EP0249752B1 - Lithographic fountain solution containing mixed colloids - Google Patents

Description

The invention relates to an acidic, aqueous dampening solution concentrate for use in planographic printing.

The planographic printing process is known. In general, printing is done from a flat plate or cylinder, which is usually made of anodized aluminum and has essentially no height differences on its surface. The ability to print depends on different properties of the image and non-image areas on the surface. For planographic printing, the image to be reproduced is applied to the plate by one of several known methods in such a way that the non-image areas become hydrophilic, while the image areas become hydrophobic. For this purpose, a light-sensitive layer is generally used. After imagewise exposing the photosensitive layer, the latent image is developed and part of the layer is removed from the plate. The plate is then treated with a preservative, whereupon it becomes hydrophilic in the areas freed from the light-sensitive layer. An aqueous dampening solution is then applied to the plate surface during the printing process. The dampening solution keeps all parts of the plate surface which are not covered by the hydrophobic image moist. In addition, it prevents the disk from toning, i. that is, it prevents the non-image areas from at least partially taking on color.

The dampening solution can be composed in such a way that it cleans the plate surface just enough and keeps it hydrophilic, that sharp contours are retained and rapid wear is prevented.

In a conventional system, the dampening solution is applied to the plate by one or more rollers. Thereafter, at least one ink roller covered with an oil-based ink comes into contact with the entire surface of the plate, but only transfers the ink to the image areas because the hydrophilic non-image areas repel the ink. Each time a print is made, the planographic printing plate is first wetted with the aqueous dampening solution and then colored with printing ink. Alternatively, the dampening solution and at least part of the greasy ink are applied to the plate simultaneously with an application roller. With this system, other rollers, usually smaller in diameter than the first roller, can then contact the plate to distribute the ink more evenly. The color in the picture is finally transferred directly to a sheet of paper or another recording material to be printed or is first transferred to an offset cloth made of rubber or plastic, which in turn then transfers the printing to the final recording material. The dampening solution can be prepared by the printer itself from a dampening solution concentrate by adding water and, if appropriate, 10 to 30% by volume of isopropyl alcohol. Acid, aqueous dampening solution concentrates according to the prior art generally contain a water-soluble resin or colloid, such as. As gum arabic or cellulose gum, and may also contain a hydrophilizing agent, buffering agent and wetting agent.

Acid fountain solutions are usually designed to perform different tasks. By using hydrophilic colloids, such as. B. gum arabic, the non-image areas of the plate should remain hydrophilic during production printing. Acidic hydrophilizing agents can also be added, so that the non-image areas remain sharper and repel the ink. With surfactants or combinations of surfactants and alcohols, such as. B. isopropyl alcohol, the surface tension of the solution should be reduced so that the plate and roller surfaces can be wetted better. This property can also be achieved without surfactants if more than the amounts of hydrophilic colloids required to maintain the hydrophilicity of the non-printing plate surfaces in combination with at least one metal salt, e.g. B. with nitrates, chromates and the like., And / or be used with alcohols or glycols.

The use of surfactants in such mixtures can lead to problems such as foaming or emulsification of the printing ink during printing. Fountain solutions with surfactants also tend to dampen the paper more when printing than solutions without surfactants, especially when working with colors. The paper becomes sponge-like and the ink runs a little on the paper so that the images are no longer very sharp. If the amount of colloid used is greater than necessary, the plate may be toned or run blind, and the ink may also tend to emulsify. Salts such as B. nitrates, in conjunction with an excess of colloid (gum arabic) can further exacerbate the problem of blind running, since these substances tend to form complexes with gum arabic and thereby form gum arabic salts, which usually lead to the blind running of the plate. If large amounts of alcohols, e.g. Isopropyl alcohol, used in dampening solutions, so the solubility of other components in the solution can be reduced. These components can then accumulate on roller or plate surfaces when they are removed from the solution. Isopropyl alcohol is also undesirable because it is toxic, flammable and has a low vapor pressure so that it evaporates quickly.

Variants of such dampening solution compositions are known. According to US Pat. No. 4,266,481, a mixture with a polyacrylamide polymer is used, for example, instead of dampening agents with gum arabic. This is intended to prevent, among other things, the formation of emulsions with the printing ink, which dampening agents based on gum arabic tend to do. Emulsifying the ink weakens the print resolution and contributes to the toning of the plate and repulsion of the ink from the ink rollers. The inclination of gum arabic, a blind running of the plat bringing about (a process in which gum arabic settles on the printing surface of the plate and thus makes it partially hydrophilic) is described in US Pat. No. 4,400,481 in connection with water-based preservatives. The patent teaches that this problem can be solved in such compositions by using a mixture of three components, namely gum arabic, polyvinylpyrrolidone and polyacrylamide. However, preservatives as described in this patent would not readily be suitable for use in a dampening solution because they tend to cause problems with emulsification and ink transfer and cause excessive solids build-up on the dampening roller systems.

It is therefore an object of the present invention to provide a dampening solution for offset printing which not only minimizes the toning or blind running of the printing plates, but also offers other advantages, such as e.g. Stabilization of the pH value, resistance to the accumulation of solution residues on ink transfer rollers or roller coatings, improved print quality, less tendency to wet the printing paper, faster drying of the printing ink and better water flow.

• a) 0,1 bis 1,5 Gew.-% eines wasserlöslichen, filmbildenden hydrophilen Kolloids, das nicht Polyvinylpyrrolidon ist,
• b) 0,75 bis 8,5 Gew.-% Polyvinylpyrrolidon,
• c) 5,0 bis 30,0 Gew.-% eines einen pH-Wert von 2,0 bis 3,5 erzeugenden wasserlöslichen, sauren Hydrophilierungsmittels,
• d) 20,0 bis 50,0 Gew.-% eines wasserlöslichen oder mit Wasser mischbaren Glykols und
• e) 0,01 bis 1,25 Gew.-% eines Phosphat/Phosphorsäure-Puffersystems in einem wäßrigen Gemisch enthält.

The above task can be solved by a dampening solution concentrate, which are essential components

• a) 0.1 to 1.5% by weight of a water-soluble, film-forming hydrophilic colloid which is not polyvinylpyrrolidone,
• b) 0.75 to 8.5% by weight of polyvinylpyrrolidone,
• c) 5.0 to 30.0% by weight of a water-soluble acidic hydrophilizing agent which produces a pH of 2.0 to 3.5,
• d) 20.0 to 50.0% by weight of a water-soluble or water-miscible glycol and
• e) contains 0.01 to 1.25% by weight of a phosphate / phosphoric acid buffer system in an aqueous mixture.

This concentrate can be composed so that it contains 12 to 60 wt .-% water and can be further diluted by the consumer with water or a mixture of water and less than 10 vol .-% isopropyl alcohol, so that dampening solution with a content of at least 99% by weight, preferably 99.1 to 99.9% by weight, of water or water / alcohol mixture and accordingly 0.1 to 0.9% by weight of non-aqueous / non-alcoholic components. The mixture forming the solution or the concentrate essentially consists of the specified components. By this it is meant that the solution is functional as described and does not contain a substantial amount of surfactant or nitrate or chromate salt as a hydrophilizing agent.

The fountain solutions according to the invention preferably have a surface tension (measured with a surface tensiometer) in the range from 30 to 40 dynes / cm ² , which can be achieved without having to add large amounts of gum arabic or surfactants and / or nitrate salts to the mixture. The dampening solutions generally have a low solids content and can generally be applied to the printing plates in smaller amounts than known solutions and still achieve the effectiveness of a quality dampening solution, ie they wet the printing plate sufficiently and prevent toning or blind running. This in turn means that the plate in turn moistens the printing paper to a lesser extent and less solids can accumulate on the plate itself or on the dampening felt rollers used to apply the solution.

The water-soluble, film-forming hydrophilic colloids which are suitable according to the invention include gum arabic, tragacanth, carboxymethyl cellulose, sodium alginate, carboxymethyl starch, methyl cellulose and similar colloids which impart hydrophilic properties when applied to the non-image areas of the planographic printing plate. Gum arabic is preferred in the context of this invention and the colloid content of the concentrate is 0.1 to 1.5% by weight.

The second hydrophilic film-forming component of the dampening solution is polyvinylpyrrolidone, which is contained in the concentrate in an amount of 0.75 to 8.5% by weight.

The ratio of polyvinylpyrrolidone to colloid in the mixture is important if the advantages of reducing the blinding or toning of the plate as much as possible are to be achieved and at the same time it should be possible to apply the smallest amounts of water to the printing plate during the print run. It has been found that the best results are achieved if the ratio of polyvinylpyrrolidone: colloid on the basis of the dry weight is at least 1: 1. The ratio of these components is preferably in the range from 10: 1 to 4: 1, very particularly preferably in the range from 8: 1 to 6: 1.

The acid component of the dampening solution serves a dual purpose: it is supposed to stabilize the pH value as well as to hydrophilize the plate. Preferred acids are water-soluble weak acids, such as. B. citric, ascorbic, sulfanilic, tartaric, lactic, acetic, maleic and phosphoric acid, although dilute sulfuric acid can be used. The preferred acid is citric acid. Sufficient acid is removed to give the concentrate a pH in the range from 2 to 3.5, which, when further diluted with water for use as a dampening solution, gives a pH in the range from 4 to 4.5. The amount of acid used is 5.0 to 30.0% by weight, preferably 10 to 25% by weight, based on the aqueous concentrate.

The fourth component of the dampening solution is a water-soluble or water-miscible glycol that serves as a wetting agent. The wetting agent should distribute the dampening solution more easily over the non-image areas of the plate surface, and it should also delay the transfer of ink to the roller surface (felt coverings) of the rollers used to apply the dampening solution gladly. Suitable glycols are alkanediols with 2 to 6 carbon atoms in the main chain, such as. B. ethylene glycol, butanediol, hexanediol, propylene glycol, neopentyl glycol and alkyl-substituted diols such as 2-ethyl-1,3-hexane-diol. The concentrate can contain 20 to 50% by weight of glycol, the preferred range being 30 to 45% by weight. Ethylene glycol is the preferred glycol.

The fifth component of the fountain solution is a phosphate / phosphoric acid buffer system that improves the stability of the mixture and appears to work synergistically with the polyvinylpyrrolidone component, keeping the pH of the fountain solution constant regardless of the degree of dilution or water quality. The phosphate component can consist of a mono-, di- or trisodium phosphate or mixtures thereof in combination with phosphoric acid, the ratio of phosphate: phosphoric acid preferably being 10: 1 to 2: 1.

The concentrate can also be a bactericide, e.g. B. ( ^R ) Dowizil-75 (manufacturer Dow Chemical Company) in an amount sufficient to prevent or delay the bacterial attack in general from 0.01 to 0.4% by weight, preferably 0.05 to 0.125% by weight , based on the concentrate. There may also be other inert ingredients such as. B. an indicator dye may be contained in the concentrate.

The above-mentioned components are processed into a concentrate by dissolving them in water, preferably demineralized water. The concentrate preferably contains 40 to 85% by weight, particularly preferably 50 to 75% by weight, of water-soluble and water-miscible components in water.

The printer can produce a ready-to-use fountain solution by adding about 25 to 90 g of the concentrate described above in 4 liters of water. The dilution ratio is preferably about 40 to 60 g of concentrate to 3.75 l of water. To further improve the wetting properties of the solution, up to 10% by volume of the water can optionally be replaced by isopropyl alcohol.

The dampening solution according to the invention has the particular advantage that certain additives contained in conventional dampening solutions are not necessary in order to achieve good water flow and the resistance of the plate against blind running and toning when used in the printing process. Conventional hydrophilizing agents such as nitrates and chromates are e.g. B. not necessary, and therefore there is a lower tendency to accumulate solids on the printing rollers during printing, due to the precipitation of these substances from the solution. However, this does not mean that it is not possible to include smaller amounts of these substances in the recipe than usual, if this is desired.

Furthermore, when the dampening solution concentrate is diluted before use in the printing process, good surface tension and good wetting properties of the solution can be achieved without the addition of isopropyl alcohol. In some cases, however, it may be desirable to add small amounts of less than 10 volume percent isopropyl alcohol to the solution so as to further improve its surfactant properties.

The invention is explained in more detail by the following examples, but without being limited to the embodiments shown:

example 1

To 40.0 parts by weight (pbw) of fully deionized water in a beaker, 0.5 pbw of phosphoric acid (85%) are added with constant stirring. Then 10.5 pbw of powdered gum arabic and 2.5 pbw of disodium phosphate are added. The solution is heated to 65 _° C. for 30 minutes with constant stirring.

• 3,8 Gt Polyvinylpyrrolidon (K-15)
• 19,0 Gt Citronensäure
• 34,3 Gt vollentsalztes Wasser
• 38,1 Gt Ethylenglykol

After 30 minutes, the heating is stopped and 46.5 pbw of demineralized water are added to the solution. During the addition of water and throughout the rest of the process, stirring is continued. 4.8% of the solution prepared as described above are then mixed with the following substances, the speed or sequence of addition not being given particular attention:

• 3.8 pbw of polyvinylpyrrolidone (K-15)
• 19.0 pbw of citric acid
• 34.3 pbw of demineralized water
• 38.1 pbw of ethylene glycol

The mixture is stirred until all components have dissolved. It is then cooled to room temperature and forms the dampening solution concentrate.

Example 2

To prepare a dampening solution, approximately 250 g (8.75 oz) of the concentrate from Example 1 are mixed with approximately 19 liters (5 gallons) of completely deionized water. This solution (solution A) is filled into the one fountain solution box of a two-color printing machine, model Miller TP 29S. In the same way, a commercially available dampening solution concentrate is provided for sheet-fed printing machines. This concentrate is a solution based on gum arabic with salts and surfactants. About 250 g (8.75 oz) of concentrate are placed in a mixture of water and 25 vol.% Isopropanol. This solution (solution B) is filled into the other dampening solution box of the printing press. Each box is operated and controlled independently and supplies dampening solution to a separate printing unit. The two-color printing machine prints two colors in succession on each sheet of paper fed.

The ink and dampening solution doses are set independently of each other. Two N-50 printing plates are appropriately exposed, developed and preserved. The N-50 plates are wet roughened, anodized, negative working printing plates manufactured by Enco Printing Products, a subsidiary of American Hoechst Corporation, Somerville, New Jersey, USA.

The two prepared N-50 plates are properly clamped onto the two plate cylinders. A coated offset offset enameled stock type paper is used and both dampening solution supply systems are set to 36 at the start of the printing process. The ink supply is adjusted so that a wet color density of 1.35 results. When the color balance is reached, the dampening solution setting is switched back two positions, and this process continues until the printed sheet shows signs of toning, i. that is, until the print quality turns out to be insufficient. However, the dampening solution supply should be set as low as possible in order to keep the amount of water transported to a minimum. The greater the amount of water delivered to the paper, the greater the risk of wetting the paper and thus distorting the print. In addition, additional amounts of water mixed with the printing ink can emulsify the ink and / or cause the rollers to run bright, both of which poor print quality.

The N-50 plate processed with the commercial dampening solution (solution B) begins to tone at a setting of 30, while the plate processed with the dampening solution according to the invention (solution A) begins to tone only at a setting of 22 to 24. The dampening solution settings are switched back to the initial position and the experiment is repeated several times. The printing plate running with the commercial dampening solution (solution B) can no longer recover after the third test sequence, while the plate running with the dampening solution according to the invention (solution A) still prints cleanly even after the fourth sequence.

New plates are prepared and printed with a dampening solution setting of 32. The press is stopped for an hour. After this downtime, printing continues. The plate used with the commercially available dampening solution (solution B) prints a dark haze, which cannot be removed by increasing the amount of dampening solution. The plate must first be cleaned with a plate cleaner before it delivers copies of acceptable quality.

The plate used with the fountain solution according to the invention (solution A) runs cleanly and delivers good copies after less than 10 prints.

From this it can be seen that with the dampening solution according to the invention it is possible to print cleanly with a lower dampening solution setting and this setting results in good water flow. In addition, the plate background is kept more hydrophilic during the idle time of the printing press, which enables a better and easier freewheeling.

Example 3

All experiments are repeated as described in Example 2, but using a different commercially available fountain solution instead of the one used in Example 2. This dampening solution is particularly suitable for use in sheet-fed printing machines. It is a solution based on gum arabic with a content of salts, surfactants and an alcohol. About 175 g (6.25 oz) of the dampening solution concentrate is added to about 19 liters (5 gallons) of a mixture of water and 20 vol.% Isopropanol. The dampening solution settings are reduced until there is a toning. It can be seen that this commercially available dampening solution tones at the setting 30, while the dampening solution according to the invention only tones at 22 to 24.

Even after the downtime, the same results are obtained as in Example 2.

Example 4

The evaluation is carried out as described in Example 2, but here the N-50 plates are replaced by N-250 plates. The N-250 plates are electrochemically roughened, anodized, negative working printing plates from the company Enco Printing Products.

In this case it is found that the plate used with the commercial dampening solution tones at the setting 28, whereas the plate working with the dampening solution according to the invention only tones at the setting 20.

The properties after the downtime of the printing press correspond to those described in Example 2.

Example 5

This example describes the printing results obtained using a fountain solution concentrate prepared according to Example 1, but in which polyvinylpyrrolidone has been omitted.

A dampening solution concentrate is prepared as in Example 1, but the polyvinylpyrrolidone is omitted. All other components and their parts remain unchanged. The concentrate is diluted as stated in Example 2 and filled into the first dampening solution box of the printing press (solution C). The other box contains the commercially available dampening solution from example 2 (solution B).

The N-50 plate running with the commercial dampening solution (solution B) starts to tone at setting 30, and the dampening solution without polyvinylpyrrolidone (solution C) also leads to the plate being toned at setting 30. The dampening solution settings are returned to the initial position and the test is repeated several times. The plates running with both dampening solutions cannot recover after the third test sequence.

New plates are produced and used at setting 32 for printing. The press is turned off for an hour. After this downtime, printing is carried out again with the non-preserved and uncleaned plates. Both plates print a dark veil. With the one with the commercially available dampening solution (solution B) used plate, the veil can not be removed even with increased dampening solution supply. The plate used with solution C cleans somewhat, but not to the extent that it can be considered acceptable. Both must be treated with a plate cleaner before satisfactory copies can be obtained.

Example 6

This example describes the printing results obtained when using a fountain solution concentrate prepared according to Example 1, but in which gum arabic was omitted.

A dampening solution concentrate is produced as in Example 1, but without gum arabic. All other components and their parts remain unchanged. The concentrate is diluted as indicated in Example 2 and poured into the first dampening solution box of the printing press (solution D). The commercially available dampening solution from example 2 (solution B) is filled into the other box.

The N-50 plate running with the commercially available dampening solution (solution B) starts to tone at setting 30, and the dampening solution without gum arabic (solution D) also leads to the plate being toned at setting 30. The dampening solution settings are returned to the initial position and the experiment is repeated several times. The plate running with the commercial dampening solution can no longer recover after the third test sequence, the plate running with solution D after the fourth test sequence.

New plates are produced and used at setting 32 for printing. The press is turned off for an hour. After this downtime, printing is carried out again with the non-preserved and unpurified plates. Both plates print a dark veil. With the plate used with the commercially available dampening solution (solution B), the veil cannot be removed even when the dampening solution is intensified. The plate used with solution D cleans somewhat, but not to the extent that it can be considered acceptable. Both must be treated with a plate cleaner before satisfactory copies can be obtained.

Claims (11)

1. Lithographic fountain solution concentrate, composed of a mixture of

a) 0.1 to 1.5% by weight of a water-soluble, film- forming hydrophilic colloid other than polyvinylpyrrolidone,

b) 0.75 to 8.5% by weight of polyvinylpyrrolidone,

c) 5.0 to 30.0% by weight of a water-soluble, acidic hydrophilizing agent producing a pH from 2.0 to 3.5,

d) 20.0 to 50.0% by weight of a water-soluble or water-miscible glycol,

e) 0.01 to 1.25% by weight of a phos- phate/phosphoric acid buffer system,

f) if desired, further additives,

g) the remainder being water to make up to 100% by weight.

2. Concentrate according to Claim 1, characterized in that the b) to a) ratio is 10:1 to 4:1, relative to the dry weight of the constituents.

3. Concentrate according to one of Claims 1 or 2, characterized in that the hydrophilic colloid is gum arabic.

4. Concentrate according to one of Claims 1 to 3, characterized in that the glycol is ethylene glycol.

5. Concentrate according to one of Claims 1 to 4, characterized in that the glycol content is 30 to 45% by weight.

6. Concentrate according to one of Claims 1 to 5, characterized in that the acidic hydrophilizing agent is citric acid, ascorbic acid, sulfanilic acid, tartaric acid, lactic acid, maleic acid, phosphoric acid, acetic acid, dilute sulphuric acid, or a salt thereof.

7. Concentrate according to Claim 6, characterized in that the content of hydrophilizing agent is 10 to 25% by weight.

8. Concentrate according to one of Claims 1 to 7, characterized in that it has a pH from 3.0 to 3.5.

9. Concentrate according to one of Claims 1 to 8, characterized in that the water content is 25 to 50% by weight.

10. Dilute concentrate according to one of Claims 1 to 9, characterized in that the solids content of the constituents is 0.1 to 0.9% by weight.

11. Use of a concentrate according to one of Claims 1 to 10 in a dilute form in the offset printing process.