US3342621A - Electrostatic precipitation process - Google Patents

Description

p 1957 M. A. R. POINT ETAL 3,342,521

ELECTROSTATIC PRECIPITATION PROCESS Filed Aug. 5, 1965 United States Patent 3,342,621 ELECTRGSTATIC PRECIPITATION PROCESS Marcel A. R. Point, Grenoble, and Guy Nicolas, Meylan, France, assignors to SAMES--Soeiete Anonyme de Machines Electrostatiques, Grenoble, France, a jointstock company of France Filed Aug. 5, 1963, Ser. No. 299,758 Claims priority, application France, Aug. 3, 1962, 905,977, Patent 1,347,012 1 Claim. (Cl. 117-17) This invention relates to a coating method and more particularly to a method for electrostatically coating the surfaces of articles with divided substances, such as pulverulent solids or divided liquids, for example, by precipitation.

There has been developed an electrostatic-type system for coating various articles which is extremely rapid in action and highly efiicient. Representative methods and apparatus in accordance with this system are disclosed, for example, in French Patents 1,302,965 and 1,302,415. In such methods and apparatus, the particles of the coating substance are electrically charged and are deposited on the surface to be coated by the action of an electrostatic field. The deposited coating is then customarily subjected to heat or other bonding treatment to partly fuse the particles and bond them to the surface.

In the practice of electrostatic coating methods and apparatus of the type employed heretofore, difliculties often have been experienced in applying coatings having thicknesses exceeding certain limits. It has been found that, after the coating layer built up to a particular depth on the surface being coated, further amounts of material in many cases were not retained on the surface. This has resulted from the fact that electrostatic charges carried by the individual coating particles to a large extent were retained thereby subsequent to the time the particles settled on the surface. An electric field was thus created which progressively increased in strength and Was in opposition to the field attracting the particles to the surface. After a quantity of the particles were applied, a state of electrical equilibrium was reached, and further particles were repelled from the surface. Although in many situations the repulsion of the further particles took place after a sufiiciently thick coating layer had been applied, particularly in cases in which comparatively thick layers were desired, the repulsion of the particles prevented the realization of the appropriate thickness.

One general object of this invention, therefore, is to provide a new and improved method for electrostatically coating the surfaces of articles with divided substances.

More specifically, it is an object of this invention to provide such a method and apparatus which enables the application of a coating to the articles of substantially any desired thickness.

Another object of the invention is to provide an electrostatic coating process for applying a smooth and uniform coating to various articles in a rapid and straightforward manner.

A further object of the invention is to provide a novel electrostatic process for coating a series of articles which is economical and thoroughly reliable in operation.

In accordance with the invention, electrically charged coating particles are precipitated electrostatically on the surface of an article to be coated in the presence of a fluid having a resistivity lower than that of the particles. This fluid is effective to progressively evacuate the charges on the particles from the coating during its formation. As a result, any tendency toward the establishment of a counter-field repelling additional particles is substantially retarded and in some cases eliminated.

According to certain particularly advantageous embodiments of the invention, the fluid is in the form of a moist spray and is highly conductive, when compared with the conductivity of the charged particles. As the particles reach the surface being coated, the charges are progressively drawn ofi, thus enabling the formation of a smooth and uniform coating layer of substantially any desired thickness.

In some good arrangements, the spray is discharged directly in liquid form from a plurality of nozzles which are arranged adjacent a spray gun or other apparatus employed to direct the particles toward the article. In other cases, the auxiliary fluid is carried in a suitable container which is heated to establish a body of conductive vapor surrounding the surface being coated.

In accordance with certain embodiments, the highly conductive fluid is charged to a high potential simultaneously with the charging of the coating particles. The fluid and the particles are charged to a potential of the same polarity and exhibit a tendency to adhere to the surface of the article. Because of its higher electrical conductivity, the fluid serves to progressively dissipate the charges on the coating being formed to provide an exceedingly smooth coating layer of almost any desired thickness.

The foregoing and other objects and advantages of the invention will appear more clearly and fully from the following detailed description of various preferred embodiments, when read in conjunction with the accompanying drawing, in which:

FIGURE 1 is a diagrammatic representation, partly in section, of one illustrative embodiment of a coating apparatus for the execution of the improved method in accordance with the invention;

FIGURE 2 is a diagrammatic representation in general similar to FIGURE 1 but showing another illustrative embodiment of the invention; and

FIGURE 3 is a diagrammatic representation of the type shown in FIGURES 1 and 2 but depicting a further illustrative embodiment of the invention.

Referring to FIGURE 1, there is shown a DC. generator 1 of the electrostatic type. This generator is of conventional construction and is arranged to produce a high DC. potential at its output terminal which illustratively is of negative polarity and of the order of about 90,000 volts. The opposite terminal of the generator is connected to ground.

The output terminal of the generator 1 is electrically connected to an electrostatic spray gun shown schematically at 2. The gun 2 is supplied with a finely divided coating substance in a stream of air or other carrier gas. The coating substance and carrier gas are stored in a suitable container 3 and are fed to the gun 2 under a pressure which illustratively is about 1.5 p.s.i.g. Although a wide variety of powders or other divided particles may be employed as the coating substance, particularly good results have been obtained in the use of pulverulent varnish, various polymer materials, or other particles having comparatively high electrical resistivity.

As more fully described in the French patents referred to above, the finely divided coating particles are charged by the spray gun 2 to a high DC. potential and are discharged from the output nozzle 4 of the gun in the form of a cloud of particles within the carrier gas. The gun applies the charged particles to the surface of an article 9 to be coated which is supported in spaced relationship with the nozzle 4. The article 9 is maintained at a potential different from that applied to the particles and in the various illustrated embodiment is at ground potential. Because of the charges on the particles, an electrostatic field is established between the particles and the article which causes the particles to settle over the surface of the article and form a coating layer thereon.

Positioned immediately beneath the article 9 is an open vat or tank 7 containing water or other suitable fluid.- Although a wide variety of fluids are useful in connection with the invention, in many cases the fluid is highly conductive, having a resistivity substantially lower than the resistivity of the cloud of particles 5. The tank 7 is heated by a burner apparatus 8 to a temperature suflicient to generate water vapor 6. This vapor is directed toward the article 9 simultaneously with the application of the charged particles thereto from the spray gun 2. The vapor substantially envelopes the comparatively cool surface of the article during the coating operation.

As the negatively charged particles reach the article 9 being coated and begin to build up on the surface thereof, in the absence of the vapor 6 the coating would exhibit a tendency to acquire a negative potential of gradually increasing strength which in time would repel further negatively charged particles from the gun 2 and thus limit the thickness of the coating. However, because of the comparatively high conductivity of the vapor settling over the cooler coated surface of the article, the negative charges on the coating progressively leak off and are gradually dissipated to ground over the moist surface of the article. As further charged particles are directed toward the article from the spray gun, they electrostatically adhere to the layers of particles being built up on the surface. The action of the vapor is such that any tendency toward the establishment of an inverse electrostatic field repelling further particles from the gun is substantially reduced. As a result, the particles may be applied to the article in considerably greater thicknesses than have been previously obtainable.

In many preferred embodiments, the volume resistivity of the material for the coating particles is substantially greater than that of the highly conductive fluid 6. In some cases, the volume resistivity of the material preferably is above about 100,000 ohms/ cm. for best results. With this arrangement, the possibility of substantial current flow between the particles and an article being coated is maintained at a minimum, thus insuring optimum adhesion between the particles and the article, while the charges on the particles are readily and progressively dissipated to the fluid 6.

The coating applied to the various articles is highly uniform even on articles having irregular or complex shapes. The individual charged particles of coating material are attracted by the grounded articles and tend to seek out thinly or uncovered areas. As the coating builds up, the charges on the particles are progressively evacuated through the water vapor 6 to ground, thereby substantially retarding the establishment of any counter-field which might otherwise limit the coating thickness. In addition, the moist vapor serves to facilitate the aggregation of the individual particles over the surface of the article.

After the layer of coating material applied to the article 9 has reached the desired thickness, the article is transferred from its position above the tank 7 and is heated or otherwise treated in a conventional manner to firmly bond the coating particles to the article surface and form the finished coating.

The embodiment of FIGURE 1 is of particular utility in the application of coating layers of widely varying thicknesses to one or more articles through the use of apparatus including conventional spray gun equipment. In the modified installation shown in FIGURE 2, wherein the general arrangement is the same as in FIGURE 1 and corresponding parts are similarly designated, there is provided a plurality of auxiliary spray nozzles 10 which coaxially surround the spray gun 2 and are maintained in fixed relationship'therewith by an annular member 15. This spray gun is the same as that described heretofore and is arranged to charge the coating particles from the container 3 to a predetermined D.C. potential and to apply the charged particles to the article 9.

The nozzles 10 are supplied with water or other conductive fluid under pressure through the annular member and a conduit 11 connected to a suitable supply tank (not shown in FIGURE 2). The nozzles are arranged to direct a series of jets 6 of water vapor at substantially room temperature in the general direction of the article 9 being coated. These jets are applied to the article from substantially the same direction as that of the charged particles from the spray gun 2. As the spray from the nozzles reaches the surface of the article, the charges on the coating particles thereon are progressively evacuated to substantially eliminate any buildup of a counter-field which might otherwise tend to repel further particles from the gun 2.

Referring to FIGURE 3, there is shown a spray gun 2 having a nozzle 4 for directing a cloud 5 of negatively charged coating particles suspended in a carrier gas to the surface of the article 9. The gun 2' and the nozzle 4' V are substantially the same as those of FIGURES 1 and 2 and described heretofore but are arranged such that the electrostatic charges applied to the particles also are applied to the water spray nozzles 10. The nozzles 10 are coaxially supported around the spray gun 2' on an annular member 16 such that the nozzles are mechanically affixed to the spray gun. The member 16 is generally similar to the member 15 of FIGURE 2 but is of somewhat smaller diameter. The nozzles 10 are connected by the conduit 11 to an overhead water tank 12 which is insulated with respect to ground, as shown schematically in FIGURE 3 by the reference character 13.

The spray gun 2 is electrically connected to the water nozzles 10 through the annular member 16 such that the spray 6" from the nozzles is electrically charged with a high D.C. potential of the same polarity as that of the coating particles 5 from the gun. With this arrangement, the particles of water are attracted toward the grounded article 9 and exhibit a tendency to adhere thereto, as do the particles of coating material. Because the electrical conductivity of the water particles is higher than that of the particles of coating material, the charges on the coating are progressively evacuated through the water to ground. As a result, any tendency of the coating to repel further coating particles from the gun is eliminated, and the resulting coating may be of substantially any desired thickness.

Although water and water vapor have been specifically referred to as illustrative examples of the fluid useful in carrying away the electrostatic charges from the coating being precipitated on the article, other divided fluids having a resistivity lower than that of the coating also may be used with good effect. Such fluids may be in either liquid or gaseous form, or a suitable mixture thereof, and in some cases may include particular components which also serve to produce physical or chemical effects beneficial to the characteristics of the final coating provided on the article.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

In a method for electrostatically coating a surface, in combination, electrically charging a series of particles of given resistivity to a predetermined D.C. potential, applying a cloud of the charged particles at room temperature to the surface to be coated to form a coating thereon, heating a liquid having a resistivity lower than that of said particles to vaporize the same, directing the vapor into contact with said surface, and condensing said vapor on said surface during the application of the particles thereto, to thereby facilitate the progressive evacuation of the charges on said particles from said coating.

(References on following page) References Cited UNITED STATES PATENTS Starkey 118-630 X Bede 117-1051 Ransburg 118-624 X Spiller 117-93.4

Spiller 11793.4

6 3,112,218 11/1963 Spiller 11793.4 3,129,112 4/1964 Marvin 2393 X ALFRED L. LEAVITT, Primary Examiner. 5 MURRAY KATZ, Examiner.

A. GOLIAN, Assistant Examiner.

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