WO2005037569A1

WO2005037569A1 - Process and composition for gumming lithographic printing plates

Info

Publication number: WO2005037569A1
Application number: PCT/EP2004/010898
Authority: WO
Inventors: Ulrich Fiebag; Tobias Wittig; Tanja Ebhardt
Original assignee: Kodak Polychrome Graphics Gmbh
Priority date: 2003-09-30
Filing date: 2004-09-29
Publication date: 2005-04-28
Also published as: DE10345388A1

Abstract

The process for gumming lithographic printing plates comprises (a) providing a lithographic printing plate; (b) providing a gumming solution comprising (I) water and (II) at least one component selected from water-soluble starch and water-soluble starch derivatives; (c) applying the gumming solution onto the printing plate provided in step (a), and is characterized in that the gumming solution has a pH value >7.

Description

Process and composition for gumming lithographic printing plates

The present invention relates to a process for gumming lithographic printing plates, in particular a process using a gumming composition with a pH value higher than 7. Furthermore, the invention relates to alkaline gumming compositions for gumming lithographic printing plates.

The technical field of lithographic printing is based on the immiscibility of oil and water, wherein the oily material or the printing ink is preferably accepted by the image area, and the water or fountain solution is preferably accepted by the non-image area. When an appropriately produced surface is moistened with water and a printing ink is applied, the background or non-image area accepts the water and repels the printing ink, while the image area accepts the printing ink and repels the water. The printing ink in the image area is then transferred to the surface of a material such as paper, fabric and the like, on which the image is to be formed. Generally, however, the printing ink is first transferred to an intermediate material, referred to as blanket, which then in turn transfers the printing ink onto the surface of the material on which the image is to be formed; this technique is referred to as offset lithography.

A frequently used type of lithographic printing plate precursor comprises a photosensitive coating applied onto a substrate on aluminum basis. The coating can react to radiation such that the exposed portion becomes so soluble that it is removed during the developing process. Such a plate is referred to as positive working. On the other hand, a plate is referred to as negative working if the exposed portion of the coating is hardened by the radiation. In both cases, the remaining image area accepts printing ink, i.e. is oleophilic, and the non-image area (background) accepts water, i.e. is hydrophilic. The differentiation between image and non- image areas takes place during exposure, for which a film is attached to the printing plate precursor under vacuum in order to guarantee good contact. The plate is then exposed by means of a radiation source part of which is comprised of UV radiation. When a positive plate is used, the area on the film corresponding to the image on the plate is so opaque that the light does not reach the plate, while the area on the film corresponding to the non-image area is clear and allows light to permeate the coating, whose solubility increases. In the case of a negative plate, the opposite takes place: The area on the film corresponding to the image on the plate is clear, while the non-image area is opaque. The coating beneath the clear film area is hardened due to the incident light, while the area not affected by the light is removed during developing. The light-hardened surface of a negative working plate is therefore oleophilic and accepts printing ink, while the non-image area that used to be coated with the coating removed by the developer is desensitized and therefore hydrophilic.

Alternatively, the plate can also be image-wise exposed digitally without a film. According to recent developments, plate precursors with heat-sensitive layers are used wherein by image-wise direct heating or irradiation with LR radiation that is converted into heat, differences in the developer solubility of the heated and unheated areas of the coating are generated.

For a clean printed image it is necessary that the image areas (i.e. the image-wise remaining coating) accept the printing ink well, while the non-image areas (i.e. the image-wise revealed substrate, such as e.g. an aluminum substrate) should not accept the printing ink. In order to protect the image-wise revealed substrate, such as e.g. an aluminum substrate, against fingerprints, the formation of aluminum oxide, corrosion and mechanical attacks, such as scratches, when, the printing plate is mounted in the printing machine, i.e. to maintain and possible improve the hydrophilic nature of the non-image areas, the developed printing plate is usually subjected to a "gumming" treatment (also referred to as "finishing"). Gumming the plate prior to storing it or before long periods of standstill of the printing machine ensures that the non-image areas remain hydrophilic. When printing is started, the gumming solution has to be able to be washed off the plate quickly with the fountain solution so that the image areas are capable of accepting ink immediately. Gumming solutions have been known for a long time and are often based on gum arabic. DE 29 26 645 Al describes a process for the production of a gumming solution for offset printing plates which comprises gum arabic. According to this process, an acidic solution of gum arabic (pH 2 or less) is heated and the pH value is then adjusted to a value of preferably 4.5 to 4.7 by the addition of alkali hydroxide.

DE 20 42 217 Al discloses the treatment of a planographic printing plate with an aqueous solution whose pH value is between 1 and 5. The solution comprises alkali metal polyphosphate, phosphoric acid or citric acid and a salt selected from sodium or potassium nitrate, perchlorate, permanganate or persulfate.

US 4,880,555 Al describes a finisher for lithographic printing plates comprising maltodextrin prepared by enzymatic hydrolysis, a polyol, hydrocarbons, a mixture of long-chain alcohols and aminated alcohol sulfate, substituted phenoxypoly(oxyethylene) ethanol and an ethanolamine, and having a pH value in the range of 2.5 to 6.5.

US 4,033,919 Al describes an aqueous gumming solution comprising a polymer that contains units derived from acrylamide and 1 to 25 wt.-% units with carboxyl groups. The solution furthermore comprises an acidic material which adjusts the pH value of the solution to a value below 5.5; examples thereof include phosphoric acid, citric acid and tartaric acid.

A finisher in the form of an emulsion is described in US 4,162,920 Al. The emulsion comprises an aqueous phase containing tapioca dextrin, a salt that is non-corrosive to aluminum and water, and a solvent phase containing a hydrocarbon solvent and a surfactant soluble therein. The pH value is given as 3 to 5.

The gumming compositions described in the prior art all have in common that they comprise a water-soluble colloid or binder, such as e.g. gum arabic, polyvinyl alcohol, polyvinyl pyrrolidone, carboxymethyl cellulose, sorbite or polyacrylamide and are adjusted to a pH value <6.

It has now been found that the use of such gumming compositions can lead to ink acceptance and/or roll-up problems in printing plates whose precursors comprise an overcoat layer with components such as adhesion promoters that are capable of forming salts with acids present in the radiation-sensitive layer or formed therein during irradiation. The printing plates either do not accept any ink at all, or only accept ink after a large number of copies. Furthermore, the printing plates show a poor roll-up behavior in the background areas. It is usually necesary to clean the printing plate with special cleaning solutions in order to obtain acceptable prints.

It is the object of the present invention to provide a process for the prevention of a loss of the hydrophilic nature of the non-image areas of lithographic printing plates during storage and long periods of standstill of the printing press, without causing ink acceptance problems and/or poor roll-up behavior during subsequent printing in printing plates whose precursors comprise an overcoat layer with components such as adhesion promoters that are capable of forming salts with acids.

Another object of the present invention is to provide a gumming solution which does not lead to ink acceptance problems and poor roll-up behavior in printing plates whose precursors comprise an overcoat layer with components that are capable of forming salts with acids present in the radiation-sensitive layer or formed therein during irradiation; moreover, the preparation of the gumming solution should be easy and inexpensive.

The first object is achieved by a process comprising

(a) providing a lithographic printing plate,

(b) providing a gumming solution comprising (i) water and (ii) at least one component selected from water-soluble starch and water-soluble starch derivatives,

(c) applying the gumming solution onto the printing plate provided in step (a),

characterized in that the pH value of the gumming solution is higher than 7.

The process according to the present invention is characterized in that a lithographic printing plate is treated with an alkaline aqueous solution of soluble starch or a soluble starch derivative.

The term "printing plate precursor" as used in the present invention relates to an unimaged plate (i.e. that has not been image-wise exposed and developed) from which a printing plate is produced by image-wise exposure and optionally developing. The term "printing plate" as used in the present invention refers to an already imaged plate produced from a printing plate precursor (also referred to as "printing form").

Basically, all types of hthographic printing plates can be used in the process according to the present invention which should be protected by gumming for storage or a standstill of the printing press in order to maintain the hydrophilic nature of the background areas. Both printing plates produced from positive working precursors and printing plates produced from negative working precursors can be used; printing plates that are imaged with UV/vis radiation and printing plates that are imaged with IR radiation or direct heat can be used. It is not intended to use a lithographic printing plate produced by an electro-photographic process.

According to a preferred embodiment, a gumming solution for a printing plate is provided whose precursor comprised an overcoat layer with a component capable of forming salts with an acid (present in the radiation-sensitive layer of the precursor or generated therein by radiation). Such overcoat layers are often water-soluble oxygen-impermeable overcoats which are e.g. intended to protect a layer beneath them comprising free-radical polymerizable components against undesired polymerization with atmospheric oxygen.

The components of the overcoat layer capable of forming salts with acids are for example so- called adhesion promoters, such as polyethylene imine, polyvinyl imidazole and polyvinyl pyrrolidone, as well as their copolymers with (meth)acrylic acid and/or (meth)acrylates.

The gumming composition according to the present invention is an aqueous alkaline solution comprising at least one component selected from water-soluble starch and water-soluble starch derivatives. It can either be a ready-to-use solution or a concentrate which is diluted before use. The starch derivative should be water-soluble, preferably cold-water-soluble and is selected from starch hydrolysis products such as dextrins and cyclodextrins, starch esters, such as phosphate esters and carbamate esters, starch ethers, such as e.g. cationic starch ethers and hydroxypropyl ethers, carboxymethyl starch and acetylated starch; dextrins are preferred.

The starch used as a starting product for the starch derivatives can be of various origins, it can e.g. be obtained from corn, potatoes, rye, wheat, rice, manioc, tapioca, chestnuts or acorns; corn starch and potato starch are preferred.

In its ready-to-use state, the gumming solution according to the present invention preferably comprises 1 to 50 wt.-% starch/starch derivative, based on the solids content of the solution, especially preferred 2 to 10 wt.%.

The pH value of the gumming solution according to the present invention is higher than 7; preferably, it is in the range of 7.5 to 10 and especially preferred 7.5 to 8.5, most preferred, the pH value is about 8. Within the framework of the present invention, the pH value is the pH value measured at 20°C with a commercially available pH meter. The alkaline pH can for example be achieved by adding phosphates (in particular alkali metal phosphates), carbonates (in particular alkali metal carbonates), hydroxides (in particular alkali metal hydroxides) and/or borates (in particular alkali metal borates such as sodium tetraborate). According to a preferred embodiment, the alkaline pH value is adjusted with substances that can at the same time function as buffer agents. According to an especially preferred embodiment, a borate or a mixture of borates is used, in paticular alkali metal tetraborate(s).

The gumming solution according to the present invention can optionally also comprise one or more common additive, e.g. selected from commonly used anti-foaming agents, biocides, complexing agents, surfactants and buffer substances. It is not intended to use silicates in the gumming solution.

Suitable anti-foaming agents include e.g. the Silicone Antifoam Emulsion SE57 (Wacker), TRITON^® CF32 (Rohm & Haas), AKYPO^® LF (ethercarboxylic acid Chem Y), Agitan 190 (Mϋnzing Chemie), TEGO^® Foamese 825 (modified polysiloxane, TEGO Chemie Service GmbH, Germany). Silicone-based anti-foaming agents are preferred. They are either dispersible or soluble in water. The amount of anti-foaming agent in the ready-to-use gumming solution is preferably 0 to 1 wt.-%, based on the weight of the gumming solution, especially preferred 0.01 to 0.5 wt.-%. One anti-foaming agent or a mixture of two or more can be used.

The biocides should be effective against bacteria, fungi and/or yeasts. Suitable biocides include e.g. N-methylol-chloracetamide, benzoic acid, phenol or its derivatives, formalin, imidazol derivatives, isothiazoHnone derivatives, benzotriazole derivatives, amidines, guanidine derivatives, quaternary ammonium salts, pyridine, quinoline derivatives, diazine, triazole derivatives, oxazoles and oxazine derivatives and mixtures thereof. Their amount is not particularly restricted and preferably accounts for 0 to 10 wt.-% in the ready-to-use gumming solution, based on the total weight of the solution, especially preferred 0.1 to 1 wt.- %. One biocide or a mixture of two or more can be used.

Examples of suitable complexing agents include: Aminopolycarboxylic acid and salts thereof, such as ethylene diamine tetraacetic acid and potassium or sodium salts thereof, diethylene triamine pentaacetic acid and potassium or sodium salts thereof, triethylene tetraamino- hexaacetic acid and potassium or sodium salts thereof, hydroxyethyl ethylene diamine triacetic acid and potassium or sodium salts thereof, nitrilotriacetic acid and potassium or sodium salts thereof, 1,2-diaminocyclohexane-tetraacetic acid and potassium or sodium salts thereof and l,3-diamino-2-propanol-tetraacetic acid and potassium or sodium salts thereof, and an organophosphonic acid, phosphonoalkane tricarboxylic acid or salts thereof, such as 2- phosphonobutane-l,2,4-tricarboxylic acid and potassium or sodium salts thereof, phosphonobutane-2,3, 4- tricarboxylic acid and potassium or sodium salts thereof, phosphonoethane-2,2,2-tricarboxylic acid and potassium or sodium salts thereof, aminotris- (methylene-phosphonic acid) and potassium or sodium salts thereof and sodium gluconate. The complexing agents can be used individually or as a combination of two or more. Organic amine salts of the above-mentioned complexing agents can be used instead of the potassium or sodium salts thereof. The amount of complexing agent preferably accounts for 0 to 5 wt- % in the ready-to-use gumming solution, based on the total weight of the solution, especially preferred 0.01 to 1 wt.-%. Suitable surfactants include anionic agents, such as fatty acid salts, abietic acid salts, hydroxyalkane sulfonic acid salts, dialkyl-sulfosuccinate salts, alkyl-naphthalene sulfonate salts, alkyl-phenoxy-polyoxyethylene propylsulfonate salts, polyoxyethylene- alkylsulfophenylether salts, sodium salt of N-methyl-N-oleyltaurine, disodium salt of N- alkylsulfosuccinic acid amide, petroleum-sulfonic acid salts, sulfatized castor oil, sulfatized tallow, sulfuric acid ester salts of fatty acid alkyl esters, alkylsulfate ester salts, polyoxyethylene-alkylether-sulfuric acid ester salts, fatty acid monoglyceri.de sulfuric acid ester salts, polyoxyethylene-alkylphenylether sulfuric acid ester salts, polyoxyethylene- styrylphenylether sulfuric acid ester salts, alkylphosphate ester salts, polyoxyethylene- alkyletherphosphoric acid ester salts, polyoxyethylene-alkylphenylether phosphoric acid ester salts, partially saponified styrene maleic acid anhydride copolymers, partially saponified olefin maleic acid anhydride copolymers and condensates of naphthalene sulfonic acid salt and formalin; alkylbenzolsulfonates, alkanesulfonates, alkylsulfates and alkylethersulfates, non-ionic agents such as glycerin, ethylene glycol, triethylene glycol, sorbite fatty acid ester, alkyl-phenolethoxylates, fatty alcohol ethoxylates, alkyl-polyglucosides and N- methylglucamides; and cationic agents such as quaternary ammonium compounds with one or two hydrophobic groups and salts of long-chain primary amines. The amount of surfactants is not particularly restricted, but it preferably accounts for 0 to 5 wt.-% in the ready-to-use gumming solution, based on the total weight of the solution, especially preferred 0.1 to 1 wt- %. One surfactant or a mixture of two or more can be used.

Suitable buffer substances include e.g. tris(hydroxymethyl)-aminomethane (TRIS), phosphates, glycine, 3-(cyclohexylamino)-propane sulfonic acid (CAPS), carbonates and borates. When suitable buffer substances are added in suitable amounts, an additional adjustment of the pH value to >7, for example with NaOH, is usually not necessary.

An especially preferred gumming solution according to the present invention comprises one or more dextrins and one or more alkali metal borates (in particular sodium tetraborate).

The gumming solution can be applied using common methods that are known to the person skilled in the art. These methods include for example common coating processes, such as application with a sponge during manual gumming, with a doctor blade, by means of squeezing rolls, spraying or a dipping method. The viscosity of the gumming solution is not particularly restricted, however, it can influence the selection of the coating process (and vice versa).

The gumming solution can be applied onto a printing plate immediately after imaging, i.e. before the printing plate is used for printing. This way, the surface of the printing plate is protected in case the plate is not used for printing immediately after imaging but stored for a certain amount of time.

The gumming solution can also be applied onto a printing plate that has already been used for printing. This is especially advisable if the printing machine is stopped for a prolonged period of time during printing, for example over the weekend.

The gumming thickness is usually at most 3 μm. Depending on the application method, the solids content of the gumming and the type of plate, the wet weight of the gumming is about 5 to 50 g/m².

The gummed printing plates can be stored for several months before they are (again) mounted in the printing machine and used for printing. In practical applications, gummed printing plates are usually stored for about a week.

The gumming solution according to the present invention can be present as a ready-to-use solution or as a concentrate that is diluted with water before use. Compared to the ready-to- use solution, the liquid concentrate can be 1 to 5 times more concentrated.

Distilled water, deionized water or tap water can be used for the preparation of the gumming solution. The same applies to the water that is used for diluting a concentrate to obtain a ready-to-use solution.

The gumming solutions according to the present invention are easy to prepare; the starch/starch derivative and optionally additives is/are simply dissolved in water and, if necessary, the pH is adjusted to a value >7. No specific polymers are necessary, as is the case e.g. in US 4,033, 919 Al, nor does the preparation of the gumming solution require specific process steps, as is the case with the emulsion described in US 4,162,920 Al.

When the gumming solution according to the present invention is used, no ink acceptance problems or poor roll-up behavior occur during printing afterwards; this means that on the one hand, the gumming adheres well to the plate and protects the background area effectively and on the other hand, it can easily be removed both from the image areas and the background areas by the fountain solution and/or a printing ink/water emulsion. This is also the case in printing plates whose precursors comprise an overcoat layer with components capable of forming salts with acids, such as adhesion promoters.

The invention will be explained in more detail in the following examples; however, they shall not restrict the invention in any way.

Examples

Examples 1 to 4 and Comparative Examples 1 to 5 (Gumming composition)

Gumming solutions were prepared from the components listed in Tables 1 and 2.

Table 1

Boraxdextrin: Dextrin produced by hydrolysis of potato starch; it comprises 5 to 10 wt.-% sodium tetraborate; available from Emsland Starke GmbH/Emlichheim, Germany

2) Dextrin, produced by hydrolysis of potato starch; available from Emsland Starke GmbH

3) Sodium 2-ρhenyl-phenolate; available from Bayer AG 4) n-Hexadecyl-diphenyloxide disulfate sodium salt; available from Dow 5) Amino-trimethylene-phosphonic acid; available from Polygon, Olten

Table 2

Examples 6 to 10 and Comparative Examples 7 to 12 (Result with negative working IR-sensitive printing plate)

A coating solution was prepared from the following components:

6.4 g loncryl 683^® (acrylic acid copolymer of the company SC Johnson & Son Inc. with an acid number of 175 mg KOH/g)

8.0 g AC 50^® (methacrylic acid copolymer of the company PCAS with an acid number of 48 mg KOH/g, 70 wt.-% solution in methyl glycol)

16.8 g of an 80% solution of a urethane acrylate in methyl ethyl ketone, prepared by reacting l-methyl-2,4-bis-isocyanate benzene (Desmodur N100^® of the company Bayer) with hydroxyethyl acrylate and pentaerythritol triacrylate having a double-bond content of 0.50 double bonds/100 g when all isocyanate groups are completely reacted

2.6 g dipentaerythritol pentaacrylate

0.65 g 3-mercapto-l,2,4-triazole 1.5 g 2-(4-methoxyphenyl)-4,6-bis-(trichloromethyl)-s-triazine

0.6 g Renol blue B2G HW^® (copper phthalocyanine pigment preparation with polyvinyl butyral of the company Clariant) 0.8 g aniline diacetic acid 0.3 g 2-[2-[thiophenyl-3-[2-(l,3-dihydro-l,3,3-trimethyl-2H-indole-2-ylidene)-ethylidene]- l-cyclohexene-l-yl]-efhenyl]-l,3,3-trimefhyl-3H-indoliumchloride

These components were dissolved under stirring in 200 ml of a mixture consisting of

90 parts by volume l-methoxy-2-propanol and 10 parts by volume acetone

After filtration of the solution, it was applied by means of common processes to an electrochemically grained and anodized aluminum foil that had been subjected to an aftertreatment with an aqueous solution of polyvinylphosphonic acid and the layer was dried for 4 minutes at 90°C. The dry weight of the resulting LR-sensitive layer was about 2 g/m².

Then an oxygen-impermeable layer with a dry layer weight of 2.0 g/m² was applied analogously by coating with a solution of the following composition:

7.5 g polyvinyl imidazole

42.5 g polyvinyl alcohol (Airvol 203^® of the company Airproducts; 12 wt.-% residual acetyl groups) 170 g water

Drying took place for 5 minutes at 90°C.

The plate precursor produced in this manner was then exposed in a Trendsetter 3244 of the company Creo/Scitex with an 830 nm laser diode. An UGRA/FOGRA Postscript test strip, version EPS, which contains different elements for evaluating the quality of the copies, was used for imaging. The printing plate precursor was then processed in a commercially available Mercury News processor from Kodak Polychrome Graphics LLC equipped with a preheating section, a pre-wash section, an immersion-type developing bath, a section for rinsing with water, a gumming section and a drying section.

The following settings were used:

Speed 120 crn/min

Preheating 630

Prewash rate 0.51/m² plate

Temperature of the developer bath 23 ±1°C

After developing, the exposed areas remained on the plate while the unexposed areas were completely removed by the developer.

The gumming section was filled with a gumming composition selected from those of Examples 1 to 5 and Comparative Examples 1 to 6.

The developed printing form was cut into 11 pieces and each piece was treated with a different gumming solution. The treated pieces were then mounted in a sheet-fed offset press.

The results with respect to ink acceptance and roll-up behavior are shown in Table 3.

Table 3

Ink acceptance test: Printing machine: Roland Favorit Fountain solution: 86 % water 10 % isopropanol 4 % Combifix XL 805409 (Hostmann and Steinberg) Printing ink: Black Cora S 8900 (Sun Chemical) Paper: book paper 80 g/m², 1.75 times the volume

** Roll-up behavior: (Intensified test on Roland Favorit printing machine) After 1,000 copies, the printing machine was stopped. After 30 minutes, the printing machine was started up again (without damping roller). The dampening system was pre- wetted thoroughly, then printing was started and the dampening system was started after the first sheet had been printed. A total of 50 sheets were printed.

Evaluation: Good roll-up behavior: After 10 sheets, the background areas are already clean. Poor roll-up behavior: The background areas are only clean after 50 sheets. No roll-up: The background areas are not clean after 50 sheets.

Although the plate in Example 10 (i.e. when the gumming solution of Example 5 is used) inks up quickly and shows good roll-up behavior, the plates of Examples 6 to 8 exhibited still a somewhat better roll-up behavior. Examples 11 to 15 and Comparative Examples 13 to 18 (Result with negative working UV-sensitive printing plate)

A coating solution was prepared from the following components:

2.1 g of a terpolymer prepared by polymerization of 476 parts by weight styrene, 476 parts by weight mefhylmethacrylate and 106 parts by weight methacrylic acid 5.24 g of an 80% methyl ethyl ketone solution of a urethane acrylate prepared by reacting Desmodur N100 (Bayer) with hydroxyethyl acrylate and pentaerythritol triacrylate; amount of double bonds: 0.5 double bonds per 100 g when all isocyanate groups have completely reacted 1.29 g dipentaerythritol pentaacrylate

0.6 g 2,4-trichloromethyl-6((4-ethoxyethylenoxy)naphtyl) 1 ,3,5-triazine 0.16 g 4,4'-N,N-diethylaminobenzophenone 0.2 g benzophenone 0.19 g 3-mercapto-l,2,5-triazole 0.12 g Renol blue B2G-HW 0.1 g leuco crystal violet

35 parts by volume methyl glycol

25 parts by volume methanol

40 parts by volume methyl ethyl ketone

After filtration of the solution, it was applied by means of common processes to an electro- chemically grained and anodized aluminum foil that had been subjected to an aftertreatment with an aqueous solution of poly vinylphosphonic acid and the layer was dried for 5 minutes at 90°C. The dry weight of the resulting IR-sensitive layer was about 2.2 g/m².

Then an oxygen-impermeable layer with a dry layer weight of 1.7 g/m was applied analogously by coating with a solution of polyvinyl alcohol (Airvol 203 of the company Airproducts) and polyvinyl imidazole (cf. Examples 6 to 10). Drying took place for 5 minutes at 95°C as well.

The coated aluminum foil was cut into 11 pieces of 515 to 790 mm each. The test pieces were all exposed through a negative mask using a metal halogenide lamp (MH burner, available from Sack) with an energy of 10 mJ/cm².

Then, the test pieces were processed in a processor unit as used in Examples 6 to 10. The following settings were used:

Developer used: KPG 953 (developer available from Kodak Polychrome Graphics) Temperature of the developer bath: 27 ± 1°C Dwell time in the developer: 30 s

The gumming section was filled with a gumming solution selected from those of Examples 1 to 5 and Comparative Examples 1 to 6. Each test piece was treated with a different gumming solution. The treated test pieces were then mounted in a sheet-fed offset press. The results with respect to ink acceptance and roll-up behavior are shown in Table 4; the tests were carried out as described in Examples 6 to 10.

Table 4

Example Gumming Ink acceptance test Start-up composition behavior* 11 Example 1 Plate inks up immediately <10 sheets <10 sheets 12 Example 2 Plate inks up immediately <10 sheets <10 sheets 13 Example 3 Plate inks up immediately <10 sheets <10 sheets 14 Example 4 Plate inks up immediately <10 sheets <10 sheets 15 Example 5 Plate inks up immediately <10 sheets <10 sheets Comp. 13 Comp. 1 Plate hardly inks up >10 sheets >50 sheets

As was the case with Examples 6 to 10, it was found here as well that the roll-up behavior of the plates that had been treated with the gumming solutions of Examples 1 to 4 was slightly better than that of the plate that had been treated with the gumming solution of Example 5.

If one compares Example 10 with Comparative Example 7 (or Example 15 with Comparative Example 13) it becomes clear, surprisingly, that the ink acceptance and roll-up behavior of the gummed printing plates depends strongly on the pH value; while an alkaline pH value leads to a very good ink acceptance and roll-up behavior, an acidic pH value results in a poor ink acceptance and roll-up behavior.

A comparison of Examples 6 to 10 with Comparative Example 11 (or of Examples 11 to 15 with Comparative Example 17) surprisingly shows that not only the pH value of the gumming solution but also the starch component of the gumming solution is essential to the present invention.

Claims

1. Process for gumming lithographic printing plates comprising

(a) providing a lithographic printing plate;

(b) providing a gumming solution comprising (i) water and (ii) at least one component selected from water-soluble starch and water-soluble starch derivatives;

(c) applying the gumming solution onto the printing plate provided in step (a), characterized in that the pH value of the gumming solution is higher than 7.

2. Process according to claim 1, wherein the pH value of the gumming solution is in the range of 7.5 to 8.5.

3. Process according to claim 1 or 2, wherein component (ii) of the gumming solution is a dextrin.

4. Process according to any of claims 1 to 3, wherein the pH value has been adjusted to >7 by the addition of phosphates, carbonates, hydroxides or borates.

5. Process according to any of claims 1 to 4, wherein the gumming solution comprises one or more tetraborates.

6. Process according to any of claims 1 to 5, wherein the gumming solution additionally comprises one or more additives selected from biocides, surfactants, complexing agents, anti-foaming agents and buffer substances.

7. Process according to any of claims 1 to 6, wherein the lithographic printing plate can be obtained by image-wise exposure and, if necessary, developing of a printing plate precursor whose overcoat comprises a component capable of forming salts with acids.

8. Process according to any of claims 1 to 7, wherein the provided lithographic printing plate has already been used for printing prior to gumming.

9. Process according to any of claims 1 to 7, wherein the provided lithographic printing plate is treated with the gumming solution immediately after imaging.

10. Gumming solution for lithographic printing plates comprising

(a) water and (b) at least one component selected from water-soluble starch and water-soluble starch derivatives, characterized in that the pH value of the solution is higher than 7.

11. Gumming solution according to claim 10, wherein the pH value is in the range of 7.5 to 8.5.

12. Gumming solution according to claim 10 or 11, wherein component (b) is a dextrin.

13. Gumming solution according to claim 12, wherein the dextrin has been prepared from potato starch.

14. Gumming solution according to any of claims 10 to 13, wherein the gumming solution comprises one or more borates.

15. Gumming solution according to claim 14, wherein the borate is an alkali metal tetraborate.

16. Gumming solution according to any of claims 10 to 15, additionally comprising one or more additives selected from surfactants, biocides, complexing agents, anti-foaming agents and buffer substances.

17. Gumming solution according to any of claims 10 to 16, wherein the solution is a concentrate.

18. Gumming solution according to any of claims 10 to 16, wherein the solution is a ready- to-use solution.

19. Use of an aqueous dextrin solution having a pH of more than 7 for gumming lithographic printing plates.

20. Use according to claim 19, wherein the dextrin solution furthermore comprises one or more borates.

21. Use according to claim 20, wherein the borate is an alkali metal tetraborate.