EP0089508A1 - Process for the electrolytic roughening of aluminium and its application as a support for offset printing plates - Google Patents

Abstract

The process for the electrochemical roughening of plate, sheet or strip material made of aluminum or its alloys is carried out in an aqueous electrolyte which contains nitric acid and oxalic acid as an additive; Boric acid, aluminum nitrate and / or hydrogen peroxide may also be present as further additives. The material roughened according to this method is preferably used as a carrier material for offset printing plates carrying a radiation-sensitive layer, it being possible for the roughened surface to also be anodically oxidized.

Description

The present invention relates to a method for the electrochemical roughening of plate, sheet or strip material made of aluminum and its use as a carrier material for offset printing plates.

A long-known method for producing offset printing plates from printing plates consists in coating an aluminum plate serving as a carrier material with a radiation-sensitive mixture suitable for later printing purposes, then exposing this layer to a template and finally developing it. The oleophilic image areas remaining after development take on ink and transfer them during the printing process, whereas the surface exposed during development forms the hydrophilic non-image areas, which take on water or aqueous solutions during printing and thus repel the bold printing inks. It has also been known for a long time that the number of copies that can be achieved with a printing form can be increased considerably if the surface of the aluminum carrier is either mechanically, eg. B. with wire brushes or finely divided slurries, and / or electrochemically in aqueous acid or salt solutions, such as. As nitric acid or hydrochloric acid roughened.

The electrochemical roughening of aluminum has numerous advantages over the mechanical roughening. For certain applications, a very fine and uniform roughening is desirable, especially if the aluminum is to be used as a carrier material for offset printing plates. A fine and uniform roughening can be achieved in an electrolyte consisting of an aqueous hydrochloric acid solution, although the current density must be kept quite small, is to be elicited if no scarring _A luminiumoberfläche, and due to the low current density of the roughening process takes a relatively long time. The state of the art, for example from US Pat. Nos. 3,980,539, 3,072,546, 3,073,765, 3,085,950, 3,935,080, 3,963,594 and 4,052,275, is the electrochemical roughening of aluminum plates with hydrochloric acid or nitric acid known: One problem with this type of roughening is the tendency for scarring on the aluminum surface caused by electrolysis. This is a disadvantage for the production of offset printing plates, since such serious irregularities in the surface lead to uneven adhesion of coatings applied later and to an inadequate ink / water balance during the printing process. Also from DE-B 21 49 899 (addition of boric acid or borates to HCl and / or HN0 ₃ containing baths) or DE-B 22 18 471 (addition of amines, aldehydes, amides, urea, chromic acid or nonionic surfactants) HCl) known additives to the acids do not yet lead to surfaces that are sufficient, in particular, for the requirements in the printing plate field, this applies in particular to higher concentrations of the base acids.

The additives known from the following publications have also not yet led to success, this applies in particular if, instead of the aluminum types with a content of more than 99.5% by weight Al (for example "pure aluminum"), these too with a low aluminum content (for example of 99.0% by weight and less than the types "3003" or "A-19") are to be used; It is precisely in these types that the known processes often cause troublesome formation of smear and / or scars.

DE-B 26 50 762 (= US-A 4 087 341) describes a process for the electrochemical roughening of aluminum substrates for lithographic printing plates, in which an alternating current of a special current form (with respect to the ratio of cathodic current to anodic current and cathodic half-period and anodic half period) is used. The electrolytes are either aqueous hydrochloric acid solutions, optionally with an inhibiting or stabilizing additive, or aqueous nitric acid solutions with additives such as nitrates, amines, diamines, aldehydes, complexing agents such as ethylenediaminetetraacetate or acids such as phosphoric acid, chromic acid and sulfosalicylic acid.

From DE-A 28 16 307 (= US-A 4 172 772) a process for the electrochemical roughening of aluminum supports for printing plates is known in which an electrolyte based on a dilute hydrochloric acid solution containing an alkanoic acid from C _i to C ₄ (especially acetic acid) is used.

It is therefore an object of the invention to propose an aluminum support with a finely roughened, essentially scar-free surface, on which radiation-sensitive coatings in particular adhere firmly, from which, however, the parts of the layer which cover the later non-image areas of the printing plate can be removed during development. The surface of the aluminum support should also be able to be adequately wetted with aqueous solutions during printing so that bold printing inks cannot adhere to it. In particular, this process is said to be suitable for the use of aluminum alloys containing less than 99.5% of Al.

The invention is based on the known method for the electrochemical roughening of plate, film or strip material made of aluminum or its alloys in an aqueous electrolyte based on nitric acid and at least one further additive. The process according to the invention is then characterized in that oxalic acid is used as an additive.

The process according to the invention accordingly comprises an electrochemical treatment of aluminum in an aqueous electrolyte solution which contains nitric acid and oxalic acid in sufficient quantities to produce a fine, uniform and essentially scar-free roughening. The respective optimal concentration of nitric acid and oxalic acid is dependent on various factors, such as. B. the current density, the temperature of the electrolyte and the properties of the aluminum product to be roughened, and can be easily determined by a few simple tests. The aqueous electrolyte can optionally also contain boric acid, aluminum nitrate and / or H ₂ 0 ₂ . The concentration of nitric acid in the aqueous electrolyte is generally 3 to 20 g / l, in particular 8 to 20 g / 1 and preferably 10 to 15 g / l. With quantities above 20 g / l there is no significant difference in the roughening until the roughening effect then begins to decrease at around 500 g / l. The oxalic acid is generally present in the electrolyte in a concentration of 1 to 80 g / l, in particular 5 to 45 g / l, preferably 8 to 20 g / l. If boric acid is added, 1 g / l is generally added to this acid up to the saturation point of the electrolyte, but in particular 5 to 15 g / l, preferably 8 to 12 g / l. The proportion of hydrogen peroxide - if it is added - is usually at a concentration of 1 to 60 g / l, in particular 10 to 30 g / 1 and preferably 15 to 20 g / l. If even aluminum nitrate is added may whose _K on- concentration approximately to the saturation point of the solution at this salt correspond, it is preferably 65 to 70 g / l, g / l preferably about 65th

The current density in the process according to the invention, which can be carried out batchwise or continuously, is generally from 30 to 120 A / dm ² , in particular from 45 to 80 A / dm ² and preferably at 45 to 60 A / dm ² . The roughening time is between 20 and 180 s, in particular between 20 and 90 s and preferably between 20 and 60 s.

The aluminum surface and the inert electrode, which generally consists of graphite, chromium or lead, are preferably arranged at a distance of up to 1.5 cm, in particular between 1 and 1.5 cm, from one another. Alternating current is particularly suitable for the roughening process, a frequency above about 55 Hz giving the best roughening effect. The frequency is best between 60 and about 300 Hz.

In the process according to the invention, the materials to be roughened can, for example, be those with an aluminum content of> 99.5, 99.2,> 98.5 or 98.3%, which contain Fe, Si, Cu and optionally Zn as admixtures , Ti, Mn and / or Mg contain; these are traded, for example, with the designation "pure aluminum", "1100", "3003" or "A-19". The thickness of these materials is generally in the range of about 0.1 to 0.65 mm. However, the method is preferred for such alloys containing less than 99.5% of Al, in particular less than 99.0% of Al, such as the "3003" type, in which a uniformly roughened surface has essentially no pitting (Scars) can be achieved, which was not yet known with the previous electrolytes or electrolyte additives. This aluminum alloy has so far - probably because of its Mn - Salary - causes greater difficulties because a roughened film - presumably from a manganese oxide hydrate - forms during the roughening; this no longer occurs in the method according to the invention.

When a plate of bright rolled aluminum alloy in _O ffsetqualität (for example, the types of "3003" or "1100" of the firm Alcoa or Type "A-19" of the company _C onalco), in particular "3003" electrolytically under the above conditions only in aqueous nitric acid is roughened, the surface after the treatment has, for example, a surface area increased by about 40%. The surface is relatively flat and so soft that the plate must be hardened by anodizing before it can be used for printing. In contrast, a roughened plate, the aqueous roughening electrolyte of which contains, for example, 13 g / l of nitric acid, 20 g / 1 of oxalic acid and 65 g / 1 of aluminum nitrate, has a surface area which is approximately 250% larger than that of the bright-rolled plate. To improve the surface hardness and thus to increase the print run, the plate can optionally be anodized, but this is not necessary.

If a plate is roughened in an aqueous electrolyte containing 13 g / 1 nitric acid, 20 g / 1 oxalic acid and 10 g / l boric acid, the surface area of its surface also increases by about 625% compared to that of bright rolled aluminum . The surface shows an extremely fine pore structure, with the pore walls are relatively thick. Anodic oxidation is no longer absolutely necessary.

It is believed that during the roughening process, the aluminum reacts with the nitric acid to form aluminum nitrate. In order to stabilize the amount of aluminum nitrate present during the process, which is carried out continuously, for example, aluminum nitrate is advantageously added to the aqueous electrolyte from the outset. Aluminum nitrate is best contained in the electrolyte up to the saturation point of the solution in this salt, so that additional aluminum nitrate formed during the process is only deposited on the bottom of the electrolyte vessel, while the concentration of the solution in this salt remains relatively constant.

After the electrochemical roughening, the plate can optionally also be anodically oxidized, for example in an aqueous electrolyte which contains sulfuric and / or phosphoric acid.

In the preferred use of the roughened aluminum according to the method of the invention in the production of offset printing plates, it is advantageous to subject the roughened or the roughened and anodized plate to a hydrophilizing intermediate treatment before the photosensitive coating is applied. Such intermediate treatments have the purpose, among other things, of better surface adhesion of the coating bring about and make the aluminum surface increasingly hydrophilic, for example, polyvinyl phosphonic acid, sodium silicate or alkali zirconium fluorides are used. Radiation-sensitive compounds suitable for offset printing for the radiation-sensitive coatings are, for example, the known aromatic diazonium salts, quinonediazides and photopolymerizable compounds. These are usually mixed with resins to increase the print run of the plate. These resins include, for example, polyurethanes and phenol formaldehyde resins, as well as many other known resins.

The percentages in the examples below are based on weight.

Comparative Example V1

A "1100" aluminum alloy plate is treated with a conventional alkaline degreasing solution and then immersed in a 1.5% aqueous nitrate solution. A system in which the aluminum forms one electrode and a lead plate forms the other electrode carries alternating current at a frequency of 60 Hz. The electrodes are spaced 1.0 cm apart and a current density of 45 _{A /} dm ^{2 is used} for a period of 60 s. The resulting roughened surface is examined with a scanning electron microscope at 1000, 2000 and 5000 times magnification. It shows that it has been roughened completely, but very unevenly. There are areas with extensive, unwanted scars education available, border on the more uniform areas, but which have scars of different diameters. The surface area of this surface is 40% larger than that of untreated aluminum. Such a surface is unsuitable for quality printing.

Comparative example V2

A plate made of the aluminum alloy "3003" is treated according to the instructions of Comparative Example VI. The results are similar, i. H. Extensive scarring can be seen under the scanning electron microscope.

Comparative Example V3

A plate made of the aluminum alloy "A-19" is treated according to the information in Comparative Example V1. Similar results are obtained, but here the scarring is more extensive and more noticeable.

Comparative Example V4

A plate made of aluminum alloy "1100" is treated with a conventional alkaline degreasing solution and then immersed in an aqueous solution containing 1.5% nitric acid and 6.5% aluminum nitrate. AC is conducted through a system in which the aluminum forms one electrode and a lead plate forms the other electrode. The electrodes are spaced 1.0 cm apart and a current density of 45 A / dm ^{2 is used} for a period of 60 s. When examining under the The scanning electron microscope shows that the sample has been roughened to some extent evenly. The surface is flat and has holes of different sizes with rather thin walls. There are signs of scarring, compared to untreated aluminum a surface area increased by 45% is measured. Because of the brittleness of the holes, this surface must be anodized if one wants to achieve an acceptable pressure behavior.

Comparative Example V5

A plate made of the aluminum alloy "3003" is treated according to the information in comparative example V4. The results are similar, i.e. There are holes of different sizes and scarring.

Comparative Example V6

A plate made of the aluminum alloy "A-19" is treated according to the information in Comparative Example V4. The surface is extremely poor, 5 since it has an uneven hole size and also has some un-roughened areas and severe scarring.

example 1

A "1100" aluminum alloy plate is treated with a conventional alkaline degreasing solution and then immersed in an aqueous solution containing 1.5% nitric acid, 6.5% aluminum nitrate and 3.0% oxalic acid. Through a System in which the aluminum sample is one electrode and a lead plate is the other electrode, AC is conducted. The electrodes are spaced 1.0 cm apart and a current density of 45 A / dm ^{2 is used} for a period of 60 s.

A magnification of 500 to 10,000 times reveals a surface under the scanning electron microscope that is roughened uniformly and has holes of uniform size. The surface area of the surface is 250% larger than that of untreated aluminum. The perforated walls are thicker than in the previously described surfaces, there is no need to connect anodic oxidation to produce printing plates.

Example 2

A plate made of the aluminum alloy "3003" is treated as described in Example 1. The results are similar, i.e. the surface is roughened very evenly.

Example 3

A plate made of aluminum alloy "A-19" is treated according to the instructions in Example 1. The results are similar, i. H. the surface is also roughened very evenly.

Example 4

A plate made of the aluminum alloy "1100" is treated as described in Example 1, but in an aqueous solution containing 1.5% nitric acid, 6.5% aluminum nitrate, 3.0% oxalic acid and 1.0 % of boric acid immersed. The grained plate is under the scanning electron microscope a Oberflä _- che, which corresponds in its uniformity and its overall picture of the Example 1. Fig. However, the holes are smaller and more numerous here. At a magnification of 10,000 times, a modification of the surface that has not yet been observed in the above examples is shown, because smaller holes occur within the larger holes. As in Example 1, it is no longer necessary to anodize in order to obtain a functional printing plate. The increase in the area compared to the untreated aluminum is determined at 625%.

Example 5

A plate made of the aluminum alloy "3003" is treated as described in Example 4. The results are similar, i. H. the surface is roughened very evenly.

Example 6

A plate made of aluminum alloy "A-19" is treated according to the instructions in Example 4. The results are similar, i. H. the surface is roughened very evenly.

Claims (6)

1 process for the electrochemical roughening of plate, foil or tape-shaped material made of aluminum or its alloys in an aqueous electrolyte based on nitric acid and at least one further additive, characterized in that oxalic acid is used as an additive. 2 The method according to claim 1, characterized in that the electrolyte contains 3 to 20 g / 1 of nitric acid and 1 to 80 g / l of oxalic acid. 3 The method according to claim 1 or 2, characterized in that the electrolyte additionally contains boric acid, aluminum nitrate and / or hydrogen peroxide. 4 The method according to claim 3, characterized in that the electrolyte contains 1 g / 1 until the electrolyte is saturated with boric acid. 5 The method according to claim 3, characterized in that the electrolyte contains 1 to 60 g / 1 of hydrogen peroxide. 6 Use of the roughened material according to one of claims 1 to 5, optionally after performing an anodic oxidation of the roughened surface, as a carrier material for a radiation-sensitive layer-bearing offset printing plates.