CA1131080A - Electrostatic spraying - Google Patents

Abstract

Abstract of the Disclosure A process for electrostatically depositing metal powder on dry electroconductive substrates characterized by selecting metal powder to be deposited from powders having particle sizes of about 5 to about 150 microns and from a named group and their equivalents.

Description

~:~L3~

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 3L~3~

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 -,4 -~ ,3 ~3~

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 ~33~

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 . - 6 -.~ "
~ j, ., ~3~L~8~

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 c,~

113~

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 ~L~L3~

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 ~3~

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.

.

Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for electrostatically depositing metal powder comprising passing a fluidized mass of gas and metal powder through a zone wherein said metal powder is charged by a voltage of about 25 to about 100 kilovolts with respect to ground potential to an electrostatic potential above ground potential and impacting said fluidized mass on a dry, clean electroconductive substrate maintained at an electrostatic potential lower than that of said metal powder, said metal powder being one or more of Belmont spherical zinc powder AlcanTM MD 301 Silicon powder AlcanTM MD 301 Manganese powder Union Carbide Corp. Chromium powder Attrited MA 956 E powder Fe-65 Al powder U.S. Bronze Corp. No. 8500 Cu-25 Ni Flake powder Alloy 600 flake powder AlcanTM Aluminum Flake Powder MD 101 New Jersey Zinc Flake NJZ 1222 powder AlcanTM MC 104 Lead Tin Solder powder having particle sizes in the range of 5 to 150 microns whereby the metal powder particles adhere to said dry, clean substrate in the as-deposited condition with a force of the order of magnitude of the force adhering plastic particles to said dry, clean substrate when said plastic particles are in the electrostatically as-deposited condition.

2. A process as in claim 1 wherein the metal particles are zinc particles, the substrate is steel and the zinc particles after electrostatic deposition are sintered to and densified on said steel substrate.

3. A process as in claim 1 wherein the fluidized mass of gas and metal powder also contains plastic powder in an amount up to about 25% by weight of the metal powder.

4. A process as in claim 1 wherein subsequent to electrostatic deposition of metal, plastic powder is deposited on the substrate coated with metal powder.