EP0194429B1 - Process for the electrochemical graining of aluminium for printing plate substrates - Google Patents

Description

The invention relates to a method for the electrochemical roughening of aluminum for printing plate supports, which is carried out with alternating current in an acidic electrolyte containing chloride and ammonium ions. Printing plates (this term means offset printing plates in the context of the present invention) generally consist of a carrier and at least one radiation-sensitive reproduction layer arranged on it, this layer either by the consumer (in the case of non-precoated plates) or is applied by the industrial manufacturer (in the case of pre-coated boards) to the layer support.

Aluminum or one of its alloys has established itself as a layer material in the printing plate field. In principle, these substrates can also be used without a modifying pretreatment, but they are generally modified in or on the surface, for example by mechanical, chemical and / or electrochemical roughening (sometimes called grain or etching in the relevant literature), a chemical one or electrochemical oxidation and / or treatment with hydrophilizing agents.

In the modern, continuously operating high-speed systems of the manufacturers of printing plate supports and / or precoated printing plates, a combination of the above-mentioned modification types is often used, in particular a combination of electrochemical roughening and anodic oxidation, optionally with a subsequent hydrophilization step.

The roughening is carried out, for example, in aqueous acids such as aqueous HCI or HN0 ₃ solutions or in aqueous salt solutions such as aqueous NaCl or Al (NO ₃ ) ₃ solutions using alternating current. The thus obtainable surface roughness (expressed for example as the average roughness depth R _z) of the roughened surface are in the range of about 1 to 15 ₁ 1m, in particular in the range of 2 to 8 microns. The roughness depth is determined in accordance with DIN 4768 (as of October 1970). The roughness depth R _z is then the arithmetic mean of the individual roughness depths of five adjacent individual measurement sections.

The roughening is a. therefore carried out in order to improve the adhesion of the reproduction layer on the layer support and the water flow of the printing plate resulting from the printing plate by irradiation (exposure) and development. By irradiation and development (or de-coating in the case of reproduction layers working electrophotographically), the image points which carry color during later printing and the water-bearing non-image points (generally the exposed carrier surface) are produced on the printing plate, as a result of which the actual printing form is created. Very different parameters have an influence on the later topography of the roughened aluminum surface. For example, the following references provide information:

In the article "The Alternating Current Etching of Aluminum Lithographie Sheet" by AJ Dowell in Transactions of the Institute of Metal Finishing, 1979, Vol. 57, pp. 138 to 144, basic explanations are given on the roughening of aluminum in aqueous hydrochloric acid solutions, the following process parameters varied and the corresponding effects were examined. The electrolyte composition is changed with repeated use of the electrolyte, for example with regard to the H ⁺ (H ₃ O ⁺ ) ion concentration (measurable via the pH value) and the AI ^{3 + -} ion concentration, with effects on the surface topography being observed. The temperature variation between 16 _° C and 90 _° C shows a changing influence only from about 50 _° C, which is expressed, for example, by the sharp decline in the formation of layers on the surface. The roughening time change between 2 and 25 min also leads to an increasing metal dissolution with increasing exposure time. The variation of the current density between 2 and 8 A / dm ² results in higher roughness values with increasing current density. If the acid concentration is in the range 0.17 to 3.3% of HCI, then between 0.5 and 2% of HCI only minor changes occur in the hole structure, below 0.5% of HCI there is only a local attack on the Surface and at the high values an irregular dissolution of aluminum instead. The addition of S0 ₄ ^2-- or CI - ions in salt form [e.g. B. by adding Al ₂ (S0 ₄ ) ₃ or NaCl] can also influence the topography of the roughened aluminum. The rectification of the alternating current shows that obviously both half-wave types are required for a uniform roughening.

The use of hydrochloric acid as an electrolyte for roughening aluminum substrates can therefore be assumed to be known. A uniform grain size can be obtained which is suitable for lithographic plates and is within a useful roughness range. In pure hydrochloric acid electrolytes, the setting of a flat and uniform surface topography is difficult, and it is necessary to maintain the operating conditions within very narrow limits.

• -die DE-A 22 50 275 (= GB-A 1 400 918) nennt als Elektrolyten bei der Wechselstrom-Aufrauhung von Aluminium für Druckplattenträger wäßrige Lösungen eines Gehalts von 1,0 bis 1,5 Gew.-% an HN0₃ oder von 0,4 bis 0,6 Gew.-% an HCI und gegebenenfalls 0,4 bis 0,6 Gew.-% an HsP04,
• -die DE-A 28 10 308 (= US-A 4 072 589) nennt als Elektrolyten bei der Wechselstrom-Aufrauhung von Aluminium wäßrige Lösungen eines Gehalts von 0,2 bis 1,0 Gew.-% an HCI und 0,8 bis 6,0 Gew.-% an HNOs.

The influence of the composition of the electrolyte on the roughening quality is also described, for example, in the following publications:

• - DE-A 22 50 275 (= GB-A 1 400 918) names as electrolytes in the AC roughening of aluminum for printing plate supports aqueous solutions with a content of 1.0 to 1.5% by weight of HN0 ₃ or from 0.4 to 0.6% by weight of HCl and optionally 0.4 to 0.6% by weight of HsP04,
• DE-A 28 10 308 (= US-A 4 072 589) names as electrolytes in the AC roughening of aluminum aqueous solutions with a content of 0.2 to 1.0% by weight of HCl and 0.8 to 6.0% by weight of ENT.

• -die DE-A 28 16 307 (= US-A 4 172 772) den Zusatz von Monocarboxysäuren, wie Essigsäure zu Salzsäureelektrolyten,
• - die US-A 3 963 594 von Gluconsäure,
• -die EP-A 0 036 672 von Citronen- und Malonsäure und
• -die US-A 4 052 275 von Weinsäure.

Additions to the HCI electrolyte have the task of preventing an adverse local attack in the form of deep holes. So describes

• DE-A 28 16 307 (= US Pat. No. 4,172,772) the addition of monocarboxy acids, such as acetic acid, to hydrochloric acid electrolytes,
• US Pat. No. 3,963,594 to gluconic acid,
• EP-A 0 036 672 of citric and malonic acid and
• US-A 4 052 275 of tartaric acid.

All these organic electrolyte components have the disadvantage that they are electrochemically unstable and decompose at high current loads (voltage).

Inhibiting additives, as described in US Pat. No. 3,887,447 with phosphoric and chromic acid, in DE-A 25 35 142 (= US Pat. No. 3,980,539) with boric acid, have the disadvantage that the protective action frequently breaks down locally and there can be individual, particularly pronounced scars.

JP application 91 334/78 describes AC roughening in a combination of hydrochloric acid and an alkali halide to produce a lithographic base material.

DE-A 16 21 115 (= US-A 3 632 486 and US-A 3 766 043) describes a direct current roughening e.g. B. for decorative cladding in dilute hydrofluoric acid with cathodic switching of the aluminum.

DE-C 120 061 describes a treatment for producing a water-attracting layer by using electricity, which can also take place in hydrofluoric acid.

JP application 93 108/78 describes the production of a capacitor film; it is first roughened in an electrolyte from 0.3 to 1.5% hydrochloric acid and 15 to 25% ammonium acetate with alternating current (with 200 to 400 C / dm ² ) and then further electrolyzed in HCl with pulsed current.

DE-B 22 18 471 (= US-A 3 775 116) describes the addition of amines in concentrations of 0.05 to 5% as an anti-corrosive agent to a 1 to 3% hydrochloric acid electrolyte for the production of printing plate supports. At least one of the following substances, magnesium chloride, aluminum chloride, zinc chloride or ammonium chloride, in a total chloride concentration of up to 8% by weight, based on the total weight of the electrolyte, is used as the secondary pickling agent.

From the equivalent treatment of the secondary mordants it is clear from the examples that the surface is homogenized by the above-mentioned anticorrosive agents and not by a specific effect, for. B. the ammonium ions.

As the examples also show, these anticorrosive agents increase the roughness depth compared to pure hydrochloric acid electrolytes.

In JP application 105471/78, in addition to the 15 to 25% ammonium acetate, 0.3 to 1.5% HN0 ₃ or 1 to 30% citric acid are also claimed.

Such treatment in electrolyte systems with a pH value greater than 4.5, however, leads to coarsely pitted and / or non-area-roughened surface structures which are completely unsuitable for lithographic purposes (see comparative examples V41 to V48). In contrast to the enlargement of the surface when used in condensers, roughening for printing plate supports serves to anchor the layer and to guide the water and must therefore be very homogeneous and scar-free.

• - der Einsatz von Wechselstrom, bei dem die Anodenspannung und der anodische coulombische Eingang größer als die Kathodenspannung und der kathodische coulombische Eingang sind, gemäß der DE-A 26 50 762 (= US-A 4 087 341), wobei im allgemeinen die anodische Halbperiodenzeit des Wechselstroms geringer als die kathodische Halbperiodenzeit eingestellt wird; auf diese Methode wird beispielsweise auch in der DE-A 29 12 060 (= US-A 4 301 229), der DE-A 30 12 135 (= GB-A 2 047 274) oder der DE-A 30 30 815 (= US-A 4 272 342) hingewiesen, wobei bei letzterer auch Ammoniumchlorid bis maximal 30g/1 eingesetzt wird,
• - der Einsatz von Wechselstrom, bei dem die Anodenspannung deutlich gegenüber der Kathodenspannung erhöht wird, gemäß der DE-A 14 46 026 (= US-A 3 193 485),
• - die Unterbrechung des Stromflusses während 10 bis 120 sec und ein Stromfluß während 30 bis 300 sec, wobei Wechselstrom und als Elektrolyt eine wäßrige 0,75 bis 2 n HCI-Lösung mit NaCI-oder MgCI₂-Zusatz eingesetzt werden, gemäß der GB-A 879 768. Ein ähnliches Verfahren mit einer Unterbrechung des Stromflusses in der Anoden-oder Kathodenphase nennt auch die DE-A 30 20 420 (= US-A 4 294 672).

Another known possibility of improving the uniformity of the electrochemical roughening is to modify the current form used, these include, for example

• - The use of alternating current, in which the anode voltage and the anodic coulombic input are greater than the cathode voltage and the cathodic coulombic input, according to DE-A 26 50 762 (= US-A 4 087 341), generally the anodic half-period the alternating current is set less than the cathodic half-period; this method is also used, for example, in DE-A 29 12 060 (= US-A 4 301 229), DE-A 30 12 135 (= GB-A 2 047 274) or DE-A 30 30 815 (= US Pat. No. 4,272,342), ammonium chloride up to a maximum of 30 g / l being used in the latter,
• the use of alternating current, in which the anode voltage is significantly increased compared to the cathode voltage, according to DE-A 14 46 026 (= US-A 3 193 485),
• the interruption of the current flow for 10 to 120 seconds and a current flow for 30 to 300 seconds, alternating current and an aqueous 0.75 to 2N HCl solution with NaCl or MgCl ₂ additive being used as the electrolyte, according to the GB A 879 768. DE-A 30 20 420 (= US Pat. No. 4,294,672) also calls a similar process with an interruption of the current flow in the anode or cathode phase.

The methods mentioned can be used to make relatively evenly roughened aluminum surfaces lead, but they sometimes require a relatively large amount of equipment and can only be used within very narrow parameter limits.

The object of the present invention is therefore to propose a method for the electrochemical roughening of aluminum for printing plate supports with alternating current, which results in a uniform, scar-free and area-wide roughening structure and which can be dispensed with a large expenditure on equipment and / or particularly narrow parameter limits.

The invention relates to a process for the electrochemical roughening of aluminum or its alloys for printing plate supports in an acid electrolyte containing chloride ions and ammonium ions under the action of alternating current. The process according to the invention is characterized in that an ammonium ion-containing compound is added to the electrolyte in a concentration of 40 g / 1 up to the saturation limit and the pH is adjusted to less than 4.5.

In a preferred embodiment, an HCI electrolyte is used, the hydrochloric acid concentration being between 0.01 and 50.0 g / l, particularly preferably between 0.01 and 30.0 g / l, and the concentration of the ammonium compound preferably between 70.0 g / I and the saturation limit.

Ammonium chloride is used as the preferred compound containing ammonium ions. Within the scope of the invention it is also provided to use combinations of ammonium salts as long as the requirement is met that the pH value is adjusted to <4.5, preferably <3.0.

It has proven to be particularly advantageous if aluminum salts are added to the electrolyte, preferably in an amount of 20 to 150 g / l. However, the amount of hydrochloric acid liberated by hydrolysis of the aluminum chloride used is already sufficient to carry out the process according to the invention (see also Examples 35 and 36).

The result of a surface produced by the method according to the invention is an extremely flat (R _z = 2 to 4 J.1m) highly uniform carrier surface with excellent lithographic properties.

The process according to the invention is carried out either discontinuously or preferably continuously with strips made of aluminum or its alloys. In general, the process parameters in continuous processes during roughening are in the following ranges: the temperature of the electrolyte between 20 and 60 _° C, the current density between 3 and 130 A / dm ² , the residence time of a material point to be roughened in the electrolyte between 10 and 300 s and the electrolyte flow rate at the surface of the material to be roughened between 5 and 100 cm / s. In discontinuous processes, the current densities required tend to be in the lower part and the residence times are in the upper part of the ranges specified; the flow of the electrolyte can also be dispensed with.

In addition to the current forms mentioned in the illustration relating to the prior art, superimposed alternating current and low-frequency currents can also be used.

• - "Reinaluminium" (DIN-Werkstoff Nr. 3.0255), d. h. bestehend aus mehr als 99,5 % AI und den folgenden zulässigen Beimengungen von (maximale Summe von 0,5 %) 0,3 % Si, 0,4 % Fe, 0,03 % Ti, 0,02 % Cu, 0,07 % Zn und 0,03 % Sonstigem, oder
• - "AI-Legierung 3003" (vergleichbar mit DIN-Werkstoff Nr. 3.0515), d. h. bestehend aus mehr als 98,5 % Al, den Legierungsbestandteilen 0 bis 0,3 % Mg und 0,8 bis 1,5% Mn und den folgenden zulässigen Beimengungen von 0,5 % Si, 0,5 % Fe, 0,2 % Ti, 0,2 % Zn, 0,1 % Cu und 0,15 % Sonstigem.

In the process according to the invention, the following can be used as materials to be roughened, for example, which are either in the form of a plate, film or tape:

• - "Pure aluminum" (DIN material no. 3.0255), ie consisting of more than 99.5% AI and the following admissible admixtures of (maximum sum of 0.5%) 0.3% Si, 0.4% Fe, 0.03% Ti, 0.02% Cu, 0.07% Zn and 0.03% others, or
• - "AI alloy 3003" (comparable to DIN material no. 3.0515), ie consisting of more than 98.5% Al, the alloy components 0 to 0.3% Mg and 0.8 to 1.5% Mn and the following admissible admixtures of 0.5% Si, 0.5% Fe, 0.2% Ti, 0.2% Zn, 0.1% Cu and 0.15% other.

However, the method according to the invention can also be applied to other aluminum alloys.

After the electrochemical roughening process according to the invention, an anodic oxidation of the aluminum can then follow in a further process step to be used, for example to improve the abrasion and adhesion properties of the surface of the carrier material.

Before the anodizing step, an alkaline, but preferably an acidic, intermediate pickling can be carried out to remove any coating or to improve the water flow.

• - Das Gleichstrom-Schwefelsäure-Verfahren, bei dem in einem wäßrigen Elektrolyten aus üblicherweise ca. 230 g H₂S0₄ pro 1 Liter Lösung bei 10 bis 22 _°C und einer Stromdichte von 0,5 bis 2,5 A/dm² während 10 bis 60 min anodisch oxidiert wird. Die Schwefelsäurekonzentration in der wäßrigen Elektrolytlösung kann dabei auch bis auf 8 bis 10 Gew.-% H₂S0₄ (ca. 100 g/I H₂S0₄) verringert oder auch auf 30 Gew.-% (365 g/I H₂S0₄) und mehr erhöht werden.
• - Die "Hartanodisierung" wird mit einem wäßrigen H₂S0₄ enthaltenden Elektrolyten einer Konzentration von 166 g/I H₂S0₄ (oder ca. 230 g/I H₂S0₄) bei einer Betriebstemperatur von 0 bis 5 _°C, bei einer Stromdichte von 2 bis 3 AIdm², einer steigenden Spannung von etwa 25 bis 30 V zu Beginn und etwa 40 bis 100 V gegen Ende der Behandlung und während 30 bis 200 min durchgeführt.

The usual electrolytes such as H ₂ S0 ₄ , H ₃ P0 ₄ , H ₂ C ₂ 0 ₄ , amidosulfonic acid, sulfosuccinic acid, sulfosalicylic acid or mixtures thereof can be used for anodic oxidation. Reference is made, for example, to the following standard methods for the use of aqueous electrolytes containing H ₂ S0 ₄ for the anodic oxidation of aluminum (see, for example, BM Schenk, Material Aluminum and its Anodic Oxidation, Francke Verlag, Bern 1948, page 760; practical electroplating technology , Eugen G. Leuze Verlag, Saulgau 1970, pages 395 ff. And pages 518/519; W. Hübner and CT Speiser, The Practice of Anodic Oxidation of Aluminum, Aluminum Verlag, Düsseldorf 1977, 3rd edition, pages 137 ff.) :

• - The direct current sulfuric acid process, in which in an aqueous electrolyte from usually approx. 230 g H ₂ SO ₄ per 1 liter of solution at 10 to 22 _° C and a current density of 0.5 to 2.5 A / dm ² is anodized for 10 to 60 min. The sulfuric acid concentration in the aqueous electrolyte solution can also be reduced to 8 to 10% by weight H ₂ S0 ₄ (approx. 100 g / IH ₂ S0 ₄ ) or to 30% by weight (365 g / IH ₂ S0 ₄ ) and more can be increased.
• - The "hard anodization" is carried out with an aqueous electrolyte containing H ₂ S0 _{4 with} a concentration of 166 g / IH ₂ S0 ₄ (or approx. 230 g / IH ₂ S0 ₄ ) at an operating temperature of 0 to 5 _° C, at a current density from 2 to 3 Aldm ² , a rising voltage of about 25 to 30 V at the beginning and about 40 to 100 V towards the end of the treatment and for 30 to 200 min.

In addition to the processes for anodic oxidation of printing plate support materials already mentioned in the previous paragraph, the following processes can also be used, for example: B. can the anodic oxidation of aluminum in an aqueous H ₂ S0 ₄ containing electrolyte, the Al3 ^{+ -} ion content is set to values of more than 12 g / I (according to DE-A 28 11 396 = US-A 4 211 619 ), in an aqueous electrolyte containing H ₂ S0 ₄ and H ₃ P0 ₄ (according to DE-A 27 07 810 = US-A 4 049 504) or in an aqueous H ₂ S0 ₄ , H _s P0 ₄ and Al3 + ion-containing electrolytes (according to DE-A 28 36 803 ₌ US-A 4 229 226) are carried out.

Direct current is preferably used for the anodic oxidation, but alternating current or a combination of these types of current (eg direct current with superimposed alternating current) can also be used. The layer weights of aluminum oxide range from 1 to 10 g / m2, corresponding to a layer thickness of approximately 0.3 to 3.0 µm. After the stage of electrochemical roughening and before an anodic oxidation, a modification can also be applied which causes surface abrasion from the roughened surface, as described, for example, in DE-A 30 09 103. Such a modifying intermediate treatment can u. a. allow the build-up of abrasion-resistant oxide layers and a lower tendency to tone when printing later.

The stage of anodic oxidation of the aluminum printing plate support material can also be followed by one or more post-treatment stages. Aftertreatment is understood to mean in particular a hydrophilizing chemical or electrochemical treatment of the aluminum oxide layer.

A light-sensitive reproduction layer is applied to the material. In principle, all layers are suitable as light-sensitive reproduction layers which, after exposure, possibly with a subsequent development and / or fixation, provide an image-like area from which printing can take place and / or which represent a relief image of an original.

The materials for printing plate supports roughened by the process according to the invention have a very uniform topography, which has a positive influence on the support stability and the water flow during printing of printing forms made from these supports. Compared to the use of pure hydrochloric acid electrolytes, "scars" (compared to the roughening of the surroundings: distinctive depressions) occur less frequently, and these can even be completely suppressed; in particular, it is also possible to produce flat, scar-free supports with the methods according to the invention. Comparative examples 9 to 12 and 41 to 49 show, in comparison with the other examples, the effect of the addition of ammonium while maintaining a pH value of <4.5 as an aid for achieving flatter and nevertheless uniform surfaces. These surface properties can be realized without any great expenditure on equipment.

Examples

An aluminum sheet (DIN material no. 3.0255) is first pickled for 60 s in an aqueous solution containing 20 g / l NaOH at room temperature. The roughening takes place in the specified electrolyte systems at 40 _° C.

However, there is no restriction to the exemplary embodiments.

The classification into the quality classes (surface topography with regard to uniformity, freedom from scars and area coverage) is carried out by visual assessment under the microscope, whereby a homogeneously roughened and scar-free surface is assigned quality level "1" (best value). Quality level "10" (worst value) is assigned to a surface with thick scars of a size of more than 30 11 m and / or an extremely unevenly roughened or almost rolled surface.

Claims (16)

1. Process for electrochemically graining aluminium for use in printing plate supports, by means of an alternating current in an acidic electrolyte containing chloride ions and ammonium ions, wherein at least one compound containing ammonium ions is added to the electrolyte in a concentration of 40 g/I up to the saturation limit and the pH is adjusted to < 4.5.

2. A process as claimed in claim 1, wherein the total concentration of chloride ions in the electrolyte is adjusted to be higher than 8% by weight.

3. A process as claimed in claim 1 or 2, wherein hydrochloric acid is used as the electrolyte containing chloride ions.

4. A process as claimed in any of claims 1 to 3, wherein the hydrochloric acid concentration in the electrolyte is adjusted between 0.01 and 50 g/I.

5. A process as claimed in claim 4, wherein the hydrochloric acid concentration is adjusted between 0.01 and 30 g/l.

6. A process as claimed in claim 5, wherein the concentration of the compound containing ammonium ions is adjusted in the range from 70 g/I to 400 g/i.

7. A process as claimed in any of claims 1 to 6, wherein the pH is adjusted to < 3.0.

8. A process as claimed in any of claims 1 to 7, wherein at least one ammonium salt of an inorganic acid is used as the compound containing ammonium ions.

9. A process as claimed in any of claims 1 to 8, wherein ammonium chloride is used.

10. A process as claimed in any of claims 1 to 9, wherein at least one aluminum salt is additionally added to the electrolyte.

11. A process as claimed in claim 10, wherein the aluminum salt of an inorganic acid is added.

12. A process as claimed in claim 11, wherein aluminum chloride is added.

13. A process as claimed in any of claims 10 to 12, wherein the aluminum salt is used in a concentration from 20 to 200 g/I based on the electrolyte.

14. A process as claimed in any of claims 1 to 13, wherein a current density greater than 30 A/dm2 is used.

15. A process as claimed in any of claims 1 to 14, wherein graining is carried out for a period from 3 to 30 seconds.

16. A process as claimed in any of claims 1 to 15, wherein an alkaline or preferably an acidic intermediate pickling is carried out after graining and before anodizing.