US3579429A - Process for applying a white paint electrophoretically - Google Patents

Abstract

A WHITE PAINT FILM APPLIED TO AN IRON OR STEEL SURFACE BY THE ELECTROPHORETIC METHOD OF PAINTING TENDS TO BE OF A MUDDY, OFF-WHITE OR YELLOWISH COLOR; ALSO, FOREIGN DEPOSITS ON THE SURFACE TO BE PAINTED SHOW-UP IN THE WHITE PAINT FILM AS COLOR IMPERFECTIONS. THESE PROBLEMS ARE AVOIDED BY PRETREATING THE IRON OR STEEL SURFACE WITH AN AQUEOUS OLUTION OF COPPE TO APPLY THERETO COPPER DEPOSITS AND THEREAFTER PAINTING THE PRETREATED SURFACE WITH A WHITE PAINT BY THE ELECTROPHORETIC METHOD OF PAINTING.

Description

United States Patent O 3,579,429 PROCESS FOR APPLYING A WHITE PAINT ELECTROPHORETICALLY Frank E. Manson, Lansdale, and Lester Steiubrecher, Southampton, Pa., assignors to Amchem Products, Inc., Ambler, Pa.

No Drawing. Continuation-impart of application Ser. No. 630,839, Apr. 14, 1967. This application Jan. 6, 1970, Ser. No. 1,056

Int. Cl. B0111 5/02; C231) 13/00 US. Cl. 204-181 20 Claims ABSTRACT OF THE DISCLOSURE A White paint film applied to an iron or steel surface by the electrophoretic method of painting tends to be of a muddy, off-white or yellowish color; also, foreign deposits on the surface to be painted show-up in the white paint film as color imperfections. These problems are avoided by pretreating the iron or steel surface with an aqueous solution of copper to apply thereto copper deposits and thereafter painting the pretreated surface with a white paint by the electrophoretic method of painting.

PUBLICATIONS (A) Bogart, Harold N., G. L. Burnside and George E. F. Brewer, The Concept of Development of the Ford Electrocoating System, Society of Automotive Engineers, Preprint 998A for International Automotive Engineering Congress, Detroit, Mich., January 1115, 1965.

(B) Burden, I. P., and V. H. Guy, The Development and Evaluation of Paints for Electrophoretic Deposition, Transactions of the Institute of Metal Finishing, 1963, vol. 40.

(C) Gloyer, S. W., Donald P. Hart, and Robert E. Cutforth, Electrodeposition Theory and Practice, Ofiicial Digest, February 1965. (Publication of the Federation of Societies for Paint Technology).

(D) Hutchinson, C. 0., Some Aspects of Electrodeposited Organic Coatings, Products Finishing, December 1965.

(E) Hutchinson, C. 0., Some Aspects of Electrodeposited Organic Coatings, Society of Automotive Engineers, Reprint 650511, Chicago, 111., May 17-21, 1965.

(F) LeBras, Louis R., Electrodeposition Theory and Mechanisms, Journal of Paint Technology, Vol. 38, No. 493, February 1966.

(G) New Process Electrodeposits Paint, Products Finishing Staff Report, Products Finishing, June 1964.

As set forth in the above publications, the process of electrophoretic painting offers many advantages to the finishers of metal surfaces. Utilizing such process, the thickness of the paint film that is applied to the surfaces of the articles, including such surfaces as corners and sharp edges, can be closely controlled. It is also possible to avoid runs, sags, beads or tears in the painted surface. Other 3,579,429 Patented May 18, 1971 advantages include the ability to coat recesses in the surface to which the paint is applied and the ability to utilize virtually of the paint. Also, the electrophoretic paint process is ideally suited to automation and, in addition, there is a minimal hazard from fire and toxic vapors. Furthermore, the Wet deposited paint film has slight tack which facilitates handling.

Although there are a number of advantages that can be realized by utilizing the electrophoretic method of applying paint to metal surfaces, there have been certain disadvantages connected with this method. It has been observed that the corrosion resistance of ferriferous surfaces which have been painted electrophoretically is somewhat less than desired. This problem has been encountered even when the surface has ben precoated with a corrosion resistant phosphate coating of the conventional type. It has been observed also that there is a tendency for the electrodeposited paint to not adhere well to the ferriferous surface or to ferriferous surfaces which have been pretreated with another coating, for example, phosphate treated, prior to the application of the paint.

One approach that has been taken to avoid the above problems involves the modification of conventional phosphate coating solutions of the type which form corrosion resistant and paint bonding coatings. For example, the aforementioned application Ser. NO. 630,839 discloses a method for coping with the aforementioned problems by pretreating the ferriferous surface with a zinc phosphate or zinc/ calcium phosphate coating solution which contains dissolved copper in an amount within the range of about 10 to about 300 parts per million and preferably about 50 to about 100 parts per million (p.p.m. hereafter); thereafter the coated surface is painted electrophoretically. The copper in the phosphate coating solution functions to improve the corrosion resistant and paint adherent properties of the electrophoretically painted surface.

Another example of a modification of phosphate coating solutions for the purpose of avoiding the aforementioned problems is the addition of fluoride ion to zinc phosphate coating solutions. It has been disclosed that the corrosion resistant properties of an iron or steel surface which is painted electrophoretically can be improved if the surface is first coated with a zinc phosphate coating solution which contains at least 0.1 gram per liter of F- and then rinsed with an aqueous solution containing hexavalent chromium. This pretreatment of the surface is said to avoid the formation of depressions or holes which would otherwise form in the electrophoretic paint film.

A relatively recent development in the electrophoretic paint field has been the development of improved electrophoretic paint formulations which do not lead to the problems referred to above. As a consequence of this development, there are presently available improved electrophoretic paints which form films having improved corrosion resistant and adherent properties. From the standpoint of improving the corrosion resistant and adherent properties of the electrophoretically painted surface, there is little, if any, benefit to be gained by pretreating the ferriferous surface with a copper-container or other modified type of phosphate coating solution if the coated surface then has applied to it an electrophoretic paint of the improved type.

There is, however, a severe problem of a very different type that is encountered in electrophoretic painting. It is one that is peculiar to the application of a white electrophoretic paint, and it is one that is prevalent even with the improved electrophoretic paints that do not lead to corrosion resistant and paint adherent problems. The problem, which is disclosed also in the aforementioned application, is that a muddy or off-white color is obtained when white paints are electrophoretically deposited on iron or steel surfaces. As disclosed in the aforementioned application,

the tendency of an electrophoretically applied white paint to appear muddy or off-white in color can be minimized or completely eliminated if the ferriferous surface to which the paint is applied is precoated by subjecting the surface to a zinc phosphate or zinc/calcium phosphate coating solution which contains from about to about 50 p.p.m. of dissolved copper. The application states also that if the ferriferous surface is so precoated and then subsequently painted white electrophoretically, there is obtained a white painted surface that has a very attractive appearance, that is, a whiter white appearance, rather than an oif-white appearance.

The invention described herein is directed to problems encountered in the electrophoretic application of a white paint and encompasses the method disclosed in the aforementioned application for coping with the problem.

To expand somewhat on the unattractive appearance of an electrophoretically white painted surface, experience has shown that there is a tendency for the surface to be muddy, off-white or even yellowish in color. This undesirable color may cover the total surface uniformly or it may cover portions of the surface. In the latter case, the surface is mottled, that is, it contains blotches of a muddy, ofl-white or yellowish color.

In addition to the above problem, experience has shown that another type of problem is encountered also when a white paint is applied electrophoretically to a ferriferous surface that has thereon foreign deposits such as, for example, oil, grease, and carbonaceous materials. It appears that such deposits, often referred to categorically as smut, tend to migrate to the surface of the White paint film as it is forming. There they appear as color imperfections in the white paint film. This is aesthetically unappealing, of course.

A standard procedure in any painting operation is to clean the surface prior to applying the paint; however, some of the smut which clings tenaciously to the metallic surface often escapes removal and then detection. (The surface may appear to the naked eye to be clean, but some smut may be present thereon.) Extra processing steps which include an inordinately rigorous cleaning of the ferriferous surface to remove the deposits is time consuming and costly. Thus, inherent advantages of the electrophoretic paint process are offset by the disadvantage of color imperfections appearing on the white paint film.

[t is noted that this imperfection problem is not encountered or at least is not severe when the white paint is applied by methods other than the electrophoretic method. When the paint is applied by other methods, the paint film covers the smut which remains attached to the ferriferous surface. On the other hand, it appears that during the electrophoretic application of a white paint, the smut is detached from the metallic surface and then moves to the surface of the paint film.

The above problems are aesthetic ones in that the white painted surface has an unattractive appearance. -In addition, imperfections in the white paint film that are caused by deposits of smut and/ or the muddy, off-White or yellowish color of the painted surface can also be the source of a functional problem because they affect adversely the reflectance characteristics of the white paint film. The functional problem is encountered when the white painted article (for example a light fixture) is used in an application which requires that the reflectance characteristics of the paint film have a certain minimum value. In some applications, it has been found that the electrophoretic method of painting cannot be used satisfactorily to apply white paint to an article because the reflectance characteristics of the white paint film do not meet the required standards.

As mentioned briefly hereinabove, the aestheitc and functional problems which have been discussed above are peculiar to the electrophoretic application of a white paint; they are encountered also, but to a lesser degree when the electrophoretic paint is of an olf-white shade such as, for example, ivory, egg shell, oyster-white and cream. However, they are not encountered when the electrophoretic paint is of a dark color or of a brilliant color; for example, these problems are not encountered when a black or red paint is applied electrophoretically.

Previous efforts to avoid the above problems have been unsuccessful or have serious shortcomings. For example, it has been suggested that the discoloration of a white or pastel shade electrophoretic paint film can be avoided by pretreating an iron surface with a conductive undercoat which is formed from a water thinable paint comprised of a pigment that contains a conductive material, such as carbon black or lamp black, and a water dispersible resin of the type such as phenolic/alkylds or melamine/alkyds. The preferred method of applying the undercoat has been reported to be by electrophoretic means. The electrophoretic paint is then applied over this undercoat. Disadvantages of this method are: the materials comprising the undercoat are relatively expensive; and there is needed additional equipment and control means to apply the undercoat by the preferred electrophoretic techniques. This adds burdensomely to the cost of the overall process as materials and equipment are needed to apply two resinous paint coats to the metallic surface.

In view of the above, it is an object of this invention to provide an improved process for applying a white paint to a ferriferous surface electrophoretically.

Another object of this invention is to provide a process which would minimize or completely eliminate the tendency of a white electrophoretic paint to produce a muddy or off-white or yellowish color when deposited on iron or steel surfaces.

It is still another object of this invention to provide a process for improving a reflectance characteristics of a white paint film that has been applied to a ferriferous .surface by the electrophoretic method of painting.

It is still another object of this invention to provide an improved process for minimizing or eliminating surface color imperfections in a white paint film that has been applied to a ferriferous surface by the electrophoretic method of painting.

In accordance with this invention, it has been found that the above objects and attendant advantages, as Well as others, can be attained by applying copper to a ferriferous surface and then applying to the surface a white paint by the electrophoretic method of painting.

As will be explained more fully below, the copper can be applied to the ferriferous surface in various ways. For example, the copper can be applied by contacting the ferriferous surface with an aqueous solution which contains a soluble copper salt. Such a solution can contain justthe copper salt or it can contain other ingredients provided that they do not interfere with the result that is desired. Preferably, such other ingredients impart to the ferriferous surface a beneficial property, for example, a coating to which the electrophoretic paint adheres readily and/or which is corrosion resistant.

The copper can be applied to the bare ferriferous surface or it can be applied to a previously formed coating which covers the ferriferous surface. The invention can be practiced also by applying the copper to the bare ferriferous surface and then forming a coating, for example, a paint adherent coating, on the surface.

After the copper has been applied to the ferriferous surface, it can be painted electrophoretically with a white paint, Unless otherwise stated, as used herein in describing or defining the present invention, the term white is intended to describe or define those paints colors which give rise to the aforementioned problems and thus it covers pure shades of white, that is those which have a high reflectance value, for example about or above, and also off-shades of white, for example, ivory, oysterwhite and the like, having a lower reflectance value.

Advantages provided by applying copper to the ferriferous surface before it is painted electrophoretically with a white paint, are important and numerous. The aesthetic and functional problems mentioned above can be eliminated or substantially alleviated. Thus, white electrophoretically painted articles with improved reflectance characteristics can be produced. A uniformly white surface free of yellowish, off-white or muddy blotches can be provided. In addition, a white surface free of color imperfections caused by smut deposits on the ferriferous surface can be produced also.

Another important advantage of the invention is that it provides an effective way of coping with the problems described above without appreciably adding to the cost of the painting process. Furthermore, the means for applying the copper to the ferriferous surface can be in tegrated readily into conventional systems that are used to pre-treat the ferriferous surface before it is painted. In situations where a coating, for example, one that is paint adherent, is formed on the ferriferous surface prior to the application of the electrophoretic white paint, the copper can be added to the coating solution.

Turning now to a description of the preferred method for applying the copper to the ferriferous surface, it is preferred that the surface be precoated with a zinc phosphate or zinc/calcium phosphate coating solution containing from about 0.5 to about 50 p.p.m. of dissolved copper. Phosphate coating solutions to which the copper is added are well known and have long been used to apply corrosion resistant and paint adherent under coatings to metallic surfaces, They can be made up according to available techniques. Zinc oxide, phosphoric acid and calcium carbonate are examples of ingredients that can be dissolved in water to prepare the phosphate coating solution. The copper can be added to the solution in any form which is not detrimental to it. It is added most conveniently in the form of a soluble copper salt such as, for example, copper nitrate, copper acetate and copper chloride.

In addition to containing conventional amounts of phosphate ion, zinc or zinc and calcium ions, the phosphate solution will also generally contain an oxidizing ion such as nitrate, nitrite, chlorate, etc. Furthermore, to this class of phosphate solutions there can be added other ingredients which are generally known to olfer to the user one or more advantages.

The conditions under which the phosphate solution bath can be operated are those well known in the art. In general, the pH of the bath will range from about 1.8 to about 3.5. The free and total acidity of the bath should generally be kept within the range of from about 0.3 to about 10 and from about to about 100 respectively. By the terms free acidity and total acidity, it is meant that values attained by titrating a ml, sample of the working bath with 0.1 normal sodium hydroxide solution using Brom Phenol Blue and Phenolphthalein as indicators respectively. The number of -mls. required to titrate the bath sample to the Brom Phenol Blue endpoint is spoken of as the free acidity and the number of mls. required to titrate the bath to the phenolphthalein endpoint is spoken of as the total acidity.

In practicing this invention, it is also preferred to maintain the coating bath substantially free of ferrous iron in accordance with the known state of the art such as, for example, by having present in the bath a suflicient quantity of an oxidizing agent which maintains the bath substantially free of ferrous iron. It has been found that if the quantity of ferrous iron is allowed to build up in the bath, very loose powdery and unsuitable coatings can be obtained. Thus, it is preferred to maintain the bath substantially free of ferrous iron in accordance with the known state of the art.

A number of important advantages can be attained by applying the copper to the ferriferous surface according to the preferred method described above. One important advantage is that it is easier to control the amount of copper that is deposited on the ferriferous surface when it is applied from a phosphate coating solution. As will be discussed more fully below, if excess amounts of copper are applied to the metallic surface, the color of the paint film can turn out to be a distinctly bluish color rather than the desired White or blue-tinted white color. Excess amounts of copper can be deposited more readily when the copper is applied from an aqueous/copper salt solution which contains no other ingredients; consequently, care should be taken to avoid this when utilizing such a solution. This problem is not en countered when utilizing the preferred method described above.

Another advantage that is attained when copper is applied to the metallic surface in accordance with the preferred method described above is that a coating solution which forms a corrosion resistant and paint adherent coating is applied simultaneously with the application of copper. As mentioned above, it is a standard operating procedure to apply a metallic surface, prior to painting it, a coating which is paint adherent and one which is also corrosion resistant in the event that the painted article is going to be used in an environment which can cause corrosion. In accordance with the preferred method described herein, such a coating and the copper can be applied in a one-step treatment.

It is noted that zinc phosphate and zinc/ calcium phosphate coating solutions containing copper are Well known and have been used to apply undercoatings to metallic surfaces, but to surfaces which are painted subsequently by a method other than the electrophoretic method of applying a white paint. (For example, see US. Pat. Nos. 1,791,715; 1,869,121; 1,873,363; 1,888,189; and 2,813,- 812.) Copper in such coating solutions functions as an accelerating agent, that is the weight of the phosphate coating deposited on the metallic surface over a given period of time is increased due to the presence of the copper. However, experience has shown that when relatively high amounts of copper are present in the phos phate coating solution, and when the phosphate coating is painted by a non-electrophoretic method, not only is the corrosion resistance of the coated surface poor, but additionally, the coating is a relatively poor adhesive surface for other subsequently applied coatings such as paint. (From a reading of the above patents, it can be seen that a great deal of previous work has been done to modify copper-containing phosphate coating solutions, in a manner such that coatings produced therefrom would have better corrosion resistant and adhesion properties.) It would have been expected that these same problems would be encountered when utilizing the preferred method of this invention to deposit the copper; however, they are not and this is indeed surprising.

The copper can be deposited on the ferriferous surface by methods other than the preferred method. For example, it can be deposited from an iron phosphate coating solution of the type that is used for applying corrosion resistant and paint adherent coatings to a metallic surface. Iron phosphate coating solutions are often used instead of zinc phosphate or zinc/calcium phosphate coating solutions in applications where it is desired to impart paint adhesive properties to the metallic surface, but where corrosion resistant properties are not so important. The corosion resistant properties of iron phosphate coatings are generally not so good as zinc and zinc/calcium phosphate coatings. In general, the concentration of the copper in the iron phosphate coating solution should be Within the range of about 0.5 to about 50 p.p.m. The concentration of the other ingredients that comprise the solution and the conditions under which they are applied are well known in the art. Generally speaking and by way of example, such solutions contain water, about two g./l. to about 20 g./l. of monosodium phosphate and an accelerator, such as sodium molybdate in an amount within the range of about 0.05 to about 0.5 g./l. Exemplary application conditions include spraying the iron phosphate coating solution onto the surface for a period of time ranging from about one to about two minutes and at a temperature within the range of about 140 F. to about 160 F; or the metallic surface can be immersed in a bath of the coating solution for about two to about five minutes and at a temperature of about 150 F. to about 170 F.

Another method for depositing the copper on the ferriferous surface is to subject the bare metal surface to an aqueous solution that contains nothing more than a soluble copper salt. In general, the concentration of the copper in the solution can range from about 100 to about 10,000 ppm. After the copper has been deposited on the ferriferous surface, it can then be electrophoretically painted with a white paint; on the other hand, a suitable coating, for example a phosphate coating of the zinc, zinc/calcium or iron type, can be applied after copper deposition and then the ferriferous surface can be painted white electrophoretieally.

Still another method for depositing the copper is to apply it from a copper solution to a ferriferous surface that has been coated first with a suitable coating, for example, a phosphate coating of the zinc, zinc/calcium or iron type; thereafter the surface can be painted white electrophoretically. This method for applyingthe copper can be fitted in very easily with other coating process steps. For example, it is conventional to rinse a phosphate coated surface with water right after the phosphate coating has been applied to the surface for the purpose of removing therefrom excess or spent coatmg solution. A water soluble copper salt can be added to the rinse water and the resulting solution used to rinse the coated surface and apply the copper to it. Thus, the copper 1s apphed without setting up another treatment stage 1n the production process. This same advantage can be attained also by adding the copper to other types of nose solutions as long as ingredients of the solution do not interfere with deposition of the copper. I

It should be understood that the invention contemplates also the application of copper from an aqueous solution thereof to a ferriferous surface that has been coated previously with a suitable undercoating which has been rinsed; in other words, a copper solution can be applied at another treatment stage after the surface has been coated and rinsed.

When applying copper to a previously coated ferriferous surface from a copper containing rinse, care should be taken to avoid degrading or stripping the coating from the surface. Copper solutions are acidic; if they are too ,acid, they can degrade the undercoating or even strip it from the ferriferous surface. While this may not be detrimental from the standpoint of depositing the copper on the surface for the purpose of improving the properties of the White paint film, it can affect adversely or eliminate the desired properties of the undercoating. A satisfactory pH range for the copper solution will depend on the type of undercoating that is present and it can be determined best by subjecting samples of the coated metal to copper solutions of varying pH.

Regardless of the method used to apply the copper to the ferriferous surface, it should be understood that available techniques that are used usually to clean or otherwise pretreat ferriferous surfaces prior to the application thereto of an undercoating or to the application of an electrophoretic paint can be used to prepare the ferriferous surface for the deposition of the copper and the subsequent application of the electrophoretic paint. This will rid substantially the metallic surface of unwanted oxide scales, other corrosion products and other undesirable deposits which otherwise might interfere with the deposition of the copper or the application of the electrophoretic white paint. However, precautionary steps or extra rigorous cleaning of the surface to avoid color imperfections in the paint film caused by tenaciously clinging deposits are obtained due to the improvements provided by this invention.

It should be understood also that when the copper is applied to the ferriferous surface from a copper solution which contains coating-forming ingredients, available techniques can be used to rinse the coated surface. For example, when the copper is deposited from a zinc or zinc/calcium phosphate coating solution, the coated surface can be rinsed first with water and then with a rinse solution, such as, for example, one containing hexavalent or trivalent chromium or mixtures thereof.

After depositing the copper on the ferriferous surface by one of the exemplary methods described above, it can be painted electrophoretically with a suitable electrophoretic white paint. Available techniques and paints can be utilized. By way of example, it is noted that electrophoretic paints can be prepared from water soluble or dispersible resins such as methacrylic, acrylic, alkyd, melamine and epoxy resins. Preferably an anionic resin which will deposit on the ferriferous surface which is made the anode is used. The solids content of the electrophoretic paint can be within the range of about 8 to 17 percent by weight and the paint can have a pH within the range of about 7.5 to about 9.5. Exemplary conditions of application of the electrophoretic paint to the ferriferous surface include: a voltage of about 50 to about 450 volts; a current density of about 0.1 to about 10 amperes per square foot; a paint temperature ranging from room temperature to about F.; and a contact time ranging from about 0.5 minute to about 5 minutes. It should be understood that the above descriptions of the electrophoretic paints and their conditions of application are exemplary.

After the paint has been applied, the painted surface can be rinsed and then cured according to available techniques. For example, the painted surface can be rinsed with water and then heated to an appropriate curing temperature, for example 300 F. to 400 F. for about 20 to about 30 minutes.

As set forth hereinabove, the properties of the electrophoretically applied white paint film can be improved due to the presence of copper on the pretreated ferriferous surface. The amount of copper deposited influences the results that are obtained. For example, if sufficient copper is not deposited, the white paint film can have a muddy, off-white or yellowish color; in addition, color imperfections in the paint film caused by deposits on the metallic surface may be evident also. (The term blemished is used herein to describe paint films which manifest these color problems.) On the other hand, if too much copper is deposited on the ferriferous surface, the aforementioned problems will not be encountered, but the paint film may be more blue in color than is considered desirable.

it is not only diflicult, but also impractical to give guidelines on the minimum and maximum amounts of copper that should be deposited on the metallic surface. The difliculty arises because of numerous variables that are lnherent in the process, including both the pretreatment and painting steps of the process. The variables can used; the conditions under which the white paint is depostion of a specific amount of copper under one set of conditions may give a different result under a different set of conditions. The following are examples of variables that can have a bearing on the amount of copper to be deposited; the particular white electrophoretic paint that is used; the conditions under which the white paint is deposited; the thickness of the white paint film that is applied; the specific type of ferriferous metal that is being painted; and the specific method used for depositing the copper on the surface.

As to the impracticalities of determining the amount of copper deposited on the ferriferous surface, it is noted that it is much easier to adjust the concentration of the copper in the coating solution until the desired result is achieved than it is to analyze the surface for the amount of copper thereon.

Notwithstanding the various factors that may have to be taken into account in depositing appropriate amounts of copper on the ferriferous surface, it has been found from experience that a satisfactory white film can be obtained if the ferriferous surface is subjected to a phosphate coating solution which contains from about 0.5 to about 50 p.p.m. copper for a period of time ranging from about 0.25 to about 2 minutes. When a solution containing nothing more than a soluble copper salt is used and in which the copper concentration of copper is relatively high, for example about 100 to about 10,000 p.p.m. copper, then shorter contact times should be used, for example about one second to a few seconds. In general, longer contact times should be used for relatively dilute solutions and shorter contact times should be used for relatively concentrated solutions. Speaking generally, this will be effective to deposit a suflicient amount of copper on the metallic surface to avoid the aforementioned problems, but not an excess amount which will produce a paint film which is more blue than white. However, it should be understood that greater or somewhat smaller amounts of copper can be used in the solution and/or the contact time between solution and the ferriferous surface varied in order to achieve the desired result. Sufiice it to say that the amount of copper deposited on the surface should be an amount such that the electrophoretically applied paint forms a film having a white unblemished surface. The term white unblemished as used herein means a paint film that is substantially true in color, that is a white color or a white color having a bluish tint; one that has substantially no blotches; and one that has substantially no visible color imperfections caused by deposits on the ferriferous surface.

Some general observations can be made concerning the shade of the white color produced when suitable amounts of copper have been deposited on the ferriferous surface. In general, the greater the amount of copper on the surface, the bluer will be the tint of the white paint film-other variables held constant. To state this conversely, the white paint film will have a whiterwhite appearance for lesser amounts of copper. It is diflicult to distinguish with the naked eye the whiterwhite shade from the blue-tinted white shade. However, it can be distinguished readily by comparing the painted surface with a white color standard. It is noted also that the reflectance characteristics of the painted surface are higher for the whiter-white shades than the blue tinted white shades. Thus, when painting an article which is used in an application where this characteristic is of primary importance, it is recommended that the amount of copper deposited on the surface be a minimal amount, that is an amount just suflicient to produce an un blemished painted surface. This can be achieved by applying the copper to the ferriferous surface from a copper solution containing minimal amounts of copper, for example, a zinc or zinc/calcium phosphate solution containing about 0.5 to about 5 p.p.m. of copper and preferably about 0.5 to about 2.5 p.p.m. copper.

In applications where it is desired to produce a blue tinted white shade of attractive appearance, the copper can be applied from a more concentrated solution of copper, for example one containing about to about 50 p.p.m. of copper. As mentioned above, the more copper deposited on the surface the lower will be the reflectance properties of the white paint film. However, in applications where reflectance characteristics are not important, a more eye-appealing white color can be obtained if the paint film has a bluish cast. In general, experience has shown that the eye prefers a bluishwhite over a pure white even though the bluish-white color has less reflectance than the pure white color. In order to obtain this bluish effect, paint manufacturrs often add to White paints a material which imparts a bluish tint to the white paint. Ultramarine blue is an example of such a material. In the practice of the 10 invention, the bluish tint can be obtained with white paints that do not contain such materials. (It is noted that the presence of such pigments in white electrophoretic paints does not avoid the blemished paint film of the type which is avoided by the practice of this invention.)

In applications where there is used a white paint which leads also to corrosion resistant and/or paint adherent problems, then it is recommended the copper be applied from a zinc or zinc-calcium phosphate coating solution which contains about 10 to about 50 p.p.m. copper and preferably about 20 to about 50 p.p.m. This Will not only improve the corrosion resistant and/or paint adherent properties but also improve the aesthetic properties of the paint film.

It should be understood that the concentration ranges of the copper in the coating solutions described above may have to be adjusted (for example the minimum or maximum concentration values decreased or increased) in order to obtain the desired result in view of the other previously mentioned factors that influence the results that are obtained. If the desired result is not obtained initially, it is recommended that trial runs be made in which the concentration of the copper in the solution or perhaps the contact time of the solution with the ferriferous surface is varied.

In pretreating the ferriferous surface, an aqueous solution containing copper ions can be applied to the surface by available application techniques which are suitable. For example, it can be applied by immersing the ferriferous surface in a bath of the copper solution; or the ferriferous surface may be sprayed with a copper solution; or the copper solution can be applied by rollers. It should be understood that other application techniques can be used also.

When ferriferous surfaces are immersed in a bath of the copper solution, care should be taken to maintain the concentration of the copper ions in the solution at the desired level. Accordingly, the concentration of the copper in the bath should be determined periodically and copper should be added to the bath as needed.

The concentration of the copper in the bath can be determined according to available techniques. For example, a sample of the bath can be analyzed for its copper content by the use of neocuprine (2,9-dimethyl-1,lO-phenanthroline). This material combines with copper to form a yellow stable complex which can be used to make a colormetric determination of copper concentration. It is preferred that the copper concentration in the bath be determined by the use of a cupric ion activity electrode. Such an electrode and attendant equipment can be used to make a direct reading of the copper concentration. Such an electrode and attendant equipment are known and are sold by Orion Research, Inc.

It will be appreciated that satisfactory amounts of copper can be applied to the ferriferous surface from solutions which contain extremely small amounts of copper. Maintaining such small amounts of copper in an operating bath in which ferriferous surfaces are immers d continuously can be somewhat difiicult. In the practice f this invention, it is preferred that the desired concentratron of copper be maintained in the bath by automatically feeding copper into the bath as copper is depleted therefrom and as the need arises for additional copper. In accordance with this invention, it has been found that this can be accomplished as described below.

A probe consisting of a cupric ion activity electrode (such as the one mentioned hereinabove and sold by Orion Research, Inc.) and a standard Calomel reference electrode is positioned in the coating bath containing copper. The electrodes should be positioned in the bath in contact with that portion of the coating solution that is being worked, that is the portion of the solution into which the ferriferous surfaces are being immersed. This will ensure that the electrodes are positioned in that part of the bath where the copper is being depleted. A voltage is developed across the probe. The voltage reflects the amount of cupric ion activity in the coating solution.

The leads from each of the electrodes making up the probe are connected to a self-balancing potentiometer type recorder controller. The controller in turn is connected to a feed pump. The feed pump is used to pump a concentrated aqueous solution of copper into the bath as copper is needed in the bath. The concentrated aqueous solution can be made-up from copper nitrate or other suitable copper salt and it can be positioned in a container located conveniently near the bath. The controller is set to initiate operation of the pump to add concentrated copper solution to the bath at those times when the probe indicates a drop in the copper content of the bath. The apparatus can be set-up so that the controller activates the pump directly on signal from the probe; or it may activate a timer which can be placed between it and the pump so that when the probe voltage drops, the controller will trigger the timer, which in turn will activate the feed pump for a predetermined interval of time. The overall result is that the concentration of copper in the bath is maintained at the desired level by automatically feeding copper into the bath as copper is depleted therefrom and as the need arises for additional copper.

A theory which attempts to explain the workings of the invention described herein has been proposed. First by way of background, it is noted that in the electrophoretic method of painting, the metallic surface to be painted is made the anode usually and the metallic container holding the paint is made the cathode. An electrical current is passed through the paint bath by means of these electrodes as the anode is immersed in the paint bath and voltage is applied. As to the blemished appearance that is obtained when a white paint is applied to the ferriferous surface (the anode), it is believed that the oxidation which takes place at the anode, includes the oxidation of metallic iron to anionized form thereof from which iron hydrates are formed. It is believed also that the iron hydrates becomes trapped in or are carried by the paint film as it forms and that hydrates are even present on the exterior surface of the paint film. The explanation for this is that the exterior surface of the paint film is made up of the paint particles that were deposited first on the ferriferous surface. In other words, as paint particles deposit on the ferriferous surface, they are pushed away from the surface by subsequently deposited paint particles. Thus, the particles last deposited on the ferriferous surface make-up the inside surface of the paint film and the particles first deposited make-up the exterior or visible surface of the film. This inside out formation of the paint film is peculiar to the electrophoretic method of painting. (For example, it is not encountered when paint is applied by a brush because the particles of paint first deposited on the surface remain there and are covered over by subsequently applied particles.) The iron hydrates caught up in the paint film are dark in color and they can discolor it. In addition, the hydrates can form dark iron oxides when the paint film is subjected to heat for the purposes of curing the paint. The oxides can cause the unsightly appearance of the surface because their dark color dominates or shows through the white paint film.

As to the function of the copper, it is believed that it too is oxidized at the anode to copper ions and that they combine with an ingredient in the white paint to form a material which effectively masks the color of the aforementioned iron hydrates and iron oxides. For example, the copper ions can combine with an amine compound to form a copper complex (theoretically Cu(NH 'which is blue in color. (Amine compounds are used in electrophoretic paints as solubilizers.) It is theorized that the blue complex is trapped in or carried by the paint film as it is formed (similar to the iron hydrates) wherein it functions to cancel or mask the undesired colors of the iron hydrates and oxides. The result is an unblem- 12 ished paint film that is a whiter-white color or of a bluetinted white color depending on the amount of copper that was deposited on the surface.

The above theory can be used also to explain how the copper functions to minimize or completely eliminate color imperfections caused by deposits on the ferriferous surface. The inside out formation of the electrophoretic paint film carries at least some of the deposits to the surface of the film where they appear as color imperfections. However, it is believed that the blue copper complex masks the color imperfections.

Examples set forth below are illustrative of the practice of this invention. Comparative examples are set forth also for the purpose of illustrating the improvements provided by this invention.

The first example shows the electrophoretic application of white paint to steel panels which were pretreated with phosphate coating solutions containing copper and, for the purposes of comparison, the pretreatment of steel panels with the same phosphate coating solutions, except that they did not contain copper.

EXAMPLE I A series of steel panels was prepared by one of the following methods:

(A) Alkali cleaned, water rinsed, zinc phosphated, water rinsed and dried.

(B) Same as (A) above except 20 p.p.m. copper ion added to the zinc phosphating bath.

(C) Alkali cleaned, water rinsed, zinc/calcium phosphated, water rinsed and dried.

(D) Same as (C) above except 20 p.p.m. copper ion added to the zinc/ calcium phosphating bath.

The panels were then painted with Glidden 65197 White Electrocoat Enamel, manufactured by the Glidden 00., Cleveland, Ohio, at 200 volts for 2 minutes, rinsed in distilled water and baked at 350 F. for 20 minutes.

All the panels were then visually rated for whiteness. The results are summarized in Table I below. The panels that had the Whitest white color were rated as (1). Those with the most off-white, yellowish or light brown appearance were rated at (10).

Table I.-Results of pretreatments on whiteness of Gliddcn 65197 White Electrocoat Enamel Whiteness rating Pre-treatment, including (1)=most white type of phosphate coating: (10)=most ofi-white (A) Copper-free zinc phosphated steel (10) (B) Same as (A) above with 20 p.p.m. copper added to zinc phosphating bath (l) (C) Alkali cleaned, zinc/calcium phosphated steel (5) (D) Same as (C) above With 20 p.p.m. copper added to zinc/ calcium phosphating bath (1) It is interesting to note that every substrate listed in Table I when subsequently conventionally spray painted with Sherwin-Williams Kem Fast White Bake Enamel #E73W2, manufactured by the Sherwin-Williams Co., Chicago, 111., had whites that looked alike and no substrate painted had a whiter white than another.

Examples set forth in Table II below are illustrative of various methods that can be used to apply copper to the ferriferous surface. Comparative examples are set forth also. In this grou of examples, there were used 4 inch x 6 inch commerical grade steel panels. The panels were first cleaned in an alkali solution and then rinsed with water. (Those panels which were treated subsequently with a zinc phosphate coating solution were also treated with a grain refiner, after which they were rinsed with water.) Thereafter, they were treated with the aqueous coating solution set forth in Table II below and in the manner set forth. After the panels were so pretreated, they Were painted with an electrophoretic white 13 paint (Powercron White W20020PPG Industries, Inc.), then water rinsed and baked until the paint cured. The thickness of the paint film that was applied was in the range of 1.0 to 1.3 mils. The appearance of the paint film is set forth in the table.

TAB LE II Example Aqueous coating solution used in Appearance of paint No. pretreatment film 2 Iron phosphate coating solution Yellowish-white in containing no copper, but concolor with more taining 6.5 g./l. of monosodium pronounced yellowphosphate, 0.16 g./l. of NaMoO4, ish colored stains 0.64 g./l. of a nonionic wetting along the edges and agent, 0.32 g./1. of oxalic acid, and more pronounced 0.4 g./l. of NaF; and having a pH yellowish colored of 3.7; sprayed for one minute. blotches over remainder of the surface.

3 Iron phosphate coating solution of Bluish-white in Example 2 and including 50 p.p.m. color; no stains copper added as cupric nitrate; and no blotches. sprayed for one minute.

4 Iron phosphate coating solution of Uniformly White with Example 2 and including 10 p.p.m. bluish tint; not as of copper added as cupric nitrate; blue-white in color sprayed for one minute. as Example 3; no

stains and no blotches.

5 Zinc phosphate coating solution Similar to paint containing no copper but confilm of Example 2 taining 0.15% zinc, 7.5 g./l. of except that the HzPO-r, 0.61 g./l. of NaClOa, 0.24 yellowish-White g./l. of NaNOz, and 0.15 g./l. of color was somewhat Ni; sprayed for one minute. lighter and there were fewer blotches appearing on the surface.

6 Zinc phosphate coating solution of Uniformly bluish- Example 5 (sprayed for one white in color; minute) followed by rinsing with somewhat more a copper solution containing 500 bluish than p.p.m. copper added in the form Example 3; no

stains or blotches. Similar to Example 6.

of copper sulfate.

Copper solution containing water and 1,000 p.p.m. of copper added in the form of copper sulfate; immersed for seconds.

8 Immersed in copper solution of Example 7 for 10 seconds followed by immersion in zinc phosphate coating solution of Example 5 for one minute.

With reference to the above table, it can be seen that the painted panels of Examples 2 and 5, that is those not pretreated with copper, had a very poor appearance. On the other hand, the painted panels of Examples 3 and 4 and 6-8, that is those pretreated with copper in one way or another, were all uniform in color and thus did not contain blotches and stains. The painted panels with the greater amount of copper deposits had a more bluish tint. It is noted that Examples 3 and 4 show the application of copper to the panel from an iron phosphate coating solution; Example 6 shows its application from an aqueous copper rinse solution to a panel coated previously with a zinc phosphate coating; Example 7 shows its application to a bare metal surface from a water/ copper solution followed by painting; and Example 8 shows its application from a water/ copper solution to a base metal surface followed by immersion in a zinc phosphate coating solution. It is noted also that a painted panel pretreated with a zinc phosphate coating solution of Example 5 to which there was added 25 p.p.m. of silver in the form of silver nitrate had a blemished surface similar in appearance to that of Example 5.

The next group of examples are illustrative of the application of copper to steel panels from zinc phosphate coating solutions which contain different amounts of copper. The panels used in this group of examples were commercial grade steel and each was subjected to the following sequence of steps:

( 1) Cleaned with a strong alkali cleaner; (2) Rinsed with tap water; (3) Rinsed with a conventional grain refiner;

(4) Immersed for one minute in a Zinc phosphate coating solution having a temperature of F. and containing:

(f) Copper in the amount indicated in Table III below.

(5 Rinsed with water;

(6) Rinsed with chromic acid solution; (7) Rinsed with deionized Water;

(8) Dried with high pressure air.

Each of the panels was then painted electrophoretically with a white electrophoretic (Powercron White 97675- PPG Industries, Inc.) and thereafter the painted panel was rinsed with tap water, then deionized water, and then each was baked until the paint was cured. The thickness of the paint film on each of the panels was in the range of 1.3 to 1.5 mils. Table III below sets forth the results that were obtained for each of the panels immersed in the copper-containing zinc phosphate coating solution and for the purposes of comparison, the results for a panel which was immersed in the above coating solution which did not contain copper.

TABLE III 5 Like Example 11, except more bluish in color.

appearance with more pronounced Likei Example 12, except more bluish in co or.

Like1 Example 13, except more bluish in or. Like Example 14, except more bluish in color.

From the above table it can be seen that the panel which was precoated in the phosphate coating solution that conta1ned no copper had a blemished paint film of an unattractive appearance, whereas those panels to which the copper was applied had an unblemished paint film of attractive appearance.

The next example shows the application of copper to a steel panel from a zinc/ calcium phosphate coating solution containing copper.

EXAMPLE 16 A commercial grade steel panel was subjected to the following sequence of steps:

(1) Cleaned with a strong alkali cleaner;

(2) Rinsed with Water;

(3) Sprayed for 1 minute with a zinc/calcium phosphate coating solution having a temperature of F. and containing:

Ingredients: Amount, g./l. (a) Calcium 1.0 (b) Zinc 2.0 (c) H PO 7.0 (d) NO 4.2 (e) Cu++ 0.005

(4) Rinsed with tap water;

(5) Rinsed with a chromic acid solution; (6) Rinsed with deionized water; and

(7) Dried with high pressure air.

The panel was then painted electrophoretically with a white electrophoretic paint (Powercron white 97675 PPG Industries, Inc.) after which it was rinsed with tap water and then deionized Water; thereafter, it was baked until the paint was cured. The thickness of the paint film was about 1.25 mils. It was observed that the paint film was uniformly white in color with a bluish tint; it had no color imperfections. On the other hand, another commercial grade steel panel which was subjected to the same process steps as described above except that it was immersed in a zinc/calcium phosphate coating solution which contained no copper, had a paint film that was yellowish in color and it contained blotches of a darker yellowish color. The overall yellowish color was not quite as pronounced as that of Example 9 above, but nevertheless it was evident and the overall appearance of the panel was quite unattractive.

In summary, it can be said that this invention provides a very economically and functionally efiicient way of coping with a problem that has been a deterent to the wide spread use of electrophoretically applying a white paint to ferriferous articles. Unblemished white paint films of attractive appearance can be attained without appreciably adding to the cost of the overall painting process.

We claim:

1. In the method wherein a white paint film is applied to a ferriferous surface by painting the surface electrophoretically and wherein the white paint film has a blemished appearance, the improvement comprising applying copper to said ferriferous surface and thereafter painting said surface electrophoretically with a white paint, wherein the amount of copper applied to the surface is sufficient to provide a white unblemished paint film.

2. A method according to claim 1 wherein said copper is applied to said surfaces by contacting it with a phosphate coating solution containing copper.

3. A method according to claim 2 wherein said phosphate coating solution is selected from the group consisting of zinc phosphate and zinc/ calcium phosphate coating solutions.

4. A method according to claim 1 including forming on said ferriferous surface a phosphate coating and thereafter applying said copper to the ferriferous surface.

5. A method according to claim 4 wherein said copper is applied to said surface by rinsing said coating with an aqueous rinse solution containing copper.

6. A method according to claim 1 including applying a phosphate coating to said surface after said copper has been applied and thereafter applying said white paint electrophoretically.

7. A method according to claim 1 wherein the amount of copper applied to said surface is such that said white unblemished paint film has a bluish tint.

8. A method for pretreating and painting electrophoretically a ferriferous surface with a white electrophoretic paint comprising applying to said surface an aqueous solution containing copper to deposit copper thereon and thereafter electrophoretically painting said surface with a white paint wherein the amount of copper deposited on said surface is an amount at least sufiicient to provide a white unblemished paint film.

9. A method according to claim 8 wherein said copper is applied from an aqueous solution containing from about 0.5 to about 50 p.p.m. of copper.

10. A method according to claim 9 wherein said aqueous solution is a coating solution selected from the group consisting of zinc phosphate coating solution and zinc/ calcium phosphate coating solution.

11. A method according to claim 10 wherein the 1 6 amount of said copper in said solution is within the range of about 0.5 to about 5 ppm.

12. A method according to claim 11 wherein the amount of said copper in said solution is within the range of about 0.5 to about 2.5 ppm.

13. A method for pretreating and painting electrophoretically a ferriferous surface comprising immersing said surface in an aqueous solution containing about 0.5 to about 50 ppm. to deposit copper thereon, withdrawing said ferriferous surface from said solution and thereafter painting said surface electrophoretically with a white paint.

14. A method according to claim 13 including maintaining the concentration of said copper in said solution at the desired level by automatically feeding copper into said solution as copper is depleted therefrom and as the need arises for additional copper.

15. In the process of painting a ferriferous surface electrophoretically with a white paint, wherein prior to the application of said paint, there is formed on said surface a phosphate coating by applying to said surface a phosphate coating solution selected from the class consisting of zinc phosphate coating solution and zinc/calcium phosphate coating solution, the imporvement which comprises establishing and maintaining in said coating solution during its application to said surface dissolved copper in a concentration between about 10 and about 50 parts per million to apply thereto a phosphate coating and thereafter applying electrophoretically said white paint to said phosphate coated surface.

16. A method in accordance with claim 15 and further comprising maintaining said solution substantially free of ferrous iron during treatment of said surface by addition thereto of an oxidizing agent capable of oxidizing ferrous iron.

17. A method for improving the whiteness of an electrophoretically white painted ferriferous surface comprising a phosphate coating on a ferriferous surface by applying thereto a phosphate coating solution containing about 0.5 part per million to about 50 parts per million of dissolved copper, wherein said solution is selected from the class consisting of zinc phosphate coating solution and zinc/calcium phosphate coating solution, wherein the pH of said solution is within the range of about 1.8 to about 3.5, wherein the free acidity of said solution is within the range of about 0.3 to about 10, wherein the total acidity of the solution is within the range of about 5 to about 100, and wherein said coating solution contains an oxidizing agent capable of oxidizing ferrous iron for maintaining said solution substantially free of ferrous iron, and electrophoretically applying a white electrophoretic paint to said phosphate coated surface thereby forming thereon a White unblemished paint film.

18. An article comprising a ferrifer-ous surface coated and painted according to the method of claim 17.

19. A method according to claim 17 wherein the concentration of said copper in said solution is within the range of about 0.5 to about 5 p.p.m.

20. A method according to claim 17 wherein the concentration of said copper in said solution is within the range of about 0.5 to about 2.5 p.p.m.

References Cited UNITED STATES PATENTS 3,109,757 11/1963 Reinhold 204181 3,467,589 9/1969 Ransch et al 204-481 3,502,511 3/ 1970 Forsberg 204-181 HOWARD S. WILLIAMS, Primary Examiner UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pat n N 3.579. +29 Dated Mag 18, 1911 Inventor(s) Frank E. Manson and Lester Steinbrecher It is certified that error appears in the above-identified patent; and that said Letters Patent are hereby corrected as shown below:

Column 2, line 15, "ben" should read -been-.

Column 8 line 1, "obtained" should read -obviated--.

Column 8, line 58, "used; the conditions under which the white paint" should read -influence the results that are obtained, that--.

IN THE CLAIMS Claim 2, line 2, "surfaces" should read --surface--.

Claim 17, line 3, after "ing" read --forming-.

REFERENCES CITED "Ransch" should read --Rauech--.

inned and sealed this 26th day of October 1 971 (SEAL) Attest:

EDWARD M.F'[ETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Patents