US3540990A - Electrocoating process - Google Patents

Description

Nov. 17, 1970 YolcHlRo oNlsHl ETAL 3,540,990

, ELECTROCOATING PROCESS Filed Feb. 28, 1967 United States Patent O U.S. Cl. 204-181 9 Claims ABSTRACT OF THE DISCLOSURE Cataphoresis is utilized to precipitate an insulating coating on a conductive member in a bath of insulating varnish dispersed into water. The varnish may be composed of polystyrene resin or emulsied acrylonitrile resin. The precipitated coating is applied with a coagulant composed of an organic solvent soluble or partially soluble in water, for example, phenol or n,n'dimethyl formamide. The coagulant is preferably maintained at at least room temperature. The coating applied with the coagulant is preliminarily hardened in the order of 70 C. and finally hardened in the order of 180 C. Alternatively, it may be fully hardened through a single operation. Also apparatus for carrying out the above steps are illustrated and described.

This invention relates to a process of forming electrically insulating coatings on electrically conductive members from electrically insulating varnish of aqueous dispersion type lthrough cataphoresis,

In order to form insulations on electrically conductive members such as electrically conductive wires, there have been already proposed various processes such as dipping or flow coating processes using electrically insulating varnishes dissolved or diluted in non-aqueous solvents; dipping or ow coating processes using electrically insulating varnishes dispersed into aqueous solvents; dipping, ow coating or cataphoretic processes using electrically insulating varnished dissolved or diluted in aqueous solvents and the like.

The dipping or ow coating processes using insulating varnishes dissolved or diluted into aqueous solvents are not only disadvantageous in that the varnishes involved are expensive and easy inflammable but also can only provide thin coatings having a thickness in the order of from 0.003 to 0.005 mm. for dipping and a maximum thickness in the order of from 0.005 to 0.010 mm. for ilow coating. The dipping or ow coating processes using insulating varnishes of aqueous dispersion type can provide thicker coatings but their thickness only ranges from approximately 0.03 to 0.05 mm. Further the dipping, ilow coating or cataphoretic processes using insulating varnishes dissolved or diluted in aqueous solvents each can provide a thickness in the order of 0.1 mm. If it iS attempted to form coatings having a thickness in excess of 0.1 mm., -it is diflicult to provide the good quality of coatings because of their porosity. In order to avoid this diiculty, it may be possible to increase the coating thickness to a certain extent by the addition of various fillers. However the resulting coatings after having been hardened tend generally to decrease in adhesiveness to the associated grounds to which they were attached.

It is accordingly the chief object of the invention to eliminate the disadvantages as above described.

It is another object of the invention to provide an improved coating process of safely and inexpensively forming on electrically conductive members electrically insulating coatings having a thickness of from approxi- -3,540,990 Patented Nov. 17, 1970 ice@ mately 0.2 to 0.5 mm. or more and good properties or substantially free from any irregularity and porosity.

These and other objects of the invention which will be apparent as the description proceeds, are accomplished by providing a coating process comprising the steps of precipitating an electrically insulating coating on an electrically conductive member in a bath of an electrically insulating varnish of aqueous dispersion type lthrough cataphoresis, applying an organic solvent soluble or partially soluble in water to the coating and thereafter drying and hardening the coating.

Preferably, the organic solvent may be applied to the coating while maintaining the solvent at at least room temperature.

Advantageously, after the application of the organic solvent, the coating may be preliminarily dried at a ternperature of from 40 C. to the nal hardening temperature for from 10 minutes to l hour and then finally hardened at the iinal hardening temperature.

As above described, the invention comprises the use of electrically insulating varnishes dispersed into Water or those of aqueous dispersion type. The preferred examples of such varnishes involve nely divided synthetic resins, such as polystyrene resins dispersed or suspended into aqueous dispersion medium, and emulsiiied insulating varnishes as those of acrylonitrile resins and the like.

It has been found that with the cataphoretic process used, the insulating varnishes of aqueous dispersion type can provide considerably thicker insulating coating than do the varnishes dissolved or diluted into aqueous solvents. However, the coatings as precipitated are ditlicult to be hardened or cured into uniform, continuous structures, it is necessary to add any suitable coagulant to the varnish. If the particular dipping or tlow coating process is used to form an insulating coating from the corresponding varnish dissolved or diluted into an aqueous solvent, then it is required to coagulate or coalesce the coating-forming material or the precipitated dispersedparticles in the stage of evaporizing the associated dispersion medium. To this end, any suitable coagulant or coalescing agent may be sometimes added to the dispersion medium.

The coatings precipitated through the cataphoretic process each have a very small proportion of the dispersion medium remaining as compared with layers of insulating varnishes formed through dipping or flow coating processes due to the fact that the cataphoretic stage is accompanied by electroendosmosis. This leads to a shortage of the dispersion medium and hence of the coagulant required for the coating forming stage. Therefore the coatings can be in most cases cracked in the hardening stage. Accordingly, in order to harden a layer precipitated from the corresponding varnish of aqueous dispersion type through the cataphoretic process into an excellent coating, it is required to supply coagulant due to the shortage thereof.

This may be accomplished by preliminarily adding the coagulant in an amount larger than that in effecting the normal dipping or flow coating process. The results of experiments, however, indicated that with such a measure the varnish was required to have the coagulant added thereto in an amount of from 30 to 300% based upon the weight of the non-volatile ingredient of the varnish and also that selection of both the dispersion medium and the coating forming material is generally subject to limitations in terms of the stability, viscosity, and non-volatile ingredients, of the dispersoid and the conditions for forming coatings.

It has been found that the coagulant is advantageously applied in the required amount, to the layer of coating forming material precipitated from the corresponding insulating varnish of aqueous dispersion type through cataphoresis followed by drying and hardening. It has also been found that for satisfactory results, the coagulant should be maintained at room temperature or above and preferably at 15 C. or above. This appears to result from the fact that the precipitated layer is somewhat dissolved in the coagulant While at the same time the latter enters the precipitated layer although the exact mechanism cannot now be fully understood.

The results of numerous experiments indicated that the coagulants used in the invention should be organic solvents soluble or partially soluble in water and capable of dissolving somewhat the associated insulating layers precipitated from the varnishes involved. The preferred examples of the organic solvents involve phenol for varnishes of polystyrene resins and dimethyl formamides and dimethyl acetamides for varnishes of acrylonitrile reslns.

The final step is to heat dry the insulating layer having applied thereto the coagulant. It has been found that, before the finally hardening operation, a pre-drying operation may be preferably performed at a temperature of from 40 C. to the final hardening temperature for from minutes to l hour. This measure prevents the coating from being cracked due to the thermal shock occurring when the insulating layer is being heated at the iinal hardening temperature from the beginning of the heating operation. It has also been found that during the hardening operation, a suitable amount of the coagulant may be advantageously applied to the insulating coating being hardened followed by re-hardening. In this way cracks and irregularities that might initially formed on the insulating layer can be effectively removed.

The manner in which the invention can be carried out will now be described in conjunction with the accompanying drawing in which:

FIG. 1 is a schematic view, partly in longitudinal section of a coating device suitable for carrying out the invention; and

FIG. 2 is a view similar to FIG. 1 but illustrating a modification.

While the invention will be described as being applied to electrically conductive members inthe form of an elongated continuity such as electrically conductive wires, it is to be understood that the invention is equally applicable to any desired shape of electrically conductive members.

Since elongated continuous members are generally produced through drawing operations using dies it is normally diiiicult to dip the members in a bath of coagulant such as previously described Without coating precipitated on the member brought mechanically into contact with the coagulant. Under these circumstances, certain means are necessarily provided for applying the coagulant to the coating on the conductive member, in contrast to workpieces iinite in dimension in all directions. To this end, an arrangement shown in FIG. 1 includes means for applying to a coating precipitated on a continuous conductive member a coagulant by means of a sprinkling or spraying technique while that shown in FIG. 2 includes means for passing the precipitated coating through an atmosphere saturated with a vapor of the coagulant for the purpose of applying the coagulant to the coating. If desired, both arrangements may be combined with each other for the same purpose.

In FIG. 1, an elongated, continuous, electrically conductive member such as an electrically conductive wire 10 is fed in liquid tight relationship into an inclined container 12 containing a liquid of coating forming material 14. Within the container 12 the wire 10 passes through an opposed electrode 16 of hollow cylindrical shape suitably disposed in the container to be coated with the liquid through the cataphoretic process. Then the coated Wire 10 leaving the container 12 enters a coagulation chamber 18 substantially aligned with the container 12. Within the chamber 18 the wire 10 passes between two opposed series of sprinkling or spraying nozzles 2 0 suitable supported in the chamber to be applied with a coagulant spouted from the nozzles. The excess amount of the coagulant is collected in a reservoir 22 disposed on that end portion of the chamber 18 near to the container 12. The coagulant in the reservoir 22 is fed into the series of nozzles 20 through pumps 24 and associated lines. The wire 10 leaves the chamber 18 and then heated on a heating chamber (not shown) until the coating is hardened in the manner as previously described.

In FIG. 2 wherein the same reference numerals designate the components identical or corresponding to those shown in FIG. l, there are provided means for increasing a temperature of a coagulant which is intended to be applied to a precipitated coating. A coated wire 10 leaving a coating chamber 18 of the same construction as the container 12 shown in FIG. l, passes through an inclined coagulation chamber 18 having an atmosphere of a saturated coagulant vapor. As shown in FIG. 2 a heating chamber 26 including a heating coil 28 is disposed below the chamber 18 and has an opening communicating with the lowermost end of the chamber 18 and another opening communicating with the intermediate portion of the chamber to permit the vaporized coagulant to fill the chamber. The chamber 18 also is provided in the exit portion and that portion of the outer wall adjacent the exit portion with the respective cooling coils 30 and 32 for cooling the outer wall of the charnber thereby to prevent the coagulant from escaping, as its vapor, from the chamber through its exit 34. That portion of the coagulant not applied to a coating on the particular wire passing through the chamber is liquidized and then passed to the lowermost end of the chamber until it falls into the heating chamber 26 to be again vaporized.

The arrangement shown in FIG. 2 is advantageous in that the coagulant can raise its temperature and that the appearance of the final coatings is more grossy than those formed through the dipping processes.

The following examples illustrate the practice of the invention.

EXAMPLE I A mixture of 400 parts by weight of Water, 200 parts by weight of styrene monomer, 2 parts by weight of potassium persulfate, 2 parts Iby weight of sodium hydrogen sulte, 4 parts by weight of sodium salt of lauric acid ester was emulsifed and polymerized at 80 C. for 4 hours to form a polystyrene resin. The resulting polystyrene resin was dispersed into water to prepare a bath of varnish of aqueous dispersion type containing a controlled amount of 20% of the non-volatile matter.

A piece of sheet mild-'steel as a workpiece and an electrode opposed thereto were placed in the bath of varnish and a direct current voltage of 30 volts was applied across the workpiece and the opposed electrode for 30 seconds with the workpiece used as an anode electrode whereby a layer of the resin was precipitated on the workpiece through the cataphoresis.

Then the workpiece coated `with the resinous layer Was dipped into phenol, serving as a coagulant, maintained at a temperature of 50 C. for 30 seconds after which the resinous layer on the piece was partially hardened on a furnace maintained at C. for l hour. The partially hardened resinous layer was then completely hardened at 200 C. for l hour. The resulting hardened layer was glossy and 0.300 mm. thick.

The results of tests indicated that the layer or coating had good properties.

EXAMPLE II An emulsiiied acrylonitrile varnish commercially available under the trademark Lect-ron, RK-6311 was diluted into water to produce a bath of varnish containing 20% by -weight of the non-volatile matter. A piece of sheet mild-steel cleaned by alkali degreasing and pickling and an opposed electrode were placed in the bath thus produced. A direct current voltage of 20 volts was applied across the piece of sheet mild-steel and the opposed electrode for 1 minute with the piece used as an anode electrode whereby a coating was precipitated on the piece through cataphoresis.

Then the piece was removed from the bath of varnish and subsequently dipped in n,n'-dimethyl formamide serving as a coagulant at room temperature for 1 minute. After the removal from the coagulant, the piece was heated in a furnace at 70 C. for 1 hour to partially harden the coating thereon. Then the partially hardened coating was finally hardened at 180 C. for 4 hours whereby a uniform, continuous coating having a thickness of 0.220 mm. resulted.

The resulting coating 'was tested in the arc resisting property according to the standard test specified in the ASTM, No. D 495 and found to have such a property corresponding to 180 seconds or more. It was also tested in hot-softening resisting property in the following manner. A length of bare copper Wire having a diameter of 0.6 mm. was placed perpendicularly on one edge of a 10 x 10 x 100 mm. test piece of mild-steel having a resinous coating formed thereon in the manner as above described and loaded with a weight of 500 grams. Then the assembly thus prepared increased in temperature at rate of 200 C. per hour within a furnace until a short circuit between the copper wire and the test piece was caused to complete an electric circuit. A temperature at which the electric circuit was just completed was measured to provide a measure of the hot-softening resisting property. The coating formed in Example II was determined to have such a temperature of 200 C. or more.

EXAMPLE III The procedure of Example II was repeated excepting that a direct current voltage of 10 volts was applied across a workpiece and an opposed electrode for minutes and that the final hardening time was 2 hours. The resulting coating was uniform and continuous structure and a thickness of 0.275 mm.

The coatings thus formed were tested in dielectric breakdown strength and volume resistivity according to the standard tests specied in the ASTM, Nos. D 149 and D 257 respectively. The results of tests indicated that the dielectric breakdown strength was 4 kilovolts per a tenth of one millimeter for a thickness of 0.300 mm. in dried state and 2.5 kilovolts per a tenth of one millimeter for a thickness of 0.310 mm. in wet state. Also the volume resistivity was determined to be 5.1 1015 ohms-cm. in dried state and 4.9 1014 ohms-cm. in Wet state.

EXAMPLE 1V The procedure of Example II was repeated excepting that a direct current voltage of 20 volts was applied across a workpiece and an opposed electrode for 50 seconds and dimethyl acetamide was used as an coagulant. The resulting coating had a thickness of 0.220 mm. and good properties.

EXAMPLE V The procedure of Example II was repeated excepting that a direct current voltage of volts was applied across a workpiece and an opposed electrode for 5 minutes to form on the kworkpiece a coating which in turn, had n,ndimethyl formamide at 70 C. dropped thereupon for 30 seconds. The resulting coating had good properties and was 0.205 mm. thick.

EXAMPLE VI The procedure of Example II was repeated excepting that a direct current voltage of 18 volts was applied across a workpiece and an opposed electrode for 4 minutes to form on the workpieces a coating which in turn was maintained in an atmosphere supersaturated with n,n'dimethyl formamide vapor for 1 hour. A good coating 0.310 mm. thick resulted.

6 EXAMPLE VII Within the same bath of varnish as in Example II a zinc plated piece of sheet mild-steel was subject to the cataphoresis at a direct current voltage of 25 volts for 2 minutes. A coating precipitated on the piece was dipped into n,n-dimethyl formamide at 70 C. for 30 seconds and then hardened in the same manner as in Example II. This resulted in a hardened coating having a thickness of 0.195 mm.

EXAMPLE VIII The procedure of Example II was repeated to precipitate on a suciently polished piece of sheet aluminum, a zinc immersion-plated piece of sheet aluminum, and piece of sheet aluminum plated with zinc and copper in the named order, coatings through the cataphoresis at a direct current voltage of 20` volts for 2 minutes. The hardened coatings thus formed had thicknesses of 0.210, 0.240 and 0.230 mm.

EXAMPLE IX Within the same bath of varnish as in Example II except for 22% by weight of the non-volatile matter, the cataphoresis was eifected at a voltage of 20 volts for 1.5 minutes to precipitate a coating on a workpiece. The coating was dipped into n,ndimethyl formamide at C. for 30 seconds and then maintained in an atmosphere supersaturated with n,ndimethyl formamide vapor for 1 hour followed by hardening in a furnace at from 150 to 180 C. for 4 hours. The resulting coating was very glossy and 0.160 mm. thick.

For purpose of comparison, an example will now he described using a bath of varnish having a coagulant preliminarily added thereto.

A cataphoretic bath was composed of the above-mentioned Lecton containing 28% by weight of the non-volatile matter and n,n-dimethyl formamide added thereto in an amount of 250% based on the weight of the nonvolatile matter. A piece of sheet mild-steel and an electrode opposed thereto were placed in the bath and a direct current voltage of 20 volts was applied across the pieces and the electrode for 10 seconds with the piece used as an anode electrode whereby a resinous coating was precipitated on the piece through the cataphoresis. The coating was preliminarily hardened at C. for 30 minutes and finally hardened at 180 C. for 2 hours to provide a hardened coating having a thickness in the order of 0.270 mm. Approximately 30% of the hardened coatings produced in the manner just described had good properties. This offers a proof that the resulting coatings were very unstable although the stability dependents upon the electrically conductive material involved.

However, it is to be noted that if the coatings as precipitated is applied with the present coagulant in any of the manners as previously described in the above examples that the resulting coatings have the excellent properties.

As the process is repeated, the coagulant will progressively adsorb water originating from the precipitated coatings. However the coagulant containing water can be rened through fractional distillation. It has been found that a content of Water up to 20% lby weight in the coagulant does not greatly affect the present process.

From the foregoing, it will be appreciated that the invention has provided an effective process of cataphoretically precipitating a relatively thick coating on an electrically conductive member in a bath of varnish dispersed into water through a single operation or a few operations and hardening the coating into a uniform continuous structure.

The invention has various applications. For example, it can be applied to form insulations of suitable thickness on commutator segments of electrical rotary machines. This eliminates the necessity of interposing between the adjacent segments expensive mica pieces which were previously used, and accordingly the operation of interposing the mica pieces between the segments is omitted resulting in the simplication of their assemblage. However, it is to be understood that the invention is not limited to such insulation and that it is equally applicable to electrically conductive wires and any other electrically conductive member required to be insulated.

While the invention has been described in conjunction with the embodiments and examples thereof, it is to be understood that various changes and modifications may be resorted to without departing from the spirit and scope of the invention. For example, the application of a coagulant to a coating as precipitated described in any one of the above examples may be repeated as desired. Alternatively, any combination of the applications of the coagulant as described in the examples may be employed. As previously described, a coating as precipitated in a bath of varnish containing a coagulant may be applied with the same or dissimilar coagulant in the manner described in any one of the examples. Further, during the preliminarily hardening operation, the coagulant may be again applied to the coating as by dipping treatment and then subject to the preliminarily hardening treatment. This measure is eifective particularly in the case the coating is deficient in content of coagulant prior to the first preliminarily hardening operation.

What we claim is:

1. A process of forming an electrically insulating coating on an electrically conductive member comprising the steps of (a) depositing an electrically insulating coating on the electrically conductive member in a bath of electrically insulating varnish dispersed in water,

(b) then applying a coalescing agent comprising an organic solvent at least partially soluble in water, to said deposited coating in the uncured state, and

(c) thereafter curing the resulting coating.

2. A process as claimed in claim 1 wherein the varnish is selected from the group consisting of polystyrene resins and acrylonitrile resins and in which said organic solvent is selected from the group consisting of phenol, dimethyl formamide and dimethyl acetamide.

3. A process as claimed in claim 1, wherein said curing step comprises a two step procedure in which said coating CJI is preliminarily dried at a temperature of from 40 C. to the final curing temperature for from about 10 minutes to 1 hour and a nal step in which the preliminarily hardened coating is nally cured at the final curing temperature.

4. In a process of forming an electrically insulating coating on an electrically conductive member, comprising the step of depositing an insulating varnish coating on said conductive member in a bath of insulating Varnish dispersed in water through cataphoresis and then curing said coating, the improvement which comprises applying to said coating prior to said curing a coalescing agent in which comprises an organic solvent which is at least partially soluble in water.

5. A process as claimed in claim 4 in which said coalescing agent is sprayed on said coating.

6. A process as claimed in claim 4 in which said coalescing agent is applied in vapor form to said coating.

7. A process as claimed in claim 4, wherein said organic solvent is applied at ambient temperature or above.

8. A process as claimed in claim 4, wherein the varnish is selected from the group consisting of polystyrene resins and acrylonitrile resins.

9. A process as claimed in claim 4, wherein the organic solvent is selected from the group consisting of phenol, dimethyl formamide and dimethyl acetamide.

References Cited UNITED STATES PATENTS 2,142,968 1/1939 Stoesser 117-63 2,386,634 10/1945 Robinson 204-181 2,478,322 8/ 1949 Robinson et al 204-181 2,561,513 10/1958 Horback et al. 117-63 3,159,558 12/1964 McCoy 204-181 OTHER REFERENCES Fink: Electrodeposition and Electrochemistry of the Deposition of Synthetic Resins, in Transactions of the Electrochemical Society, vol. 94, 1948, pp. 309-310.

Shyne: Electrophoretic Application of Organic Finishes, in Organic Finishing, May 1956, vol. 17, No. 5, pp. 12-14.

HOWARD S. WILLIAMS, Primary Examiner

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