US3200058A

US3200058A - Cyclical current reversal for an electrophoretic deposition

Info

Publication number: US3200058A
Application number: US133441A
Authority: US
Inventors: Thomas H Oster
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1961-08-23
Filing date: 1961-08-23
Publication date: 1965-08-10
Anticipated expiration: 1982-08-10
Also published as: BE640626A

Description

Aug. 10, 1965 'r. H. OSTER 3,200,058

CYCLICAL CURRENT REVERSAL FOR AN ELECTROPHORETIC DEPOSITION Filed Aug. 23, 1961 OWER SOURCE THOMAS h. OSTER INVENTOR.

A TTORNEKS United States Patent 3 200,058 CYCLICAL CURRENT REVERSAL FGR AN ELEQTROPHORETEC DEPOSETION Thomas H. Oster, Dearboru, Mich, assignor to Ford Motor Company, Dear-born, Mich, a corporation of Delaware Filed Aug. 23, 1961, Ser. No. 133,441 5 Claims. (Cl. 204-481) This invention relates to the coating of articles by electrophoretic deposition. More particularly, this invention relates to the repeated periodical reversal of the electric current and electric field during the electrophoretic coating process,

The improvement of the electrophoretic coating process of this invention comprises the repeated periodic reversal of the electric field during the coating of an article which has been placed in a coating bath containing a colloid. Upon reversal of the electrical field, the colloid of the coating bath which has been previously migrating towards the surface of the article now migrates away from the surface of that article. This reversal of the electric field causes particles which are in the immediate vicinity of the submerged article to move away from the surface of the article. Charged particles which have been deposited upon the surface of the article maintain their position on the article. Thus, the deposition is irreversible in the sense that the deposited particles lose their tendency to return into the colloidal dispersion notwithstanding the reversal of the electrical rfield. When the electric field is again reversed, the particles will again tend to migrate toward the article being coated. The areas of the article most sparsely covered by deposited colloid particles will tend to exert the greatest attraction for the migrating particles since these areas are at the highest potential; hence, areas not completely covered by colloidal particles will tend to Ibe coated upon returning the electric held to its initial condition. The foregoing process of migration toward the article to be coated and migration away from the article to be coated will result in a surface coating of substantially uniform thickness. Further, the coated surface is virtually free firom any gas bubbles that are generally formed during the coating process.

It should be understood that the use of repeated current reversal is disclosed in the United States Patent 2,678,909 and is well known in the electr c-plating industry; however, large and fundamental differences exist between the former process and the process hereinafter described. It is known to those skilled in the art that an electro-deposit has a tendency to redissolve deposited metal in the baths during the application of the reverse current, while electroph-oreti-c depositions, to which the process hereinafter described applies, results in an irreversible deposition. Thus, in electroplating metal dissolves .from the anode while the piece receives its protective coating. During the reverse cycle the metal dissolves from the piece coated and is redeposited on the anode. The practical consequence of these facts for the electroplater is that the total plating coulombs must be larger than the total reverse or deplating coulombs. Further, it should be noted that the reason for applying the reverse current in electro-plating is not applicable to electrophoretic depositing. In electroplating the reversing of the current causes the high point in the deposition to be eroded away from the surface of the material, thereby smoothing the surface. In electrophoretic deposition this erosion away does not take place, The foregoing phenomena is clearly evidenced by FIGURES 2 and 3 of the above cited U.S. Patent 2,678,909. Further, the depositing cycles used in the instant invention and the voltages 32%,858 Patented Aug. 10, 1965 used would not be satisfactory in a plating process where metal ions are being deposited.

The characteristic advantages of the hereinafter described invention are:

(1) The coulombs used for cleaning can be equal or even greater than the coulombs used for electrophoretic deposition.

(2) The cleaning period of one piece can be used as part of the coating period for another piece in the same bath.

(3) The resulting electrophcretic coatings are of superior quality and appearance.

The object of the invention is to provide an electrophoretic process of coating whereby a smooth surface resultsr Another object of the invention is to provide an elec trophoretic process of coating materials whereby a surface free of gas bubbles results.

Another object of the invention is to provide an electrophoretic coating process whereby an article is coated with paint and the coating is free from gas bubbles.

Another object of the invention is to apply cyclical current reversal techniques to a coating process involving an irreversible deposition.

Another object of the invention is to apply cyclical current techniques to an electrophoretic coating process.

Other objects of this invention will in part be obvious and will in part appear hereinafter.

In the figures:

FIGURE 1 is a schematic drawing depicting the apparatus used in performing the cyclical current reversal process of electrophoretic deposition.

FIGURE 2 is a schematic diagram of another embodiment for accomplishing the cyclical current reversal process of electrophoretic deposition.

Referring to FIGURE 1, chemically resistant tank 1 contains a coating bath 5. The articles to be coated 6, 7 are placed upon hangers 9, The hangers 9 are supported on conductor bars 3. The conductor bars 3 are connected to the source 2 by a conductor 4. The conductor 4 has a switch 8 placed in the circuit in order to initiate .and terminate the coating process. It should be understood that the current source 2 may be a direct current source such as a battery or rectifier combined with suitable relays, reversing switches and resistancescontrolled by a timing mechanism to reverse the current at predetermined times and vary its value in accordance with the requirements of the articles being coated, and the coating bath. The electric potential is above, preferably substantially above, the value at which the electrolysis of water occurs. Alternative electrical controls are shown in Electropla't-ing Engineering Handbook, published by Reinhold Publishing Co, A. K. Graham, Editor, pages 578-580, (1955 Each deposition cycle has a coating period and a cleaning period. The coating period is that portion of the cycle during which article 6 is being coated. The cleaning period is that portion of the cycle during which colloidal particles are migrating away from article 6. The current source 2 generates as asymmetrical electrical signal, that is, the duration of the electrical signal which causes the coating period of each cycle is greater than the duration of the signal which causes the cleaning period of each cycle. This is ordinarily necessary in order that there be suflicient time for colloidal particles to reach the article being coated, once the particles in the immediate vicinity of the article have been deposited. However, when a bath agitating means is used, it is not necessary to use an asymmetrical current. Further, when long coating and cleaning periods are used, it is not usually necessary to use an asymmetrical current source.

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The coating bath is a colloidal suspension consisting of colloidal particles dispersed in a liquid. The colloidal particles hereinafter described carry a negative charge. However, the teaching of this invention is not confined to the deposition of negatively charged particles. The electrophoretic technique is equally applicable to the deposition of positively charged colloidal particles. The colloidal particles, can be dispersed in any of the Well-known electrophoretic vehicles, and the instant invention is not limited to any particular liquid vehicle. The behavior of pigment particles in various liquid vehicles is discussed in the' article, Electrophoretic Behavior of Pigment Suspension, by I-l. Brintzinger, R. Haug and G. Sachs, Farbe and Lack, volume 58, pages 10 (published January 1952).

One red primer which has been used as a colloidal particle and which has been electrophoretically deposited as a prime coating on an electrically grounded steel panel is formulated as follows:

A film-forming material consisting of a styrene-allyl alcohol copolymer 63 percent by weight is mixed with linseed fatty. acid 37 percent by weight. This mixture is esterlfied at 500 F. to an acid number of 5 and a maximum viscosity of 2.5 poises, measured when the copolyrner is reducedto 60 percent nonvolatile with xylene. A portion of this film-forming mixture 51 percent by Weight is intimately blended in a roller mill with red oxide pigment 45 percent by weight and linseed fatty acid 4 percent by weight. This blend 38 percent by weight is then letdown by further blen'ding it with an additional portion of the film-forming material of the above composition 50 percent by weight, melamine formaldehyde 11.8 percent by weight, and cobalt naphthanate .2 percent by weight.

This paint mixture is then emulsified by the addition of an emulsifing agent which consists of chemically pure concentrated ammonium hydroxide (28 percent strength) 3.5 percent by weight and dernineralized water 96.5 percent by .weight. This diluted ammonium hydroxide solution is gradually added until the so-called inversion point is reached, at which time the viscosity of the paint mixture drops suddenly after reaching a high point. The balance of the diluted ammonium hydroxide is then added. The resulting emulsion is then further refined in a colloid mill until a more stable emulsion is reached.

The paint composition of the above example is then utilized to prepare an aqueous coating bath for electropho retically coating an article. One example of the electrophoretic coating process utilizing this paint composition is as follows:

Example 1 Two parts of the automotive primer paint composition described above is thoroughly mixed with 6 parts of water and A; part of concentrated ammonium hydroxide to form a colloidal dispersion to be used as a coating bath. This coating bath has a specific resistance of approximately 5.() 10 ohm centimeter.

The foregoing coating bath is placed in the chemically resistant tank 1. The articles to be coated are placed on the support bars 3 by means of the hangers 9 and are immersed in the bath. The hangers 9, support bars 3, conductor 4'and closed manual switch 8 form an electrical connection to the current source 2. The current source 2 causes a potential difference of. 100 volts to be applied across the articles 6 and 7 being coated. This potential difference initially results in article 6 being positively charged and article 7 initially being negatively charged. The potential difference is maintained for four seconds, during which negatively charged colloidal particles are deposited upon the surface of positively charged article 6. Following the initial four second depositing cycle, the positive 100 volts is applied to article 7 for two seconds whereupon article '7 becomes more positively charged than article 6 and the negatively charged colloidal 2 particles migrate away from article 6 and are deposited on the article '7. 7

The foregoing cycle is repeated for 70 seconds; thus, 12 four-second coating periods and 11 two-second cleaning periods are applied to tire articles. Following the completion of the twelfth four-second period, the article 6 is removed and another article is placed on hanger 9.-

The above coating process is repeated and then both articles are removed.

Example 2 Examples 3 and 4 The foregoing examples are applied to a coating bath consisting of automotive primer paint composition heretofore described mixed with 6 parts of Water 'alfld' flg part of concentrated ammonium hydroxide and 19, part of a soap detergent to form a colloidal dispersion. The specific resistance of this coating is approximately 1.0x 10 ohm centimeter.

' A second embodiment of this invention is illustrated in FIGURE 2. The articles '20 to be coated are trans ported by a conveyor 21 to a chemically resistant tank 22 containing the coating bath 23. The articles 20 are hung from the conveyor 21 by a hook 224, which has an electrically insulated portion 25 to isolate the article 20 from ground. A contact plate 26 is attached to the paint hook 24 below insulated portion '25. Upon passing into the coating bath, contact plate 2% makes electrical contact with one the segments of segmented bus bar 27. The segmented bus bar 27 has segments 28 which are connected to the power source 3%; The segmented bus bar 27 also has segments 2% which are connected to ground. The paint hooks 24 are so spaced that there are always at least two articles in the coating baths. The segments of the segmented bus bar 27 are so spaced'tha-t one article in the coating bath will be in contact with a segment 28 connected to the power source while the other article is in contact with a segment 29 connected to ground.

The speed of the conveyor carrying the articles and the length of each segment will control the cycle time. For example, if the articles were carried by the. conveyor at the rate of 15 feet per minute, the segments would have to be 2.5 feet long for a '10 second coating cycle and 2.25 feet long for a 9 second cleaning cycle.

. It should be understood that an electrode, which is not an article to be coated, can be utilized in accordance with the teachings of the invention.

In order to prevent shorting of the positive and negative segments, suitable insulatorsegments (not shown) are inserted between each segment of opposite polarity.

If the voltage and current used are high enough to cause extensive arcing, it would be necessary to provide a meansof minimizing this arcing. For this purpose the segmented bus bar would be fastened in an insulated trough and covered with a layer of transformer oil. The trough would be open atthe top so that the contact plate can make good electrical connection with the segmented bus bar under, the layer of oil. 7 The trough would be as long as the segmented bus bar plus additional s ace for the arm contacts to enter and leave. The ends of the trough would be sealed in order to keep the oil at the proper level. a I i r p In experiments performed in accordance Withthe invention, the resulting coated surfaces have been virtually free of any gas bubbles. .i a

It is intended that all the matter contained in the above description shall be deemed to be illustrative and not limiting.

I claim:

1. In a method of applying a coating to a conducting surface of an article comprising the steps of causing the surface of said article to contact an aqueous bath having organic film-forming material dispersed therein, said article serving as a first electrode, causing a second electrode to contact said aqueous bath, and causing a direct electric current to flow between said first and second electrodes and through said aqueous bath at an electric potential substantially above the value at which the electrolysis of water occurs until a coating of said film-forming material is electrically deposited upon said article from said aqueous bath, the improvement which comprises causing the direct-ion of flow of said direct current between said first and second electrodes to be reversed after a significant period of time for a predetermined and significant period of time, and repeating the reversal while said article is in contact with said aqueous bath.

2. In a method of applying a coating to a conducting surface of an article comprising the steps of causing the surface of said article to contact an aqueous hath having charged, colloidal coating material dispersed therein, said material including a film-forming organic resin binder and a pigment, said article serving as a first electrode, causing a second electrode to contact said aqueous bath at an electric potential substantially above the value at which the electrolysis of water occurs until a coating of said material is electrophoretically deposited upon said article from said aqueous bath, the improvement which com rises reversing the direction of flow of said direct current after a significant period of time, causing the resulting reversed current to flow for a predetermined and significant period of time, reversing the direct-ion of said reversed current, repeating such reversals while said article is in contact with said aqueous bath.

3. In a method of applying a coating to a conducting surface of an article comprising the steps of causing the surface of said article to contact an aqueous bath having charged particles of organic coating material dispersed therein, said article serving as a first electrode, causing a second electrode to contact said aqueous bath, and cansing a direct electric current to flow between said first and second electrodes and through said aqueous bath at an electric potential substantially above the value at which the electrolysis of water occurs until a coating of said coating material is electrophoretically deposited upon said first electrode from said aqueous bath, the improvement which comprises causing the direction of flow of said direct current between said first and second electrodes to be reversed after a predetermined and significant period of time for a predetermined and significant period of time and repeating such reversals after comparable predetermined periods of time while said electrodes are in contact with said bath until said first electrode and said second electrode are coated with an electrically irreversible coating of said organic coating material.

4. In a method of applying paint to a metallic article comprising the steps of causing the surface of said article to be immersed in an aqueous bath having a paint formulation comprising charged particles of a film-forming, organic resin dispersed therein, causing a second metallic article to contact said aqueous bath, and causing a direct electric current .to fiow between said first and second articles and through said aqueous bath at an electric potential substantially above the value at which the electrolysis of water occurs until a Water insoluble coating of said resin is electrically deposited upon said first article, the improvement which comprises reversing the direction of flow of said direct electric current between said first article and said second article after a predetermined andv significant period of time for a second significant period of time, reversing said direction of flow after said second period and repeating the reversal of flow after comparable periods of time while said first and second articles are in contact with said bath.

5. In a method of applying paint to a metallic article comprising the steps of causing the surface of said article to be immersed in an aqueous bath having a paint formulation comprising charged particles of a film-forming, organic resin dispersed therein, said articles serving as a first electrode, causing a second electrode to contact said aqueous bath, and causing a direct electric current to flow between said first and second electrodes and through said aqueous bath at an electric potential substantially above the value at which the electrolysis of water occurs until a water insoluble coating of said paint formulation is electrolphoretically deposited upon said first electrode, the improvement which comprises reversing the direction of flow of said direct electric current between said first and second electrodes after a predetermined and significant period of time for a predetermined and significant period of time of different duration, reversing said direction of flow after said period of different duration, and repeating the reversal of flow after corresponding periods of time while said first and said second electrodes are in contact with said bath.

References Cited by the Examiner UNITED STATES PATENTS 1,769,659 7/30 Williams 204--182 2,314,604 3/43 Van der Horst 204-26 2,678,909 5/54 Jernstedt 204-41 FOREIGN PATENTS 482,548 3/38 Great Britain.

WINSTON A. DOUGLAS, Primary Examiner. MURRAY TILLMAN, JOHN R. SPECK, Examiners.

Claims

1. IN A METHOD OF APPLYING A COATING TO A CONDUCTING SURFACE OF AN ARTICLE COMPRISING THE STEPS OF CAUSING THE SURFACE OF SAID ARTICLE TO CONTACT AN AQUEOUS BATH HAVING ORGANIC FILM-FORMING MATERIAL DISPERSED THEREIN, SAID ARTICLE SERVING AS A FIRST ELECTRODE, CAUSING A SECOND ELECTRODE TO CONTACT SAID AQUEOUS BATH, AND CAUSING A DIRECT ELECTRIC CURRENT TO FLOW BETWEEN SAID FIRST AND SECOND ELECTRODES AND THROUGH SAID AQUEOUS BATH AT AN ELECTRIC POTENTIAL SUBSTANTIALLY ABOVE THE VALUE AT WHICH THE ELECTROLYSIS OF WATER OCCURS UNTIL A COATING OF SAID FILM-FORMING MATERIAL IS ELECTRICALLY DEPOSITED UPON SAID ARTICLE FROM SAID AQUEOUS BATH, THE IMPROVEMENT WHICH COMPRISES CAUSING THE DIRECTION OF FLOW OF SAID DIRECT CURRENT BETWEEN SAID FIRST AND SECOND ELECTRODES TO BE REVERSED AFTER A SIGNIFICANT PERIOD OF TIME FOR A PREDETERMINED AND SIGNIFICANT PERIOD OF TIME, AND REPEATING THE REVERSAL WHILE SAID ARTICLE IS IN CONTACT WITH SAID AQUEOUS BATH.