Patent Publication Number: US-7217444-B2

Title: Process for electrostatic powder coating an article using triboelectrically charged powder with air jet assist

Description:
BACKGROUND 
     An exemplary embodiment of this application relates to an electrostatic powder coating process and apparatus for coating an article with charged powder particles by forming a powder cloud with one or more air jets, so that the cloud of charged particles surrounds and coats the article. More particularly, the exemplary embodiment relates to an electrostatic powder coating process and apparatus for coating a grounded article with triboelectrically charged powder particles contained in a chamber, advancing the charged powder particles onto at least one rotatable donor roll, rotating the donor roll past an air jet whereat a combination of electric fields and/or air jets detach the charged powder particles from the donor roll to form a powder cloud that is directed to the grounded article. 
     The process of coating metal or conductive surfaces with dry powder coatings is well known. The process has been used since the mid 1950&#39;s. The initial applications of electrostatic powder coating involved the coating of pipe and electric motors. By utilizing the natural principle of opposite charges attract, this coating technology offered manufacturers an alternative to solvent-based paints. With the growing need to reduce air pollution from solvent-based paints, the demand for electrostatic powder coating has increased over the years. Although the cost of powder and liquid coatings is comparable, dry powder coating is advantaged because spray booths are easily cleaned, no solvents are used eliminating the need for air pollution control equipment, over spray can be collected for reuse, thicker film coatings can be obtained in a single application, and powder has no surface tension, so it will penetrate into small gaps precluded by liquid coatings. 
     Powder particles for powder coating apparatus are typically delivered to spray guns that electrically charge the expelled powder particles by means of ion corona discharge or triboelectric charging. The powder feed system for the spray guns generally requires fluidization of the powder particles. The powder generally does not contain surface flow additives to improve fluidization because the quality of powder coating film would be compromised during the oven-curing step. With this materials constraint, the powder coating industry generally uses powder particles having an average diameter of 30 to 40 μm, since particles of this size are easily fluidized. The fluidized powder particles are pneumatically transported to a triboelectric or ion charging spray gun for charging the powder and directing the cloud of powder particles to the article to be coated. For triboelectric spray guns, the powder collides with triboelectric-active materials placed in a tortuous path within the gun. For ion spray guns, a voltage of approximately 100 kV applied to a needle electrode generates a corona and the ions are captured on the powder particles in the powder cloud. However, to achieve higher coating quality with thinner layers, there is a need for an improved process and apparatus for powder coating articles with powder particles having an average diameter of approximately 10 μm. This requirement is particularly desired for top gloss coat in automobile coating. 
     There are many existing powder coating systems wherein a powder is air fluidized in a reservoir and pneumatically fed to a spray gun where the powder is charged. A combination of electrostatic and pneumatic forces transports the charged powder to the article to be coated. Electrostatic forces attract the charged powder to the article. The coated article is typically baked in an oven for approximately 10 minutes at about 400° F. The powder coating melts and flows into a durable film. Such typical powder coating systems are provided by, for example, Wagner International AG, Ransburg Corporation, Nordson Corporation, and Gema Volstatic AG. 
     U.S. Pat. No. 6,342,273 discloses an improvement over the typical powder coating processes and apparatus. In this reference, electrophotographic development system technology is described to triboelectric or induction charge powder paint particles for electric field transfer directly to flat substrates for subsequently curing, such as, a continuous or discontinuous band, sheet or web substrates. One substrate example was an unwound flat coil. According to a preferred embodiment, the powder paint particles are mixed with magnetic or non-magnetic carrier particles to obtain friction charging. Then the mixture is transported adjacent the substrate to be coated by a transport means, and the charged powder particles are extracted from the mixture and applied to the substrate by means of an electric field between the substrate and the means of transport. The advantage of such a process is that it is possible to apply powder particles having particle sizes between 5–30 μm. Unfortunately, such a process is limited to coating directly onto closely adjacent and confronting flat substrates in a manner similar to developing an electrostatic latent image on a photoreceptor in the field of electrophotography. Thus, the process of this patent cannot adequately coat three-dimensional articles, such as those articles needed in the industry for appliances, automobiles, and the like. 
     U.S. Pat. No. 5,518,546 discloses process and apparatus for applying electrostatically charged powder resin particles to a substrate to be coated by utilizing a fluidized bed for inductively charging the powder resin particles through a high voltage means disposed at one portion of the fluidized bed and a grounded electrode disposed in another portion of the fluidized bed, so that an electric field is created therebetween. Fluidizing air is applied to the powder resin particles to establish an electrostatically charged powder cloud within the fluidized bed, and then pneumatically conveying the charged powder particles from the fluidizing bed to a dispensing nozzle or gun for directing the charged powder particles onto the substrate. The coating on the substrate is subsequently fused or cured to form a permanent film thereon. The powder resin particles are mixed with at least one modifying agent that promotes charging of the mixed particles, but does not alter the melt or durability characteristics of the powder resin particles. 
     U.S. Pat. No. 5,078,084 discloses using a powder material for coating large objects, such as, vehicle bodies. An electrical charge is applied to sprayed powder which is directed toward grounded vehicle bodies to be coated by spray guns. The application of powder material onto the automotive or truck bodies is performed in a spray booth, so that over spray powder that is not deposited on the vehicle body can be collected. An exhaust system that creates a negative pressure in the spray booth aids in containment of the over spray powder and causes the over spray powder to be drawn into a powder collection and recovery system. The recovered powder is saved for reuse by the powder spray guns. 
     U.S. Pat. No. 5,743,958 discloses apparatus to collect and reuse over spray powder that contains a proportionately greater percentage of smaller particles or “fines” than virgin powder, since a greater percentage of larger particles have adhered to the object to be coated. The system for applying powder coating material onto large objects, such as, vehicle bodies, includes a powder spray booth, a powder kitchen containing a mixing hopper, and a number of feed hoppers, which receive the powder material from the powder kitchen and supply it to powder spray guns. The mixing hopper in the kitchen maintains the selected ratio of recovered over spray powder and virgin powder material. 
     U.S. Pat. No. 4,805,069 discloses an electrostatic powder painting apparatus including a powder charging apparatus therein. The powder painting apparatus has a pair of plasma electrodes disposed in an insulative tubular passage for transporting powder carried by gas. A DC voltage is intermittently applied between the plasma electrodes to form one space where mainly desired polarity ions exist that are drawn from the plasma electrodes and another space where mainly opposite polarity ions exist, thereby assuring stable and strong charging without adhesion and accumulation of powder to and on either one of the pair of plasma electrodes. 
     U.S. Pat. No. 4,330,567 discloses an electrostatic fluidizing bed coating apparatus for work pieces, especially those of continuous length, such as metal wires. The coating apparatus includes a housing with a planar horizontal porous support member therein defining a fluidization chamber in the housing above the porous support member and a plenum below it. Gas is introduced into the plenum for passage upwardly through the porous member to effect fluidization of particulate coating material in the chamber. A means is provided for ionizing the gas in the plenum to effect electrostatic charging of the fluidized coating material. An electrically conductive grid is mounted in the plenum between the porous support member and ionizing means and has means to control its electrical potential. The work piece is passed through the housing between the porous support member and the cloud control grid, so that the grid may be used to affect the deposition of powder upon the work piece. 
     U.S. Pat. No. 3,680,779 discloses an electrostatic powder sprayer for depositing powder material on an article to be coated. A metering roller at the bottom of a powder reservoir is rotated to dispense powder. A first circuit is connected to the metering roller and a conductor located between the article and the metering roller establishes a primary AC electric field to accelerate the powder from the metering roller. At least one electrode is spaced a greater distance from the metering roller than the conductor. A second circuit is connected between the metering roller and the electrode to establish an auxiliary electric field therebetween. The second circuit is provided to further control the movement of powder from the area of the primary electric field to the article, so that a more uniform powder is laid on the article and the amount of powder “fly around” is reduced. 
     U.S. Patent Application Publication No. 2002/0127332 discloses apparatus and method for applying powder to the interior surface of a hollow object. A powder discharge device having a powder discharge outlet is positioned within the hollow object and a stream of electrostatically charged powder is directed through the discharge outlet. The object is rotated, so that the stream of charged powder contacts the interior surface of the rotating object. 
     U.S. Patent Application Publication No. 2002/0160123 discloses a method for electrostatic powder coating or painting of non-conductive polymer surfaces by applying a conductive layer, such as a metal foil on the side opposite the non-conductive polymer surface to be painted, so as to provide sufficient conductivity to enable electrostatic powder painting thereof. With the conductive layer in place, the non-conductive article can be painted electrostatically. After painting, the conductive layer can be optionally removed without affecting the painted surface. 
     U.S. Patent Application Publication No. 2003/0003813 discloses metallic studs of a dynamoelectric machine that are coated with a powder resin to form an electrical insulator between the rotor and the studs. The powder coating is applied by several disclosed methods. In the preferred method, an electrostatic fluidized bed is provided and air is passed through the powder disposed in the bed to obtain a rolling boil of the powder. An applied high potential is provided in the air to produce free electrons that are passed through the powder. The stud is located in the powder cloud in the fluidized bed and as the powder is charged, the electrostatic potential enables the powder to be attracted to the studs. The powder coating on the stud is then cured. Instead of disposing the stud in a powder cloud, a spray gun may be utilized. 
     SUMMARY 
     It is an object of an exemplary embodiment of this application to provide an electrostatic powder coating process and apparatus for triboelectrically charging powder particles having an average diameter of about 10 μm, loading a uniform layer of the charged powder particles on a donor roll, and detaching the charged powder particles from the donor roll by means including an air jet to generate a cloud of charged powder particles for electrostatic attraction and coating of articles, such as, for example, appliances and automobiles. 
     In one aspect of the exemplary embodiment, there is provided a process for electrostatic powder coating of an article, comprising the steps of: triboelectrically charging powder particles contained in a chamber, said powder particles having an average diameter of about 10 μm; advancing said powder particles from said chamber onto at least one rotatable donor roll; metering said powder particles on said donor roll while the donor roll is being rotated to provide a uniform layer of powder particles thereon; providing means for detaching said powder particles from the at least one donor roll and forming a cloud of charged powder particles, said means for detaching including an air jet; directing said cloud of charged particles toward an article to be coated by said air jet; and grounding said article, so that said charged powder particles are attracted to and coat the outer surface of said article. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of this application will now be described, by way of example, with reference to the accompanying drawings, in which like reference numerals refer to like elements, and in which: 
         FIG. 1  is a schematic isometric view of an electrostatic powder coating system incorporating the powder coating process and apparatus of this application; 
         FIG. 2  is a schematic elevation view of the powder coating apparatus of this application shown in cross-section as viewed along line  2 — 2  in  FIG. 1 ; 
         FIG. 3  is a schematic elevation view of another powder coating apparatus of this application shown in cross-section and similar to  FIG. 2 ; and 
         FIG. 4  is a schematic elevation view of still another powder coating apparatus of this application shown in cross-section and similar to  FIG. 2 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  schematically depicts the various components of an illustrative powder coating system  10 , incorporating the electrostatic powder coating apparatus  12  of this application. Inasmuch as the art of electrostatic powder coating systems are well known, the operation of the system shown in  FIG. 1  will be only briefly discussed. 
     The electrostatic powder coating system  10  shown in  FIG. 1  includes a powder coating booth  14  that ensures that the powder cloud produced by the electrostatic powder cloud generating apparatus  12  is contained therein. The powder cloud generating apparatus  12  may contain one or more stationary or moving units for producing the powder cloud. Articles  16  to be coated, such as, for example, an I-beam, may be manually placed in the coating booth through the door  13  or, as illustrated, may be placed in the coating booth automatically by a conveyor or monorail  18  on movable supports  17 . A slit  19  having a flexible seal  20  is provided in the top wall of the coating booth to accommodate the movable support carrying the article  16  to be coated while preventing the escape of the powder cloud. Once the article is coated with powder, it is removed through a back door (not shown) and transported into an oven  22  for curing through the opening  21  covered by sliding doors  24  moved in directions indicated by arrows  23 . Instead of an oven, another curing enclosure (not shown) could be provided for low temperature curing, such as, for example, UV curing. 
     The coating booth  14  may be constructed with stainless steel or a non-metallic material. Cleaning air jets  26  interconnected between one side of the coating booth and a manifold  25  of an air compressor  27  are provided to enable clean up of the over spray powder in the coating booth. A cyclone-type powder extractor  28  connected to the coating booth may be activated to collect the unused powder in the coating booth for reuse. On all coating booths, manual or automatic, air velocities should be sufficient to prevent powder from escaping and causing contamination of the work area. In other words, there should be air streaming into the coating booth at every opening during a coating operation of at least 0.5 m/sec. Usually, the powder recovery system includes the air withdrawal or suction necessary to prevent powder from escaping from the coating booth. 
     In  FIG. 2 , a schematic cross-sectional elevation view of the powder coating apparatus  12  is shown as viewed along line  2 — 2  in  FIG. 1 . The powder coating apparatus comprises a housing  30  containing a supply of powder material  50  in a lower section or sump  52  of the housing. The housing  30  is attached to the coating booth  14  with an aperture  49  therebetween. The powder material  50  comprises powder particles  51  and magnetic beads (not shown). The powder particles range in size from about 5 to 40 μm in diameter and are preferably about 10 μm. The magnetic beads range in size from about 30 to 300 μm in diameter and are preferably about 60 to 80 μm. The beads can have a partial surface coating of a polymer such as poly(methyl methacrylate) or polytetraflouroethylene and mixtures thereof for the purpose of controlling the triboelectric charging polarity and magnitude of the powder particles. 
     The housing sump  52  includes at least one auger  53 , and preferably two, that is rotatably mounted therein and is rotated by any suitable drive means, such as, for example, an electric motor (not shown). The auger serves to disperse and mix the powder particles and magnetic beads, to transport the powder material to appropriate locations within the sump  52 , and to agitate the powder material within the sump to triboelectrically charge the powder particles, so that they adhere to the magnetic beads. 
     A magnetic brush roll  54  transports powder material  50  from the sump  52  to the loading nips  55  of a pair of donor rolls  56 . The donor rolls are parallel to each other and mounted for rotation about their respective parallel axes  47 . The donor rolls may be identical or have different sizes. The donor rolls  56  are rotatably driven in the direction of arrows  45  by any suitable means, such as, for example, one or more electric motors (not shown). The donor rolls contact the magnetic brush roll  54  and provide loading nips  55  along the length of the donor rolls. A layer of powder material is sandwiched between the magnetic brush roll and the donor rolls. 
     Magnetic brushes are well known, so the construction of a magnetic brush roll need not be described in great detail. Briefly, the magnetic brush roll  54  comprises a rotatable tubular member or sleeve  58  within which is located a stationary magnetic cylinder  59  having a plurality of alternately polarized magnetic pole pieces  57  impressed around its outer surface. A non-magnetic member  60  is impressed among the magnetic poles on the magnetic cylinder  59  at a location downstream from the point of contact by the last or downstream donor roll, in order to remove the powder material  50  from the magnetic brush roll. As the tubular member  58  of the magnetic brush roll  54  rotates in the direction of arrow  63 , magnetic beads of the powder material, together with powder particles adhering triboelectrically thereto, are attracted to the magnetic brush roll except in a region adjacent the non-magnetic member  60 . 
     Thus, the powder material  50  is conveyed from the sump  52  to electrically grounded donor rolls  56  through loading nips  55  whereat the powder particles are extracted from the magnetic beads and deposited on the donor rolls by an AC/DC electrical bias on the rotating tubular member  58  through voltage source  68  connected to the axially located shaft  61  of the magnetic cylinder  59 . Shaft  61  is parallel to the donor roll axes  47 . As the magnetic brush roll continues to rotate passed the last donor roll, the magnetic beads, which are partially denuded of powder particles, encounter the non-magnetic member  60 , causing the powder material, though having a reduced amount of powder particles, to drop from the magnetic brush roll and into the sump. The ratio of powder particles to magnetic beads are maintained at a relative constant amount by well known concentration monitoring devices (not shown) of the type typically used in the electrophotography industry. 
     A metering blade  62  removes excess powder material  50  from the magnetic brush roll  54  and ensures an even depth of coverage with powder material before arrival at the loading nips  55 . The donor rolls  56  have a resistive overcoating on each of their outer surfaces that provides dissipation of any charge accumulation thereon. An array of self-spaced AC biased wires  64  having a diameter of about 60 μm detaches the charged powder particles  51  from the electrically grounded donor rolls to generate a powder cloud  66  in the vicinity of the wires  64 . The AC voltage from AC voltage source  65  relative to the donor rolls is typically a square wave having an amplitude of 600 to 800 volts peak-to-peak and a frequency of 10 kHz. A low DC voltage (not shown) can also be included with the AC applied to the wires to prevent powder particle accumulation on the wires. For negative charged powder particles, a negative DC potential up to approximately 100 volts reduces powder particle accumulation on the wires. For positive charged powder particles, a similar positive DC potential is advantageous. 
     For well-defined articles, such as, rolls and flat coil stock (not shown), the electrostatic coating apparatus can be positioned close to the articles, for example, up to a few millimeters away with the rolls or coil stock being moved past the electrostatic coating apparatus. For this situation, a DC electrical bias (not shown), in combination with an AC voltage that has an amplitude up to several kilovolts at a frequency of several kilohertz, may be applied between the coating apparatus  12  (In this case, the donor rolls are not electrically grounded.) and the article  16  to be coated to cause the charged powder particles to deposit on the article. This deposition process on uniform surfaces of articles, generally equally spaced from the coating apparatus, minimizes powder particle over spray from the powder cloud  66  that requires collection for reuse. 
     For irregular-shaped articles  16 , such as I-beams and automobile and appliance components, the spacing between the coating apparatus and articles to be coated must be tens of centimeters to several meters. For this type of article to be coated, air jets  67  are positioned near the powder cloud  66  of AC fringe electric field detached powder particles  51 , in order to transport the charged powder particles  51  closer to and surround the article to be coated. The electrostatic forces cause the deposition of the charged powder particles in the powder cloud  66  onto the exposed exterior surfaces of the irregular-shaped article  16 . Generally, the article  16  to be coated is electrically grounded. Although primary reference is made to coating metallic objects that are held at ground potential, it is understood that a wide variety of substrates can be coated, including composites of carbon nanotubes/nanofibers and other conductive fibers in polymeric materials, wood, medium density fiberboard (MDF), etc. 
     Most of the powder particles  51  on the donor rolls  56  is removed by the AC fringe field detachment wires  64 , so that the donor rolls must be reloaded in a single pass by the magnetic brush  54  having the powder material of magnetic beads and powder particles. To readily achieve this, the beads should be conductive in order to provide high-deposition electric fields. 
     Although  FIG. 2  illustrates AC biased wires  64  as a preferred method of subjecting an AC fringe electric field to the layer of powder particles on the donor rolls  56 , other fringe field generating configurations may be effective in detaching charged powder particles from donor rolls and generating a powder cloud. For example, the Scavengeless Electroded Donor Development system disclosed in U.S. Pat. No. 5,504,563 may be used in which an AC bias is applied between neighboring interdigitated electrodes (not shown) embedded in a rotating donor roll  56  or belt (not shown). Also, a stationary AC biased interdigitated electrode assembly positioned behind a moving thin dielectric donor belt (neither shown) may be used as disclosed in U.S. Pat. No. 5,276,488. The relevant portions of U.S. Pat. No. 5,504,563 and U.S. Pat. No. 5,276,488 are hereby incorporated by reference regarding the use of AC fringe fields to detach charged powder particles from donor rolls. 
     For triboelectric or ion spray guns used in the present powder coating industry, the maximum spraying rate per gun is about 600 gm/min. The powder coating apparatus of  FIG. 2  achieves a maximum spraying rate of 600 gm/min with a powder particle coverage on the donor rolls  56  of 1 mg/cm 2 , the length of each of the two donor rolls is 40 cm, and the surface speed of each donor roll is 125 cm/sec. For high powder particle throughput requirements, it may be necessary to increase the number of mixing augers  53  or to include a paddle wheel (not shown) in the sump  52  in order to provide adequate triboelectric charging of the powder particles. To circumvent the need to periodically replace aged magnetic beads, the replenishing powder particles can contain an optimum percentage of magnetic beads for the continual replacement of aged beads via a well-known technique of providing a trickle port (not shown) on the housing  30 . 
     The typical charge-to-mass ratio of 30 to 40 μm electrostatic powder particles used in the powder coating industry is about 1 μC/gm. This level of charging is low compared to that used in electrophotography, even after scaling the ratio according to the inverse of the particle size (i.e., for 10 μm powder particles, the charge-to-mass ratio would be about 3 μC/gm). The maximum charge-to-mass ratio is limited by air breakdown within the deposited layer on the article to be coated. If finer powder particles enable thinner coatings, higher charge-to-mass ratios could be used without the air breakdown limitation that causes coating defects. For a breakdown field of about 10 V/μm, the maximum charge-to-mass ratio is about 5 μC/gm for about a 2 mg/cm 2  coverage of powder particles on the article. This charge-to-mass ratio can be obtained by selecting a coating material for the magnetic beads. 
     If magnetic bead carry out on the donor rolls  56  is excessive, permanent magnets  78  could be placed adjacent the donor rolls to collect the beads in a region above the loading nips  55  and a sufficient distance from the magnetic brush roll  54  to avoid picking up any beads therefrom. 
     Referring to  FIG. 3 , another embodiment of the electrostatic powder coating apparatus  12 A of this application is shown in a cross-sectional, schematic elevation view similar to that of  FIG. 2 . The electrostatic powder coating apparatus in  FIG. 3  is substantially identical to that in  FIG. 2 , except there are more donor rolls  56  mounted for rotation about their respective axes  47  in the direction of arrows  45  than in the configuration shown in  FIG. 2 . In  FIG. 3 , there are three donor rolls shown. As in the previous embodiment, the donor rolls are in substantial tangential contact with the surface of the magnetic brush  54  to form respective loading nips  55 . The donor rolls are adjacent and confront the aperture  49  that interconnects the interior of housing  30  with the interior of coating booth  14 . The donor rolls are each either electrically grounded or biased by electrical voltage source  69 , and each has a resistive overcoating that provides dissipation of any charge accumulation. An air jet  67  near each powder particle covered donor roll detaches the charged powder particles  51  from the donor roll in a direction through the aperture  49  and towards the article  16  to be coated. The air jets can be generated through a slot  70  or a linear array of apertures (not shown) in elongated tubular members  71  connected to a source of compressed air (not shown). The space-charge electric field from the powder cloud  66  generated by the air jets  67  promotes powder particle deposition onto the grounded article  16 . The powder particles on the donor rolls are substantially totally removed by the air jets, so that they must be reloaded in a single pass by the magnetic brush containing the powder material comprised of magnetic beads and powder particles. This is readily accomplished if the magnetic beads are also electrically conducting to provide high deposition electric fields. 
     The maximum rate of charged powder particles generated by the electrostatic powder coating apparatus of  FIG. 3  is about 900 gm/min, which is more than the maximum spraying rate of existing triboelectric or ion spay guns. To achieve this rate of charged powder particles generated by the embodiment in  FIG. 3 , the powder particle coverage on each of the donor rolls should be about 1 mg/cm 2 . In addition, the length of each donor roll should be at least 40 cm, the minimum number of donor rolls should be three, and the surface speed of each donor roll should be 125 cm/sec. Again, for high powder particle throughput requirements, it may be necessary to increase the number of mixing augers  53  or to include a paddle wheel (not shown) for mixing in the sump  52  to provide adequate powder particle charging. Also, the replenishing powder material could contain an optimum percentage of magnetic beads for the continued replacement of aged beads by way of trickle port (not shown) in the housing  30 . This technique is well known in the electrophotography field, and circumvents the need to periodically replace all of the aged magnetic beads in the powder material. 
     If magnetic bead carry out on the donor rolls  56  is excessive, permanent magnets (not shown) may be positioned adjacent the donor rolls in a region above the loading nips  55  and at a location sufficiently distanced from the magnetic brush roll  54 , as described for the embodiment of  FIG. 2 , in order to collect the beads from the donor rolls. 
     In  FIG. 4 , a schematic elevation view of another embodiment of a powder coating apparatus of this application is shown in cross-section. In this embodiment, the powder coating apparatus  12 B comprises a housing  29  having a chamber  31  therein containing a supply of powder particles  51 , as a single component, in an arcuate lower section, as seen in cross-section, that is referred to as sump  32 . The housing  29  is attached to the coating booth  14  with an aperture  49  therebetween. The powder particles range in size from about 4 to 40 μm in diameter and are preferably about 10 μm in average diameter. The sump  32  includes a rotatable paddle  33  that is fixedly mounted on shaft  34 . The shaft is located at about the center of the paddle, and the paddle is rotated about shaft  34  in the direction of arrow  73  by any suitable means, such as, for example, an electric motor (not shown). The chamber also has a second arcuate lower section  35  that contains a rotatable, conformable paddle roll  36 . The second lower section  35  is adjacent the sump  32  and the sump and second lower section have a common wall  37  that has a height selected to enable powder particles  51  to be urged from the sump into the second lower section  35  by the paddle  33 . 
     The conformable paddle roll  36  is mounted on a shaft  38  and consists of any suitable compliant material, such as, for example, a conductive rubber. In the preferred embodiment, the paddle roll has a cylindrical shape with parallel grooves  39  in its outer surface, and the shaft  38  is axially located therein. The conformable paddle roll is positioned in contact with a rotatable donor roll  56 A, and when the paddle roll is rotated about its shaft  38  in the direction of arrow  40 , powder particles  51  are delivered in the grooves  39  to the donor roll  56 A. The paddle shaft  34 , the paddle roll shaft  38 , and donor roll shaft  41  are parallel to each other. The donor roll is rotated about its shaft  41  in the same direction as the paddle roll, as shown by arrow  42 . The paddle roll  36  and donor roll  56 A may be rotated individually or together by any suitable means, such as, for example, an electric motor (not shown). 
     An electrical bias is applied to the paddle roll by voltage source  76  to control the charging polarity and magnitude of the charged powder particles  51  deposited on the electrically grounded donor roll  56 A. The paddle roll is rotated at a different speed than the donor roll. The differential speed is used for the paddle roll in order to promote the loading of the powder particles onto the donor roll. Though the paddle roll is shown as being rotated in the same direction as the donor roll, the paddle roll may be rotated either with or against the direction of the donor roll. 
     A flexible metering blade  43  is loaded against the donor roll  56 A to meter the powder particles  51  on the donor roll and provide a uniform layer thereon. The metering blade also provides additional triboelectrical charging of the powder particles. The amount of metering of the powder particles on the donor roll is controlled by the degree of extension of the metering blade beyond the contact line of the metering blade with the surface of the donor roll. A greater extension provides a thicker powder particle layer. If the flexible metering blade is made conducting, an electrical bias (not shown) can be applied to the metering blade to assist in obtaining the desired level of powder particle charging. Therefore, the metering blade is made conducting in the preferred embodiment. 
     The donor roll  56 A has a resistive overcoat layer (not shown) on its outer surface that provides dissipation of any charge accumulation. An air jet  44  near the powder-particle covered donor roll  56 A detaches the charged powder particles  51  from the donor roll and produces a powder cloud  66 . The air jet directs the powder cloud  66  through the aperture  49 , located between the housing  29  and coating booth  14 , and towards the article  16  to be coated. The air jet  44  is generated along the length of the donor roll from an elongated tubular member  46  that is parallel to the donor roll. The tubular member may have a long narrow slot  48  or a linear plurality of small apertures (not shown) in the elongated tubular member  46 . A typical compressed air source (not shown) is connected to one end of the tubular member  46 , and the compressed air is regulated by well known means (not shown) to provide the desired air jet velocity for detaching the charged powder particles. 
     The space-charge electric field from the powder cloud  66  of charged powder particles  51  promotes powder particle deposition onto the grounded article  16  to be coated. Since essentially all of the powder particles on the donor roll  56 A is removed by the air jet  44 , the donor roll is reloaded in a single pass by paddle roll  36  and metering blade  43 . 
     For the powder coating apparatus  12 B illustrated in  FIG. 4 , it is estimated that a maximum rate of charged powder particles delivered from the donor roll is about 300 gm/min. For this delivery rate, the powder particle coverage on the donor roll is 1 mg/cm 2 , the length of the donor roll is 40 cm, and the surface speed of the donor roll is 125 cm/sec. If a higher delivery rate is desired, multiple powder coating apparatuses can be ganged together. 
     The maximum charge-to-mass ratio is limited by air breakdown within the deposited layer of powder particles on the article  16 . The typical charge-to-mass ratio of 30 to 40 μm electrostatic powder particles used in the powder coating industry is about 1 μC/gm. For powder particles having an average diameter of 10 μm, the charge-to-mass ratio is about 3 μC/gm. If finer powder particles enable thinner coatings, higher charge-to-mass ratios could be used without the air breakdown limitation that causes coating defects. For a breakdown field of about 10 V/μm, the maximum charge-to-mass ratio is about 5 μC/gm for about a 2 mg/cm 2  coverage of powder particles on the surface of the article. This charge-to-mass ratio can be readily obtained by material choices for the paddle roll, donor roll, and metering blade. 
     The exemplary embodiments described above for powder particle coating apparatus has one major advantage over the prior art. These embodiments eliminate the need for fluidization of the powder particles, so that finer powder particle may be used to coat irregular, three-dimensional articles with thinner coats. 
     Although the foregoing description illustrates the preferred embodiment, other variations are possible and all such variations as will be apparent to those skilled in the art are intended to be included within the scope of this application as defined by the following claims.