Note: Descriptions are shown in the official language in which they were submitted.
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
--4--
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
--10--
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.
--11--
~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,
-12-
, .
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.
-13-
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
- 14 -
3S8
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
-16-
., ~ ,
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
-17-
~ ~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
18-
~, . . .
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
--19--
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
-20-
. .
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
5358
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.
-23-
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
-24-
.
~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
-26-
., , ~ ~ . .
-
~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
- 28 -
~:~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
- 29 -
, , ~. , . ~
.. .,. ; :
~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.
-30-
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.
-31-
~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
.
-32-
~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~
-33-
.
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
53S8
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--
.
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.
-36-
. ~ . ~ ~, ..
: . : .
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.
-37-
, .
: , , , ~ ;
, ,. , ~ .
