DE2002599B2 - PROCESS FOR APPLYING ORGANIC COATS OF VARNISH AND COLOR TO METALLIC SURFACES - Google Patents

Description

The invention relates to a method for applying organic lacquer and paint coatings metallic surfaces, in which on the metallic surface or on the one provided with a chrome layer A chromium hydroxide coating is applied to the metallic surface.

A method of the aforementioned type is known from British patent specification 1046434, for example. In this process, a metal, for example steel, a chrome and a applied non-metallic layer consisting of chromium hydroxide. The respective layer thicknesses as well as the total thickness of the coating depend on the duration of the treatment. After coating the coating of metallic chromium and chromium hydroxide should have a high level of corrosion resistance, Have paintability and impact resistance.

A similar process to the electrochemical Application of a coating of chromium hydroxide to metals is from the German Auslegeschrift 1236 898 known; it works with an electrolyte, which in terms of a higher current efficiency and a better throwing power and a stabilized coating contains certain fluorine compounds.

The above-mentioned coatings inherently have good adhesion if the respective organic lacquer and paint coating is applied immediately after the application of the chromium hydroxide layer. The high resistance to corrosion is due to the fact that chromium hydroxide coatings are extremely chemically active. With the high chemical activated ^ tivity of the coating, however, is the risk of adsorption of foreign bodies or contaminants during transport, storage and further processing of the coated Guts connected. This risk is unavoidable if the coated material comes into contact with oily substances for reasons of further handling or further processing. This cannot be avoided at all in deep-drawing, since deep-drawing is hardly possible without deformation-relieving agents, in particular oily lubricants. As a result, although the chromium hydroxide layer is excellent in adhesiveness for a paint coating immediately after application, this adhesiveness deteriorates as organic substances are adsorbed on the chromium hydroxide layer. This applies in particular to the very important melamine alkyd varnishes.

The invention is therefore based on the object of maintaining the inherently good adhesion of the chromium hydroxide layer with its high adsorptive capacity and of eliminating the disadvantageous consequences, in particular of greasy surface contamination. In the case of a method of the type mentioned at the outset, this object is achieved in that, according to the invention, an aqueous solution of a binder is applied directly to the wet chromium hydroxide coating and dried. The binding agent forms a protective or barrier layer, which is able to counteract any impairment of the natural adhesion due to adsorbed contamination ^; avoid tendencies, especially lubricants. This works particularly well when oleoresins, alkyd resins, aminoalkyd resins, phenolalkyd resins and acrylic resins are used individually or in a mixture as water-soluble binders. It is important that the binder does not impair the adhesion of the paint and that it polymerizes into the paint layer itself. In addition, the binder must be water-soluble so that it is adsorbed uniformly while the chromium hydroxide layer is still wet and free from other contaminants after washing with water. Furthermore, after normal drying, the binder should be essentially insoluble and resistant to vegetable oils which are used in subsequent process stages to protect against abrasion and scratches. Finally, the binder must not produce poor paint adhesion caused by lubricants used in molding, and must also allow these lubricants to be easily removed before the paint is applied.

The method according to the invention is suitable for all metals which, after the application of a chromium hydroxide coating are corrosion-resistant such as steel, zinc, tin, lead, chrome, aluminum, Copper, magnesium, silver and their alloys, regardless of whether the coating is electroless or is applied galvanically.

When immersing, the metal to be treated is immersed in an aqueous solution, which is the main constituent individually or in groups of sodium dichromate, potassium and sodium chromate and chromic anhydride with additions of sulfuric acid, sulfates, nitric acid and fluorine compounds.

When electroplating the metal is cathodically in an aqueous solution of chromic anhydride or Brought sodium dichromate, which also contains additives such as sulfuric acid, sulfates, fluorine compounds, May contain nitric acid, manganates, tungstates and molybdates. Electroplating can

also as electrolytic treatment and chrome plating of steel in an aqueous solution of chromic anhydride with additives such as phosphoric acid, sulfuric acid, boric acid, sulfonic acid, Acetic acid, hydrochloric acid, bromic acid, hydrofluoric acid and their salts and fluorine compounds take place.

In the method according to the invention, the chromium hydroxide coating is first applied to the metal surface brought, washed with water, for a short time in an aqueous solution of the water-soluble Dipped or sprayed binder while the surface layer is still wet, whereupon eventually the workpiece is squeezed between rollers to create a thin even layer of the to achieve water-soluble binder on the layer of chromium hydroxide.

The water-soluble binders according to the invention, since they are resins, are water-soluble paints form, compatible with the paints applied to the metal and polymerize after the Painting itself in a coating layer, so that there is better adhesion of the paint. In addition, the thin layer of the water-soluble layer formed on the surface of the chromium hydroxide layer The binder not only adheres firmly, but also becomes practically insoluble after drying is not attacked by oily substances such as lubricants, so that degreasing can also be carried out with ease can be carried out without adversely affecting the adhesion of the organic coating.

The water-soluble binders of the invention can be broken down into the five listed below Divide groups.

1. Oleoresins.

These resins are made by mixing rosin, ester gum, alkylphenol resin, coumarone resin, Cyclopentadiene resin, petroleum resin and mahogany nut shell oil or epoxy resin with castor oil, wood oil, flaxseed oil or soybean oil and by subsequent reaction with maleic acid or instead with fumaric acid, Itaconic acid, acrylic acid or citraconic acid.

2. Alkyd resins.

These polyesters are made from polyalcohols such as glycerine, trimethylolethane, trimethyiolpropane, Pentaerythritol, sorbitol, diglycerin, dipentaerythritol, epichlorohydrin. Allyl glycerol ether, phenyl glycerol ether as well as epoxy resin and dibasic acids such as phthalic acid, maleic acid, fumaric acid, succinic acid, adipic acid, itaconic acid, Citraconic acid or, instead of the above-mentioned dibasic acids, from polybasic acids Acids such as benzene tricarboxylic acid, benzene tetracarboxylic acid, phosphoric acid, additions of rosin and maleic anhydride and maleic oils. Polyesters with polyoxymethylene bonds are also suitable, by using polyether as polyalcohol, such as polyethyleneglycol and propylene glycol are formed.

3. Phenolic alkyd resins.

These include alkyd resins modified with resol, the crude component of which is kr., Oil, xylenol or is butylphenol; also with, mem pentaerythritol ester of diphenolic acid "modified alkyd resins and finally with a water-soluble one Resin made by incorporating carboxyl and then methylol groups modified alkyd resins.

4. Aminoalkyd resins.

This is with methoxymethylmelamine, for example dimethoxymethyltrimethylolmelamine and hexamethoxymethylolmelamine modified alkyd resins, as well as partially etherified polymethylolmelamine or polymethylolurea modified alkyd resins as well as polymethylolmelamine, Alkylenpolymaine, Hydroxypolyar.iine and Gu-

lu anidine modified alkyd resins.

5. Acrylic resins.

These are copolymers of acrylic or methacrylic acid with methyl acrylate, Ethyl acrylate or methyl crotonate; further around

π copolymers of the alkyl ester, the acrylic

or methacrylic acid with vinyl toluene; furthermore to hydroxymethylated copolymers of acrylamide or methacrylamide with N-vinyl lactam; and finally to mixed polymer

sate of N-methylolamide with ethyl acrylate.

The water-soluble binders listed above can either be used individually or in Combination can be used.

Provided a water soluble binder on its own some paints can be used due to the low affinity of the binder used result in only poor adhesion with the base resin. To avoid this, it is therefore desirable to use two or more water-soluble binders together. Good adhesion to all types of Paints result from the use of alkyd resin binders, in which epoxy resin is used as polyalcohol.

The amount adsorbed can be adjusted by adjusting the

.15 Binder concentration in the aqueous solution can be varied as desired. However, the preferred concentration is 0.1 to 8% or 3 to 100 mg / nr in the adsorbed amount of the binder. These amounts are small in comparison with those which are conventionally used for water-soluble paints, in which these values are, for example, 10 to 20% in the concentration of the binder or 50 to 500 mg / dm ² . The binders used according to the invention have neither the purpose of underprotection nor that of a pore filler in the conventional sense. The amount of binder used, namely 3 to 100 mg / m ^2, is completely sufficient to form a thin layer; baking is not required, just a simple removal of the water by an ordinary drying process. The layer is extremely thin, i.e. less than 0.1μ, so it is colorless and transparent without affecting the appearance. It does not act as a protective film, but only acts as a barrier to the adsorption of oily substances.

The lower limit of the adsorbed layer, namely 3 mg / m ^2, is to be regarded as the minimum limit necessary to completely cover the entire surface; smaller amounts do not produce the desired effect. The adsorbed amount of 3 mg / u ^{2 of} the water-soluble binder corresponds to a concentration of 0.1% in a solution. Below this concentration, the paint adhesion is poor

b5 sufficient if, for example, a lubricant is applied to the metallic material. On the other hand, an amount of binding agent exceeding the upper limit of 100 mg / m ² leads to such a thick one

Binder layer that often occurs when rolling or squeezing an uneven compression, which has an adverse effect on the appearance. The adsorbed amount of 100 mg / m ² corresponds to a concentration of 8% in a solution. A higher concentration and thus a larger adsorbed amount does not produce the desired effect, but affects the appearance and leads to incomplete drying. For these reasons, the amount of the water-soluble binder is limited to 3 to 100 mg / m ² .

The metals to be treated according to the invention can be bent or drawn, with lubricants can be used; they were then degreased and finally covered with a varnish or paint. It is essential that the thin binder layer adheres firmly to the chromium hydroxide layer so that they is practically insoluble and does not adsorb lubricant, so that complete degreasing is possible. The make coat is coated with common stoving paints such as B. alkyd, melamine alkyd, epoxy, Compatible with epoxy melamine alkyd, acrylic and vinyl paints. This is due to the property of the binder to be polymerized into the applied organic lacquer layer and the adhesion of the paint.

The thin layer of binder on the metal is practically insoluble and can never get through the usual Removed solvent degreasing such as trichlene degreasing and emulsion degreasing will. However, partial removal from the surface is possible if treated with a strong alkaline degreaser containing sodium hydroxide or sodium silicate. Even when the fat is completely removed, part of the binding agent remains and thus improves the adhesion of the colors.

Even when an oily substance is used in an amount of 3 to 10 mg / m ^{2 in} order to avoid abrasion and scratching of the metallic surface, direct painting is possible without the need for further treatment for good paint adhesion. This is probably due to the fact that the oil-containing substance is not adsorbed by the thin binder layer, but rather diffuses into the paint without any adverse effect.

The oils used according to the invention also include di-2-ethylhexyl sebacate, cottonseed oil and flaxseed oil.

A better understanding of the invention can be obtained from the examples given below in conjunction with the drawing. It shows

1 shows a cross section of a metallic material according to the invention,

Fig. 2 shows a corresponding cross section of a metallic Material, but the metallic base body is provided with a coating layer,

Fig. 3 shows a further cross section, wherein on the an oil layer is applied to the metallic material shown in FIG. 1,

FIG. 4 shows a further cross section, with an oil layer on the metallic material according to FIG. 2 ii is upset.

In Figs. 1 and 2, 1 is the metallic base body, with 2 the chromium hydroxide layer and with 3 clic a thin layer of the water-soluble binder hc / cichncl. In ilen FIGS. 3 and 4, 4 is an intermediate layer of an over / .ugsmctalls / wipe the

metal base body 1 and the chromium hydroxide layer 2, while 5 is an oil film is.

Some examples based on the teaching according to the invention are shown below.

example 1

A cold-rolled steel strip was cathodically treated in an aqueous solution containing 60 g / l chromic anhydride and 0.55 g / l sulfuric acid in order to form a double layer of chromium hydroxide and metallic chromium on the steel strip. Thereafter, the steel strip was washed with water and immersed for two seconds in a 4% strength aqueous solution of an oleoresin as a binder obtained by heating a mixture of 75 parts of castor oleic anhydride, 5 parts of an epoxy resin and 20 parts of maleic anhydride. After compacting by means of rollers to adjust the adsorbed amount of the binder to 30 mg / m ² , the steel strip was dried at room temperature. Spindle oil (No. 60) was used in an amount of 2 g / m ² for the surface protection of this sample. After degreasing with Trichlendampf for two minutes, a white melamine alkyd varnish was applied in a thickness of 25 μm and baked. The Erichsen test with an A "mark gave a value of 0/100 for the exfoliated surface. In contrast, a similarly treated steel without adsorbed binder adheres spindle oil to its surface; in this case the Erichsen test gave a value of 97/100 .

Example 2

A cold-rolled steel strip was electrolytically treated in a chromic acid solution as in Example 1, washed with water and then sprayed for two seconds with a 0.5% strength aqueous solution based on the weight of the water-soluble alkyd resin binder, the binder being produced by heating a mixture , which consisted of 40 parts of castor oil anhydride, 15 parts of trimethylolpropane and 45 parts of phthalic acid. The steel strip was then squeezed between rollers to adjust the adsorbed amount of the binder to 10 mg / m ² , then dried with hot air at 50 ° C. and finally provided electrostatically with cottonseed oil in an amount of 8 mg / m ² . After a month, an alkyd coating (hammer paint) was applied in a thickness of 25 μm by baking. The Erichsen A 'test gave 0/100 of the exfoliated surface, while the same test on a sample without the use of a binder gave a value of 100/100.

Example 3

After one month the samples were pressed according to Example 2 using a No. 620 pressing oil at a draw ratio of 2 to a cylinder with a diameter of 40 mm and flat bottom. After one week, the samples were immersed in cold trichons for one minute and degreased and then exposed to trichloro steam for two minutes. A mclaminalkydlack was in a Thickness of 30 μπι applied and baked. With X-shaped scribing according to Erichsen on the bottom and on the side wall of the cylinder, the sample with the resin binder gave a value of 0/100, while

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the test on the same cylinder of the other sample, in which no binder was used, on Bottom 96/100 and 50/100 on the side wall resulted.

Example 4

An electrolytically tinned plate was treated as a cathode in a 3% solution of sodium dichromate to apply a chromium hydroxide coating. The plate was then washed with water and immersed for one second in a 0.2% strength aqueous solution of a binder which consisted of 80 parts of a water-soluble alkyd and 20 parts of hexamethoxymethylmelamine. The alkyd was obtained by heating a mixture of 60 parts of linseed oil, 10 parts of glycerin and 30 parts of maleic acid. The wet plate was squeezed between rollers to adjust the adsorbed layer to 5 mg / m ² . After drying at room temperature, di-2-ethylhexyl sebacate was applied in an amount of -'o 5 mg / m ² using turpentine oil substitute as the solvent.

A white melamine alkyd varnish was applied to this sample applied and baked, in a thickness of 10 μπη, whereupon the product with a Draw ratio of 2 to a 40mm diameter cylinder with a flat bottom was deep drawn. The Erichsen test gave values on the bottom and on the side wall of the cylinder of 0/100 and 1/100, during the same experiment JO on a cylinder made from a sample without a binder was drawn deep, gave values of 43/100 and 82/100.

Example 5 _J5

A cold-rolled steel strip was electroplated with a 0.01 μ thick layer of chromium hydroxide and metallic chromium in an aqueous solution of 30 g / l chromic anhydride and 0.1 g / l chromium sulfate. Immediately after washing, an aqueous solution containing 6% of a binder was sprayed on, which consisted of 40 parts of partially butoxylated methylolurea and 60 parts of a water-soluble alkyd resin according to Example 2. Thereafter, the steel strip was squeezed between rollers to adjust the adsorbed amount of the binder to 70 mg / m ² , and dried by blowing hot air at 60 ° C. Cotton seed oil was then applied electrostatically in an amount of 3 mg / m ² and rust protection oil in an amount of 5 g / m ² to. After three months, the sample was deep-drawn with a draw ratio of 2 to a cylinder 40 mm in diameter with a flat bottom. The cylinder was subjected to two seconds of Sprühentfettung at 60 ° C with a 3% solution of an al-5 ^ kaiischen detergent. Then a melamine alkyd clear lacquer was applied in a thickness of 30 μm and stoved. The Erichsen test was carried out on the bottom and on the side wall of the cylinder and gave a value of w> 0/100. The same treatment was carried out with a comparative sample, but without a binder. This resulted in incomplete degreasing, so that the flaked area gave values of 100/100 and 93/100. Furthermore, a salt spray test b5 of the sample according to the invention without an organic coating at 35 ° C. after 20 hours immediately after the application of cottonseed oil resulted in no rust, while the same test on a binder-free sample showed a rust ratio of 2.5%.

Example 6

A galvanized steel strip was electrolytically treated by the chromate method in an aqueous solution of 30 g / l of chromic anhydride and 0.2 g / l of sulfuric acid to form a layer of chromium hydroxide on the steel strip. Immediately after washing with water, a 1% aqueous solution was sprayed from phenol-modified alkyd resin and then squeezing the tape between rollers to the adsorbed amount of the binder to 15 mg / ^m: set. After drying, machine oil No. 120 was applied in an amount of 5 mg / m ^2. After three months, the sample was degreased for one minute by immersion in cold Trichlene and then for a further minute in Trichlene steam. A melamine alkyd resin varnish was then applied in a thickness of 25 μm and baked. The Erichsen test gave a value of 0/100, while a comparison sample without a binder showed a value of 38/100.

Example 7

A cold-rolled steel strip was first coated with a chromium layer 0.025 μm thick and then electrolytically treated by the chromate process in a solution containing 60 g / l chromic anhydride, 5 g / l acetic acid and 0.5 g / l sodium fluorosilicate. The layer mainly contained chromium hydroxide. Immediately after washing with water, the tape was immersed for one second in an aqueous solution containing a 1% water-soluble acrylic resin made from a copolymer of N-methylolamide and acrylic acrylate. The tape was then squeezed between rollers to adjust the adsorbed amount of the binder to 15 mg / m ² . After drying at room temperature, the sample number was. Thermoformed 640 press oil into a cylinder of 80 mm diameter and 60 mm depth, was immersed one minute each in cold trichlene and 2% sodium hydroxide solution at 6O ⁰ C for degreasing. Then a phthalic acid alkyd varnish of 15 μm thickness was applied and stoved. The Erichsen test gave values of 0/100 on the bottom and on the side wall of the cylinder, while the same test on a cylinder made from a similar but binder-free sample gave a value of 100/100 on the bottom and a value of 97 on the side wall / 100 resulted.

Example 8

A cold-rolled steel strip was subjected to an electrolytic chromic acid treatment, and in an aqueous solution containing 50 g / l chromic anhydride, 0.2 g / l sulfuric acid and 0.4 g / l sodium fluorosilicate, around a double layer of chromium-containing chromium hydroxide and metallic chromium to raise. Immediately after washing with water, the tape was placed in a 1.0% strength aqueous solution of a binder mixture of 50 parts of a water-soluble oleoresin according to Example 1 and 50 parts of a water-soluble alkyd resin according to Example 2 for one minute. The tape was then squeezed between rollers to adjust the adsorbed amount of the binder to 15 mg / m ² , and then at

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20th

Room temperature dried. A white melamine alkyd varnish was applied in a thickness of 25 μm and baked. The Erichsen experiment gave 0/100. A rust preventive oil was also applied to the sample in an amount of 5 mg / m ² and the sample was stored for three months. Thereafter, the sample was degreased by immersion in cold Trichlene for one minute and subsequent treatment with Trichlene steam for two minutes and a white melamine alkyd varnish applied to a thickness of 25 μm and baked. The Erichsen test gave a value of 0/100 for the exfoliated surface.

The Erichsen test on a similar sample to which no binder had been applied gave 23/100 if the same white lacquer was applied directly and 100/100 if the above Anti-rust oil has been applied for three months.

Example 9

A cold-rolled steel strip was coated with a 0.025 µm thick chromium layer and subjected to the electrolytic chromate process in an aqueous solution containing 80 g / l sodium dichromate and 2 g / l ammonium sulfate to apply a chromium hydroxide layer. Immediately after washing with water, the tape was immersed for one second in a 4% strength aqueous binder solution which consisted of 70 parts of a water-soluble alkyd resin according to Example 2 and 30 parts of a water-soluble acrylic resin according to Example 7. The tape was then squeezed between rollers and in this way the adsorbed amount of the binding agent was ^{adjusted to 45 mg / m 2} ). After drying at 60 ° C., di-2-ethylhexyl sebacate was then applied in an amount of 5 mg / m ² . Two samples prepared in this way were provided with an alkyd white lacquer and an acrylic white lacquer in a thickness of 30 μm and fired. The Erichsen test produced a value of 0/100 for the paint layer.

Two further samples to which the binder and di-2-Äthylhexylsebacat were applied, were at a draw ratio of 2 using a machine oil no. 120 into a cylinder of 40 mm diameter and a flat bottom deep-4 ^r> subjected. Degreasing took place for one minute by

40 dip in cold trichlene and two minutes with trichlene steam. Then a white alkyd varnish and a white acrylic varnish were applied in a thickness of 30 μm and baked. The Erichsen test on these two samples showed no peeling of the coating on the bottom and the side wall.

On the other hand, similar samples to which no binder was applied gave di-2-ethylhexyl sebacate directly and white alkyd varnish and a white acrylic varnish were applied, values of 97/100 or 8/100. Further Comparative samples to which no binder but D.O.S. was applied directly, were after Deep drawing into a cylinder with a flat bottom, degreased and coated. The Erichsen attempt on this both samples gave values of 83/100 for the bottom and 45/100 for the side wall of the cylinder with the Alkyd varnish and values of 17/100 and 10/100 in the case of acrylic varnish.

Example 10

A cold-rolled steel strip was coated with chromium in an aqueous solution containing 150 g / l chromic anhydride and 1.5 g / l sulfuric acid, around a metallic chromium layer of 0.32 g / m ² on the strip and an aqueous chromium oxide layer of 8 mgCr / m ² to be applied to the chrome layer. Immediately after washing with water, the tape was sprayed with a 1% strength aqueous solution of a binder which consisted of 5 parts of a half-ester of an epoxy resin and trimellitic anhydride and 1 part of polymethylolmelamine. The tape was squeezed between rollers to adjust the adsorbed amount of the binder to 9 mg / m ² , then dried, and then provided with 3 mg / m ^{2 of} linseed oil.

Samples of a tape treated in this way were coated with a white alkyd varnish, a white melamine alkyd varnish, a white acrylic varnish or polyester coating in a thickness of 30 μm. The Erichsen test yielded values of 0/100 for these samples.

In contrast, similar samples, on which no binder, but similar coatings were found plotted directly, values of 20/100, 35/100, 9/100 and 13/100.

1 sheet of drawings

Claims (5)

Patent claims: 1. Process for applying organic lacquer and paint coatings to metallic surfaces, with the one on the metallic surface or on the metallic one provided with a chrome layer Surface a chromium hydroxide coating and after deforming in the presence of a lubricant a lacquer or paint coating is applied is characterized in that directly on the wet chromium hydroxide coating an aqueous solution of a binder is applied and dried. 2. The method according to claim 1, characterized in that the water-soluble binder Oleoresins, alkyd resins, aminoalkyd resins, phenol alkyd resins and acrylic resins, individually or as a mixture be used. 3. The method according to claim 1 or 2, characterized in that the binder layer is applied ^{in a thickness of 3 to 100 mg / m 2.} 4. The method according to any one of claims 1 to 3, characterized in that on the binder layer an oil film is applied. 5. The method according to claim 3, characterized in that on the binder layer di-2-ÄthyIhexylsebacat, Cottonseed oil or flaxseed oil is applied.