DE2125014A1 - Hydrophilizing agent for electrostatically obtained offset printing plates - Google Patents

Description

Hydrophilizing agent for electrostatically obtained offset printing plates.

The invention relates to the production of copies or prints by electrographic processes using an electrostatic offset or planographic printing plate that a coating of photoconductive zinc oxide or other photoconductive finely divided material within an organic having resinous binder. With the agent according to the invention, the surface of the information-carrying Printing plate insoluble in water in the non-image areas, repellent for oleophilic printing inks and hydrophilic made.

It is known (US Pat. Nos. 2,987,395 and 3,018,772) to produce a film suitable as a printing plate made of a metal sheet, a plastic film or paper, this film material having a coating of photoconductive Has zinc oxide in a resin binder. This layer has the necessary photoconductive properties for

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Development of an electrostatic latent image obtained by electrostatic methods. The development is carried out in this case with a water-repellent and ink-accepting developer, so that the printed surface of a treatment must be subjected to the non-image areas from their oleophilic state to a hydrophilic state - that is water-receptive and ink-repellent - to convert so that _t the information-bearing printing plate can be used in the context of offset or planographic printing for the production of printed products.

So far one has used such hydrophilization solutions that either only potassium ferrocyanide or preferably in conjunction with a humectant such as glycerin or glycol (U.S. Patent 5 oo1 872) · These known agents often contained buffering substances to adjust the corresponding pH like primary ammonium phosphate, sodium phosphate or sulfuric acid.

Is has been found to be present in such solutions the ferroeyanide rapidly decomposes to form the unstable ferricyanide and hydronium ions. The rate of decomposition is determined by the action of light and / or Air increases · The extent of decomposition is further accelerated by higher acidity. In the case of hydrophilization the acidity required for the surfaces in question, the decomposition is already noticeable Reason it is desirable to use a hydrophilizing solution with a pH value of at most 5, preferably between 4 and 4.5, available · The decomposition is accompanied by a rapid and permanent darkening of the solution and precipitation insoluble substances.

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As part of the application, the hydrophilization solution applied in a thin layer to the information-bearing surface of the printing film in order to remove the non-image areas of to convert olyophilic to hydrophilic. With this thin application, the decomposition takes place even faster and that during the decomposition The products formed have a very detrimental effect on the hydrophilization and the shelf life of those that already carry information Printing film off.

One of the main problems with such electrostatic printing films is that they are flawless to manufacture With information provided printing foils, extremely strict process conditions must be adhered to. the Instability of the hydrophilization solution is one of the essential factors for the limited applicability of the electrostatic printing foils to obtain a wide variety of high quality prints.

The invention now relates to a hydrophilization solution for electrostatic printing plates or foils that do not show rapid decomposition during storage and use and thus the use of these plates or foils is permitted to a far greater extent. The solution according to the invention remains effective Relatively stable level, so that you get consistent results in image development and reproduction Production of high quality prints. The solution according to the invention remains stable even when applied in a thin layer on the information-bearing film surface, reducing the quality of the prints and the storage time of the information-bearing Foil is improved.

According to an older proposal, the hydrophilization solution contains in addition to potassium ferrocyanide, a chelating agent that allows the iron (H) and iron (IH) ions to be trapped without removing them from solution. That way

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inactivated ions are maintained and in equilibrium conditions held. It can be assumed that this delays the continued decomposition of the ferrocyanide will.

It was found that an improvement in the stability of the hydrophilization solution in an acidic / aqueous medium can be achieved in that it also contains alkali metal sulfite such as potassium, sodium or ammonium sulfite.

Satisfactory results are obtained with proportions of 0.1 to 5% by weight of sulfite in solution, preferred Sulphite ranges are between 0.25 and -2.5 wt.

The ferrocyanide can be the ammonium or alkali salt, preferably the potassium salt, use in amounts between 0.5 and 5 wt .-%, preferably 1 to 2.5 wt .-%.

As mentioned above, the hydrophilizing solution can optionally contain a buffer substance such as primary ammonium phosphate, to adjust the pH value below 5, preferably to 4 to 4.5, included in order to an acidic environment guarantee. Phosphoric acid, for example, is suitable for this.

The mechanism on which the stability improvement of the hydrophilizing solution is based has not yet been clarified. It is believed, however, that the sulphite causes the conversion of the perrocyanide to the much more unstable one Ferrieiyanid delays and with it the decomposition and that Decreasing the uselessness of the solution. Whatever the cause, it is known that the presence of sulfite ions that were introduced into the solution as alkali sulfites, the stability of the hydrophilization solution improved to such an extent that it can be exposed to sunlight for long periods of time without rendering it ineffective will.

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The invention is illustrated in more detail by the following examples. The amounts given are% by weight.

Example 1;

Aqueous solution of 2 $ potassium ferrocyanide, 8 $ primary ammonium phosphate, 0.1 $ disodium ethylenediaminetetraacetate and 1 ia potassium sulfite in 88.9 $ water was adjusted to a pH of 4.5 with the aid of phosphoric acid.

Example 2 :

A solution of 2 $> potassium ferrocyanide, 8 $ primary ammonium phosphate, 0.5 ^ potassium sulfite in 89.5 $> water was adjusted to a pH of 4 with the help of phosphoric acid. Even after exposure to light for 30 hours, the solution was clear and pale yellow and did not show any ferricyanide ions in the analysis.

Example 3:

A solution of 1.5 g potassium ferrocyanide, 6 g primary ammonium phosphate, 0.1 g of disodium ethylenediamine tetraacetate and 1.5 g of sodium sulfite in 91.9 g of water was treated with phosphoric acid adjusted to a pH between 4 and 4.55. After 30 hours of exposure to light, the solution was still clear and weak yellow and when analyzed gave negative values for ferricyanide.

The solutions of Examples 1 to 3 were wiped onto the layer of photoconductive zinc oxide on the planographic printing film after developing the electrostatic image with the aid of a liquid toner containing hydrophobic particles separates in the image areas. After applying the hydrophilizing solution the information-bearing printing film was mounted on the offset printing unit. So it turned out high quality prints after the printing surface first with a water-repellent wiping agent and then with provided with an oily printing ink and this was transferred to the printing medium by the printing process.

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The solutions of Examples 1 ″ to 3 had good stability and storage time, one could still see a perfect conversion of the non-image areas into the hydrophilic ones Reached state after the solution has been exposed to sunlight for 30 hours.

Comparison;

A solution of 2 g of potassium ferrocyanide and 8, ο prim - Ammonium phosphate in 90 g of water was adjusted to a pH of 4-3 with phosphoric acid. After 3o h in the light the solution was dark brown and showed considerable amounts of precipitate. Analysis revealed the presence of Ferricyanide. The hydrophilization was with this solution not satisfactory.

Even if this solution is adjusted to a pH of 401, it was also dark brown after 30 hours Coloration and a considerable precipitate. The analysis for ferricyanide was positive and the hydrophilization not satisfactory.

But even when this solution was adjusted to a pH of 4.9, the solution turned orange-yellow and showed a low pH Precipitation after 3 hours. The analysis for ferric anide was positive, the hydrophilization inadequate.

The above comparison shows the significantly better properties of the solution according to the invention with regard to stability and shelf life, from which it follows that such printing films can be used on a much larger scale and thereby obtain a large number of high quality prints.

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Claims (5)

Claims Stable hydropolisation solution for printing plates or foils which are electrostatically provided with images and text for offset ^ - or flat printing consisting of a carrier and a resin-bonded layer of a photoconductive material for hydrophilizing the non-image areas in the form of a aqueous solution of a perrocyanide characterized in that it is a sulfite, preferably a Contains dissolved alkali or ammonium sulfite. 2. hydrophilization solution according to claim 1, characterized in that the sulfite content 0.1 to 5 wt .-%, preferably 0.25 to 2.5 wt .-%. 3. hydrophilizing solution according to claim 1 or 2, characterized in that the content of ferrocyanide in the form of alkali ferrocyanide is 0.5 to 5 wt. 4-. Hydrophilizing solution according to claims 1 to 3 characterized characterized in that it has a pH value below 5, preferably between 4 and 4.5. 5. hydrophilization solution according to claim 4, characterized in that the pH value with Phos phosphoric acid is adjusted. , 81 / IV _ ,. _f ■ - "■" ■ 109849/1745