EP0184756A2 - Electrolyte for the electrochemical treatment of metal plates, and process for the manufacture of anodised metal plates, especially for the application as printing plate supports - Google Patents

Abstract

The present invention relates to an electrolyte for the anodization of metal plates, in particular made of aluminum or its alloys, which consists of a water-soluble organic acid from the group consisting of phosphonic, phosphoric, sulfonic or at least tripasic carboxylic acid or mixtures thereof, the electrolyte being one with this compatible base, in particular alkali or ammonium hydroxide, is added, the addition being chosen so that a pH of 3 to 10, preferably 4 to 8, in particular 6 to 7, is set. The invention also relates to a method for anodization using the electrolyte and the use of the metal plate as a support for lithographic purposes.

Description

Electrolyte for the electrochemical treatment of metal plates and method for producing anodized metal plates, preferably for use as a printing plate carrier

The present invention relates to the electrolytic surface treatment of metal plates and the use of the products obtained.

The treated metal plates have increased corrosion resistance and are suitable for use in planographic printing, among other things. When used as a support for planographic printing plates, they are notable for increased adhesive strength of the photosensitive layers, longer print runs, less wear on the image and non-image areas, longer shelf life and improved hydrophilicity in the non-image areas. This is especially true when using aluminum or aluminum alloys.

Anodic oxidation is an electrolytic process in which the metallic workpiece is treated as an anode and treated in a suitable electrolyte. When electrical current flows from a cathode through the electrolyte to the metallic workpiece, the surface of the metal is converted to one of its oxide forms with decorative, protective, or other properties.

The cathode is made of metal or graphite, and the only significant reaction that occurs on it is the evolution of hydrogen. The oxide layer is formed from the solution side, that is, outside the metal, so that the oxide formed is directly adjacent to the metal. The oxygen required for oxide formation comes from the respective electrolyte.

• Zum einen gibt es die sogenannte Sperrschicht, die entsteht, wenn der verwendete Elektrolyt nur eine geringe Fähigkeit zum Anlösen der Oxidschicht besitzt. Diese Schichten sind im wesentlichen nicht porös. Die Dicke ist begrenzt auf etwa 13 A/Volt. Nach Erreichen dieser Grenzdicke bildet die Schicht eine wirksame Sperrschicht gegen weiteren Ionen- oder Elektronenfluß. Der Strom fällt auf einen geringen Wert ab, und die Oxidbildung hört auf. Für dieses Verfahren werden Bor- und Weinsäure als Elektrolyten eingesetzt.

Oxide layers produced on aluminum anodically are divided into two main classes:

• On the one hand there is the so-called barrier layer, which is created when the electrolyte used has only a slight ability to dissolve the oxide layer. These layers are essentially non-porous. The thickness is limited to approximately 13 A / volt. After reaching this limit thickness, the layer forms an effective barrier layer against further ion or electron flow. The current drops to a low value and the oxide formation stops. Boric and tartaric acid are used as electrolytes for this process.

On the other hand, the oxide layer does not reach its final limit thickness if the electrolyte exerts a noticeable solution on the oxide: current continues to flow and the oxide layer contains a "porous" structure. Porous structures can be quite thick, that is, they can reach a thickness of a few 10 pm, but a thin barrier layer made of oxide is always retained at the boundary between metal and oxide.

Investigations under the electron microscope have shown that small, closely spaced oxide cells are present on the entire oxide layer and generally extend perpendicular to the interface between the metal and oxide layers.

Sulfuric acid is used as the electrolyte in most cases, and phosphoric acid is also frequently used. Anodically produced aluminum oxide layers are harder than oxide layers obtained under the influence of air.

If the anodic oxidation takes place for decorative or protective reasons or to improve the adhesion, strong electrolytes such as sulfur and phosphoric acid are used. A mixture of these two electrolytes is used in US Pat. No. 2,703,781.

US Pat. No. 3,227,639 describes the use of a mixture of sulfophthalic and sulfuric acids for producing protective and decorative anodic oxide layers on aluminum. Other aromatic sulfonic acids are used in accordance with US Pat. No. 3,804,731 in a mixture with sulfuric acid.

Anodization is followed by an aftertreatment step in numerous processes in which the porous surface is sealed, as a result of which the final properties of the layer are determined. For example, pure water can be used for this at high temperatures. It is believed that part of it the oxide layer is dissolved and precipitated again as a voluminous hydroxide (or hydrated oxide) within the pores. Other aqueous sealants contain metal salts, the oxides of which can be precipitated together with the aluminum oxide.

In US-A 3 900 370 a mixture of calcium ions and a water-soluble phosphonic acid is used for sealing, which forms a complex with the divalent metal ion, whereby the anodized aluminum (or the anodized aluminum alloys) is protected against corrosion. Polyacrylamide is proposed as a sealant.

According to US Pat. No. 3,915,811, an organic acid (acetic acid, hydroxyacetic acid or aminoacetic acid) is added to a mixture of sulfuric and phosphoric acid in order to produce an anodic oxide layer on a plate which is then to be galvanized.

US Pat. No. 4,115,211 describes the anodic oxidation of aluminum by means of alternating current or superimposed alternating and direct current. The electrolyte used contains a water-soluble acid and a water-soluble salt of a heavy metal. The water-soluble acid can be oxalic, tartaric, citric, malonic, sulfuric, phosphoric, sulfamic or boric acid.

US Pat. No. 3,988,217 describes the use of an electrolyte which contains quaternary ammonium salts or aliphatic amines and a water-soluble thermosetting resin and which is used for the anodic oxidation of aluminum for protective and decorative purposes and to prevent corrosion.

The advantages offered by anodized aluminum as a substrate for planographic printing plates have long been known, and numerous processes have been proposed in the past in which sulfuric and phosphoric acid are used as electrolytes. The acids are used either alone or in a mixture or in succession. Before the anodic oxidation, the carrier is optionally roughened mechanically, electrochemically or chemically. In order to increase the adhesion of the photosensitive layer and the hydrophilicity of the non-image areas, it has proven expedient to apply a corresponding underlayer before applying the photosensitive layer. In US Pat. No. 3,181,461, the treatment with an aqueous alkaline silicate solution is carried out after the anodization.

From US-A 2 594 289 it can be seen that porous anodized layers are suitable for planographic printing, while non-porous anodized layers are unsuitable because the porous layer gives the non-image areas on the plate a surface with improved water flow and the adhesive strength of the material forming the image areas on the plate surface is increased by the fact that it can penetrate into the pores.

US Pat. No. 3,511,661 describes aluminum foils for planographic printing which have been anodized in aqueous phosphoric acid, the oxide layers having a structure with pore-like openings, which has aluminum oxide cells whose average diameter is approximately 200 to 700 A, and on the surface of which a layer of about 10 to 200 mg / m ^{2 of} aluminum phosphate is located.

US Pat. No. 3,658,662 describes an electrochemical silicatization of a cleaned, etched aluminum plate to improve the hydrophilicity.

According to US Pat. No. 3,902,976, an aluminum foil which has been anodized in a conventional manner is electrolytically aftertreated in an aqueous solution of sodium silicate in order to obtain a hydrophilic, abrasion and corrosion-resistant layer and thus a presensitized support suitable for planographic printing.

According to US Pat. No. 4,022,670, the anodic oxidation of aluminum foils is carried out in an aqueous solution of a mixture of a polybasic mineral acid, for example sulfuric acid, and a larger proportion of a polybasic aromatic sulfonic acid, for example sulfophthalic acid, whereby anodically produced porous Oxide layer is formed, which can be directly coated with a light-sensitive coating.

US-A 4 090 880 describes a two-stage process in which a cleaned aluminum foil with an intermediate layer, e.g. from alkasilicate, fluorides of metals from group IV-B, polyacrylic acid or alkali zirconium fluoride, and then anodized in a conventional manner in aqueous sulfuric acid. The plate pretreated in this way is said to have an increased shelf life after being coated with light-sensitive diazo compounds.

According to US Pat. No. 4,153,461, a conventional anodic oxidation, which is carried out up to an oxide layer thickness of at least 0.2 pm, is followed by an aftertreatment with aqueous polyvinylphosphonic acid, at temperatures between 40 and 95.degree. The treatment improves the adhesion of a subsequently applied coating, the shelf life of the plate and the hydrophilicity of the non-image areas after exposure and development and the print run.

Similar electrolysis processes are described in US-A 4,383,897, 4,399,021, 4,448,647 and 4,452,674.

Significant commercial success has been achieved with plates of the type described above, especially when a diazo compound is used for the photosensitive Layer is used. However, some improvements appear desirable.

It is therefore an object of the present invention to provide an electrolyte and a method for the production of metal plates, by means of which defects in the coating which occasionally occur on the plates and occasional unpredictable, rapid wear in the printing press are excluded and faster, more reliable freewheeling behavior in the Printing machine can be achieved. The process also strives for an even higher print run, the process being more economical than the conventional anodic oxidation, which is followed by a second operation for sealing and post-treating the oxide layer before the photosensitive coating can be applied.

In cases where the anodic oxidation takes place for protective and decorative reasons, better corrosion resistance and greater economy compared to known methods are desired.

The above object is achieved by an electrolyte for anodizing metal plates by means of direct current, which contains at least one water-soluble organic acid which is capable of forming an insoluble organometallic oxide complex, the characteristic feature of which is that the electrolyte is compatible with it contains compatible base and a pH from 3 to 10 and an electrochemical process with the aid of which a firmly adhering, insoluble complex of metal oxide and organic component is applied to a metal surface, the metal serving as the anode and the electrolyte used as a water-soluble mixture based on polybasic organic acid, the a characteristic feature is that a compatible base is added in an amount sufficient to adjust the pH of the electrolyte in the range between about 3 and 10.

The polybasic acid can be a polyvinylphosphonic, polycarbonate or polysulfonic acid and is advantageously a polymeric acid. Polyvinylphosphonic acid (PVPS) is preferably used as the electrolyte. The process is operated with direct current. The insoluble complex of metal oxide and organic component formed during electrolysis consists of a combination of anodically generated oxide and polyacid, which forms a protective layer on the metal with improved corrosion resistance. The complex of metal oxide and organic component is well suited for anchoring light-sensitive layers. Printing plate carrier materials produced according to the invention result in a longer shelf life, better printing properties and a longer print run than carriers produced using conventional methods.

The manufacturing method according to the invention also includes cleaning and adhering to a metal object finally, the anodic oxidation of the metal object by means of direct current in an aqueous organic solution in which a water-soluble organic acid or a mixture of two or more organic acids, the acid in the case of carboxylic acids in each case should be at least three-base, and a base, where the amount of base is chosen so that the pH of the solution is between 3 and 10. The electrolysis is carried out under conditions such that an insoluble complex of metal oxide and organic component is formed in which the organic acid is contained and which adheres firmly to the surface of the metal object. An examination of the surface of the product produced according to the invention shows that it is essentially non-porous.

The metal supports are cleaned before the electrochemical treatment according to the invention. This can be done using a variety of different solvents or aqueous alkaline solutions, the choice of which depends on the type of metal used and the purpose for which the support is to be used.

Typical alkaline degreasing agents are, for example, hot aqueous solutions which contain alkalis such as sodium hydroxide, potassium hydroxide, trisodium phosphate, sodium silicate, aqueous, alkaline products and wetting agents. Such a mixture is, for example, ( ^R ) Ridolene 57 (manufacturer Amchem Products, Pennsylvania). Degreasing treatments with solvents such as trichlorethylene, For some time, 1,1,1-trichloroethane and perchlorethylene have been used less frequently because of health and ecological concerns. Degreasing with solvents is done by dipping, spraying or steaming.

Suitable metals that can be treated according to the invention are steel, magnesium and preferably aluminum and aluminum alloys. Suitable carriers for planographic printing plates are e.g. the aluminum alloys 1100, 1050, 3003 and A-19, which are offered by the companies Alcoa and Consolidated Aluminum Company, among others.

Typical aluminum alloys for planographic printing plates contain the following components (in% by weight):

The chemical composition of the alloy used probably also has a certain influence on the effectiveness of the electrolytic deposition of organic electrolytes. Other alloy components that are not normally analyzed can also have an impact.

The surface of the metal to be treated can be smooth or roughened. The roughening is done with the help Known methods, such as chemical roughening in alkaline or acidic etching solutions, dry brushing, wet brushing with abrasive suspensions, roughening with abrasive balls and electrochemical roughening. Each of these processes results in a different roughening and surface topography.

The best results are achieved with the electrolyte and the method according to the invention if the cleaned surface is treated electrolytically immediately, that is to say before an air oxide layer can form. Before immersing the cleaned, degreased and possibly roughened plate in the organic electrolytic solution for electrolytic deposition, the plate should be etched to remove air oxide. The etching is done in a known manner, e.g. in one of the acidic or alkaline media or electrolytes described. The plate can also be removed with an etchant, e.g. a solution of phosphoric and chromic acid. It is preferred to rinse the metal surface with water immediately after cleaning and, if necessary, to roughen it, and to subject it to the electrolytic treatment according to the invention while it is still wet, although satisfactory results are also achieved if the procedure is not so careful.

A conventional anodic oxidation of the metal optionally follows the cleaning and any roughening before the electrolytic deposition process according to the invention is carried out.

• Nitrilotriessigsäure, 1,2,4,5-Benzoltetracarbonsäure, das Kondensationsprodukt aus Benzolphosphonsäure und Formaldehyd (Polybenzolphosphonsäure), ein Copolymer verschiedener Molekulargewichte aus Methylvinylether und Maleinsäureanhydrid, ein Copolymer aus Methylvinylether und Maleinsäure, Polyvinylsulfonsäure, Polystyrolsulfonsäure, Phytinsäure, Alginsäure, Poly-n-butylbenzolsulfonsäure, Polydiisopropylbenzolsulfonsäure, Polyvinylphosphonsäure, Dodecylpolyoxyethylenphosphorsäure, Tridecylbenzolsulfonsäure, Dinonylnaphthalindisulfonsäure, 2,2-Dinitro-4,4-stilbendisulfonsäure, Diisopropylpolynaphthalindisulfonsäure, 2-Ethylhexylpolyphosphorsäure, Dodecylnaphtalindisulfonsäure, Di-n-butylnaphthalindisulfonsäure, Polydecylbenzolsulfonsäure, Polyacrylsäure, Polymethacrylsäure, Diethylendiaminpentaessigsäure, Polynaphthalinsulfonsäure, Ethylendiamintetraessigsäure, Hydroxyethylethylendiamintriessigsäure sowie Gemische aus den genannten Verbindungen, die alle wasserlöslich sind.

The organic electrolytes which are suitable according to the invention for increasing the corrosion resistance include aqueous solutions of sulfonic, phosphoric and carboxylic acids which are at least three-base and can be monomeric or polymeric, and mixtures thereof. The following are examples of electrolytes:

• Nitrilotriacetic acid, 1,2,4,5-benzenetetracarboxylic acid, the condensation product of benzenephosphonic acid and formaldehyde (polybenzenephosphonic acid), a copolymer of different molecular weights from methyl vinyl ether and maleic anhydride, a copolymer of methyl vinyl ether and maleic acid, polyvinylsulfonic acid, polystyrene sulfonic acid, phytic acid, alginic acid, poly-n- butylbenzenesulfonic acid, Polydiisopropylbenzolsulfonsäure, polyvinyl Dodecylpolyoxyethylenphosphorsäure, Tridecylbenzolsulfonsäure, dinonylnaphthalene disulfonic acid, 2,2-dinitro-4,4-stilbenedisulfonic acid, Diisopropylpolynaphthalindisulfonsäure, 2-Ethylhexylpolyphosphorsäure, Dodecylnaphtalindisulfonsäure, di-n-butylnaphthalindisulfonsäure, Polydecylbenzolsulfonsäure, polyacrylic acid, polymethacrylic acid, diethylenetriaminepentaacetic acid, polynaphthalenesulfonic acid, ethylenediaminetetraacetic acid, Hydroxyethylethylenediamine triacetic acid and mixtures of the compounds mentioned, all of which are water-soluble.

A high degree of hydrophilicity and good adhesion of the image are required for planographic printing. Preferred electrolytes are therefore the condensation product from Ben zolphosphonsäure and formaldehyde, low molecular weight copolymers of methyl vinyl ether and maleic anhydride, copolymers of methyl vinyl ether and maleic acid, polyvinyl sulfonic acid, phytic acid, polyvinylphosphonic acid, Dodecylpolyoxyethylenphosphorsäure, Diisopropylpolynaphthalinsulfonsäure, 2-Ethylhexylpolyphosphorsäure, ethylenediaminetetraacetic acid, hydroxyethylethylenediaminetriacetic acid, or mixtures of several of these compounds.

The condensation product of benzenephosphonic acid and formaldehyde, phytic acid, polyvinylphosphonic acid, 2-ethylhexylpolyphosphoric acid and mixtures of these compounds are particularly preferably used in planographic printing.

A very suitable electrolyte mixture, for example, a mixture of phytic acid and Polyvinylphosphon _- acid.

The properties of the coated metal depend essentially on the concentration of the electrolyte and the electrolysis conditions, such as voltage, current density, time or temperature.

The expert will combine this process parameters in each individual case so that the desired Oberflächenbe _- will have to create comprehensive.

A compatible base is contained in the electrolyte solution according to the invention in an amount such that the solution has a pH in the range from 3 to 10. In particular, the pH is between 4 and 8, very particularly preferably between 6 and 7. At extremely high or low pH values, the anodic oxide layer undesirably dissolves. It was found that the closer the pH is to the neutrality limit, the better the adhesion between the metal workpiece and the acid component. In addition, the workpiece does not need to be rinsed off after the anodic oxidation. By promoting the metallic bond, the anchoring of the acid component in the anodic oxide layer is also improved. This can be seen in a higher print run of a printing plate made from a carrier treated according to the invention. Bases suitable according to the invention are, for example, hydroxides, such as sodium, lithium, potassium and ammonium hydroxide. It is possible that a harder anodic oxide layer will form in the specified pH range due to the decreasing solubility in aluminum oxide.

The adhesion of an electrodeposited layer is much better than with a conventional thermal immersion process, which is applied after the anodic oxidation. A 1.0 N NaOH solution removes most of such a thermally deposited layer, while one is electrolytically deposited Layer remains practically intact; this means that the latter is insoluble in reagents of the same or less aggressiveness.

Plates intended for planographic printing are subjected to a test immediately after the electrolytic deposition of the complex of metal oxide and organic component, that is to say before the photosensitive layer has been applied. The plates are stained wet or dry, with the second test being stricter. After staining, the plate is rinsed under running water or sprayed with water and lightly rubbed off. The ease and completeness with which the paint can be removed is a measure of the hydrophilicity of the plate surface.

The printing ink runs off completely when rinsed with water from printing plates produced according to the invention, which have been colored dry and dried at 100.degree. In contrast to this, printing plates which have not been anodized or are conventionally anodized and have been subjected to a thermal immersion treatment in an aqueous polyvinylphosphonic acid solution give rise to an irreversible fogging, even if milder test conditions are used in the heat treatment.

In the dyeing tests, plates with and without a light-sensitive coating are subjected to an aging treatment over different periods of time and at different temperatures. Then it is examined to what extent it have retained their hydrophilic properties. Plates coated with various diazo compounds are examined after the treatment with regard to step wedge resistance, image resolution, retention of the hydrophilicity at the image background sites and easy developability. Suitable photosensitive materials are described below.

Finally, the printing plates according to the invention, together with comparison samples, are used in a printing press. The differences in surface abrasion, the resolution of the grid wedge, the speed and purity of the freewheel and the height of the print run are assessed.

In general, plates which have been treated electrolytically according to the invention, with concentration, duration, temperature, voltage and current density being able to be varied within wide limits, are always superior to conventional plates. The relationship between the variables obviously does not play a decisive role here. However, some variables have been found to be more important than others according to the invention, and certain parameters of these variables are in turn more critical to achieving optimal results than others. This will be discussed in more detail later.

• Als Elektrolyt dient beispielsweise 1,5 %ige Polyvinylphosphonsäure, die mit Natriumhydroxid auf einen pH-Wert von 6 eingestellt wurde, bei einer Temperatur von 23 °C und einem Gleichstrom von 60 Volt, wobei eine gereinigte und geätzte Aluminiumplatte des Typs 1050 als Anode und ein Kohlestab als Gegenelektrode dient.

The following is a description of the processes that occur during a typical electrodeposition attempt:

• The electrolyte used is, for example, 1.5% polyvinylphosphonic acid, which has been adjusted to a pH of 6 with sodium hydroxide, at a temperature of 23 ° C. and a direct current of 60 volts, with a cleaned and etched aluminum plate of the 1050 type as the anode and a carbon rod serves as a counter electrode.

The complex of aluminum oxide and organic compound that makes up the surface layer initially forms very quickly. After a second, the layer thickness is already more than 0.12 pm. After three seconds it is up to 0.17 pm and after five seconds the increase in the layer thickness slowly stops at 0.20 pm. Even after 120 seconds there is no further appreciable increase.

The voltage is kept practically constant during the entire period of electrolytic deposition.

The amperage is not an independent variable, but depends on the other process conditions, in particular the voltage and electrolyte concentration. The amperage begins to drop very shortly after the start of electrolysis.

This gives the impression of a self-limiting process in which an electrolytically deposited barrier layer is formed from a complex of metal oxide and organic compound, through which the further current flow is throttled. However, the throttling is not as strong as for anodization with Q boric acid, where the maximum layer thickness is 13 to 16 A / V. This was found out with the help of common surface investigation methods (eg Auger analysis), coupled with ion sputtering.

Based on tests at different voltages and time periods, it is assumed that the layer consisting of the complex of metal oxide and organic compound on the plate surface acts as a capacitor. As long as the dielectric strength is not exceeded during the electrolysis, there will be no further weight gain as the time progresses and the layer will be retained throughout. If the dielectric strength is exceeded, the layer is perforated and its integrity is lost. While it is believed that this assumption accurately describes the facts, it is based purely on speculation and is in no way the basis for the present invention. The breakdown mentioned depends primarily on the voltage, with a potential of 70 V quickly breaks down. However, a certain breakdown is already registered at 30 V if the time is extended to over 250 seconds.

The limits of the conditions for the breakdown thus depend on the respective variables of the process from. The optimum conditions for carrying out the method according to the invention and for producing corresponding products lie within these limits, which are determined by means of the methods described. In addition, however, products are obtained in a much larger range of relatively insignificant conditions, which still have decisive advantages over the prior art.

The concentration of the electrolyte used is between about 0.01% and the saturation limit and is essentially independent of its chemical composition. Solutions with a concentration of more than 30% are generally not used, while at concentrations close to the lower limit the conductivity of the solution is very low; in the case of 0.001% polyvinylphosphonic acid it is e.g. at 61,000 a. Nevertheless, even at a concentration of 0.05%, a layer of a complex of metal oxide and organic component is formed, thanks to which products are obtained which are different from known products, such as e.g. Aluminum plates, which have been anodized in a conventional way and then heat-sealed with a polyvinylphosphonic acid solution, are characterized by better corrosion resistance, durability, hydrophilicity and printing properties.

The current conductivity of the electrolyte grows rapidly with increasing concentration, which leads to shorter treatment times and lower voltage requirements.

Products obtained at a concentration between 1% and 5% show only slight differences in their properties, but characteristic properties are still obtained at a concentration of 30%, despite the high viscosity that the electrolyte then has. With the voltage remaining constant, the extent of layer formation decreases as the concentration increases. Taking into account the properties obtained, the ease of processing, the layer thickness achieved and the costs for the electrolyte, the preferred concentration range is between about 0.8% and 5%.

There is an almost linear relationship between the basis weight of the layer of insoluble complex of metal oxide and organic component and the DC voltage used. As soon as the voltage is more than 5 volts, the electrodeposited layer gives the material a corrosion resistance and printing properties that are superior to conventional materials.

If the voltage rises above 70 V (direct current), the closeness of the layer is destroyed and the layer is "perforated" in places, which is probably due to the fact that the dielectric strength is exceeded. The best corrosion resistance is therefore achieved at voltages below 70 V.

Direct current is required for the method, but it can also be superimposed by alternating current. Pulsed direct current can also be used. Square waves from pulsed anodizing current sources are particularly suitable.

At constant voltage, the amperage is greatest at the start of the electrical deposition and, over time, it decreases with increasing thickness of the layer composed of the complex of metal oxide and organic compound on the metallic carrier, thus reducing the current conductivity. Within 30 seconds it drops to a level at which the further power consumption is minimal. This contributes significantly to the economy of the process, since the desired useful layer has already been deposited at this point.

When using a 1.5% solution of polyvinylphosphonic acid, the pH of which had been adjusted to 6 with sodium hydroxide, the amperage initially rose to about 10 A / dm ² and fell to about 0.1 A / dm ² after 5 seconds from. This drop to very low current values is characteristic of processes in which the organic electrolytes according to the invention are used. In contrast, the current drop in conventional anodizing processes with strong electrolytes is slow and settles at around 10 to 15 amperes.

The ampere number is therefore a dependent variable, the size of which is determined by the independent variables electrolyte composition, concentration and voltage. Current densities between about 1 and 5 A / dm ² are characteristic of favorable process conditions and are therefore preferred.

The process can be carried out at temperatures from about -2 ° C (near the freezing point of the electrolyte) to about 60 ° C. Measured against the criteria of surface hardness, image adhesion, hydrophilicity and durability, the best results are obtained at 10 ° C, although the performance drop at temperatures between 10 ° C and room temperature, up to 40 ° C, is only very slight. In order to carry out the process at very low temperatures, complex cooling devices would be necessary. For reasons of economy and because of the low power consumption, the preferred temperature range is around 10 to 35 ° C, in particular around 20 to 25 ° C.

More than 60% of the layer consisting of the complex of metal oxide and organic component is formed in the first 5 seconds of the electrolytic process. A treatment time of more than 5 minutes is not necessary for planographic printing, since no further layer build-up takes place; it also does no harm as long as the voltage is kept low. The treatment time is preferably about 0.16 to 1 minute.

In terms of process technology, the short period of time, the low temperature (room temperature, so that hardly any additional heating or cooling devices are required) and the low power consumption represent favorable economic factors in comparison to conventional anodizing processes, which as a rule also include additional heat treatment of the carrier material.

Iminoquinonediazides, o-quinonediazides and condensation products of aromatic diazonium compounds in combination with suitable binders are suitable as photosensitive mixtures which can be applied to the layers according to the invention from the complex of metal oxide and organic component for the production of printing forms. Such connections are e.g. in U.S. Patent Nos. 3,175,906, 3,046,118, 2,063,631, 2,667,415 and 3,867,147. The mixtures described in the latter document are generally preferred. Furthermore, photopolymer systems based on ethylenically unsaturated monomers with photoiniators, which may contain matrix polymer binders, can be used. Photodimerization systems, e.g. Polyvinyl cinnamates and systems based on diallyl phthalate prepolymers are also suitable. Such systems are described in U.S. Patents 3,497,356, 3,615,435, 3,926,643, 2,670,286, 3,376,138 and 3,376,139.

The examples given for light-sensitive systems which are suitable according to the invention relate to customary mixtures see. In principle, all of the mixtures specified are suitable, but the diazo compounds are generally preferred since they generally adhere best to the complex of metal oxide and organic component and give a higher image resolution when printed.

The invention is illustrated by the following example, but there is no restriction to the example.

example

Two plates of a 3003 aluminum alloy suitable for printing purposes and roughened in an abrasive suspension are treated for 30 seconds in a 1 N sodium hydroxide solution and rinsed with distilled water at room temperature.

• 1. Wäßrige Polyvinylphosphorsäurelösung von 1 Gew.-% mit einem pH-Wert von 2;
• 2. Wäßrige Polyvinylphosphorsäurelösung von 1 Gew.-%, deren pH-Wert mit konzentriertem Ammoniumhydroxid (29 %) auf etwa 6,5 eingestellt wurde.

Each of the two plate pieces was anodized in one of the electrolyte solutions described below:

• 1. A 1% by weight aqueous polyvinyl phosphoric acid solution with a pH of 2;
• 2. 1% by weight aqueous polyvinyl phosphoric acid solution, the pH of which was adjusted to about 6.5 with concentrated ammonium hydroxide (29%).

The anodization is carried out for 60 seconds with lead as the counter electrode at a voltage of 30 V and a maximum of 5 A / d _m ² .

The plate pieces treated in this way are spin-coated with the following mixture, which is dissolved in a suitable solvent:

The dry layer weight is approximately 750 mg / m ² . The printing plates obtained are exposed with an exposure device (Berkey Ascor) through a suitable template so that a fully covered step 7 is obtained on a Stauffer wedge. The plates are developed with a developer (Enco Negative Subtractive Developer) and then preserved (Enco Subtractive Finisher). Both plates are used under tough test conditions (excessive pressure on the plates and printing ink with strong abrasion properties) in a printing machine (Miehle). The following conditions apply to printing: uncoated paper, dampening solution with a pH value of 4.35, relative humidity 53%, non-alcoholic dampening system.

Measured by the density changes of the printed Stauffer exposure indicator, the wear of the plate (2) was 46% less than that of the plate (1). When the test was repeated, the wear was reduced by 38%. From the two experiments, there is an average improvement in the printing performance of plate (2) by 42% compared to the performance of plate (1).

It can be concluded from this that plates which are anodically oxidized at a higher pH value produce a higher pressure output than the same plates which are only anodically oxidized at a lower pH value.

Claims (19)

1. Electrolyte for anodizing metal plates by means of direct current, which contains at least one water-soluble organic acid which is capable of forming an insoluble organometallic oxide complex, characterized in that the electrolyte contains a base which is compatible with it and has a pH of 3 to 10. 2. Electrolyte according to claim 1, characterized in that it contains an acid from the group of phosphonic, phosphoric, sulfonic or an at least three-base carboxylic acid or mixtures thereof as acid. 3. Electrolyte according to one of claims 1 or 2, characterized in that it is an acid nitrilotriacetic acid, 1,2,4,5-benzenetetracarboxylic acid, a condensation product of benzene phosphonic acid and formaldehyde (polybenzene phosphonic acid), a copolymer of methyl vinyl ether and maleic anhydride, a copolymer Methyl vinyl ether and maleic acid, polyvinylsulfonic acid, polystyrene sulfonic acid, phytic acid, alginic acid, poly-n-butylbenzenesulfonic acid, polydiisopropylbenzenesulfonic acid, polyvinylphosphonic acid, dodecylpolyoxyethylenephosphoric acid, tridecylbenzenesulfonic acid, dinonylnaphthalenedisopropyl acid, 2,2-dinidonic acid, 2,2-din polynaphthalenedisulfonic acid, 2-ethylhexylpolyphosphoric acid, dodecylnaphtalinedisulfonic acid, di-n-butylnaphthalenedisulfonic acid, polydecylbenzenesulfonic acid, polyacrylic acid, polymethacrylic acid, diethylenediaminepentaacetic acid, polynaphthalenesulfonic acid, ethylenediaminetetraethylacetic acid, containing hydroxylenediamine tetraacetic acid, or containing hydroxylenediamine tetraacetic acid, ethylenediamine tetraacetic acid or diamine triethyl acetic acid, or hydroxylenediamine tetraacetic acid. 4. Electrolyte according to one of claims 1 to 3, characterized in that it contains alkali and / or ammonium hydroxide as the base. 5. Electrolyte according to one of claims 1 to 4, characterized in that it has a pH of 4 to 8. 6. Electrolyte according to claim 5, characterized in that it has a pH of 6 to 7. 7. A method for anodizing metal plates, wherein the plates are cleaned and then subjected to electrolytic treatment by means of direct current in an electrolyte which contains at least one water-soluble organic acid which is capable of forming an insoluble organometallic oxide complex, characterized in that that a base compatible with this is added to the electrolyte and the pH is adjusted to 3 to 10. 8. The method according to claim 7, characterized in that the base is added in such an amount that the pH is adjusted to 4 to 8. 9. The method according to claim 8, characterized in that the pH is adjusted to 6 to 7. 10. The method according to any one of claims 7 to 9, characterized in that an acid from the group of phosphonic, phosphoric, sulfonic or an at least tripasic carboxylic acid or mixtures thereof is added as the acid. 11. The method according to any one of claims 7 to 10, characterized in that the acid nitrilotriacetic acid, 1,2,4,5-benzenetetracarboxylic acid, a condensation product of benzene phosphonic acid and formaldehyde (polybenzene phosphonic acid), a copolymer of methyl vinyl ether and maleic anhydride, a copolymer of methyl vinyl ether and maleic acid, polyvinyl sulfonic acid, polystyrene sulfonic acid, phytic acid, alginic acid, poly-n-butylbenzenesulfonic acid, Polydiisopropylbenzolsulfonsäure, polyvinyl Dodecylpolyoxyethylenphosphorsäure, Tridecylbenzolsulfonsäure, dinonylnaphthalene disulfonic acid, 2,2-dinitro-4,4-stilbenedisulfonic acid, Diisopropylpolynaphthalindisulfonsäure, 2-Ethylhexylpolyphosphorsäure, Dodecylnaphtalindisulfonsäure, di- n-butylnaphthalenedisulfonic acid, polydecylbenzenesulfonic acid, polyacrylic acid, polymethacrylic acid, diethy lendiaminepentaacetic acid, polynaphthalenesulfonic acid, ethylenediaminetetraacetic acid, hydroxyethylethylenediamine triacetic acid or mixtures thereof. 12. The method according to any one of claims 7 to 11, characterized in that alkali and / or ammonium hydroxide is added as the base. 13. The method according to any one of claims 7 to 12, characterized in that treating plates made of aluminum or its alloys. 14. The method according to any one of claims 1 to 13, characterized in that with an acid concentration of 0.5% to 30.0%, at 1 to 90 V, a current density of 1 to 10 A / dm ² , for a time from 0.08 to 5 minutes, anodized at a temperature of - 2 to 60 ° C. 15. The method according to claim 14, characterized in that with an acid concentration of 0.8% to 5%, at 5 to 70 V, a current density of 1 to 5 A / dm ² , for a period of 0.16 to 1 minute , anodized at a temperature of 10 to 35 ° C. 16. The method according to any one of claims 7 to 15, characterized in that a light-sensitive layer is applied after the anodization. 17. The method according to claim 16, characterized in that one applies a layer selected from the group o-quinonediazides, condensed aromatic diazonium compounds and photopolymers. 18. Use of an electrolyte according to claims 1 to 6 for the anodization of metal plates for lithographic purposes. 19. Use of an electrolyte according to claim 18 for anodizing plates made of aluminum or its alloys for lithographic purposes.