Electrostatic Spray Apparatus And Method

Abstract

An electrostatic spray apparatus and method for spraying coating materials having conductivities ranging from moderately conductive to highly conductive, which is characterized by freedom from electrical shock and ignition hazards occasioned by a grounded operator inadvertently contacting the hose and/or grounding it during use in an explosive atmosphere. Included is a spray gun which can be moved relative to a stationary source of coating material, an improved shock-free and ignition-free conduit for transporting under pressure moderately and/or highly electrically conductive coating materials from the stationary supply to the movable gun, and a source of high voltage unidirectional current for electrostatically charging the coating material prior to deposition thereof onto the article being coated.

Description

This invention relates to electrostatic spray coating, and more particularly to an apparatus and method for electrostatically spray coating moderately and highly electrically conductive coating materials which is relatively free of shock and ignition hazards occasioned by inadvertent contact of the coating supply conduit by a grounded operator and/or grounding of the conduit in an explosive atmosphere.

Coating materials sprayed on objects to be coated can be categorized, from the standpoint of their electrical conductivity, as falling into one of three categories, namely, low, intermediate or moderate, and high conductivity coatings. Coating resistivities in the general range of 2.times.10.sup.5 ohm-centimeters to 10.sup.6 ohm-centimeters are considered to be in the intermediate or moderate conductivity range, while coating materials having electrical conductivities below and above this range are viewed as falling in the low and high conductivity categories, respectively. While specific conductivity values have been used to define low, intermediate and high conductivity ranges, it is understood that these conductivity values are arbitrary and relative, and employed only for the purpose of illustration. Accordingly, a coating material having a resistivity above or below the range of 2.times.10.sup.5 -- 10.sup.6 ohm-centimeters could conceivably be considered as an intermediate or moderate conductivity coating material, notwithstanding that it falls near, although without, the specific numerical range noted. Similarly, a coating material with a resistivity between 2.times.10.sup.5 ohm-centimeters and 10.sup.6 ohm-centimeters, although near one or the other of these limits, may possibly be considered as either a high or low conductivity coating depending on to which end of the intermediate range it is closest.

Heretofore there has been no commercially feasible method or apparatus for electrostatically spraying moderate conductivity and high conductivity coating materials which is free from shock and ignition hazards occasioned by inadvertent contact with the coating supply hose by a grounded operator or inadvertent grounding of the hose in an explosive environment. While there have been proposals generally aimed at achieving electrostatic spraying of moderate and high conductivity coating materials, none have been entirely free of safety hazards.

For example, it has been proposed to electrostatically spray high conductivity coating materials with apparatus which includes, among other things, a spray gun equipped with a high voltage antenna proximate the spray nozzle for electrostatically charging emitted coating particles, and a remote spray tank which is electrically isolated from ground potential and connected to the gun by a hose. However, in accordance with this proposal, since the coating material is highly conductive, the high voltage applied to the coating material by the charging antenna is transmitted back through the hose to the stationary supply tank by the column of highly conductive coating material in the hose, with the result that the coating in the hose along the entire length thereof is at the high electrical charging potential. The high voltage column of coating material in the hose produces, along the entire length of the hose, a large voltage across the hose wall in a radial direction. This large radial electrical potential produces current leakage radially through the hose wall along the length thereof due to the fact that the hose wall does not have infinite resistivity. The leakage current accumulates electrical charge on the exterior surface of the hose wall. Should a grounded operator inadvertently contact the hose at any point along its length, the accumulated electrical charge will discharge to ground through the operator, electrically shocking the operator. Similarly, should the electrically charged hose inadvertently be grounded anywhere along its length, and thereby discharge, a safety hazard is created by virtue of the possibility of ignition should the discharge occur in an explosive atmosphere as is the case when flammable, solvent-based coatings are used.

In an alternate proposal for electrostatically spraying highly conductive coatings, which avoids the need for a high voltage charging antenna in the gun, the coating material is electrostatically charged at the supply tank and transmitted through the hose to the gun in an electrically charged condition. However, the same safety problem exists as with the charged gun antenna since the entire column of paint in the hose is at a high electrical potential, and steps must be taken to avoid, at any point along the hose, inadvertent hose contact by a grounded operator and/or accidental hose grounding if operator shock and/or ignition is to be avoided.

Even in electrostatic spray coating applications where the coating material is only moderately conductive, the high voltage electrostatic charging potential applied to the gun antenna, while not transmitted back along the entire length of the hose by virtue of the increased resistance of the column of moderately conductive coating material in the hose, is nevertheless a safety hazard. Specifically, because of the moderate conductivity of the coating material, a voltage approximating the high charging voltage present at the gun antenna will be applied for some distance back through the hose column of coating material in that portion of the supply hose adjacent the gun, with a gradual reduction in voltage as the distance from the gun increases. The presence of this gradually diminishing high voltage in the supply hose results in a substantial voltage being applied radially across the hose wall for a significant distance from the gun. This voltage produces a leakage current radially through the hose wall due to the small, yet not insignificant hose conductivity. This leakage current accumulates measurable electrical charge on the exterior surface of the hose for a substantial portion of its length. Should an operator inadvertently contact the hose anywhere in the length which is electrically charged, the operator will be subjected to an electrical shock. Similarly, should the electrically charged hose inadvertently be grounded anywhere along the charged length, and thereby discharged, there is a possibility of ignition as a consequence of the flammable nature of the atmosphere which exists when flammable solvent-based coatings are used.

Theoretically, the electrically charged hose problems described above could be obviated by increasing the radial thickness of the hose to a point such that radial current leakage through the hose wall, occasioned by the presence therein of electrically charged coating material, is reduced to zero or a nonhazardous value near zero. However, from a commercial standpoint such a solution is impractical, or at the very least highly disadvantageous. For example, if the radial thickness of the hose is increased to a point where current leakage in a radially outward direction is zero or a nonhazardous value near zero, the wall thickness of the hose will be so large as to render the hose relatively inflexible. Since a vast number of electrostatic spray coating applications require that the spray gun be susceptive of movement relative to the stationary coating supply tank, it is absolutely essential in these applications that the coating supply hose be flexible to permit the required movement of the gun relative to the tank.

A further disadvantage of increasing the radial thickness of the supply hose wall to a point such that radial current leakage through the hose wall is zero or a nonhazardous value near zero is that over a period of use the coating material will permeate radially through at least a portion of the supply hose wall due to the fact that the supply hose is not completely impermeable to the coating material. As a consequence, the resistivity of the supply hose wall now permeated by the coating is reduced to a point where measurable radial current leakage through the hose wall exists, producing a charged hose exterior which, as noted, produces hazards of electrical shock and/or ignition should the hose be accidentally contacted by the operator or grounded in a flammable atmosphere.

Similarly, and due to the fact that the hose wall is not completely chemically inert to the coating material, at least some portion of the interior hose wall will be dissolved by the coating material. This increases the voltage gradient across the wall which, if sufficient, electrically breaks down the wall. Even if dielectric breakdown does not occur, dissolution of a portion of the hose wall reduces the electrical path length radially through the wall, resulting in increased current leakage through the wall for a given applied voltage, in turn increasing the likelihood of shock or ignition hazards. Thus, in practice, even a hose wall initially so thick as to avoid hazardous current leakage will, eventually, become hazardous due to permeation and/or dissolution of the hose wall by the coating material.

It has been a principal objective of this invention to provide an apparatus and method for electrostatically spraying moderately and/or highly conductive coating materials which are free of safety hazards occasioned by inadvertent contacting by the operator of the coating supply hose and/or accidental grounding of the hose in an explosive environment. This objective has been accomplished in accordance with certain principles of this invention by a very simple and effective, yet highly unobvious, approach which comprehends interconnecting a spraying device, such as a gun or the like, and the remotely located coating supply tank, pump or similar coating source, with a coating supply conduit characterized, first, by having an inner, coating-encircling dielectric zone sufficient to withstand dielectric breakdown when subjected to the charging voltage by electrically charged coating material in contact therewith; and, second, by having an outer, dielectric-encircling electrically conductive zone to remove from the exterior of the conduit electrical charge flowing radially through the dielectric zone, thereby avoiding charge accumulation on the conduit exterior and the attendant increase in shock and ignition hazards.

The coating supply conduit of this invention tolerates radial charge leakage through the coating supply conduit wall, which leakage in practice cannot be avoided, at least where flexible hoses and extended useful hose life are desiderata, and yet avoids the accumulation of charge on the outer surface of the conduit. As a consequence of this radial charge leakage tolerance, the conduit accommodates some permeation and dissolution of the inner hose wall by the coating material, permitting the conduit to be safely used for an extended period free of hazards in the form of operator electrical shock should the operator inadvertently contact the conduit and/or ignition should the conduit inadvertently be grounded in an explosive atmosphere.

Since the conduit of this invention safely conducts away electrical charge radially transported through the conduit wall, thereby avoiding hazardous charge accumulation on the conduit exterior, the conduit wall need not have a thickness of such proportions that radial current flow through the conduit wall is zero or near zero. Since a zero radial current leakage wall thickness is not necessary, a wall thickness can be selected which permits flexibility of the conduit as is necessary in many electrostatic spray coating applications where the spray gun must be moved relative to the coating supply tank, pump or the like.

In accordance with a preferred embodiment of the invention, the dielectric zone of the conduit wall is faced, on its interior surface, with material which is substantially chemically inert, as well as relatively impermeable, to the coating material. By virtue of the impermeable and inert interior surface of the dielectric zone, the dielectric breakdown resistance zone can be fabricated of a dielectric material which itself is somewhat more permeable and/or chemically active with respect to the coating material. This is often desirable, particularly where flexibility of the conduit is essential, because impermeable and inert dielectrics are often stiff, and it therefore is advantageous to use as the dielectric breakdown resistant wall material a somewhat more permeable and chemically active dielectric which as a rule is generally more flexible.

These and other advantages and objectives of the invention will become more readily apparent from a detailed description of the preferred embodiment taken in conjunction with the drawings in which:

FIG. 1 is a vertical cross-sectional elevational view (taken through the electrical and coating flow passages) of a manually operated electrostatic air spray gun and coating material supply tank shown connected by the novel conduit of this invention to provide an electrostatic spray coating system capable of safely spraying moderately and highly conductive coating materials;

FIG. 2 is a vertical cross-sectional elevational view of the gun of FIG. 1 taken through the air flow passages; and

FIG. 3 is an end elevational view of the spray gun of FIGS. 1 and 2, showing the lines along which the sectional views of FIGS. 1 and 2 are taken.

Electrostatic spray coating systems of the general type to which this invention relates typically include as a principal component thereof an electrostatic spray gun. The gun has a handle designed to be manually grasped in use by the operator and a barrel which at its forward end terminates in a nozzle. A spray of finely divided, or atomized, particles of coating material, such as paint, lacquer, enamel, or the like flows from the gun nozzle toward the object being coated when an actuator on the handle, such as a trigger, is actuated by the operator. An electrode, electrically insulated from the gun handle, trigger, and barrel is mounted in the nozzle and maintained at a high d.c. potential, e.g., 75 Kv, for electrostatically charging the coating particles as they leave the nozzle. Electrostatic charging of the particles enhances, for well known reasons, the deposition of the coating on the article being coated, which is typically maintained at ground potential. A source of coating material is connected to the barrel of the gun via a flexible conduit, hose, or supply line. Actuation of the trigger activates a flow valve in the gun to permit the flow of coating material to the nozzle whereat it is atomized and emitted as a spray. Electrostatic spray systems also typically include an electrical power pack or booster supply for transforming commercially available low voltage power to high d.c. voltages which are applied to the gun electrode for electrostatically charging the coating particle.

Depending upon whether or not the gun is of the "air" type, wherein atomization of the liquid coating stream is effected by impact of an air stream with the liquid coating stream, a source of air may or may not be connected to the gun via an air line for impinging air on the liquid stream in the region of the nozzle. If the spray gun is of the "airless" type, wherein atomization of the coating particles in the region of the nozzle is effected hydraulically, the air line may be omitted.

With reference to the drawings, an electrostatic spray gun 10 is illustrated which is of the air-operated type, relying upon the impact of an air stream with a liquid coating stream to effect atomization of the coating material. While the preferred embodiment of the invention is described as applied to an air gun, it should be understood that the invention is equally applicable to other types of electrostatic manual and automatic spray guns and systems.

The gun 10, considered in more detail in connection with FIGS. 1 and 2, comprises an electrically conductive metal handle assembly 11 contoured to be conveniently grasped by the operator, an elongated electrically insulative barrel assembly 12 and an insulative nozzle assembly 13. The handle assembly 11 is generally made from a metal casting, such as aluminum, and includes an air inlet 16, a trigger actuated air flow control valve 17, and a trigger 18 for controlling the flow of air through the valve 17. There is also an adjustable air valve 20 in the gun handle 11 for controlling the shape or "fan" of the coating spray emitted from the gun. The air inlet port 16 opens into a generally vertical air passage 21 which communicates with a transverse counterbored air valve passage 22. The air passage 22 in turn communicates with a pair of horizontal air lines 23, 24 in the handle assembly 11 which in turn communicate with horizontal passages 25, 26 of the barrel assembly 12.

The barrel assembly 12 is made from an electrically insulative material, such as one of the common plastics, and includes the main body section 27 through which the pair of air lines or passages 25, 26 extend, as well as the material flow control passage 35 and an electrical flow control passage 36. The material flow control passage 35 is intersected by an inclined passage 37 through which coating material is supplied from a coating supply tank 33 to the passage 35 via the novel coating conduit 38 of this invention, to be described in more detail later, which has its end 38-1 connected to the gun. The section of coating hose 38 adjacent the lower end of the handle is supported by an electrically conductive bracket assembly 40 which establishes electrical continuity between the outer surface, skin, or zone 38A of the conduit 38 and the electrically conductive handle 11. The bracket assembly 40 includes an angled conductive metal strap 40A having its one end fastened to the handle butt 11A via a threaded screw fastener 39. The other end of the strap 40A is apertured to receive an externally threaded electrically conductive metal collar 40B. Collar 40B has a flared bore section 40C which slidingly engages the exterior surface of conduit sheath, skin, or zone 38A, and an unflared bore section 40D. A second electrically conductive metal collar 40E is provided which threadedly engages the collar 40B. A frusto-conical section 40F of collar 40E fits between flared bore 40C of collar 40B and the exterior surface of conduit sheath 38A. As collar 40E is screwed down over collar 40B, collar section 40F is wedged against the exterior conduit sheath 38A, to establish an electrical connection between sheath 38A and collar 40E. Since collars 40E and 40B, as well as strap 40A, are electrically conductive, the sheath 38A is electrically connected to conductive handle 11.

Air flow in the passage 26 of the barrel is controlled by the trigger actuated valve 17, while air flow in the passage 25 is regulated by the valve 20 of the handle. At the forward end of the barrel body 27, a passage (not shown) through the nozzle assembly 13 communicates between the air flow passage 26 and a passage 41 of the nozzle assembly 13. This latter passage 41 is located between a fluid nozzle 42 and an air nozzle 43, and is open at the front so that it defines an annular air passage 44 around the coating orifice (not shown) of the fluid nozzle 42. Air issuing from this air passage 44 impinges with the stream of coating issuing from the coating orifice of the nozzle 42 and at least coarsely atomizes coating the stream. There may be additional ports of the air nozzle 43 connected with the air passage 26 to further atomize the coating stream. There are also a pair of fan-shaping ports (not shown) located in a pair of horns 45 of the air nozzle 43 which communicates through a passage of the air nozzle 43 with the passage 25 of the barrel 27. Adjustment of the valve 20 controls the amount of flow of air issuing from the horns 45 of the nozzle and thus the degree of "fan" formed by the atomized spray.

Flow of coating through the coating nozzle 42 is controlled by a valve 50 located at the forward end of the passage 35. The valve comprises a metal seat 47 located in a counterbored recess 48 and a movable metal needle 51. At its forward end the needle 51 is receivable in an axial aperture of the seat 47 to close the valve and preclude the passage of coating into and through the nozzle assembly 13. The needle 51 at its opposite end is attached to the trigger 18 so that in addition to controlling the flow of air through the gun, the trigger controls the flow of coating through the gun. The connection of the trigger to the air valve 17 and to the coating valve 50 is such that the air valve 17 always opens upon initiation of paint flow.

All of the components of the coating control valve 50 are made from insulative material except for the needle 51, a biasing spring 52, an intermediate needle section 55, and a trigger connecting needle section 53. Consequently, the nonconductive components of the coating valve 50 maintain an electrical stand-off between the electrically conductive elements at the forward end of the gun and the metal conductive elements, particularly the handle, at the rear of the gun. Also, reduction of conductive components in contact with the coating reduces the capacity of the gun for build-up of capacitive energy which could arc to ignite the volatile atmosphere or shock the operator, thereby maximizing safety in a further respect.

The coating material nozzle 42 is made from an electrically nonconductive material which is threaded into a counterbore 57 in the forward end of the body 27. It has an axial passage or bore (not shown) which opens into the rear of the counterbore 57. The rear of the counterbore 57 in turn communicates with the central aperture of the valve seat 47 via a passage 58 such that coating material passing through the aperture of the valve seat 47 may enter and pass through the axial passage in the coating nozzle 42. The axial passage in the coating nozzle 42 terminates in a small diameter outlet for discharge of a solid stream or jet of coating material.

The air nozzle 43 is also made from an electrically nonconductive material. It is threaded over a threaded sector of the barrel and has air passages which connect the ports and the horns 45 to the fan air control passage 25 of the barrel.

An antenna or electrode 61 protrudes from the orifice of the coating nozzle 42. A high unidirectional d.c. potential, e.g., 75 Kv, is supplied to the antenna 61 via an insulated electrical conductor 62 which interconnects the gun and power supply. The cable 62 is a conventional flexible coaxial cable within which there is a central electrical conductor surrounded by an insulating sheath, a conductive grounding sheath, and an encasing sheath of insulation. This cable is secured to the gun by conventional locking plug 63. The electrically conductive portion of the cable together with its electrically insulating sheath extends upwardly from the plug 63 into and through a nonconductive hose 64. This hose fits within the handle of the gun and protrudes from the forward handle end into the barrel wherein the central electrically conductive portion of the cable connects to the antenna 61 via a resistor 66 and other suitable connecting members such as spring 65, lead 67, and spring 68. The conductive grounding sheath is electrically connected to the gun handle 11 by the conductive locking plug 63 to effectively ground the handle 11, as well as ground via conductive bracket 40 the exterior sheath or surface 38A of the coating supply hose 38.

The coating supply tank 33 is conventional in design and includes a metallic container 72, preferably aluminum, capable of holding anywhere from 1 gallon to 50 gallons of coating material 73. The tank 33 is provided with a selectively removable aluminum cover 74 which in use is held in sealing engagement with the coating-containing tank via circumferentially spaced clamps 75, one of which is shown. An air conduit 76 formed in the tank cover 74 communicates the interior 77 of the tank above the level of the coating 73 with a source of pressurized air (not shown) for maintaining the coating under pressure. With the coating in the tank 33 pressurized, the coating is pressure fed to the conduit 38 and thence to the spray gun 10 via a conductive metal tube 78. Tube 78 has its upper end passing through the tank cover 74 to communicate with the remote end 38-2 of the coating conduit 38 and its lower end immersed in the pressurized coating 73. An agitating mechanism 80 in the form of an impeller 81 fixed to the end of a shaft 82 driven by a motor 83 extends into the coating material 73 for the purpose of insuring that the coating is maintained in a homogeneous state.

When moderately conductive coatings are being used, the resistance of the column of coating in the conduit 38, which often is 25 feet or longer in length, is sufficient to prevent the high voltage charging potential at the gun antenna from being applied to any appreciable extent to the tank 33 or its contents 73. Under such circumstances, the tank 33, and accordingly the coating cOntents 73 thereof, are electrically connected to ground potential via an electrically conductive strap or the like (not shown) having one end electrically connected to the tank and the other end to a source of ground potential. Such grounding will enable any electrical charge from the high voltage gun antenna which does leak to the tank via the coating column in the conduit to be safely discharged without hazardous electrical charge accumulation in the tank. When highly conductive coatings are being used, the conductive tank and coating contents are not electrically grounded inasmuch as the high voltage charging potential of the gun antenna is applied to the tank contents via the highly conductive coating column in the conduit. Instead, the tank is electrically insulated from ground potential by suitable means, such as shown in copending application Serial No. 199,114, filed Nov. 16, 1971 in the names of Hastings et al., assigned to the assignee of this application. The Hastings et al. application is incorporated herein by reference.

As noted earlier, this invention has unusual utility in the spraying of moderately conductive and highly conductive coating materials. Illustrative of the moderately conductive coating materials are solvent-based coatings such as enamels and lacquers wherein the solvent may be acetone, ethanol, methanol, methyl-ethyl-ketone, or the like. Illustrative of highly conductive coating materials are water-based enamels. Of course, it should be understood that this invention can also be utilized to spray low conductive coating materials.

The novel coating conduit, hose, or supply line 38 of this invention, in accordance with a preferred form thereof, includes an electrically conductive outer layer, surface, skin sheath, or zone 38A, an intermediate dielectric breakdown resistant layer, core, sheath, or zone 38B, and an inner barrier surface, skin, layer, sheath, or zone 38C. The inner surface or zone 38C is preferably chemically inert with respect to the coating being transported within a bore 38D defined thereby such that the surface 38C will not be significantly corroded, dissolved, eroded, or otherwise physically or chemically deteriorated by chemical interaction with the coating being conveyed through the bore. The inner surface 38C is also preferably essentially impermeable with respect to the coating conveyed through bore 38D, effectively establishing a fluid-tight barrier between the coating-transporting bore 38D and the remaining layers or zones 38B and 38C of the conduit 38. The establishment of a barrier between the bore 38D and zones 38B and 38C which, with respect to the coating being transported is substantially fluid-tight, limits possible permeation of the intermediate dielectric zone 38B by the coating should the latter zone be permeable, which is often the case where the intermediate zone is fabricated of flexible dielectric material since many flexible dielectric materials are permeable to common coating solvents. Were significant permeation of zone 38C permitted to occur, an electrically conductive path through the zone 38B could be established, assuming the latter is permeable, leading to undesirably high electrical current leakage in a radial direction through the wall of conduit 38.

The zone 38C can be fabricated of electrically conductive material, although an insulative material is preferred. One such insulative material found to be highly desirable for use in fabricating zone 38C when the conduit is used to transport coatings having solvent bases utilizing acetone, methyl-ethyl-ketone, ethanol, methanol and the like, is hollow extruded tetrafluro-ethylene tubing. Preferably, a wall thickness measured in the radial direction of no more than approximately 40 mils is provided. Since tetrafluroethylene, which is substantially inert and impermeable to common organic coating solvents, is relatively stiff, the thickness of zone 38C should be kept to a minimum to maintain conduit flexibility. Tetrafluroethylene has a resistivity of approximately 10.sup.18 ohm-centimeters and a dielectric breakdown resistance of approximately 1,000 volts/mil, and as such resists substantial radial electrical current flow and dielectric breakdown when subjected to coating at the charging potential. Obviously, other compositions can be used to fabricate the inner zone 38C, the particular composition depending upon the characteristics of the coating being transported through the bore 38D.

The intermediate core or zone 38B functions, in combination with the inner zone 38C, to establish a dielectric breakdown resistant barrier in the radial direction which withstands dielectric breakdown when the interface between the inner zone 38C and the coating in bore 38D is subjected to a high voltage as necessarily occurs when the gun antenna charging voltage is applied to the interface via the column of moderately or highly conductive coating in the bore which is in electrical contact with the gun. In a preferred form of the invention, the intermediate dielectric breakdown resistant zone or bore 38B is fabricated of extruded hollow polyethylene tubing having a thickness of approximately 85 mils. Such a construction is relatively flexible, in addition to having the desired electrical properties of low radial current leakage and high dielectric breakdown resistance. Specifically, polyethylene has a resistivity of 10.sup.15 - 10.sup.16 ohm-centimeters and a dielectric breakdown resistance of approximately 700 volts per mil and, like zone 38C, resists substantial radial electrical current leakage flow and dielectric breakdown when subjected to voltages thereacross on the order of the charging potential. Other dielectric materials could be utilized depending upon the degree to which it is desired that the intermediate zone material be chemically inert and impermeable to the coating. For example, polypropylene and vinyl plastics may be used although such are more prone to being dissolved or permeated by organic coating solvents should such permeate through the inner surface or zone 38A which, in practice, occurs to at least a very slight extent.

It has been found desirable to fabricate the inner and intermediate zones 38C and 38B of material which provides a combined, or average, dielectric strength of approximately 800 volts per mil, although average, or combined, dielectric strengths ranging between 250 volts per mil and 1,000 volts per mil are satisfactory for specific applications. If composite dielectric strengths of lesser values are used, the thickness of the conduit wall measured in the radial direction may become undesirably large, increasing the bulk and stiffness of the coating conduit.

The outer electrically conductive zone or surface 38A functions to leak, or conduct away, to a potential lower than that of the charged coating in bore 38D, e.g., to ground potential, electrical charge which has flowed radially through the inner and intermediate zones 38C and 38B. Some charge flow does occur in practice due to the less than infinite resistivity and impermeability of the materials of zones 38B and 38C, which resistivity and impermeability tend to degrade with time and use. The charge removal function of outer conductive zone 38A prevents the accumulation or build-up of electrical charge on the outer surface of the conduit 38. Such charge accumulation would present a safety hazard in the form of an electrical shock to a grounded operator inadvertently contacting the hose, or ignition if the exterior surface of the conduit were inadvertently grounded in an explosive atmosphere, since in each case the accumulated charge on the exterior of the conduit would discharge to ground.

In a preferred form, where resistance to abrasion is desired, the outer zone 38A is fabricated of an electrically conductive plastic, such as carbon-loaded polyurethane having a wall thickness measured in a radial direction of approximately 20 mils. Of course, other electrically conductive plastics may be used, and a description of such may be found in ASME Publication 66-MD-31 entitled "Conductive Plastics" by Irving Litant, and in Machine Design, "Conductive Plastics" by Irving Litant, Oct. 16, 1969, Pages 168-172. Alternatively, outer zone 38A may be an electrically conductive metallic foil, sheath, or the like of copper, silver, aluminum or similar conductive material.

Since in practice there is some current leakage radially through the wall of the conduit 38 to outer surface of the conduit from the interface of zone 38C and electrically charged coating, and further since such current leakage is distributed in varying amounts along the longitudinal axis of the conduit, the conduction to ground of the radial leakage current by the exterior zone 38A necessarily produces a voltage drop in an axial direction along the length of the conductive zone 38A. Since under normal conditions the external conductive surface 38A will be grounded at two points, namely, at opposite ends of the conduit where it communicates with the gun 10 and with the tank 33, the radial leakage current charge removal path measured in an axial direction will have a length equal to approximately one-half the total length of the conduit. With conduit lengths of 25 feet, an axial leakage current removal path of 121/2 feet will be present along the exterior zone 38A assuming the exterior zone is grounded at both its ends. Of course, if the conduit zone 38A is grounded at only one end, either the gun or tank, the leakage current removal path axially along the exterior conductive zone 38A will have a length equal to the length of the hose. Since any electrical current flow is necessarily accompanied by a voltage drop due to the resistance of the conducting material, the axial leakage current removal flow to ground potential provided by exterior zone 38A produces a voltage drop in an axial direction along the surface of the exterior zone 38A. Preferably this voltage drop should be maintained at a low value, for example, on the order of 15 volts or less. Accordingly, the resistivity and thickness of the conductive zone 38A should be selected such that for a given total radial leakage current through the zones 38C and 38B along the entire conduit length, and for a given conduit length and number of ground potential connections, the maximum voltage with respect to ground potential along the length of the conductive zone 38A is maintained at a safe minimum.

To assure that the desired safe minimum voltage drop along the length of the external surface 38A is achieved, the carbon-loaded polyurethane layer or zone 38A of the preferred embodiment may be supplemented by provision of an external sheath (not shown) of highly conductive metal, such as copper, in electrical contact therewith. Alternatively, for the carbon-loaded polyurethane zone 38A a highly conductive metallic layer such as copper, foil, braid, or the like may be substituted. However, and as noted, a conductive plastic, such as carbon-loaded polyurethane, is preferred by virtue of its high abrasion resistant characteristic, and accordingly when a conductive plastic is utilized, its wall thickness measured in a radial direction will be determined in part by the thickness required to give the desired abrasion resistance. Of course, the wall thickness of the exterior surface 38A measured in a radial direction will also be determined by the annular cross-sectional area thereof required to give the desired resistance per unit axial conduit length such that the voltage drop in the axial direction along the exterior surface of the conduit will not be undesirably high.

As noted, conductive conduit zone 38A preferably is grounded at both its opposite ends 38-1 and 38-2. Where moderately conductive coating is used and tank 33 is grounded, conduit zone 38A at end 38-2 is electrically connected to the tank by an electrically conductive fitting 85 having an integral crimpable collar 85A which surrounds the conductive zone 38A and is in intimate electrical and physical contact therewith. Fitting 85 is threaded to an electrically conductive manually operated shut-off valve 87 which itself is threaded to the conductive coating feed pipe 78 secured to, and in electrical contact with, grounded tank cover 74. If tank 33 is not grounded, as when highly conductive coatings are used, zone 38A at conduit end 38-2 can be grounded independently by a separate ground cable (not shown). Of course, suitable electrical stand-offs should be provided between the high voltage zone 38C and the grounded zone 38A at end 38-2.

Conduit zone 38A at conduit end 38-1 is grounded via conductive bracket 40 mounted to conductive handle 11, as described earlier.

To provide the desired electrical stand-off between grounded zone 38A of conduit end 38-1 and the inner zone 38C in contact with the charged coating in bore 38D, insulative fitting 88 is used. Fitting 88 includes an upper insulative end section 89 threaded into the passage 37 of insulative barrel 12, and a lower insulative end section 90 threaded to the upper end section 88. Inner conduit zone 38C at its end surrounds an insulative tubular insert 91 located in bore 38D of the fitting section 89. The tapered upper end of fitting section 90 surrounds the end of conduit zone 38A and when threaded into fitting section 89 clamps conduit zone 38A around insert 91 to seal the connection between bore 38D and gun barrel passage 37. Conductive conduit zone 38A is received in the bore 92 of fitting section 90 and sealed by an O-ring. The distance between the foreshortened end of outer conductive conduit zone 38A and the end of inner conduit zone 38C which contacts the charged coating in bore 38D provides the necessary electrical stand-off at conduit end 38-1 between conduit zones 38A and 38C. Intermediate dielectric zone 38B terminates at a point between the ends of zones 38A and 38C.

In the preferred embodiment, a three-layer composite conduit structure was described to afford the characteristics of a) flexibility of the overall conduit, b) chemical inertness and c) impermeableness of the inner zone to the coating, d) resistance of the conduit to dielectric breakdown when subjected to voltages applied by the charged coating transported by the conduit, and e) requisite conductivity of the outer zone for conducting away to a lower potential radial current leakage through the conduit wall. It is to be understood that such a three-layer composite conduit is not the only suitable approach. For example, the inner and intermediate zones 38C and 38B can be substituted by a single layer or zone providing such single layer or zone has the requisite flexibility if such is necessary in the particular application, inertness and impermeableness to the coating, resistance to dielectric breakdown when subjected to a high voltage by the charged coating transported in bore 38D, and a radial electrical resistance which will limit current leakage in a radial direction from the charged coating in the bore to a relatively low value, e.g., below 1 microampere.

The composite conduit of this invention having an inner dielectric breakdown resistant zone and an outer charge-removing conductive zone can be used, when filled with an electrically conductive fluid, as a fluid-state electrical conductor for transporting high voltage power between two spaced points interconnected by the conduit. For example, instead of using a conventional solid-state electrical conductor, such as an insulated copper wire, to conduct high voltage power from the power pack to the coating charging gun antenna, the composite conduit of this invention, filled with a conductive fluid, to thereby establish a fluid-state electrical conductor, could be employed. The conduit fluid at one end of the conduit bore would be electrically connected to the gun antenna while the conduit fluid at the other end of the conduit bore would be electrically connected to the high voltage power supply. In this manner, the high voltage at the power supply would be applied to the gun antenna via the conductive fluid in the conduit bore. Depending upon whether or not the conductive fluid is an electrolyte, the mechanism of electrical conduction, or electrical charge transport will be effected either by ion migration in the case of an electrolyte or free electron transport if the conductive fluid is not an electrolyte. The outer conductive conduit zone would be electrically connected to a source of potential substantially lower than the potential of the high voltage power supply.

When the conduit of this invention is used as a fluid-state electrical conductor, the safety features noted in connection with its use to transport electrically charged coating material from a tank to a gun are fully realized. Specifically, the inner dielectric wall or conduit zone resists dielectric breakdown when subjected to high voltages by the high voltage conductive fluid in contact therewith. Additionally, the outer conductive conduit zone removes charge which would otherwise tend to accumulate on the exterior of the conduit as a consequence of radial current leakage through the conduit wall. Since charge accumulation on the conduit exterior is avoided, electrical shock and ignition hazards due to inadvertent contacting of the conduit by a grounded operator or accidental grounding of the conduit are eliminated.

From the foregoing disclosure of the general principles of the present invention and the above description of the preferred embodiment, those skilled in the art will readily comprehend various modifications to which the present invention is susceptible. Accordingly, we desire to be limited only by the scope of the following claims:

Claims

We claim:

1. A method of electrostatically spraying at least moderately electrically conductive coating material comprising the steps of:

electrically charging the coating material to a high voltage prior to deposition thereof on an object to be coated,

supplying the coating material to a spraying device through a hollow conduit having a wall structure in contact with said coating material which is substantially chemically inert and impermeable to said coating, said conduit through which said coating is supplied being constructed to withstand dielectric breakdown when subjected to the coating charging voltage, and

removing, from the exterior surface of said conduit, electrical charge which has been conducted radially through said conduit from electrically charged coating material in said conduit.

2. The method of claim 1 wherein said charge-removing step includes conducting electrical charge from the exterior surface of said conduit by electrically connecting said exterior conduit surface at a point proximate said spray device to a potential source having a potential substantially lower than the coating charging potential.

3. The method of claim 1 wherein said charge-removing step includes conducting electrical charge from the exterior surface of said conduit by electrically connecting said exterior conduit surface at a point proximate the coating supply to a potential source having a potential substantially lower than the coating charging potential.

4. The method of claim 1 wherein said charge-removing step includes conducting electrical charge from the exterior surface of said conduit by electrically connecting said exterior conduit surface at a first point proximate said spray device and at a second point proximate said coating supply to a potential substantially lower than the coating charging potential.

5. An electrostatic spray coating system comprising:

a supply of at least moderately electrically conductive coating material,

a source of high voltage unidirectional electrical current,

a coating spraying device located remote from said coating supply for spraying coating material onto an object to be coated, said object being maintained at an electrical potential different from said high voltage source,

an electrically conductive element electrically connected to said high voltage source and located to electrically charge said coating material prior to coating said object, and

a coating conduit interconnecting said spraying device and said coating supply having a bore through which coating material is transported from said coating supply to said spray device, said conduit including

a. an inner zone surrounding said bore, said inner zone including material which is substantially chemically inert and impermeable to said coating material,

b. an outer zone of electrically conductive material surrounding said inner zone, said outer zone being electrically connected to a source of electrical potential substantially lower than said potential of said high voltage source, and

c. an intermediate zone between said inner and outer zones, said intermediate zone including dielectric material,

said inner and intermediate zones having dielectric constants and radial thicknesses selected to avoid dielectric breakdown therein when subjected to the potential difference existing between 1) the interface of said transported coating and inner zone and 2) said outer zone,

said inner and intermediate zones having resistivities collectively permitting electrical current leakage from said transported coating in said bore to said outer zone in an amount which, if allowed to accumulate, would electrically charge said outer zone to a measurable level and produce unsafe operator electrical shock and/or explosive atmosphere ignition hazards.

6. An electrostatic spray coating system comprising:

a supply of at least moderately electrically conductive coating material,

a source of high voltage unidirectional electrical current,

a coating spraying device located remote from said coating supply for spraying coating material onto an object to be coated, said object being maintained at an electrical potential different from said high voltage source,

an electrically conductive element electrically connected to said high voltage source and located to electrically charge said coating material prior to coating said object, and

a coating conduit interconnecting said spraying device and said coating supply having a bore through which coating material is transported from said coating supply to said spray device, said conduit including

a. a first zone surrounding said bore, said first zone including electrically conductive material connected to a source of electrical potential substantially lower than said potential of said high voltage source, and

b. a second zone between said bore and said first zone, said second zone including material which is substantially chemically inert and impermeable to said coating material, said second zone including dielectric material constructed to avoid dielectric breakdown therein when subjected to the potential difference existing between 1) said transported coating and 2) said first zone, said second zone having a resistivity permitting electrical current leakage therethrough from said transported coating in said bore to said first zone in an amount which would, if allowed to accumulate, electrically charge said second zone to a measurable level and produce unsafe operator electrical shock and/or explosive atmosphere ignition hazards.

7. An electrostatic spray coating system comprising:

a supply of at least moderately electrically conductive coating material,

a coating spraying device located remote from said coating supply for spraying coating material onto an object to be coated, said object being maintained at an electrical potential different from said high voltage source,

coating charging apparatus for electrically charging said coating material prior to coating said object, and

a coating conduit interconnecting said spraying device and said coating supply, said conduit including a bore through which coating material is transported from said coating supply to said spray device, said conduit including

a. a first zone surrounding said bore in electrical contact with said transported coating, said first zone including dielectric material for avoiding dielectric breakdown of said first zone when contacted by electrically charged coating material transported in said bore, and

b. a second zone surrounding said first zone, said second zone including electrically conductive material connected to an electrical potential source of lower potential than the potential of said transported coating for preventing accumulation of electrical charge radially interiorly of said second zone occasioned by electrical current conduction radially through said first zone.

8. An electrostatic spray coating system comprising:

a supply of at least moderately electrically conductive coating material,

a coating spraying device located remote from said coating supply for spraying coating material onto an object to be coated, said object being maintained at an electrical potential different from said high voltage source,

coating charging apparatus for electrically charging said coating material prior to coating said object, and

a coating conduit inteconnecting said spraying device and said coating supply, said conduit including a bore through which coating material is transported from said coating supply to said spray device, said conduit including

a. a dielectric wall surrounding said bore and having a zone in contact with the coating which is substantially chemically inert and impermeable to said coating, said dielectric wall withstanding electrical breakdown when subjected to the coating charging potential, said dielectric wall permitting only an insubstantial electrical current flow in a radial dirction, and

b. charge conduction means for removing charge from said wall occasioned by said radial electrical current conduction.

9. An electrostatic coating spray system comprising:

a coating spray device,

an antenna fixed relative to said spray device,

a source of high voltage unidirectional current, said source electrically connected to said antenna to electrostatically charge coating material prior to deposition thereof on an object to be coated,

a source of moderately conductive coating material,

a hollow coating conduit interconnecting said coating source and said spray device for transporting coating material to said spray device, said conduit having a dielectric wall with a zone in contact with the coating which is substantially chemically inert and impermeable to said coating, said wall constructed to withstand dielectric breakdown when subjected to an electrical potential by electrically charged coating material in said conduit, and

charge removal means electrically connected to the exterior surface of the said dielectric wall for removing therefrom electrical charge which has been conducted radially through said wall from electrically charged coating material in said conduit.

10. An electrostatic coating spray system of claim 9 wherein said dielectric wall has an electrical resistance sufficiently low to permit electrical current to flow therethrough from said charged coating in said conduit in an amount which would, if allowed to accumulate, electrically charge said exterior surface of said dielectric wall to a measurable level and produce unsafe operator electrical shock and/or explosive atmosphere ignition hazards.