medium has adequately dried, the masking film is easily removed by applying to the coated article a suitable remover, for example by spraying or immersion of the coated article Various petroleum-derived hydrocarbon distillate fractions are useful for such purpose. However, it is important to select a remover which is not so aggressive in solvent action as to attack to protective coating and yet which has sufficient solvent strength to remove the masking film. An example of a too aggressive solvent is petroleum-based paint thinner. On the other hand, mineral spirits is not strong enough in solvent action to remove the masking film.

The products those commonly referred to as naphthas or solvent naphthas, are useful for such purpose, for example, the mild solvent known as V.M & P. Naphtha. Such product can be obtained, for example, from Piedmont Paint Manufacturing Division, Whittaker Corporation, Greenville, S.C. Such product has a boiling point of 240.degree.-285.degree. F. at 760 mm Hg; a vapor pressure of about 15 mm Hg at 100.degree. F., a specific gravity of 3.8 (air =1.0), is composed of 100% volatiles and has an evaporation rate of 9.20 (ether =1.0). On treating the dried protectively coated article with such masking film removers, the masking composition is effectively removed into suspension in the naphtha, and there is no measurable effect on the coating medium.

The masking film compositions of the invention are particularly useful in connection with the dip coating of metal surfaces with paints. For example, my pending U.S. application Ser. No. 641,437 for "Apparatus and Method for Liquid Treatment of Article Surfaces," filed January 15, 1991, describes an apparatus and method for paint-coating articles by rotating the articles in two axes of roatation for more effective coating, particularly of metal articles such as castings and worked equipment components having a plurality of cast or drilled internal passageways which otherwise can be coated only ineffectively and with great difficulty. My copending U.S. patent application for "Method of Masking a Bore Surface During Liquid Treatment of a Component Containing the Bore," more particularly describes a masking plug for use in the coating of such articles having difficultly-coatable internal passageways, for example, railroad brake apparatus. As described therein, a masking film is especially useful for coating the outer surfaces of such masking plugs so as to seal small gaps purposely provided between such outer plug surfaces and the surfaces of internal passageways into which the plug is inserted to mask the passageway against unwanted entry of coating medium.

The compositions of the invention also may be usefully applied to various plastic surfaces to be masked against deposition thereon of coatings applied thereto.

The inventive compositions may be applied by hand, since they are non-corrosive and otherwise generally environmentally safe in case of human contact. They also may be applied by brush or by roller in the case of large surfaces to be masked.