US 3670699 A
Electrostatically charged fluidized bed apparatus having a porous membrane through which a continuous stream of air flows upwardly to fluidize powder in a bed above the membrane. A conductive web consisting of a layer of conductive particles incorporated into the membrane is maintained at a potential of at least 5,000 volts so that the stream of air carries electrical charges to powder suspended in the fluidized bed. Alternatively, the conductive web may be a fabric of conductive fibers mounted beneath and close to the membrane.
Description (OCR text may contain errors)
I United States Patent 1151 3,670,699
Sargent 1 June 20, 1972  ELECTROSTATICALLY CHARGED 3,396,699 8/1968 Beebe et al. ..117/17 FLUIDIZED BED APPARATUS 3,537,426 11 1970 Spiller et al ..1 18/629 3,496, 11 21970 Ch 1 ..1186  Inventor: Jerald P. Sargent, Lake Elmo, Minn. 9 me ar 2]  Assignee: Minnesota Mining and Manufacturing Primary Exam ner-Mer in Stein Company, St. Paul, Minn. Assistant ExaminerLeo Millstein  Filed: June 24 1970 Attorney-Kinney, Alexander, Sell, Steldt & Delahunt 211 Appl. No.: 49,241 57 ABSTRACT Electrostatically charged fluidized bed apparatus having a  U.S. Cl. ..118/629, 1 l7/l7, l l8/DlG. 5, poro membrane through which a continuous stream of air H8/627 flows upwardly to fluidize powder in a bed above the mem-  Int. Cl ..B05b 5/02 brarm A conductive web consisting f a layer f conductive  Field of Search ..1 18/628, 629, 639, 640, 400.5, particles incorporated into the membrane is maintained at a l 18/ 624; 1 17/17 potential of at least 5,000 volts so that the stream of air carries electrical charges to powder suspended in the fluidized bed.  References Cited Alternatively, the conductive web may be a fabric of conduc- UNITED STATES PATENTS tive fibers mounted beneath and close to the membrane.
3,248,253 4/1966 Barford et al ..1 18/4005 6 Claims, 2 Drawing figures PATENTEnJuH 20 me IIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIII T m .NME m T I/A 0 M. mm M 3 m ELECTROSTATICALLY CHARGED FLUIDIZED BED APPARATUS FIELD OF THE INVENTION This invention relates to electrostatically charged fluidized beds for coating articles using powdered resins.
BACKGROUND TO THE INVENTION It is known that an electrostatically charged fluidized bed provides certain advantages over conventional fluidized beds for coating articles with powdered resins. For example, the article to be coated need not be preheated, and powder can easily be selectively removed from the article before heating to fuse the deposited powder. If there is a defect in the fused coating, powder is readily applied to the defect and removed from areas already of adequate coating thickness, after which the article is heated to fuse the newly deposited powder. With a little practice and care, it is difiicult to detennine that the coating was ever defective.
Electrostatically charged fluidized bed devices are shown in U.S. Pat. Nos. 3,248,253 (Barford et al.), 3,336,903 (Point) and 3,339,699 (Beebe et al.). In these devices, the powder is electrostatically charged by means of metal electrodes located within the fluidized bed. A device of this general nature marketed by Electrostatic Corporation has a metal grid suspended in the fluidized bed a short distance above the porous membrane through which gas is forced upwardly to fluidize powder in the bed. When a grounded article is positioned above such a fluidized bed, powder is attracted to and deposited upon the article, but not in desirably uniform thickness.
Another shortcoming of electrostatically charged fluidized beds of the prior art is the high voltage required for efficient performance. Manufacturers of devices now on the market recommend the use. of voltages up to about 120,000 volts.
THE PRESENT INVENTION The present invention differs from devices of the prior art by the unique manner in which the powder of the fluidized bed is electrostatically charged and by the uniformity of resultant coatings. Although the present invention still requires a high voltage, good efficiency is attained at voltages relatively lower than are needed in the prior art, e.g., only l0,000-20,000 volts, and the user is better protected from the high voltage.
These advantages are realized by incorporating into the porous membrane of an otherwise conventional fluidized bed apparatus a web of moderately conductive particles or fibers, which web may have a resistance of about l-l ,000,000 ohms and contains innumerable sites for charges when subjected to a high voltage. The conductive web may be incorporated into the porous membrane by coating one or both sides of the membrane with a dispersion of graphite particles or fibers, using any coating technique, e.g., by spraying, brushing or dipping. Whatever coating technique is used, it is important that the conductive particles of the dried coating be contiguous over the entire coated area, but not at such a density as to appreciably interfere with the gas-transmissive nature of the membrane. Alternatively, the chargeable web may consist of a woven or nonwoven fabric of graphite fibers positioned horizontally beneath and closely adjacent to the porous membrane in the path of the gas which fluidizes the powder, again such that the upward flow of gas through the web and porous membrane is not appreciably inhibited.
THE DRAWING FIG. 1 is a schematic central section of an electrostatically charged fluidized bed apparatus embodying the present invention, and
FIG. 2 is a schematic central section of another embodi ment of the present invention employing a pair of fluidized beds.
The fluidized bed apparatus illustrated in FIG. I has four rectangular side walls 9, a bottom wall 10 and a porous membrane 11 to provide a container 12 and a chamber 13 into which a gas such as air is continuously supplied under pressure through a conduit 14. The gas flows upwardly through the membrane 11 at a volume suflicient to suspend free-flowing powder 15 as a fluidized bed within the container 12. The walls 9 and 10 are made of an insulating material such as a thermoplastic resin. The membrane 11 rests on Supports 16 integrally formed with the side walls 9, and a bead of thermosetting epoxy resin 17 around the entire upper periphery of the membrane 11 holds the membrane in place and seals it against substantial leakage at its peripheries.
The entire under face of the membrane 11 has been coated to provide a continuous web 18 of conductive particles. One post of a high-voltage resistor 19 is electrically connected to the conductive web 18 by a conductive coil spring 20 which is under mild compression and held in place by a globule of thermosetting epoxy resin to insure good electrical contact with the web. The other post has an electrical lead 21 to a source of high voltage.
Another embodiment of this invention is shown in FIG. 2 employing a pair of identical fluidized beds which are of the same construction as the apparatus of FIG. 1 except as noted. Each of the pair has a porous membrane 1 1a sealed between a low side wall 9a and a high side wall 9b. A thin adhesive tape having a metal foil backing 25 provides a smooth joint between each membrane 110 and high side wall 9b. The entire exposed surface of each membrane 11a, metal foil 25 and high side wall 9b is coated with conductive particles.
When a grounded article 26 is held in the position shown and the conductive webs 18a are charged to the same potential, powder 15a is attracted from both fluidized beds, thus coating all surfaces of the entire article 26 without changing its position. This makes the apparatus of FIG. 2 especially adapted to conveyor lines. In contrast, it is usually necessary EXAMPLE I A fluidized bed apparatus as illustrated in FIG. 1 has been constructed using polymethylmethacrylate of one-fourth inch thickness for the side walls 9 and base wall 10. The porous membrane 11 was porous high-density polyethylene of threesixteenths inch thickness having a particle retention of 25 microns and larger which is marketed as a fluidizing type membrane by Porous Plastics Limited, Essex, England under the tradename Vion." The container 12 measured 6 inches on a side and was 4.5 inches in height, and the chamber I3 was 2.5 inches in height.
The entire under surface of the membrane 11 was coated to provide a moderately conductive web 18 by wiping the surface with a cloth soaked in a dispersion of graphite particles in alcohol, specifically dag" dispersion No. 154 of Acheson Colloids Company, Port Huron, Michigan. The dried web 18 completely masked the white color of the membrane 11 and exhibited a resistance of about 50,000 ohms at an electrode separation of 1 inch. None of the black color of the graphite particles penetrated to the opposite surface of the web. The resistor 19 had a resistance of I60 megohms and was rated for 30 watts. The lead 21 was connected to a negative DC potential of 13,000 volts.
Used in this apparatus was a powder comprising solid bisphenol-epichlorohydrin-type epoxy resin, isophthalyl dihydrazide and a catalytic amount of dicyandiamide plus flow-control agents. Fifty-five percent of the powder would pass 325 mesh (44-micron openings). Starting with an inch of powder, a flow of dry air of 0.9 cubic foot per minute fluidized the powder to a height of about 2 inches.
When a grounded copper bar at room temperature was placed one-half inch above the surface of the fluidized bed for 6 seconds, it was uniformly covered with powder which fused when the article was heated in an oven at 200 C for 15 minutes to provide a tough cured electrically insulating protective coating having a uniform thickness of about. 6 mils (0.15 mm).
Good results were obtained when the article being coated was a different metal or was moderately conductive or was a dielectric such as ceramic or plastic. In each case adequate grounding of simple articles such as the copper bar was obtained through the body of the person holding the article above the electrostatically charged fluidized bed. Articles containing sharp inside corners or small openings are desirably connected to a grounded strap.
This apparatus has been operated with the conductive web at a potential ranging from 5,000 to l60,000 volts. Below ,000 volts, the rate of powder deposit is unduly slow for commercial use, and above 160,000 volts, there is danger of arcing to the article being coated, although resistor 19 minimizes the chance of injury to the operator. A good rate of powder deposit is readily achieved at l0,00030,000 volts, and there usually is no advantage to increasing the potential above 30,000 volts.
EXAMPLE 2 EXAMPLE 3 When the apparatus described in Example 1 was modified by reversing the porous membrane 1 1 so that the graphite particle web 18 was on the upper face, no difference was noted in performance. Neither was any difference noted when a graphite particle web was applied to both faces of the porous membrane, and both webs were connected to the 13,000-volt potential. Of course, care must be taken in applying more than one coating of graphite particles that the gas-transmissivity of the web is not reduced to the point that the powder can no longer be fluidized under moderate air pressure.
EXAMPLE 4 The apparatus described in Example 1 was modified by substituting an identical porous membrane for membrane 11 except that instead of being continuous, the conductive web was provided by a graphite coating applied in a pattern of a circle 1 inch in diameter plus three concentric rings, each one-half inch in width and spaced from each other and from the circle by one-half inch. With each member of the pattern electrically connected to the l3,000-volt potential, even better uniformity of powder deposit was realized, as compared to the apparatus of Example 1, but at some expense in rate of coating buildup. To increase the coating rate to equal that of Example 1, it was necessary only to increase the DC potential to lead 19. However, since it is desired to keep that potential as low as possible in the interest of economy and the operator's peace of mind, it is normally preferred to coat all or most of the face of the porous membrane with conductive particles, unless absolute uniformity of coating is required.
The graphite particles have been coated on the membrane in other patterns, including a small circle at the center of the membrane, a small square at the center and a snowflake. In each case powder from the fluidized bed deposits on a grounded article at a somewhat lower rate, and no advantage is seen in using such patterns. Regardless of the pattern of the conductive web, its area should be at least 2 percent of the area of the membrane, and preferably at least 20 percent.
EXAMPLE 5 Apparatus identical to that described in Example 1 was constructed except that the porous membrane was uncoated and a second uncoated porous membrane was added to provide means for mounting a fabric of graphite fibers. The conductive web provided by this carbonaceous fabric was coextensive with the membranes, and separated the two membranes by its thickness, i.e., about one thirty-second inch. The fabric had a resistance of about 2-3 ohms measured at an electrode separation of one inch, and was connected to the resistor 19 by a lead wire extending through the lower membrane.
When the fabric was at a potential of 13,000 volts, the performance was equivalent to that obtained with the apparatus described in Example l. A bed of the powder of Example 1 fluidized to a height of 2 inches deposited sufficient powder on a grounded copper bar held one-half inch above the powder for 6 seconds to provide, after curing in an oven, a uniform coating about 8 mils (0.20 mm) in thickness. Increasing the exposure time to 15 seconds provided a coating which when cured in an oven had a thickness of about 24 mils (0.61 mm).
When a similar unheated grounded bar was held 2.5 inches above the surface of the bed for 15 seconds and then placed in an oven, a cured coating was obtained having a uniform thickness of about 17 mils (0.43 mm).
When additional powder was added to raise the height of the fluidized bed to 4 inches, an unheated grounded bar placed one-half inch above the bed for 15 seconds received a coating which when cured had a uniform thickness of about 26 mils (0.66 mm).
EXAMPLE 6 The apparatus of Example 5 was modified by sealing an uncoated porous polyethylene membrane to the four side walls 9 2 inches above the position of membrane 1 1. The free-flowing powder was placed on the added upper membrane, thereby spacing the conductive fabric from the fluidized bed by about 2 inches. With the fabric at 13,000 volts, the powder deposited on grounded articles held above the fluidized bed at a rate slightly less than 50 percent of that obtained with the apparatus described in either Examples 1 or 4. This apparent loss of efficiency can be compensated by increasing the applied voltage, not so desirable from a point of view of safety, as well as economy.
EXAMPLE 7 When the apparatus of Example 6 was modified by spacing the extra uncoated membrane only one-half inch above the carbonaceous fabric, the efficiency was only slightly reduced as compared to the apparatus of Example 5.
EXAMPLE 8 The apparatus of Example 6 was modified by replacing (a) the carbonaceous fabric and the two membranes which held it with (b) the single membrane of Example 1 and its graphite particle web. The same loss of a little over 50 percent in efficiency was observed, which loss could likewise be compensated by increasing the applied voltage.
EXAMPLE 9 Any coating of particles having a resistance of 1-1 ,000,000 ohms measured at an electrode spacing of 1 inch should be useful, as long as the flow of the fluidizing gas is not unduly restricted. To illustrate, the apparatus of Example 1 was modified: instead of the coating of graphite particles, the under surface of the porous web 11 was coated with Ransprep No. 100," a transparent coating material marketed by Ransberg Electrostatic Corporation which dried to provide a moderately conductive coating having a light amber color and a thickness of less than 1 mil (25 microns). The resistance of the dried coating was 750,000 ohms measured at an electrode spacing of 1 inch.
This coating interfered slightly with the flow of air through the porous membrane, requiring slightly increased pressure at the air supply to provide a 2-inch fluidized bed. At the increased air pressure the coating efficiency was essentially equal to that in Example 1.
EXAMPLE l The apparatus of Example 1 was modified by replacing the membrane 11 with an uncoated porous stainless steel plate of one-eighth inch thickness having l5-micron openings. The resistance of this plate was less than 0.1 ohm measured at an electrode separation of one inch. When 13,000 volts were applied to the stainless steel plate, the fluidized bed was charged so that articles could be coated electrostatically. However, the efficiency was very poor. From this and other experiments, it was determined that the resistance of the conductive web 18, or of the porous membrane if the membrane is constructed of conductive material in lieu of providing a conductive web, should not be less than about 1 ohm.
To provide a porous membrane which is useful for the purposes of this invention without a separate conductive web, one should construct the membrane out of material of only moderate conductivity such as conductive polyethylene.
in the apparatus of each of the foregoing examples, best results were attained at a fluidized bed depth of about 1-3 inches and with the article to be coated about k to 4 inches above the surface of the fluidized bed. Closer distances permit rapid buildup. However, articles having sharp inside corners or small openings should be held at greater distances to insure reasonably uniform powder buildup on both external and internal surfaces. Surfaces as much as two feet from a fluidized bed operating as described in Example 1 receive an appreciable deposit of powder, but it is desirable for commercial purposes to hold the article so that all of its surfaces are much closer to the bedthan that distance.
When the side walls of the apparatus used in Example 1 were extended and sufficient powder was added so that the fluidized bed was inches in height, it was necessary to hold the article within about one-fourth inch of the surface of the bed of powder to obtain reasonable efficiency at 13,000 volts.
At a distance of one-half inch, the rate of powder buildup was reduced to a third. Greatly increased voltage was necessary to enable reasonably efficient deposit of powder on areas of the article which were not very close to the surface of such a deep fluidized bed.
EXAMPLE 1 1 Apparatus as illustrated in FIG. 2 was constructed in the same manner as in Example 1 except as indicated below. Each of the high side walls 9b was 12 inches in height and extended 7% inches above the membrane 1 la. The low side walls 9a extended 2% inches above the membrane 11a. Each bed was 9 inches wide between side walls 9a and 9b and 43 inches in length. The entire upper surface of each membrane 11a was coated with the same dispersion of graphite particles used in Example 1, but the web of graphite particles did not extend up the high side walls 9b.
With the powder used in Example 1 fluidized to a height of 2 inches and both of the conductive webs 18a at a positive potential of 13,000 volts, a grounded copper bus bar measuring 3 feet by 4 inches by V4 inch was moved horizontally past the fluidized beds in the position of article 26 as shown in FIG. 2, with the large dimension horizontal and the intermediate dimension vertical. All surfaces of the bus bar received a desirably uniform deposit of powder which cured to provide a tough, uniform protective coating when the bus bar was heated in an oven.
Apparatus of the type described in this example has been modified as shown in HQ 2 to continue the conductive membrane 18a up the high side walls 9b. As a result, powder was deposited at a more rapid rate on grounded articles in the position of article 26in FIG. 2. Since some of the powder falls through the space between the two fluidized beds, it is desirable that there be means for collecting and reusing it.
1. Fluidized bed coating apparatus having side walls of insulating material and a horizontal porous membrane sealed to the side walls to form a container and means for producing an upward stream of gas through the membrane for providing a fluidized bed of free-flowing powder in the container, which apparatus is characterized by the improvement comprising a gas-transmissive, moderately conductive web consisting of a layer of contiguous conductive particles incorporated into the porous membrane or a fabric of conductive fibers mounted horizontally beneath and close to the porous membrane in the path of gas produced by the means for producing a stream of gas, which web contains innumerable sites for charges when subjected to a high voltage, and means for applying to the web a potential of at least 5,000 volts so that gas flowing upward through the charged web carries electrical charges to powder suspended in the fluidized bed.
2. Fluidized bed coating apparatus as defined in claim 1 wherein the conductive web consists of a layer of graphite particles applied to the under face of the porous membrane.
3. Fluidized bed coating apparatus as defined in claim 1 wherein the conductive web consists of conductive particles incorporated into the porous membrane, which particles are fibers.
4. Fluidized bed coating apparatus as defined in claim 1 wherein the conductive web consists of a woven or nonwoven fabric of graphite fibers.
5. Fluidized bed coating apparatus as defined in claim 1 wherein the conductive web has a resistance of about 1l,000,000 ohms measured at an electrode spacing of 1 inch.
6. A pair of closely spaced fluidized bed coating apparati as defined in claim 1, each having a relatively low opposite side wall positioned adjacent the relatively low side wall of the other, such that electrostatically charged powder in both beds is attracted to a grounded article intermediate the two relatively low side walls to deposit powder on the article from both sides.