US3645872A - Process for depositing organic coatings - Google Patents

Abstract

A PROCESS FOR DEPOSITING ORGANIC COATINGS UPON METALLIC OR OTHER CONDUCTIVE SUBSTRATES WHEREIN THE ORGANIC SUBSTANCE IS A DISPERSION OF INCOMPLETELY, PARTIALLY OR COMPLETELY POLYMERIZED MATERIAL IN AN AQUEOUS ELECTROLYTE. THE SUBSTRATE IS CONNECTED AS AN ANODE OR A CATHODE IN THE DISPERSION AND JUXTAPOSED WITH A COUNTERELECTRODE. AN ELECTRICAL POTENTIAL IS APPLIED AND THE CONDUCTIVITY OF THE DISPERSION IS INITIALLY ESTABLISHED AT 5,000-100,000 UMHO-CM.-1 AT 20*C. THE DISPERSION INCLUDES ONE OR MORE ORGANIC SUBSTANCES WHICH DEPOSIT UPON THE SUBSTRATE UNDER THE INFLUENCE OF THE ELECTRICAL POTENTIAL SUCH THAT WITHIN 25 SECONDS AFTER THE CURRENT-DENSITY PEAK DEVELOPS, THE ELECTRICAL RESISTANCE AT THE DEPOSITION-RECEIVING ELECTRODE INCREASES TO REDUCE THE CURRENT DENSITY TO A LEVEL BELOW 20% OF THE PEAK THEREOF. PREFERABLY, THE CURRENT DENSITY IS REDUCED BELOW 20% OF ITS PEAK LEVEL WITHIN 10 SECONDS AFTER THE CURRENT DENSITY PEAKS.

Description

United States Patent 01 fice 3,645,872 Patented Feb. 29, 1972 3,645,872 PROCESS FOR DEPOSITING ORGANIC COATINGS Jiirgen Weigel, Illingen, Germany, assignor to ECM Gesellschaft fur Elektrochemische Materialbeschichtung mbH & Co. KG, Frankfurt am Main, Germany No Drawing. Filed Jan. 14, 1969, Ser. No. 791,160 Claims priority, application Germany, Jan. 17, 1968, P 16 46 037.0 Int. Cl. B01k 5/02; C23b 13/00 US. Cl. 204-181 20 Claims ABSTRACT OF THE DISCLOSURE A process for depositing organic coatings upon metallic or other conductive substrates wherein the organic substance is a dispersion of incompletely, partially or completely polymerized material in an aqueous electrolyte. The substrate is connected as an anode or a cathode in the dispersion and juxtaposed with a counterelectrode. An electrical potential is applied and the conductivity of the dispersion is initially established at 5,000l00,000 ,amho-cmr at 20 C. The dispersion includes one or more organic substances which deposit upon the substrate under the influence of the electrical potential such that within 25 seconds after the current-density peak develops, the electrical resistance at the deposition-receiving electrode increases to reduce the current density to a level below 20% of the peak thereof. Preferably, the current density is reduced below 20% of its peak level within seconds after the current density peaks.

The present invention relates to an improved process for the electrochemical deposition of organic coatings, from an aqueous dispersion on conducting materials, on a substrate.

In my copending application Ser. No. 373,970 filed June 10, 1964, now US. Pat. 3,424,663, I have described and claimed a process for the electrophoretic deposition of an organic material, especially a synthetic resin, upon a metallic body wherein the resin is dispersed in an aqueous medium to form negatively polarized particles which are attracted toward a surface of a metallic substrate adapted to receive the deposit and immersed in the electrolyte and poled anodically with respect to a counter-electrode. In order to prevent the interaction of the resin particles with the metal anions released by the anode into the solution, the cations released by the metallic surface are neutralized or reversed in polarity to anions by interaction with a complexing agent having the necessary aflinity for the metal involved. The synthetic-resin particles are solids of one or more high-molecular-weight polymers formed by addition, substitution or condensation polymerization, while the liquid medium may further include conductive particle's deposita-ble on the substrate concurrently with the synthetic-resin particles so that the layer does not act exclusively as an insulator terminating electrophoretic current flow. In other words, these conductive particles insure the maintenance of the electric-current flow between the collecting surface of the substrate and the liquid electrolyte medium or phase. It may be noted that these particles can be ionic particles whose effective electrical charge has been reversed in polarity by complexing or complex formation. The substrate, according to the principles of the invention, described in this application,

can be composed of a metal which is anodizable concurrently with deposition of the synthetic resin so that an oxide layer is formed on the collecting surface simultaneously with the deposition of the synthetic resin. This arrangement is especially advantageous for aluminum substrates since the oxide layer may function in part as a complex former to tie up metal ions as noted earlier. The anodizing step may also impart the desired color to the substrate. Additionally, the synthetic-resin dispersion can include particles of a coloring agent or pigment which are deposited as part of the resin layer. When the substrate is rendered anodic, as is usually the case because of the generally negative charge carried by resin particles, an anionic dyestuif may be employed; cationic dyestulfs can be used when the substrate has a negative charge.

In my subsequent application Ser. No. 745,371, filed July 17, 1968 as a continuation-in-part of application Ser. No. 373,970, and also copending herewith, but since issued as U.S. Pat. 3,497,440 I have described a further improvement in the coating of metals with organic materials. -In this system, one or more organic surfactants are solubilized in the electrochemical medium and the polarity across the electrodes is chosen so as to be opposite the polarity of the lange-molecular portion of the solubilized surface active agent on the substrate. I have found that it is thus possible to deposit upon the substrate from solution a coating of a surface-active agent 'which appears to bond chemicals to the metallic surface and provide selfcontained anticorrosion protection or substitutes a base for the deposition of other materials, e.g. the electrophoretically deposited resins mentioned earlier. The surfaceactive agent is baked onto the substrate at a temperature well above ambient, preferably C. to 200 C. Depending upon the polarity of the substrate, I deposit a surface-active agent of opposite polarity, e.g. anionic or cationic surface active agents. Among the anionic surfactants or anionic-active surface-active agents I have found most satisfactory are the fatty-acid condensation products, salts of alkyl sulfosuccinates, salts of ricinus oil-sulfuric acid half ester, alkyl sulfates, alkyl phosphates and alkyl or alkyl aryl 'sulfonates. Among the cationic or cation-active surface-active agents, I have found that best results are obtained with alkyl pyridinium salts and alkyl ammonium salts.

Numerous attempts by others have been disclosed to electro-chemically deposit organic materials, such as elastomers including natural and synthetic rubber, synthetic and natural resins and polymeric plastic materials, from aqueous dispersions on metals or other electrically conducting articles. Of these aqueous coating compositions, those containing dissolved synthetic resins have been used to an increasing extent in recent time. Synthetic resins which contain numerous acid groups in their molecular structure and which have been neutralized or rendered slightly alkaline with ammonia and/or amines so as to be soluble, have proved particularly satisfactory as syntheticresin binders (electrophoretic painting).

In the prior-art processes, the organic coating is mainly deposited by electrophoretic techniques. To this end, the electrical conductivity of the suspensions, emulsions or solutions of the material to be deposited is kept as low as possible. In the known processes, the electrical conductivity of the baths is generally in the range of about 400-2000 micro-mho-cmf imho-cmf The electrophoretic deposition of paint is often adversely affected when the bath has a higher conductivity; e.g., pinholes may be formed in the paint film. For this reason, special process measures are often adopted to prevent an ingress of electrolytes into the bath.

These prior-art procedures are often unsatisfactory. For instance, the films of plastics materials and synthetic rcsins which are electrochemically deposited often do not have an adequate bond strength and adhesion. In many cases, the material is deposited in sponge form. Relatively long treatment times, of several minutes or more, are required in most cases for a deposition of the desired coatings. Another disadvantage is the sensitivity of the elecrophoretic coating baths to pH changes so that special supervision is required.

It is an object of the present invention to provide an improved method of depositing organic coatings upon metallic and other conductive substrates which will obviate the aforementioned disadvantages and yield more economical coatings of improved quality with less supervision than has been the case heretofore.

A further object of the instant invention is to extend the principles originally set forth in the above-identified copending applications to further improved systems for the coating of metallic substrates with organic layers strongly adherent to the substrates.

Another object of my invention is the provision of an improved method of rapidly coating substrates with organic layers.

It has now been found that firmly bonded organic coatings of improved quality can be electrochemically deposited in very short treatment times on electrically conducting articles, particularly metals, if the electrically conducting article is connected as an anode or cathode and immersed in an electrolytically acting, aqueous dispersion of partly and/or completely reacted syntheticresin materials and an electric potential is applied to the articles. According to the invention,'the dispersion is adjusted to have an electrical conductivity of 000100,000 (5 X -10 preferably 10,000-30,000, micro-mho-cmf at C. and has added to it one or more organic substances which are depositable on the deposition-receiving electrode in response to the application of an electric potential and within seconds after the current-density peak is reached develops an electrical resistance on the deposition-receiving electrode so as to reduce the current density below 20% of the peak current density.

It has been found to be particularly advantageous to ensure that the current density is reduced below 20% of the peak current density within 10 seconds, preferably within a period less than 5 seconds, after the curerntdensity peak is reached. The current density should suitably decrease to or below 500 milliamperes per square decimeter (ma./dm.

The synthetic-resin materials contained in the aqueous dispersions which are used in accordance with the invention may consist of any desired polymeric natural substances or modified polymeric natural substances, synthetic (addition) polymers or copolymers, polycondensates or polyadducts or mixtures thereof. It is not required to use completely reacted products which contain virtually no reactive groups. Partly reacted products may also be used. These include synthetic-resin materials which still contain in the molecule a relatively large number of reactive groups, such as carboxyl, carbonyl, hydroxyl, amino or epoxide groups, or contain reactive multiple bonds or chemical-bonding sites. Suitable synthetic-resin materials include polyalkylenes, polystyrenes and their copolymers, polyvinyl-chloride, polyvinylidenechloride, fluorine-containing polymers, polyvinylacetates, polyvinylethers, polyvinylcarbazoles, polyacrylates, polyesters, alkyd resins, polyamides, polyamines, polyurethanes, polycarhonates, epoxide resins, formaldehyde resins and silicone resins.

The selection of the synthetic-resin materials to be used will essentially depend on the nature of the electrically conducting material, the purpose for which the coat- 4 ings are intended and the desired technological properties of the deposited film. The synthetic-resin materials used in the dispersion preferably have acid number below 10. Polystyrene and/ or copolymers containing polystyrene, as well as polyacrylate resins, have proved particularly suitable.

The synthetic-resin material is preferably selected or prepared such that the sintering temperature of the material lies below the boiling temperature of the aqueous dispersion. In many cases I may reduce the sintering temperature of the plastics materials used in the dispersions, for example with the aid of specific plasticizers. The boiling point of the dispersion may also be increased by intro.- duction of additives, such as polyols. The use of syntheticresin materials having a sintering temperature below the boiling point of the dispersion is desirable because the bath conditions used according to the invention is characterized by a distinct temperature rise in the boundary layer which adjoins the electrode to be coated. As a result the coatings being deposited are sintered to coherency and are baked onto the substrate or previously deposited layers as they are built up.

Powdered solid synthetic-resin materials may be used in making the dispersions. The particle size should be in the range of about 0.1-10 microns. It is also possible to use polymer dispersions which are formed in the production of synthetic-resin materials; thus the solids formed during suspension polymerization may be employed directly, i.e. without recovering from the reaction medium. The concentration of synthetic-resin material in the aqueous dispersion may vary within a wide range. A solids content as low as 1 gram per liter in the dispersion has been found basically to sufiice. However, solids concentrations in excess of 10 grams per liter are preferred. The concentration may be increased up to about 60%. Excessively high solids concentrations may involve undesirably high losses as the workpieces are removed.

The synthetic-resin materials which are employed must be maintained in the aqueous medium in a dispersion which is as stable as possible. The usual physical and chemical measures may be adopted toward that end.

As noted earlier, the dispersion of synthetic-resin material which is employed is adjusted according to the invention to an electric conductivity of 5000100,000 micro-mho-cmr at 20 C. The conductivity is preferably established at l0,00030,000 micro-mho-cm.- 'which yields a particularly good and fast coating action. All substances which dissociate in the dispersion can be used to elfect this adjustment, provided that they are compatible with the constituents of the dispersion and do not adversely affect the stability of the dispersion or the electrode material.

It has been found to be particularly desirable to establish the required conductivity at least in part with dissociating substances which react with the metal ions that enter into solution at the anode to form a reaction product consisting of a neutral compound or an ion having the same polarity as the particles of syntheticresin material which are deposited. Suitable dissociating substances for adjusting the required electric conductivity include soluble carbonates, phosphates, chromates, halides, cyanides and salts of low organic monoor polycarboxylic acids. Measuring instruments which are on the market may be used to measure the electrical conductivity. In such measurements the electrical resistance of the aqueous medium is determined with the aid of a Wheatstone bridge and alternating current.

As has been mentioned above, another feature according to the invention resides in that the dispersion contains one or more organic substances, which are depositable on the deposition-receiving electrode in response to the application of an electric potential and within 25 seconds after the current density peak build up an electrical resistance on the deposition-receiving electrode so as to reduce the current density below of the peak current density.

This requirement is met by organic substances which dissociate in the aqueous dispersion and under the conditions of use are capable of forming a passive layer on the deposition-receiving electrode within the very short time. Anionic, cationic and amphoteric organic compounds having surface-active properties have proved particularly desirable (see my application Ser. No. 745,371 U.S. Pat. 3,497,440. The selection will mainly depend on the nature of the material to be coated and partly on the syntheticresin material which is used. It is easy to determine in a preliminary test whether a given substance is suitable for a certain electrode material. To this end, one ascertains whether the substance in an aqueous medium develops a high electrical resistance on the tested electrode material within the short time required when an electrical potential at a constant voltage is applied. The use of dissociating compounds having surface-active properties has the advantage that the ions formed in the dispersion at least in part will be located on the synthetic-resin particles and impart to the latter a certain electric charge.

Organic substances which are suitable for the purposes of the invention include alkali soaps, amine soaps, alkyl sulfonates, alkyl sulfates, aryl sulfonates, alkylaryl sulfonates, alkali salts and ammonium salts of esters of sulfosuccinic acid, alkali and ammonium salts of sulfuric acid esters of castor oil, fatty acid condensates, casein, gelatine, lactalbumin, zein and gum arabic, provided that they met the above-mentioned requirements.

Amphoteric compounds, such as casein, gelatine, zein or their derivatives, and anion-active sulfuric acid esters of castor oil, have proved particularly suitable for use in the coating of aluminum as a cathode. Proteins are preferable for coating of iron as a cathode. Casein may be added for the anodically coating of noble metals.

The concentration of the passivating organic additives used according to the invention is generally about 1-10% of the synthetic-resin material which is used. In the case of casein concentrations of 2-10 grams per liter of the dispersion are satisfactory. Alkyl sulfonates or alkylaryl sulfonates may be used in concentrations of 2-50 grams per liter.

To eliminate oxygen which is contained in the water and to combine oxygen or hydrogen which is formed by electrolysis where certain metals are employed, it may be necessary to add acceptors to the dispersion in concentrations of 1-30 grams per liter, e.g. hydrazine as an acceptor for oxygen and peroxides as acceptors for hydrogen.

Inorganic and/or organic coloring matter may be added to the dispersion of synthetic-resin material for a coloring effect. Inherently colored plastic materials may be used in making the dispersion, or the coloring matter may be dissolved in any plasticizer to be added.

Where the dispersions of plastic material are not sufficiently resistant to chemical, physical or bacterial action, the stabilizers known for these purposes in plasticmaterial technology may be added to these dispersions.

The process according to the invention may be used to coat various conducting materials, preferably metal surfaces. The electrochemical deposition may be effected on the anode or the cathode. It has been found that anodizing is more desirable because it results generally in coatings having a higher bond strength.

The coatings deposited by the process according to the invention may be colorless or colored, transparent or opaque. Hence, the process has a wide field of application. The coatings afford a very good protection against corrosion because they have a high bond strength and density. For this reason, the process according to the invention may be used to advantage for coating workpieces which are to be subjected to severe influences of the weather and temperature, such as roofing materials, and aluminum facings for buildings. The coatings will also facilitate the cold-working of high-grade steel. Owing to their high wear resistance and their decorative appearance, transparent or colored coatings may be used on ornamental articles. Coatings having a high electrical insulation characteristic may be made with the aid of suitable synthetic-resin materials.

It is a special advantage of the process according to the invention that the pH value of the dispersion is not limited to a narrow range. Dispersions used for anodizing may have a pH value in a range of 6-13. In the coating of aluminum, best results are obtained with a pH value of 7-11. pH values of 10-12 have proved desirable in the coating of steel and noble metals. Cathodizing may be effected with a pH value of 1-9, preferably 2-5. Owing to the wide range of permissible variation mentioned above, the changes of the pH value in the deposition bath caused by reactions at the electrode generally have no adverse effect.

The conducting workpieces to be coated are introduced into the aqueous dispersions and connected to a current source. In the anodic deposition of the organic coating, the workpiece will be connected as an anode; in the cathodic deposition the workpiece is connected as a cathode. The counterelectrode may consist of the bath tank, or a separate counterelectrode may be introduced into the dispersion.

The bath tanks used in the electrochemical deposition of synthetic-resin materials may be provided with a circulating device in order to ensure an improved wetting of the workpieces immersed into the bath and to dissipate surplus heat from the electrode. A circulation may also be desirable if pigments are added to the dispersions of plastics material.

In the electrochemical deposition of plastics material, the voltage may be maintained in the range of 10-250 volts, preferably 40-80 volts. The peak current densities which occur during the coating operation are usually in the range of 1-10 amperes per square decimeter. Deposition times of 1-25 seconds are sufficient for the formation of a satisfactory layer. Deposition times below 10 seconds or even below 5 seconds are usually sufficient.

Because only very short deposition times are required in the process according to the invention, it may be desirable to apply the electrical potential in intervals of 3-4 seconds in succession to workpieces which are disposed one beside the other or one opposite the other. This practice will enable the use of a power source having a lower current rating because it avoids the simultaneous deposition over large surface areas, which would require a high current value.

The temperature of the deposition baths may be selected as desired within certain limits. The treatment temperatures essentially depend on the stability of the dispersion of synthetic-resin material and the requirements as to the desired thickness of deposited film of the synthetic-resin material. Higher bath temperatures result in a faster sintering of the deposited particles onto the substrate so that thinner coatings are formed. Temperatures below 15 C. are less desirable.

The coatings deposited on the workpieces which have been removed from the deposition bath are non-marring. This is particularly desirable in the continuous coating of tubing or wire, which is mechanically guided after it has left the bath.

The coated workpieces are generally rinsed with water to remove adhering residues of the bath. To avoid formation of crusts by drying electrolyte residues, e.g., where hard rinsing water is employed, it is recommended to spray the pieces finally with de-ionized water.

If the deposited coating has not yet been fully cured, after-curing may be required and maybe effected, e.g. by baking at workpiece temperatures of -200 C. The baking conditions will depend on the nature of the synthetic-resin material which is employed. The baking time is generally about 3-30 minutes. For certain syntheticresin materials, however, a full curing may be effected in much shorter times. The coating may also be cured by different methods known in the art, e.g. by a treatment with electrode rays, chemical curing treatment, etc.

8 EXAMPLE III A dispersion was prepared as in Example H but with out the addition of diphenoxyethylformal. The sintering temperature of the styrene-acrylonitrile copolymer was The dispersions used in the process according to the p l invention must be replenished as required to ensure a 140 grams per 112131901 T' added a pagrt of uniform quality of the deposited coating even when dep- PP W to Prof/Ids a dlsperslon F hav1 ng hlghet osition-receiving material has passed through the bath for bolhn'g pomt- Alllmmflm sheets were coated dlspersfon a relatively long time 10 E and F as descrlbed in Example II. pnly a non-adhering powder remained on the surface which had been treated EXAMPLE I in dispersion E. The use of dispersion F resulted in a smooth, pore-free and firmly bonded coating having a An aqueous dispersion A was made which contained thickness of about 12 microns. 10% b Weight styrene-acrylonitrile copolymer (particle size 0.210 microns, specific gravity 1.1 grams per 15 EXAMPLE IV cubic centimeter), 12% by weight diphenoxyethylformal An aqueous dispersion G was Prepared, Which com (bolhng range Q 9 grams Per hter tngly tained 12% by weight polymethacrylic ester (particle size col and 2 grams per liter caseln. The pH value of the 0.l-0.5 micron specific gravity 1.08 gram cubic centidlspersion was ad usted to 12 w th triethylamme. The meta. acid number 0), 8% by weight dibutylphthalate sintering temperature of the plasticized res n was about (boiling range 160473 C) 5 grams per liter Sodium T electncal conductolvlty of the dlsperslon was salt of a sulfuric acid ester of castor oil, 6 grams per liter 1500 20 KF and 0.9 gram per liter hydrazine. The pH value of T Provlde a 115136151011 B havmg electrical conduc' the solution was adjusted to 8.0 with ammonia. The sintertfvlty of 6000 20 gram Per ing temperature of the plasticized resin was about 90 11ml 29 P gram hter w C. The specific conductivity of the dispersion was 12,000 part of dispersion A. To provide a dlspersion C having at an electrical conductlviPy of 12,000 at Sheet aluminum was coated in dispersion G as described C, grams P hter Nazcos g 1 g per hter in Example II. The peak current density was 5 amperes KCN adqed to another Part of dlsperslon 30 per square decimeter. 1.5 seconds after the current density Articles having a gold-plated surface andconnected as peak, the current density amountad to 05 ampere per an anode at a D.C. voltage of volts are lmmersed for Square decimeter The coatings wem rinsed and than 1W0 Sscollds into dispersions A, B and Tespsctively, at baked at 200 C. for 15 minutes. The baked coatings 25 C. 182-8 chromium-nickel steel was used as hi3 cathwere smooth and free of pores and had a density of 25 ode. After this treatment, the articles were rinsed with 35 microns, tap water and subsequently with completely deionized EXAMPLE V water and were then heated in an air-circulating oven at 200 C, for 10 inutes, An aqueous dispersion H was prepared, which con- The current density conditions in the bath and the chartained 10% by weight styrene-acrylonitrile copolymer acteristics of the resulting coating are indicated in Table 1. 4 0 (particle size 0.210 microns, specific gravity 1.1 grams TABLE 1 Dispersion Peak current density, ampere/sq. dm 0.3 1.2 3.0. Current density 1 second after peak, About 0.2... About 0.2"-.. About 0.5.

ampere/sq. dm. Smooth and Coating Peels when Smooth and free of pores.

rinsed. free of pores. Thickness of coating, microns About 65 About 15.

EXAMPLE II per cubic centimeter), 10% by weight diphenoxyethylformal (boiling range 190-200 C.), 2 grams per liter aqueous dlspe'rslon D was PreParefL Whlch casein, 5 grams per liter NaOH, and 2.7 grams per liter tamed by Weigh? styreneacFylommlF copolymer hydrazine. The sintering temperature of the plasticized ti l 3116 01-10 microns, speclfic gravity 1.1 grams resin was about 70 C. The dispersion had a pH value of P I cubic centimeter), 12% y Welght diphenoxyethyl' 10 and an electrical conductivity of 12,000 micro-inhoformal (boiling range 190-200 C.), 10 grams per liter -1 at triglycol, 2 grams P liter s 5 grams P lite! KF RRSt 1405 m. sheet steel (DIN 1623) was anodized in and gram P liter hydrazme- The P Value of the dispersion H for 2 seconds using a cathode of 18:8 dispersion was adjusted to 8 With ammsnia- The sintering chromium-nickel steel at a D.C. voltage of 80 volts and temperature Of the plasticized resin was about 70 The a bath temperature of 25 C. The peak current density fllectric Conductivity of the dispersion Was 12,000 micfowas 4 amperes per square decimeter. 1.5 seconds after the mho-cm. at 20 C. current density peak, the current density had dropped Sheets of A1 99.5 aluminum (99.5% purity) were con- 65 to 5 ampere per Square d i nected as an anode at a D.C. voltage of 80 volts and When rinsed and baked for 10 minutes at 200 C., immersed into dispersion D at 25 C. for a duration of the plates had pore-free, smooth, firmly bonded coatings 2 seconds. The cathode consisted of 18.8 chromiumhavingathickness of 14 microns. nickel steel. The peak current density was 4 amperes per Iclaim: square decimeter. 1.5 seconds after the current density 1. A method of depositing a synthetic resin upon a peak, the current density was 0.5 ampere per square deciconductive substrate, comprising the steps of dispersing meter. The sheets taken from the coating bath were rinsed solid particles of said synthetic resin in an aqueous with tap water and then with completely desalinated medium capable of functioning as anelectrolyte; introwater and subsequently heated in an air-circulating oven ducing said substrate into the dispersion of solid particles at 200 C. for 10 minutes. The sheets had firmly bonded, and applying an electric potential of 10 to volts across smooth, pore-free coatings in a thickness of 7 microns.

said substrate and a counterelectrode in contact with the bath to electrophoretically deposit the solid particles of the synthetic resin in a layer on said substrate; and adjusting the composition of the dispersion to an initial electrical conductivity of 5000 to 100,000 micromhocmr at 20 C. and such that the layer formed on said substrate increases in resistance to reduce the current density at said substrate to a level below 20% of the peak current density within about 25 seconds after the current density peak develops.

2. The method defined in claim 1 wherein the composition of the dispersion is initially adjusted to a conductivity of substantially 10,000 to 30,000 micromho-cmr at 20 C.

3. The method defined in claim 1 wherein the composition of the dispersion is adjusted such that the current density at said substrate is reduced below 20% of the peak current density within about seconds of the development of the current density peak.

4. The method defined in claim 1 wherein the composition of the dispersion is adjusted such that the current density at said substrate is reduced below of the peak current density within about 5 seconds of the development of the current density peak.

5. The method defined in claim 1 wherein the composition of said dispersion is adjusted such that the current density at said substrate is reduced below 500 milliamperes per square decimeter as a result of the increased electrical resistance of said layer.

6. The method defined in claim 1 wherein said particles have a sintering temperature below the boiling point of said aqueous dispersion.

7. The method defined in claim 1 wherein said synthetic resin particles have a particle size of about 0.1 to 10 microns and are composed of a material selected from the group which consists of polyalkylenes, polystyrenes and their copolymers, polyvinylchloride, polyvinylidenechloride, fluorine-containing polymers, polyvinylacetates, polyvinylethers, polyvinylcarbazoles, polyacrylates, polyesters, alkyd resins, polyamides, polyamines, polyurethanes, polycarbonates, epoxy resins, formaldehyde resins and silicone resins.

8. The method defined in claim 7 wherein the synthetic resin has an acid number below 10.

9. The method defined in claim 7 wherein said synthetic resin is a polystyrene or a copolymer thereof.

10. The system defined in claim 7 wherein said synthetic resin is a polyacrylate.

11. The method defined in claim 7 wherein the composition of said dispersion is adjusted by adding thereto an organic substance other than said synthetic resin.

12. The method defined in claim 11 wherein said organic substance is a surface-active agent and is used in an amount of 1 to 10% by Weight of the synthetic resin.

13. The method defined in claim 12 wherein said organic substance is selected from the group which consists of alkali soaps, amine soaps, alkyl sulfonates, alkyl sulfates, aryl sulfonates, aryl sulfates, alkylaryl sulfonates, alkali-metal and ammonium salts of esters of sulfosuccinic acids, alkali-metal and ammonium salts of sulfuric acid esters of castor oil, fatty acid condensates, casein, gelatin, lactalbumin, zein, other proteins and gum arabic.

14. The method defined in claim 13 wherein said organic substance is casein.

15. The method defined in claim 13 wherein said organic substance is a sulfuric acid ester of castor oil.

16. The method defined in claim 13, further comprising the step of introducing into said dispersion a pigment of dyestulf for incorporation into said layer.

17. The method defined in claim 13, further comprising the step of adding to said dispersion between 1 and 30 g./liter of a hydrogen or oxygen acceptor.

18. The method defined in claim 17 wherein said acceptor is hydrazine or a peroxide.

19. The method defined in claim 13, further comprising the step of baking said layer onto said substrate upon removing same from said dispersion at a temperature of to 200 C. for a period ranging up to 30 minutes and sufiicient to complete curing of the layer.

20. The method defined in claim 13 wherein the composition of the dispersion is adjusted by adding thereto at least one substance selected from the group which consists of water-soluble carbonates, phosphates, chromates, halides, cyanides and salts of low molecular weight monoor polycarboxylic acids.

References Cited UNITED STATES PATENTS 3,428,586 2/1969 Coats 204-181 FOREIGN PATENTS 1,073,965 6/1967 Great Britain 204181 743,070 9/1966 Canada 204181 HOWARD S. WILLIAMS, Primary Examiner