US3855106A - Process for electrodeposition of paint - Google Patents

Abstract

In the electrodeposition of base solubilized, resinous materials from an aqueous dispersion onto a metal, anodic substrate immersed in the dispersion by passing a direct electrical current through the dispersion and substrate, generally speaking there are two stages in the deposition of the film. There is a first stage in which the current density increases due to a fixed or increasing voltage and relatively low film resistance; and a second stage in which the current density tends to decrease because of the increased resistance of the growing film. In accordance with our invention the overall deposition process is improved by maintaining the current density during the second stage of film deposition at a maximum value, predetermined by small scale experiments, by increasing the deposition voltage above the nominal rupture voltage of the film as required to maintain the relatively high current density value, which is a maximum value suitable for obtaining rapid film deposition and good film throw without damaging the film by boiling aqueous liquid or low boiling cosolvents therein.

Description

United States Patent 1191 Campbell et al. I

[11 3,855,106 1451 Dec. 17, 1974 Appl. No.: 366,580

PROCESS FOR ELECTRODEPOSITION OF PAINT Inventors: Gregory A. Campbell, Romeo; I William B. Brown, Birmingham, both of Mich.

57 ABSTRACT materials from an aqueous dispersion onto a metal, anodic'substrate immersed in the'dispersion by passing substrate, generally speaking there are two stages in Assigneei Mntors Corporanon, the deposition of the film. There. is a first stage in Detron, Mlcnwhich the current density increases due toa fixed or Filed: June 4, 1973 increasing voltage and relatively low film resistance;

and a second stage in which the current density tends growing film. In accordance with our invention the [52] U.S.-Cl. 204/181 overall deposition Process is improved by maintaining 51 Int. Cl C23b 13/00 current density during the Second Stage of film [58] Field of Search 204/181 Position at a'maximum value, predetermined by Small scale experiments, by increasing the deposition volt- [56] References Cited age above the nominal rupture voltage of the filmas required to maintain the relatively high'current den- UNITED'STATES PATENTS sity value, which is a maximum value suitable for ob- 3,355,374 I 11/1967 Brewer et al. 204/l8l mining rapid film deposition and good film throw $323"; a I without damaging the film by boiling aqueous liquid or 3:647:658 3/1972 Hofling et a]. 204/l8l low bo'lmg cosolvems therem' 4 Claims, 3 Drawing Figures Primary Examiner-John H. Mack Assistant Examiner-Aaron Weisstuch v Attorney, Agent, or Fir mGeorge A. Grove 9) V o0 U a so 2 60 g 8 I :2 E J '-40 3 g a: 7-20 0 1'0 4b 5b 6b 76 8b 9b I00 I no TIME, SECONDS In the electrodeposition of base solubilized, resinous a direct electrical current through the dispersion and to decrease because of the increased resistance of the mesa-.106

PATENTED DEC] H574 Q mmamiz Ht y 1 TIME SECONDS E om crcmzua ,hzwmmnu 5 D N w, E 5 m m. 4 m 0 u. mm bfimnifi m w m m m .m n I rm t o, .0 I 8 m rm -w -w M -w -m m s 2 v o E F523 #2556 TIME, SECONDS PROCESS FOR ELECTRODEPOSITION OF PAINT This invention pertains to the electrophoretic deposition of base solubilized, polycarboxylic acid resins on electrically conductive, anodic electrode substrates. More specifically, this invention pertains to a method of carrying out such a painting process to achieve better paint coverage without producing imperfections in the paint film, commonly known as rupture, by measuring the current density particularlyduri'ng the later stages of the painting of an article and maintaining the current density at a predetermined maximum value throughout the final buildup of the paint film to a suitable thickness.

The art of painting electrically conductive substrates by electrophoretically depositing pigmented, base solubilized, synthetic resin onto the substrate is now widely practiced. One significant advantage of such a painting process is that, in principle, a uniform film buildup is obtained over a substantial portion of the substrate. This is particularly important for corrosion resistance,

. 2 ing articles to be painted containing an aqueous dispersion of suitable base solubilized, polycarboxylic acid resin and suitable pigments, solvents and the like as may be required for a particular application. Either the painting tank is rendered cathodic in a direct current electrodeposition circuit or special cathodes are placed in the tank. The articles to be painted are immersed in the bath and rendered anodic and a direct current is a I voltage. Initially the current density will increase rapfor example, on automobile car bodies. It is known will rupture to produce holes or nonuniforrnities of thickness in the paint film which either permit corrosion to take place or are unsightly, or both. The nominal rupture voltage varies for each resin system employed and, in fact, also is a function of the composition of the bath in which the resin is dispersed. Some resins have higher rupture voltages than others. Presently, high rupture voltages are considered to be in the range of 300 to 500 volts. However, it is desired to' deposit the paint resin at still higher voltages or to otherwise obtain more complete coverage on a car body or other articles for purposes or corrosion resistance and the like. It is also desirable to accomplish the paint film buildup as rapidly as possible, consistent with good quality paint film formation, so that the residence time of the article to be painted in the paint tank is minimized.

It is an object of the present invention to provide a method of depositing a base solubilized, polycarboxylic acid resin containing paint on an anodic electrode substrate wherein the current density during the deposition, particularly in the final stages thereof, is maximized so as to obtain rapid film buildup and throw, but to avoid film rupture.

It is a more specific object of the present invention to provide a method of electrophoretically depositing suitable base solubilized resins on an electrically conductive substrate wherein the deposition voltage is increased toward the nominal rupture voltage as film buildup occurs and, thereafter, the deposition voltage is further increased as necessary to maintain a predetermined maximum current density which will maximize the rate of paint film buildup and the throw of the film without rupturing or otherwise damaging the film due to boiling of water or cosolvent in the film.

In accordance with a preferred embodiment of our invention, these and other objects and advantages are accomplished by initially providing a tank for immersidly, but as the electrical resistance of the growing film increases, the current density levels off and decreases unless the voltage is further increased. Usually no film rupture occurs during the first stage of the deposition because the film is not thick enough to heat up and cause bath liquids entrained therein to boil. During the last stage of the film deposition process we allow the current density to fall to a predetermined maximum value and thereafter maintain current density at said value for the rest of the deposition period by increasing the deposition voltage as necessary. This maximum current density value is determined experimentally as will be described in more detail by conducting testson a laboratory scale. The maximum current density is that value at which the film can be deposited without heating the film and the liquids entrained therein above the boiling point of the liquids. At such high current density and deposition voltage the film throw and rate of film buildup are maximized.

These and other objects and advantages tion will be more-fully appreciated from a detailed description thereof which follows. In the description reference will be made to the drawings, in which:

FIG. 1 is a graph of current density and film temperature versus time in a specific electrophoretic painting experiment in which rupture occurred in the paint film;

FIG. 2 is a graph of current density and film temperain 'which film rupture did not occur; and

FIG. 3 is a graph of current density and film temperature versus time depicting another embodiment of our invention in an electrophoretic painting process in which film rupture did notoccur.

During the electrodeposition of water-based paints,

v defects are encountered when film rupture occurs. This rupture has been referred to as blowing or gassing of the coating and results when the local current density and film resistance combine to exceed a critical value. The film rupture produces a hole in the paint film or roughness due to differences in paint film thickness. In electrodepositing a particular resin composition at increasing voltages without interruption there is eventually reached a maximum voltage, the rupture voltage, at which the described defects in the film occur. By measuring and following both the temperature of the film as it is deposited and the value of the current effecting the deposition we have found that rupture is caused when the temperature of the film exceeds the of our invenboiling point of the liquid entrained therein. This liquid typically comprises water and low boiling organic cosolvents dispersed therein. When the liquid boils, portions of the film are removed. If the voltage is then quite high, substantial current flow may take place to produce widespread destruction of the film. In our experience rupture usually does .not occur until a large percentage of the film has already been deposited.

In a commercial electrophoretic painting operation, of course, painting baths containing hundreds or even thousands of gallons of paint are employed. During the painting of a single article neither the temperature of the bath nor its resistance to electrical conductivity may change appreciably. However, as the bath ages, its conductivity may change (typically increases) due to ionic impurities which are carried into the bath or fonned therein. As the painting of a particular article begins, the initial film has little electrical resistance because it is very thin. Therefore, there is little film heating due to PR losses. The metallic substrate acts as a heat sink tocool the film. However, as the film increases in thickness, film heating increases and the film and liquids entrained therein can become quite hot in a short period of time if the current flow is not controlled. Heretofore,, such control has been accomplished by maintaining the deposition voltage below the so-called rupture voltage, or by interrupting the current flow altogether so that the film has a chance to cool before further film buildup is attempted.

The following examples will illustrate how we have improved upon prior art electrophoretic painting processes to maximize film throw and rate of deposition while avoiding film rupture due to film overheating an boiling of liquids entrained therein.

A laboratory paint bath was prepared containing a commercial amine solubilized, styrene-allyl alcoholbased alkyd resin body primer pigmented with iron' oxide particles. A conventional throw box was assembled to serve as the article to be electrocoated. This device comprised two zinc phosphated, cold rolled steel panels, l8 inches by 4 inches in dimensions. The edges of the panels were inserted into two rubber gaskets, each running the full length of the panels, so that the panels were held spaced apart from each other by as inch in parallel relationship. This assemblage of panels and rubber gaskets formed a box (throw box) which was open only at the top and the bottom. A thermocouple was applied to the outer surface of one of the panels for the purpose of measuring the terrnperature of the film deposited on the panel. The thermocouple as 0.015 inch in diameter and enclosed in a stainless steel sheath. The throw box was immersed in the paint with only the top portion extending above the bath level. An electric circuit was prepared comprising a direct current potential source, anarnmeter, the phosphated steel panels as anodic substrates, the bath as electrolyte and cathodic electrodes immersed in the 4 a minute. At the completion of the two minute painting operation the throw box was removed from the bath, disassembled and the panels examined. The outer surfaces of the panels were covered with the paint film. The paint film had also been formed on the inside surfaces of the panels to a distance of ll inches up from the bottom edge of the box. The throw was-thus 11. A scratch was made on the panels throughthe paint film to the base metal. The scratched panels were then sprayed for 336 hours with a salt solution. Rusting was observed to have occurred at the. scratch, and rust formed a distance of 7/32 of an inch back under the painted surface. This indicated that the paint filmwas not fully continuous. Some porosity hadapparently been formed in the paint film due to rupture during the painting operation.

A second experiment, similar to that described above, was performed withtwo new phosphated steel panels formed into a throw box, except that in this case an uninterrupted,'constant deposition voltage of 400 volts DC was imposed for 120 seconds. At the completion of the deposition process the panels were disassembled from the box and examined. A paint film throw of 12 inches was obtained which represented an increase of 1 inch over that obtained at a constant deposition voltage of 350 volts DC. However, the panels fared much worse in the salt spray scribe test. Corrosion was observed under the paint film a distance of 15/32 of an inch from the initial scribe. A substantial A third experiment was performed by following a practice in accordance with'our invention. Two new phosphated, cold rolled steel panels were selected and formed into a throw box as described above. A paint bath of the above composition was employed. Electrophoretic deposition was commenced by imposing a voltage of volts DC for about 10 seconds. The current flow and film temperature were monitored during this experiment and the values'are summarized in FIG. 1. The current density is traced as the solid line and the film temperature as the broken line. At 50 volts the current density initially shot up to about 2% amperes bath spaced from the steel panels. The dimensions of the panels were such that the two external surfaces thereof made up a total of one square foot of surface area. Thus an ammeter reading in amperes was in effect a current density reading in amperes/square foot. The rupture voltage of the paint was nominally 375 volts.

A direct current voltage of 350 volts was impressed for 120 seconds. The initial current was over 10 amperes. However, it rapidly decreased to below one ampere within 20 seconds and below ampere in about persquare foot and quickly'fell off in a period of about 10 seonds. to less than one ampere per square foot. As the film thickness increased, so did its electrical resistance. The film temperature at this time was of-the order of 30C. At about 10 seconds from the commencement of painting, the deposition voltage was abruptly increased to volts' DC. The current flow responded immediately to a value of about 3% amperes per square foot and fell off during the next 10 seconds to about one ampere per square foot. After a total of about 20 seconds from the start of painting, the voltage was increased to 350 volts. The current density in-v creased to a value just under 3 amperes per square foot and began to decrease. At about 50 seconds from the beginning of the painting operation the current had decreased to about 0.8 ampere per square foot. From this point on the current density was maintained at 0.8 am-- pere per square foot by gradually further increasing the voltage as required. .The temperature of the film initially climbed to about 40 C. until the plating operation had continued for some 85 seconds. The film temperature then rather abruptly increased to over 100 C., at which point liquids entrained in the film boiled and substantial film rupture occurred. In other words, one could not suitably deposit paint film of this composition at the current density of 0.8 ampere per square foot in the terminal stage of painting without damaging the film produced thereby.

A fourth experiment was performed generally following the electrodeposition procedure described with respect to the third experiment above. The current density' and film'temperature were measured and recorded throughout the deposition. The respective values are summarized in the graph of FIG. 2, with the solid line representing the current density and the broken line indicating film temperature. As in the third experiment, the initial deposition voltage was 50 volts. This was increased to 150 volts after about seconds of film deposition and then to about 350 volts after an additional 10 seconds of film deposition. However, in this experiment the current density was allowed to fall, after the voltage increased to 350 volts, to a value of 0.55 ampere per square foot. It was then necessary to increase the deposition voltage above 350 volts to maintain the current density at 0.55 ampere per square foot. It is noted that the film temperature in this terminal stage of the deposition, from about 50 seconds on, remains below 40 C.

At the completion of the deposition the throw box was disassembled and the two panels examined. The outer surfaces of the two panels were fully coated. The inner surfaces of the panels were coated up from the bottom of the original box to a distance greater than the throw values obtained in experiments 1 and 2 above. Furthermore, when the panels were subjected to the salt spray scribe test as described above, no rust was formed under the paint film adjacent the scratch mark. In other words, the salt spray scribe value was zero.

Thus it is seen that in electrocoating with a resin composition of the type specifically herein employed one can advantageously obtain good film throw and corrosion resistance by maintaining a current density during the last stage of film deposition of 0.55 ampere per square foot or slightly above. However, if one maintains a current density of 0.8 ampere per square foot during this period, film rupture occurs due to boiling of bath liquids in the deposited film. In accordance with our invention one would electropaint with described resin composition in a production scale operation at a current density of about 0.55 to 0.65 ampere per square foot during the last half of the deposition to obtain good film throw and corrosion resistance. As the production scale bath ages itmay become contaminated with electrically conductive chemical species and current densities in this range may then be maintained at slightly lower voltages. If one continued to employ the original higher voltages, film rupture could occur. However, by measuring and depositing at the predetermined current density one can safely obtain good film coverage without risk of film rupture.

In accordance with another embodiment of our invention the deposition voltage can be gradually increased toward the nominal rupture voltage in the first stage of deposition rather than increased stepwise as described above. Current densities and film temperatures representative of this procedure are depicted in -FIG. 3, with the solid line representingcurrent density and the broken line representing film temperature. The deposition voltage is gradually increased to about 350 volts (when using the resin composition identified in the above experiments) during this first 20 to 30 seconds of film deposition. With the increasing voltage the rupture voltage has been reached, the current density is permitted to decrease until it reaches the desired predetermined maximum current density desired for the final stage of the film deposition. At this point the deposition voltage is increased above the nominal rupture voltage as required to maintainthe desired maximum current density for the second stage of the deposition.

In FIG. 3 this maximum current density was about 0.65 ampere per square foot. i

From the foregoing it will be appreciated that in general there are two significant discernible stages in they electrodeposition of water dispersible paint films. In the first stage the voltage may be increased stepwise or gradually, as desired, toward the nominal rupture voltage and the current density will increase and possibly intermittently decrease depending upon how the voltage increase is accomplished. Generally speaking, after about the first S0 to seconds of film deposition .the film thickness will have increased to the point at which further increases in deposition voltage mustbe undertaken with care toavoid film rupture. In accordance with our invention further voltage increases are effected only to maintain a suitable predetermined maximum current density consistent with the objectives of obtaining. maximum film throw and corrosion resistance and avoiding film rupture due to boiling of bath liquids entrained therein. Since the bath liquids usually contain water and low boiling organic cosolvent liquids dispersed therein, it is usually preferred to avoid heating the film above about C. In this'way the formation of small holes in the film due to the evolution of low boiling organics can be avoided and a more corrosion resistant film produced.

Organic resin coating materials which may be em- 5 Such materials may include or be employed with other organic monomers and/or polymers including, but not by way of limitation, hydrocarbons and oxygen substituted hydrocarbons, such as ethylene glycol, propylene glycol, glycerol, monohydric alcohols, carboxylic acids, ethers, aldehydes and ketones. The film-forming material may include or be employed with pigments, metallic particles, dyes, drying oils, etc., and may be dispersed as a colloid, emulsion or emulsoid. Coating materials adapted for anodic deposition mayinclude one or more of the aforementioned resins having a plurality of free carboxyl groups or their equivalent in their'polymeric structure. Dispersion of these resins in water can be efiected by the addition of a suitably basic material, such as ammonia, water soluble amines, mixtures of 7 monomeric and polymeric amines, potassium hydroxide and the like.

While our invention has been described in terms of a few specific embodiments thereof, it will be appreciated that other forms can readily be adapted by one skilled in the art. Accordingly, our invention is to be limited only by the following claims.

What is claimed is:

1. In the method of depositing a base solubilized, resinous, polycarboxylic acid material from an aqueous dispersion thereof onto an electrically conductive, anodic substrate immersed in said dispersion by passing a direct electric current through said dispersion and substrate whereby a resinous film of predetermined thickness is deposited on said substrate, there being two discernible stages in the process of depositing said film: (a) a first stage in which the current density increases due to an initially fixed or increasing voltage, below the nominal rupture voltage of said material, and

relatively low film resistance and (b) a second stage in which the. current densty decreases to a low generally constant value because of increased resistance of the growing film, the improvement comprising maintaining said current density during said second stage of the film deposition process at a predetermined value greater than said low value by increasing the deposition voltage to above said nominal rupture voltage as required to maintain said current density value for a time sufficient to complete the film deposition, said predetermined current density value being a maximum value found experimentally to be suitable for obtaining maximum film throw without damaging the film by boiling aqueous or cosolvent liquid therein.

2. In the method of depositing a base solubilized, resinous, polycarboxylic acid material from an aqueous dispersion thereof onto an electrically conductive, anodic substrate immersed in said dispersion bypassing a direct electric current through said dispersion and substrate whereby a resinous film of predetermined thickness is deposited on said substrate, there being two discernible stages in the process of depositing said film: (a) a first stage in which the current density in creases due to an initially fixed orincreasing voltage, below the nominalrupture voltage of said material, and relatively low film resistance and (b) a second stage in which the current density decreases to a low generally constant value because of increased resistance of the growing film, the improvement comprising maintaining said current density during said second stage of the film deposition process at a predetermined value greater than said low value by increas-- tally to be suitable for obtaining maximum film throw without heating the deposited resin film above C. a

3. A method of depositing a base solubilized, resinous, polycarboxylic acid material from an aqueous dispersion thereof onto an electrically conductive anodic substrate,

. said resin having a predetermined,

voltage, comprising immersing a said substrate in-the aqueous resin dis- 1 persion,

passing a direct electric current through said dispersion and substrate at a first voltage value below said nominal rupture voltage for a first period of seconds to commence deposition of a resin film on said substrate, the current density initially increasing abruptly and thereafter gradually decreasing,

increasing the deposition voltage toa second value greater than said first value but less than said nominal rupture voltage and continuing to deposit said resin at said voltage for a second period of seconds, the current density again increasing abruptly and thereafter gradually decreasing,

increasing the deposition voltage to about said nominal rupture voltage while said current density abruptly increases and thereafter gradually decreasing,

and then maintaining said current density at a predetermined value by further increasing said voltage to complete the deposition of a resin film of predetermined thickness on said substrate, said predetermined current density value being a maximum value found experimentally to be suitable for obtaining maximum film throw without damaging said film by boiling aqueous or cosolvent liquid therein.

nominal rupture 4. A method of depositing a base solubilized, resinous, polycarboxylic acid material from an aqueous dispersion thereof onto an electrically conductive anodic substrate, said resin having a predetermined, nominal rupture voltage, comprising immersing a said substrate in the aqueous resin solution, passing a direct electric current through said dispersion and substrate by employing an initial deposition voltage below said nominal rupture volt-' age and thereafter increasing said deposition voltage toward said nominal rupture voltage to effect deposition of a resin film on said substrate, the current density initially increasing and thereafter gradually decreasing,

and then maintaining said current density at a predetemrined value by further increasing said voltage to complete the deposition of a resin film of predetermined thickness on said substrate, said predetermined current density value being a maximum value found experimentally to be suitable for obtaining maximum film throw without damaging said film by boiling aqueous or cosolvent liquid therein.

Claims (4)

1. IN THE METHOD OF DEPOSITING A BASE SOLUBILIZED, RESINOUS, POLYCARBOXYLIC ACID MATERIAL FROM AN AQUOUS DISPERSION THEREOF ONTO AN ELECTRICALLY CONDUCTIVE, ANODIC SUBSTRATE IMMERSED IN SAID DISPERSION BY PASSING A DIRECT ELECTRIC CURRENT THROUGH SAID DISPERSION AND SUBSTRATE WHEREBY A RESINOUS FILM OF PREDETERMINED THICKNESS IS DEPOSITED ON SAID SUBSTRATE, THERE BEING TWO DISCERNIBLE STAGES IN THE PROCESS OF DEPOSITING SAID FILM; (A) A FIRST STAGE IN WHICH THE CURRENT DENSITY INCREASES DUE TO AN INITIALLY FIXED OR INCREASIBG VOLTAGE, BELOW THE NOMINAL RUPTURE VOLTAGE OF SAID MATERIAL, AND RELATIVELY LOW FILM RESISTANCE AND (B) A SECOND STAGE IN WHICH THE CURRENT DENSITY DECREASES TO A LOW GENERALLY CONSTANT VALUE BECAUSE OF INCREASED RESISTANCE OF THE GROWINNG FILM, THE IMPROVEMENT COMPRISING MAINTAINING SAID CURRENT DENSITY DURING SAID SECOND STAGE OF THE FILM DEPOSITION PROCESS AT A PREDETERMINED VALUE GREATER THAN SAID LOW VALUE BY INCREASING THE DEPOSITION VOLTAGE TO ABOVE SAIDC NOMIAL RUPTURE VOLTAGE AS REQUIRED TO MAINTAIN SAID CURRENT DENSITY VALUE FOR A TIME SUFFICIENT TO COMPLETE THE FILM DEPOSITION, SAID PREDETERMINED CURRENT DENSITY VALUE BEING A MAXIMUN VALUE FOUND EXPERIMENTALLY TO BE SUITABLE FOR OBTAINING MAXIMUM FILM THRWO WITHOUT DAMAGING THE FILM BY BOILING AQUEOUS OR COSOLVENT LIQUID THEREIN.

2. In the method of depositing a base solubilized, resinous, polycarboxylic acid material from an aqueous dispersion thereof onto an electrically conductive, anodic substrate immersed in said dispersion by passing a direct electric current through said dispersion and substrate whereby a resinous film of predetermined thickness is deposited on said substrate, there being two discernible stages in the process of depositing said film: (a) a first stage in which the current density increases due to an initially fixed or increasing voltage, below the nominal rupture voltage of said material, and relatively low film resistance and (b) a second stage in which the current density decreases to a low generally constant value because of increased resistance of the growing film, the improvement comprising maintaining said current density during said second stage of the film deposition process at a predetermined value greater than said low value by increasing the deposition voltage to above said nominal rupture voltage as required to maintain said current density value for a time sufficient to complete the film deposition, said predetermined current density value being that value found experimentally to be suitable for obtaining maximum film throw without heating the deposited resin film above 80* C.

3. A method of depositing a base solubilized, resinous, polycarboxylic acid material from an aqueous dispersion thereof onto an electrically conductive anodic substrate, said resin having a predetermined, nominal rupture voltage, comprising immersing a said substrate in the aqueous resin dispersion, passing a direct electric current through said dispersion and substrate at a first voltage value below said nominal rupture voltage for a first period of seconds to commence deposition of a resin film on said substrate, the current density initially increasing abruptly and thereafter gradually decreasing, increasing the deposition voltage to a second value greater than said first value but less than said nominal rupture voltage and continuing to deposit said resin at said voltage for a second period of seconds, the current density again increasing abruptly and thereafter gradually decreasing, increasing the deposition voltage to about said nominal rupture voltage while said current density abruptly increases and thereafter gradually decreasing, and then maintaining said current density at a predetermined value by further increasing said voltage to complete the deposition of a resin film of predetermined thickness on said substrate, said predetermined current density value being a maximum value found experimentally to be suitable for obtaining maximum film throw without damaging said film by boiling aqueous or cosolvent liquid therein.

4. A method of depositing a base solubilized, resinous, polycarboxylic acid material from an aqueous dispersion thereof onto an electrically conductive anodic substrate, said resin having a predetermined, nominal rupture voltage, comprising immersing a said substrate in the aqueous resin solution, passing a direct electric current through said dispersion and substrate by employing an initial deposition voltage below said nominal rupture voltage and thereafter increasing said deposition voltage toward said nominal rupture voltage to effect deposition of a resin film on said substrate, the current density initially increasing and thereafter gradually decreasing, and then maintaining said current density at a predetermined value by further increasing said voltage to complete the deposition of a resin film of predetermined thickness on said substrate, said predetermined current density value being a maximum value found experimentally to be suitable for obtaining maximum film throw without damaging said film by boiling aqueous or cosolvent liquid therein.

Images