US3575832A

US3575832A - Method for electrocoating small objects

Info

Publication number: US3575832A
Application number: US733916A
Authority: US
Inventors: Olin B Johnson
Original assignee: Ford Motor Co
Current assignee: EIDP Inc
Priority date: 1968-06-03
Filing date: 1968-06-03
Publication date: 1971-04-20
Anticipated expiration: 1988-04-20
Also published as: BE733975A; DE1927436C3; JPS556118B1; FR2010054A1; DE1927436B2; JPS5125445B1; GB1207469A; DE1927436A1

Abstract

METHOD FOR ELECTRODEPOSITING PAINT UPON SMALL, FERROMAGNETIC OBJECTS WHICH INCLUDES SUPPORTING THE WORKPIECE THROUGHOUT COATING AND CURING WITH ELECTROMAGNETIC SUPPORT MEANS AND IMPRESSING THE COATING VOLTAGE VIA THE MAGNETIC CONNECTION.

Description

20, E973 o. B. JOHNSON METHOD FOR ELECTROCOATING SMALL OBJECTS 2 Sheets-Sheet l @MV f @mmf Filed June 3, 1968 pr 20, 1971 o. B. JOHNSON METHOD FOR ELECTROCOATING SMALL OBJECTS 2 Sheets-Sheet 2 Filed June 3, 1968 l f l I f f f f Of: /A/ 5. JU/fNSUA/ INVENTORS BY v 5M 7" TQQ/V5 V5 m@ mw United States U.S. Cl. 204-181 4 Claims ABSTRACT F THE DISCLOSURE Method for electrodepositing paint upon small, ferromagnetic objects which includes supporting the workpiece throughout coating and curing with electromagnetic support means and impressing the coating voltage via the magnetic connection.

This invention is concerned with providing a more effective method for employing electrodeposition to the coating of small objects. When paint is electrodeposited from a liquid bath, the object to be painted serves as one electrode of the electrodeposition cell, i.e. the anode in anodic deposition. Such arrangement assumes electrical connection between the workpiece and the appropriate lead of an external circuit which includes a direct current, e.g. a rectified alternating current, power source. Inherently, the area of electrical connection is not painted. To paint by electrodeposition large numbers of small, ferromagnetic objects, e.g. springs, bolts, screws, brackets, etc., it has been suggested to suspend a large number of such objects in an electrically conductive wire basket and thereby paint large numbers of such objects at one time. It was soon discovered that items painted in this manner exhibited unpainted or lightly painted spots at three or more locations where such objects made contact with the basket or with each other. Further, in such a method, the location of such spots cannot be controlled.

It is one object of this invention to provide method and means for electropainting small conductors while minimizing the uncoated area of the workpiece, i.e. reducing the number of contact areas during painting to one and minimizing the area of such single contact.

It is another object of this invention to provide method and means for electropainting small conductors while controlling the location of the luncoated area incidental to contact with the coating circuit.

In this method, one or more, preferably a large number, of the objects to be painted are contacted at the desired location thereon by electromagnetic contact means and held thereby while immersed in the coating bath and While the electrodeposited coating undergoes curing. The magnetic force is released and the painted object is discharged into receiving means. For electrodeposition, electrical connection between workpiece and external coating circuit is made through the electromagnetic contact assembly.

One problem associated with electrodeposition of paint is that an exposed conductor in electrical connection with the workpiece will be painted if immersed in the coating bath. Since the connection is maintained during curing in the instant method, such coating is polymerized in the same manner as that on the workpiece. Thus, if no intervening element is employed, care must be exercised to avoid contact between supporting conductor and the bath thereby avoiding the necessity for frequent removal of polymerized paint from such conductor. Thus, in one embodiment of this invention, the electromagnetic contact and conductor rst picks up a disposable shield of suitable metal, advantageously a thimble-shaped foil, before picking up the object to be painted. ln another embodiment, a plastic shield encompasses the connecting end of the electromagnetic contact and conductor except for the required contact area.

Electrodeposition of paint from a liquid bath now accounts for a substantial portion of industrial painting. While it is recognized that paints can be chemically constituted so as to be either anodically or cathodically depositable, substantially all industrial painting by electrodeposition to date has taken the form of anodic deposition from an aqueous dispersion of a polycarboxylic acid resin comprising paint. Both the method and paints especially prepared therefor are exemplified by U.S. Pat. 3,230,162 to A. E. Gilchrist. In those embodiments wherein the coating is to be cured by irradiation with an electron beam, the preferred binder resins comprise polycarboxylic acid resins having alpha-beta olefinic unsaturation. Depending upon molecular weight and the type and degree of oleiinic unsaturation, it may be necessary to employ vinyl monomers with such resins to provide adequate crosslnking. Electron beam curing of paint is described by W. I. Burlant in U.S. Pat. 3,247,012.

This invention will be more fully understood from the following detailed description and the accompanying drawings wherein:

FIG. 1 is a schematic illustration of one embodiment of the method of this invention and one embodiment of apparatus for carrying out such method;

FIG. 2 is an enlarged, partially cutaway View of one of the electromagnetic loading means shown in FIG. 1;

FIG. 3 is a sectional view of one embodiment of the core assembly of the electromagnetic loading means shown in FIG. 2 illustrating a centrally positioned, electrically insulated, electrical conductor for establishing electrical connection between the workpiece and the coating circuit; and

FIG. 4 is a schematic illustration of another embodiment of the magnetic loading means constructed and arranged to cause the workpiece to revolve while passing through the curing station and provided with plastic shielding means.

Referring now to FIGS. 1,2 and 3, there is shown an electromagnetic loading assembly 11. Assembly 11 comprises a core assembly 11-1 which makes contact with workpiece, a housing 112, an electrically conductive coil 11-3 surrounding core assembly 11-1 and a tubular support member 11-4. In this embodiment, core assembly 11-1 comprises a tubular soft iron core 11-11, a conductor 11-12, and an insulator 11-13. The ends of coil 11-3 are in electrical connection with a direct current, electrical power source, not shown, via electrical connections 11-5 and 11-6. Coil 11-3 and core 11-11 form the electromagnet of loading assembly 11-1. Conductor 11-12 provides conduction means for completing the coating circuit hereinafter described. In another embodiment, the core is unitary and functions both as the core of the electromagnet for lifting and as conductor for completing the coating circuit.

Loading assembly 11 is duplicated by

loading assemblies

17, 19, 21, 2.3l and 25 shown to the right of assembly 11 in FIG. 1. The operation of such assemblies is hereinafter described with relation to the functional step carried out at the indicated locations.

Tubular support member 11-4 and duplicate support members of

assemblies

17, 19, 21, 23 and 25 are supported by and reciprocal within a continuous conveyor assembly a portion of which is indicated at 27. In one embodiment, conveyor 27 is adapted for intermittently continuous movement with spaced stops. In another embodiment, the loading assemblies represented by 11, 17, 19, 21, 23 and 25 are automatically detached from the conveyor at the coating and/or curing stations hereinafter described and automatically such functions are achieved.

In the embodiment here illustrated, the electromagnet of a loading assembly in the position of assembly 11 is deactivated.

In the rst operative position, indicated by loading assembly 17, the loading assembly is lowered by conventional power means, not shown, c g. mechanical means, pneumatic means, etc. Upon reaching its lower position, the core 17-1 contacts a positioned, disposable, ferromagnetic foil thimble 29, e.g. steel foil about 0.002 inch thickness. Such thimbles are fed into position for such contact by conventional feeder means, not shown, via channel 31. The electromagnet of assembly 17 is energized, thimble 29 is attracted thereto, and assembly 17 is raised to its upper position. The electromagnet remains energized until the loading assembly reaches the position indicated by loading assembly 25.

In the second operative position, indicated by loading assembly 19, the loading assembly is lowered as before and the lower surface of thimble 33 contacts the top of bolt 35. Bolt 35 is moved into position for such contact by conventional feeder means, not shown, via channel 37. Bolt 35 is attracted to and held by the magnet of loading assembly 19 which is then raised to its upper position.

In the third operative position, indicated by loading assembly 21, the loading assembly is lowered to its lower position where bolt 41 is fully immersed in an aqueous coating bath 43. The lower portion of thimble 45 is also immersed in bath 43. Bath 43 is retained in a coating tank which serves as the cathode of an electrodeposition cell. Tank 47 is in electrical connection with the negative lead of a direct current electrical power source, not shown, via conductor 49. Bolt 41 is in electrical connection with a positive lead of this power source via thimble 45, conductor 21-12 of assembly 21, and conductor 51` Bath 43 comprises an aqueous dispersion of polycarboxylic acid resin, water soluble base, particulate pigment and water. The impressed coating voltage is in the range of about 50 to about 500, preferably about 100 to about 300 volts. Residence time in the bath is advantageously in the range of about 0.2 to about 2.0 minutes. Upon completion of coating, loading assembly 21, with bolt 41 attached and bearing a wet coat of paint, is raised to its upper position and moved Ionto the coating station.

In the fourth operative position, the curing station 53, there is shown in outline loading assembly 23 with thimble and bolt attached. Polymerization of the coating deposited in bath 43 is here effected by suitable force curing means, e.g. heat curing or if suitable coating material is employed by irradiation with an electron beam.

When a loading assembly reaches the final operative position, indicated by loading assembly 25, the electromagnet is deenergized releasing thimble 55 and bolt 57 to fall into a receiving bin 59. The loading assembly then returns to the position occupied by loading assembly 11.

Referring now to FIG. 4, there is shown an electromagnetic loading assembly 61 which differs from the aforedescribed loading assemblies in the following particulars and illustrates two other embodiments of this invention. Mounted on tubular support member 61-4 is a splined or gear-like sleeve 61-7 which is adapted to mesh with and be rotated by

gears

63 and 65 mounted on. shafts 67 and y69 respectively. Shafts `67 and 69 are rotatably mounted on conveyor means and operatively connected with drive means adapted to cause

shafts

67 and 69 to rotate. This arrangement provides rotation of the loading assembly and hence of the workpiece while passing through the curing zone. Such rotation is particularly useful when the coating is to be cured by an electron beam. Loading assembly 61 has a core assembly `61-1 similar to the corresponding core assemblies shown in FIG. 1. Aixed to the sides of core assembly 61-1 is a plastic sleeve v71 having a flexible lip 7l-1 which protects the lower portion of core assembly 61-1, excepting the workpiece-contact area, from contacting the coating bath.

reengaged when Sleeve 71 is preferably a low surface energy plastic such as polytetrauoroethylene. In the embodiment wherein sleeve 71 is employed, the use of the disposable foil thimbles is eliminated.

To simplify description, the method and means of this invention are illustrated in the drawings with each loading assembly picking up a single object to be painted. It will be understood by those skilled in the art that the electromagnet of each loading assembly can be designed in both size and configuration to pick up and permit simultaneous processing of large numbers of small items. The size and electrical input to such magnets can be tailored to operations of a given installation. Likewise the magnetic contact area may be a single surface or a series of needle-like points, e.g. three points in triangular formation.

It will be understood that modifications in the method and means hereinbefore disclosed may be made to suit varying conditions and uses without departing from the scope of this invention as defined by the claims appended hereto.

EXAMPLE 1 Coil springs (steel) of about 2.5 inches in length and having an outside diameter of about 5/8 inch are painted using the method hereinbefore described employing a coating bath prepared in the following manner:

An extended coupled glyceride drying oil paint binder is made by reacting in an agitation tank 8,467 parts of alkali-refined linseed oil and 2,025 parts of maleic anhydride, i.e. heating the same together at 232.2 C. for about three hours until an acid number of -90 results, cooling this intermediate to 157.2 C., adding 1,789 parts of vinyl toluene containing 48 parts of ditertiary butyl peroxide and reacting at 218.3D C. for about one hour.

The resulting vinyl toluenated material is then cooled to 157.2 C. and 5,294 parts of non-heat reactive, thermoplastic, oil-soluble phenolic resin are added, the temperature raised to 232.2 C. and held one hour. The phenolic resin is a condensation product of about equimolar quantities of para tertiary butyl phenol and formaldehyde and is added as a solid lump resin having softening point of 1Z0-150 C., specic gravity of LOB-1.05 at l20 C., and has been stripped to remove excess phenol and low molecular weight materials. The electrical equivalent weight of the resulting acid resin as extended is about 1,640, and it has acid number of 65.

The material then is cooled to 93.3 C., and 1,140 parts are taken for forming a paint dispersion. To these 1,140 parts, parts of water are added, then 13.6 parts of triethylamine, the mixture agitated for a few minutes, then 74 more parts of water and 92.5 parts diisopropanol amine added. This mixture is further reduced with 1,825 parts water and 32.5 parts diethylene triamine while agitation is continued.

To this paint dispersion there is added 50 parts of a treating mixture of mineral spirits, a light hydrocarbon liquid having A.P.I. gravity of 45-49.5, specific gravity of 15.6 of 0.78-080, flash point (Cleveland Open Cup) between 37.8-46" C., a negative doctor test and no acidity, 12 parts of a wetting agent (the oleic ester of sarcosine, having a maximum of 2% free fatty acid, a specific gravity of 0.948, color on the Gardner scale of 6, and a molecular weight of 340-350).

A pigment grind is made from 123 parts of vinyltoluenated, maleic-coupled linseed oil made in the same manner as the resin hereinabove shown in this example (except that the resulting polycarboxylic acid resin is not extended with the phenolic resin), 8.4 parts of diisopropanol amine, 0.7 part of an antifoam agent (a ditertiary acetylenic glycol with methyl and isopropyl substitution on the tertiary carbon atoms), 233 parts of fine kaolin clay, parts of pigmentary titanium dioxide, 7.8 parts of fine lead chromate, 15.5 parts of ne red iron oxide, 16.9 parts of carbon black, and 201 parts of water. The resulting pigment grind is then blended with the forca going paint dispersion and treating mixture to make a concentrated paint. The resulting paint is reduced further with water in the ratio of one part of the resulting paint per parts of water to make an initial painting bath for electropainting operations. The resulting bath has resin solids (non-volatile matter) concentration of 7.24%. The total of amine equivalents used in making up the initial bath is about 4.5 times the minimum amount necessary to keep this polycarboxylic acid resin, once dispersed, in anionic polyelectrolyte condition in the bath and about 1.25 times full neutralization of the acid resin with respect to its acid number (determined by the pyridine method described hereinbefore).

The replacement paint solids are made by dispersing 1,140 parts of the same kind of extended polycarboxylic acid resin with 100 parts of water and 13.6 parts of triethylamine. To this is added the mineral spirits, the wetting agent, and the foregoing pigment grind, all of the same compositions and in proportions as are used to make up the original paint dispersion for the bath.

The impressed difference of potential between the immersed workpieces and the cathode is about 200 volts. The coated workpieces are cured by baking in an atmospheric oven at about 190 C. for about 15 minutes.

EXAMPLE 2 Threaded steel bolts approximately 3 inches in length are painted using the method hereinbefore described employing a coating bath prepared in the following manner:

A silicone-modified, polyester type, polycarboxylic acid resin is prepared from the following components:

Moles Grams Ml.

Maleic anhy drlde 0. 63

Tetrahydrophthalic anhydride 1. 6l

Neopentyl glycol 2. 48

Polysiloxane (25% by weight) 175. 0

ylene Hydroquinone 0. 2827 The polysiloxane employed is a commercially available (Dow Corning Z-6018) hydroxy-functional, cyclic, polysiloxane having the following properties:

Hydroxy content, Dean Stark:

Percent condensible 5.5 Percent free 0.5 Average molecular weight 1600 Combining weight 400 Refractive index 1,53 l-1,539 Softening point, Durrans:

The glycol, the polysiloxane and the xylene are added to a four neck liter ask, heated to a temperature of about 160 to about 165 C. for about 2 hours while being stirred and under a nitrogen atmosphere. The reaction mixture is cooled to about 125 to about '130 C., the maleic anhydride, the tetrahydrophthalie anhydride and the hydroquinone are added and the temperature is increased slowly to about 190 to about 200 C. which is maintained for about 3.5 hours and to an acid number of about 47.7. The heating is stopped, the xylene is stripped, and the mixture is cooled to about 80 C. About 45.0 grams styrene and about 45.0 grams methylmethacrylate are added. The acid number of this binder solution is then determined to be about 43.4. This binder solution is hereinafter termed Binder A.

6 A mill base is prepared from the following components:

Grams Titanium dioxide Binder A 50.8 Styrene 7.0 Methylmethacrylate 7.0

This mixture is placed in a ball mill and milled for approximately 38 hours. This mill base is hereinafter termed Mill Base 1.

A resin-monomer dispersion is prepared from the following components:

mixer. The mill base and the binder are premixed and then poured -into the vortex of the aqueous mixture of amine and water. The resultant mix is blended for 10 minutes. Blending is stopped for 5 minutes and then continued for 5 minutes. Blending is stopped for 20 minutes and then continued for 5 minutes. After 5 minutes, the dispersion is stirred with 400 grams of distilled Water. The resulting emulsion has about 10 wt. percent solids.

The electrodeposition step is carried out with a difference of potential between workpieces and cathode of about volts. The electrodeposited coatings are polymerized by rotating the coated bolts while the same are passed through an electron beam. The conditions of irradiation employed are as follows:

Voltage--275 kv.

Currentl5 milliamperes Total dose-10 megarad Atmosphere-nitrogen Distance: electron window to work-7 inches EXAMPLE 3 Wood screws about 1.5 inches in length are painted using the method hereinbefore described employing a coating bath comprising an aqueous dispersion of diisopropanolamine, methyl methacrylate and a vinyl copolymer of methylrnethacrylate, ethylacrylate, glycidyl methacrylate, and methacrylic acid. The coating is cured by electron beam irradiation as in the previous example using a potential of about 295 kev., a current of about 1 milliampere and a total dose of about l0 megarad.

I claim:

1. In a method of painting wherein a lm of paint binder resin is electrodeposited upon an electrically conductive, ferromagnetic object from an aqueous coating bath having said resin dispersed therein while said object is immersed in said bath and serving as one electrode of the electrical circuit of electrodeposition, said object is subsequently removed from said coating bath, and said lm is cured thereon, the improvement wherein a contact is established between said object and electromagnetic support means and said electromagnetic support means provide (a) electrical connection between said object and said electrical circuit through said contact while said resin is being electrodeposited on said object, and (b) uninterrupted support for said object through said contact from the time of its immersion in said coating bath until said film is substantially cured.

2. In a method of painting wherein a lm of paint binder resin is electrodeposited upon an electrically conductive, ferromagnetic object from an aqueous coating bath having said resin dispersed therein while said object is immersed in said bath and serving as one electrode of the electrical circuit of electrodeposition, said object is subsequently removed from said coating bath, and said iilm is cured thereon, the improvement which comprises the combination of (l) establishing contact between said object and electromagnetic support means and holding said object with said electromagnetic support means through said contact,

(2) providing electrical connection between said object and said electrical circuit of electro-deposition through said contact,

(3) maintaining uninterrupted said contact while said object is immersed in said bath, said film is electrodeposited thereon, said object is removed from said bath, and said lm is force cured, and

(4) interrupting said contact and releasing said object from said electromagnetic support means.

3. In a method of painting wherein a lm of paint binder resin is electrodeposited upon an electrically conductive, ferromagnetic object from an aqueous coating bath having said resin dispersed therein while said object is immersed in said bath and serving as one electrode of the electrical circuit of electrodeposition, said object is subsequently removed from said coating bath, and said lm is cured thereon, the improvement wherein there is interposed between said object and electromagnetic support means a ferromagnetic foil thimble and said electromagnetic support means through said thimble provide (a) electrical connection between said object and said electrical circuit through said thimble while said resin is being electrodeposited on said object, and (b) uninterrupted support for said object through said thimble from the time of its immersion in said coating bath until said lm is substantially cured.

4. In a method of painting wherein a lm of paint binder resin is electrodeposited upon an electrically conductive, ferromagnetic object from an aqueous coating bath having said resin dispersed therein while said object is immersed in said bath and serving as one electrode of the electrical circuit of electrodeposition, said object is subsequently removed from said coating bath, and said lm is cured thereon, the improvement wherein a contact is established between said object and electromagnetic support means, said electromagnetic support means provide (a) electrical connection between said object and said electrical circuit through said contact while said resin is being electrodeposited on said object, and (b) uninterrupted support for said object through said contact from the time of its immersion in said coating bath until said lm is substantially curred, and a plastic sleeve surrounds that portion of said electromagnetic support means in contact with said object when said object is immersed in said bath.

References Cited UNITED STATES PATENTS 3,464,906 9/ 1969 Ridley et al. 204-181 FOREIGN PATENTS 351,659 7/ 1931 Great Britain 204-297 424,246 11/ 1933 Great Britain 204-297 HOWARD S. WILLIAMS, Primary Examiner U.S. Cl. X.R.