Note: Descriptions are shown in the official language in which they were submitted.
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The present lnvention is concerned with dry electro-
static deposition of metal particles and more ~articularly with
dry eIectrostatic deposition of metal particles on dry, electro-
conductive substrates.
PRIOR ART
It is generally recognized in the art that nonin-
sulated metal powder cannot be successfully electrostatically
coated on dry conductive surfaces because (1) the metal powder
tends to short-out electrodes in spray guns and fluid beds, and
(2) the deposited metal powder, at best, only weakly adheres
to the surface on which it is deposited. Table I lists currently
available literature containing such disclosures.
TA~LE I
1. U.S. Patent 4,027,366, John M. Millar and
William P. Moran, assignee Beatrice Foods
Co., column 15, lines 59-68.
2. Product Finishing, January 1972, Daniel
R. Savage, "Powder Coating: A Look at
Equipment and Materials".
3. Fundamentals of Powder Coating, E. P. Miller
and D. D. Taft, editors, Chapter D, Page 21,
Society of Manufacturing Engineers 1974.
4. "The Electrostatic Deposition of Conducting
and Semi-conducting Powders, R. P. Corbett,
page 52, Science and Technology of Surface
Coatings, B. N. Chapman and J. C. Anderson
1974.
5. U.S. Patent 3,868,925 by Edwin J. Smith,
Robert B. Rerf, assignee National Steel
Corporation.
U.S. Pat.
No. Inventor Date
6~ 3,513,810 Jackson May 1970
7. 3,628,501 Jackson et al December 1971
8. J. A. Cross IEE Conf. Public No. 133 Machining
Forming and Coating No. 1975, pp. 46-52.
9. Bright et al IEE Conf. Pub. Ibid pp. 38-45
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10. Corbett, R. P. Electrical Methods or ~lachinery,
Forming and Coating IEE Conf. Pub. 61 London
1971 pp. 111~
11. Bright et al IE~ Conf. Pub. 61 London 1971
pp. 119-127.
PROBLEMS
Weak adherence of electrostatically deposited metal
particles as disclosed in the prior art presents a severe pro-
blem. If no adhering agent, e.g., water or glue, is used, the
as-deposited metal particles can be readily displaced on or from
the surface on which they are deposited during transport from
a depositing station to a rolling or sintering station. This
sort of displacement can result in a substantial amount of
defective production. When water or other adhering agent is
used, this adhering agent must be removed slowly from the coated
substr~e under quiescent conditions in order to prevent dis-
placement of the electrostatically deposited metal powder.
Furthermore, some metal powders such as aluminum can react with
water or other adhering agents. The need for slow removal of the
adhering agent increases the cost and complexity of equipment
auxiliary to the elec~rostatic deposition equipment. Additional
investment and control are needed to uniformly apply the adher-
ing agent prior to electrostatic depositionO
_SCOVERY
It has now been found that by employing a combina-
tion of concepts dry electrostatic metal coating can be success-
fully accomplished on a commercial scale.
OBJECTS
It is an object of ~he invention to provide a means
for electrostatic spraying or coating metal powder on conductive
substrates.
It is further and more specific object of the present
invention to provide a means for electrostatic spraying or
coating zinc on steel substrates.
GENERAL DESCRIPTION
Generally speaking the present invention contemplates
electrostatic coating or spraying of metal powder onto conduc-
tive (e.g., metal~ substrates which comprises conveying
selective metal particles of a size or sizes within the range
of about 5 to about 150 microns and particularly specified
hereinafter in Table II through a zone ionized by application
of a positive or negative voltage of about 25 to about 100
kilovolts (kV) with respect to ~round to a binder-free elec-
troconductive substrate to be coated which is maintained at
said ground or a voltage lwer than that induced on the
particles.
Surprisingly and despite all the prior teachings
to the contrary, it has been found that these metal particles
within the size range specified do not have a significant
tendency to short-out ionizing electrodes in conventional
electrostatic spray guns such as the GEMATM HP 720 made by
GEM AG, St. Gallen, Switzerland so long as the internal high
voltage electronic connections are isolated from the gas
stream conveying metal particles to the ionizing zone and
the powder feed rates are adjusted to prevent accumulation
of powder in the feed lines. An electrostatic fluid bed
apparatus identified as Model No. 500 Desk Style Powder
Coater of Electrostatic Equipment Corporation of 80 Hamil~on
Street, New ~aven, Conn. 0~511 has also been successfully
used with zinc and nickel powder. In this equipment, the
ionized zone is maintained in a separate chamber separated
by a porous membrane from the bed of metal powders. Ionized
gas passes into the bed so that shorting problems
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at the high voltage terminals are minimized. However care must
be taken to ensure that the charged powder bed does not self
discharge. This is especially true of metal powers with a
low explosive limit (e.g., Al).
When particular metal particles of the proper size
as described hereinbefore or more preferably in the ran~e of
about 15 to 80 microns are passed through the ionizing zone and
then impacted on a clean conductive substrate (e.g., steel, aluminum,
conductive [carbon loaded] plastics) surprisingly they adhere
strongly. As a practical matter the adherence is sufficiently
great that a coated substrate passes a "shake and tap test". The
shake and tap test consists of first shaking the coated test panel
in air and then tapping the coated panel two or three times against
a convenient hard surface such as a desk or table edge. Passing
of the tests require that no significant amount of coating powder
falls off the shaken and tapped panel. This is a rather severe
test and shows an adhesion of electrostatical]y deposited metal
particles of the same order of magnitude as the adhesion oE electro-
statica~ly deposited plastic particles. The Cross article cited
hereinbefore shows that plastic particles in the electrostaticallv
as-deposited condition adhere so strongly to a steel substrate
that a force of about 3000g's on the average is required to dis-
lodge them. In contrast, aluminum metal particles deposited
electrosta ically on steel by Cross adhere with a force that, on
the average, can be overcome by a force of lOOg's. Passing of
the shake and tap test assures that electrostatically deposited
powder on, for example, a steel substrate can be conveyed by
ordinary means to a sintering or rolling station without an
undue amount of defective product being produced.
Applicants would very much like to give a theory
which accounts for their success contrary to the disclosures
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and expectations of the prior art but are unable to do so.
Table II lists specific powders which have been successfully
electrostatically sprayed.
TABLE II
Example CommercialU.S. Screen
No. Powder SourceSize Range
l Zn (Spherical Belmont -325
2 Si (MD301) Alcan Aluminum Corp. -325
3 Mn (MD301) Alcan Aluminum Corp. -325
4 Cr Union Carbide Corp. -325
Attrited MA 956E* *** -200
6 Fe-65 Al *** -200
7 Cu-25 Ni Flake U.S, Bronze Corp.
No. 8500 --
8 Alloy 600 Flake** *** --
9 Al Flake MD ?01 Alcan Aluminum Corp. --
Zn Flake NJZ1222
MD 104 New Jersey Zinc Co. -100
11 Lead Tin Solder Alcan Aluminum Corp. -200
* Composition in percent by weight 20% Cr, 4% Al, bal. Fe
** Composition in percent by weight 72% Ni, 15~ Cr, bal. Fe
*** Experimental Product of The International Nickel Co., Inc.
In order to attempt to explain their success, appli-
cants applied highly sophisticated tests in order to attempt
to specify the surface characteristics of metal powders that
are operable for purposes of the invention without success.
As an example, BelmontTM zinc powder (-325), the best spraying
of zinc powders tested was subjected to spectrographic analysis
which showed traces of chromium, iron, nickel, silicon and
lead in the metal. Additional analysis on the Auger Electron
Spectrometer which indicated the atomic species on the surface
oE a specimen showed zinc and about 7~ silicon (as SiO2~ as
the sole materials of the powder particle surEace persisting
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.,
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for about 2000 A into the powder. Other zinc powders which
were operable in spraying contained no silica on the surface.
While the presence of silica on the surface of one sample may
be significant with respect to an enhancement of sprayability,
on the overall, neither bulk analysis of a spectrum of powders
nor sufrace analysis of the same spectrum of powders gave any
indication to applicants of the mechanism whereby the present
invention operates.
The process of the present invention is a first step
overall in coating metal substrates. After electrostatic
spraying for example on steel, the coating on the substrate
can be mechanically densified hot or cold, heat treated or the
like to produce final coated products such as zinc-coated steel,
terne plate, solder-coated nickel, nickel-chromium-iron coated
steel, copper-nickel coated steel, aluminum-coated steel,
nickel-coated steel and the like. The steps conducted after
electrostatic coating, while included in the following examples
for purposes of showing practical utility of the invention are
conventional and depend upon the type of substrate coated and
the coating metal. For some purpos~s, for example spra~ing
complexly shaped objects, it can be advantageous to include
plastic powder in the metal spray stream or as an overcoat.
When used in the metal spray stream the plastic powder can be
up to about 25~ by volwme of total metal and plastic.
EXAMPLES
In order to give a grea~er appreciation of the inven-
tion -the ~ollowing examples are given. In these examples metal
powder was sprayed tnrou~h a GEMATM HP 720 spray gun modified
only to provide a rubber O-ring seal oE internal high voltage
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electric contacts. The powder was passed through the gun in a
gaseous fluidized stream using dry air as the carrying gas at
an effective pressure of about 10 to about 30 psig (about 1.66
to 3 atmospheres absolute). The gun was energized to the voltage
set forth in Table III and held at a distance of approximately
18 inches (0.45 meter) from an electroconductive substrate held
at ground potential. In all cases in the examples the substrate
was clean, dry, slightly roughened (pickled) steel although
phosphate-coated and chromate-coated steel can often be used as
a substrate with advantage.
Table III sets forth electrostatic spraying data
with respect to examples identified in Table II.
TA~LE III
Example Powder Air Air Pressure
No. Powder _ ~etting Setting psi Atm (abs.)
1 Zn 0.5 9.5 10 1.66
2 Si 0.0 9.5 15 2.00
3 Mn 0.5 9.2 20 2.33
4 Cr 0.5 9.2 10 1.66
Attrited
MA 956E 0.4 9.3 20 2.33
6 Fe-65 Al 0.2 9.0 20 2.33
7 Cu-25Ni Flake 0~0 9.4 20 2.33
Alloy 600
8 Flake 0.75 9.5 30 3.00
Al Flake
9 MD 201 0.25 8.5 20 2.33
Zn Flake 0.5 9.0 20 2.33
11 Pb-Sn solder 0.2 9.5 20 2.33
Example I is particularly advantageous with respect to s~eel coat-
ing because after sintering for example at about 400C for about
2 minutes and densification by cold rolling a zinc-coated product
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is obtained which is essentially equivalent to or better than
hot-dipped galvanized product. This result is obtainable at
a capital cost which is only a fraction of the capital cost of
a hot dip line of equivalent capacity. Lead-tin solder coatings
made from the powder of Example 11 sprayed at voltages of 50
to 75 kilovolts and melted onto the steel surface at about 200~C
is another example of a coating which can be completed, after
electrostatic spraying by means of a heat treatment at a relatively
low temperature. Still further the zinc coating made with the
powder of Example 1 or Example 10 can be oversprayed with a
layer of plastic powder. The composite coating is then heated
at about 200C for about 15 minutes so as to melt the plastic
and adhere the zinc powder to the steel.
Examples 2 to 8 represent powders which require high
temperature sintering. Successful sintered coatings after appllca-
tion by electrostatic spraying are made by sintering at about
800C to about 1350C for about 15 to about 120 minutes, longer
; times being used with lower temperatures and vice versa and,
generally, higher temperatures being used with metals and alloys
having higher melting points.
For some metals it is advantageous to densify the
as electrostatically sprayed coating, for example, by cold
rolling prior to sintering. In this case, it is advantageous
to discharge any residual electric charge in the powder layer
just prior to the coated substrate entering the rolling stage.
This can be done by steam treatment, humidity control, ion dis-
charge, etc.
In addition to the utility disclosed hereinbefore,
the process of the present invention can be used for surface
enrichment, for example enrichment of nickel or cobalt base
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superalloys with chromium or silicon (or both) for enhanced
oxidation resistance. Also the process oE the present inven-
tion can be used in catalyst maufacture, for example, coating
mixtures of copper-nickel-chromium on substrates such as wire
mesh; in battery manufacture; in producing magnetic, decorative,
wear-resistant, or gall-resistant coatings; and in weldrod manu-
facture.
Although the present invention has been described in
conjunction with preferred embodiments, it is to be understood
that modifications and variations may be resorted to without
departing from the spirit and scope of the invention as those
skilled in the art will readily understand. Such mcdifications
and variations are considered to be within the purview and
scope of the invention and appended claims.
.
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