The invention relates to the dampening of printing plates. More specifically, it relates to an improved method for the preparation of a dampening water or fountain solution for moisturing lithographic printing plates, in particular presensitized lithographic printing plates (also known as "PS plates").

Lithographic printing technique makes the best use of the fact, that water and oil are essentially incompatible with one another. The printing surface of a lithographic printing plate comprises areas which receive water and repel an oil ink and those which repel water and receive an oil ink, the former serving as non-image areas and the latter serving as image areas. The non-image areas become damp with dampening water used in lithographic printing to thus enhance the difference in surface chemical properties between the image areas and the non-image area and hence to increase both the ink repellency of the non-image areas and the ink receptivity of the image areas.

The dampening water or fountain solution is generally a homogeneous mixture of organic solvents in water and in which various additives are dissolved such as acids, salts, naturally occurring polymeric materials, such as gum Arabic and/or a variety of synthetic polymers.

Various organic solvents are used. For example in the Dahlgren dampening system or film dampening systems in general, an aqueous solution containing about 20 to 25 vol% of isopropyl alcohol (IPA) is used together with other non-volatile additives. Although the use of IPA for technical reasons is preferred as very uniform dampening can be achieved, IPA is apt to evaporate and therefore the use of a special device is required for keeping the concentration of isopropyl alcohol constant. Furthermore IPA usage should be reduced for health safety and environmental reasons.

U.S. 3,877,372 describes a dampening water composition that allows avoiding the use of IPA containing a mixture of ethylene glycol monobutyl ether and at least one of hexylene glycol and ethylene glycol. U.S. 4,278,467 discloses an IPA free dampening water composition containing at least one of n-hexoxydiethylene glycol, n-hexoxyethylene glycol, 2-ethyl-1,3-hexanediol, n-butoxyethylene glycol acetate, n-butoxydiethylene glycol acetate and 3-butoxy-2-propanol. EP0066176 discloses a dampening water composition containing 2-ethyl-1,3-hexanediol and at least one of completely water-soluble propylene glycol, ethylene glycol, dipropylene glycol, diethylene glycol, hexylene glycol, triethylene glycol, tetraethylene glycol, tripropane glycol and 1,5-pentanediol.

In a regular printing process, the method for the preparation of the dampening water solution is by dissolving a dampening solution concentrate, often called a fountain etch, in water just before using it in the printing process. This method has as a disadvantage, that it is not very flexible because adjusting of the dampening water can only be done by increasing the amount of concentrate or water. Furthermore the dampening concentrate tends to be unstable upon longer time storage due to the presence of significant amounts of organic solvents required for an efficient dampening, and an incorrect composition can be formed by selective evaporation of one or more of the solvents. In U.S. 3,877,372 a fountain etch (dampening solution concentrate) is mixed with a formula containing the ethylene glycol monobutyl ether and hexylene glycol or ethylene glycol to which finally a sufficient amount of water is added to make up the desired concentration. In U.S. 4,278,467 n-hexoxydiethylene glycol or n-hexoxyethylene glycol or 2-ethyl-1,3-hexanediol, is mixed with a fountain etch (dampening solution concentrate) and the resulting solution was mixed with an appropriate amount of water. EP 0066176 does not make use of a dampening solution concentrate but rather mixes all ingredients such as gum Arabic, phosphoric acid solution, water, 2-ethyl-1,3-hexanediol and propylene glycol in one step together.

There remains therefore a need for an easy and flexible method to make a dampening water solution with a reduced amount of IPA giving a uniform dampening in the lithographic printing process.

An object, therefore, of the present invention is to provide a more easy and convenient method for making of a dampening water solution.

A further object is to provide a more flexible method for preparing a dampening water solution.
A further object is to prepare this dampening water solution with a reduced amount of IPA, and in which the composition of the dampening water solution can be adjusted accurately. It is a further object of this invention to have a sufficient control on the composition of the dampening water solution.
It is also an objective to provide a method for making a homogeneous dampening water composition ready for use in the printing process.

These and other objects are met by providing a method for the preparation of a ready to use dampening water solution for use in a lithographic printing process comprising the steps of

• dissolving a dampening-solution-concentrate in water giving a pre-dampening solution,
• adding to the pre-dampening solution, at least two organic solvents, giving a dampening water solution.
• mixing the dampening water solution until a homogeneous mixture is obtained.
  The invention is also directed to a mixture of organic solvents which can be used preferably in the method of this invention.

The contents or amounts of the components for use in the dampening water composition described in this specification are expressed on the basis of the total weight of the dampening water composition when used in a printing process, unless otherwise specified. This is not be confused with contents or amount of components in dampening solution concentrates that prior to the actual use need to be properly diluted.

The values for surface tension mentioned throughout this description are relate to the surface tension at 25°C.

In the context of this invention with the term organic solvents is meant materials that are liquid under standard conditions and comprise carbon- and hydrogen atoms and optionally also comprise oxygen-, nitrogen- and/or sulphur atoms, and optionally also, although in this invention not preferred halogen atoms.
In the context of this invention with a low surface tension of an organic solvent is meant a surface tension of below 40 mN/m (= dynes/cm.)

As already mentioned, in general a dampening solution concentrate is as such not ready for use, namely the concentrate will be dissolved in demineralised or tap water until the required concentration is obtained, subsequently the resulting solution is used in the lithographic printing process. The present inventors have found, that such an embodiment has serious drawbacks, because often the concentrate is unstable upon storage due to the presence of a high concentration of organic solvents in the concentrate, required for good dampening and because in this method there is only limited flexibility to adjust the composition of the dampening water solution when for example selective evaporation occurs by one or more solvents.

In the present invention, the dampening solution concentrate only contains a limited amount of organic solvents or no organic solvents at all. This has the advantage, that the stability upon storage of the concentrate is increased considerably. As a first step in the method of the invention this dampening solution concentrate is dissolved in water, generally demineralised water. The dissolving step is done just before printing. To this pre-dampening solution are then added the organic solvents.

In one embodiment the preparation of the dampening water solution is done just before and/or during the printing process. In another embodiment the at least two organic solvents are pre-mixed before addition to the pre dampening solution, giving an organic solvent mixture. It is preferred that the at least two organic solvents have or the organic solvent mixture has a surface tension below 40 mN/m, preferably below 35 mN/m and even more preferably below 30 mN/m. In accordance with one embodiment of the present invention the concentration of organic solvents in the dampening water solution is between 1 and 8 vol% and preferably between 2 and 6 vol%.

Upon the use of IPA in the dampening water solution, the resulting homogeneous mixture has a lower density than the pre-dampening water solution and therefore the concentration of the IPA can be controlled by density measuring means, which upon deviations of the density automatically can be adjusted by increasing or decreasing the amount of IPA added to the pre-dampening solution.

In the past in the Dahlgren dampening system, or film dampening systems in general, as much as 25 vol% IPA is used to obtain the required dampening characteristics. Modifications of printing equipment and optimisations in dampening water solutions compositions, have resulted in that nowadays the amounts of IPA in dampening water solutions is around 10%. In the printing process during the usage of the dampening water or fountain solution the IPA tends to evaporate which results in an incorrect composition of the dampening water solution. For this reason during the printing process IPA is added to the dampening water solution to compensate for this evaporation. The amount to be added is determined by the difference between a preset value for the density and the measured density value by density measuring means. If the measured density is higher due to the evaporation of IPA, the density is then restored by adding an amount of IPA equivalent to the evaporated IPA amount resulting in a correct IPA content. The density measuring means and density controlling means can be selected from those known to a person skilled in the art.

It is not clear, what the exact mechanism is for the beneficial behaviour of IPA in the dampening water solution but one of the contributing mechanisms could be that the surface tension of the dampening water solution is lowered by the included IPA in the dampening water solution. By lowering the surface tension the wetting of the dampening rollers and the lithographic printing plate will improve.

In the literature a vast amount of compounds is described for lowering surface tension, see for instance EP-A-1216842 and also herein below. It is not very practical to add these compounds to the pre-dampening solution, as these compounds generally have a limited solubility in water and upon addition to and mixing with the pre-dampening solution a non homogeneous mixture can form, consisting of a solvent- and a water based phase or often a precipitate forms. Furthermore the densities of most of these compounds come near to the density of water and the used amounts are therefore difficult to control using the density measuring means as mentioned above.

Furthermore, although in the method of this invention it is not necessary to include IPA, it cannot be denied however that when it comes to printing results including IPA in the dampening water solutions for some reason very often if not always gives superior results.

The present inventors now surprisingly have found, that mixtures of above mentioned surface tension lowering compounds with IPA will solve completely the earlier mentioned disadvantages.

Furthermore, surprisingly it was found that the total amount of organic solvents could be reduced by 50% or more by using IPA and an organic solvent having a surface tension of less than 40 mN/m. In particular if one of the compounds (or mixture of compounds) mentioned above was used together with IPA, the total amount of organic solvents could be reduced with 50% or more. Thus in one embodiment one of said at least two organic solvents is iso-propyl alcohol.

In one method the IPA and one or more of the organic solvents mentioned above are added separately to the pre-dampening solution, where the amount of organic solvents used is between 1 and 8 vol% and preferably between 2 and 6 vol%. The density of the dampening water solutions formed can be controlled in the same way as done when IPA is used alone, meaning by density control. The amount of IPA used as a function of the total amount of used organic solvents is between 20 and 90 vol%. For ease of use, IPA amounts over 50 vol% are preferred. It was found, that one of the methods to determine the optimal amount of organic liquids used is the measurement of the surface tension. It was observed, that in general the surface tension of the resulting dampening water should be below 50 mN/m in order to have homogeneous and continuous moisturing of the dampening rollers and of the lithographic printing plate. Such a low surface tension will only be obtained upon the use of low surface tension organic solvents, some of which have been indicated above. These solvents are characterised by their surface tension being lower than 40 mN/m. Preferred solvents have a surface tension below 35 mN/m, and even more preferred organic solvents have a surface tension of below 30 mN/m. The use of these low surface tension organic solvents preferably together with IPA will give a dampening water solution with a surface tension below 50mN/m. Thus, in one embodiment the concentration of iso-propyl alcohol in said at least two organic solvents is between 20 and 90 wt%, and in a further embodiment further organic solvents of said at least two organic solvents have a surface tension below 40 mN/m, preferably below 35 mN/m and even more preferably below 30 mN/m.

As already mentioned in one embodiment the organic solvents are premixed before addition to the pre-dampening solution. This method has various advantages, as only one liquid needs to be metered, which results in a stable dampening water solution, where the possibility of formation of separate phases by mixing the pre-mix of low surface tension organic solvents with the pre-dampening solution is significantly reduced. The mixture of organic liquids preferably contains between 20 and 90% of IPA and preferably IPA is used in amounts over 50 vol% in the organic liquid mixture. The density of the dampening water solutions formed can be controlled in the same way as done when IPA is used alone, meaning by density control. In order to have an accurate controlling method, the difference in density between the pre-dampening solution and the mixture of organic solvents should be as close as possible to the difference using IPA alone. In terms of accuracy we have observed, that an acceptable accuracy can be obtained when the difference in density between the organic solvent mixture and the pre-dampening solution is at least 0.1 g/cm³, more preferably more than 0.13 g/cm³ and most preferably more than 0.17 g/cm³. Another method of characterising the preferred densities of the organic solvent mixture is to compare this density with the density of pure IPA. For a skilled person in the art in terms of dosing accuracy the use of IPA is preferred. IPA has a density of 0.78 g/cm ³(25°C). A preferred mixture of low surface tension organic solvents should than have a density as close as possible to the density of pure IPA. As a rule of thumb it can be stated, that when this difference is within 12% acceptable dosing accuracy can be obtained This difference is calculated by dividing the density of the mixture by the density of IPA subtracting 1 and multiplying the outcome with 100%. Thus in a further aspect the invention concerns an organic solvent mixture suitable for mixing with a pre-dampening solution just before and/or during the lithographic printing process, wherein the solvent mixture comprises isopropyl alcohol and at least one further organic solvent having a surface tension below 40 mN/m, preferably below 35 mN/m and even more preferably below 30mN/m. In a preferred embodiment the concentration of iso-propyl alcohol is between 20 and 90 vol %. Also it is preferred that the organic solvent mixture according to the invention has a density below 0.9 g/cm³, preferably below 0.83 g/cm³.

The concentration of the organic liquids in the resulting dampening water solution in this method is between 1 and 8 vol% and preferably between 2 and 6 vol%. The surface tension of the dampening water solution, is preferably below 50 mN/m.

Suitable (further) organic solvents for use in the method of the invention described above and in the organic solvent mixture of the invention described above are selected from the group consisting of ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, ethylene glycol monoisobutyl ether, diethylene glycol monoisobutyl ether, triethylene glycol monoisobutyl ether, ethylene glycol monopropyl ether, diethylene glycol monopropyl ether, triethylene glycol monopropyl ether, ethylene glycol mono-tertiary-butyl ether, diethylene glycol mono-tertiary-butyl ether, triethylene glycol mono-tertiary-butyl ether, ethylene glycol monohexyl ether, diethylene glycol monohexyl ether, triethylene glycol monohexyl ether, ethylene glycol monophenyl ether, diethylene glycol monophenyl ether, triethylene glycol monophenyl etherpropylene glycol monomethyl ether, dipropylene glycol monomethyl ether, tripropylene glycol monomethyl ether, propylene glycol monoethyl ether, dipropylene glycol monoethyl ether, tripropylene glycol monoethyl ether, tetrapropylene glycol monoethyl ether, propylene glycol monopropyl ether, dipropylene glycol monopropyl ether, tripropylene glycol monopropyl ether, propylene glycol monoisopropyl ether, dipropylene glycol monoisopropyl ether, tripropylene glycol monoisopropyl ether, propylene glycol monobutyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monobutyl ether, dipropylene glycol monoisobutyl ether, tripropylene glycol monoisobutyl ether, propylene glycol mono-tertiary-butyl ether, dipropylene glycol mono-tertiary-butyl ether, tripropylene glycol mono-tertiary-butyl ether,2- ethyl- 1,3-hexanediol 2-methyl-2,4-pentanediol, 2,5-dimethylehexane-2,5-diol, 2,5-dimethyl-3-hexyne-2,5-diol and mixtures thereof.

The (further) organic solvents, which gave the best behaviour when used in the method of the invention described above and in the organic solvent mixture of the invention described above are selected from the group consisting of propylene glycol monopropyl ether, dipropylene glycol monopropyl ether, tripropylene glycol monopropyl ether, propylene glycol monoisopropyl ether, dipropylene glycol monoisopropyl ether, tripropylene glycol monoisopropyl ether, propylene glycol monobutyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monobutyl ether, dipropylene glycol monoisobutyl ether, tripropylene glycol monoisobutyl ether, propylene glycol mono-tertiary-butyl ether, dipropylene glycol mono-tertiary-butyl ether, tripropylene glycol mono-tertiary-butyl ether,2- ethyl- 1,3-hexanediol, 2-methyl-2,4-pentanediol, 2,5-dimethylhexane-2,5-diol, 2,5-dimethyl-3-hexyne-2,5-diol and mixtures thereof. Using these organic solvents in the methods described above resulted in the best dampening behaviour and using these solvents in combination with IPA the amount of total organic liquids used in the dampening water solution can be reduced most significantly.

In the combination with the organic solvents mentioned above, the amount of IPA used in the dampening water can be reduced significantly as described above, but also other low density solvents can be mixed with the organic liquids mentioned above. Suitable organic liquids as replacement of IPA can be amongst others methanol, ethanol, n-propanol, in general organic liquids having a density below 0.82 g/cm³ and which are completely mixable with water and the organic solvents mentioned above.

The pre-dampening water concentrate can comprise amongst others

• a further agent for improving the wetting,
• a hydrophilic polymeric compound
• a pH-buffering agent
• an odor-masking agent
• an antiseptic agent
• a chelating agent,
• a coloring agent,
• an anti-corrosion agent,
• an anti-foaming agent).

Suitable agents can be found amongst others in EP-A-1216842 and US-A-5637444 and are included herewith as reference.

For usage in the lithographic printing process the various components which should form the dampening water solution are conveniently combined in a kit of parts. The parts combined in the kit are mixed with water after which a ready to use dampening water solution is obtained. The kit of the present invention comprises a dampening solution concentrate and a mixture of organic solvents as described above, i.e a mixture comprising at least IPA, which mixture has a surface tension of 40 mN/m, preferably 35 mN/m and more preferably of 30 mN/m or less and preferably a density of 0.90 g/cm³, preferably 0.83 g/cm³ or less. Suitably the organic solvent mixture in the kit comprises as a further solvent at least one of the solvents mentioned above. The kit described above gives, when properly mixed with water, excellent dampening water solutions having a low IPA content for use in the lithographic printing process.

In an embodiment the kit of parts further comprises instructions how to use the components in the kit, preferably in accordance with the method as described herein above, i.e. instructions to first mix the dampening solution concentrate with a suitable amount of water to form a pre-dampening solution and then to add to this pre-dampening solution the mixture of organic solvents. Another embodiment of this invention is a kit of parts comprising a mixture of organic solvents comprising at least IPA, which mixture has a density of 0.90 g/cm³, preferably 0.83 g/cm³ or less and a surface tension of 40 mN/m, preferably 35 mN/m and more preferably of 30 mN/m or less and instructions how to use the mixture of organic solvents, preferably in accordance with the method as described herein above, i.e. instructions to first mix a dampening solution concentrate with a suitable amount of water to form a pre-dampening solution and then to add to this pre-dampening solution the mixture of organic solvents. It is to be understood however that instructions to first mix a dampening solution concentrate with the mixture of organic solvents comprising at least IPA as defined above and then to add water to form a ready to use dampening water solution is also encompassed by this invention.

Dampening solution concentrates that can suitably be used in this invention are commercially available. Examples of suitable dampening solution concentrates are Acedin 2004, Dynafount 426 from Bousfield, Dampstar 4.6/Dampstar 5.8 from Solco, Combifix and Hydrofix from Hostmann and Steinberg, Vegra Red and Blue from Vegra, and the like.

A further aspect of the invention is the use of the organic solvent mixture as defined above or the kit of parts defined above for the preparation of a ready to use dampening water solution for use in a lithographic printing process.

Also an aspect of the present invention is a dampening water solution for use in a lithographic printing process having a surface tension of less than 50 mN/m and comprising water, for a dampening water solution customary additives and 0.2-7.2 vol% iso-propylalcohol and at least one further organic solvent having a surface tension below 40 mN/m.

Measurement of the surface tension:
The surface tension can be measured in various ways which are described in the literature for example in The Physical Chemistry of Surfaces by A. W. Adamson; John Wiley & Sons 1982 pg 4-46.
In the present invention the surface tension is measured according to the ring method which is generally named the Du Nouy ring method which is one of the most easy and fastest methods to use. The Du Nouy ring method measures the force required to pull a small platinum ring through the surface of the liquid.
The force to pull a ring of radius R through the surface may be measured by a balance and may be related to the surface tension Y by the equation $$\mathrm{F}=2\left(2\mathrm{\pi }\mathrm{R}\right)\mathrm{Y}$$
Y= surface tension in mN/m, or dynes/cm

A Roland 700 4 color sheetfed press, used for high quality medicine box printing, printed with a mixture in water of 9 vol% of IPA and vol3% of Dampstar 5.8 as dampening additive, on conventional positive printing plates. Dosing of the IPA and Dampstar 5.8 as pre-dampening concentrate was done by two different dosing pumps directly into the dampening water.
In this process, the IPA was replaced by 3 vol% of a mixture of 75 vol% of IPA and 25vol% of propyleneglycol-n-butylether.
Dosing of the solvent mixture was done with the dosing pump earlier used for the IPA, also directly into the dampening water, and monitored by a density measurement device from Technotrans, called Combicontrol, which kept the concentration within the required limits. Dissolution of the solvent mixture in the pre dampening solution went smoothly without the need for extra stirring. The typical "oily" droplets indicating a separate solvent phase of non-dissolved solvent on the surface where not found, indicating complete dissolution.
The density of the mixture of 25 vol % propyleneglycol-n-nbutylether and 75 vol % of IPA is 0.81 g/cm³, compared to 0.78 g/cm³ (25°C)of IPA alone. This gives an acceptable difference of 2.5%. Dosing of the Dampstar 5.8, as the dampening concentrate was done at a constant dosage of 3 vol% and was controlled by conductivity measurement, well known in the art as a standard way to control the dosage of low- or non- solvent containing dampening concentrates in aqueous solution.
Print quality, resolution in micro lines, 12 um not broken, was excellent, dot gain was even better than with the higher dosage of IPA.
Surface tension of the dampening water solution was 45.6 mN/m and showed a uniform wetting of the rubber dampening rollers.

A KBA Rapida 4 color, format 3B, printing with 10 vol% of IPA and 3 vol % of Acedin 2004 as pre-dampening additive, was changed to 4 vol% of the mixture of 60 vol% IPA and 40 vol% propyleneglycol-n-propylether. In this mixture the difference in density measurement compared to pure IPA is 3.8%, which is acceptable. Dissolution of the solvent mixture in the pre dampening solution went equally well as described in example 1.
Density control was done here by Baldwin Balcontrol BC713. Plates used were Kodak Brillia CTP plates.
Visual observation of the rubber dampening rollers showed an even and undisturbed wetting of the surface at low and high printing speeds. Surface tension of the dampening water solution was 42.2 mN/m

On a KBA 408 heatset web printing press, printing with 3 vol% of Acedin Heat 2060 as the pre-dampening additive and 9 vol% of IPA was done on LWC paper. Plates were conventional baked positive plates. Baked plates are plates which are extra hardened by a heat treatment in order to render them resistant to solvents. However this only is done on web presses.
The 9 vol% of IPA was replaced here by 3 vol% of a mixture of 50 vol% of tripropyleneglycol-mono-methylether and 50 vol% of IPA. Dosing was done by the regular dosing pump, and density control was done manually by means of an areometer The density of this solvent mixture was 0.88 g/cm3, introducing a measurement difference of 11.4% compared to pure IPA. This is near the defined limit, but in the art of printing still acceptable. Although a higher content of solvent other than IPA was used in this mixture, dissolution in the pre dampening solution was good..
Printing went well, printers did not notice any difference as dampening and ink settings remained unchanged and no measurable densitometric difference could be detected. Surface tension of the dampening water solution was 41.6 mN/m and gave also at the highest printing speed a uniform wetting of the dampening rubber rollers. Various printing jobs with run-lengths from 25.000 to 85.000 copies were produced during one day. Average print speed was between 25.000 and 30.000 copies per hour.

In order to demonstrate the importance of the low surface tension in the mixture of IPA and the described solvents, a printing test was done using a mixture of 75 vol% of IPA and 25 vol% of 1,2- ethyleneglycol. This mixture has a surface tension when diluted at 3 vol% in water of 57,2 dynes/cm, instead of 45,6 dynes/cm of the mixture described in example 1. The surface tension of 1,2 ethyleneglycol as pure liquid is 47.99mN/m .
A printing test on the KBA Rapida from example 2 using 3 vol% of Acedin 2004 as the dampening solution concentrate and 3 vol% of the above described mixture did not result in a good printing result.
As the dampening rollers were not sufficiently well wetted by the dampening water solution, the dampening roller speed had to be increased from 40 to 85 on the dial on printing press console in order to increase the dampening water transport. This resulted in an uneven dampening water distribution across the dampening rollers an excess ink emulsification, especially at the edges of the ink form roller. This resulted in an unstable ink water equilibrium and unacceptable variation of print quality from print to print.

Low dampening roller speed was obtained again and even dampening water distribution when the dosage of the solvent mixture was increased from 3vol% to 9vol%, resulting in a decrease of the surface tension of the dampening water solution from 57,2, dynes/cm to 42,0 dynes/cm. This restored the required stable ink water equilibrium, necessary for good, stable high quality printing having the disadvantage of the usage of a high amount of organic solvents.
This example demonstrates the need for a sufficiently low surface tension of the dampening water for good and stable printing, and the principle of this invention.