GB2166255A - Composition for coating lithographic substrate plates - Google Patents

Abstract

An aqueous alkaline solution containing an alkali-soluble silicon compound; an alkali-soluble inorganic compound of an element of Group IIb, IIIb or VIb of the Periodic Table; a water-soluble organic compound of a water-soluble thermosetting polymeric compound or a methylolmelamine; and, dispersed in the solution, a water-insoluble inorganic material. Such a solution can be applied on to a substrate, and hardened, to provide a support for a photosensitive layer in a lithographic presensitised plate.

Description

SPECIFICATION Composition for coating lithographic substrate plates This invention relates to a composition for coating lithographic substrate plates; once applied as a coating, and hardened, a photosensitive resin layer can be applied thereon.

Lithography is a printing method which utilizes the repulsion between water and oil on the upper surface of a plate for printing. A presensitized plate for use in lithography comprises a supporting plate (B) comprising a layer (A) of the type provided by the invention and a substrate plate (C); a photosensitive resin layer is disposed on the surface of the layer (A), and this forms an image.

Aluminium plates have been widely used as the substrate plate (C). For this purpose, aluminium plate has been subjected to various treatments. For instance, in order to strengthen the adhesiveness between the aluminium plate and the photosensitive resin layer, one of the surfaces of the aluminium plate is chemically or mechanically ground, thereby forming microscopic unevenness thereon, and the treated surface is subjected to anodic oxidation, to harden the surface, and then treatment with, say, an aqueous silicate solution, to improve the desired hydrophilicity of the surface. Such treatments cause problems of stability of the quality of the supporting plate (B), difficulty in maintaining complete quality control, and high costs of installation and maintenance.

Plastics or paper sheets are generally unsatisfactory for use as the substrate plate (C).

According to the present invention, an aqueous alkaline solution contains an alkali-soluble silicon compound; an alkali-soluble inorganic compound of an element of Group llb, Illb or Vlb of the Periodic Table; a water-soluble organic compound of a water-soluble thermosetting polymeric compound or a methylolmelamine; and, dispersed in the solution, a water-insoluble inorganic material.

A solution of the invention (sometimes described herein as "the present composition") can satisfactorily be used to produce a presensitised plate for use in lithography. Only one coating layer of the solution on the substrate is necessary, before hardening and application of a photosensitive resin layer, irrespective of the substrate material.

A solution of the invention may be prepared by (a) mixing the silicon compound and the alkali-soluble inorganic compound with an aqueous solution of the organic compound; (b) dispersing the water-insoluble inorganic material in the resultant liquid mixture; (c) vigorously stirring the resultant dispersion; and (d) removing air bubbles from the stirred mixture. One or more components selected from metal-chelating agents, pigments, dyestuffs and viscosity-raising agents may be added to the dispersion.

The silicon compound is, for example, potassium metasilicate, sodium metasilicate, sodium orthosilicate, silicon monoxide, silicon monosulphate, orthosilicic acid or metasilicic acid.

The alkali-soluble inorganic compound is, for example, a compound of zinc such as zinc hydroxide, zinc chloride, zinc nitrate, zinc phosphate, etc., a compound of cadmium such as cadmium hydroxide, cadmium chloride, etc., a compound of boron such as borax, etc., a compound of aluminum such as aluminum hydroxide, aluminum nitrate, aluminum phosphate, etc., a compound of galium such as galium hydroxide, galium sulfide, etc., a compound of chromium such as chromium hydroxide, potassium chromate, potassium bichromate, etc. and a compound of molybdenum such as molybdenum oxide, etc.

may be exemplified. Every one of the alkali-soluble inorganic compounds acts on the silicon compound when alkali-soluble inorganic compounds are heated, thereby forming a water-insoluble and hydrophilic membrane easier.

As the water-soluble organic compound according to the present invention, (1) a water-soluble, thermosetting polymeric compound, which is initially soluble in water and becomes water-insoluble after being treated at high temperatures, such as water-soluble melamine oligomer, and water-soluble polybutadiene modified by maleic anhydride, and (2) methylolmelamines may be mentioned. In addition to those mentioned above, as the water-soluble, thermosetting polymeric compound, water-soluble vinyl polymer, water soluble polyester, water-soluble polyamide, etc. may be exemplified. For instance, a water-soluble polyester resin which obtained by modifying a polyester with an acrylic compound (made by GO-O Chem. Ind. Co., Ltd. under the registered trade name of PLASCOAT), a nylon modified by Nmethoxymethylation, polystyrene modified by maleic anhydride, saponified polyvinyl acetate may be used.

Further, as methylolmelamines, di- and tri-methylolmelamine, and a mixture thereof are used preferably.

As the water-insoluble inorganic powdery material according to the present invention, powdery material which is hydrophilic and insoluble or hardly soluble in water or an aqueous alkaline solution may be mentioned. For instance, oxides of metals such as zinc oxide, aluminum oxide, antimony oxide, calcium oxide, chromium oxide, tin oxide, titanium oxide, iron oxide, copper oxide, lead oxide, bismuth oxide, magnesium oxide, manganese oxide, etc., salts such as calcium carbonate, calcium sulfate, etc., silicon compounds such as colloidal silica, natural pigments such as kaolin, bentonite, clay, etc. and powdery metals such as iron powder, zinc powder, aluminum powder, etc. may be used. Of the above-mentioned water-insoluble inorganic powdery materials, colloidal silica is particularly suitable for the purpose of the present invention.

The production of the present composition is carried out, for instance, by the following process.

In the first place, an alkali-soluble silicon compound and an alkali-soluble inorganic compound are dissolved in an aqueous solution of alkali such as sodium hydroxide and potassium hydroxide, thereby preparing an aqueous solution (hereinafter referred to a "A liquid"). Also, the alkali-soluble inorganic compound may be dissolved in an aqueous solution of water glass without adding an alkali to prepare the A liquid because water glass is an aqueous alkaline solution of silicon.

Separately, a water-soluble organic compound of a water-soluble, thermosetting polymer compound or methylolmelamine is dissolved in water or an aqueous alkaline solution, and optionally, a hardening catalyst, for instance, guanadin sulfamate and the like is added to the thus prepared solution, thereby preparing an aqueous solution (hereinafter referred to as B liquid).

Then, A liquid is mixed with B liquid, and a water-insoluble inorganic powdery material is added to the mixture.

After adding optionally a metal-chelating agent, pigment(s), dyestuff(s), a viscosity-raising agent, etc., to the thus obtained dispersion, the whole dispersion is vigorously stirred and then the thus formed air bubbles are removed from the whole dispersion, thereby obtaining the present composition. The sum of the amount of the water-soluble silicon compound and that of the alkali-soluble inorganic compound is 1 to 40 parts by weight, preferably 3 to 20 parts by weight to 100 parts by weight of the water-soluble organic compound of the water-soluble, thermosetting polymeric compound or methylolmelamines, and the amount of the water-insoluble inorganic powdery material is 5 to 80 parts by weight, preferably 20 to 50 parts by weight to 100 parts by weight of the water-soluble organic compound of the water-soluble, thermosetting polymeric compound or methylolmelamines.In the case where the sum of the amount of the water-soluble silicon compound and that of the alkali-soluble inorganic compound is below one part by weight, the hydrophilicity of the surface of the layer (A) in which the picture-image is not formed becomes insufficient and as a result, there is a tendency that the non picture-image parts hold printing ink to cause the so-called ground blur on the printed surface. On the other hand, in the case where the sum is over 40 parts by weight, the hydrophilicity of the surface of the layer (A) becomes excessive and, as a result, the adhesion of the photosensitive resin layer in which picture-image parts are to be formed to the layer (A) becomes poor and accordingly, the photosensitive resin in which the picture-image parts have been formed exfoliates.

In the case where the amount of the water-soluble inorganic powdery material is below 5 parts by weight, the state of the unevenness of the surface of the layer (A) is apt to become scattered resulting in the poorness of the reproducibility of the net points on the picture-image parts. On the other hand, in the case where that amount is over 80 parts by weight, the permeated amount of the solution of the photosensitive resin into the layer (A) is increased resulting in the increase of the amount of the solution of photosensitive resin used and accordingly, the case becomes non economic.

It is preferable to use the water-insolubie inorganic powdery material passing through the sieve of Tylon mesh of 325 (diameter of powder: less than 43 ELm), however, it is more preferable to use the finer one.

As the substrative plate (C) of the supporting plate (B), to which the present composition is applied on the surface thereof, every material which is inexpensive and fulfills the performances required for the plate for lithography may be used and for instance, foils or plates of a metal such as aluminium, iron, copper, tin, zinc, lead etc., or alloys thereof, plastic films or sheets of polyester, polypropyrene, poiyimide, polyacrylonitrile, polycarbonate, polyamide, polyvinyl chloride, polyvinylidene chloride, polystyrene, polyethylene, etc., synthetic papers, art papers, coated papers, cardboards, thin papers, etc. may be exemplified. On the metals, aluminum, zinc, iron etc. are suitable, and of the plastic materials, those of relatively high in dimensional stability such as polyester, polyimide and polycarbonate are suitable.

Of the paper materials, synthetic paper, art paper, coated paper, cardboard, etc. are suitable.

In the case of applying the present composition on the surface of a metal plate for substrative plate (C), a metal-chelating agent such as 5-sulfosalicylic acid, ethylenediamine-tetraacetic acid, trans-cyclohexane1,2-diaminetetraacetic acid etc. may be added to the present composition for the purpose of reinforcing the adhesion of the layer (A) formed from the present composition to the substrative plate (C).

It is not necessary to carry out a specific treatment of the substrative plate (C) before the application of the present composition thereonto as far as the surface thereof is not polluted by oily substance.

However, the physical treatment of the surface of the substrative plate (C) such as wet- or dry honing, ball-grinding and brush-grinding and/or the chemical treatment by acid(s) and alkali(s) may be effected in order to broaden the specific surface area thereof.

The printing plate for lithography can be prepared by using the present composition as follows.

For instance, the present composition is applied on the surface of the substrative plate (C) by a bar coater, etc., and the thus treated plate is subjected to thermal treatment for 0.5 to 6 min at a temperature of 120 to 250so, thereby hardening the thus applied present composition on the plate (thickness: 2 to 5 Ism). Thus a supporting plate (B) for use in lithography is formed.

After applying a solution of a photosensitive resin on the surface of the thus formed layer (A) of the supporting plate (B) according to a conventional manner, the thus treated supporting plate is dried, thereby obtaining a presensitized plate (hereinafter referred to as "PS plate") having a layer of the photosensitive resin thereon.

By exposing the thus prepared PS plate to lights and developing the thus exposed PS plate, a printing plate for use in lithography is available.

The alkali-soluble inorganic compound, the alkali-soluble silicon compound and the water-soluble organic compound of the water-soluble thermosetting polymeric compound or methylolmelamines contained in the present composition form a membrane which firmly adheres to the substrative plate (C) in the step of thermal treatment, although the details of the membrane-formation has not been elucidated.

However, it is considered that particularly, the alkali-soluble inorganic compound is hardened together with the water-soluble thermosetting polymeric compound during the step of thermal treatment and as a result, a substance having the still remaining hydrophilic groups is fixed to the surface of the thus hardened membrane, thereby exhibiting the function of retaining the hydrophilicity required for the layer (A) of the supporting plate (B) for a long time period.

On the other hand, it is considered that the water-insoluble inorganic powder material forms a suitable unevenness on the surface of the layer (A) of the supporting plate (B), as the result, the layer (A) adheres firmly to the layer of the photosensitive resin which is to be applied thereon by the physical adhesive function of the thus formed unevenness.

As has been described as above, the present invention provide a composition which can fulfill the performance required to the supporting plate (B) for use in lithography by only one step of applying the composition on the surface of the substrative plate (C), thereby unnecessitating the surface-treatment steps such as surface grinding, anodic oxidation, hydrophillizing, etc. which have been hitherto carried out independently to each other for fulfilling the performance required for the supporting plate (B).

The present invention will be explained more in detail while referring the following non-limitative examples and the following comparative examples. The present invention may include various variations of the above-mentioned description and also include the following examples within the scope of the present invention.

Examples 1 to 7 A composition for forming the layer (A) of the supporting plate (B) for use in lithography was produced by (1) dissolving 25 g of a methylolmelamine (made by Nippon Carbide Ind. Co., Ltd., under the registered trade name of NICARESIN) into 30 g of water and further adding 1 g of guanadine sulfamate as a catalyst, thereby obtaining an aqueous solution, (2) adding to the thus obtained aqueous solution, an amount (shown in Table 1) of water glass (Water glass No. 3 of Japanese Industrial Standards, containing 28 to 30% by weight of SiO2 and 9 to 10% by weight of Na2O) saturated with zinc hydroxide, thereby obtaining an aqueous mixture, (3) dispersing an amount (shown in Table 1) of colloidal silica (made by Nippon Aerosil Ind.Co., Ltd. under the index of OX-50), in the thus obtained aqueous mixture while stirring for 2 hours, and (4) removing air bubbles formed in the thus obtained dispersion.

TABLE 1 Amount of Amount of Amount of Example methylolmelamine water glass* colloidal silica (part by wt.) (part by wt.) (part by wt.) 1 100 5 20 2 100 10 20 3 100 20 20 4 100 30 20 5 100 5 10 6 100 5 30 7 100 5 40 Note: * saturated with zinc hydroxide, containing 7% by weight of zinc hydroxide.

Examples 8 to 13 A composition for forming the layer (A) of the supporting plate (B) for use in lithography was produced by (1) preparing a mixture of an alkali-soluble silicon compound, an alkali-soluble inorganic compound and an alkali shown in Table 2, (2) admixing the thus prepared mixture with the methylolmelamine of Example 1 at the weight ratio of the methylolmelamine to the mixture of 100:5, (3) further admixing colloidal silica (refer to Example 1) and titanium oxide (made by Teikoku Kako Co., Ltd., and obtained as MT-150W, in an amount of 10 parts by weight per 100 parts by weight of the methylolmelamine, respectively, and (4) adding water to the thus-obtained mixture, thereby obtaining a composition having a concentration of ingredients of 50% by weight.

Examples 14 to 18: The procedure of Example 1 was repeated in each case, but using the components given in Table 3.

The amount of each component is given in parts by weight. The water-glass used in Example 14 at least is Water glass No. 3 (JIS). The modified polymer used in Example 16 is a water-soluble polybutadiene modified by maleic anhydride (BN-3010, Nippon Soda Co., Ltd.). The water-soluble polyester used in Example 17 is Plascoat Z-802 (GO-0 Chem. Ind.Co., Ltd.). TABLE 2 Component Example Weight alkali-soluble alkali-soluble ratio of silicon compound inorganic compound alkali components 8 Sodium metasilicate Aluminum hydroxide Sodium hydroxide 1:1:1 9 Potassium metasilicate Borax Potassium hydroxide 1:0.3:1 10 Water glass* Zinc hydroxide Sodium hydroxide 1:1:1 11 Water glass* Chromium hydroxide Sodium hydroxide 1:0.5:1 12 Water glass* Sodium metasilicate Sodium hydroxide 1:10.2:1 Molybdenum sulfide 13 Sodium orthosilicate Cadmium chioride Sodium hydroxide 1:0.3:2 Note: * Water glass No. 3 (Japanese Industrial Standards) TABLE 3 Example Component 14 15 16 17 18 alkali-soluble Water Water glass Water glass sodium water glass silicon glass metasilicate compound 1.0 1.0 3 5 2 alkali-soluble Zinc Aluminum Cadmium Aluminum Zinc innorganic hydroxide hydroxide chloride hydroxide hydroxide compound 0.2 1.0 1 1 1 Water-soluble methylol- methylol- modified polyester methylol organic melamine melamine polymer melamine compound 25.0 25.0 30 20 15 Catalyst Guanadine Guanadine Guanadine sulfamate sulfamate - - sulfamate 1.0 1.0 0.7 Inorganic Clay 1.0 Zinc dust Kaolin 8 Calcium Kaolin 1 powdery 5.0 sulfate 5 CaCO3 2 material Colloidal Colloidal Colloidal Colloidal Awl203 1 silica 1.0 silica 1.0 silica 0.5 silica 0.5 Colloidal silica 1 Alkali - - - Potassium Sodium hydroxide hydroxide 4 4 Water 30.0 35.0 40 50 40 Test examples 1 to 13: Tests on the printing plates prepared by using the present composition Supporting plates for use in lithography were prepared by applying each one of the present compositions produced in Examples 1 to 13 on a surface of each one of the de-fatted aluminum thin plates (Japanese Industrial Standards A-1100) of 200 m/m in length, 300 m/m in width and 0.15 m/m in thickness while using a #6 bar-coater and the thus coated composition was hardened by heating for 3 min in a hot wind drier at a temperature of 1800C and, after applying a solution of a negative type photosensitve resin on the thus coated surface of each of the thus obtained supporting plate, the thus coated plate were dried in a hot wind drier at 1200C for one min to obtain the presensitized plate (PS plate).

After placing a negative film (test pattern of 175 lines with the area of net-points of from 3 to 97%) on the surface of the photosensitive resin layer of each one of the PS plates and setting the thus treated PS plate in a vacuum printing frame in a usual manner, the PS plate was exposed to ultraviolet rays from a hyperpressure mercury lamp (made by OOKU Works, 4kW) at a distance of 1 m for 30 sec. By developing the thus exposed PS plate while using a liquid developer, printing plates for lithography were obtained.

After washing the printing plates with water and draining off water thereon by using a squeezer, inking was carried on the printing plate by pouring a protect ink on the thus treated surface thereof while using a sponge. Thereafter, the degree of the adhesion of the ink to the picture-image parts was observed.

After washing the printing plates again with water, draining off water thereon by using the squeezer and drying the printing plates, the printing plates were subjected to the following tests.

Adhesion of the picture-image parts to the supporting plate was judged on the basis of the degree of exfoliation of the net points after 1000 times of abrasion while using a Taber rotary abrader (truck wheel: CS-17 type, load of 1000 g).

Reproducibility of the image was judged by the reproducibility of the test pattern (175 lines, the area of net points of 3 to 97%).

Sensitivity was judged by the number of steps of the step tablet (made by Codak Co., No. 2 provided with 21 steps.) Hydrophilicity of the non-picture image parts was judged by (1) the state of spoiling by the protect ink after the inking thereof and (2) the yes or no of coming off of the offset ink by pouring water after applying the offset ink directly on the non-picture image parts.

The results of the tests are shown in Table 4.

Comparative test Example 1*: By applying a solution of the same negative-type photosensitive resin as in Test Example 1 on a surface of the same thin aluminum plate as in Test Example 1, a printing plate for use in lithography was obtained, and the specific properties thereof was observed in the same tests as in Test Example 1, the results being also shown in Table 4.

Comparative test example 2*: A printing plate for use in lithography was prepared in the same manner as in Test Example 1 while using a composition obtained by the same method as in Example 1 except for using 5 parts by weight of water glass No. 3 and 20 parts by weight of colloidal silica to 100 parts by weight of metylolmelamine (NICARESIN). The specific properties thereof were measured in the same manner as in Test Example 1, the results being also shown in Table 4.

Comparative test example 3.':': A printing plate for use in lithography was prepared in the same manner as in Test Example 1 while using a composition obtained by the same method as in Example 1 except for using 20 parts by weight of colloidal silica to 100 parts by weight of methylolmelamine (NICARESIN). The specific properties thereof were measured in the same manner as in Test Example 1, the results being also shown in Table 4.

In Table 4, the evaluations are graded as A (excellent), B (good) and C (poor).

TABLE 4 Test Adhesion Adhesion Reproduci- Hydrophili of ink to of city of picture- picture- bility of Sensi- non-picture Example image image image parts parts parts image tivity 1 A A A A A 2 A A A A A 3 A A A A A 4 A A A A A 5 A A A A A 6 A A A A A 7 A A A A A 8 A A A A A 9 A A A A A 10 A A A A A 11 A A A A A 12 A A A A A 13 A A A A A Printing test example 1: A supporting plate (B) for use in lithography was prepared by continuously applying the composition of Example 2 at a velocity of 30 m/min onto a de-fatted surface of aluminum foil of 500 m/m in width and 0.15 m/m in thickness while using a reverse coater and hardening the thus coated composition at 230"C.

A presensitized plate (PS plate) was prepared by applying a solution of negative-type photosensitive resin onto the surface of the thus hardened layer in a usual manner and drying thereof.

After cutting off a piece from the thus obtained PS plate, a negative image film was placed on the photosensitive resin layer of the piece, and the PS plate was exposed by ultraviolet light from a hyper pressure mercury lamp of 4 kW at a distance of 1 m for 40 sec. After subjecting the thus exposed PS plate to development by a developing liquid (Nippaku PS developer NN-221) and washing the developed PS plate with water, the washed PS plate was dried to obtain a printing plate for use in lithography.

Printing was carried out on sheets of high quality paper by using the thus obtained printing plate and a printing machine (Heidel KORD). More than 50,000 sheets could be printed without any trouble.

Printing test example 2: A PS plate was prepared-by applying a solution of positive-type photosensitive resin on the surface of the hardened layer of the supporting plate (B) prepared in the same manner as in Printing Test Example 1, and drying the thus coated supporting plate (B). After placing a positive image film on the PS plate, the thus treated PS plate was exposed by ultraviolet light from a hyper pressure mercury lamp of 4kW from the distance of 1 m for 50 sec. After subjecting the thus exposed PS plate to development by a positive-type developer (made by OHKA Ind. Co., Ltd., under the index of PL-101) and washing the thus developed PS plate with water, the washed PS plate was dried to obtain a printing plate for use in lithography.

Printing was carried out by using the thus prepared printing plate in the same manner as in Printing Test Example 1. More than 50,000 sheets could be printed without any trouble.

Printing test examples 3 and 4: By using each of the two compositions produced respectively in Examples 9 and 13, two printing plates were prepared in the same manner as in Printing Test Example 1. By using each of the thus prepared printing plates, printing was carried out.

In both the cases, more than 50,000 sheets could be printed without any trouble.

Printing test examples 5 and 6: Two supporting plates were prepared by applying the composition produced in Example 14 on the two de-fatted aluminum plates and hardening the thus coated composition by heating for 2 min at 200"C.

A negative type printing plate and a positive-type printing plate were prepared by using each one of the thus prepared supporting plates, in the same manners as in printing Test Examples 1 and 2, respectively.

Printing was carried out by using each of the thus prepared printing plates. In both the cases, more than 50,000 sheets could be printed without any trouble.

Printing test example 7: A printing plate for use in lithography was prepared by (1) sticking a de-fatted aluminum foil of 25 micrometers in thickness with a sheet of synthetic paper of 0.2 mm in thickness, (2) applying the compo sition obtained in Example 15 on the surface of the aluminum foil and hardening the thus coated compo sition by heating for 3 min at 170"C, thereby obtaining a supporting plate for use in lithography and (3) treating the supporting plate in the same manner as in Printing Test Example 1.

As a result of subjecting the thus prepared printing plate to actual printing, more than 50,000 sheets of paper could be printed without any trouble.

Printing test example 8: A supporting plate was prepared by applying the composition obtained in Example 16 on the surface of a polyester film of 100 um in thickness, the surface having been blasted preliminarily by air-honing, and hardening the thus coated composition by heating for 3 min at 1600C.

A printing plate was prepared by using the thus obtained supporting plate in the same manner as in Printing test Example 1, and the thus obtained printing plate was used for printing.

As a result, more than 50,000 sheets of paper could be printed with the thus prepared printing plate without any trouble.

Printing test example 9: A supporting plate was prepared by applying the composition obtained in Example 17 on the surface of an iron foil of 50 um in thickness, the surface having been subjected to de-fatting treatment, and hard ening the thus coated composition by heating for 12 min at 200"C.

A printing plate was prepared by using the thus obtained supporting plate in the same manner as in Printing test Example 1, and the thus prepared printing plate was used for printing.

As a result, more than 50,000 sheets of paper could be printed without any trouble.

Printing test example 10: A supporting plate for use in lithography was obtained by applying the composition obtained in Exam ple 18 on the surface of a substrative plate prepared by sticking polycarbonate film of 25 m in thickness onto onto the both surfaces of a defatted iron foil of 50 ym in thickness, and hardening the thus coated com- position by heating for 3 min at 170"C.

A printing plate for use in lithography was prepared by using the thus obtained supporting plate in the same manner as in Printing Test Example 1, and subject to printing.

As a result, more than 50,000 sheets of paper could be printed without any trouble.

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Claims (13)

1. An aqueous alkaline solution containing an alkali-soluble silicon compound; an alkali-soluble inor ganic compound of an element of Group llb, Illb or Vlb of the Periodic Table; a water-soluble organic compound of a water-soluble thermosetting polymeric compound or a methylolmelamine; and, dis-.

persed in the solution, a water-insoluble inorganic material.

2. A solution according to claim 1, wherein the silicon compound is selected from potassium metasili cate, sodium metasilicate, sodium orthosilicate, silicon monoxide, silicon monosulphide, orthosilicic acid and metasilicic acid.

3. A solution according to claim 1 or claim 2, wherein the element is zinc, cadmium, boron, alumin ium, gallium, chromium or molybdenum.

4. A solution according to claim 3, wherein the alkali-soluble inorganic compound is selected from zinc hydroxide, zinc chloride, zinc nitrate, zinc phosphate, cadmium hydroxide, cadmium chloride, borax, aluminium hydroxide, aluminium nitrate, aluminium phosphate, gallium hydroxide, gallium sulphide, chromium hydroxide, potassium chromate, potassium dichromate and molybdenum oxide.

5. A solution according to any preceding claim, wherein the thermosetting polymeric compound is selected from melamine oligomer, polybutadiene modified by maleic anhydride, polystyrene, vinyl polymers, polyesters and polyamides.

6. A solution according to any preceding claim, wherein the methylolmelamine is dimethylolmelamine or trimethylolmelamine.

7. A solution according to any preceding claim, wherein the water-insoluble inorganic material is selected from zinc oxide, aluminium oxide, antimony oxide, calcium oxide, chromium oxide, tin oxide, titanium oxide, iron oxide, manganese oxide, calcium carbonate, calcium sulphate, colloidal silica, kaolin, bentonite, clay, aluminium, iron and zinc.

8. A solution according to any preceding claim, which comprises 100 parts by weight of the organic compound and 1 to 40 parts by weight of, in combination, the silicon compound and the alkali-soluble inorganic compound.

9. A solution according to any preceding claim, which comprises 100 parts by weight of the organic compound and 5 to 80 parts by weight of the water-insoluble inorganic material.

10. A solution according to claim 1, substantially as described in any of the Examples.

11. A process for producing an aqueous alkaline solution according to any preceding claim, which comprises (a) mixing the silicon compound and the alkali-soluble inorganic compound with an aqueous solution of the organic compound; (b) dispersing the water-insoluble inorganic material in the resultant liquid mixture; (c) vigorously stirring the resultant dispersion; and (d) removing air bubbles from the stirred mixture.

12. A process according to claim 11, which additionally comprises adding to the dispersion one or more components selected from metal-chelating agents, pigments, dyestuffs and viscosity-raising agents.

13. A method for producing a presensitized plate for use in lithography, which comprises applying a coating of a solution according to any of claims 1 to 10 on to the surface of a substrate plate, hardening the coating, and applying a photosensitve resin on to the surface of the hardened coating.