EP0215422A1 - Process for electrochemically roughening aluminium for printing plate supports - Google Patents

Abstract

The invention relates to a process for the electrochemical roughening of aluminum or its alloys for printing plate supports, wherein one works with an acidic electrolyte containing β-diketo compounds; hydrochloric or nitric acid with addition of acetylacetone 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 carried out with alternating current in an acidic electrolyte containing β-diketo compounds.

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 thereon, this layer either by the consumer (in the case of non-precoated plates) or by 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. The addition of SO₄²⁻ ions or Cl⁻ ions in salt form [z. B. by adding Al₂ (SO₄) ₃ or NaCl] can also affect the topography of the roughened aluminum. The experimental 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.

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) calls 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 HNO 3 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 monocarboxylic 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 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 often 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 anodic 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 with an alternating current in an electrolyte of 0.3 to 1.5% hydrochloric acid and 15 to 25% ammonium acetate and then further electrolyzed in HCl with pulsed current.

In JP application 105 471/78 in addition to the 15 to 25% ammonium acetate, 0.3 to 1.5% HNO₃ or 1 to 3.0% citric acid are also claimed.

Such treatment in electrolyte systems with a pH greater than 4.5, however, leads to coarse-grained and / or surface roughened surface structures which are completely unsuitable for lithographic purposes. In contrast to the enlargement of the surface when used in condensers, the roughening for printing plate supports serves to anchor the layers and to guide the water and must therefore be very homogeneous and scar-free.

The use of acetylacetone in simple metal cleaners is e.g. B. described in DE-A 19 26 809. However, the object of the present invention is to produce a support material suitable for lithographic purposes with an extremely homogeneous surface topography.

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 sec and a current flow for 30 to 300 sec, alternating current and an aqueous 0.75 to as an electrolyte 2 N HCl solution with NaCl or MgCl₂ addition are used, according to GB-A 879 768. A similar process with an interruption of the current flow in the anode or cathode phase is also called DE-A 30 20 420 (= US A 4,294,672).

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.

The object of the present invention is therefore to propose a method for the electrochemical roughening of aluminum for printing plate supports, which results in a uniform, scar-free and area-covering roughening structure and which can be dispensed with a large outlay 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 acidic electrolyte under the action of electrical current. Alternating current is preferably used. But as can be seen from Examples 30 to 32, good lithographic surfaces can also be produced by using anodic direct current in the electrolyte according to the invention.

The process according to the invention is characterized in that an acidic electrolyte is used, to which a β-diketo compound is added.

In a preferred embodiment, one works with an HCl or HNO₃ electrolyte, the acid concentration between 0.01 and 50.0 g / l, particularly preferably between 0.01 and 30.0 g / l, and the concentration of the β- Diketo compound is between 3 g / l and the saturation limit, particularly preferably between 40.0 g / l and the saturation limit.

Acetylacetone is used as the preferred β-diketo compound. Within the scope of the invention it is also provided that combinations of β-diketo compounds are used as long as the requirement is met that the pH is adjusted to be acidic.

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.

The amount of hydrochloric acid released by hydrolysis of the aluminum chloride used may already be sufficient to carry out the process according to the invention.

The result of a surface produced by the method according to the invention is an extremely flat (R _z = 2 to 5 µm) 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 / dm², the residence time of a material point to be roughened in the electrolyte between 10 and 300 sec and that Electrolyte flow rate on the surface of the material to be roughened between 5 and 100 cm / sec. 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. The usual electrolytes such as H₂SO₄, H₃PO₄, H₂C₂O₄, 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 H₂SO₄ for the anodic oxidation of aluminum (see, for example, M Schenk, material aluminum and its anodic oxidation, Francke Verlag, Bern 1948, page 760; Praktische Galvanotechnik, Eugen G. Leuaze 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 anodic oxidation of printing plate support materials already mentioned in the previous paragraph, the following methods can also be used, for example: 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 anodic oxidation, however alternating current or a combination of these types of current (e.g. 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 include 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 in particular to mean a hydrophilizing chemical or electrochemical treatment of the aluminum oxide layer, for example 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 ensure that the hydrophilicity is already sufficient for many areas of application To increase the aluminum oxide layer even more, at least the other known properties of this layer are 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 a template. 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 layers such as those e.g. 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 are isomerized, rearranged, cyclized or crosslinked during exposure (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-diazoquinones such as naphthoquinonediazides, p-diazoquinones or diazonium salt condensates (Kosar, Chapter 7).

Suitable layers also include the electrophotographic layers, ie those which are inorganic contain 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, which are described, 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 at least two aromatic carboxylic and / or heterocyclic nuclei-containing compound which is capable of condensing at least one position in an acidic medium with an active carbonyl compound. 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 (for example 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 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 ethers, multinuclear quinones, acridine derivatives, phenazine derivatives, quinoxaline derivatives, quinazoline derivatives or synergistic mixtures of various ketones. A variety can be used as binders soluble organic polymers are used, for example polyamides, polyesters, alkyd resins, polyvinyl alcohol, polyvinylpyrrolidone, 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 the photosensitive compound and a high molecular weight polymer with pendant alkenylsulfonyl or cycloalkenylsulfonylurethane groups as the binder.

It is also possible to use photo-semiconducting layers such as e.g. in DE-C 11 17 391, 15 22 497, 15 72 312, 23 22 046 and 23 22 047 are described, are applied to the support materials, whereby highly light-sensitive, electrophotographic layers are formed.

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 supply when printing 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; 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 V4, V13 and V29 show in comparison with the other examples play the effect of the addition of β-diketo compounds while maintaining an acidic pH value as a tool for achieving flatter, yet 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.

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 µm and / or an extremely unevenly roughened or almost rolled surface.

Claims (14)

1. A process for the electrochemical roughening of aluminum for printing plate supports in an acidic electrolyte, characterized in that at least one β-diketo compound is added to the electrolyte. 2. The method according to claim 1, characterized in that hydrochloric acid or nitric acid is used as the acid in the electrolyte. 3. The method according to claim 1 or 2, characterized in that one adjusts the acid concentration in the electrolyte between 0.01 and 50 g.l. 4. The method according to claim 3, characterized in that one adjusts the acid concentration between 0.01 and 30 g / l. 5. The method according to any one of claims 1 to 4, characterized in that the concentration of the β-diketo compound is adjusted to 3.0 g / l up to the saturation limit. 6. The method according to claim 5, characterized in that adjusting the concentration of the β-diketo compound to 40.0 g / l to 400 g / l. 7. The method according to any one of claims 1 to 6, characterized in that acetylacetone is used. 8. The method according to any one of claims 1 to 7, characterized in that at least one aluminum salt is added to the electrolyte. 9. The method according to claim 8, characterized in that one adds the aluminum salt of an inorganic acid. 10. The method according to claim 9, characterized in that aluminum chloride or aluminum nitrate is added. 11. The method according to any one of claims 8 to 10, characterized in that the aluminum salt is used in a concentration of 20 to 200 g / l based on the electrolyte. 12. The method according to claim 1 to 4, characterized in that alternating current is used as the electrical current. 13. The method according to any one of claims 1 to 12, characterized in that one works with a current density greater than 30 A / dm². 14. The method according to any one of claims 1 to 13, characterized in that the roughening is carried out for a period of 3 to 30 seconds.