US3348965A - Electrostatic spraying - Google Patents

Description

Oct 24, 1967 E. w. DRUM 3,348,965

ELECTROSTATIC SPRAYING Filed Feb. 4, 1964 10 2o 30 wv. AVAILABLE ON ELECTRODE ADNI-IDHAB NOIlISOdBG NI mawaAoadwl 1N3aa3 INVENTOR EDWARD W. DRUM ATTORNEYS United States Patent 3,348,965 ELECTROSTATIC SPRAYING Edward W. Drum, Indianapolis, Ind., assignor to Ransburg Electro-Coating Corp., Indianapolis, Ind., a corporation of Indiana Filed Feb. 4, 1964, Ser. No. 342,488 7 Claims. (Cl. 117-93.4)

ABSTRACT OF THE DISCLOSURE Organic solvent based paint containing electrically conductive metallic particles tends to short out when the paint is mechanically atomized and sprayed through a zone in which the atmospheric ions are in a state of high spatial concentration to charge the spray particles and improve paint deposition efficiency. The conductive particles in the paint tend to become oriented to cause this shorting to take place. When this condition is encountered, conductive agents such as polar organic solvents or ionic surface active agents are incorporated in the liquid continuum of the paint to increase its conductivity and cause the electrostatic potential to drain off through the paint to lower the potential which is available and thereby avoid the shorting out incidental thereto.

The present invention relates to coating processes in which an organic solvent-based paint containing dispersed electrically conductive metallic particles is mechanically sprayed using an electronstatically charged surface or electrode in the immediate vicinity of the spray to charge the sprayed paint and cause it to deposit with increased efficiency, the paint being modified in accordance with the invention to avoid short-circuiting while preferably producing increased deposition efficiencies in comparison with the corresponding deposition efliciency obtained by reducing the electrostatic potential below that level which causes short-circuiting.

The invention is particularly adapted to coating processes in which the liquid material is mechanically atomized and passed through a zone in which the atmospheric'ions are in a state of high spatial concentration while the spray particles are also in a state of high spatial concentration. This is conveniently accomplished by providing a compressed air spray gun having charging electrode means which create a high concentration of atmospheric ions in a zone close to the place of formation of the spray so that the material passing through the zone of concentrated atmospheric ions is also concentrated. Devices which spray in this fashion are disclosed in the copen'ding applications of James W. Juvinall et al., Serial No. 60,657, filed October 5, 1960, now Patent No. 3,169,882, and James W. Juvinall, Serial No. 148,793, filed October 25, 1961, now Patent No. 3,169,883, the disclosures of which are hereby incorporated by reference.

It is of interest to note that the compressed air spray equipment of copending application Serial No. 60,657, now Patent No. 3,169,882, includes charging electrode means in direct electrical contact with part of the column of paint which is to be atomized so that the capacity of the column of paint to drain away a portion of the electrical potential on the charging electrode will be immediately apparent. Indeed, structures in which the direct electrical contact noted above is present are preferred in the invention because this permits adequate drain-off of electrical potential with minimum alteration of the paint. However, the invention is also applicable whenever mechanical force is used to move material being sprayed or the particles in a state of high spatial concentration through a zone in which the atmospheric ions are in a state of high spatial concentration even when the electrode charging means is not in direct contact with any part of the column of paint which is supplied. This is because the close proximity between the charging electrode and the column of paint supplied creates an electrical continuity causing voltage drain-off through a conductive paint.

As will be evident, the problem of the invention is the short-circuiting of the electrical potential through the column of paint which is supplied. Quite obviously, whenever the electrostatic spray structure is such as to avoid the imposition of high electrical potentials on the column of paint which is supplied, then the problem of the invention would not be present, and the directions provided by the invention would have no purpose.

Electrostatic spray processes of the type under consideration have met with substantial acceptance, but relatively few metallic paints could be used in the process, and these were usually paints containing low concentrations of metallic particles, such as powders or flakes. When the metallic paints were formulated to contain high concentrations of the metallic particles, the electrostatic process would short out. This can be attributed to an alignment or linkage of the metallic particles occurring as high voltage is passed from the electrode, down the paint column to the grounded handle of the gun. Thus, once this linkage occurred, a metal path was provided for all of the voltage to ground, hence, shorting out.

It has been noted in past experiments that a reduced voltage on the electrode would allow continuous spraying without shorting out. The voltage applied then would have to be determined by the shorting voltage at which the particular metallic paint being used occurred. In general, the heavier the metallic concentration, the lower the applied voltage that could be used.

In other words, some of the more desirable and more heavily pigmented metallic paints become too conductive under the influence of high voltage so that the electrostatic forces, in the mechanical spray system under consideration, short out through the paint instead of improving transfer efficiency from the spray gun to the target. Accordingly, and when such paints are encountered, it is possible to proceed by greatly reducing the voltage supplied to the electrode, but equipment for this purpose is not usually available and, more important, significant voltage reduction in this manner greatly lowers the deposition efliciency of the system.

Referring to standard voltages used in electrostatic spray systems of the type under consideration, voltages varying from about 60 kv. to about kv. are common, 60 kv. being the voltage which is most usually employed and the voltage supply equipment is normally designed to supply this voltage without capacity for regulation. Many of the desirable metallic paints will short out at this standard voltage and, indeed, will continue to short out until the voltage is lowered below 35 kv., and even below 25 kv. Now, if one could avoid the shorting problem by decreasing voltage a small amount, e.g., from 60 kv. to 40 kv. or higher, then the decrease in transfer efficiency might be tolerable, since for this range of applied voltage, the decrease in transfer efiiciency is a modest one. However, further losses of transfer efliciency with further decrease in applied voltage becomes intolerable making it necessary to find other techniques to permit efficient electrostatic spray operation.

It is also known that electrostatic spray efi'lciencies are affectedlby the conductivity of the paint being sprayed since paints which are excessively conductive do not respond well. In the electrostatic mechanical spray system under consideration and applying the standard kv. potential to paints free of metallic pigments, it is noted that when the paint conductivity is high enough to cause a portion of the charge to drain off to ground through the.

paint to reduce the voltage on the electrode, that small reductions in electrode voltage, e.g., from 60 kv. to 50 kv. are accompanied by a rapid reduction in transfer efficiency. Using a conventional paint which will accept 60 kv., one can thin to a given viscosity using a highly conductive solvent such as nitroethane, or a nonconductive solvent such as xylene or mineral spirits. When the paint is thinned with nonconductive solvent and the voltage reduced to 50 kv. by operation of a variable power control on the power supply, the reduction from 60 kv. to 50 kv. is accompanied by only a small reduction in transfer efiiciency. Using conductive solvent such as nitroethane to reduce the electrode voltage to 50 kv. causes a significantly greater reduction in transfer efficiency.

Accordingly, there is good reason to avoid the incorporation of conductive solvents and additives to the liquid continuum of a paint system when such incorporation leads to an increase in the conductivity of the paint continuum which causes a significant drain-off to ground of the applied voltage. The term drain-off is used as a term of convenience to denote a lossof voltage due to current flow through the paint.

Curiously, and preferred practice of the present invention is based upon this finding, when metallic paints comprising metal particles suspended in an organic solventbased paint continuum and which have a pronounced tendency to short out at voltages which are well below standard electrostatic spray voltages (4O kv. or lower) arelmodified to increase the conductivity of the paint continuum so that about half or more of the applied voltage drains off to ground through the conductive paint, that the problem of shorting out may be avoided by the so-lowered electrode voltage. Moreover, the transfer efficiency obtained by lowering electrode voltage in this manner is markedly superior to the transfer efficiency obtained by adjusting the power supply to produce the same lowered electrode voltage and without increasing the conductivity of the paint continuum, even when equipment to control the power supply is available.

There is presented herewith a graph which shows the improved deposition efliciency obtained by the invention.

The graph was obtained by taking numerous commercial metallic paints which tend to short out at 60 kv. and thinning these to desired spray viscosity with xylene, and then spraying these using a variable power control on the power supply to reduce the electrode voltage until shorting out was avoided. The same commercial metallic paint was then thinned to substantially the same spray viscosity using a mixture of xylene and a conductive agent such as nitroethane to obtain a more conductive paint producing a drain-off of voltage during use which lowers the electrode voltage from about 60 kv. to about the same level which avoided shorting out when the variable power control was relied upon. In each instance, tests were run to determine how much paint was deposited on a target under given conditions and, using power control variations as a standard, the percent of improvement or lack of improvement was calculated and graphed against the electrode voltage which pertained in the comparison.

The curve which is presented represents an average of many runs with many different paints.

In these tests, a specific grounded target is selected and exposed to the spray for a given period of time, after which one measure the weight of. material which is deposited upon the target. It will be understood that the exact extent of improvement will vary with the specific test procedure adopted, just as it will in actualpractice; but the fact of improvement is essentially independent of details of the test procedure. Accordingly, and while the specific percentage of improvement reported may have little meaning in an actual situation, any positive percentage is indicative of improved electrostatic deposition, and the greater the percentage, the greater is the extent of improvement. In other words, if a given test shows a 10% improvement by the use of the invention, this would not determine whether a 5, 10, or 15% improvement in deposition would be obtained in any given industrial situation, though it would determine that an improvement would be obtained.

The marked improvement in deposition efficiency up to about 35 kv. should be self-evident from the graph, especially in the range of from 10-25 kv. which represents paints more poorly sprayable by the mechanicalelectrostatic system under consideration.

From the standpoint of safety and reliability, the provision of equipment on the power supply capable of regulating the level of the electrostatic potential which is generated is not desirable. In such instance, the present invention is useful to avoid shorting out even when the electrode potential at which this valuable result is obtained is above 35 kv. or close to that potential which is normally generated. Thus, and despite the fact that there is a direct technique of somewhat superior efficiency possible, the fact that it is seldom available makes the invention of value even when a small reduction in electrode potential is all that is necessary.

Also, and from the standpoint of equipment, it should be observed that using more conductive paints in accordance with the invention causes a steady drain-off of voltage through the column of paint whenever voltage is applied. When the voltage is only applied while spraying takes place, the problem of overheating resistive components of the gun is not significant. Of course, if no at rangement is made to cause the applied voltage to be turned on andoif with the mechanical spray means, then care must be taken to manually correlate the imposition of electrical potential with mechanical spraying or resistive components capable of withstanding higher temperatures should be used or heat-dissipating means employed. Any combination of these mechanical techniques to avoid overheating may be used as desired.

It should be understood that increasing the conduc tivity of the paint continuum to cause a partial drain-off of electrode potential may cause some lowering of the electrode potential at which shorting out takes places, but this effect is small and does not significantly detract from the value of the invention. In some instances the opposite eifect may be noted and this is advantageous when it takes place.

As should be evident from the. foregoingflhe present invention is based upon a phenomenon associated with the interaction between an electrostatic field established by the potential on an electrode, a zone of spray particles in a state of high spatial concentration and which are to be charged by the electrostatic field and the capacity of a column of paint to drain off part of the potential on said electrode by virtue of the conductivity of its liquid continuum. While the precise nature of this interaction may not be fully apparent, it is clear that the specific nature of the agent which is dissolved in the organic solvent continuum of the paint to provide the desired increased conductivity is not material so long as it is not injurious to the performance of the paint.

Thus, any polar solvent may be used so long as it is compatible with the organic solvent continuum of the paint. Similarly, any agent which ionizes to increase conductivity may be used so long as it is compatible with the organic solvent continuum of the paint and effective to cause an increase in electrical conductivity. Surface active agents, either anionic or cationic, are effective for this purpose.

Other polar solvents are represented by 2-nitropropane, nitromethane, methyl ethyl ketone, acetone, acetonitrile, butyl acetate, butanol, etc. Cationic surface active agents which may be used are illustrated by quaternary and diquaternary ammonium halides having the formula:

in which R is preferably a hydrocarbon radical containing from -22 carbon atoms and X is a divalent hydrocarbon chain of from 1-10 carbon atoms (preferably CH CH CH A partirularly preferred cationic surface active agent is N-tallo-N,N-dimethyl, N,N',N-trimethyl-1,3-propylene diammonium chloride.

Appropriate anionic surface active agents are alkyl aryl sulfonates and phosphonates and the amine salts thereof such as the propyle amine salt of dodecyl benzene sulfonic acid.

It should also be understood that the paints which are the subject of the invention are those which become excessively conductive when subjected to a unidirectional electrostatic potential of the level of magnitude contemplated herein and by virtue of highly conductive metallic particles suspended therein which become oriented by the electrostatic forces. Thus, the excessive conductivity of these paints is not a characteristic of the paint continuum in the absence of the metallic particles.

Example The invention is illustrated by the specific example which follows in which a conventional long oil alkyd is pigmented with conventional aluminum paste to provide a metallic paint which presents the shorting out problem of the invention.

A raw paint is provided by intimately admixing: (1) 60.6 parts by weight of a long oil soya alkyd resin formed by reacting 1 mol of phthalic anhydride with 1.25 mols of glycerin and 1.75 mols of soya oil fatty acid; (2) 29.1 parts by weight of xylene; and (3) 10.3 parts by weight of a commercial aluminum paste constituted by a dispersion of nonleafing aluminum flake in hydrocarbon naphtha having a flash point of 105 C. and a Kau-ri-butanol value of 60, the paste containing 65% by weight of the flake aluminum. The nonleafing aluminum flake possesses a water covering power of 25,000 square centimeters per gram.

1000 cc. of this raw paint are thinned to 19 seconds (Zahn No. 2 Cup at 80 F.) with 400 cc. of xylene. The above paint thinned with xylene as indicated can be electrostatically sprayed without danger of shorting out with an available electrode voltage of 27.3 kv. The same raw paint, when thinned to 18.5 seconds (Zahn No. 2 Cup at 80 F.) by combining 1000 cc. of raw paint with 450 cc. of nitroethane and 30 cc. of the propylamine salt of dodecyl benzene sulfonic acid, provides sufiicient conductivity to cause an automatic reduction in the electrode voltage to 29.0 kv. from a 60.3 kv. power supply and was sprayable without danger or shorting out at this electrode voltage.

An improvement in deposition efliciency of 16.8% is achieved by using increased paint conductivity to provide automatic voltage reduction in comparison with the utilization of a variable power supply to provide a similar lowered electrode voltage to permit the spraying of the less conductive paint thinned with xylene.

From the standpoint of completeness, the two experiments referred to above were performed using a commercially available electrostatic spray gun available from the Ransburg Electro-Coating Corporation and identified as Ransburg 301-E R-E-A Gun with a Ransburg 229F power supply. The R-EA Gun referred to hereinbefore is constructed in accordance with the teachings of said copending application Serial No. 60,657, now Patent No. 3,169,882.

It should be observed that the paint rendered conductive is sprayed at an electrode voltage of 29.0 kv., whereas the paint of lesser conductivity is sprayed at 27.3 kv. Aside from the fact that this small increase in electrode voltage would not account for the approximately 17% improvement in deposition efiiciency which is obtained, the reason why the nonconductive paint is not sprayed at 29.0 kv. is because it tends to short out at this higher electrode voltage. As a result of this, it is necessary to run it at the highest voltage possible in the absence of shorting out, e.g., at the 27.3 kv. value reported hereinbefore.

As has been observed previously, in some instances, the paints modified for increased conductivity will spray without shorting out at a higher electrode voltage than the unmodified paints, and sometimes at a lower electrode voltage than the unmodified paint. In either event, the present invention provides improved operation. The improvement is somewhat greater where, as in the present example, modification of the paint for increased conductivity permits application of a higher electrode voltage without shorting out.

The invention is defined in the claims which follow.

I claim:

1. In a coating process in which a liquid organic solvent-based paint is mechanically atomized to form spray particles and in which the material being sprayed is in a state of high spatial concentration when passed through a zone in which the atmospheric ions are in a state of high spatial concentration to cause the spray particles to become electrostatically charged to thereby improve the efiiciency of deposition of the paint being sprayed upon a target, said paint containing electrically conductive metallic particles dispersed in an organic solvent-based liquid continuum and said particles tending to become oriented to cause the electrostatic potential generating said atmospheric ions to short out through the paint and the electrostatic potential used being sufficient to short out through said paint, the improvement which comprises incorporating in said liquid continuum of said paint an agent which increases the conductivity thereof to cause said continuum to become sufficiently conductive so that a portion of said electrostatic potential drains off through the paint to thereby lower the electrostatic potential to a level incapable of causing said orientation of conductive particles and the shorting out incidental thereto.

2. The improvement recited in claim 1 in which said agent is a highly polar organic solvent.

3. The improvement recited in claim 1 in which said agent is an ionic surface active agent.

4. The improvement recited in claim 1 in which at least about half of the electrostatic potential which is supplied is drained oflf through said paint.

5. In a coating process in which a liquid organic solvent-based paint is mechanically atomized to form spray particles and in which the material being sprayed is in a state of high spatial concentration when passed through a zone in which the atmospheric ions are in a state of high spatial concentration to cause the spray particles to become electrostatically charged to thereby improve the efficiency of deposition of the paint being sprayed upona target, said paint vcontaining'electrically conductive metallic particles dispersed in van organictsolvent-based liquid continuum and saidtpart-icles tending to become oriented to cause electrostatic potentials up to about 40 -kV. generating said atmospheric ions to-short out through the paint, the improvement which comprises incorporating in said liquidcontinuum-of said paint an agent which increases the conductivity thereof to cause said continuum tobecome sufi-iciently conductive :so that a portion of said'electrostatic potential drains offthrough the paint to thereby lower the electrostatic potential to a level of less than about 35 kv., said level being incapable of causing said orientation of conductive particles and the shorting out incidental thereto.

6. The improvement recited in claim 5 in which particles tend to become oriented to cause electrostatic potentials up to about 30 kv. generating said'atmospheric ions References Cited UNITED STATES iPATENTS 3,129,112 4/1964 Marvin 117-934 3,130,066 4/1964 Brady 11793.43 3,210,316 10/1965 'Merck et a1 ll793.4 X 3,251,551 5/1966 Walberg 117-934 X 15 ALFRED L. LEAVITT, Primaly Examiner.

A. GOLIAN, Assistant Examiner.

Claims (1)

1. IN A COATING PROCESS IN WHICH A LIQUID ORGANIC SOLVENT-BASED PAINT IS MECHANICALLY ATOMIZED TO FORM SPRAY PARTICLES AND IN WHICH THE MATERIAL BEING SPRAYED IS IN A STATE OF HIGH SPATIAL CONCENTRATION WHEN PASSED THROUGH A ZONE IN WHICH THE ATMOSPHERIC IONS ARE IN A STATE OF HIGH SPATIAL CONCENTRATION TO CAUSE THE SPRAY PARTICLES TO BECOME ELECTROSTATICALLY CHARGED TO THEREBY IMPROVE THE EFFICIENCY OF DEPOSITION OF THE PAINT BEING SPRAYED UPON A TARGET, SAID PAINT CONTAINING ELECTRICALLY CONDUCTIVE METALLIC PARTICLES DISPERSED IN AN ORGANIC SOLVENT-BASED LIQUID CONTINUUM AND SAID PARTICLES TENDING TO BECOME ORIENTED TO CAUSE THE ELECTROSTATIC POTENTIAL GENERATING SAID ATMOSPHERIC IONS TO "SHORT OUT" THROUGH THE PAINT AND THE ELECTROSTATIC POTENTIAL USED BEING SUFFICIENT TO "CHORT OUT" THROUGH SAID PAINT, THE IMPROVEMENT WHICH COMPRISES INCORPORATING IN SAID LIQUID CONTINUUM OF SAID PAINT AN AGENT WHICH INCREASES THE CONDUCTIVITY THEROF TO CAUSE SAID CONTINUUM TO BECOME SUFFICIENTLY CONDUCTIVE SO THAT A PORTION OF SAID ELECTROSTATIC POTENTIAL DRAINS OFF THROUGH THE PAINT TO THEREBY LOWER THE ELECTROSTATIC POTENTIAL TO A LEVEL INCAPABLE OF CAUSING SAID ORIENTATION OF CONDUCTIVE PARTICLES AND THE SHORTING OUT INCIDENTAL THERETO.

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