EP0167751B1 - Process for treating aluminium surfaces - Google Patents

Description

The present invention relates to a method for treating the surface of aluminum foils, by means of which the foil surface is made suitable for use in planographic printing plates.

The production of printing plates by coating the surface of a support made of aluminum foil with a photosensitive mixture suitable for planographic printing and exposure by means of a template and subsequent development has long been known. The remaining oleophilic image areas take on and transfer color when printed, while the hydrophilic non-image areas take on water or aqueous solutions during printing and repel the bold ink.

It has also long been known that by roughening the aluminum surfaces, be it mechanically, e.g. using wire brushes or abrasive suspensions, or electrochemically using electrolytic acid solutions, e.g. Nitric acid solutions, the print run of a plate can be increased significantly.

Electrolytic roughening processes of aluminum have many advantages over mechanical roughening (see e.g. U.S.-A-3,072,546 and 3,073,765). A particularly fine and even roughening is desired for certain purposes. These properties are of particular importance if the aluminum is to be used as a support for planographic printing plates. Very fine roughening can be achieved in an aqueous hydrochloric acid electrolyte, but the current density must be kept quite low, otherwise pitting will occur on the aluminum surface. Due to the low current density, the roughening times are relatively long.

The electrolytic roughening of aluminum foils with hydrochloric or nitric acid is known and is used, among other things. in US ― A ― 3 980 539, 3 072 546, 3 073 765, 3 085 950, 3 935 080, 3 963 594 and 4 052 275.

The surface is greatly enlarged by the electrochemical roughening, which leads to an improvement in the printing properties. However, difficulties arise with these methods. Because the surface is structured evenly and has a large area, but the roughening is relatively flat. This creates two problems: 1. The printing plates are poorly sucked in a vacuum frame, which leads to under-radiation; and 2. the plates have insufficient water absorption capacity, which causes difficulties in maintaining a wide range of color / water balance during the printing process. The consequences can be a loss of quality in printing.

From US-A-4 242 417 a method for roughening aluminum carriers is known, in which the carriers are first mechanically roughened, e.g. with the aid of a wire brush or an abrasive suspension, and then in a saturated aqueous solution of an aluminum salt of a mineral acid, which may also contain up to 10% of a mineral acid. The roughening effect of this solution can optionally be enhanced by electrolysis.

This process has several disadvantages. First, the surface is structured in a direction-oriented manner due to the roughening in the abrasive suspension, as a result of which the print quality and the ink / water balance are impaired. Second, tiny particles of the abrasive contaminate the surface. Third, the roughening process using an abrasive suspension is subject to constant fluctuations. The bristles of the brushes will shorten due to use, and the abrasive suspension loses its abrasion ability in use, so that fresh material must be added. The purity of the aluminum surface depends on the time, since A) (OH) g, Al ₂ 0 ₃ and aluminum particles are constantly formed. All this leads to surfaces of different quality.

GB-A-1 027 695 discloses a four-stage process for the production of printing plate supports, in which an electrolytic roughening is carried out in the second stage. The electrolyte contains nitric acid, sulfite waste liquor, aromatic aldehydes or aromatic ketones, their derivatives or pine oil.

The object of the present invention is to obtain the advantages of electrochemical roughening without having to accept the disadvantages of the prior art or mechanical roughening, and to produce printing plate supports which have a uniform and good topography.

• a) mit einem wäßrigen Bad bestehend lediglich aus
  • I) 1 bis 25 Gew.-% an Salz- und/oder Salpetersäure und
  • 11) 1 bis 25 Gew.-% an einer fluorionenhaltigen Säure oder einem Salz davon und
  • 111) gegebenenfalls wenigstens einer Verbindung aus der Gruppe Ammonium-, Kalium-, Natrium- oder Lithiumpersulfat, -peroxydisulfat, -disulfat oder Sulfonsäure so lange behandelt bis eine Ätzung hervorgerufen wird und
• b) eine elektrochemische Behandlung bei einer Stromstärke von 30 bis 120 Aldm² in einem wäßrigen Elektrolyten enthaltend Salz- oder Salpetersäure, gegebenenfalls unter Zusatz von Salzen dieser Säuren, wobei der Elektrolyt frei ist vor organischen Lösemitteln, vornimmt und
• c) in einem wäßrigen Elektrolyten enthaltend Schwefelsäure oder Phosphorsäure anodisiert.

The present invention is based on a process for the treatment of aluminum surfaces for printing plate supports, first roughening and then anodizing. The process is characterized in that the aluminum surface

• a) with an aqueous bath consisting only of
  • I) 1 to 25 wt .-% of hydrochloric and / or nitric acid and
  • 11) 1 to 25% by weight of a fluorine-containing acid or a salt thereof and
  • 111) optionally treated at least one compound from the group consisting of ammonium, potassium, sodium or lithium persulfate, peroxydisulfate, disulfate or sulfonic acid until an etching is caused and
• b) an electrochemical treatment at a current of 30 to 120 Aldm ² in an aqueous electrolyte containing hydrochloric or nitric acid, optionally with the addition of salts of these acids, the electrolyte being free from organic solvents
• c) anodized in an aqueous electrolyte containing sulfuric acid or phosphoric acid.

The surfaces treated according to the invention are thus matted by the chemical etching process, thereby avoiding the directional orientation that can be clearly observed in mechanical roughening processes. The chemical etching is followed by an electrochemical roughening step in which an additional grain is applied to the etched surface. A surface with an increased surface area and better capillary wetting is obtained, which is expressed by an improved ink / water balance of the printing plates produced with the supports according to the invention.

Aluminum foils which are suitable according to the invention are those which partially contain alloying constituents due to contamination, such as e.g. Aluminum Association alloys 1050, 1100 and 3003. The thickness of the aluminum foils used according to the invention is in the range customary for these purposes and is e.g. between 0.01 and 0.064 cm (0.004 and 0.025 inches). The choice of the best thickness of the aluminum foil is left to the expert.

• 1. 1 bis 25 Gew.-% Salz- und/oder Salpetersäure und
• 2. 1 bis 25 Gew.-% einer anorganischen fluorionenhaltigen Säure oder einem ihrer Salze, insbesondere HF, H₂SiF₆, HPF₆, HBF₄, K₂ZrF₆, K₂TiF₆, NH₄F oder NH₄HF₂ getaucht wird.

In the practical implementation of the method according to the invention, the cut or continuous aluminum foil is first degreased and then chemically etched, for which purpose the foil is placed in an aqueous bath containing only

• 1. 1 to 25 wt .-% hydrochloric and / or nitric acid and
• 2. 1 to 25% by weight of an inorganic fluorine-containing acid or one of its salts, in particular HF, H ₂ SiF ₆ , HPF ₆ , HBF ₄ , K ₂ ZrF ₆ , K ₂ TiF ₆ , NH ₄ F or NH ₄ HF ₂ is dipped.

The amount of hydrochloric and / or nitric acid used is preferably between 5 and 18% by weight, in particular between 7 and 12% by weight.

The amount of the fluorine ion-containing compound used in the bath is preferably 3 to 15% by weight, in particular 5 to 12% by weight.

To improve the surface properties of the film, the bath described above may also contain other constituents, such as ammonium, potassium, sodium or lithium persulfate, peroxydisulfate or disulfate. The bath can also contain sulfonic acids. The temperature of the bath is between 10 and 95 ° C, preferably between 20 and 80 ° C, in particular between 25 and 60 ° C. The immersion time is preferably about 5 s to 3 min. Longer times are possible, but not expedient, since excessive aluminum dissolution takes place in these cases. The immersion time is preferably 20 to 120 s, in particular 40 to 80 s. The fact that the aluminum foil and a further electrode in the bath may be connected to a power source means that the immersion process can be converted into electrolysis, but this is a question of the application and a question of cost. In the case of electrolysis, alternating or direct current with a density of 30 to 45 A / dm ^{2 can be used} for 30 to 60 s, the aluminum foil being the cathode. The nature of the etching solution does not change, since the aluminum is etched to form a water-insoluble aluminum fluoride that can be constantly removed by simply filtering it off. After the etching, the film is preferably rinsed off before the electrochemical roughening of stage b) begins.

According to the invention, the next process step is an electrolytic roughening of the aluminum in an aqueous electrolyte solution which is free from organic solvents and contains nitric and / or hydrochloric acid. The optimal concentration of hydrochloric acid and / or nitric acid depends on various factors, such as the respective current density, the temperature of the electrolyte solution and the properties of the aluminum object to be roughened. The cheapest parameters can easily be determined in a few simple tests.

Optionally, the electrolytic solution can also contain oxalic acid, aluminum nitrate, aluminum chloride or hydrogen peroxide (see US ― A ― 4,336,113), boric acid (see US ― A ― 4,374,710) or another of the many additives known for electrochemical roughening processes .

In the electrochemical roughening stage, the concentration of nitric acid is preferably 3 to 20 g / l, particularly preferably 8 to 20 g / l, in particular 10 to 15 g / l. At a concentration of more than 20 g / l to about 500 g / l there is no noticeable difference in the effect; from about 500 g / l the effect begins to wear off. The concentration of hydrochloric acid in the electrochemical roughening step is preferably 3 to 100 g / l, particularly preferably 5 to 60 g / l, in particular 8 to 15 g / l. The concentration of the optionally used oxalic acid is preferably 1 to 80 g / l, particularly preferably 5 to 45 g / l, in particular 8 to 20 g / l.

The concentration of hydrogen peroxide optionally used is preferably 1 to 60 g / l, particularly preferably 10 to 30 g / l, in particular 15 to 20 g / l.

The concentration of aluminum nitrate optionally used is preferably at the saturation limit, particularly preferably 65 to 70 g / l, in particular 65 g / l.

The concentration of optionally used aluminum chloride is preferably 1 to 10 g / l, particularly preferably 1 to 8 g / l, in particular 1 to 5 g / l.

The concentration of the boric acid optionally used is preferably 1 g / l to the saturation limit, particularly preferably 5 to 15 g / l, in particular 8 to 12 g / l.

In the process according to the invention, the current density in the electrolyte is 30 to 120 A / dm ² , preferably 45 to 80 A / dm ² , in particular 45 to 60 A / dm ² .

The electrolysis time is preferably 20 s to 3 min, particularly preferably 20 to 90 s, in particular 20 to 60 s.

The distance between the aluminum surface and the inert electrode, which is preferably made up of Graphite, chrome or lead is preferably up to about 1.5 cm, in particular 1 to 1.5 cm.

The roughening is preferably carried out with alternating current, the best roughening effect being achieved at a frequency above 50 Hz. The frequency range particularly preferably extends from 60 to 300 Hz.

The electrochemical roughening [stage b)] is followed by an anodic oxidation [stage c)] of the film. For this purpose, the film is passed through an anodizing bath containing sulfuric or phosphoric acid.

The acid concentration is preferably between 10 and 20% by weight. The temperature of the anodizing bath is 20 to 80 ° C, with the best results being achieved at temperatures between 20 and 40 ° C. Very good results are also achieved if the film in the anodizing bath under the action of direct current has a density of 0.11 to 11.1 Aldm ² (1 to 100 A / ft ² ), preferably 1.1 to 5.6 Aldm ² (10 up to 50 A / ft ² ). The anodic oxidation takes 0.5 to 30 minutes, but usually no longer than 1 to 2 minutes.

In the production of planographic printing plates, it is advantageous to subject the surface treated according to the invention to a hydrophilizing aftertreatment before the photosensitive layer is applied. The application of such an intermediate layer improves the adhesion of the light-sensitive layer to the plate surface and the hydrophilicity of the aluminum surface. Polyvinylphosphonic acid, sodium silicate, alkali zirconium fluorides, such as potassium zirconium hexafluoride, and hydrofluoric zirconic acid are usually used for the hydrophilizing treatment.

These materials are described in US-A-3 160 506 and 2 946 683 as being suitable for the preparation of aluminum substrates for the inclusion of a photosensitive layer.

Photosensitive mixtures suitable for planographic printing typically include aromatic diazonium salts, quinonediazides and photopolymerizable compounds known from the prior art. These are normally used in a mixture with resinous binders to increase the print run. Examples of the large number of known suitable binders of this type include polyurethanes and phenol / formaldehyde resins.

The invention is explained in more detail with reference to the following examples, but without being restricted to the exemplary embodiments shown.

Comparative Example A (prior art)

An aluminum plate made of an alloy of type 1100 was degreased in a common aqueous alkaline degreasing solution and with alternating current of 900 ° C. in an aqueous solution containing 13 g / l of HN0 ₃ and 65 g / 1 of Al (N0 ₃ ) ₃ electrolytically roughened and then rinsed off. The aluminum surface was then anodized using direct current of 240 ° C., the aluminum serving as the anode. The electrolyte contained 150 g / l of H _Z S0 ₄ . The anodized surface was rinsed off and hydrophilized by treatment with a solution containing 2.2 g / l of polyvinylphosphonic acid at 65 ° C. for 30 s. The plate was again rinsed, dried and coated with a negative working solution consisting of a polyvinyl formal / acetate / alcohol terpolymer, phosphoric acid, a phthalocyanine pigment and a diazonium condensation product of 3-methoxy-4-diazo-diphenylamine sulfate and 4,4'- Bismethoxymethyldiphenylether, isolated as mesitylene sulfonate, existed. The solution was applied so that a layer weight of 700 mg / m ² resulted.

After drying, the coated plate was exposed and developed to give a fully covered step 7 on a 21 step Stauffer wedge. The developed and preserved plate was used under normal printing conditions on a sheetfed press using Dahlgren fountain solution and a medium toughness ink. A comparison experiment to determine the color / water balance consisted in reducing the amount of water until the plate began to tone, or increasing the amount of water until the plate was completely flooded. In the first case, the amount of water was not sufficient for the plate to run clean. As a result, color got on the non-image areas of the printed copy. In the second case, an excess of water accumulated in the ink system, which resulted in undesired ink clogging and bare running of the rollers. The measuring scale of the ink / water balance, which ranges from 0 to 100, is a relative scale that varies from press to press, but it reliably indicates the respective balance for each machine. In the present case, the minimum is 36 and the maximum is 40.

With the printing plate produced in this way, only 150,000 flawless prints could be produced until defective image areas appeared.

Part of the plate was checked for surface roughness using a scanning electron microscope (SEM) and a perthometer. At 240x, 1200x and 6000x magnification, it was found that the surface consisted of uniform holes with a diameter of 2 to 6 pm. In addition, the surface was essentially flat because the roughness was practically the same everywhere. On average, it was 4.5 pm.

Comparative Example B (prior art)

A plate was treated, coated and tested as described in Comparative Example A, except that 8 g / l of HCI and 40 g / l of AICI _{3 were} used instead of HNO ₃ / Al (NO ₃ ) ₃ for the electrochemical roughening of the aluminum. Under practical pressure conditions, the plate was used for fountain solution values between 36 and 42 achieved only moderately satisfactory results. Only 180,000 flawless prints could be made.

When viewed by the SEM, it was found that the surface had fewer individual pores than when roughened with HN0 ₃ . The surface consisted of uniform holes with a diameter of 6 to 9 pm. It was also found that the surface was substantially flat, ie there were no noticeable differences in the roughness. The average roughness was 5.25 pm.

Comparative Example C

An aluminum plate made of an 1100 type alloy was degreased and rinsed in a common aqueous alkaline degreasing solution. The plate was then immersed for 60 s at a temperature of 60 ° C. in a solution which contained 100 g / l of HN0 ₃ (100%) and 100 g / l of NH ₄ F. The treated plate was rinsed well and dried. When evaluated by an SEM at 240x, 1200x and 6000x magnification, it was found that the surface was strongly structured. It had evenly distributed bumps that were about 10 µm in diameter and about 8-10 µm high; the distance from tip to tip was 40-50 pm. It also appeared to be obvious that the effect of the etching solution on the aluminum began at the intermetallic limit and resulted in a substantially directional topography.

Part of the aluminum treated in this way was anodically oxidized and coated in accordance with Comparative Example A. The exposed and developed plate was printed in a printing press to determine the bandwidth of the color / water balance. The result was a range from 28 to 52. The printing plate became unusable after less than 50,000 copies, which is due to the rough structure of the surface.

Comparative Example D

A 1100 aluminum alloy plate was treated as described in Comparative Example C, but an alternating current of 900 ° C was also used to accelerate the etching with HN0 ₃ and NH ₄ F. After the treatment, the plate was rinsed thoroughly and dried. When evaluated by an SEM at 240x, 1200x and 6000x magnification, it was also found that the surface was strongly structured. It was characterized by evenly distributed elevations, which had a diameter of about 10 µm and a height of about 6-8 pm; the distance from tip to tip was 35―45 pm. The topography appeared even and essentially directional.

It was also found that it had a very fine-pored structure that evenly covered the entire surface.

A piece of the plate treated in this way was anodically oxidized, hydrophilized and likewise coated in accordance with the information from comparative example A. The exposed and developed plate was used in a printing press for printing in order to determine the range of the color / water balance. A range of 28 to 56 was found. The printing test showed that only about 80,000 good prints can be obtained before the plate becomes unusable.

example 1

A plate made of an aluminum alloy of type 1100, which was etched in a solution containing HN0 ₃ and NH ₄ F according to the information of comparative example C, was additionally electrochemically roughened, anodically oxidized and hydrophilized, as described in comparative example A. The plate treated, rinsed and dried in this way was coated, exposed, developed and used for printing in a sheet-fed printing machine as described there. 220,000 good prints were obtained before the plate was unusable. The color / water balance range ranged from 28 to 56.

A piece of the plate was evaluated under an SEM at 240x, 1200x and 6000x magnification. The surface consisted of uniform holes with a diameter of 2―4 µm. It was also found that the surface was not flat, but rather three-dimensional. The mean roughness was 6.2 pm.

Examples 2 to 5 and Comparative Examples E to G

Vergleichsbeispiel A gibt ein vom Stand der Technik bekanntes übliches Verfahren zur Herstellung eines mittels HNO₃/Al(NO₃)₃ elektrochemisch aufgerauhten Trägermaterials aus Aluminium an.The examples show the effect of treatment with various combinations of mordants followed by electrochemical roughening. In all examples, plates made of aluminum alloy 1100 were degreased according to comparative example A, anodically oxidized and hydrophilized. When HNO ₃ / Al (NO ₃ ) _{3 was used} as the roughening electrolyte, the process parameters were the same as in Comparative Example A. When using HCl / AIC1 ₃ as the roughening electrolyte, the process parameters were the same as in Comparative Example B.

Comparative Example A specifies a conventional method known from the prior art for producing an aluminum base material roughened electrochemically by means of HNO ₃ / Al (NO ₃ ) ₃ .

Comparative example B specifies a conventional process known from the prior art for producing an aluminum base material which has been electrochemically roughened by means of HCI / AICI ₃ .

Comparative examples C and D show the advantages of an etching treatment prior to electrochemical roughening. In contrast to Comparative Examples A and B, the surface is given a three-dimensional structure by increasing the roughness. This effects the ink / water balance over a larger area. However, the printing tests show that the electrochemical roughening cannot be replaced by this method, since the print run decreases.

Example 1 illustrates the advantage which is achieved according to the invention if etching is first carried out to obtain an enlarged, three-dimensional surface and then electrochemically roughened to obtain a very pore-rich surface. In this case, both the improved ink-water balance and the print run are increased.

Examples 2, 3, 4 and 5 demonstrate the benefits of using an acid and a fluoride-containing compound as a mordant prior to electrochemical roughening to improve color / water balance and print run.

Comparative Examples E, F and G show that the use of an acid without the addition of a fluorine-containing compound or a fluorine-containing compound without the addition of an acid is by no means sufficient to achieve significantly better results than in the other comparative examples.

Claims (16)

1. Process for the treatment of aluminium surfaces for printing plate supports, which are first roughened and then oxidized anodically, characterized in that the aluminium surface is treated

a) first with an aqueous bath merely consisting of

I) 1 to 25% by weight of hydrochloric and/or nitric acid and

II) 1 to 25% by weight of an acid containing fluoride ions or a salt thereof and

III) if desired, at least one compound from the group consisting of ammonium persulphate, potassium persulphate, sodium persulphate or lithium persulphate, ammonium peroxydisulphate, potassium peroxydisulphate, sodium peroxydisulphate or lithium peroxydisulphate, ammonium disulphate, potassium disulphate, sodium disulphate or lithium disulphate or sulphonic acid, until etching is effected, and

b) then electrochemically with a current intensity of 30 to 120 A/dm² in an aqueous electrolyte containing hydrochloric or nitric acid, if appropriate with the addition of salts of these acids, the electrolyte being free of organic solvents, and

c) finally in an aqueous electrolyte containing sulphuric acid or phosphoric acid to effect anodization.

2. Process according to Claim 1, characterized in that the constituent II) in process step a) is HF, H₂SiF₆, HPF₆, HBF₄, K₂ZrF₆, K₂TiF₆, NH₄F or NH₄HF₂.

3. Process according to one of Claims 1 or 2, characterized in that at least one compound from the group consisting of oxalic acid, aluminium nitrate, aluminium chloride, hydrogen peroxide or boric acid is additionally added to the electrolyte of step b).

4. Process according to one of Claims 1 to 3, characterized in that at least one compound from the group consisting of ammonium persulphate, potassium persulphate, sodium persulphate or lithium persulphate, ammonium peroxydisulphate, potassium peroxydisulphate, sodium peroxydisulphate or lithium peroxydisulphate, ammonium disulphate, potassium disulphate, sodium disulphate or lithium disulphate or sulphonic acid is additionally added to the bath of step a).

5. Process according to one of Claims 1 to 4, characterized in that the bath contained in step a) is set to 5 to 25% by weight.

6. Process according to one of Claims 1 to 5, characterized in that the treatment in step a) is carried out for at least 5 seconds.

7. Process according to one of Claims 1 to 6, characterized in that the treatment in step a) is carried out at 10 to 95°C.

8. Process according to one of Claims 1 to 7, characterized in that in step a), alternating or direct current of 30 to 45 Aldm² is applied.

9. Process according to one of Claims 1 to 8, characterized in that the electrolyte in step b) contains nitric acid in a concentration of 3 to 500 g/I.

10. Process according to one of Claims 1 to 9, characterized in that the electrolyte in step b) contains hydrochloric acid in a concentration of 3 to 100 g/I.

11. Process according to one of Claims 1 to 10, characterized in that the electrochemical treatment in step b) is carried out at a current intensity of 30 to 120 A/dm^2.

12. Process according to one of Claims 1 to 11, characterized in that the anodization in step c) is carried out at an acid concentration of 10 to 20% by weight and at a temperature of 20 to 80°C.

13. Process according to one of Claims 1 to 12, characterized in that the anodization is followed by hydrophilizing.

14. Process according to Claim 13, characterized in that the hydrophilizing is carried out using a compound from the group consisting of polyvinyl phosphonic acid, sodium silicate, hydrofluorozirconic acid or alkali metal fluorozirconate.

15. Process according to one of Claims 1 to 14, characterized in that finally a light-sensitive layer is applied.

16. Process according to Claim 15, characterized in that a light-sensitive layer is applied, which contains a compound from the group consisting of aromatic diazonium salt or quinone diazide.