U.S. patent number 3,853,580 [Application Number 05/245,132] was granted by the patent office on 1974-12-10 for methods for electrogasdynamic coating.
This patent grant is currently assigned to The National State Bank. Invention is credited to Meredith C. Gourdine.
United States Patent |
3,853,580 |
Gourdine |
December 10, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
METHODS FOR ELECTROGASDYNAMIC COATING
Abstract
Method for applying coating materials to articles, using
electrogasdynamic apparatus providing a flow channel having a
dielectric boundary which has a length/width ratio of greater than
2.5. Gas containing particulate coating material is ionized to
create an electrical discharge field for imparting an electrical
charge to the particles prior to passage through the dielectrically
bounded flow path and thereby creating a high space charge
potential. In special applications, a free-radical forming monomer
gas or a fusible particle substance is carried in an inert gas into
the electrical discharge field and through the flow channel toward
the article to be coated. In any case, the charged particles are
subjected to an axial charge repelling field (due to space charge
effects) in the flow channel to raise the electrical potential of
the particles, and thereby the potential gradient between the
particles and the workpiece.
Inventors: |
Gourdine; Meredith C. (East
Orange, NJ) |
Assignee: |
The National State Bank
(Elizabeth, NJ)
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Family
ID: |
26937018 |
Appl.
No.: |
05/245,132 |
Filed: |
April 18, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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837562 |
Jun 30, 1969 |
3673463 |
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601270 |
Nov 15, 1966 |
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512083 |
Dec 7, 1965 |
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Foreign Application Priority Data
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Apr 21, 1971 [JA] |
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46-25242 |
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Current U.S.
Class: |
427/485; 310/309;
118/621; 239/692; 361/226; 264/439; 264/460; 264/483 |
Current CPC
Class: |
B05B
5/001 (20130101); B05B 5/08 (20130101); B05B
5/032 (20130101) |
Current International
Class: |
B05B
5/025 (20060101); B05B 5/03 (20060101); B05B
5/08 (20060101); B05b 005/02 () |
Field of
Search: |
;117/93.4R,93.41,93.42,93.43,93.44,17,1B,93.1GD,93.1CD,161UZ
;239/3,15 ;264/24 ;118/621,622,627 ;310/5,6 ;317/3,4 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Martin; William D.
Assistant Examiner: Newsome; John H.
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
This is a division of application Ser. No. 837,562, METHODS AND
APPARATUS FOR ELECTROGASDYNAMIC COATING, filed June 30, 1969,
itself a division of application Ser. No. 601,270 for
ELECTROGASDYNAMIC SYSTEMS AND METHODS filed Nov. 15, 1966, which in
turn is a continuation-in-part of application Ser. No. 512,083,
ELECTROGASDYNAMIC SYSTEMS MAINTAINING CONSTANT GAS SPEED, filed
Dec. 7, 1965. Application Ser. No. 837,562 is now Pat. No
3,673,463, and applicatiaon Ser. Nos. 601,270 and 512,083 are now
abandoned.
Claims
I claim:
1. A method of applying a fixed coating to a substrate, including
the steps of:
flowing a gas in a bounded flow path,
including in the gas a coating substance,
establishing an ionizing field between at least two electrodes at
an upstream location in said bounded flow path, thereby subjecting
the gas to said ionizing field to provide charged particles of the
coating substance,
forcing the gas and entrained charged particles of the coating
substance downstream through a portion of the bounded flow path to
an exit from the bounded flow path,
emitting the charged particles out of the bounded flow path by said
exit to a location less constricted than the bounded flow path,
thereby creating, in the vicinity of the exit, a space charge field
at an electrogasdynamically increased potential and separated from
the charging electrodes by said portion of flow path,
continuing to force gas entrained charged particles through the
bounded flow path portion and out of said exit in opposition to
said space charge field, and
disposing the surface of the substrate to be coated beyond said
exit so as to be contacted by said particles,
said step of forcing the gas and entrained charged particles
through a bounded flow path portion comprising flowing the gas and
charged particles along a path portion sufficiently long to permit
an electrogasdynamic potential increase to a very high potential
without incurring dielectric breakdown.
2. A method as defined in claim 1, in which:
the substrate to be coated is the inside surface of a conduit,
and
the flow path is located interiorly of the conduit.
3. A method as defined in claim 2, in which:
the axis of the flow path is substantially parallel to the axis of
the conduit.
4. A method as defined in claim 1 further comprising the step
of:
providing the substrate with a reference potential different from
the space charge potential to thereby create a mutual electrical
force between the substrate and the charged particles.
5. A method as set forth in claim 4, wherein:
the substrate to be coated is an object of a dielectric material,
and
the reference potential is provided interiorly of the object
relative to the surface to be coated.
6. A method as defined in claim 5, wherein:
the substrate to be coated is located within a chamber having walls
at said reference potential.
7. A method in accordance with claim 1, in which:
the bounded flow path has an aspect ratio of length L to width W of
at least 2.5 where width W is taken as the maximum cross-sectional
dimension of the channel normal to the direction of flow at the
maximum space charge concentration.
8. A method according to claim 1, further comprising the steps
of:
flowing a monomer gas with the ionizable gas down the flow path;
and
depositing on the substrate free radical material and partculate
matter, whereby the free radical material polymerizes to form a
mixed coating of the particulate matter and the polymer of the
monomer.
9. A method as recited in claim 1, further comprising the steps
of:
adding heat to the particles carried by the gas prior to contact
with the substrate to be coated, thereby to condition the particles
for fusion upon contact with the substrate.
10. A method as recited in claim 1, further comprising the step
of:
applying heat to the substrate, thereby to fuse together the
particles contacting the surface.
11. A method as recited in claim 1, wherein the substrate to be
coated is elongate and substantially flat, the method further
comprising the steps of:
moving the flat substrate in the direction of its elongation to
thereby subject the substrate to continuous coating by the
particulate coating substance; and
stripping the coating from the substrate to thereby form a
continuous sheet of the substance forming the particles.
12. A method of coating the surface of a substrate with a substance
initially in the form of a monomer gas, comprising the steps
of:
flowing the monomer gas to an electrical discharge field in the
flow path to thereby form charged and uncharged free radical matter
of the monomer,
flowing and emitting primarily only the uncharged of the free
radical matter through the flow path and out the exit to a less
constricted location,
restraining substantially all of the charged radical matter from
flowing through the flow path and the exit by field opposition,
and
disposing the surface of the substrate beyond said outlet to be
contacted and coated by the uncharged free radical matter.
13. A method of coating first particles with a substance initially
in the form of second particles, comprising the steps of:
providing first and second bounded flow paths having, first and
second exits, respectively,
establishing a first ionizing field between at least two electrodes
at an upstream location in the first bounded flow path,
establishing a second ionizing field between at least two
electrodes at an upstream location in the second bounded flow
path,
flowing a gas in the first path and including the first particles
therein,
flowing a gas in the second path and including the second particles
therein,
subjecting the gas and first particles in the first flow path to
the first ionizing field therein to thereby impart charges of one
polarity to the first particles,
subjecting the gas and second particles in the second flow path to
the second ionizing field therein to thereby impart charges of an
opposite polarity from said one polarity to the second
particles,
forcing the gas and entrained charged first and second particles
downstream through a portion of the respective first and second
bounded flow paths to respective exits,
emitting the charged particles out of the bounded flow paths by
said exits to a location less constricted than the bounded flow
paths, thereby creating, in the vicinity of each exit, a space
charge field at electrogasdynamically increased potential and
separated from the respective charging electrodes of the associated
flow path by said portion of the associated flow path,
continuing to force gas entrained first and second charged
particles through the bounded flow path portions out of said exits
in opposition to said space charge fields, and
admixing the first and second particles outside of their respective
paths to thereby effect the combination of the particles bearing
opposite charges due to the mutual electrical forces
therebetween,
said steps of forcing the gas and entrained charged particles
through a bounded flow path portion comprising flowing the gas and
charged particles along respective path portions sufficiently long
to permit an electrogasdynamic potential increase to a very high
potential without incurring dielectric breakdown.
14. A method as defined in claim 13, in which:
the charges are imparted to the first and second particles by
subjecting them to respective electrical discharge between corona
and attractor electrodes, wherein the electrical polarity of each
corona electrode corresponds to the polarity of the charge imparted
to the particles in the associated flow path.
Description
BACKGROUND OF THE INVENTION
Electrostatic coating techniques have been used for some time to
deposit on a workpiece a homogeneous even coating of charged
particles.
The apparatus used to carry out such coating generally consists of
a hand-held or mounted spray gun device for atomizing a liquid
supply of paint or other coating material, and for subsequently (or
simultaneously) charging the coating particles. The particles are
then drawn to the coating surface by electrostatic lines of force
between the particles and the article. One disadvantage of known
methods, however, is the undesirable "spreading" of the spray
pattern, sometimes improved by implementing secondary electrostatic
pattern-controlling fields.
Another disadvantage of such known apparatus is their failure to
generate sufficient space charge (particle) potential to establish
effectively strong field gradients between the particles and the
surface of the object to be coated. To create strong gradients, it
has been the practice to use electrostatic atomization, or to
impose an external field (using extremely high voltages) between
the spray source and the coating object. Where high external
electric fields are used, the possibility of arcing is enhanced,
necessitating sometimes elaborate safety precautions.
SUMMARY OF THE INVENTION
The present invention offers improved techniques and apparatus for
applying a variety of particulate coating substances to surfaces by
charging coating particles and passing them through a
substantially, non-conducting flow path boundary to increase the
space charge potential due to the electrical charge on the
particles. Preferably, the flow channel has a minimum aspect ratio
of length/width of 2.5, and the particles to be charged may be
liquified by heat prior to charging, or can be formed by subjecting
a monomer gas to an electrical discharge.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of these and other aspects of the
invention, as well as the objects and advantages thereof, reference
may be made to the following detailed description and to the
drawings, in which:
FIGS. 1-3 are schematic illustrations in cross-section, showing
various applications of electrogasdynamic ionizing apparatus in
accordance with the invention to the coating of object surfaces;
and
FIG. 4 is a cross-sectional view of a two-channel electrogasdynamic
ionizer in accordance with the invention, in which the outlets of
the channels are arranged to intermix the fluids exiting
therefrom.
The principles of electrogasdynamic precipitation may be utilized
in the coating of object surfaces, and because of the versatility
and simplicity of electrogasdynamic apparatus, EGD coating systems
may be operated on gas streams seeded with both organic and
inorganic materials. Moreover, both insulative and metallic
surfaces can be coated. EGD precipitation techniques are applicable
to coating techniques because, in effect, the object to be coated
serves as a collector electrode. Moreover, as in EGD precipitators,
it is desirable to charge particles and create in a flow channel a
high space charge potential without, at the same time, causing
dielectric breakdown of the gas or coating aerosol.
In the above-noted application Ser. No. 601,270, EGD apparatus are
disclosed, all of which have an aspect ratio of length (L) to width
(W) exceeding 2.5, where the width W is taken as the maximum
cross-sectional dimension of the channel (normal to the direction
of flow) at the maximum space charge concentration. Alternately,
for flow channels of cylindrical cross-section, the aspect ratio is
expressed as L/R, where R is the cylinder radius; and the
corresponding miminum aspect ratio is 5.
It has been found that similar limitations apply to EGD coating
apparatus, where the flow channel should have an aspect ratio of
L/R greater than 5, or L/W greater than 2.5 in order to achieve
optimum space charge potentials without incurring dielectric
breakdown. These aspect ratios take into account not only the
maximum safe operating potentials for most coating operations, but
also the maximum desirable transverse electric field within the
channel due to space charge alone, while allowing for sufficient
length to carry the charged particles away from the ionizing
electrodes toward the workpiece.
The schematic illustration of FIG. 1 depicts the use of an
electrogasdynamic ionizer 164 in a system for coating solid objects
positioned within a chamber 165. The ionizing apparatus is in the
form of a gun and consists of a single flow channel defined between
parallel dielectric plates 166, corona and attractor electrodes 167
and 168, respectively, and a power supply 169 for applying an
ionizing potential between the electrodes 167 and 168. The
electrodes are of the type shown and described in detail in
application Ser. No. 601,270.
The gun 164 is supported by a suitable structure, such as the
conduit 170 having an inlet 171 receiving a carrier gas in which a
coating substance in particle form is entrained. The gas and
particle mixture enters the electrogasdynamic flow channel inlet
172 where it becomes charged in the electrical discharge field
between the corona and attractor electrodes 167, 168. Ions and
charged particles move downstream and are exhausted into the
chamber 165, creating a space charge field. However, because the
channel is relatively long, the charged particles exit a sufficient
distance downstream of the electrode to inhibit the tendency of the
particles to return to the gun structure at a lower electric
potential.
For purpose of explanation, the chamber 165 is shown to contain
three solid objects of arbitrary shape, the first object 173 being
metallic and connected by a conductor 173a to a metal wall 165a of
the chamber, which is referenced to ground. Another object 173b is
of a dielectric material and is referenced to ground by the
conductor 173c, which terminates internally of the mass 173b.
Finally, a third object 173d, constructed of either metal or an
insulative material, has no connection to a reference potential. As
the charged particles are emitted from the gun 164 into the chamber
165, the objects 173 and 173b will both receive an even coating
over their total surfaces, irrespective of the direction in which
the surfaces of these objects face, since the space charge field in
the chamber 165 is effective to induce the charged particles to
seek a potential lower than the space charge potential. The object
173d, on the other hand, will receive a coating only on those
surfaces exposed directly to the flow of particles from the outlet
of the flow channel in the gun 164.
In FIG. 2 the electrogasdynamic gun 164' is shown supported by
suitably resilient guides 175 at the interior of a conduit or pipe
176 of which the inner surface 176a is to be coated. An air and dry
powder mixture is fed into a pump or compressor 178 and
subsequently into a heating unit 179 where the individual particles
are liquified by the application of heat. From the heater 179, the
air and liquified particles continue into the flow channel of the
gun 164' where the particles are charged and sprayed into the
interior of the conduit 176. A power supply 169' excites the
ionizing electrodes of the gun. Preferably, the conduit 176 is
suitably connected to ground, as indicated. Upon reaching the
interior of the conduit 176, the charged particles set up a space
charge field and are deposited evenly over the total interior
surface 176a. Thus, the conduit 176 itself functions as the
collector electrode of an electrogasdynamic precipitator.
FIG. 3 shows an extension of the foregoing principles to the
coating of a flat object 180. There, the electrogasdynamic
apparatus includes a pair of dielectric plates 181, 182, with the
longer of the plates 182 serving to direct the flow over the
surface 180a. Further, as charged particles are blown into the
space between surface 180a and the plate 182, the space charge
field drives some of the charged particles to the dielectric plate
182. This plate soon reaches a condition of charge saturation and
produces an electrical field gradient normal to the plate 182 in a
direction tending to aid the space charge field in depositing the
particles on the surface 180a.
At the inlet to the electrogasdynamic gun, an aerosol, such as an
air and dry powder mixture of a desired material, is intermixed
with a free radical-producing gas, e.g., a monomer gas. In the
ionizer 183, the powder particles are charged and the monomer gas
is broken down into free radicals, both charged and uncharged. The
charged aerosols, along with the uncharged free radicals, are
carried downstream into the "collector" which, in the FIG. 3
device, is formed between the dielectric plate 182 and the surface
180a of the object to be coated. By precipitating action, (as well
as by diffusion), the uncharged free radicals and charged aerosol
particles are deposited on the surface 180a, where the free
radicals polymerize to form a thin film coating and assist in the
fusion of the powdered particles. As an example, the monomer gas
may be ethylene and the powder polyethylene. When the ethylene free
radicals and polyethylene powder are deposited on the surface 180a,
they form a thick film without the application of heat.
As a further example, a monomer gas alone may be used for surface
coating. In such case, an inert gas, such as argon or neon, may be
used as a carrier for the free radicals formed in the corona
discharge of the EGD gun. Molecular ions and the charged free
radicals formed in the corona discharge are not carried downstream
by the flow to any appreciable distance, since they possess
relatively high mobilities and are quickly attracted by the
attractor electrode in the ionizer 183. Therefore, primarily only
the uncharged free radicals exit into the collecting region. This
phenomena is of advantage, since after an initial coating is built
up on the object surface, charged particles thereafter deposited
can burn the coating by discharging through the coating to the
coated surface. It will be appreciated, that any of a number of
monomer gases, such as styrene and propylene, may be employed.
The device of FIG. 4 operates identically to those shown in FIG. 1
to charge the particles dispersed in two separate thin flow
channels 185a, 185b formed by the parallel dielectric plate
structure 186, 187, 188. Each plate has associated therewith a
plate attractor electrode 189, all of which can be electrically
connected to be at the same potential, viz., the potential on the
conductor 190. Ionization excitation sources 191a, 191b of opposite
polarity are attached between the common conductor 190 and
respective corona electrode arrays 192a, 192b in the flow paths to
yield ionization fields of corresponding opposite electrical
charges. As depicted, the gas in the channel 185a becomes
positively ionized and the particles carried thereby positively
charged, while the gas and particles flowing through the channel
185b are associated with negative charges.
At the exit end of the device, the flow channels 185a, 185b are
directed toward each other in such a way that the flow through the
one channel intermixes with the flow through the other channel.
Thus, at the flow channel exits, the positively and negatively
charged particles are in close proximity whereby they are mutually
attractive. If, for example, particles carried in the stream in the
channel 185a are liquid and those particles in the channel 185b are
solid, the (negative) solid particles become coated with the
(positive) liquid particles, the charge on at least one of the
attractive particles becoming neutralized upon physical
combination. It is apparent, of course, that the apparatus of FIG.
4 can be used in any manner according to the invention to bring
about charging and combining of gas entrained particles in whatever
physical state, whether liquid or solid. Moreover, the device is
further advantageous in effecting ionization of the gas flowing in
each of the separate flow channels whereby ions of the ionized gas
become mutually attractive until chemical or electrical combination
occurs. Accordingly, the term "particle" is used in its broadest
sense.
The foregoing principles may be applied to the formation of a thin
sheet of material by stripping the coating from the coated object
in a well-known manner. For example, in FIG. 3 the flat object
might represent one run of an endless belt so that the surface 180a
thereof moves past the particle discharge. Subsequently, the
coating may be peeled from the surface 180a by a knife-edge (not
illustrated) to form a continuous sheet of the coating
material.
* * * * *