EP0292801A2 - Process for the electrochemical graining of aluminium for supports for printing plates - Google Patents

Abstract

The invention relates to a method for the electrochemical roughening of aluminum for printing plate supports, wherein one works with an electrolyte containing sulfate ions and aluminum chloride; sulfuric acid and aluminum chloride are preferred. The printing plate carriers roughened by means of the process have a particularly uniform, scar-free and area-wide roughening structure.

Description

The invention relates to a method for the electrochemical roughening of aluminum for printing plate supports, which is preferably carried out with alternating current in an electrolyte containing sulfuric acid, chloride and aluminum ions.

Printing plates (this term means offset printing plates in the context of the present invention) generally consist of a support and at least one radiation-sensitive reproduction layer arranged thereon, this layer either from the consumer (in the case of non-precoated plates) or from the industrial one Manufacturer (for pre-coated boards) is applied to the substrate.

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 types of modification 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 HCl or HNO₃ solutions or in aqueous salt solutions such as aqueous NaCl or Al (NO₃) ₃ solutions using alternating current. The roughness depths that can be achieved in this way (given, for example, as the mean roughness depths R _z ) of the roughened surface are in the range from about 1 to 15 μm, in particular in the range from 2 to 8 μm. 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 carried out, inter alia, in order to improve the adhesion of the reproduction layer on the substrate and the water flow of the printing plate resulting from the printing plate by irradiation (exposure) and development. By irradiation and development (or decoating in the case of reproductive layers working electrophotographically), the image points which carry color during later printing and the non-image points which carry water (generally the exposed carrier surface), which creates the actual printing form. 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 Lithographic 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⁺ (H₃O⁺) ion concentration (measurable via the pH) and the Al³⁺ 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 / dm² results in higher roughness values with increasing current density. If the acid concentration is in the range 0.17 to 3.3% of HCl, then only insignificant changes in the hole structure occur between 0.5 and 2% of HCl, below 0.5% of HCl there is only a local attack on the Ober surface and with the high values an irregular dissolution of aluminum instead. If a direct current is used instead of alternating current, it is evident that both types of half-wave are obviously required for a uniform roughening. Already in this review it is pointed out that the addition of sulfate ions increasingly leads to undesired, coarse, non-homogeneous roughening structures which are not suitable for lithographic purposes.

The use of hydrochloric acid 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.

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 HNO₃ or 0 , 4 to 0.6% by weight of HCl and optionally 0.4 to 0.6% by weight of H₃PO₄,
- 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 O.8 to 6, O wt .-% of HNO₃.

Additions to the HCl 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 / or 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-A 17 580/80 describes an AC roughening in a combination of hydrochloric acid and one Alkali halide for the production of 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.

DE-A 29 34 597 (= US-A 4 201 836, 4 242 417 and 4 324 841) describes the optionally electrochemical roughening of aluminum using a saturated aluminum salt solution, to which up to 10% of a mineral acid can be added. In the examples, aluminum chloride is used as the salt and hydrochloric acid is optionally added.

Such a saturated aluminum chloride solution (≧ 500 g / l AlCl₃ x 6 H₂O), especially in the acidic range, represents an extremely high risk of corrosion for the materials used. In particular, the surface quality achievable using added sulfuric acid as an added mineral acid in the examples would be , as the comparative examples V24 to V33 show, very pitted and not usable for lithographic applications.

JP-B 006 571/76 describes the alternating current roughening of an aluminum sheet for lithographic printing plates in electrolytes which contain 1 to 4% HCl and 0.1 to 1% H₂SO₄. In this electrolyte concentration range, as the comparative examples V34 to V53 show, only irregularly roughened surface profiles that do not correspond to the prior art can be achieved.

GB-A 1 392 191 describes the influence of sulfate ions in concentrations of more than 10 to 15 ppm in hydrochloric acid electrolytes as harmful for the production of a lithographic support material and uses a phosphoric acid additive as a remedy.

For use as a printing plate support material, EP-A 132 787 describes a roughening of aluminum in 1000 to 40000 ppm nitric acid, which contains 50 to 4000 ppm (up to 0.4%) sulfate ions; again the harmful influence of higher concentrations is spoken of. Roughening is even prevented from 5000 ppm.

US Pat. No. 1,376,366 describes the electrochemical lowering of metals in particular steel with direct current in a solution of ammonium chloride, sulfuric acid and nitric acid. The aim is to shape a workpiece. The aim of roughening for lithographic surfaces, on the other hand, is a very fine (1 to 10 µm), coating-free structuring of the surface, which ensures good copying layer anchoring and retention of the dampening water during the printing process task. The coating during roughening can be suppressed by using alternating current.

US-A 3 284 326 describes the roughening of an aluminum foil for capacitor production using direct current to achieve a high capacitance. A solution of chloride and phosphate is used as the electrolyte, the type of cation for the capacitor foil roughening - with the exception of the disadvantageous aluminum - being immaterial. Up to 10 mol% of the cation can also be replaced by H⁺; however, the text emphasizes that it is not a good idea to start with an acidic electrolyte.

For use as a capacitor film, roughening is carried out in systems containing aluminum chloride and sulfate in the following publications: US Pat. No. 4,427,506, US Pat. No. 4,395,305, JP-A 76 100/80, JP-B 39 169/78, JP- A 141 444/77 and JP-B 25 142/74.

In contrast to the sole goal of a large surface enlargement when used in condensers, however, the fundamentally different roughening for printing plate supports serves to anchor the layer and to guide the water and must therefore be very homogeneous and scar-free in a narrow roughness depth range.

In US Pat. No. 4,427,506, it is expressly pointed out for the production of capacitor foils that a sulfate ion content of> 500 ppm is harmful.

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-A 4,272,342),
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 2 N HCl solution with NaCl or MgCl₂ addition are used as electrolyte, according to 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.

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

It is therefore the object of the present invention 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-covering roughening structure and which involves a large outlay on equipment and a special choice of materials for reasons of corrosion protection and / or particularly narrow parameter limits can be dispensed with.

The object is achieved by a process for the electrochemical roughening of aluminum or its alloys for printing plate supports by means of alternating current in an electrolyte containing sulfate and chloride ions, the acidic, sulfate-containing electrolyte containing chloride ions in the form of aluminum chloride.

As the comparative examples V58-59 and example 57 show, the presence of aluminum ions for uniformizing the surface is absolutely advantageous for the process according to the invention for the production of printing plate supports. As examples V60 and V61 show, the use of direct current also leads to very grained surfaces that are absolutely unsuitable for lithographic purposes. In addition, an undesirable white coating occurs and the sheets are not roughened all over.

The electrochemical roughening for the production of lithographic printing plates with sulfate ions in a relatively high concentration of 5 to 100 g / l is surprisingly achieved by adding chlorides in the form of aluminum chloride. Lower concentrations of e.g. B. sulfuric acid cause an uneven surface structure.

In a preferred embodiment, an H₂SO₄ electrolyte is used, the sulfate ion concentration between 5 and 100 g / l, particularly preferably between 20 and 50 g / l, and the concentration of the chloride ions between 1 and 100 g / l, particularly preferably between 10 and is 70 g / l.

In a preferred embodiment, chloride ions are used as AlCl₃ x 6 H₂O in a concentration between 20 and 250 g / l, particularly preferably between 50 and 200 g / l.

Higher chloride ion concentrations increase the local attack in the form of unwanted scars. The invention also provides for combinations of different compounds containing chloride ions to be used.

Following the electrochemical roughening, in a preferred treatment step there is also chemical removal by means of a pickle, which cleans the surface of any coating. A solution containing sulfuric acid or removal in sodium hydroxide solution is particularly preferred.

The result of a surface produced by the method according to the invention is a highly uniform support surface with excellent lithographic properties that can be varied over a wide roughness depth range (R _z = 2 to 5 μm).

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 180 A / dm², the residence time of a material point to be roughened in the electrolyte between 10 and 300 seconds and that Electrolyte flow rate on the surface of the material to be roughened between 5 and 100 cm / sec. Due to the continuous driving style and the simultaneous release of Al ions and the consumption of H⁺, constant adjustment of the electrolyte composition by the corresponding diluted acids is necessary.

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.

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% Al 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
- "Al 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.

For anodic oxidation, the usual electrolytes such as H₂SO₄, H₃PO₄, H₂C₂O₄, amidosulfonic acid, sulfosuccinic acid, sulfosalicylic acid or mixtures thereof can be used be set. For example, reference is made to the following standard methods for the anodic oxidation of aluminum (see, for example, BM Schenk, Material aluminum and its anodic oxidation, Francke Verlag, Bern 1948, page 760; Practical Galvanotechnik, 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 about 230 g of H₂SO₄ per 1 liter of solution at 10 to 22 ° C and a current density of 0.5 to 2.5 A / dm² for 10 to 60 min is anodized. The sulfuric acid concentration in the aqueous electrolyte solution can also be reduced to 8 to 10% by weight of H₂SO₄ (approx. 100 g / l H₂SO₄) or increased to 30% by weight (365 g / l H₂SO₄) and more.
- The "hard anodization" with an aqueous H₂SO₄ containing electrolyte at a concentration of 166 g / l H₂SO₄ (or about 230 g / l H₂SO₄) at an operating temperature of 0 to 5 ° C, at a current density of 2 to 3 A / dm² , 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 minutes.

In addition to the methods for the anodic oxidation of printing plate supports already mentioned in the previous paragraph For example, the following methods can also be used: z. B. can the anodic oxidation of aluminum in an aqueous H₂SO₄ containing electrolyte, the Al³ + ion content is set to values of more than 12 g / l (according to DE-A 28 11 396 = US-A 4 211 619), in an aqueous electrolyte containing H₂SO₄ and H₃PO₄ (according to DE-A 27 07 810 = US-A 4 049 504) or in an aqueous electrolyte containing H₂SO₄, H₃PO₄ and Al³⁺ ions (according to DE-A 28 36 803 = US-A 4,229,226).

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, causing a surface removal 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. Hydrophilicity in particular becomes after-treatment chemical or electrochemical treatment of the aluminum oxide layer, for example an immersion treatment of the material in an aqueous polyvinylphosphonic acid solution according to DE-C 16 21 478 (= GB-A 1 230 447), an immersion treatment in an aqueous alkali silicate solution according to DE- B 14 71 707 (= US-A 3 181 461) or an electrochemical treatment (anodization) in an aqueous alkali silicate solution according to DE-A 25 32 769 (= US-A 3 902 976). These post-treatment stages serve in particular to additionally increase the hydrophilicity of the aluminum oxide layer, which is already sufficient for many areas of application, the remaining known properties of this layer being at least retained.

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. They are applied either by the manufacturer of presensitized printing plates or by so-called dry resists or directly by the consumer to one of the usual carrier materials.

The light-sensitive reproduction layers include such as z. B. in "Light-Sensitive Systems" by Jaromir Kosar, John Wiley & Sons Verlag, New York 1965, are described: The layers containing unsaturated compounds in which these compounds in Exposure isomerized, rearranged, cyclized or crosslinked (Kosar, Chapter 4), such as cinnamate; the layers containing photopolymerizable compounds, in which monomers or prepolymers optionally polymerize during exposure by means of an initiator (Kosar, Chapter 5); and the layers containing o-diazo-quinones such as naphthoquinonediazides, p-diazo-quinones or diazonium salt condensates (Kosar, Chapter 7).

The suitable layers also include the electrophotographic layers, ie those which contain an inorganic or organic photoconductor. In addition to the light-sensitive substances, these layers can of course also contain other constituents, such as, for example, resins, dyes, pigments, wetting agents, sensitizers, adhesion promoters, indicators, plasticizers or other customary auxiliaries. In particular, the following light-sensitive compositions or compounds can be used in the coating of the carrier materials:
positive-working, o-quinonediazide, preferably o-naphthoquinonediazide compounds, which are described, for example, in DE-C 854 890, 865 109, 879 203, 894 959, 938 233, 1 109 521, 1 144 705, 1 118 606, 1 120 273 and 1 124 817;
Negative-working condensation products from aromatic diazonium salts and compounds with active carbonyl groups, preferably condensation products from diphenylamine diazonium salts and formaldehyde, for example in DE-C 596 731, 1 138 399, 1 138 400, 1 138 401, 1 142 871, 1 154 123, US-A 2 679 498 and 3 050 502 and GB-A 712 606;
Negative working, mixed condensation products of aromatic diazonium compounds, for example according to DE-A 20 24 244, which each have at least one unit of the general types A (-D) _n and B connected by a double-bonded intermediate member derived from a condensable carbonyl compound. These symbols are defined as follows: A is the remainder of a compound containing at least two aromatic carbocyclic and / or heterocyclic nuclei, which is capable of condensing with an active carbonyl compound in an acidic medium at at least one position. D is a diazonium salt group attached to an aromatic carbon atom of A; n is an integer from 1 to 10 and B is the residue of a diazonium group-free compound capable of condensing with an active carbonyl compound in at least one position of the molecule in an acid medium;
positive-working layers according to DE-A 26 10 842, which contain a compound which cleaves off on irradiation, a compound which has at least one COC group which can be cleaved by acid (e.g. an orthocarboxylic acid ester group or a carboxylic acid amide acetal group) and optionally a binder;
Negative working layers of photopolymerizable monomers, photoinitiators, binders and optionally other additives. The monomers used here are, for example, acrylic and methacrylic acid esters or reaction products of diisocyanates with partial esters of polyhydric alcohols, as described, for example, in US Pat. Nos. 2,760,863 and 3,060,023 and DE-A 20 64 079 and 23 61 041. Suitable photoinitiators include benzoin, benzoin ether, multinuclear quinones, acridine derivatives, phenazine derivatives, quinoxaline derivatives, quinazoline derivatives or synergistic mixtures. A variety of soluble organic polymers can be used as binders, e.g. B. polyamides, polyesters, alkyd resins, polyvinyl alcohol, polyvinyl pyrrolidone, polyethylene oxide, gelatin or cellulose ether;
Negative working layers according to DE-A 30 36 077, which contain a diazonium salt polycondensation product or an organic azido compound as a photosensitive compound and a high molecular weight polymer with pendant alkenylsulfonyl or cycloalkenylsulfonylurethane groups as a binder.

It is also possible to apply photo-semiconducting layers, such as are described, for example, in DE-C 11 17 391, 15 22 497, 15 72 312, 23 22 046 and 23 22 047, to the support materials, thereby producing highly light-sensitive, electrophotographic layers.

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. Undesirable "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. The comparative examples V24 to V33 and V34 to V53 show in comparison with the other examples the effect of the electrolyte system according to the invention to achieve flat 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 seconds in an aqueous solution of 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 in terms of 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 µm and / or an extremely unevenly roughened or almost rolled surface.

Claims (12)

1. A process for the electrochemical roughening of aluminum or its alloys for printing plate supports by means of alternating current in an electrolyte containing sulfate and chloride ions, characterized in that the acidic, sulfate-containing electrolyte contains chloride ions in the form of aluminum chloride. 2. The method according to claim 1, characterized in that sulfuric acid is used as the sulfate-containing electrolyte. 3. The method according to any one of claims 1 or 2, characterized in that the sulfate ion concentration in the electrolyte is set between 5 and 100 g / l. 4. The method according to claim 3, characterized in that one adjusts the sulfuric acid concentration between 20 and 50 g / l. 5. The method according to any one of claims 1 to 4, characterized in that the concentration of the chloride ions is adjusted to 1 to 100 g / l. 6. The method according to claim 5, characterized in that adjusting the concentration of the chloride ions to 10 to 70 g / l. 7. The method according to any one of claims 1 to 6, characterized in that one additionally uses aluminum salt in a concentration of 20 to 200 g / l, based on the electrolyte. 8. The method according to any one of claims 1 to 7, characterized in that one works with a current density greater than 40 A / dm². 9. The method according to any one of claims 1 to 8, characterized in that the roughening is carried out for a period of 3 to 30 sec. 10. The method according to any one of claims 1 to 9, characterized in that further acids and / or salts are added to the electrolyte. 11. The method according to any one of claims 1 to 10, characterized in that after the electrochemical roughening and before the anodization, an intermediate step is optionally carried out. 12. The method according to any one of claims 1 to 10, characterized in that the pH of the electrolyte is less than 2.