Production of support for lithographic printing plate

Details Production of support for lithographic printing plate Production of support for lithographic printing plate

2001-07-02

2003-04-01

Wong, Edna (Department: 1741)

Electrolysis: processes, compositions used therein, and methods

Electrolytic coating

Treating substrate prior to coating

C205S205000, C205S206000, C205S220000

Reexamination Certificate

active

06540901

ABSTRACT:

This invention relates to a method for the production of a support for use as a substrate for a lithographic printing plate. More specifically, the invention provides a method for the surface treatment of a support material, whereby a substrate having particularly favorable lithographic properties may be obtained.
The material used as the support material depends upon the specific purpose for which the printing plate is to be used and may be, for example, a metal, paper or plastics material. Generally for printing plates, however, the preferred substrate is aluminum, most preferably electrochemically roughened aluminum which includes a surface layer of anodic aluminum oxide. Optionally, said aluminum may be laminated to another metal, such as copper or zinc, or to a plastics material, for example a polyester material such as poly(ethylene terephthalate).
Conventionally, aluminum substrates intended for use as support materials for lithographic printing plates and their precursors have been subjected to surface treatments prior to application of a light sensitive coating material. These treatments serve to improve the lithographic properties of the aluminum, in particular, its hydrophilicity. This is important during printing operations, since the basis of lithography is the ability of the lithographic plate to accept ink in image areas whilst rejecting ink and accepting water in background (non-image) areas, so that the printed image remains free from dirt and other contamination in said background areas. Thus, the light-sensitive coating of a lithographic printing plate precursor is imagewise exposed to radiation in order to change the solubility characteristics of the coating in the radiation-struck areas. The soluble areas are subsequently dissolved away by treatment with a developing solution, to expose the aluminum surface which must be capable of rejecting ink and accepting water.
A typical surface treatment comprises an initial graining treatment, wherein the aluminum surface is roughened by either mechanical or electrochemical means, and a subsequent anodizing treatment, by means of which a layer of aluminum oxide is formed on the surface of the aluminum. Anodizing treatments may, for example, be carried out by passing a grained aluminum web through a bath of a suitable anodizing acid, such as sulfuric or phosphoric acid, or a mixture thereof, whilst an electric current flows through the anodizing bath and the web serves as the anode. The presence of a surface anodic layer greatly enhances the hydrophilicity of the aluminum surface, and the adhesion of the subsequently formed image layer is found to be much improved when the surface of the aluminum is subjected to a graining treatment prior to anodizing.
Additionally, there is generally a requirement for a further surface treatment following the anodizing process. Such a treatment—referred to as a post-anodic dip—is usually applied in order to improve specific lithographic printing properties of the substrate, such as clean up of background areas, coating adhesion or corrosion resistance, and will typically involve treating the aluminum with a solution, often an aqueous solution, of the chosen reagent. Commonly used post-anodic dips include aqueous solutions containing, for example, various inorganic salts or organic derivatives such as poly(acrylic acid) or various aqueous-soluble copolymers.
Thus, EP-A-567178 discloses the treatment of grained and anodized aluminum with an aqueous solution of an alkali metal bicarbonate, whilst the use of solutions containing anions including chloride, fluoride, nitrate, carboxylate, sulfate and phosphate for application to anodized aluminum is described in JP-A-10129143. In addition, GB Patent No 1128506 deals with a process wherein anodized aluminum is treated with an aqueous solution of titanium, zirconium or hafnium tetrachloride—or the corresponding double fluoride formed with, for example, alkali metal fluorides—and subsequently dipped in an aqueous alkaline solution of potassium tetrapyrophosphate. Indeed, the successful use of various fluoride derivatives of titanium, hafnium and zirconium for post-anodic dip treatments has been widely reported; for example, GB Patent No 1504503 teaches the use of potassium titanium fluoride in combination with a vegetable tannin compound and a soluble lithium compound in the treatment of anodized aluminum surfaces in order to improve corrosion resistance, whilst EP-A-178020 discloses a treatment process for otherwise untreated aluminum which provides excellent corrosion resistance and paint adhesion characteristics and involves sequentially contacting the surface with (1) an aqueous acidic solution containing hafnium, zirconium and/or titanium ions, fluoride ions, a tannin compound and a sequestering agent and (2) a solution comprising a polyphenolic compound or acid salt thereof.
The present inventors have found, however, that the use of fluoride derivatives of this type can give rise to problems during the platemaking process, thereby resulting in the production of printing plates of inferior quality. Specifically, it was observed that the ease of performing deletions was adversely affected, such that corrections to the plate surface could only be carried out with difficulty. As a consequence, costly delays were experienced during platemaking, and the vigorous treatments required in order to effect the corrections gave rise to damage to the plate surface, with a resulting deleterious effect on printing performance.
It is, therefore, an object of the present invention to provide a post-anodic dip treatment for grained and anodized aluminum which eliminates the problem of poor ease of deletion during printing platemaking which is associated with the use of fluoride derivatives of titanium, hafnium and zirconium, whilst at the same time retaining the advantageous properties which are associated with the use of these materials.
Initially, the present inventors studied the possible addition of further materials to the post-anodic dip to study their effects in combination with the said fluoride derivatives. Specifically, a series of experiments was carried out with an aqueous solution of potassium hexafluorozirconate to which various other salts had been added; the intention was to examine the potential benefits of several alternative anions in combination with the hexafluorozirconate. It was found that particularly beneficial results were achieved when orthophosphate salts were present in the post-anodic bath, and that the resulting printing plates showed good ease of deletion. Unfortunately, however, severe practical difficulties were associated with this procedure, and the addition of orthophosphate salts to the post-anodic bath in this way was shown to give rise to severe precipitation problems during manufacture, to the extent that the resulting process could not be considered to be commercially viable. Hence, the inventors sought an alternative process, by means of which the beneficial results of the treatment could be maintained, whilst at the same time eliminating the said practical problems.
According to the present invention there is provided a process for the manufacture of a substrate for use in the production of lithographic printing plates, said process comprising the steps of:
(a) providing an aluminum substrate;
(b) graining at least one surface of said substrate;
(c) applying an anodic layer to said at least one grained surface;
(d) treating said at least one grained and anodized surface with an aqueous solution comprising at least one salt of a metal from Group IB, IIB, IVA, IVB, VB, VIA, VIB, VIIB or VIII of the Periodic Table; and
(e) treating said at least one treated surface with an aqueous solution comprising at least one orthophosphate salt of an alkali metal.
Said treatment of said at least one grained and anodized surface with an aqueous solution comprising at least one salt of a metal from Group IB, IIB, IVA, IVB, VB, VIA, VIB, VIIB or VIII of the Periodic Table is preferably carried out by treating said substrate wit

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