US3826425A

US3826425A - Electrostatic apparatus

Info

Publication number: US3826425A
Application number: US00264967A
Authority: US
Inventors: J Scharfenberger; E Drum
Original assignee: Ransburg Corp
Current assignee: Ransburg Corp
Priority date: 1972-06-21
Filing date: 1972-06-21
Publication date: 1974-07-30
Anticipated expiration: 1991-07-30
Also published as: DE2331418A1; CA1016739A; GB1435182A; FR2336855A5; AU5366673A; JPS4952233A; GB1435181A; BE797119A

Abstract

An apparatus for and a method of electrostatically charging coating material. The apparatus includes a composite member terminating in an extended edge adjacent with coating material is formed into a charged spray. The composite member is constructed of an electrically non-conducting binder material such as a polymeric material, for example, phenolic resin and includes in the binder material an electrically conducting material such as graphite or copper, and reinforcing material such as glass fibers or woven hemp or flax (canvas). The composite member has desirable properties such as low effective electrical capacitance despite a necessary large physical size, good impact and tensile strength, and good chemical and mechanical resistance to the action of the ingredients of the coating material. The composite member is adapted to be connected to a high voltage source such as a direct current voltage source capable of supplying up to 100,000 volts or more to the composite member. Means such as a coating material feed tube and nozzle supplies to the composite member a suitable liquid such as paint, or dry coating material such as powder. A suitable means is used to move the coating material across the composite member to the extended edge in the vicinity of which the coating material is formed into a charged spray. The last named means may include an inner hub and an outer hub for cooperatively retaining the composite member.

Description

United States Patent 1 1 Scharfenberger et al.

[111 3,826,425 [451 July 30, 1974 ELECTROSTATIC APPARATUS [75] Inventors: James A. Scharfenberger; Edward W. Drum, both of Indianapolis, Ind.

[73] Assignee: Ransburg Corporation, Indianapolis,

Ind.

22 Filed: June 21,1972

[21] Appl. No.: 264,967

[52] US. Cl. 239/15 [51] Int. Cl. B05b 5/00, F23d 11/28 [58] Field of Search 239/15 56] References Cited UNlTED STATES PATENTS 2,901,177 8/1959 Norris 239/15 2,901,178 8/1959 Norris 239/15 3,010,428 11/1961 Sedlacsik 239/15 3,083,911 4/1963 Griffiths, .Ir 239/15 3,085,749 4/1963 Schweitzer ct a 239/15 3,128,045 4/1964 Gauthier 239/15 3,279,429 10/1966 Fclici et a1 239/15 3.358.931 12/1967 Wirth 239/15 3.572589 3/1971 Norris 239/15 Prinmry Eraminer-Lloyd L. King Almrm'y, Agent, or Firm-Merrill N. Johnson; Harry E. Downer; David H. Badger [57] ABSTRACT An apparatus for and a method of electrostatically charging coating material. The apparatus includes a composite member terminating in an extended edge adjacent with coating material is formed into a charged spray. The composite member is constructed of an electrically non-conducting binder material such as a polymeric material, for example, phenolic resin and includes in the binder material an electrically conducting material such as graphite or copper, and reinforcing material such as glass fibers or woven hemp or flax (canvas). The composite member has desirable properties such as low effective electrical capacitance despite a necessary large physical size, good impact and tensile strength, and good chemical and mechanical resistance to the action of the ingredients of the coating material. The composite member is adapted to be connected to a high voltage source such as a direct current voltage source capable of supplying up to 100,000 volts or more to the composite member. Means such as a coating material feed tube and nozzle supplies to the composite member a suitable liquid such as paint, or dry coating material such as powder. A suitable means is used to move the coating material across the composite member to the extended edge in the vicinity of which the coating material is formed into a charged spray. The last named means may include an inner hub and an outer hub for cooperatively retaining the composite member.

4 Claims, 5 Drawing Figures This invention generally relates to an apparatus for and a method of electrostatically charging coating material, and more particularly, to an apparatus including a composite member terminating in an extended edge adjacent which coating material is formed into a charged spray, the composite member having low effective electrical capacitance despite a necessary large physical size, good impact and tensile strength (mechanical strength), and good chemical and mechanical resistance to the action of the ingredients of the coating material. The composite member is constructed of an electrically non-conducting binder material and included in the binder material, an electrically conducting material and a reinforcing material. The apparatus includes an inner hub and an outer hub for cooperatively retaining the composite member.

The apparatus of the present invention is intended primarily for use in the electrostatic charging of either liquid coating materials such as paint, or dry coating material such as powder. The charged particles of coatin g material, whether liquid or dry, are attracted to and deposited upon the surface of an adjacent object or article maintained at a coating material attracting potential. The coating material deposited upon the surface of the object or article provides a layer thereon.

Several types of systems are commercially available that employ electrostatic principles to aid in the deposition of coating material particles upon a surface to be coated. These systems differ from each other in several respects including the means and methods used to atomize and electrostatically charge the liquid coating material, and in the case of dry material to eject and electrostatically charge the dry coating material.

One highly successful commercial system known as the Ransburg No. 2 Process" establishes a high voltage deposition and charging electric field between the surface to be coated and an exposed body of liquid material formed on an atomizer having an extended thin edge portion. The exposed body of liquid material is made coextensive with the extended edge of the atomizer, and the electric field has a high gradient at the extended edge of the liquid material. The electric field forms the extended edge of the liquid material into a number of liquid cusps which extend outwardly beyond the edge of the atomizer. Each of these cusps is highly charged and, therefore, a strong electrical repulsion exists between the main liquid body and the liquid in the tip of the cusps. Conversely, a strong electrical attraction exists between the liquid in the tips of the cusps and the surface to be coated. Therefore, a small portion of the liquid at the tip of each cusp is electrostatically urged to escape the forces of surface tension binding it to the main body of liquid material. The cusp tip is pulled away from the cusp when the repulsive force exceeds the surface tension force and is released into the electric field as a highly charged particle to be deposited on the article surface.

The Ransburg No. 2 Process often employs an electrically charged, physically large, rotating metal disk as the atomizer. Coating material is supplied to the surface of the disk at its center and is moved to the atomizer edge by rotating the disk. Under the action of the electric field, the coating material is dispersed adjacent the disk edge as a spray of finely divided, highly charged particles. The voltage at the edge of the metal disk is substantially independent of the spacing between the disk and the article being coated. Generally, the metal disk is physically large in order to obtain the necessary edge extent, and, therefore, has high effective electrical capacitance. That is, the metal disk has the propensity or ability of causing a disruptive discharge to an adjacent object at a potential relative to the metal disk which encourages such a discharge. Substantially the entire capacitance of a metal disk can be essentially instantaneously discharged. Therefore, one disadvantage of the metal disk is that if a person accidentally approaches too close to the disk, he may experience a high energy electrical shock. Further, if the discharge has sufficient energy, it may ignite the solvent fumes associated with conventional paint materials. In any event, and particularly with manually operated spray apparatus, it is desirable to have a minimum of metallic conductive material associated with the particle charging portion of the apparatus so as to minimize the effective electrical capacitance thereof. The advantages of minimizing the effective electrical capacitance are disclosed in U.S. Pat. No. 3,048,498, granted to Juvinall et al. The safety features disclosed in that patent are desirably incorporated in the apparatus of the present invention.

An electrostatic apparatus which attempts to overcome the high effective capacitance associated with metal atomizer is shown in U.S. Pat. No, 3,072,231, granted to Point et al. This patent discloses an atomizing head having a detachable collar with a sharp or tapering atomization edge. The detachable collar is made of a high resistivity material such as epoxy resin filled with lamp black or carbon black. The collar made of this material is very fragile and experiences erosion during use due to the mechanical action of the ingredients of coating material such as paint. Damage to the collar may adversely affect electrical charging of the liquid coating material.

An electrostatic apparatus which overcomes many of the problems associated with an electrostatic apparatus using a detachable collar made of epoxy resin filled with lamp black or carbon black is disclosed in U.S. Pat. No. 3,128,045, granted to Gauthier. Disclosed is an atomizing head including a pair of annular electrically insulating members of epoxy bonded fiber glass and an electro-conductive member of finely divided carbon in a thermosetting resin interposed between the pair of insulating members. The three elements used to provide the laminate atomizing head are brought together by the application of heat and pressure. This often leads to warping of the laminate head. Such laminate atomizing heads, because they are rotated at high speed, must be well balanced if they are to serve their intended function. The warping in many instances renders such disks unusable. To overcome this problem, the atomizing head of U.S. Pat. No. 3,128,045 is made of thicker elements and, therefore, is bulky, heavy and difficult to manufacture when compared to the atomizing head of the apparatus of the present invention.

It is, therefore, a desideratum to provide an electrostatic apparatus that overcomes the above difficulties and that effectively electrically charges a variety of coating materials; that has sufficiently low effective electrical capacitance despite its necessary large physical size so as to minimize harmful shock to personnel and to minimize solvent vapor ignition hazards and yet have sufficient electrical conductance to permit a high niently replaced in the apparatus when desired. The apparatus disclosed herein fulfills the above desideratum.

According to one form of the invention, there is provided an apparatus for electrostatically charging coating material including a composite member, for example a disk, having a surface terminating in an extended edge adjacent which coating material is formed into a spray. The composite member includes an electrically conducting material and reinforcing material in a binder of electrically non-conducting material. The composite member terminates in an extended edge and includes an electrically conducting material such as graphite or copper, and reinforcing material such as glass fibers or woven hemp or flax (canvas), in an electrically non-conducting binder material such as a polymer material (phenolic resin). The composite member may include minor amounts of impurities which do not harmfully affect the desired electrical, mechanical and chemical characteristics of the above-mentioned constituents. The composite member has properties such as low effective electrical capacitance when compared to a metal disk, good impact andtensile strength (mechanical strength), good chemical and mechanical resistance to the action of the ingredients of the coating material, thinness, lightweight and mechanical flexibil ity which allows it to rotatein such a manner as to minimize oscillation. The composite member is adapted to be connected to a high voltage source. A means provides the composite member with a suitable liquid such as paint, or dry coating material such as powder. A suitable means is used to move the coating material across the composite member to the edge adjacent which the coating material is formed into a charged spray.

The appended drawings illustrate several apparatus embodying the concepts of the present invention constructed to function in the most advantageous modes presently devised for the practicalapplication of the principles involved in the invention. In the drawings:

FIG. 1 is a diagrammatic illustration of an apparatus embodying the concepts of the present invention;

FIG. 2 is a partial cross sectional view taken of the spraying head of the apparatus shown in FIG. 1;

FIG. 3 is an enlarged partial cross sectional view of the disk of the spraying head of FIG. 2;

FIG. 4 is a top view of another form of spraying head embodying the concepts of the present invention for use in the apparatus as shown in FIG. 1; and

FIG. 5 is an enlarged partial cross sectional view of the head shown in FIG. 4.

Referring now to FIGS. 1 and 2 of the drawing, an electrostatic apparatus incorporating the concepts of the present invention is indicated by thereference numeral 10. The electrostatic apparatus includes means 11, hereinafter referred to as the spraying head or head 11, having a surface 12 that terminates in an extended edge 13 adjacent which coating material is formed into a charged spray. A means 14, such as the combination of a coating material feed tube 25 and nozzle means 26, is adapted to provide the surface 12 of the spraying head 11 with a suitable liquid such as paint, or dry coating material such as powder. Means 15 such as an air motor is used to rotatably displace the head 11 to move the coating material (paint) across the surface 12 to edge 13 adjacent which the coating material is formed into a charged spray.

Preferably, the articles 16 to be coated with coating material are transported by conveyor means 17 into spray zone 18 and around the head 11 in a path that generally conforms to the Greek letteromega. Preferably, shroud means 19 is employed to partially enclose the spray zone 18. While the articles 16 are in the coating zone 18,. each is maintained at a coating material particle attracting potential by suitable electrical connection to ground or earth potential through hangers 20 of conveyor means 17.

Head 11 is rotated at about 900 revolutions per minute or higher and is connected to the ungrounded terminal of direct current voltage supply 21 through cable 22, multi-megohm resistor 23, the series combination of conductive foam 63 and semiconducting rod 62 and semiconducting ring 61, air gap 39, the series combination of semiconducting plastic rods 24 and semiconducting plug 60, and conductive ink 38 as shown in FIG. 2. The multi-megohm resistor 23 has a value of several megohms per kilovolt with a substantial portion of the resistance adjacent the head 11. The voltage supply 21 is capable of supplying up to 100,000 volts or more at the ungrounded output terminal of the voltage supply to the head 11 through the multi-megohm resistor 23.

Coating material such as paint is fed to about the center of the rotating head 11 by means 14 including feed tube 25 and nozzle 26. The coating material is moved outwardly over the surface 12 of therotating head 11 to the edge 13. Atomization of the paint occurs from cuspsor filaments of paint pulled out from the coating material at edge 13 under the influence of the electric field extending to the articles 16 in the coating zone 18. The paint particles possess a high charge-to-mass ratio and v are electrostatically attracted to and deposited upon the grounded or earthed articles 16 in the coating zone 18.

Articles 16 of extended vertical length may be coated by repeatedly moving head 11 over the articles by a reciprocator means 27. Since the reciprocator means 27 and air motor 15 may be of any known suitable form, of which many are conventionally used, a detailed showing of each has been omitted from the drawing in the interest of a clearer showing of the inventive portion of this disclosure.

Referring to FIGS. 2 and 3 of the drawing and more particularly to FIG. 3, a cross section of one form of disk member 28 of head 11 is shown. The disk member 28 is in the form of a ring having distributed high electrical resistivity. The disk member 28 is a composite material including an electrically conducting material 29 and a reinforcing material 30 both included in an electrically non-conducting binder material 31. Preferably, the conducting material 29 is a carbon containing material, and more preferably, a particulate graphite having an average particle size about 40 to 50 microns. In lieu of particulate graphite, other electrically conducting materials such as carbon black or carbon fiber may be used. Likewise, it may be possible to incorporate the electrically conducting characteristics of the composite into the reinforcing material 30 by using conductive glass fibers or carbon fibers. However, particulate graphite is preferred because of its good chemical and mechanical resistance to the action of the ingredients contained in paint. If good chemical and mechanical resistance to the action of the ingredients of the paint is not a consideration, then other suitable electrically conducting materials may be substituted for the particulate graphite.

The weight percent of graphite in the composite member 28 should be such as to provide the composite with the desired surface resistivity. A suitable weight percentage of graphite to the total weight of the composite is about 4 to 8 weight percent, and preferably about 5 to 7 weight percent of the total weight of the composite. Composites containing more than about 8 percent by weight graphite appear to be too conducting, whereas composites containing less than about 4 percent by weight graphite appear to be too nonconducting.

In order to provide the composite with the desired mechanical properties such as good impact and tensile strength and dimensional stability, a suitable reinforcing material 30 is incorporated into the binder material 31 of the composite. Desirably, the reinforcing material 30 includes the properties of heat and electrical resistance, and chemical and mechanical resistance to the action of the ingredients of the coating material. Such reinforcing materials include, but are not limited to, organic fiber reinforcements such as hemp, flax and sisal; synthetic reinforcements such as fibers of Nylon and Orlon; and mineral reinforcements such as fibrous glass and asbestos. The preferred reinforcing materials include fibers of hemp, flax, Nylon, Orlon and glass. The reinforcing material 30 may be either woven or nonwoven with woven fiber preferred over non-woven fiber. 1n the event good chemical and mechanical resistance of the reinforcing material 30 to the action of the ingredients of the coating material is not a consideration, then reinforcing materials such as maceratedlchopped fibers of cotton, or fibers of rayon, fluorocarbon alpha cellulose and the like may be used in the binder material to reinforce the composite. Preferably, the weight percent of the reinforcing material to the total weight of the composite is about 40 to 44 weight percent. The weight percent of the reinforcing material to the total weight of the composite may be varied as long as the reinforcing material performs its intended function.

The binder material 31 of the composite should possess such properties as good heat and electrical resis tance, and good chemical and mechanical resistance to the action of the ingredients of the coating material. A polymeric material such as a rigid thermosetting resin is satisfactory. Suitable rigid thermosetting resins include epoxy, polyester, phenolic and melamine resins with phenolic resin being the most preferred resins. Desirably, the weight percent of binder material to the total weight of the composite is about 52 weight percent. The weight percent of the binder material to the total weight of the composite may be varied as long as the binder performs its intended function.

A

disk

28, 10 inches in outer diameter with a central aperture of 4% inches in diameter and about onesixteenth of an inch thick and including 6.3 weight percent of particulate graphite, 52 weight percent canvas (hemp or flax) woven reinforcement, and 41.7 weight tances measured at 1,000 volts, direct current between a single point on the circumference of the aperture of disk 28 and a single point on the extended edge 13 is about 15.8 megohms/inch.

An average resistance/square inch measured at 1,000

volts, direct current, between a first annular metal band contacting the entire circumference of the aperture of disk 28 and a second annular metal band contacting the entire outer circumference (edge 13) of the disk, is about 0.018 megohms/- 15 square inch.

The resistance/inch from a single point on the circumference of the aperture of disk 28 to a single point on the extended edge 13 on substantially the same radius line is determined by using a circuit comprised of a direct current, high voltage source having one terminal grounded and an output voltage of 1,000 volts; a Hallmark Standards voltage meter connected between the output of the voltage source and ground; a first probe having its input connected to the output of the voltage source and a probe tip on the edge 13 of the disk; and a second probe having a probe tip on the circumference of the aperture of the disk and is connected through an ammeter to ground. A plurality of current and voltage readings were taken around the disk by moving the probe tip of the first probe on the edge 13 and the probe tip of the second probe on the circumference of the aperture of the disk and these readings were used to calculate resistances which were 5 averaged to arrive at the above-recited megohms/inch.

13 of disk 28 and a second annular metal band contacting the entire circumference of the aperture of the disk is determined by using a circuit comprised of a direct current high voltage source having one terminal grounded and an output voltage of 1,000 volts; a Hallmark Standards voltage meter connected between the output of the voltage source and ground; the first annular band contacts the extended edge 13 and is connected to the output of the high voltage source; the second annular band contacts the entire circumferential surface of the aperture of disk 28 and is connected through an ammeter to ground. A plurality of readings were taken of current and voltage and these readings were used to calculate resistances which were averaged to obtain the above-recited megohms/square inch.

The above resistances were ascertained in a realtive humidity of about 62 percent at a temperature of about 78 F.

The surface resistivity is determined of a plurality of materials each about one-sixteenth of an inch thick and including 6.3 weight percent of particulate graphite, 52 weight percent canvas (hemp or flax) woven reinforcement, and 41.7 weight percent phenolic resin binder with a minor amount of undetermined impurities in a relative humidity of about 62 percent at a temperature of about 78 F. Each such material is to be used as a composite disk member. The circuit used to measure the surface resistivity of each such material is a circuit comprised of a direct current, high voltage source having one terminal grounded and an output voltage of about 1,000 volts; a Hallmark Standards voltage'meter connected between the output of the voltage source and ground; a first -inch long, 0.0075 inch wide knife edge of SAE type 01 tool steel connected to the output of the high voltage source and engaged with the surface of the material; a second 5-inch long, 0.0075 inch wide knife edge of SAE type 01 tool steel with its knife edge in spaced parallel relationship with the first knife edge and separated by an insulator and engaged with the surface of the material, the spacing between the knife edges is I inch; and an ammeter connected in series with the second knife edge and ground. The force at knife edges is about 4,223 grams. The high voltage source is activated so as to apply about 1,000 volts direct current to the series connected circuit. As the knife edges are moved to various positions on the surface of each of the materials, ammeter readings are obtained which, when converted to ohms/square, vary between about megohm/square and about 200 megohms/square. The average of the readings across the surfaces of the materials is about 3.2 megohms/square. The value of 3.2 megohms/square was obtained by averaging the readings across each material separately and then averaging the average readings of each material.

As shown in F 1G. 2, the upwardly presented surface 12 of the composite disk member is generally coplanar with the coating-delivery face of inner hub 32 of electrically non-conducting material. The inner hub 32 includes an outer circumference of a raised, circular rim shoulder 33 that overlies a portion of the surface 12 of the disk 28. Rotating the disk 28 causes coating material such as paint fed by the nozzle 26 through feed tube 25 from paint supply 36 and pump 37 to the circular trough 34 formed in the inner hub 32 to, under the action of centrifugal force, climb circular wall 35 and flow smoothly over the raised circular rim shoulder 33 onto the surface 12 of disk member 28. The centrifugal force created by the rotation of the composite disk member 28 causes the coating material to move across the upper surface 12 of the composite disk member in operative association with the upper surface l2'to the extended edge 13 of the disk. In most cases coating materials are not extremely electrically conducting, and where such materials are used, the coating material supply 36 and pump 37 and preferably some intermediate point of the feed tube 25 extending from paint supply 36 should be suitably electrically grounded or earthed.

High voltage is applied to the upper surface 12 of the composite disk member 28 through electrically conducting ink 38 of the type disclosed in US. Pat. No. 3,021,077, granted to Gauthier, the series combination of semiconducting rods 24 and semiconducting plug 60, air gap 39, the series combination of semiconducting ring 61 and semiconducting rod 62 and conductive foam 63, multi-megohm resistor 23, and cable 22 from high voltage power supply 21. The coating material moving to the extended edge 13 of the composite disk member 28 comes into operative association with the surface 12 and moves radially outwardly and to the extended edge 13 adjacent which it is dispersed as a spray of finely divided, electrically charged particles for deposition on the grounded articles 16 to be coated. A high voltage of at least 40,000 volts, and preferably 70,000 volts or more at the output terminal, is supplied from the voltage supply 21 (normally from the negative terminal thereof) through the multi-megohm resistor 23, preferably of the order of several megohms per kilovolt applied, with at least a portion of the resistance immediately adjacent to inner hub 32 for establishing the required electrical gradient between the extended edge 13 of composite disk member 28 and the articles 16 to be coated. If the apparatus 10 is used with a voltage supply 21 having an output voltage which decreases appreciably as current increases, series resistance may be reduced.

To effect rotation of head 11, it is interconnected to a drive shaft 41 by an assembly 42 all of whose components at the head end are preferably made of electrically non-conducting materials capable of withstanding stresses associated with the highest voltages associated with the voltage power supply 21 without accompanying breakdown or rupture of the material. Such an assembly 42 includes a recess 43 formed in drive shaft 41, keyway 44, key 45 in the keyway 44 and abutting the shaft 41, and an annular cover 46. As shown, the driving torque of the shaft 41 is transmitted to the inner hub 32.

Suitably connected to the inner hub 32 by means of fastening means 47 such as bolts and the like is outer hub 48. Annular flange 49 of the outer hub 48 and the rim 33 of the inner hub 32 are used to compressively retain and thereby suitably support the composite disk member 28 in position with respect to the other components of the spraying head 11 during the charging of the coating material. It is seen that the disk 28 may thus be easily and quickly removed and/or replaced by re moving bolts 47 and outer hub 48, replacing the disk with a different disk, reinserting bolts 47 into the outer hub and tightening the bolts to the position shown in FIG. 2.

The electrostatic apparatus 10 is illustrated and has been described herein in an upright horizontal position. However, it may be oriented in any plane, and it may be oriented in an inverted position with the coatingdelivery surface 12 presented upwardly.

Referring now to FIGS. 4 and 5 of the drawing, another modification of disk member 50 embodying the concepts of the present invention is shown. The disk member 50 includes a surface 55 that terminates in an extended edge 51 adjacent which coating material is formed into a charged spray. The disk 50 shown in FIGS. 4 and 5 is annular and includes a reinforcing material 53 in a binder material 54 as in the composite disk member 28 of FIG. 3; however, the electrically conducting material 52 in the embodiment of FIGS. 4 and 5 is localized and recessed in the disk 50 as a thin strip of electrically conducting material 52 including a ring element 56 closely adjacent the extended edge 51 and a radial element 57 extending from about the aperture 58 of the disk to the ring element 56.

Desirably, the reinforcing material 53 includes the properties of good heat and electrical resistance, and good chemical and mechanical resistance to the action of the ingredients of paint. Such materials include, but are not limited to, organic fiber reinforcements such as hemp, flax and sisal; synthetic reinforcements such as fibers of Nylon and Orlon; and fluorocarbon mineral reinforcements such as fibrous glass and asbestos. The preferred reinforcing materials include fibers of hemp, flax, Nylon, Orlon and glass. The reinforcing material 53 may be either woven or non-woven with woven fiber preferred. In the event good chemical and mechanical resistance of the reinforcing material 53 is not a consideration, then reinforcement materials such as macerate chopped fibers of cotton or fibers of rayon, fluorocarbon, alpha cellulose and the like may be used in the binder material. The weight ratio of reinforcing material to binder material is about 1:1. The weight ratio of reinforcing material to binder material may be varied as long as each material performs its intended function.

The binder material 54 should possess such properties as good heat and electrical resistance, and good chemical and mechanical resistance to the action of the ingredients of paint. A polymeric material such as a rigid thermosetting resin is satisfactory. Suitable rigid thermosetting resins include epoxy, melamine, phenolic and polyester resins with phenolic resin being the most preferred resin.

The conducting metal strip material 52 including

elements

56 and 57 is desirably physically small and in the form of a one-sixty-fourth of an inch wide metal strip insert recessed in the disk 50. Ring 56 of conducting material is located a short distance from and coextensive with the extended edge 51 of the disk 50. The radial strip 57 of the same dimension as ring element 56 extends from aperture 58 of the disk 50 to ring element 56. The exposed film of liquid coating material at the extended edge 51 is charged to a high voltage by virtue of the conductance of the liquid between the ring element 56 and the extended edge 51. The resistance of the paint layer between the metal strip material and the extended edge 51 thus becomes an additive part of the total resistance in the electrical circuit between the high voltage supply 21 and the disk edge 51. At the same time, the effective electrical capacitance of the disk 50 is small compared to a metal disk and its rigidity and tensile strength is better than a similarly shaped non-reinforced unit.

After a coating of paint of the desired thickness is deposited on the articles 16, each article is moved from the spray zone 18, and the coating changed to a solid by either air drying or heat drying at suitable elevated temperatures.

The following example is given to further illustrate the invention:

EXAMPLE Unheated cylindrical metal tubes 16 having a length of about 42 inches, a diameter of about 1 inch and arranged on about 3-inch centers are conveyed in an arc of about 270 in extent around means 11 at a rate of about 9 to 10 feet per minute. About 500 cc per minute of Lilly White Paint 565 manufactured by Lilly Industrial Coatings, Inc. of Indianapolis, Ind., USA. is delivered to disk 28 having about a 25-inch diameter and rotating at about 900 rpm. The disk 28 includes graphite particles and woven canvas in a binder of phenolic resin. The spacing between the disk edge 13 and tubes 16, when the tubes are adjacent the disk 28 and in spray zone 18, is about to 12 inches. The articles are in the spray zone for about 45 to 60 seconds. The voltage applied to the spraying head 11 including disk 28 from the direct current power supply 21 is approximately 100 kilovolts. The resistance value of the multimegohm resistor 23 is about 500 megohms. A reciprocator moves the disk 28 vertically over the length of the tubes 16 at about times per minute. After spraying the tubes 16 with the Lilly White Paint 565 and baking at about 300 F for about 20 minutes, a permanent film about one mil thick is formed on each tube. No destructive discharge between the disk 28 and the tubes 16 is observed when the tubes are intentionally moved into contact with the extended edge 13 of the composite disk member 28.

As regards electrostatic coating, it appears that, in the alternative, articles 16, if carried by electrical insulators, can be connected to the high voltage power supply 21 and the spraying head 11 can be grounded. In such alternative, the electrostatic field exists between the spraying head 11 and the articles 16; such electrostatic field appears to have substantially the same effect on the paint and the powder particles as does the electrostatic field provided when the article is grounded and the spraying head is connected to the power supply 21. Preferably, however, the spraying head 11 is connected to the high voltage power supply 21 and articles 16 are grounded. In the event the surfaces of articles 16 are electrically non-conductive, the surfaces may be rendered sufficiently electrically conductive using a suitable pretreatment coating. For example, the insulative surfaces of wood, fiberwood, Masonite hardboard, polyethylene, Nylon, and the like may be pretreated as described in US. Pat. No. 3,236,679, granted to Spiller, to thereby render such surfaces sufficiently electrically conductive for electrostatic liquid or powder coating.

The dimensions of the disks 28 and S0 and the operating parameters of the electrostatic apparatus incorporating the disks are not given by way of limitation but for the purpose of illustration only. Such dimensions and operating parameters may vary over a considerable range with respect to one another. Further, the concepts of the present invention apply not only to disk elements, but also to bell-like elements as described in US. Pat. No. 3,048,498, granted to Juvinall et al.

The disk structure of the present invention has increased mechanical strength because of the presence of the reinforcing material; accordingly, a much thinner and lighter weight disk can be used in electrostatic apparatus for charging coating material than was heretofore possible. Such a lighter weight and thinner disk tends to rotate without harmful oscillation and, therefore, provides more consistently uniform coating material atomization characteristics. A disk made from the material taught is more rugged and less likely to suffer from abusive handling than a disk made from nonreinforced materials. Such a reinforced disk, because it has better tensile strength than a non-reinforced disk, can be rotated at five to six times the annular velocities normally employed with non-reinforced disks. The higher rotational speed is beneficial since it allows greater quantities of coating material to be atomized per unit of edge length.

Paint," as that term is used herein, includes liquid or semi-liquid material which may be applied to surfaces in relatively thin layers and which change to a solid coating with the passage of time. The change to a solid may or may not be reversible and may occur by evaporation of solvent, by chemical reaction, or by combination of the two. Paints usually consist of a vehicle or binder, a pigment which contributes opacity, color, hardness and bulk to the film, and a solvent or thinner which controls the consistency.

As disclosed hereinbefore, the concepts of the present invention may be utilized to electrostatically charge particles of a dry coating material such as powder. Powder, as that term is used herein, means and includes thermoplastic dry powders such as polyester, polyvinyl chloride, polypropylene, polyethylene, Nylon, cellulose acetate butyrate; thermosetting dry powders such as epoxies, polyesters, acrylics; other dry powders such as starch, talc, vitreous enamel, and the like.

Many powders, and particularly synthetic thermosetting and thermoplastic powders, when fused, provide films which have characteristics, such as corrosion resistance, color and dielectric strength which make them desirable as coating materials. For example, epoxy resins may be applied and fused to pipe and fittings used in handling corrosive materials; and fused polyesters, butyrates and acrylics may be used as protective and decorative coatings for large flat surfaced articles such as automobile parts, appliance parts and the like, or tubular articles such as bicycle frames and the like. Still other powders such as powdered fluxes can be applied to a surface to be used ultimately in the powder form. Also, powdered talc can be applied to prevent self-adhesion of the surfaces being coated. The particular powder used to coat the article will depend on, among other things, the purpose for which the coating is to be used, the nature of the finish required, the environmental conditions to which the article is to be subjected and the like.

Generally, powders are prepared by grinding bulk material, usually at a low temperature. Powder having a particle size in the order of 20 to 200 microns is preferred for electrostatic powder spraying, however, the powder may be coarser or finer, depending on the particular material and application thereof.

The

disks

28 and 50 illustrated in FIGS. 2 and 4 respectively, may be used to electrostatically charge the powder formed into a spray.

While the invention is illustrated and described in its presently preferred embodiment, it will be understood that modifications and variations may be effected without departing from the scope of the novel concepts of this invention and as set forth in the appended claims.

We claim:

1. In an electrostatic coating apparatus including a conductive member having high resistance and forming an annular electrostatic atomizer, a central member connected to a rotating shaft and carrying said conductive member, a resistive high voltage connection between a source of high voltage and said conductive member, and an insulating support for said rotating shaft and said high voltage connection, the improvement in which the central member is comprised of two separable parts, one part of which is connected to said rotating shaft and includes semiconducting connection connected with said high voltage connection, and the other part of which is fastened to said one part to hold the conductive member removably therebetween and connected with said semiconducting connection.

2. The apparatus of claim 1 wherein the high voltage connection includes a semiconducting ring surrounding the rotating shaft, and the semiconducting connection is a semiconducting rod carried adjacent the semiconducting ring within the interior of said one part of the hub.

3. The apparatus of claim 2 wherein the semiconducting rod is oriented within said one part with its end opposite said ring and said one part and rod are spaced from said ring, said space forming a current path with low conductivity.

4. The apparatus of claim 3 wherein the insulating support encloses resistors forming the resistive high voltage connection, the semiconducting ring, and a resistive connection therebetween.