Method For Manufacturing Painted Metal Sheet

Abstract

A method for manufacturing a painted metal sheet by successive paintings comprises the steps of (a) subjecting a metal sheet plated with zinc or zinc alloy to an electrophoretic treatment with a water-soluble resin series paint to provide an undercoating, then (b) painting the steel sheet having the electro-deposited paint coating with a water-soluble or solvent-soluble paint to provide a final coat, and then (c) baking and drying the undercoat and the final coat at the same time. Step (a) may optionally be preceded by a step of subjecting the initial metal sheet, plated with zinc or zinc alloy, to a chemical-synthetic treatment to form a foundation for subsequent painting.

Description

The present invention relates to a method for manufacturing a painted metal sheet by a method involving baking the steel metal sheet after the application of successive coatings of paint thereon.

Heretofore, in manufacturing a painted metal sheet there has usually been adopted a so-called "one coating -- one baking" method, using a sprayer or a roll coater. In this case, however, as the painting is carried out only one time, the product of this method was defective in corrosion resistance due to an unavoidable formation of pin holes. Consequently, when anticorrosiveness and workability are particularly required, a so-called "two coating -- two baking" method had to be adopted.

Besides, there has been applied another method, for instance, a "two coating -- one baking" method, in which a final painting and a subsequent baking are carried out immediately after an undercoating has been completed, while the resultant undercoat is still in the wet state. By applying this method, it is made possible to reduce pin holes but, on the other hand, it is not possible to manufacture a thick coat because of the necessity of painting the metal sheet while rotating a painting roll in the same direction as the moving direction of the metal sheet, and further it is also not possible to obtain the improvement in coat properties to be expected from duplicated paintings, because heterogeneous paints can not be used.

On the other hand, when applying the "two coating -- two baking" method to the painting of a metal sheet or coil, there can be obtained following advantages that pin holes can be reduced, any suitable paints can be selected for the undercoating and final coating respectively according to the use, and a thick coat can be formed; hence a painted metal sheet having excellent anticorrosiveness as well as adhesiveness can be obtained. However, this method also has disadvantages in that it requires two roll coating machines and two baking furnaces with the result of a sharp increase in equipment costs, the control of the painting becomes difficult on account of the increased length of the painting lines, the frequency of producing inferior products becomes high, and metal sheets to be scrapped because of inferior products increase.

The aim of the present invention is to eliminate the various drawbacks of conventional methods as the above-mentioned ones; and the basic idea underlying the present invention resides in that at first an undercoating is provided with a water-soluble paint by an electrophoretic painting step, and immediately after rinsing and wind-drying the resultant undercoat a water-soluble or solvent-soluble paint is applied to the undercoat in a proper manner and thereupon the thus-painted steel sheet or coil is baked once to produce a final product.

That is, one of the main objects of the present invention is to provide a painting method, in which successive formations of two or more paint coats on the surface of a metal sheet can be carried out without interposing a baking between the undercoating and the final coating by subjecting the metal sheet to an electrophoretic painting with a water-soluble or water-dispersed paint to form an undercoat, because the water content of the paint for the undercoating can be made so small by the electrophoresis that the whole surface of the undercoat can be wind-dried in a very short period of time, which makes it possible immediately to apply the final coat to the undercoat in succession without baking the undercoat. This method of the present invention is attended with the advantages that, as a paint of another kind can be used for the final coating, excellent anticorrosiveness and adhesiveness of the painted metal sheet can be obtained, the baking can be carried out with one baking furnace and the painting line can be made short, which bring favorable effects on the control of equipment and of the painting operation.

The second main object of the present invention is to provide a painting method, in which flat metal sheet or strip coil is subjected to an electrophoretic painting with a water-soluble acrylic resin series paint to form an undercoat and immediately thereupon the final painting is carried out by applying a solvent-soluble thermosetting paint to the undercoat without baking the electrodeposited undercoat.

The third main object of the present invention is to provide a painting method, which is the most practical method for successively painting a metal sheet and in which a steel sheet or strip coil plated with zinc or zinc alloy (hereinafter referred to as "galvanized steel sheet") is optionally subjected to a chemical-synthetic foundation treatment such as a phosphate treatment or chromate treatment before it is subjected to the electrophoretic painting. Then, the chemical-synthetically treated galvanized steel sheet or the galvanized steel sheet is dipped in an electrodepositing bath to obtain an electrodeposited undercoat. Immediately after the electrodeposition treatment, the steel sheet is dried by simply blowing air on it (without baking) and then a solvent-soluble thermosetting paint, e.g. one of the phenol series, melamine series, alkyd series, acrylic series and vinyl series is applied on the electrodeposited undercoat. Finally, both coats, that is, the electrodeposited undercoat and the finish solvent-soluble paint coat, are baked at a single stroke, thereby to obtain colored galvanized steel sheet.

As to the electrodepositing bath to be used in the above-mentioned third process step of the present invention, that of the following composition is to be recommended as the most effective one, that is, a bath which is mainly composed of a water-soluble resin, containing as main ingredients lower alkyl ester of acrylic acid and/or lower alkyl ester of methacrylic acid and their ammonium salt or amine salt, and a water-soluble chromium compound added in an amount of 0.01 to 5.0 weight percent to the said water-soluble resin.

The role of this water-soluble electrodepositing bath to be played in the method of the present invention is overwhelmingly important and is inseparably connected with the method of the present invention.

In the following, therefore, the electrodepositing bath (hereinafter referred to also as "electrodepositing paint" or "electrophoretic paint") will be explained in detail.

In conventional methods of coating metal articles with organic compounds by electrophoresis, there have been mainly used water-dispersion series paints and water-soluble water-solution series paints. As for the water-dispersion series paint a dispersion or emulsion of high organic high molecular compound is employed, and as for the water-solution series paint alkali metal salts of the acrylic series, melamine series, alkyd series or maleic oil series compounds are employed, and many of them are already known through various patents relating thereto and others.

However, recently various investigations have been made to improve the painting method by treating a steel sheet in a treating bath, in which a water-soluble chromium compound is added to a water-soluble high molecular compound to obtain a painted steel sheet having excellent anticorrosiveness and durability after baking the painted steel sheet.

The present invention is a development of these investigations and is distinguished by the following features:

The first feature of the present invention resides in the novelty of the electrodepositing bath employed in the process of the present invention, in which the compounding ratio of water-soluble acrylic copolymer or methacrylic copolymer containing water-soluble chromium compound added thereto is specified, whereby it is made possible to attain an increased insolubilization reaction, the formation of a particularly excellent anticorrosive coat, an increase in electrodepositing efficiency and an excellent throwing power and the like, when carrying out the electric-deposition in the said bath. The second feature of the present invention is that the undercoat obtained by the electrodepositing paint itself not only has an excellent atmospheric corrosion resistance but also imparts an excellent working adhesiveness and anticorrosiveness to an ordinary paint coat formed on the undercoat. The third feature of the present invention, which is the most important feature of the present invention, is that the most serious problem any conventional anodic chemical-synthetic treatment by means of electrophoresis must confront, that is, the difficulty in treatment control caused by strong alkaline ingredients concentrated on the cathode in the conventional treatment, does not occur in practically operating the method of the present invention. This seems to be attributable to the excellent stability of the treating bath of the present invention at a high temperature.

The paint used in the electrodepositing operation of the present invention is a water-soluble organic high molecular compound such as copolymer of alkali metal salt of acrylic acid or amine or ammonium salt of methacrylic acid usually copolymerized with lower acrylic ester, preferably lower alkyl ester of about four carbon atoms or lower methacrylic ester, preferably lower alkyl ester of about four carbon atoms, and in some case further a third compound, if necessary. In this case, the ester radical is not subjected to any particular restriction.

The viscosity of this copolymer resin is preferably less than 10,000 CPS in 30 percent aqueous solution. The higher the degree of polymerization, the better the working adhesiveness, anticorrosiveness and durability of the coat obtained thereby. However, if the viscosity is more than 10,000 CPS, there occurs a remarkably difficulty in the treatment and further the electrodeposition of the paint becomes excessive, resulting in a deterioration in the rinsing effect. The range of the viscosity is advantageously in the range of from 2,000 to 6,000 CPS. This range is the most suitable from the points of working adhesiveness and other properties of the deposited coat, and an amount of electrodeposition and a rinsing effect. The alkali salt in the copolymer is necessary to be less than 50 percent from the stand point of properties of the coat obtained, but is usually used in an amount of about 3 to 5 percent from the necessity of water-solubilization.

Metals to be treated in the bath used in the process of the present invention cover iron and steel products, that is, iron and steel themselves, melting zinced steel sheet, electro-galvanized steel sheet, tin-plate sheet, aluminum-plated steel sheet, turnplate sheet. More precisely, the electrophoretic treatment of the present invention is applied not only to the surface of iron and steel sheet, but also of aluminum-, zinc- or tin-plated steel sheet and, more advantageously to zinc- or zinc alloy-plated steel sheet which is further coated with a chemical-synthetic film by a foundation treatment for painting such as phosphate treatment or chromate treatment. In these cases, metal ions flow out in the solution and react immediately with copolymer of acrylic aid or methacrylic acid and with water-soluble chromic acid ions or bichromic acid ions, and the resultant salt is uniformly dispersed in the coat, which accelerates the gelation of the coat and later acts as an oxidizing agent for effecting and accelerating the bridging reaction by the subsequent treatment of drying the steel sheet by heating. Therefore, the mixing ratio of chromium compound to be added to a water-soluble organic high molecular compound such as acrylic acid copolymer or methacrylic acid copolymer is sufficient with only about one-fifth to one-tenth of that in the case of a simple chromate treatment. More concretely, a proper mixing ratio of chromium compound to be applied in the anodic treating bath (electrodepositing paint) is 0.01 to max. 5 weight percent of the above-mentioned water-soluble organic high molecular compound, as calculated in terms of chromium (if below 0.05 weight percent, the deposited coat will be sporadic). An excellent anticorrosiveness can be obtained specially in the range of 0.01 to 1 weight percent.

If the chromium ion concentration is increased to some degree, a passivating tendency appears at the time of electrodeposition, whereby it becomes difficult to obtain a sufficient thickness (for instance, 10 to 100 mg/dm.sup.2), causing a deterioration of the anticorrosiveness, and if the chromium ion concentration is lowered too much, the chromic acid concentration in the paint coat is reduced, whereby the bridging of the paint coat will be insufficient, which causes the deterioration of working adhesiveness and anticorrosiveness.

As for chromium compounds to be added there may be enumerated water-soluble chromate or bichromate of ammonium, alkali metal, alkaline earth metal, or the third group metal. Practically chromates or bichromates of ammonium, magnesium, strontium, gallium, aluminum or the like are used.

The treating bath containing 1 to 30 percent of organic high molecular compound (in terms of the non-volatile part) is practically used, but is not subjected to a particular limitation from the point of the formation of the coat. The pH value of the treating bath is used mainly in the alkaline state. The pH value which is most suitable for practical use is about 8.0.

If the content of the organic high molecular compound (in terms of the non-volatile part) is about 30 weight percent, the viscosity of the surface coat of the article taken out from the treating bath will become too high, while if the content is below 1 weight percent, the efficiency of electrodeposition will be deteriorated.

The concentration of water-soluble resin of the electrodepositing paint suitable for practical use is in the range of 5 to 30 weight percent, preferably 10 to 15 weight percent. If the concentration of the vehicle exceeds 30 weight percent, the viscosity of the paint will increase rapidly and the painting loss will increase and also the minuteness of the coat will be lost. On the other hand, if the concentration of the vehicle is below 5 weight percent, the Coulomb's efficiency will be remarkably reduced, the thickness of the paint coat obtained at a normal electrodeposition voltage will become thin, resulting in the reduction of the anticorrosiveness.

The electrodeposited coat is required to be of such a thickness that it can be dried in a short period of time by drying (hot wind-drying or compulsive wind-drying) which is to be carried out after rinsing subsequent to the electrodeposition. That is, the electrodeposition should be carried out to such a degree that the resultant coat may reach the thickness of max. 150 mg/dm.sup.2, desirably 20 to 90 mg/dm.sup.2. The electrodeposition is followed by rinsing and drying, and thereupon the final painting by a solvent-soluble or water-soluble paint is carried out. As for the paint for the final painting there may be enumerated those of the thermosetting acrylic resin series, vinyl-denatured acrylic resin series, alkyd-denatured resin series, vinyl chloride resin series, and denatured alkyd resin series. Among these, vinyl-denatured acrylic resin series paints show the best results in adhesiveness and anticorrosiveness.

In the following we shall explain the basic process of the present invention with reference to the attached drawing showing the process steps. As is shown in the accompanying FIGURE of drawing illustrating the steps of the process of the present invention in series, a steel sheet 1 loosened by an uncoiler 2, said steel sheet being subjected to or not yet subjected to a surface treatment, is transferred through a guide roll 3 and a pinch roll 4 to a vessel 5 for degreasing and hot-water rinsing, wherein physical and chemical surface regulations are carried out, and then transferred through a roll 6 made of nylon series resin to a bonderising vessel 7. After the bonderising, the steel sheet 1 is then passed through a machine 8 for electrophoretic painting, wherein the water-soluble paint is regulated to be of a concentration suitable for the electrophoretic painting (for instance, to the concentration of about 15 weight percent), and there is applied an electric current of a determined voltage turned to a coil speed of the line so that an amount of the paint deposited on the steel sheet is 10 to 100mg/dm.sup.2.

As for water-soluble paint, any paint of the acrylic ester series, phenol alkyd series and the like may be used. After the steel sheet is coated with a determined water-soluble paint in the machine 8 for electrophoretic painting, it is then rinsed in a vessel 9 for rinsing and then lightly dried in a drying machine 10, which is provided with a wind box to dry the steel sheet by wind at room temperature. In this case, the drying is carried out by properly regulating the amount and temperature of the wind in accordance with the speed of the line.

The steel sheet, the undercoating of which has thus been completed, is then subjected to a finish painting according to per se conventional methods, for instance, it is led by guide rolls 11 and 12 to a roll coating machine 13, wherein a finishing is performed.

As for the paint for the finishing there may be mentioned paints of the thermosetting synthetic resins such as phenol, melamine, alkyd, acryl, vinyl chloride and the like, said paints being so diluted by solvents to such a viscosity as suitable for the finishing (for instance, 30 to 150 sec./20.degree. C. according to Fordcup No. 4). The paint as above-mentioned is applied on the steel sheet by regulating distances of the roll coater so that the dried coat has the thickness of 1 to 30 microns. 14 is a roll coating machine for painting the reverse (however, by carrying out the electrophoretic painting double painting of the reverse may be omitted). The finish-painted steel sheet is passed through a drying oven 15 having a hot wind heated up to a temperature of 150.degree. to 250.degree. C., wherein both the undercoat and the final coat are simultaneously baked.

The baked steel sheet is water-cooled in a water-cooling vessel 16 and transferred to a recoiler 18 through a pinch roll 17 to coil the steel sheet.

The bonderising step included in the above-mentioned process is a particularly advantageous foundation treatment, but is not a requisite condition. Therefore, it can be replaced by a chromate treatment or any other chemical-synthetic treatment, or it may be even omitted as the case may be. Further, it is, of course, evident that as for the paint for the finishing a water-soluble paint of a kind different from that for the undercoating may be applied.

However, in the practical operation of the process of the present invention, besides the basic steps as above-mentioned, the following three additional steps facilitate and secure the operation.

The first additional process is as follows:

In the successive painting of a strip steel sheet plated with zinc or zinc alloy (including galvanized steel sheet, melting zinc-plated steel sheet and the like) according to the method of the present invention, the steel sheet coated with a zinc layer or zinc alloy layer is subjected to a phosphate treatment or other chemical-synthetic treatment or other coat treatment which has a high electric resistance prior to the electrophoretic painting with a water-soluble paint or water-dispersed paint, the method of the present invention is characterized by maintaining the electric insulation resistance at the coat formed on the surface of the galvanized steel sheet by the chemical-synthetic treatment or the coat treatment of a high electric resistance to below 10.OMEGA. cm.sup.2, when effecting the electrodeposition, and then carrying out the final painting on the electrodeposited coat and baking the electrodeposited undercoat and the final coat at the same time.

In the case of the successive painting of a metal sheet, contrary to a general electrodeposition painting, a conductor roll is required as a device for supplying electricity to the metal sheet. In this case, if an electric current and a voltage to be supplied reach great values, due to the rise in the speed of the successive painting, there occur various troubles between the conductor roll and the material to be electrodeposited such as, for instance, occurrence of arc spots and melting out of the conductor roll.

This first additional step is to eliminate these troubles which may be caused at the conductor roll. In the following this additional step will be explained in detail by referring to concrete examples.

The application of the electrodeposition painting to an iron and steel product, particularly to a galvanized steel sheet, which is further chemical-synthetically treated, has proved to show much better results in achieving the uniformity of the paint coat and high speed painting as compared with the case of a conventional painting by using usual paint.

At present, the painting speed, which is in general use, is about 30 to 60 m/min, but a project is afoot to increase the painting speed even up to 100 to 150 m/min. In such a case, advantages to be obtained by the application of the electrodepositing painting method will be increased, but trouble as above-mentioned will become acute.

That is, with a speed up of the painting speed, electrodepositing conditions become more severe, particularly, a greater voltage and electric current will be required. For instance, when carrying out an electrodeposition with a painting speed of about 60 m/min. a voltage of about 50 V for 4 seconds, an electric current of 2,000 A will be required for 5 dm.sup.2 and 3 foot width.

As a greater electric current and voltage are thus required as the result of the speed up of the painting speed, the electric behavior of the coat formed by the pretreatment as a foundation for the electrodepositing painting exerts a great influence on the electrodepositing painting. That is, with an increase in the resistance of the coat formed by a phosphate treatment or chromate treatment the phenomena of arc spots occurring and the conductor roll melting out become remarkable. Further, there must be feared the danger of the melting out of the conductor roll occurring by the electrolysis due to the formation of an electrolytic cell between the conductor roll and the metal sheet to be treated and the occurrence of arc spots, if a rinsing water is on the material subjected to the foundation treatment at the position of the conductor roll where it is brought into contact with the strip coil sheet. As the result of various investigations made minutely in view of the facts as above-mentioned on the conditions for the foundation treatment and electrodepositing painting to perform the successive paintings, it has been confirmed that the following are the requisite conditions for regularly manufacturing an electrodepositing-painted metal sheet: that is, excessive agent used for the foundation treatment and excessive rinsing water should not be attached to the surface of the metal sheet to be treated and the insulation resistance of the coat formed by the foundation treatment should be less than 100.OMEGA. cm.sup.2, desirably less than 5.OMEGA. cm.sup.2 at the surface coming into contact with the conductor roll.

Thus, the features of the first additional step can be concluded as follows: the insulation resistance is kept at less than 10.OMEGA. cm.sup.2, advantageously less than 5.OMEGA. cm.sup.2 at the surface coming into contact with the conductor roll, and the coat to be formed by the foundation treatment is properly regulated in its quality, thickness and water content so that the above-mentioned condition of the insulation resistance may be fulfilled, and then a metal sheet, for instance, a galvanized steel sheet, which has been subjected to the foundation treatment in the controlled manner as above-mentioned, is sufficiently rinsed so that excessive agent used for the foundation treatment does not remain on the treated surface of the metal, and thereupon fully dried by wind or hot wind before the metal is subjected to the electrodepositing painting through the conductor roll to form an undercoat, on the treated surface of the metal.

The second additional step of the present invention is as follows:

In the successive painting of a strip steel sheet plated with zinc or zinc alloy according to the method of the present invention the steel sheet coated with a zinc layer or zinc alloy layer is subjected to a phosphate treatment or other chemical-synthetic treatment or other coat treatment which has a high electric resistance, or not subjected to the above-mentioned treatment, prior to the electrophoretic treatment with a water-soluble paint, the second additional step is characterized in that the supply of electricity should be so regulated that a contact rolling pressure given by the conductor roll on the surface of the galvanized steel sheet or chemical-synthetically treated galvanized steel sheet is more than about 100 kg per 1 meter width of the steel sheet.

In the case of electrodepositing painting, particularly anodic electrodepositing painting, it differs from a general metal plating by the following two points: that is, an anodizing and a high treating voltage. Consequently, in this case, special consideration is to be given to the conductor roll, through which electricity is supplied for the electrodepositing painting of a steel coil. If the rolling pressure between the metal sheet to be treated and the conductor roll is low, there occurs an arc between them, and if water and the like are on the surface of the metal sheet, an electrolytic cell is formed between the conductor roll and the metal sheet, in which the conductor roll acts as the anode, while the metal sheet as the cathode, whereby the melting out of the conductor roll metal occurs. The occurrence of arc between the conductor roll and the strip coil, which is usually galvanized and then subjected to the phosphate treatment, causes not only a melting out of the conductor roll as above-mentioned, but also a sticking of zinc oxide on the surface of the roll and a damage due to the sticking on the surface of the strip coil formed by the chemical-synthetic treatment.

The second additional step of the present invention is to eliminate various troubles connected with the occurrence of an arc, which proves a hindrance to the electrodepositing painting. To obtain the above-mentioned object of the second additional step, for instance, in carrying out the electrodepositing painting of a metal coil, this metal coil is pressed on the conductor roll at the upper part, the side part and the middle part of the latter by using a pressing roll or others. In this case, the rolling pressure should be about 100 kg, preferably above 300 kg per one meter width of the coil. In this case, of course, a pressing roll for imparting the necessary pressure is not limited to only one roll. A plurality of rolls may be used, as occasion demands. The pressing roll may be replaced by an imposition of a tensile force on the coil.

The third additional step of the present invention is as follows:

In the successive painting of a strip steel sheet plated with zinc or zinc alloy according to the method of the present invention, the metal sheet coated with a zinc layer or zinc alloy is subjected to a phosphate treatment or other chemical-synthetic treatment or other coat treatment which has a high electric resistance, prior to the electrophoretic painting with a water-soluble paint, this additional step is characterized in that electricity is supplied while arranging the surface of the conductor roll, which is to be brought into contact with the steel sheet, by removing a melt stuck to the said surface of the conductor roll, which is mainly composed of zinc or zinc oxide, by means of a scraper such as a doctor blade, immediately after the melt has stuck when supplying the electricity.

The state of contact of the conductor roll with the strip coil will be deteriorated, as the line speed increases. In this case, if an electric resisting coat is formed on the surface of the strip coil in advance, to which electricity is supplied, the production of sparks between the strip coil and the conductor roll increases, whereby there occurs the trouble of the conductor roll melting out.

Particularly, in the case, when the strip coil is plated with a metal such as zinc, which has a relatively small surface hardness and a low melting point, and moreover a coat of high electric insulation resistance is formed on the plated surface by a phosphate treatment, or a chromate treatment or other resinous coat forming, there are easily caused a trouble that the plating metal is oxidized and burnt by sparks or is stuck to the surface of the conductor roll, whereby the plated metal layer on the surface of the strip coil is damaged.

As a result of investigations it has been confirmed that the melt stuck to the surface of the conductor roll can easily be removed from the conductor roll, so far as it remains still in the soft state immediately after it has stuck to the conductor roll, but with the lapse of time, it becomes harder and difficult to remove, resulting in the formation of an increasing projection of the melt stuck on the surface of the conductor roll, because of a new one being successively accumulated after another. If the melt stuck on the surface of the conductor roll is not prevented without delay from being formed into a projection, not only damages caused on the surface of the strip coil will extend in a wider range as well as become heavier, the production of sparks will be accelerated, whereby the loss of the conductor roll caused by the melting will be hastened and the life of the conductor roll will remarkably be shortened, but also the production of sufficient products will be hindered.

The third additional step of the present invention is to eliminate the above-mentioned disadvantages by using a very simple means. That is, this step is characterized in that, when carrying out an electrophoretic painting or other electric re-coating treatment of a plated metal sheet having an electric resisting coat on its surface, while pressing the conductor roll on the conductor roll, the solidified melt stuck on the surface on the conductor roll when supplying electricity is quickly and continuously scraped off by means of a doctor blade made of, for instance, stainless steel or other metal. If the doctor blade is positioned at a region of the surface of the conductor roll which is not brought into contact with the strip coil (for instance, a zinc-plated steel sheet, which is further subjected to a phosphate treatment), while facing the edge of the blade at a proper angle (.alpha.), that is, about 10.degree. to 20.degree. to the normal of the conductor roll, then the solidified melt such as zinc oxide spottedly stuck on the surface of the conductor roll can easily and effectively be scraped off by the edge of the doctor blade, while the conductor roll is rotated, consequently a painted steel sheet having a favorable surface condition is obtained, as the electrophoretic painting can thereby be applied on the steel sheet, as it is.

At the same time also the loss of the conductor roll by melting and other disadvantages as above-mentioned can effectively be reduced.

Further, in order to suppress an increasing production of sparks between the strip coil and the conductor roll, which is caused by the supply of a greater electric current as a result of the line speed being increased, and which causes the loss of the conductor roll by melting, resulting in a remarkable shortening of the life of the conductor roll, the following countermeasure is to be taken, such as forming the edge part of the conductor roll with an electric insulating material by providing the edge part of the main body of the conductor roll itself with a rubber or plastic lining layer, so that the width of the surface of the conductor roll, to which an electric current is supplied, may be made narrower than that of the strip coil (for instance, a zinc-plate steel sheet having a coat formed by a phosphate treatment or other electric-insulating coat), advantageously, the former is made 0.9 times as wide as the latter, thereby to eliminate the part of the surface of the conductor roll, where sparks are most produced.

Examples of the present invention are explained in the following: ##SPC1##

The result of paint-coating tests made about the above-mentioned examples will be compared with conventional paint-coating methods. ##SPC2##

Next, a treating bath for electrophoresis adopted by the present invention and examples of using the said bath will be explained in Examples 2 and 3.

EXAMPLE 2

A galvanized steel sheet having a clean surface was made the anode, while a stainless sheet was used for the cathode. A coat was formed with an interpolar voltage of 80 V in the treating solution of the composition as shown in the following:

Treating bath: Copolymer of ammonium acrylate and butyl ester acrylate (ratio of polymerization 3/7 10 parts Ammonium chromate 0.2 part Water 90 parts pH 7.5 Anodizing chromate: 90 volts 30 seconds After rinsing baking for one minute at 250.degree. C

the thus obtained paint coat is a transparent one having a slightly yellow-colored luster and shows no change for 250 hours in a salt water spray test.

When an anodic electrophoretic treatment is carried out in the treating solution of the present invention, an insoluble electrodeposited coat is formed on the surface of a galvanized steel sheet, which is proportioned to a time of supplying an electric current, without resulting in a decrease of the electric current and, then by drying the said coat by heating after a simple rinsing or wringing a coat completely uniform and firm and excellent in anticorrosiveness can be obtained.

The thus-obtained galvanized steel sheet which was subjected to an electrodepositing treatment is practically used either as a steel sheet having an undercoat as a foundation for a further painting or an anticorrosive steel sheet itself according to more or less amount of the deposit. Further, a colored paint coat may be formed by causing an aqueous agent for dispersing coloring pigments subjected to an anionic activator treatment to co-exist in the treating bath of the present invention. This method is also practically used as a surface treating method which answers the purpose of effecting protection and decoration at the same time.

By adding further sulfuric acid or boric acid to the treating bath of the present invention, it becomes possible to remarkably improve the throwing power of the coat formed and to impart marked and useful properties thereto.

EXAMPLE 3

Treating bath: Copolymer of three kinds of compounds: methyl ester methacrylate, ammonium acrylate and isobutyl ester acrylate (ratios of copolymerization 3/3/4 ) 15 parts Zinc chromate 0 .25 parts Methylated melamine 10 parts Water 75 parts pH 8 Anodizing conditions: 60 volts 60 seconds After rinsing Baking for 2 minutes at 200.degree. C

in this example there has been formed an electrodeposited coat superior in atmospheric corrosion resistance and particularly in corrosion resistance to that formed according to Example 2.

EXAMPLE 4

A galvanized steel sheet was used as a metal sheet to be coated with paints. After brush-grinding by using a nylon series roll, the steel sheet was subjected to a conventional zinc phosphate treatment for 10 seconds at 50.degree. C. and then rinsed for 3 to 5 seconds, while buffing by means of a cotton buff. Then, it was again rinsed for 2 to 3 seconds and thereafter wind-dried. Then, the steel sheet was subjected to an electrodepositing coating with paints, in which a water-soluble electrodepositing paint as mentioned in Examples 2 and 3 was electrolytically deposited in an amount of 30 to 40 mg/cm.sup.2. Immediately after wind-drying, the sheet was coated with a final paint in the thickness of 15 to 20 microns. Thereupon the steel was fired at about 200.degree. C. for 50 to 60 seconds and coiled after water cooling.

Line speed 45 to 50 m/min Phosphate coat resistance 4 to 5 .OMEGA.. cm.sup.2 Electrodeposition current 40 to 5 V Time 3 to 5 seconds Temperature 40 to 50.degree. C. Amount of deposition 30 to 40 mg/cm.sup.2 Final coating paint Acrylvinyl series, 15 to 20 microns Salt water spray test excellent (for 500 hrs) Atmospheric corrosion resistance excellent (for 6 months) Working adhesivesness Dupont 1 kg. 50cm excellent Erichsen 7mm excellent Bending 180.degree. excellent

In this example there occurred neither arc spots nor loss of conductor roll by melting during the process of electrodepositing coating with paint.

EXAMPLE 5

A galvanized steel sheet subjected to a phosphate treatment was coated with paint by the electrodeposition for the time of 3 to 5 seconds under the conditions of the line speed of 45 to 50 m/min, 40 to 50 V and 4 to 5 A/dm.sup.2, while a rolling pressure of 300 kg per 1 meter of coil width was imposed by means of one roll. The electrodepositing paint was the water-soluble paint for use in the electrodeposition as used in Examples 2 and 3. The bath temperature was 40.degree. to 50.degree. C. The deposition amounted to 30 to 40 mg/dm.sup.2. The electrodeposition was followed by rinsing and wind-drying. Without carrying out a baking, the steel sheet was then coated with acrylvinyl series paint in the thickness of 15 to 25 microns and then baked at 140.degree. to 270.degree. C. for 50 to 60 seconds, and further rinsed again and coiled. After rinsing the steel was coiled. In this example there was seen no occurrence of arc spots and loss of conductor roll by melting. The properties of the product were as follows:

Salt water spray resistance excellent (for 500 hrs) Moisture resistance excellent (for 500 hrs) Atmospheric corrosion resistance excellent (for 6months) Working adherence Dupont 1 kg.sup.. 50 cm good - excellent Erichsen 7 mm excellent Bending 180.degree. excellent

EXAMPLE 6

A galvanized steel sheet was used as a metal sheet to be painted. After brush-grinding the steel sheet was subjected to a conventional zinc phosphate treatment by using a commercial zinc phosphate (at 70.degree. C. for 10 seconds). Then, the metal sheet was pressed down on a conductor roll with a rolling pressure of 300 kg per one meter coil width by two rolls respectively. The electrodepositing painting was carried out for 3 to 5 seconds under the conditions of the line speed 45 to 50 m/min, 40 to 50 V, and 4 to 5 A/dm.sup.2. The electrodepositing paint was a water-soluble acrylic acid series paint as used in Examples 2 and 3. The electrodeposit amounted to 30 to 40 mg/dm.sup.2. Immediately after rinsing and wind-drying the final coating was carried out with acrylvinyl series paint and fired at a temperature of 140.degree. to 270.degree. C. for 50 to 60 seconds. After water cooling, it was coiled. There was found no occurrence of arc spots and loss of conductor rolls by melting, and the product has the following properties:

Salt water spray resistance excellent (for 500 hrs) Moisture resistance excellent (for 500 hrs) Atmospheric corrosion resistnce Excellent Working adhesiveness Dupont 1 kg.sup.. 50 cm excellent Erichsen 7 mm excellent Bending 180.degree. excellent

EXAMPLE 7

In a coil painting of a hot-dip plated steel sheet, the steel sheet was subjected to a zinc phosphate treatment at a temperature of 70.degree. C. for 10 seconds by using zinc phosphate, after the brush-grinding. The electrodepositing paint composed of a copolymer of ammonium acrylate and acrylic ester was electrodeposited in an amount of 30 to 40 mg/dm.sup.2. After rinsing and wind-drying an acrylvinyl series paint was painted in a thickness of 15 to 25 microns by using a roll coater and baked at 140.degree. to 270.degree. C. for 50 to 60 seconds. Then, the steel was water-cooled and coiled. ##SPC3##

Thus even though the method of the present invention is a two-coating and one-baking method, the paint does not come turbid in its color, but shows an excellent adhesiveness and corrosion resistance, because in the method of the present invention an aqueous electrophoretic paint, which is essentially of the property of being very easily dried, is used as an undercoating paint and then a water-soluble or solvent-soluble paint is applied as a final paint and thereupon is baked and dried for hardening. Moreover, as the undercoat and the final coat, or more than two coats, as occasion calls, may be baked simultaneously at one time, not only there is required only one baking furnace but also the painting line may be remarkably shortened in length. There are still further advantages obtained by the method of the present invention that the painting operation can easily be managed and thereby the yield of products may highly be improved.

As is shown in examples as above-mentioned, the method of the present invention can be applied not only to a successive painting of a steel coil, but also to a successive painting of a cut steel sheet.

Claims

What is claimed is:

1. A method for manufacturing a painted metal sheet by successive paintings which comprises the steps of subjecting the metal sheet to an electrophoretic painting with an aqueous paint as an undercoating painting, rinsing and wind-drying the undercoat formed by the electrophoretic painting and then subjecting the resultant undercoated metal sheet to a non-electrophoretic final painting with an organic solvent-soluble thermosetting paint and thereafter baking the resultant electrodeposited undercoat and final coat together in a single baking step.

2. A method for manufacturing a painted metal sheet by successively painting flat metal sheet or strip coil which is plated with zinc or zinc alloy, which comprises the steps of subjecting the said metal sheet or strip coil to an electrophoretic painting with a water-soluble acrylic resin series paint as a underpainting, rinsing and wind-drying the undercoat formed by the electrophoretic painting, and immediately thereupon non-electrophoretically painting the undercoated metal sheet or strip coil with at least one kind of thermosetting paint selected from the group consisting of the organic solvent-soluble phenol resin series, the melamine resin series, the alkyd resin series, the acrylic resin series and the vinyl resin series or their copolymers as the final painting and then baking the resultant electrodeposited undercoat and final coat together in a single baking step.

3. A method for manufacturing a painted steel sheet or strip coil by successively painting steel sheet or strip coil which is plated with zinc or zinc alloy, which comprises the steps of subjecting the steel sheet or strip coil to an electrophoretic painting by dipping the same into a water-soluble electrodepositing treating bath, said bath consisting of a water-soluble resin, which is composed of acrylic ester and/or methacrylic ester and its ammonium salt or amine salt, and a water-soluble chromium compound added in an amount of about 0.01 to 5.0 weight percent to the said resin, rinsing and drying the undercoat formed by the electrophoretic painting, when the coat is deposited in an amount of 150 mg/dm.sup.2, and immediately thereupon non-electrophoretically painting the undercoated steel sheet or strip coil with at least one kind of thermosetting paint selected from the group consisting of phenol resin series, melamine resin series, alkyd resin series, acrylic resin series and vinyl resin series or their copolymers as the final painting and baking the resultant electrodeposited undercoat and thermosetting final coat at the same time in a single baking step and cooling the resultant product.

4. A method as claimed in claim 3, wherein a water-soluble phosphoric acid series additive or boric acid series additive or pigment is added to the electrodepositing treating bath.

5. A method as claimed in claim 4, wherein the steel sheet or strip coil which is plated with zinc or zinc alloy, is further subjected to a phosphate treatment or chromate treatment or other coat treatment to obtain a phosphate coat or chromate coat or a coat having high electric resistance at the outermost layer.

6. A method for manufacturing a painted steel sheet or strip coil by successively painting steel sheet or strip coil, which is plated with zinc or zinc alloy, which comprises the steps of subjecting the steel sheet or strip coil to a chemical-synthetic treatment to obtain a coat having a high electric resistance, then subjecting the chemical-synthetically treated steel sheet or strip coil to an electrophoretic painting with a water-soluble paint or a water-dispersed paint, while keeping the electric resistance of the chemical-synthetically formed coat to below 10.OMEGA. cm.sup.2, rinsing and wind-drying the undercoat formed by the electrophoretic painting and then subjecting the resultant undercoated steel sheet or strip coil to a non-electrophoretic final painting with an organic solvent-soluble thermosetting paint as the final painting and thereafter baking the resultant undercoat electrodeposited and the thermosetting final coat together in a single baking step.

7. A method as claimed in claim 6, wherein the electric resistance of the chemical-synthetically formed coat is kept to below 5.OMEGA. cm.sup.2.

8. A method for manufacturing a painted steel sheet or strip coil by successively painting steel sheet or strip coil which is plated with zinc or zinc alloy, which comprises the steps of subjecting steel sheet or strip coil to an electrophoretic painting after optional chemical-synthetical treatment, in advance, while regulating the contact rolling pressure between the steel sheet or strip to be treated and a conductor roll so that it is above about 100 kg per one meter width of the steel sheet or strip coil, rinsing and wind-drying the undercoat formed by the electrophoretic painting and then subjecting the resultant undercoated steel sheet or strip coil to a non-electrophoretic final painting with an organic solvent-soluble thermosetting paint and thereafter baking the resultant undercoat electrodeposited and the thermosetting final coat together in a single baking step.

9. A method as claimed in claim 8, wherein the contact rolling pressure between the steel sheet or strip coil to be treated and the conductor roll is regulated to be more than about 300 kg per one meter width of the steel sheet or strip coil.

10. A method for manufacturing a painted steel sheet or strip coil by successively painting steel sheet or strip coil, which is plated with zinc or zinc alloy, which comprises the steps of subjecting the steel sheet or strip coil to a chemical-synthetical treatment to obtain an undercoat as a foundation for painting which has high electric resistance, and then subjecting the chemical-synthetically treated steel sheet or strip coil to an electrophoretic painting with a water-soluble paint or a water-dispersed paint, while removing solidified melt stuck to the surface of a conductor roll, to which electricity is supplied, such as zinc or zinc oxide, by means of a doctor blade, immediately after it has stuck to the surface of the conductor roll and arranging the surface of the conductor roll, which is brought into contact with the said steel sheet or strip coil, rinsing and wind-drying the resultant undercoat formed by the electrophoretic painting and then subjecting the resultant undercoated steel sheet or strip coil to a non-electrophoretic final painting with an organic solvent-soluble thermosetting paint as the final painting and thereupon baking the resultant undercoat electrodeposited and the thermosetting final coat together in a single baking step.