CA1125358A - High potential discharge control circuit including a resistive material coated electrode for induction-charging electrostatic spraying system - Google Patents

Abstract

HIGH POTENTIAL DISCHARGE CONTROL CIRCUIT INCLUDING
A RESISTIVE MATERIAL COATED ELECTRODE FOR
INDUCTION CHARGING ELECTROSTATIC SPRAYING SYSTEM

Abstract of the Disclosure A high voltage discharge control circuit is provided for an induction-charging spraying system. The circuit comprises a first re-sistance means comprising an induction-charging electrode coated with a resistive material that retards transport of electric charge across the electrode surface to electrode edges or surface discontinuities which are most susceptible to arcing to an electrical ground point. A current limiting resistor may be in series with the resistive coating and a high voltage source to inhibit current surge to the electrode surface from other circuit elements. The circuit may also contain additional current limiting series resistors in the high voltage cable and in the power supply to further inhibit current surges. Shunt resistors are provided between high potential circuit elements and electrical ground to drain accumulated charge from the circuit.

Description

Back~round of the Inveneion -Field of the Invention:
Electrostatic spraying devices which provlde spray stre~ms of charged liquid particles by an induction-charging mechanism are well known.
Of particular interest herein is circuitry for controlllng or reducing the incidence of high electric potential discharges from elements of an induction-charging spraying system.
`~ . , l~Z5358 State of the Art:
A need has long been recogni~ed for an electrostatic spray device that can safPly and efficiently deposit charged particles of paint or coating material onto a subs~rate. Corona-discharge electro-static spray systems~ for example, can provide fairly efficient depo-sition of charged paint particles onto a substrate. These corona devices, however, typically utilize needle-like elec~rodes that establish corona-producing electric fields by application of potentials of about 100~000 volts to the electrode with resulting corona-discharge currents approximating 50-300 mlcroamps. Such high-power electric discharges present potential shock ha~ards to equipment operators.
Moreover, there is great lik~lihood in corona systems of hlgh potential electric dlscharge by arcing from the elec~rode to a ground poin~ or by sparks from the electrode to air-borne particulate matter, which electric discharges can ignite flammable paint vapors. The hazard of fire and explosion from corona-discharge ignited paint vapors has, for example~ substantially precluded use by ma~or household appliance manufacturers of electrostatic spray devices for spraying organic-based paints onto the lnterior surfaces of appliance cabinets.
In response to a need or an electrostatic spray device that -can safely and efficiently spray flammable pain~s wi~hou~ hazards of fire or explosion or electrical shock to equipmen~ operators, there ha~ been recently provided an improved electrostatic spray deYlce as disclosed in U.S. Pa~ent No. 4,009,829 to J. E. Sickles. This electro-static spray device comprises an induction-charging electrode positioned exteriorly of ~ 2 -:`
. ,. ~

:
~2535~

an external-mlxing spray-forming nozzle. Since charge imposition on spray particles occurs by the method of induction, there is practically no like-lihood, under ideal conditions, oE any substantial arc or high energy corona discharge. The absence of any substantial discharge is assured by an electrode surface configuration that is devoid of sharp edges and points and by the application of high voltage potentials to the electrode of about ~5,000 volts or less, with normal current dissipation by the electrode being at a level of about 1 to 3 microamps or less. With the induction-charging electrode operating at these substantially lower voltage and current levels as compared to a typical corona-discharge electrode, any incidence of arcing or sparking is substantially reduced. Moreover, operator injury resulting from electric shock is avoided by the practically insignificant current available to be delivered by the electrode.
In addition to the aforementioned improved safety features, the described induction-charging spray device provides improved charged particle atomization. It has been found that a spray device comprising an induction-charging electrode disposed exteriorly of, or outwardly from, an external-mixing nozzle provides an assembly of particles characterized by a high degree of fineness and uniform size and having a relatively high average charge-to-mass ratio. These factors are important in achieving maximum transfer of coating material from the spray device to the target substrate and for achieving levelling or flow oE the material into an evenly deposited, uniformly coalesced film.
This unique combination of safety and deposition efficiency ~S features of the described induction-charging spray device is responsible for the significant commercial success of the device in overcoming problems inherent with corona-charging types of electrostatic spray equipment. It Z~3~

has been found, however, that because oE the rather large amounts of energy stored within capacitive elements of the charging circuit, rela-tively intense electrical dlscharges may occur from the electrode to an object at a lower electrical potential under certain less than ideal operating conditions. For example, when an energized, hand-gun mounted electrode is brought too close to an electrically grounded object, there may be a sudden arc from the electrode to the grounded object. ~lso, during use of induction-charging spraying equipment the metallic surfaces of electrodes may become nicked or scratched, thereby establishing ideal sites of surface discontinuity for corona or sparking discharges. After periods of spraying, dried paint may build up on the electrodes and pro-vide sites for producing corona or sparking, especially where the dried paint contains metallic or electrically conductive pigments.
A high intensity electrical energy discharge occurs as an arc of current between an electrode maintained at a relatively high potential and an object at a relatively lower potential. The object at a lower potential may be another portlon of the spray device, or it may be an electrically grounded article such as the target to be coated, or other object in the spraying environment. Electrical arcs or sparking may also occur between the electrode and air-borne dust or spray particles which may be at lower potentials than the electrode. ~hile these sudden dis-charges may be merely irritating to the touch of a spray operator, the arcing or sparking phenomena may have potentially lethal conse~uences where they occur in environments of flammable vapors, such as contributed by many organic paint systems.
Cne solution to the sparking problem is that of merely decreasing the voltage applied to the electrode to a level so that energy stored in the charging circuit lacks sufficient potential to cause intense current-carrying arcs to leave the electrode surface. Decreasing the electrode potential, however, also causes particle charging and coating deposition efficiencies to (iecrease.
Another solution to the dilemma of providing high charging voltages while substantially eliminating flammable vapor-igniting sparks is described in U. S. Patents No. 3,641,971 and No. 3,795,839 to Walberg.
For the Walberg corGna or contact charging spraying system, there is p-rovided a complicated, expensive circuit "deenergizing means" for dis-connecting the high potential source when a certain threshold current is reached above which arcing occurs. The Walberg circuitry, while appro-priate for corona spraying systems, which typically require heavy and expensive high voltage and current power supplies, is unsuitable for an induction-charging system which is typically adaptable to small portable power supplies. Moreover, the interruption of the charging circuit de-creases particle charging and coating deposition efficiencies.
Another hazard common to electrostatic spraying environments is the sudden discharge or sparklng that may occur when a break or shor~
circuit occurs in the high voltage circuit. For example, breaks and short circuiting to ground may occur in the high voltage cable that delivers high potential from a power supply to an induction-charging electrode. There may follow a sudden discharge of electrical energy stored in other high capacitive elements of the electrostatic system, such as the power supply, or in the induction-charging electrode or in the cable itself, each of which is capable of storing significant amounts of electrical energ-,-. Sudden electrical discharge may produce sparking conditions at the short circuit point with consequent hazards of fire or explosion of paint vapors and may also constitute a signi~icant shock hazard to equipment operators.

53~E~

Summary oE the Invention The lncidence oE illgll potentlal electric discharges from components oE an incluction-charging electrostatic spraying system may be substantlally red~lced by lncluding ln the charg:ing circuit resistance means of appropriate resistance values to retard the flow oE charge stored in one portion of the circuit to another circuit portion where short circuit or likely arc-producing conditions exist. An induction-charging electrostatic spraylng system is of a type whlch comprises llquid particle spray-Eorming means for discharging a spray stream of liquid particles along the axis of the spray-forming means, induction-charging means dlsposed ln operable association wlth the spray-forming means to establlsh a charging zone in which charge is induced on liquid particles as the particles are formed by the spray-forming means, and means for connecting a high voltage DC electric potential to the induction-charging means. A control circuit for controlllng high potential electric discharges from elements of the induction-charging electrostatic-spraying system comprises at least one induction-charging electrode disposed ex-teriorly of, or outwardly from, the spray-forming means. The electrode comprises a substrate having a wall faci~ng the axis of the spray-forming means, the wall providing a surface from which an induction-charglng field is established when an electric potential is applied to the electrode wall.
A first resistance means is connec,ed between the high voltage DC electric potential connecting means and the electrode wall, with at least a portion of the first resistance means comprising the electrode wall and providing - 25 the surface from which an induction-charging electrostatic field may be established.

535~

The flrst resistance means comprises a layer of resistive ma-terial forming the electrode wall. The resistive material layer is characteri~ed in providing a surface having sufEicient electrical surface resistivity to lnhiblt substantial transport of electric charge across the electrode surface to an edge of the electrode or to a discontinuity on the electrode surface at electrode operating voltages typical of an induction-charging system. The limitation of substantial charge flow to electrode edge portions or discontinuities reduces the tendency for high potential electric discharge by arcing from an electrode edge or discon-tinuity to an electrical ground point.
The resistive material comprising the electrode wall layer may be provided by a film formed from a coating composition which comprises a mixture of a substantially non-electrically conductive resinous component and an electrically conductive component. The ratio of proportions oE
conductive to non-conductive components may be varied to provide a wide range of compositions that form films having various suitable surface resistivities.
An electrode wall may comprise a film that, while having suitable surface resistivity for inhibiting surface charge tranYport and arc for-mation, may be of insufficient thickness to provide appropriate impedance to charge flow from other portions of the charging circuit. The first re-sistance means will then comprise a discrete resistance element connected in series with the resistive material comprising the electrode wall and the high voltage electric potential connecting means, the discrete resis-tance element being disposed in close physical proximity to ~he electrode wall resistive film. In "close physical proximity" is intended to indicate that the electrical connection between the electrode wall and the first ~53~3 resistance means is as short as possible so that any substantial capaci-tive or charge-storing capability created witllin the connection is significantly less than the capaci.tance of the electrode circuitry or of the charging circuit. The first resistance means has sufficient ohmic resistance to retard or impede the transport of electric charge to the electrode surface at the magnitude of voltage applied to the electrode required to establish the induction-charging field. ~ first resistance means of an appropriate resistance value and disposed in close physical proximity to the electrode wall will impede transfer of electric charge to the electrode surface such that substantially no electric discharges will occur which have intensities sufficient to ignite flammable paint vapors.
The incidence of a spark-ignited fire or explosion of flammable paint vapors is a function of the intensity and duration of the spark or arc, as well as the type and concentration of the vapor. ~rc intensity and duration are, in turn, related to the energy imparted to the arc by the charging circuit. If the quantum of arc energy remains below that energy required to ignite the most flammable vapor-air mixture found in typical spraying operations, then ignition and explosion are not likely : 20 to occur. It has been determined that for a saturated mixture of toluene and air at 62F., or of xylene and air at 115F., which are the most flammable concentrations of these typically utilized solvents, the threshold energy of ignition is about lS millijoules. Thus, a rela-tively safe electrostatic charging system for use in spraying -xylene-or toluene-containing paints in confined spaces may be provided by an electrode that provides substantially no electrical discharges or, if discharges do occur, by an electrode that electrically discharges arcs or sparks having energies less than l5 millijoules.

~3~

The first resistance means may comprise a discrete resistor of the conventional carbon-filled or wire-wound type that may be embedded in an electrode substrate of electrically non-conductive or dielectric mater~al. The first resistance means may comprise a slab of resistive material that may make up most of the material of the electrode suDstrate. Combinations of resistive material in series with discrete resistors may also be used.
An induction-charging electrostatic spraying system typically comprises, in addition to an induction-charging electrode, a power supply for furnishing a suitable high voltage DC electrical potential to the induction-charging electrode. In many industrial applications the power supply may be remote from the spraying device. The use of a remote power supply typically requires a cable designed for trans-mitting high voltages safely over a distance of six to twenty feet or more from the power supply to the induction-charging electrode. In other applications~ a portable power supply may be fitted to the spray device itself, eliminating the need for a high voltage conveying cable.
Second and third resistance means may be included in $he high electric potential discharg~ cont~ol clrcuit. The second resistance means comprises a current limiting resistor connected in series with ~-a high voltage ou~put ter~inal of the power supply and w~th the first resistance means associated with the induction-charging electrode.
Typically, the series resistor is located in the high voltage trans-mitting cable at a ._ g _ ~Z~i35~3 point near the connection of the cable to the high voltage connecting means of the induction-chargillg electrode. The third resistance means comprises a current llmiting resistor in series Wittl the power supply high voltage output terminal and the connecting means Eor connecting the high voltage cable to the power supply. A purpose of these current limiting resistors is to inhibit transfer of charge stored in one portion of the circuit to anocher circuit portion. With the retardation of flows of substantial charge either from the power supply to the cable or from the induction-charging electrode to the cable, the likelihood of sparking from the cable to ground points is minimized, in the event that the cable is pulled from the power supply terminal, or the cable is severed while power is being applied to the system, when significant amounts of charge remain stored in the charging circuit.
~ourth and fifth resistance means may be included in the high electric potential discharge control circuit. The fourth resistance means comprises a current limiting resistor connected between the power supply nigh voltage output terminal and an electrical ground connection. The fifth resistance means comprises a current limiting resistor connected at a point near the high voltage connecting means of the induction-charging ~ electrode and an electrical ground connectlon. A purpose of these resistors is to provide a shunt path to electrical ground to drain or bleed off accu-mulated charge stored within circuit elements.
As another aspect of the invention, there may be provided an induction-charging adapter for mounting on a spray device, which adapter includes a high elec~ric potential discharge control circuit. The adapter comprises (a) housing neans fabricated of a dielectric material, the housing means having an exterior wall and having an interior wall, (b) mounting means S3S~

on the interior wall of the housing means for detachably mounting the housing means onto a spray device so that the interior wall faces the axis of a spray device when the housing means is molmted on a spray device, (c) induction-charging means including at least one induction-S charging electrode attached to the housing means, the electrocle com-prising a substrate having a wall that faces the axis of a spray device when the housing means is mounted on a spray device, the wall providing a surface from which an induction-charging electrostatic field is estab-lished when a high voltage DC electric potential is applied to the elec-trode, (d) means for connecting a high voltage DC electric potential to the induction-charging electrode, and (e) first resistance means connected between the electric potential connecting means and the electrode wall, at least a portion of the first resistance means comprising the electrode wall and providing the surface from which the induction-charging field may be established, the first resistance means having sufficient resistance to retard transport of charge across the electrode surface at the voltages applied to the electrode required to establish the induction-charging field, whereby charge stored within the circuit that is available for discharge at the electrode surface is prevented from discharging at an energy level at or above the threshold energy level required to ignite a flammable gas-and-air mixture.
The first resistance m~ans of the adapter comprises a film or layer of resistive material forming the electrode wall and may comprise a discrete resistor or a slab of resistive material in series with the elec-trode wall and the high voltage electric potential connecting means for the purposes as set forth above in discussion of these resistor elements.
Also mounted upon the adapter housing may be a current limiting resistor of the type described above comprising the fifth resistance means. This addi-tional current limiting resistor is connected bétween the high voltage connect-ing means for the first resistance means and an electrical ground connection.

~53~8 The resistor provides a shunt path to gro~md for bleeding charge to electrical ground that is stored i.n the electrocle circuit oE the induction-ctlarging adapter.

Brief Description of the Drawin~s ., The accompanying drawings illustrate examples of embodiments of the invention constructed according to the best mode so far devised for the practical application of the principles thereof, and in which:
FIG~ l is a diagrammatic presentation of an electrostatic spraying system illustrating preferred physical locations of resistor elements;
FIG. 2 is a schematic diagram of a high voltage discharge control circuit for an induction-charging electrostatic spraying system;
FIG. 3 is a perspective view of an induction-charging adapter fitted to an external-mixing spray gun;
FIG. 4 is an e~cploded view of the nozzle assembly of the -spray gun illustrated in FIG. 3;
FIG. 5 is a rearward elevation of an induction-charging adapter with a downstream view into the adapter; ~ ~
FIG. 6 is a top vieu of aD induction-charglng adapter showing 20 : an embodiment of the induction-charging electrode in section; and ~IG. 7 is a top view of an inductlon-charglng adapter showing another embodiment of the induction-charglng electrode ln section.

Description of Preferred Embodiments An electrostatic spraying system of the induction-charging type is depicted ln FIG. 1. The system comprises a spray device 8, , .

35~3 an induction-charging adapter 9 fitted to the spray device, a power supply 10 for providing a suitable high voltage DC electrical potential at output terminal ll relat:ive to an electrical ground potential at ground connection 12, a cable 13 connecting high voltage cable connecting terminal 14 of the power supply with a high voltage input terminal 15 located on the induction-charging adapter, liquid coating material supply 16, a compressed air supply 17 and feed hoses 18 and 19 for de-livering liquid coating material and compressed air, respectively, to spray device 8.
A high electric potential discharge control circuit for use in combination with the induction-charging electrostatic spraying system is illustrated in the schematic diagram of FIG. 2. A first resistance means associated with an electrode of the induction-charging adapter is designated Rl, second resistance means in series with high voltage cable connecting terminal 14 and the high voltage input terminal 15 on adapter 9 is designated R2, within power supply 10 third resistance means connected between high potential output terminal 11 and cable connecting terminal 14 is designated R3, fourth resistance means connected betwee~n high voltage output terminal 11 and electrical ground connection point 12 is designated R4, and fifth resistance means connected between high voltage input termi-nal 15 and an electrical ground connection polnt~ is designated R5.
A more detailed description of spray device 8 may be found with reference to FIGS. 1 and 3, wherein there is illus-trated a conventional air-atomizing hand-held spray device 8 having a handle 20, a barrel 21 and a nozzle assembly 22. A trigger 23 serves to operate a valve assembly (not shown) within barrel 21 to regulate flows of liquid coating material and an atomizing gas, such as air, to nozzle assembly 22. A liquid coating.

3L~;2535~

material, such as a paint having a conductivity generally greater than 0,001 umho/cm, is fed to the spray device from paint supply 16 through paint feed hose 18, which is connected to spray device 8 by mating threaded members forming connecting means 24 for paint feed hose 18.
As indicated in Figure 1, the paint supply including its container is preferably electrically grounded. From a compressed air supply 17, feed hose 19 delivers atomizing air under pressure to connecting means 25, which again is an assembly of mating ~hreaded members.
The spray device as illustrated is a commercially available hand-held gun of the air-atomizing siphon type (Model 62, Binks Mfg.
Co., Chicago, Ill.). Nozzle assembly 22 is depicted as an external-mixing spray-forming nozzle of the type described in U.S. Pate~t No~ .
4,009,829 to J. ~. Sickles. As shown in Figure 4, nozzle assembly 22 comprises a liquid discharge noæzle 26 having a liquid-conveying passageway and comprises an air cap 27~ The assembly of liquid dis-charge nozzle 26 and air cap 27 defines an annular-shaped atomizing-air discharge port 28 that is concentric with a liquid discharge port 29. Air and liquid di3charge ports 28 and 29, respectively, lie in the plane of face 30 of nozzle assembly 22 and are disposed generally coaxially with respect to ~he axis of the liquid-conveying passageway of liquid discharge nozzle 260 Streams of atomizing air and liquid coating material diæcharged from ports 28 and 29, respectively, coact to form a spray stream of particles that is discharged generally coaxially with respect to the liquid nozzle axis and in a downstream direction with respect to nozzle face 30.
Liquid discharge nozzle 26 may be fabricated of electrically conducting or non-conducting materials. It is preferred that within the liquid-conveying passageway of liquid noæzle 26 near its discharge port 29 a conductive grounding element ~not shown) be provided in electrical contact with the stream o liquid coating material. This grounding element will be grounded to a common electrical ground shared by the paint supply and power supply as indicated ln FIG. 1. Preferably, air cap 27 is fabricated of a dielectric material, such as acetal resins, epoxy resins, glass-filled nylon resins and the like. If air cap 27 is made of a metallic or conductive material, then air cap 27 should be electri-cally isolated from ground potential. ~lso located on nozzle face 30 are additional air discharge ports 31. Projecting downstream from nozzle face 30 and integrally formed with air cap 27 is a pair of air horns 32. Located on air horns 32 on faces oriented toward the spray stream axis are additional air discharge ports 33. ~ir discharge ports 31 and 33 cooperate to shape the spray stream into a fan configuration.
Mounted on barrel 21 at its downstream~oriented or forward end is induction-charging adapter 9. The adapter comprises a housing 34 fabricated of a dielectric material. The dielectric material should be capable of withstanding stresses assoclated with the hieh voltages pro-vided by the power supply without electrical breakdown or tracking. Use-ful dielectric materials include those set forth above for~fabricating air cap 27. Housing 34 is mounted upon barrel 21 by a friction fit between a pair of shoulders 35 located at the rearward portion of housing 34 and upon interior wall 36. Each of shoulders 35 is yositioned diametrically opposed to the other and is shaped to mate wi~h a comple-mentary-shaped surface of barrel 21 so that housing 34 is rigidly secured to spray device 8. When the adapter housing is mounted upon spray device 8, housing interior wall 36 faces the axis of the spray stream discharged from nozz]e assembly 22, while housing exterior wall 37 faces generally in a direction outwardly of the spray stream axis.

.. . . .

` ~
53S~

Housing 34 is characterized in having its wall portions ex-tending downstream to form a pair oE lobes 38. ~lounted on the inner wall oE each lobe 38 is an lnduction-charging electrode 39. Each of electrodes 39 comprises a substrate 40 having an electrically con-ductive wall 41 with a surface 42 facing the spray stream a~is. When an appropriate high voltage DC voltage is applied to electrode 39, an electric field is established bètween wall surface 42 and a region sur-rounding liquid discharge port 29 through which liquid coating material is discharged. Means for connecting a high voltage DC electrical po-tential to each induction-charging electrode 39 includes a high voltage contact that forms high voltage input terminal 15 and a conductor 43 connecting terminal 15 to a resistive element typically located within electrode substrate 40. As illustrated in FIG. 5, terminal 15 may be a "banana" plug rigidly fixed within a portion of electrode substrate 40 that provides electrical connection for a suitable mating member incorpo- -rated into high voltage cable 13.
Adapter 9 is mounted upon spray device 8 such that electrodes 39 are positioned exteriorly of, or radially outwardly from, external-mixing nozzle assembly 22. Preferably, electrodes 39 are positloned with respect to nozzle assembly 22 so that at least a portion of surface 42 of elec- --trode wall 41 intersects a plane containing:liquid discharge port 29.
~; Thus, with respect to a plane containing noæzle assembly Eace 30, elec-trode wall surface 42 intersects the plane, with at least a portion of wall surface 42 extending downstream from, or forwardly of, nozzle assernbly face 30. The radial distance of electrodes 39 outwardly from the axis of liquid discharge nozzle 26 will generally determine the magnitude of the voltage required to be applied to electrodes 39 to ., ~ , 35~

provide an induction-charging field. For the adapter illustrated in FIG. 3 having each electrode 39 spaced outwardly about 3/4 lnch ~rom the liquid discharge noæzlc axis, DC voltages between about 5,000 volts to about 25,000 volts will produce an efEective induction-charging field in a region surrounding liquid discharge port 29, which field has an average potential gradient in the range Erom about 7 kllovolts per inch to about 33 kilovolts per inch. Voltages that are so high as to cause corona discharge from electrodes 39 are to be avoided. In this respect, the induction-charging electrode 39 may be characterized as one which is substantially non-corona producing, that is, electrode 39 has a configuration which is substantially free of sharp angles, points, or surface discontinuities that may tend to produce corona discllarges in the aforementioned voltage range.
In an induction-charging device such as that utilized in the present invention, liquid coating material atomization and electric charge imposition occur substantially simultaneously so as to create a stream of discrete particles bearing an induced electric charge. For example, the strearn of liquid coating material which passes through liquid discharge port 29 of nozæle assembly~22 is thrust into contact with a flow of air or gas from concentrically disposed atomizing~air discharge port 28, which flow of gas or air impinges upon and mixes with the liquid stream and tends to distort the stream into an irregular con-figuration comprising surface discontinuities. ~ormation of cusp-like, liquid stream discontinuities or "liquid termini" is aided by the high intensity electric field existing between high voltage electrode 39 and the grounded liquid strearn. The electric field fl~x lines tend to con-centrate at the sharp-pointed liquid termini and to induce electric ~ ~53S8 charge redistribution within the liquid stream, with charge oE sign oppo-site that of the high vo]tage electrode migrating to the e~treme sharp portions of the liquid termini. Slnce the chargcs on the liquid termini and on tlle electrode are opposite in sign, electrical attractive forces cooperate with the mechanical d:istresses Eurnished by the Elow o-f gas or air to separate the liquid termini from the liquid stream so as to fonn discrete coating material particles bearing electric charge.
Connected between high voltage input terminal 15 of the high DC electric potential connecting means and electrode wall surface 42 is a first resistance means, at least a portion of which comprises electrode wall 41 that provides surface 42 from which an induction-charging electro-static field is established. This first resistance means may be provided by any one of several embodiments. In a preferred embodiment, illustrated in FIG. 6, electrode wall 41 comprises a resistive fil-nl coated upon sub-strate 40. The electrode substrate 40 is fabricated of an electrically non-conductive or dielectric material of the aforementioned type for fabricating adapter housing 34. For purposes of illustration, the re-sistive film-forming wall 41 of tlle dielectric substrate 40 is shown to have a thickness of about one-sixth the total thickness of the electrode substrate and wall. In practice, a useful film may have a thickness of about 0.5 mil to about 2 mil. In the form of a layer, the resistive ma-terial may form a slab having a thickness varying from that of a resistive film to a thickness that is substantially coincident with the total thickness of the electrode substrate and wall.
Resistive films may be formed on electrode substrate 40 by a variety of methods. One preferred method is by coating elec~rode substrate 40 with a resinous composition that upon curing forr.ls a film having a predeter-mined surface resistivity sufficient to impede the flow of charge across the surface of the electrode and thereby reduce the likelihood o~ discharges from arc~producing sites at the surfaces. Suitable ~, . . .

s~

compositions for coating substrate 40 comprise mixtures of practically any substantially electrically non-conduct:ive organic resin component and an electrically conductillg component such as graphite. The ratio of the amounts conductive to non-conductive con-ponents may be varied to pro-vide a cured film having a surface resistivity suitable for a particular combination of operating parameters.
A particularly suitable composltion of the aforementioned type for forming a cured film may comprise graphite and a polyester-polyurethane resin binder. A grade of commercially-available graphite found particularly ~ suitable for making these coating compositions is designated "Micro 750"
sold by Asbury Graphite Mills, Inc., Asbury, New Jersey. Resistive films which form a portion of tlle first resistance means may be formed by spraying onto the electrode surface a two-package coating system that cures under ambient conditions to form a solvent-resistant film. One component of the two-package system may comprise a polyester polyol mixed with graphite, with the second component comprising an isocyanate-containing compound reactable with the polyester polyol to form a cured polyurethane resin-graphite matrix that provides a film of suitable resistivity. Other vola-tile organic solvents may be present in the sprayable composition with one or the other components to reduce viscosity for ease of application.
The aforementioned two-package coating system may be spray applied to an electrode substrate to form a suitable resistive coating of uniform thickness.
Another suitable class of materials for forming the resistive film or layer that makes up electrode wall 41 are the non-conductive thermoplastic polyester compositions sold by General Electric under the trademark Valox~, which compositions may be doped with graphite to achieve a material of suitable resistivity.
Tr 4rJ e J`1 c~ ~ k i3~

The aforementioned resistive materials may be cast or molded into layers or slabs of resistive elements making up a large portion of the bulk of the electrode substrate and wall. Tilus, a layer or slab of the resistive material may comprise a large portion or substantially all of the first reslstance means.
~ purpose of the resistive material that makes ~all 41, ~hether in the form oE a film, a layer or a slab, i5 to provide at surface 42 of '~1 electrode wall ~ a material having sufficient conductivity for the es-tablishment of an induction-charging electric field, while at the same time having a surface resistivity that tends to retard transfer of large amounts of current to potentially arc-producing sites on the electrode so as to suppress arcing from the electrode charged to a high DC electri-cal potential to some electrical ground point. The presence of a resistive material providing electrode surface 42 also tends to suppress corona for-mation that may occur at surEace discontinuities formed by nicks or scratches that accumulate on electrode surfaces during the life of the spray device.
Such arc-producing points may also be established by bits of dust or con-ductive paint particles which may temporarily collect on the electrode surface during painting operations. Moreover, the existence of a resistive surface on the exposed electrode charging surface reduces significantly the intensity of electrical shock to equipment operators who might come in contact with the electrode.
It is understood that resistive films that are quite thin provide corona suppression and anti-arcing capabillties as well as thicker films, provided that the total impedance, measured between any point on the film surface and the point of application of the high potential at terminal con-necting means 15, is above a tbreshold value. The value of threshold . .

impedance is dependent upon many parameters, such as maximum applied po-tential delivered by the power supply, total resistance within the circuit and the charge-storing capacity oE the circuit. Thus, a very thin resistive film on electrode substrate 40 may be suitable as one element in the high potential discharge control circuit provided that additional resistance elements are inserted in series between the resistive film and the high potential source. This additional resistance means is preferably located physically adjacent, and in close physical proximity to, the resistive film and thus may be positioned within electrode substrate 40. This resistance means may comprise one or more discrete resistors 44 embedded within elec-trode substrate 40 fabricated of an electrically non-conductive material, as indicated in FIG. 6. Resistor 44 may be a commercially available carbon-filled composition or wire-wound element. A preferred arrangement of FIG. 6 utilizes two high-voltage stable miniature resistors 44 commercially desig-I5 ~ nated as "Victoreen Minimox" resistors sold by Victoreen Instrument Div.,Cleveland, Ohio. Typlcal values for these and other types of discrete re-sistors may range from about 10 megohms to about 50 gigohms. An electrical connection to discrete resistors 44 is made at a Fontact point 45 located near a rear~iard, or upstream, po~^tion of resistive wall 41 adjacent sub-strate 40, about halfway between the upper and lower edges of electrode 39.
The other end of resistor 44, or a series of resistors 44, is connected to conductor 43.
The additional resistance of the first resistance means may com-prise a resistive material of the same or similar type used to make the resistive film. ~hus, a significant portion or all of electrode substrate 40 may comprise a slab of resistive material formed from a synthetic resin-graphite mixture. AS shown in FIG. 7, slab 46 may be supported by a portion ¦ r c~ , r /~

.

35~

of substrate 40 wh ch is fabricated of a dielectric material. When a significallt portlon of electrode substrate 40 and electrode wall 41 form a continuous slab 46 of resistive material as depicted in FIG. 7, the total resistance of this Eirst resistance means should be oE a value in the range from about 10 megohms to about 50,000 megohms.
The first reslstance means when in the form of a continuous slab 46 is electrically connected to terminal 15 of the high voltage connecting means by a conductor 43 that is connected to a contact point 47 on slab 46 near a rearward portion of the slab at a location with respect to wall 41 as described for the discrete resistor connection, above.
A purpose of the first resistance means is to inhibit or retard transfer of arc-forming amounts of current from other portions of the electrode circuitry, or cable or power supply circuitry, to potentially arc-producing sites Oll the electrode so that arcing from the electrode to an object at a lower potential is substantially suppressed~ This arc suppressing capability is related to, or dependent upon, several factors, such as electrode configuration, charging potential applied to the elec-trode, resistance of the first resistance means and the charge-storing capacity of the electrode circuitry and the capacqtance of the cable and power supply. The precise resistance value selected for the~first re- --sistance means may thus depend upon several factors. A criterion or test for selecting a proper resistance may be based upon whether, in actual practice under spraying conditions, arcs or sparks are generated from the electrode surface of sufficient intensity to ignite a concentration of flammable solvent vapors of a paint that is sprayed. Hence, a choice of resistance for a particular electrode system, which does not produce elec-trical discharges of sufficient intensity to ignite the most flammable vapor-air mixture conceivable, i9 generally a suitable choice of resistance for practically any spraying operation utilizing flammable paints. A
typically suitable Elammable solvent vapor-air mixture against which arc suppression may be tested is a concentration of saturated toluene-in-air S mixture at about 62F. Another rather sensitive test concentration of flammable vapor-in-air mixture is provided by a concentration of a saturated xylene-in-air mixture at about 115F. These test concentrations are intended to be exemplary of flammable mixtures for testing arc sup-pressing capability of the high potential discharge control circuit of the invention. More sensitive or more highly flammable mixtures may also be used to determine an appropriate resistance value for the first re-sistance means, since any resistance value from about 10 megohms to about 50 gigohms may be used as the first resistance means. In this regard it should be mentioned that the total resistance of the series resistance between the high potential output terminal 11 of the power supply and electrode wall 41 may comprise the first resistance means.
A suitable arc suppressing circuit will have a discrete re-sistor 44, if used in the circuit, in close~physical proximity to elec-trode wall 41. It is desirable that any capacitance in the connection or conducting means between the resistance means and electrode wall 41 be substantially less significant than the capacitance of the electrode circuitry. Hence, the connecting means from resistor 44 to wall 41 of the embodiment of FIG, 6 should be a relatively short conductor so that resistor 44 is in close physical proximity to electrode wall ~1. As depicted in FIG. 7, electrode wall 41 is directly adjacent to, and thus in close physical proximity with, resistive material slab 46.

S35~

It has been found that the portions oE the electrode most susceptible to arcing are the portions Eorming the downstream or Eorward ends ~l8. It is preEerred, therefore, that the dielectric substrate material which Eorms electrode substrate 40 encase the forward edges of electrode wàl] 41, as indicated in FIGS. 6 and 7.
The dielectric material may thus form a bead ~ that runs along the circumference oE electrode wall 41.
The precise resistance values of the first resistance means may be selected from a range from about 10 megohms to about 50 gigohms, with the exact choice being determined by the aforementioned parameters and criteria. A set of convenient measuring points for a particular re-sistance element incorporated in the electrode circuitry consists of a first point at high potential connecting means terminal 15 and a second point on electrode surface 42 near forward end 48 most remote from termi-nal 15. The resistance between these two points may be considered the "working" resistan e of the first resistance means. Values of this work-ing resistance may be selected from resistances in the aforementioned range. A value in a range of about 0.1 to about 5 gigohms is a typical choice for the first resistance means.
The hlgh voltage discharge control circuit may comprise second resistance means in a series circuit comprising the high voltage potential supplied by power supply 10 and the first resistance means. This second resistance means comprises a current limiting resistor 49 connected be-tween high voltage input terminal 15 on adapter 9 and the high voltage cable connecting terminal 14 of power supply 10. Typically, resistor 4S
may be physically located in high voltage cable 13 as depicted in FIG. 1.
Or, resistor 49 could be contained within the spray gun in its handle 20 .

~1~5~351~

or barrel 21; a location o.E resistor 49 in spray gun barrel 21 is pre-Eerred for systems utilizing portable, barrel-mounted power supplies as disclosed in a~orementioned reEerences.
In systems utilizing a power supply located remotely from the induction-charging adapter, as shown in FIG. 1, a conventionally-available shielded cable 13 rated to carry voltages of about 25,000 volts DC may be utilized to connect power supply 10 to the induction-charging electrodes of adapter 9. Cable 13 is usually wrapped about air feed hose 19 and thus where cable 13 includes resistor 49, the resistor may be physically at-tached to hose 19 by tape, heat shrink tubing, or other means at a location near air hose connecting means 25. Typically, resistor 49 may have a value in a range from about lO0 megohms to about 50 gigohms, depending on the choice of resistance values for other resistor elements of the circuit.
During operation of the electrostatic spray system, considerable amounts of electric charge are stored in the portion of cable 13 leading to the induction-charging electrodes and in the induction-charging electrode circuit. In the event cable 13 is disconnected from power supply terminal 14 while the spray device is in operation, this stored charge may discharge to an electrlcal ground point with the likelihood of electric sparks being gen-erated. Also, curre~nt discharge and sparking may result~upon accidental severance of the cable by heavy equipment used in many industrial spraying environments. The flow of substantial amounts of stored charge through cable 13 to some ground point can be retarded by the presence of resistor 49 in series with high voltage cabl.e 13.
The high voltage di.scharge control circuit may comprise third resistance means in a series circuit comprising the power supply and the ~s~s~

first resistance means. This third resistance means may comprise a current limiting resistor 50 connected between the high voltage output appearing at circuit point 11 within power supply 10 and high voltage cable connecting terminal 1~. Typically, the value of resistance for res:istor 50 may be in a range from about 0.1 to about 50 gigohms, the precise value depending upon the choice of resistance values oE other circuit resistors. In con-ventional spraying operations, considerable amounts of electrlc charge may be stored in the power supplies utilized. The accidental disconnection of cable 13 from power supply 10, while the power supply is in its oper-ating mode, may result in sudden discharge or arcing of electrical energy to a ground point. Current limiting resistor 50 serves to retard large surges of current stored in the power supply and thus minimizes arcing or sparking tendencies.
The high voltage discharge control circuit may comprise fourth resistance means between the high voltage output of the power supply and an electrical ground point. For example, current limiting resistor 52 may be connected from power suppIy high voltage output circuit point 11 to electrical ground connection 12. One purpose of resistor 52 is to provide a shunt path for discharging~stored energy in the power supply to a ground point. Hence, in the~event cable 13 becomes disconnected during a spraying operation or when the power supply ls routinely turned off, stored energy may be discharged safely ~ithout arcine or sparking occurrences.
Another purpose of resistor 52 is disclosed in aforementioned U. S. Patent No. 4,073,002 in that shunt resistor 52 may cooperate with series resistor 50 to pro~ide automatic regulation of voltage applied to the electrodes by a "ballast" effect. For example, during periods of low ., , ~ ~ . .

-~f~3~3 current drain, as when voltage is applied to the electrodes ~ithout simultaneous discharge oE coating material. ~rom the spray no~zle, the voltage at the power supply Ol1tpUt can increase to undesirable levels in the absence o:E a shunt or bleed reslstor as a constant load across the output. Since the current and voltage characteristics of the shunt resistor remain substantially constant during operation of the gun, .variations in the voltage imposed at the inducti.on-charging electrode effect relatively less change in the absolute load at the power supply output. Where the series resistance is large in comparison to the load resistance, the variations in the spacial impedance between the electrode and gro~1nded nozzle elements which produces changes in voltages at series resistor 50 are rendered relat:ively small. In practice, it has been found that the voltage regulation function can be achieved with a series control resistor 50 and a shunt resistor 52 of approxlmately equal ohmic value, although better regulation is provided where series resistor 50 is on the order of ten times the ohmic value of shunt resistor 52.
Typically, shunt resistor 52 will have a resistance value in a range from about 0.l to about l0 gigohms.
The high voltage discharge control circuit may cont~in fifth resistance means comprising a current limiting resistor 53 connected be-tween high voltage input terminal 15 on adapter 1l and an electrical ground point, as depicted in FIGS. l and 6. Resistor 53 is connected in co~non electrical connection to termlnal lS and connecting point 54 of the first resistance means resistors 44. The gro~und side oE resistor 53 may be con-nected in common to ground connection point 55 which is also connected to ground shields 56. ~ach of ground shields 56 comprises a conductive foil attached to adapter housing exterior wall 37. The structure and functions 53~3 of ground shields 56 are set forth more fully in U.S. Patent No.
4,009~829 to J. E. Sickles.
A purpose of resistor 53 i9 to provide a shunt path to electrical ground for charge s~ored in the induction-charging circuit ant in portions of cable 13~ Upon interruption of the high voltage applied to the electrodes, either by an operator purposefully turning off the power supply, or by one of ~he aforementioned accidental dis-connections of ~he power supply or cable 13, stored electrical energy may be safely discharged to ground without sparking or arcing from these circult elements. Shunt resistor 53 is preferably located as close as possible to the induction-charging electrodes. For example, the resisto~ may be mounted on adapter housing 9, as indicated above.
Also, resistor 53 may be mounted on the spray gun barrel 21 or upon paint feed hose 18. Resistor 53 is shown as attached to paint feed hose 18 in Figure 1 for purposes of illustration only, and i8 not intended to be a par~icularly preferred position for resistor 53.
Typically, resistor 53 has a resistance ~alue in a range from about 0.5 to about 10 gigohms.
A particularly preferred ~igh elec~ric poten~ial discharge control circuit for an induction-~harglng spraying sy~tem of the type described includes a pair of induction-charging electrodes each having a resistive material wall 41 about 50 mils in ~hickness that provides a fiurface for establishing an induction-charging electrosta~i~ field.
A preferred discharge con~rol circuit lncludes in series with each lnduction-charging electrode wall 41 ~wo discrete resis~ors each having a resistance value of about lO0 megohms mounted in the elec-trode sub6trate 40 which ls fabricated of a dielectric ~aterial, a resistor of about 1 gigohm included ~:~L2S3S~

in the high voltage cable, and a resistor of about 1 gigohm included in the po~er supply at its high voltage output. The total resistance of a preferred discharge circuit is in a range from about 2 gigohms to about 3 gigohms; a total resistance value of 2.5 gigohms is parti-cularly preferred, but in any case the total series resistance of the circuit between the power supply and the electrode may be in the range from about 1 to sbout 50 gigohms.
Within power supply 10, converter 60 provides a high potential DC output at terminal 11 from a 115 volt AC source. The high potential required to be provided by con~erter 60 should be adjustably between 5,00C and 25,000 ~olts DC. A description of an AC to DC converter suitable for an induction-charging system of the invention is found in the aforementioned U~S. Patent No. 4,073,002 to J. E. Sickles et al, The interrelationship of capacitive and resistive elements of the high potential discharge control circuit which provide arc suppre~sion for an induction-charging spraying system ~ay be found in the following example showing spproxi~ate values oP resistlve-capacitive elemen~s of a typical control circuit for an induction-charging system w~th two electrodes as depicted in Figure 1. A
typical arc-producing situation may occur when the charging surface near ~he forward end 48 of one of the two resistive-material coated elec~rodes i8 brought to a position in close proximity with a ground point. The in~ensity of ~he dlscharge is dependen~ upon the a~ount of oharge stored in the circui~ available for discharge from the resistive material coated electrode surface. Within the illustrated circuit there is a capac~tance, Cl, of about 10 pF in the arcing electrode circuitry as well , , ~. , . ~
.. .,. ; :

~Z~3~

as in the non-arcing electrode circuitry. In the high voltage cable 13 being approximately twenty feet in :Length (15 pF/ft.) there is a capaci-tance, C2, of about 300 pF. For this calculation, contributions from stray capacitance, such as from the power supply, may be neglected since this capacitance is relatively insignificant in comparison with the total quantiEied capacitance, CT = 2Cl~C2. For a charging system operating at 25 kilovolts DC, there may be a total stored energy ET, within the system of ~ ET = 1/2 CTV2 = 1/2 (320 x 10-12F) (25 x 103v)2 = 100 mi:llijoules An electric discharge in the form of an arc of this energy, ET, may be suppressed by providing sufficient series resistance between the electrode resistive surface from which the discharge may occur and the charge-storing elements of the circuit from which the currents must flow to provide the arc energy. The arc energy is thus comprised of current, IA, supplied from the arcing electrode capacitance and current, Ig, from the cable capacitance, and the current, Ic~ supplied from ~he non-arcing electrode circuitry. Current contributions from stray capacitance have been dis-regarded inasmuch as these currents are relatively insignificant. In the arcing electrode circuitry of an electrode having a wall of resistive material, there is assumed to be an average effective res stance of about 0.5 gigohm between the arcing point near electrode Eorward end ~ and a connection point to an additional resistance element that may be a discrete resistor having a resistance value of about 0.5 gigohm, all of which com- -prises the first resistance means, Rl, series connected between the resistive wall arcing point and the high potential connecting terminal 15.

A series resistance, R2, located in high voltage cable :L3 has a resistance value of about 1 gigohm. Assuming Eor purposes of calculating a resistive-capacitive time constant, rl, for thc arcing electrode d:ischarge circuit . ~ that the resistive element is--Y~R~

~1 ~ 1/2 RlCl = (0.5 x 109 ohms) (10 x 10-12 F) = 0.005 sec.

An average current, I~, representing the dissipation of charge, QA, through the resistive electrode during time constant, ~ , may be calculated as IA ~ QA = ClV = 25 x 10 volts r~l 1/2 RlCl 0.5 x 10~ ohms = 50 x 10-6 amps r rne capacitive time constant, 2~ for dissipation of charge stored in the cable through the cable resistance, R2, and through the arcing electrode resistance, Rl, to the arc point would be ~ ~ (Rl -~ R2)C2 = (2 x 109 ohms) (300 x 10 12F~

15= 0.6 sec.

The charge stored in the cable portion of the circuit would be Q2 ~ C2V = (300 x 10-12F) ~25 x 103v) = 7.5 x 10 6 Coul.

This charge, Q2, drains from the cable capacitance during time constant, 20~2, so that an average current, Ig, may be expressed as I ~ Q2 = 7.5 x 10 Coul~ = 12.5 x 10-6 amps r2 0.6 sec.

~zs~s~

Assuming for purposes of calculating a resistive-capacitive time co;stant, r3, for the circuit comprising the non-arcing electrode, that an effective resistance is about 3/2 Rl ~3 ~ 3/2 R1Cl = (1.5 x 109 ohms) (10 x 10 12F) = 0.015 sec.
so that where Q3 C1V = (10 x 10 12F) (25 x 103v) = 0.25 x 10-6 Coul.
there is provided an average dissipation current, Ic, IC ~ Q3 = 0-25 x 10 6 Coul.
r3 0.015 sec.
= 16.7 x 10-6 amps The effective current, Ieff, available for discharge at the arcing electrode surface as provided by the sum of currents from the capacitive portion of the non-arcing electrode circuitry, IA, from the cable capacitance, IB, and from the non-arcing electrode circuit, Ic, would be Ieff = IA ~~ IB ~ IC
= 50 x 10-6 amp + 12~5 x 10-6 amp ~ 16.7 x 10-6 amp = 79.2 x 10-6 amp Assuming an average electrode~potential of 20 kilovolts during the discharge period, r, in the arcing electrode discharge circuit, the resultant energy for forming a suppressed arc would be EarC = 1/2QV = 1/2 Ieff ~rlV
= 1/2 (79.~ x 10-6 amp) (0.005 sec.) (20 x 103v) = 3.96 millijoules .

~25~S8 The calculated energy of the suppressed electric discharge is thus sub-stantially less than the 15 millijoules threshold energy required to ignite a flammable test concentration oE a saturated mixture of toluene-in-air at about 62F. ~lence, an arc discharge of the calculated energy would not orclinarily present a ha~ard to use of an induction-charging system having the exemplified high potential discharge control circuit in the spraying of toluene-containing paint compositions.
The Eollowing Example describes a formulation of a two-package composition th~t may be spray-applied onto an electrode substrate to form a resistive film having a point-to-point resistivity measured at the film surface, which film is suitable for use in the control circuit of the in-vention. The term "point-to-point resistivity" relates to a measurement of the resistivity of a cured film between two pointed probes placed on the resistive film surface, which probes are spaced apart a distance of two inches. A point to point resistivity measuring device comprises a weighted cylinder fabricated of dielectric material having three metallic probes projecting outwardly from the bottom surface of the cylinder. Each of the three probes is spaced a distance of two inches from the other two probes so that the three probes form an equilateral triangle on the bottom surface of the cylinder. Each probe is connected to a separate wire, the three wires providing connecting means for a resistivity measuring instrument.
~esistivity measurements are taken by attaching a Sperry "Meg-O-Vo]t" meter in its megohm resistance-measuring mode across:any two of the probes. The three-probe arrangement on the bottom surface of the cylinder provides a stable support for the cylinder and allows application of uniform force to each probe as the cylinder rests on the resistive film surface to be measured.
In determining the resistivity oE a resistive-material film, measurements d ~ rl ~ r l~

.

3~8 are usually made at several locations on the film surface. Also, at each measuring location, resistivity values are obtained from each pair of probe wires. ~`he various meas-Irements are then usually averaged to give a characteristic "surface resistivity" of the film, usually ex-pressed in megohms/inch.
~mounts of the components set forth in the following Example are expressed in parts by weight unless otherwise specified.

EXA~IPLE

A polyester-polyurethane clear coating formulation having a graphite conductive component is prepared in a two-package sprayable system. One package comprises a polyester-polyol derived from components in the following proportions:

P ts by Weight hexahydrophthalic acid anhydride173 adipic acid 138 neopentyl glycol 136 trimethylolpropane 122 diethanolamine 10 n-butyl acetate 177 toluene 44 To a reaction vessel equipped with heating and agitating means, a fractional distillation column and means for maintaining a nitrogen blanket over a reaction mixture, there are added the hexahydrophthalic anhydride and neopentyl glycol, which are mixed and heated to 66C.

Thereafter the trimethylolpropane is added and the mixture is heated to 66C. The adipic acid is then charged to the reaction mixture, which is heated to 182C. and held for one-halE hour while ~ater is distilled off.
The mixture :is thereafter heated to 215C. ~ s-~mple taken after 7 1/2 hours is identified as a saturated polyester polyol having an acid number of 14.9 and a hydroxyl number of 143. The reaction vessel is now set for azeotropic reflux. The toluene is added carefully to cool the mixture to 150C., after which time the diethanolamine is added. The mixture is maintained as an a~eotropic boiling mixture at 146C. until an acid value of less than 5 is obtained. The n-butyl acetate is added to obtain a fluid mixture.
To 28 parts of the polyester polyol is mixed 10.4 parts of B "Micro 750" graphite (~sbury Graphite Mills, Inc., Asbury, N. J.) and 181.7 parts of a solvent consisti~g of a mixture of urethane grade butyl acetate, Cellosolve acetate and methylethylketone~ in a ratio of 36 to 56 to 8.
The second package of the two-package system is prepared by mixing 268 parts of an NC0-containing component, commercially identified as Spenlite P25-60CX (an NC0-terminated adduct of trime~hylolpropane/
neopentyl glycol/isophorone diisocyanate dissolved in a xylene/Cellosolve acetate solverlt mixture; available from Spencer-Kellogg Co~) with 32.4 parts of the solvent mixture mixed wlth the polyester polyol of the first package.
A sprayable mixture having a sprayable pot life of about 8 hours is prepared by mixing together the contents of the first package with 42 parts of the second package. The mixture is spray applied to an e]ectrode substrate fabricated of a non-conductive epoxy plastic to form a cured film on the substrate of about one-to-two mils in thickness. The electrode is suitable for use in an induction charging spraying system.
7~r c~d e~ k .

35~3 Other suitable mixtures utilizing the described binder may be prepared having var:ious ratios o:E amounts of graphite to resin binder, each mixture prov.ding a cured EilTn having a characteristic point-to-point surface resistivity. In order to determine standard resistivity values for a .Eilm formed from these various compositions, each of the compositions is sprayed upon a 4" x 12" glass plate using a Binks Model 62 non-e].ectrostatic siphon air-atomizing spray gun. The co~tings are allowed to cure on the plates at room temperature for 24 hours. Then the coated plates are heated in an oven at 250F. for 40 minutes to drive off volatile 1~ solvent. The cured dried films have thicknesses of about one mil. A
series oE point-to-point resistivity measurements is taken with surface resistivity measuring device described above~ The average measured values correlating with the various compositions are listed in Table I. As a standard of comparison, it should be mentioned that Velostat~ conductive plastic compositions t3M ~o.) have measured point-to-point surface resis-tivities in the range of 300 to 500 ohms/inch.

ABLE I

Graphite-to-Resin Binder Graphite* Point-to-Point Surface Weight Ratio (x:l) tgrade) Resistivity tmegohm/inch) ~ x 1.00 Mlcro 750 : 0.002 0.67 Micro 750 0.003 0.43 Micro 750 0.034 0.25 Micro 970 0.310 0.25 Micro 750 0.75 0.20 Micro 970 1.95 0.25 Micro 450 35 0.20 . Micro 750 . 45 0.20 Micro 450 76 0.15 Micro 750 100 0.15 Micro 970 500 * Asbury Graphite Mills, Asbury, N.J.

. ~ . ~ ~, ..
: . : .

1~53S8 It shoulcl be mentioned that in addition to the arc suppression capability of the exempliEied circuit, a suitable high potential dis-charge circuit may contain circuit elements selected according to the aEorementioned criteria such that no electrical discharges occur of any measurable energies. Also, discharge control circuits may be designed within the ambit of the invention that provide slight corona dissipation of electrical energy rather than discrete arc discharges. Additional resistance means may also be incorporated into the discharge control circuit to provide flexibility in the spraying operation. For example, additional induction-charging electrodes may be utilized having first resistance means of the aforementioned type, as depicted in phantom as element Rl'in FIG. 2. Also, additional current-limited high voltage outputs may be provided at cable connection 14' through current-limiting resistor 50', as depicted in phantom in FIG. 1.
~hose skilled in the art will appreciate that the invention can be embodied in forms other than as herein disclosed for purposes of illustration.

, .
: , , , ~ ;
, ,. , ~ .

Claims (34)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A high electric potential discharge control circuit for an induction-charging electrostatic spraying system of a type having liquid particle spray-forming means for discharging a spray stream of liquid particles along the axis of the spray-forming means, induction-charging means disposed in operable association with the spray-forming means to establish a charging zone in which charge is induced on liquid particles as the particles are formed by the spray-forming means, and high voltage connecting means on the induction-charging means for re-ceiving a high voltage potential from a power supply for application to the induction-charging means, said control circuit comprising:
at least one induction-charging electrode disposed out-wardly of the spray-forming means, said electrode comprising a substrate having a wall facing the spray discharge axis of the spray-forming means, said wall providing a surface from which an induction-charging electro-static field is established when an electric potential is applied to said electrode; and first resistance means connected between the high voltage potential connecting means and said electrode wall surface, at least a portion of said first resistance means comprising said electrode wall and providing the surface from which the induction-charging field may be established, said first resistance means having sufficient resistance to retard transport of charge across the electrode surface at the voltages applied to said electrode required to establish the induction-charging field, whereby the charge storable within the circuit that is dischargeable at the electrode surface is prevented from discharging at an energy level at or above the threshold energy required to ignite a flammable gas-and-air mixture.

2. The control circuit of Claim 1, wherein said resistive material comprises a matrix of a substantially non-electrically conductive resin binder and graphite.

3. The control circuit of Claim 2, wherein said resin binder is derived from a reactive mixture of a polyester-polyol and an NCO-containing compound.

4. The control circuit of Claim 2, wherein said resistive material matrix has a graphite-to-binder weight ratio in a range from about 0.15:1 to about 1.0:1.

5. The control circuit of Claim 2, wherein said resistive material is characterized in having a point-to-point surface resistivity in a range from about 0.002 megohm/inch to about 500 megohms/inch.

6. The control circuit of Claim 1, wherein said first resistance means further comprises at least one discrete resistor of the wire-wound or composition type connected between said wall of resistive material and said high voltage connecting means, said resistor having a resistance value in a range from about 10 megohms to about 50 gigohms.

7. The control circuit of Claim 1, wherein said first resistance means comprises electrically resistive material forming at least a portion of said electrode wall and forming at least an adjacent portion of said electrode substrate, said resistive material providing a resistance path from said high voltage connecting means to the surface of said electrode wall said resistance path having an average value in a range from about 10 megohms to about 50 gigohms.

8. The control circuit of Claim 1, further comprising a power supply having means for connecting high voltage carrying means at the high voltage output of the power supply, and further comprising second resistance means connected between said high voltage connecting means of the induction-charging means and said high voltage carrying connecting means on said power supply.

9. The control circuit of Claim 8, wherein said second resistance means has an ohmic value in a range from about 100 megohms to about 50 gigohms.

10. The control circuit of Claim 8, further comprising a gun-like housing for supporting said spray-forming means, said second resistance means mounted within said housing.

11. The control circuit of Claim 8, further comprising high voltage carrying means connectable between said power supply connecting means and said induction-charging means high voltage connecting means, said high voltage carrying means comprising a shielded cable, said cable including said second resistance means within said cable in series with the high po-tential source and said first resistance means.

12. The control circuit of Claim 8, further comprising third resistance means connected between the high potential source of said power supply and said power supply high voltage carrying connecting means.

13. The control circuit of Claim 12, wherein said third resistance means has an ohmic value in a range from about 0.1 gigohm to about 50 gigohms.

14. The control circuit of Claim 12, further comprising fourth resistance means connected between the high potential source of said power supply and electrical ground potential connecting means.

15. The control circuit of Claim 14, wherein said fourth resis-tance means has an ohmic value in a range from about 0.1 gigohm to about 10 gigohms.

16. The control circuit of Claim 1, further comprising fifth resistance means connected between said high voltage connecting means of the induction charging means and electrical ground connecting means.

17. The control circuit of Claim 16, wherein said fifth resistance means has an ohmic value in a range from about 0.5 gigohm to about 10 gigohms.

18. An induction-charging adapter for mounting on a spray-forming device capable of discharging a stream of spray particles along the axis of the spray device, said adapter comprising:
(a) housing means comprised of dielectric material, said housing means having an exterior wall and having an interior wall;
(b) mounting means on the interior wall of said housing means for detachably mounting said housing means onto a spray device, so that said interior wall faces the axis of a spray device when the housing means is mounted on a spray device;
(c) induction-charging means including at least one induction-charging electrode attached to an interior wall of said housing means, said electrode comprising a substrate having a wall that faces the axis of a spray device when the housing means is mounted on a spray device, said electrode wall providing a surface from which an induction-charging electrostatic field is established when a high voltage DC electric potential is applied to said electrode;
(d) high voltage connecting means connected to said induction-charging electrode; and (e) first resistance means connected between said high voltage connecting means and said electrode wall surface, at least a portion of said first resistance means comprising said electrode wall and providing the surface from which the induction-charging electric field may be established, said first resistance means having sufficient resistance to retard transport of charge across the electrode surface so as to suppress electrical discharges from the electrode surface at the voltages applied to said electrode required to establish the induction-charging field, whereby the charge storable within the circuit that is dischargeable at the electrode surface is prevented from dis-charging at an energy level at or above the threshold energy required to ignite a flammable gas-and-air mixture.

19. The induction-charging adapter of Claim 18, wherein said resistive material comprises a matrix of a substantially non-electrically conductive resin binder and graphite.

20. The induction-charging adapter of Claim 19, wherein said resin binder is derived from a reactive mixture of a polyester-polyol and an NCO-containing compound.

21. The induction-charging adapter of Claim 19, wherein said resistive material matrix has a graphite-to-binder weight ratio in a range from about 0.15:1 to about 1.0:1.

22. The induction-charging adapter of Claim 19, wherein said resistive material is characterized in having a point-to-point surface resistivity in a range from about 0.002 megohm/inch to about 500 megohms/
inch.

23. The induction-charging adapter of Claim 18, wherein said first resistance means further comprises at least one discrete resistor of the wire-wound or composition type connected between said wall of resistive material and said high voltage connecting means, said resistor having a resistance value in a range from about 10 megohms to about 50 gigohms.

24. The induction-charging adapter of Claim 18, wherein said first resistance means comprises electrically resistive material forming at least a portion of said electrode wall and forming at least a portion of the adjacent electrode substrate, said resistive material providing a resistance path from said high voltage connecting means to the surface of said electrode, said resistance path having an average value in a range from about 10 megohms to about 50 gigohms.

25. The induction-charging adapter of Claim 18,further comprising additional resistance means connected between said high voltage connecting means of the induction-charging means and electrical ground connecting means.

26. The induction-charging adapter of Claim 25, wherein said additional resistance means has a resistance value in a range from about 0.5 gigohm to about 10 gigohms.

27. A high electric potential discharge control circuit for an induction-charging electrostatic spraying system, the induction-charging electrostatic spraying system comprising:
(a) a spray-forming device comprising a liquid discharge nozzle having a liquid-conveying passageway terminating in a liquid dis-charge port and comprising an atomizing air discharge port concentric with said liquid discharge port, said spray-forming device capable of discharging a spray stream of liquid particles along the axis of said spray-forming device as defined by said liquid-conveying passageway;
(b) a housing fabricated of dielectric material disposed around said spray-forming device, said housing having an exterior wall and an interior wall, said interior wall facing the axis of said spray-forming device;
(c) induction-charging means including at least one induction-charging electrode attached to said interior wall of said housing, said electrode having a wall that faces the axis of the spray-forming device, said electrode wall providing a surface from which an induction-charging electrostatic field is established when a high voltage potential is applied to said electrode;
(d) high voltage connecting means connected to said induction-charging electrode;
(e) power supply means for providing a high voltage potential at an output terminal;
(f) high voltage cable connecting means located on said power supply means;

(g) electrical grounding means located on said power supply means;
(h) a high voltage transmitting cable for connecting to said power supply connecting means and for connecting to said high voltage connecting means of said induction-charging electrode;
said high electric potential discharge control circuit comprising:
(i) first resistance means connected between said high voltage connecting means of the induction-charging electrode and said electrode wall surface, at least a portion of said first resistance means comprising said electrode wall and providing the surface from which the induction-charging electric field is established;
(J) second resistance means connected between said high voltage connecting means of the induction-charging means and said high voltage cable connecting means of the power supply means;
(k) third resistance means connected between the high voltage potential output terminal of said power supply means and said high voltage cable connecting means; and (1) fourth resistance means connected between the high voltage potential output terminal of said power supply means and said electrical ground connecting means on said power supply means;

wherein the total resistance of said first, second and third resistance means has a value in a range from about 1 gigohm to about 50 gigohms.

28. The discharge control circuit of Claim 27, wherein said first resistance means comprises in series:

(a) a layer of electrically resistive material forming a portion of said electrode wall, said layer having a point-to-point sur-face resistivity in a range from about 0.002 megohm/inch to about 500 megohms/inch, and (b) at least one discrete resistor of the wire-wound or composition type.

29. The discharge control circuit of Claim 28, wherein said discrete resistor has a resistance value in a range from about 10 megohms to about 50 gigohms.

30. The discharge control circuit of Claim 27, wherein said second resistance means has a resistance value in a range from about 100 megohms to about 50 gigohms.

31. The discharge control circuit of Claim 27, wherein said third resistance means has a resistance value in a range from about 0.1 gigohm to about 50 gigohms.

32. The discharge control circuit of Claim 27, wherein said fourth resistance means has a resistance value in a range from about 0.1 gigohm to about 10 gigohms.

33. The discharge control circuit of Claim 27, further com-prising electrical ground connecting means located on said housing and further comprising fifth resistance means connected between said high voltage connecting means of the induction-charging electrode and said electrical ground connecting means.

34. The discharge control circuit of Claim 33, wherein said fifth resistance means has a resistance value in a range from about 0.5 gigohm to about 10 gigohms.