Spray application of coating compositions utilizing induction and corona charging means

Disclosed is a spray gun having a gas nozzle and a fluid nozzle, each of the nozzles being in cooperative spatial relationship with the other to cause a fluid stream issuing from the fluid nozzle to be atomized and sprayed as fluid particles by gas issuing from the gas nozzle. In a preferred embodiment the fluid nozzle orifice has therein an axially disposed rod to increase surface area from which the fluid particles can be found, the rod being a corona discharge electrode of a first polarity. The spray gun additionally has an induction charging electrode of a second polarity opposite the first polarity and disposed adjacent the gas and fluid nozzles, the induction charging electrode defining a charging zone wherein an electrostatic charge is imparted to atomized electrically-chargeable fluid particles. Relatedly disclosed is a method of applying a liquid coating composition having an electrical conductivity of less than about 0.06 .mu.mho/cm to a workpiece through utilization of both corona and induction charging in a spray gun.

BACKGROUND OF THE INVENTION 
Gas atomization of a fluid such as a paint composition to break up the 
fluid into particles for subsequent application to a workpiece to be 
coated is a technique well recognized in the art. Spray apparatus 
generally employed is a spray gun to which is supplied a fluid stream and 
a gas stream. The gas is most usually air, but can, of course, be chosen 
from other gases as required. The fluid stream issues from the spray gun 
via a fluid nozzle while the gas stream issues via a gas nozzle, with the 
gas stream intersecting or otherwise disturbing the fluid stream to 
provide atomized sprayed fluid particles. 
To improve coating characteristics of the fluid particles issuing from the 
spray gun, various techniques have been developed to electrostatically 
impart an electrical charge to these particles prior to their arrival on 
the workpiece to be coated. One such technique is induction charging. 
Briefly, and in relation to the instant invention, a method of inducing an 
electrical charge on sprayed fluid particles involves the placement of an 
induction charging electrode means adjacent the fluid and gas nozzles. 
This electrode means induces an electrical charge on the atomized fluid 
particles, which charge is opposite to the electrode's charge, as the 
particles pass within a charging zone created between the electrode means 
and the particle stream. The electrode means itself can be an integral 
fixture of the spray gun, or it can be removably connected to the spray 
gun. An example of the latter electrode means which can be fitted to a 
conventional spray gun is described in U.S. Pat. No. 4,009,829, to James 
E. Sickles, incorporated herein by reference. 
A second technique for imparting an electrostatic charge to fluid particles 
is corona charging. In this technique a needle-like electrode is disposed 
in the stream of fluid prior to atomization of the fluid into particles. 
The electrode discharges an electrical charge which is held by the fluid, 
with the subsequently formed fluid particles thus having a charge of the 
same polarity as that of the corona electrode. Voltage requirements in a 
corona charging system are, however, relatively high, generally 50 to 60 
KV, and therefore create possible safety and energy-consumption 
disadvantages. 
Copending application Ser. No. 076,014, filed on even date herewith and 
entitled "Electrostatic Spray Gun Having Increased Surface Area From Which 
Fluid Particles Can Be Formed," discloses a spray gun having disposed 
within the fluid stream means for increasing surface area from which fluid 
particles can be formed, said means being electrically grounded at least 
during fluid issue from the nozzle. Said spray gun has electrostatic 
charging means comprising an induction charging electrode means disposed 
adjacent the gas and fluid nozzles to create a charging zone wherein an 
electrical charge is induced on formed fluid particles. As related in said 
copending application, the surface area increasing means acts to provide a 
surface area for fluid particles issuing from the fluid nozzle orifice to 
be better exposed to the charging zone, and, because said means is 
grounded, to create a favorable potential gradient between the fluid 
particles and the electrode. Charging of the fluid particles therein 
described is solely accomplished by induction charging. It is known, 
however, that some fluids have a relatively medium-to-low electrical 
conductivity, generally defined as below about 0.06 .mu.mho/cm. It is also 
known that a spray stream contains fluid particles whose sizes cover a 
range from large to small. Further, it has been found that larger 
particles having such medium-to-low electrical conductivity are not as 
well charged with induction charging as are those particles whose 
electrical conductivity exceeds about 0.06 .mu.mho/cm. Smaller particles 
having medium-to-low electrical conductivity are, however, adequately 
charged to high charge-to-mass ratios. Conversely, it has been found that 
said larger particles do obtain adequate charging from a corona discharge 
means, while the smaller particles do not find optimum benefits with 
corona charging. 
SUMMARY OF THE INVENTION 
The subject of the invention disclosed and claimed herein is a spray gun 
having a gas nozzle and a fluid nozzle, each of said nozzles being in 
cooperative spatial relationship with the other to cause a fluid stream 
issuing from the fluid nozzle to be atomized and sprayed as fluid 
particles by gas issuing from the gas nozzle, with said spray gun having 
disposed within the fluid stream means for increasing surface area from 
which said particles can be formed, said means having at least one sharp 
edge and also being a powdered corona discharge electrode of a first 
polarity, and with said spray gun additionally having induction charging 
electrode means of a second polarity opposite the first polarity and 
disposed adjacent the gas and fluid nozzles, said induction charging 
electrode means defining a charging zone wherein an electrostatic charge 
is imparted to atomized electrically chargeable fluid particles. By 
providing the surface area increasing means, forming fluid particles are 
afforded greater exposure to the electrostatic field. 
In a preferred embodiment, the means for increasing surface area comprises 
an axially disposed sharply pointed rod within the orifice of the fluid 
nozzle and protruding forwardly therefrom. Examples of other surface area 
increasing means include one or more tubes, one or more screw-thread rods, 
multiple pointed rods, one or more rods with various geometries such as an 
inverse cone distally and the like, with the proviso that said surface 
area increasing means must have adequately sharp or pointed configurations 
in order to effect corona discharge. The means can be disposed within the 
fluid nozzle orifice, or can be otherwise mounted so long as said means 
resides within the fluid stream. 
The induction charging electrode means can be an integral fixture of the 
spray gun or it can be removably connected to said spray gun. As earlier 
recited, the induction charging electrode means imparts a charge on 
particles which is opposite in polarity to that of said electrode means. 
Conversely, the corona electrode imparts a charge on fluid particles which 
is of the same polarity as the corona electrode. As a result, surprisingly 
significant enhancement of coating deposition efficiency occurs due to 
improved charge distribution on the particles when the induction charging 
electrode means and the corona electrode are of opposite polarity. This 
effect is particularly advantageous where fluid of medium-to-low 
electrical conductivity (from about 0.005 to about 0.06 .mu.mho/cm) is 
being employed since the magnitude of charging obtainable on a fluid 
particle by induction charging is directly related to the particle's 
electrical conductivity and physical size. Hence, while larger fluid 
particles of a medium-to-low conductivity fluid cannot fully benefit from 
induction charging alone, the addition of corona charging results in 
further charging of said fluid to produce a more fully charged fluid 
spray. Further, the total voltage requirement need not exceed that used in 
induction charging alone, said requirement generally being considerably 
lower than that required when only corona charging is employed. 
Relatedly disclosed is a method of applying a sprayable liquid coating 
composition to a workpiece, said composition having an electrical 
conductivity between about 0.005 and about 0.06 .mu.mho/cm, said method 
comprising spraying the composition by employing a spray gun having a gas 
nozzle and a fluid nozzle, each of the nozzles being in cooperative 
spatial relationship with each other to cause a fluid stream of the 
composition issuing from the fluid nozzle to be atomized and sprayed as 
fluid particles by gas issuing from the gas nozzle, (b) electrically 
grounded means for increasing surface area from which fluid particles can 
be formed, such means being disposed within the fluid stream and having at 
least one sharp edge, and (c) induction charging electrode means disposed 
adjacent the gas and fluid nozzles and having a rear edge located rearward 
of a plane which is perpendicular to the axis of liquid flow and which 
passes through the discharge point of the fluid nozzle; the method further 
comprising supplying sufficient voltage to the induction charging 
electrode means to produce a corona discharge at the sharp edge of the 
surface area increasing means, the method producing corona charging and 
induction charging of sprayed fluid particles. 
In a preferred embodiment, the means for increasing surface area comprises 
an axially disposed pointed rod within the orifice of the fluid nozzle of 
the spray gun and protruding forwardly from said fluid nozzle. Examples of 
other surface area increasing means include one or more tubes, one or more 
screw-thread rods, multiple pointed rods, one or more rods with various 
geometries such as an inverse cone distally, and the like, with the 
proviso that said surface area increasing means must have adequately sharp 
or pointed configurations in order to effect corona discharge when liquid 
is being atomized. The means can be disposed within the fluid nozzle 
orifice, or can be otherwise mounted so long as said means resides within 
the fluid stream. 
Although the invention is described and exemplified more fully in the 
following description and accompanying drawings, it is to be understood 
that this description and these drawings are not intended to limit the 
scope of the invention, but rather that the invention shall be defined as 
set forth in the appended claims.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to FIGS. 1 and 2 of the drawings, a conventional hand-held, 
air-operated spray gun 10 is illustrated, said spray gun 10 having a 
handle portion 12, a barrel 14, a fluid nozzle 16 and a gas (air) nozzle 
18, the latter two elements shown in FIG. 2. The spray gun 10 has a 
conventional trigger mechanism 19 which operates valve means 20 comprising 
a needle valve assembly to admit fluid from a supply source (not shown) to 
the spray gun 10. The fluid is fed to the spray gun 10 through a suitable 
connector 22 threadably connectable to a corresponding connector on a 
fluid feed hose (not shown) from the fluid supply. The fluid to be sprayed 
passes through the valve means 20 and flows through a fluid passageway 24 
to the orifice 25 of the fluid nozzle 16. The needle 26 of the needle 
valve assembly moves axially in concert with movement of the trigger 
mechanism 19 to control fluid flow through the fluid nozzle orifice 25. In 
the embodiment shown in FIG. 2, a corona electrode rod 27 extends 
forwardly from the tip of the needle 26 to be axially disposed within the 
fluid nozzle orifice 25 and protrudes forwardly from said fluid nozzle 
orifice 25. 
Air or another suitable gas is applied under pressure to the gas nozzle 18 
by way of an air hose 28 and through suitable passageways in the body of 
the spray gun 10. The gas supply is divided into two separate passageways 
30 and 32, with gas flow being regulated by a manually adjustable control 
valve generally indicated at 34. A second control valve 36 permits 
adjustment of the needle 26 in passageway 24, in a manner as known in the 
art. The gas flow in one of the passageways, for example passageway 30, is 
directed to an annular chamber 38 from which the gas flows forward to a 
second annular chamber 40. The gas nozzle 18 incorporates a plurality of 
orifices such as an annulus 42 surrounding the fluid nozzle orifice 25, 
which serve to direct gas from chamber 40 to shape the flow of fluid from 
the fluid nozzle orifice 25 in known manner. The flow of gas from 
passageway 32 is directed to an annular chamber 46 which is in 
communication with passageways 48 and 50 leading to orifices disposed in 
diametrically opposed ears 52 and 54 of the gas nozzle 18. Gas flowing 
from the orifices in the ears 52 and 54 serve to direct gas toward the 
atomized fluid being discharged from fluid nozzle orifice 25 and thereby 
shape the pattern of the spray. 
In the instant embodiment the fluid nozzle 16 and gas nozzle 18 are 
constructed of a dielectric or electrically non-conductive material. The 
fluid nozzle 16 can be secured in the barrel 14 of the spray gun 10 by any 
suitable means, as by threads 56. Similarly, the gas nozzle 18 is secured 
to the barrel 14 by suitable means such as an annular nut 58 having an 
inner shoulder portion 60 which engages a corresponding shoulder on the 
gas nozzle 18 and which is threaded onto the exterior of the barrel 14 by 
means of threads 62. Fluid being supplied is electrically grounded, 
preferably at its source of supply, in order to insure proper induction 
charging. The fluid passageway 24 has a dielectric wall 44 to prevent 
current flow from the corona electrode rod 27 to the metal body of the 
spray gun barrel 14. 
Mounted on the exterior of the barrel 14 and concentric with the fluid 
nozzle orifice 25 is an induction charging adapter 64 bearing induction 
charging electrode means. U.S. Pat. No. 4,009,829, to James E. Sickles, 
fully describes the adapter 64, and said patent is included herein and 
made a part hereof by reference. As described and exemplified as a 
preferred embodiment in said patent and as illustrated in FIGS. 1 and 3 
hereof, the adapter is essentially a cylindrical housing 66 formed of a 
dielectric material and having a rearward portion 68 adapted to be secured 
to the spray gun and a forwardly extending portion 70 adapted to surround 
the path of the discharged spray material. Diametrically opposed portions 
of the forward part of the dielectric housing 66 are cut away at 72 and 74 
(See FIG. 3), leaving shaped, forwardly extending, opposed lobes 76 and 78 
remaining. The lobes 76 and 78 carry charging electrodes, for which a d.c. 
voltage is applied for inductively charging the spray particles, while the 
cutaway portions 72 and 74 prevent interference by the housing 66 with 
generally fanshaped patterns which may be produced in the spray, and 
assist in the aspiration of ambient air through the housing 66. Again, it 
will be understood that the dielectric housing may be constructed of any 
suitable material capable of withstanding the high voltages used, and in 
particular can be constructed of materials including acetal resins, epoxy 
resins, glass-filled epoxy resins, or the like. The adapter 64 is attached 
to the end of spray gun 10 by means of suitable mounts which are shaped to 
engage the outer surface of the barrel or of the annular nut 58. Although 
the exact shape of the mounts will depend upon the construction of the 
particular barrel to which the adapter is to be connected, the mounts in 
general are formed to secure the adapter in concentric relationship with 
the fluid nozzle orifice 25. Again, reference should be made to U.S. Pat. 
No. 4,009,829 in regard to mounting configurations. 
The electrostatic field by means of which the adapter 64 produces induction 
charging of the atomized fluid particles is generated by means of a pair 
of charging electrodes 96 and 98. These electrodes are mounted to the 
inner surfaces of lobes 76 and 78, respectively, of the adapter and thus 
are positioned on diametrically opposite sides of the fluid and air 
nozzles. The electrodes are spaced from the fluid nozzle and are 
concentric therewith, having curved surfaces which are equidistant from 
the longitudinal axis of the fluid nozzle 16. A high positive or negative 
voltage is supplied to the two opposed electrodes 96 and 98, and this 
voltage produces an electrostatic field between the electrodes, the fluid 
spray discharged from the spray gun, and the protruding rod. This field 
defines a charging zone within the adapter which serves to induce an 
opposite charge on any particulate fluids passing therethrough. The 
voltage can vary over a wide range, but preferably is less than about 30 
kilovolts. The magnitude of the voltage required to achieve optimum 
charging efficiency depends upon the radial distance between the surfaces 
of the electrodes and the axis of the liquid flow, on the longitudinal, or 
axial location of the adapter with respect to a plane perpendicular to the 
axis of the adapter and passing through the discharge point of the fluid 
nozzle, on the rates of air and liquid flow from the nozzle, and the like. 
Thus, as the induction charging electrodes are moved radially outwardly 
from the axis of the liquid flow, higher voltages are required to achieve 
the optimum charging efficiency. 
It has been found that optimum results are obtained when the average 
potential gradient within the charging zone, between the charging 
electrodes and the fluid nozzle, is between about 5 and about 30 kilovolts 
per inch. While the preferred embodiment described herein utilizes 
induction charging electrode means removably connected to the spray gun, 
it is to be understood that such electrode means can also be an integral 
fixture of the spray gun. 
Returning to FIGS. 2 and 4, the corona electrode rod 27 in the embodiment 
shown protrudes forwardly from the tip of the needle 26 and extends 
forward of the fluid nozzle orifice 25. The rod 27 in the embodiment shown 
is disposed within the shaft of the needle 26 to protrude forwardly from a 
forward orifice 29 in said needle 26. 
Diameter of the rod in relation to diameter of the fluid nozzle orifice can 
be selected as required in respect to viscosity of fluid being sprayed, 
fluid flow rate desired, and the like. Generally, the diameter of the rod 
will be between about 20 percent and about 70 percent of the diameter of 
the fluid nozzle orifice. 
As is shown in FIG. 4, the rod 27 is secured within the needle 26 by means 
of a needle tip 31 having an orifice 29 through which the rod 27 extends, 
with said needle tip 31 threadably securable to the shaft portion 33 of 
the needle 26. The rearward end of the rod 27 is spiraled and abuts the 
shaft portion 33 to be held in place with tension against the rear of 
orifice wall 35. When the spray gun 10 is in operation, the rod 27 must 
protrude forwardly from the fluid nozzle orifice 25 and can protrude into 
the charging zone of the electrodes 96,98. Connection wires within a cable 
43 lead from a power source (not shown) to the rod 27. Said cable 43 is 
threaded through the hollow interior of the needle 26, and carries 
electrical current to said rod 27. In operation, suitable voltage is 
supplied to the rod 27 as required for maximum electrostatic charging of 
the particular fluid being sprayed without effecting arcing or sparking 
between the induction charging and corona electrodes. The tip of the rod 
27 is preferably formed to a very sharp point or edge to assure maximum 
corona discharge. As earlier recited, a medium-to-low conductivity fluid, 
such as a paint composition having a high solids content, benefits greatly 
in regard to magnitude of charging when both corona and induction charging 
occurs since larger particles thereof are more readily charged by corona 
discharge while smaller particles thereof are more readily charged by 
induction. 
Referring to FIGS. 5 and 6 of the drawings, a conventional hand-held, 
air-operated spray gun 110 is illustrated, said spray gun 110 having a 
handle portion 112, a barrel 114, a fluid nozzle 116 and a gas (air) 
nozzle 118, the latter two elements shown in FIG. 6. The spray gun 110 has 
a conventional trigger mechanism 119 which operates valve means 120 
comprising a needle valve assembly to admit fluid from a supply source 
(not shown) to the spray gun 110. The fluid is fed to the spray gun 110 
through a suitable connector 122 threadably connectable to a corresponding 
connector on a fluid feed hose (not shown) from the fluid supply. The 
fluid to be sprayed passes through the valve means 120 and flows through a 
fluid passageway 124 to the orifice 125 of the fluid nozzle 116. The 
needle 126 of the needle valve assembly moves axially in concert with 
movement of the trigger mechanism 119 to control fluid flow through the 
fluid nozzle orifice 125. In the embodiment shown in FIG. 6, a rod 127 
extends forwardly from the tip of the needle 126 to be coaxially disposed 
within the fluid nozzle orifice 125 and protrudes forwardly from said 
fluid nozzle orifice 125. 
Air or another suitable gas is applied under pressure to the gas nozzle 118 
by way of an air hose 128 and through suitable passageways in the body of 
the spray gun 110. The gas supply is divided into two separate passageways 
130 and 132, with gas flow being regulated by a manually adjustable 
control valve generally indicated at 134. A second control valve 136 
permits adjustment of the needle 126 in passageway 124, in a manner as 
known in the art. The gas flow in one of the passageways, for example 
passageway 130, is directed to an annular chamber 138 from which the gas 
flows forward to a second annular chamber 140. The gas nozzle 118 
incorporates a plurality of orifices such as an annulus 142 surrounding 
the fluid nozzle orifice 125, which serve to direct gas from chamber 140 
to shape the flow of fluid from the fluid nozzle orifice 125 in known 
manner. The flow of gas from passageway 132 is directed to an annular 
chamber 146 which is in communication with passageways 148 and 150 leading 
to orifices disposed in diametrically opposed ears 152 and 154 of the gas 
nozzle 118. Gas flowing from the orifices in the ears 152 and 154 serve to 
direct gas toward the atomized fluid being discharged from fluid nozzle 
orifice 125 and thereby shape the pattern of the spray. 
In the instant embodiment the fluid nozzle 116 is preferably constructed of 
metal, and is grounded through the fluid sprayed. Said nozzle 116 can also 
be grounded directly, or can be constructed of an electrically 
non-conductive or dielectric material. The gas nozzle 118 is constructed 
of an electrically non-conductive or dielectric material. The fluid nozzle 
116 can be secured in the barrel 114 of the spray gun 110 by any suitable 
means, as by threads 156. Similarly, the gas nozzle 118 is secured to the 
barrel 114 by suitable means such as an annular nut 158 having an inner 
shoulder portion 160 which engages a corresponding shoulder on the gas 
nozzle 118 and which is threaded onto the exterior of the barrel 114 by 
means of threads 162. Fluid being supplied is electrically grounded, as by 
means of a ground plate 144, in order to insure proper induction charging. 
Mounted on the exterior of the barrel 114 and concentric with the fluid 
nozzle orifice 125 is an induction charging adapter 164 bearing induction 
charging electrode means. U.S. Pat. No. 4,009,829, to James E. Sickles, 
fully describes the adapter 164, and said patent is included herein and 
made a part hereof by reference. As described and exemplified as a 
preferred embodiment in said patent and as illustrated in FIGS. 5 and 7 
hereof, the adapter is essentially a cylindrical housing 166 formed of a 
dielectric material and having a rearward portion 168 adapted to be 
secured to the spray gun and a forwardly extending portion 170 adapted to 
surround the path of the discharged spray material. Diametrically opposed 
portions of the forward part of the dielectric housing 166 are cut away at 
172 and 174 (see FIG. 7), leaving shaped, forwardly extending, opposed 
lobes 176 and 178 remaining. The lobes 176 and 178 carry charging 
electrodes, for which a d.c. voltage is applied for inductively charging 
the spray particles, while the cutaway portions 172 and 174 prevent 
interference by the housing 166 with generally fanshaped patterns which 
may be produced in the spray, and assist in the aspiration of ambient air 
through the housing 166. Again, it will be understood that the dielectric 
housing may be constructed of any suitable material capable of 
withstanding the high voltages used, and in particular can be constructed 
of materials including acetal resins, epoxy resins, glass-filled epoxy 
resins, or the like. The adapter 164 is attached to the end of spray gun 
110 by means of suitable mounts which are shaped to engage the outer 
surface of the barrel or of the annular nut 158. Although the exact shape 
of the mounts will depend upon the construction of the particular barrel 
to which the adapter is to be connected, the mounts in general are formed 
to secure the adapter in concentric relationship with the fluid nozzle 
orifice 125. Again, reference should be made to U.S. Pat. No. 4,009,829 in 
regard to mounting configurations. 
The electrostatic field by means of which the adapter 164 produces 
induction charging of the atomized fluid particles is generated by means 
of a pair of charging electrodes 196 and 198. These electrodes are mounted 
to the inner surfaces of lobes 176 and 178, respectively, of the adapter 
and thus are positioned on diametrically opposite sides of the fluid and 
air nozzles. The electrodes are spaced from the fluid nozzle and are 
concentric therewith, having curved surfaces which are equidistant from 
the longitudinal axis of the fluid nozzle 116. A high positive or negative 
voltage is supplied to the two opposed electrodes 196 and 198, and this 
voltage produces an electrostatic field between the electrodes and the 
electrically grounded fluid spray discharged from the spray gun. This 
field defines a charging zone within the adapter which serves to induce an 
opposite charge on any particulate fluids passing therethrough. The 
voltage can vary over a wide range, but preferably is less than about 30 
kilovolts. The magnitude of the voltage required to achieve optimum 
charging efficiency depends upon the radial distance between the surfaces 
of the electrodes and the axis of the liquid flow, on the longitudinal, or 
axial location of the adapter with respect to a plane perpendicular to the 
axis of the adapter and passing through the discharge point of the fluid 
nozzle, on the rates of air and liquid flow from the nozzle, and the like. 
Thus, as the induction charging electrodes are moved radially outwardly 
from the axis of the liquid flow, higher voltages are required to achieve 
the optimum charging efficiency. 
It has been found that optimum results are obtained when the average 
potential gradient within the charging zone, between the charging 
electrodes and the fluid nozzle, is between about 5 and about 30 kilovolts 
per inch. While the preferred embodiment described herein utilizes 
induction charging electrode means removably connected to the spray gun, 
it is to be understood that such electrode means can also be an integral 
fixture of the spray gun. 
Returning to FIGS. 5 and 7, the rod 127 in the embodiment shown protrudes 
forwardly from the tip of the needle 126 and extends forward of the fluid 
nozzle orifice 125. The rod 127 in the embodiment shown is disposed within 
the shaft of the needle 126 to protrude forwardly from a forward orifice 
129 in said needle 126. The rod 127 is electrically conductive and 
grounded with a connection wire 143 shown in broken line from the needle 
shaft to ground plate 144. Diameter of the rod in relation to diameter of 
the fluid nozzle orifice can be selected as required in respect to 
viscosity of fluid being sprayed, fluid flow rate desired, and the like. 
Generally, the diameter of the rod will be between about 20 percent and 
about 70 percent of the diameter of the fluid nozzle orifice, but can be 
greater or less depending upon actual fluid nozzle orifice diameter and 
physical characteristics of fluid being sprayed. Because the rod is 
electrically grounded during fluid issue, the fluid in contact with the 
rod is very near ground potential, thus providing a maximum potential 
gradient between the electrode means and the fluid particles or droplets 
entering the charging zone to thereby produce maximum droplet charging. 
Furthermore, it is found that the rod acts to provide more surface area 
from which particles can be formed, resulting in formation of a greater 
number of more uniformly-sized charged particles under the combined action 
of the shearing atomization air and the applied electric field. The 
maximum potential gradient discussed above, coupled with the greater 
tendency to produce uniformly-sized droplets, also acts to distribute the 
electrical charge more evenly on the droplets and thereby yield better 
deposition of fluid particles on the workpiece being coated, said 
workpiece being understood to be electrically receptive to the charged 
spray. 
As is shown in FIG. 8, the rod 127 is secured within the needle 126 by 
means of a needle tip 131 having an orifice 129 through which the rod 127 
extends, with said needle tip 131 threadedly securable to the shaft 
portion 133 of the needle 126. The rearward end of the rod 127 is spiraled 
and abuts the shaft portion 133 to be held in place with tension against 
the rear of orifice wall 135. When the spray gun 110 is in operation, the 
rod 127 must protrude forwardly from the fluid nozzle orifice 125 and can 
protrude into the charging zone of the electrodes 196,198. 
As above described, the spray gun provides an electrostatic induction 
charging electrode which imparts an electrical charge to the sprayed 
particles substantially simultaneously with their formation, and further 
embodies a rod concentrically disposed within and protruding forwardly 
from the orifice of the fluid nozzle, said rod being electrically grounded 
as above described. It has suprisingly been found that, when a 
medium-to-low conductivity fluid is exposed to induction charging 
electrode means within a grounded pointed rod is also present in the 
stream of said fluid, a corona discharge can be effectuated off of the tip 
of said rod by increasing the induction charging electrode means' voltage 
above that voltage required for effective induction charging alone of said 
fluid. Thus, an increase in voltage causes the grounded pointed rod to 
operate as corona discharge electrode. The magnitude of voltage increase 
to the induction charging electrode means can range from that required for 
corona discharge to first occur to a value just below that which causes 
arcing or sparking between the induction charging electrode means and the 
pointed rod. This corona effect is particularly advantageous where 
medium-to-low conductivity fluid is sprayed since the magnitude of 
charging obtainable on such a fluid particle by induction charging is 
directly related to the fluid's electrical and physical parameters. While 
smaller particles of such a fluid can be more completely charged by the 
induction process, larger particles cannot. With the addition of corona 
discharge, however, these larger particles can also be more completely 
charged. 
When fluid particles have an electrical conductivity above about 0.06 
.mu.mho/cm, it has been found that the above-related corona effect does 
not occur, thus surprisingly illustrating a heretofore unknown method of 
providing both corona and induction charging to a medium-to-low electrical 
conductivity liquid coating composition. Further, because the polarities 
of the charges imparted by both the corona and induction charging means 
are the same, no charge cancellation effect can occur. 
The following Examples are incorporated herein to illustrate improved 
results in transfer efficiency to a workpiece being coated with a liquid 
coating composition having a medium-to-low electrical conductivity. 
Transfer efficiency (TE), reported as a percentage of coating composition 
deposited on a target in relation to the theoretical amount (100 percent) 
which could be deposited on said target is determined according to the 
following formula: 
##EQU1## 
In the above calculation, the designation "target speed" refers to the 
speed at which the target is passed perpendicularly to the axis of the 
fluid nozzle of the spray gun. Weight of coating composition deposited is 
determined after drying. Coating composition flow is measured at the spray 
gun. The term "coating solids" is defined as the decimal fraction of 
weight solids. In the description which follows, all transfer efficiency 
values are determined according to the above formula. Targets utilized for 
measuring transfer efficiency in the Examples herein were constructed 
according to the following description. Each of five targets used in each 
measurement of transfer efficiency consisted of a pre-weighed aluminum 
foil about 6 inches (15.24 cm) wide, 36 inches (91.44 cm) long, and 0.0015 
inch (0.0038 cm)thick. An electrically-grounded frame was provided, and 
the targets were mounted thereon in the following order. Two of the foil 
targets were mounted on a flat aluminum plate attached to the frame, thus 
providing two flat sheets. The remaining three foil targets were mounted 
on U-shape (when viewed from above) aluminum plates attached to the frame, 
thus providing three semi-tubular targets. The lateral sides of these 
targets were about 13/4 inches (4.45 cm), while the remaining portion 
(equivalent to the base of the U-shape) was about 11/8 inches (2.85 cm). 
Distance between the mid-points of said bases of the U-shape plates was 6 
inches (15.24 cm). Finally, five tube-shaped (when viewed from above) 
aluminum foil targets, not involved in transfer efficiency measurements, 
were provided to the frame to make certain that electrical attraction of 
charged particles being sprayed toward the targets was not improperly 
concentrated toward the adjacent semi-tubular target which, but for the 
tube-shaped targets, would be the final target to be sprayed. 
In the Examples which follow, a Binks Model 70 spray gun was utilized. The 
spray gun was equipped with a Binks Model N65 fluid nozzle and a center 
rod disposed within the nozzle orifice, was modified to be equipped with a 
Binks Model N63PB, air cap, and was fitted with the induction charging 
adapter of FIG. 3. The spray gun was stationary and placed so that the 
targets were 12 inches (30.48 cm) from the face of the air cap. The frame 
upon which the targets were disposed was passed at a speed of 28 feet 
(8.53 m) per minute in front of the spray gun. For each set of 
measurements, four sets of two such passes were made while heated paint 
was being sprayed. The foils were then removed from the frame, baked for 
20 minutes at 340 F., cooled to 70 F., and weighed to determine net paint 
deposition from which transfer efficiency was calculated. Paint flow rate 
is measured at the temperature at which the paint is sprayed. 
EXAMPLE 1 
A paint composition comprising the following components was prepared: 
______________________________________ 
Polyester resin (60% solids) 
35.31 lbs. (16.02 kg) 
[Polycron.RTM. Appliance Finish Resin, 
PPG Industries, Inc.] 
Dipropylene glycol methyl ester 
18.83 lbs. (8.54 kg) 
[Dowanol.RTM. DPM, Dow Chemical Co.] 
Polyethylene cuts 3.01 lbs. (1.37 kg) 
[Pennsylvania Refining Co., #3012] 
Rutile titanium dioxide 
144.53 lbs. (65.56 kg) 
Combined with 
Hexamethoxy melamine resin 
24.00 lbs. (10.89 kg) 
[Resimene X-747.RTM., Monsanto Co.] 
Dipropylene glycol methyl ester 
3.82 lbs. (1.73 kg) 
[Dowanol.RTM. DPM, Dow Chemical Co.] 
Isobutanol 4.20 lbs. (1.91 kg) 
N-butyl acetate 1.55 lbs. (703 g) 
Combined with 
Superfine fumed silica 
2.00 lbs. (907 g) 
[Cab-O-Sil.RTM., Cabot Corp.] 
Combined with 
Polyester resin (ester diol-isophthalate- 
63.23 lbs. (28.68 kg) 
90% in Cellosolve acetate) 
Epoxy resin solution (25% in toluene) 
27.66 lbs. (12.55 kg) 
Hexamethoxy melamine resin 
28.61 lbs. (12.98 kg) 
[Resimene X-747.RTM., Monsanto Co.] 
Cold pressed castor oil 
7.55 lbs. (3.43 kg) 
2-ethylhexyl acrylate homopolymer 
0.47 lbs. (213 g) 
(62.5% solids in xylene-butanol solvent) 
Organosilicane surfactant 
0.03 lbs. (13.6 g) 
[L-7500, Union Carbide Corp.] 
Combined with 
40% para toluene sulfonic acid 
1.47 lbs. (667 g) 
Carbon black tint 0.07 lbs. (31.8 g) 
______________________________________ 
The paint composition had a delivery temperature of about 180.degree. F. as 
measured at the butt of the spray gun, and was sprayed utilizing 35 psig 
atomizing air pressure, also measured at the gun butt, with transfer 
efficiency (TE) measurements made on flat sheets and on semi-tubular 
targets as above described. The temperature of the atmosphere (air) in 
which the targets were disposed and through which the spray travelled to 
said targets was 70.degree. F. Conductivity of the paint composition at 
180.degree. F. was 0.041 .mu.mho/cm; viscosity was 200 centipoise; solids 
content by weight was 80 percent. Table I shows results obtained. 
TABLE I 
______________________________________ 
Voltage (KV) Positive 
Paint 
Supplied to Temper- Paint % TE % TE 
Induction Charging 
ature Flow Rate Flat Semi- 
Electrode (F.) (gm/min) Sheet Tubular 
______________________________________ 
18 179.degree. 
199.6 59.6 17.2 
______________________________________ 
EXAMPLE 2 
The procedure of Example 1 was followed except for an increase in voltage 
supplied to the induction charging electrode. Table II shows results 
obtained. 
TABLE II 
______________________________________ 
Voltage (KV) Positive 
Paint 
Supplied to Temper- Paint % TE % TE 
Induction Charging 
ature Flow Rate Flat Semi- 
Electrode (F.) (gm/min) Sheet Tubular 
______________________________________ 
22-23 182.degree. 
200 71.1 31.0 
______________________________________ 
As is evident from the above, essentially the same conditions in Example 1 
as in Example 2 except for a voltage increase in the induction-charging 
electrode significantly increased transfer efficiency at both flat sheet 
and semi-tubular targets. It has been visually observed that utilization 
of the spray gun here employed, having a grounded rod concentrically 
disposed within its fluid nozzle orifice, in combination with a coating 
composition having a relatively low electrical conductivity causes a 
corona discharge to occur at the tip of said rod when the voltage of the 
induction charging electrode is increased above about 20 KV. It is 
believed that larger particles of the spray stream are better charged by 
said corona discharge while smaller particles of the stream are better 
charged by induction. While the paint composition exemplified in the above 
Examples was heated, it is to be understood that such heating is not 
required so long as the viscosity of the composition being sprayed is low 
enough to permit adequate sprayability of said composition. 
Those skilled in the art will recognize that the inventive quanta of this 
application can be embodied in forms other than those specifically 
exemplified herein for purposes of illustration.