BACKGROUND OF THE INVENTION In news printing, non-printing plates are mounted at those parts of the cylinder where no ink should be transferred to the web to be printed. These non-printing plates are called"dummy plates". Such dummy plates are also used in the case of multi- color printing. Dummy plates are printing plates without ink-accepting areas. The dummy plates transfer the fountain solution applied over the entire width of the cylinder to the web and at the same time must not take up the ink.

Roughened and anodically oxidized aluminum plates are commonly used as dummy plates. However, such plates are very sensitive to fingerprints and other external influences like dust; fingerprints result in ink-accepting areas and thus smearing in the printing machine. Furthermore, the hydrophilicity of such dummy plates decreases during use which again results in ink-accepting areas and thus smearing. It is therefore preferred that dummy plates be treated with a protective gum solution.

Coated dummy plates are for instance disclosed in EP-A-0 894 642 where the non-photosensitive water-soluble protective layer contains a water-soluble organic compound which contains at least one OH-or NH-acidic group having a pKa value of £ 8, and has a layer thickness which is smaller than the average roughness of

the support material.

Another coated dummy plate is described in EP-A-0 790 530 where the water-soluble non-photosensitive layer contains at least one organic polymer with a water solubility of at least 2 g/l at room temperature and at least one inorganic compound reacting as an acid.

However, the coating compositions described in these documents cannot be applied to the substrate with a slot coater but only with coaters which suffer from the disadvantage that there is a direct contact between coater and substrate so that the substrate may be damaged mechanically during coating.

In DE-C-42 01 660 granulates useful as gumming agent for offset printing plates are disclosed. A solution of said granulates could be applied to the substrate by a slot coater, however, the properties of the obtained coating are insufficient.

It is desirable that a coating composition can be applied by a slot coater since the slot coater technology has some advantages compared with for instance roll coaters. For example, since there is no direct contact between substrate and coater, the substrate cannot be damaged by the slot coater. Furthermore, in slot coater technology a greater web speed can be used compared to roll coaters and the parameters of the coating like coating thickness can be adjusted more easily with a slot coater.

Therefore, there is a need for a non-photosensitive coating composition which results in coatings with excellent properties, i. e. no longitudinal coating strips and no non-uniform coating thickness, and which on the other hand results in a fast roll-up and no toning when used as a coating for dummy plates. Furthermore, it should be possible to apply the coating composition by a slot coater in order to avoid mechanical damage of the substrate during coating.

SUMMARY OF THE INVENTION In one aspect, the invention relates to a non-photosensitive coating composition comprising (a) at least one water-soluble film-forming polymer and (b) at least one high molecular thickening agent which shows thixotropic behavior.

In another aspect, the invention relates to an offset printing plate precursor comprising an overcoat obtainable by applying the coating composition of the invention on the photosensitive layer of the plate precursor.

In still another aspect, the invention relates to a dummy plate for offset printing comprising a coating obtainable by applying the coating composition of the invention on a substrate.

In still another aspect, the invention relates to a process for the production of a printing plate precursor or a dummy plate comprising applying the coating composition of the invention by means of a slot coater.

BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a graph showing the relationship between viscosity and rpm of coating composition 1 in comparison to coating composition A.

DETAILED DESCRIPTION OF THE INVENTION The non-photosensitive coating compositions according to the present invention comprise at least one water-soluble film-forming polymer and at least one high molecular thixotropic thickener.

The water-soluble film-forming polymer can be any polymer which is usually used as water-soluble low viscous binder in the field of overcoats on the photosensitive layer of printing plate precursors, as disclosed for instance in DE 197 32 902 which is incorporated herein by reference, or water-soluble film-forming binders in the field of gums as disclosed for instance in EP-A-0 790 530 which is incorporated herein by reference.

Suitable examples of such polymers are for instance polyvinyl alcohols (PVA), polyamides such as polyvinylpyrrolidone (PVP), low viscous water-soluble cellulose and derivatives thereof, partially saponified polyvinyl acetates, water- soluble dextrines and mixtures thereof. For dummy plates dextrines are especially preferred, while PVA and PVP are most preferred for overcoats. The amount of the water-soluble film-forming polymer in the coating composition is not particularly limited and depends on the molecular weight of the polymer and the coating method

to be used. In most cases, the amount is within the range of about 0.2 to about 20 wt% based on the total coating composition, more preferably about 0.2 to about 15 wt%; for slot coater application it is most preferably from about 0.2 to about 1 wt%, while for application with a roll coater the amount is most preferably from about 5 to about 15 wt%.

The thixotropic thickener can be any highly viscous polymer which shows thixotropy. Examples are for instance xanthan gum, high viscous cellulose and cellulose derivatives like hydroxyethyl cellulose, carboxymethyl cellulose and methyl cellulose, high viscous sodium alginate, guar gum, locust bean gum, gum karaya, gum tragacanth, high viscous starch, carrageenan, casein, hectorit, high viscous polyvinylpyrrolidone, high viscous polyvinyl alcohol and mixtures thereof. Preferred are such thickeners which show thixotropic as well as pseudoplastic behavior.

Preferred thickeners are xanthan gum and high viscous hydroxy ethyl cellulose; xanthan gum is the most preferred one. The amount of the thickener in the coating composition is not particularly limited, however, the amount of the thickener is preferably such that the viscosity of the coating composition at medium shear stress is within the range of about 2.5 to about 60 means, preferably about 5 to about 50 mPa-s when measured according to Brookfield, DV-II-LV at 20°C with a UL adapter; in most cases the amount of the thickener is from about 0.02 to about 0.2 wt% based on the total composition.

According to the present invention it is possible to use a certain kind of polymer (for instance PVA) with high molecular weight as thickener and with low molecular weight as binder. For instance PVAs having a molecular weight of about 15000 to 30000 and a viscosity (measured in a 4 % solution at 20°C according to Hopper) of about 3 to about 8 mua-s are useful as binder; PVAs having a molecular weight of about 70000 to 100000 and a viscosity (measured in a 4 % solution at 20°C according to Hopper) of about 30 to about 55 mua-s are useful as thickeners.

The coating solutions contain water (preferably demineralized water), an organic solvent miscible with water (for example lower alcohols and glycols) or mixtures thereof as the solvent. Preferred solvents are water and a mixture of water and a lower alcohol (for example isopropanol). The amount of the solvent is not

limited, however. Suitably the amount of the solvent is about 80 to about 99.6 wt% based on the total composition, more preferably about 94 to about 99.5 wt%.

The coating composition may further contain at least one of preservatives, surfactants, dyes, biocides, sequestering agents, antifoaming agents and corrosion inhibitors which are usually used in gum solutions. Suitable surfactants are anionic surfactants such as sodium dodecyl sulphate, sodium dodecyl sulphonate, alkylamino carboxylates and dicarboxylates, cationic surfactants such as tetraalkyl ammonium salts, and non-ionic surfactants such as polyethylene glycols. Examples of suitable preservatives are for instance p-hydroxybenzoic acid ester and 1,2- benzisothiazolin-3-on. Useful sequestering agents are for instance polyphosphates, Trilon (available from BASF, Germany) and Sequion from Polygon. Sodium nitrate, ammonium nitrate and ammonium carbamate are suitable corrosion inhibitors.

Suitable antifoaming agents are for instance Agitan'290 (available from Münzing, Germany), Silikon-Antischaum-Emulsion SE57 (available from Wacker Chemie, Germany) and Antimussol WLN (available from Sandoz, Switzerland).

The amount of the above listed additives is not particularly limited as long as they do not deteriorate the effect of the present invention obtained by the mixture of the film-forming polymer and the thickener. The amount of the preservatives is about 0 to about 10 wt% based on the total amount of binder, preferably about 0.1 to 2 wt%, that of the surfactants is about 0 to 10 wt%, preferably about 0.05 to 0.5 wt%, that of the sequestering agents is about 0 to 5 wt%, preferably about 0.005 to 0.1 wt%, that of the antifoaming agents is about 0 to 1 wt%, preferably about 0.001 to 0.05 wt%, and that of the corrosion inhibitors is about 0 to 5 wt%, preferably about 0.1 to 1 wt% and the dyes are present in an amount of about 0 to 1 wt%, preferably about 0.005 to 0.05 wt%.

The coating compositions of the present invention are especially useful for the preparation of coated dummy plates, however, they can also be used for applying an overcoat on top of the photosensitive layer of a printing plate precursor as O2-barrier layer if the film-forming polymer is suitably chosen, i. e. is for instance polyvinyl alcohol.

Dummy plates according to the present invention comprise a substrate conventionally used for printing plates. Preferably the substrate is selected from the group consisting of mechanically and/or electrochemically grained aluminum foil or plate, grained aluminum which was subjected to an anodizing treatment and plastic foils which may optionally be laminated to an aluminum foil. Most preferably the substrate for the dummy plates is the same as the substrate of the printing plate used in a certain printing process so that both plates show the same ink-water balance.

The coating composition of the present invention is applied to said substrate by usual means known to those of ordinary skill in the art. The dry coating weight is preferably about 0.10 to about 0.25 g/m2, more preferably about 0.10 to about 0.15 glm2. Although any conventional coater like roll coater, bar coater, wire wound rod coater and air knife coater can be used, it is preferred to use a slot coater for applying the coating composition to the substrate. When a slot coater is used, the solid content of the coating composition to be applied is preferably about 0.2 to about 2 wt% of the total composition, more preferably about 0.3 to about 1. 0 wt%. For coaters using squeeguee rollers it is preferred that the solid content of the coating composition be about 5 to about 20 wt% based on the total composition, more preferably about 8 to about 12 wt%.

When applying the coating composition of the present invention to the substrate by means of a slot coater, the gap between the coating head and the substrate to be coated is held constant and is preferably about 100 to about 250 um. The substrate speed is preferably between about 20 and 80 m/min, more preferably about 30 to about 60 mlmin. The coating weight can be varied by varying the gap and the speed.

In the same way as described above for the preparation of coated dummy plates, the coating composition of the invention can be applied to the photosensitive layer of a printing plate precursor.

The dummy plates produced according to the procedure described above neither show longitudinal coating stripes or wind pattern nor an inconsistent coating thickness which appears with dummy plates produced according to previous methods with coating solutions having low thixotropy. Without being bound to any

theory, the inventor believes that the coating composition can be applied evenly due to the thixotropic properties of the used thickener since the viscosity of the composition is reduced during coating by the shear stress. Due to the thixotropic behavior, the obtained dummy plates show excellent"coating cosmetics"without any defects such as bubbles and mottles. Apart from these advantages, the dummy plates according to the present invention can be economically produced by an automized production line, they are not sensitive to fingerprints and dust, do not tend to stick, are stable over a long storage time, show excellent hydrophilic properties and are therefore not sensitive to toning. After restart of the press, the dummy plates according to the invention show immediate roll-up.

All references cited herein are incorporated by reference herein in their entirety.

The following examples describe the invention in more detail without limiting the invention.

EXAMPLE 1 1. Coating Compositions Gum solution (stock solution): Granules according to Example 1 of DE-C-42 01 660 were prepared by the use of the following components: 529.0 g dextrine (potato starch) 105.8 g sorbitol 2.5 g Marlophen 1028 N (a surfactant available from Huls AG, Germany) 0.6 g Trilon'B (tetrasodium salt of EDTA ; available from BASF, Germany) 19.8 g urea 0.1 g Agitan'290 (antifoaming agent available from Munzing Chemie, Germany) 7.9 g Parmetolt B70 (preservative available from Schule & Mayer GmbH, Germany)

0.7 g NaOH The obtained granules were dissolved in demineralized water to obtain a 20 % solution thereof.

Coating composition 1: A coating composition was prepared by mixing 94.9 wt% demineralized water, 5.0 wt% gum solution obtained as described above and 0.1 wt% Kelzan D (a xanthan gum, available from Langer & Co., RitterhudelGermany) under stirring. The viscosity of the composition at medium shear rate was determined to be 49.4 mPa-s (at 20°C) by using the Brookfield method (DV-II-LV with UL adapter).

The viscosity of the composition versus rpm is shown in Fig. 1.

Coating composition 2: A coating composition was prepared by mixing 94.87 wt% demineralized water, 5.00 wt% gum solution obtained as described above, 0.1 wt% Kelzan D and 0.03 wt% 1, 2-benzisothiazolin-3-on under stirring. The viscosity was determined to be 49.5 mua-s (at 20°C).

Coating composition 3: A coating composition was prepared by mixing 99.3 wt% demineralized water, 0.6 wt% Emdext 30 AN45 (dextrine available from Emsland-Starke GmbH, Emlichheim/Germany) and 0.1 wt% Kelzan D under stirring. The viscosity was determined to be 48 mua-s (at 20°C).

Coating composition 4: A coating composition was prepared by mixing 95.9 wt% demineralized water, 4.0 wt% gum solution obtained as described above and 0.1 wt% Rhodopolt 23 (xanthan gum available from Rhone-Poulenc Industries, Paris/France) under stirring.

Coating composition 5: This composition was prepared by mixing the following components: 84.9 wt% demineralized water 10.0 wt% isopropyl alcohol 5.0 wt% gum solution obtained as described above 0.1 wt% Kelzan'D Comparative coating compositions A and B : Coating composition A differed from composition 1 in that no thickener was present (95.0 wt% water + 5. 0 wt% gum solution as described above).

Viscosity : 2 mua-s (at 20°C).

Coating composition B differed from composition 2 in that no thickener was present (94.97 % water + 5.0 % gum solution as described above + 0.03 % 1, 2-benzisothiazolin-3-on). Viscosity: 2 mPa s (at 20°C).

2. Preparation of Dummy Plates A lithographic aluminum substrate (electrochemically grained and anodized aluminum substrate) was coated with any of the coating compositions above by means of a commercially available slot coater at a web speed of 32 m/min and a gap of 150 u. m. The obtained dry coating weight was 0.15 g/m2.

The coating properties and the behavior on press are summarized in the following Table 1.

Table 1 Coating Coating application Behavior on press composition I Very good, faultless coating Excellent 2 Very good, faultless coating Excellent A Poor coating, stripes, bubbles, mottles Defective areas pick up ink B Poor coating, stripes, bubbles, mottles Defective areas pick up ink 3 Very good, faultless coating Excellent 4 Very good, faultless coating Excellent 5 Very good, faultless coating Excellent

As is apparent from Table 1, the coating compositions according to the present invention showed excellent coating behavior. The dummy plates obtained with coating compositions 1 to 5 showed immediate roll-up.

The printing characteristics after different storage times are shown in the following Table 2.

Table 2

Storage time C o a t i n g C o a t i n g C o a ting Substrate until printing Composition 1 Composition 2 Composition 4 w i t h o u t coating Printing Fresh plate No toning No toning No toning Toning Characteristics Fast roll-up Fast roll-up Fast roll-up PrintingI week No toning No toning No toning Strong toning Characteristics Fast roll-up Fast roll-up Fast roll-up Printing 1 month No toning No toning No toning Strong toning Characteristics Fast roll-up Fast roll-up Fast roll-up Printing 3 months No toning No toning No toning Strong toning Characteristics Fast roll-up Fast roll-up Fast roll-up

As is apparent from Table 2, dummy plates according to the invention show a fast roll-up and no toning problems.