Compositions: coating or plastic – Coating or plastic compositions – Coating repellent
Reexamination Certificate
1996-07-12
2002-08-20
Koslow, C. Melissa (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Coating repellent
C101S450100
Reexamination Certificate
active
06436176
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a concentrated dampening solution suitable after dilution for printing with a lithographic printing plate obtained according to the silver salt diffusion transfer process.
Furthermore the present invention relates to a method for printing by using said dampening solution and a lithographic printing plate obtained according to the silver salt diffusion transfer process.
BACKGROUND OF THE INVENTION
Lithographic printing is carried out by feeding both water and oleophilic colored ink to the printing plate surface to allow the image areas to receive preferentially the colored ink and the non-image areas preferentially water and then transferring the ink deposited on image areas onto a substrate such as paper for example. Therefore, in order to obtain a print of good quality, it is necessary that oleophilicity of the image areas and hydrophilicity of the non-image areas are both strong enough so that when water and ink are applied, the image areas may receive sufficient amount of ink while the non-image areas may completely repel the ink. For this purpose, various methods for rendering the non-image areas insensitive to oil have been studied according to the type of printing plate materials and the printing methods.
That is, hitherto, as lithographic printing plate materials, there are presensitized plates (PS plates) with a support of metals such as aluminum, plate materials obtained by electrophotographic process (such as electro fax), so called master papers comprising a paper as substrate coated with a pigment together with a binder and photographic printing plate materials using silver salts as disclosed in Japanese Patent Kokoku No. 48-30562. Plate making methods and printing methods using solutions to render the non-image areas insensitive to oil and dampening solutions suitable for each of these materials have been studied and developed.
Lithographic printing plates which utilize transfer silver images obtained by silver complex diffusion transfer process as ink receptive areas are disclosed in Japanese Patent Kokoku No. 48-30562 and Japanese Patent Kokai Nos. 53-21602, 54-103104 and 56-9750 and are well known.
The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book “Photographic Silver Halide Diffusion Processes” by AndréRott and Edith Weyde—The Focal Press—London and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values (“DTR-image”) with respect to the black silver image obtained in the exposed areas of the photographic material.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background. For example, typical lithographic printing plates are disclosed e.g. EP-A-423399 and EP-A-410500.
The DTR-image can be formed in the image-receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR-element) or in the image-receiving layer of a so-called single-support element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image-receiving layer in water permeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.
According to the preferred embodiment of the mono-sheet DTR-offset printing plate, disclosed in e.g. U.S. Pat. No. 4,722,535 and GB-P-1,241,661, a flexible support e.g. of paper or of an organic resin is provided in the order given with a silver halide emulsion layer and a layer containing physical development nuclei serving as the image-receiving layer. After information-wise exposure and development the imaged element is used as a printing plate without the removal of the now-useless emulsion layers.
Said type of mono-sheet DTR offset printing plates is not compatible with other types of printing plates with regard to dampening solutions. In order that said mono-sheet DTR offset printing plates show no ink acceptance in the non-printing areas (no staining), use should be made of special dampening solutions containing a sizable amount of a transparent pigment usually colloidal siliciumoxide as disclosed e.g. in U.S. Pat. No. 3,829,319, U.S. Pat. No. 4,238,279 and EP-A 304.662.
For economical reasons (less packaging material, less storage space, a smaller volume to transport) practically all dampening solutions, including the above mentioned dampening solutions are sold in a concentrated form and only diluted at the moment they are used on the printing press.
Said dampening solutions should have a pH between 3 and 6. This requirement stems from the fact that otherwise said dampening solutions are not compatible with the usual lithographic inks. To maintain the required pH, said dampening solutions has to comprise a buffer because otherwise the pH will vary due to different causes e.g. the dilution with tap water, the contact with the usually alkaline printing paper etc.
However, the above mentioned concentrated dampening solutions show a lack of shelf life due to the presence of this transparent pigment in said solutions having typically a pH between 3 and 6. Indeed, the lithographic active transparent pigment, mostly colloidal silica is not stable at said pH but is only stable at an alkaline pH of around 9 to 10. Colloidal silica which is stable at a pH below 7 is not lithographic active.
Use of a dampening solution which is not stable results in a cloudy outlook of the undiluted as well as of the diluted dampening solution. Furthermore, this instability may cause a soiling of the dampening unit of the printing apparatus. Possibly, other parts of the press may also get soiled in due time. When a dampening circulation system is used, which delivers dampening solution to one or several presses, this system is also very prone to get soiled. This soiling of the dampening unit or the dampening circulation system results in the need for a more frequent cleaning of said units (a cumbersome and time-consuming operation). Apart from soiling said instability of said dampening solutions may even result in a possible staining in the non-image areas of the printed copies because less active transparent pigment is present in the dampening solution.
So there is a need for a concentrated dampening solution containing silica which is stable at a pH below 7 and so yielding a stable dampening solution but which silica is lithographic active.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a stable concentrated dampening solution having lithographic activity for printing with a lithographic printing plate obtained according to the DTR-process.
It is a further object of the present invention to provide a method for lithographic printing using the above mentioned concentrated dampening solution and a lithographic printing plate obtained according to the DTR-process.
Still further objects of the present invention will become clear from the description hereinafter.
According to the invention there is provided a concentrated dampening solution for use in a lithographic printing process having a pH between 3 and 6 and comprising a water-soluble organic solvent, a phosphate salt and a transparent pigment, characterized in that said transparent pigment is a modified silica in which the silica particles have a number average size of 0.003 to 0.100 &mgr;m and in which the silica particles are coated
Mortelmans Paul
Van Hunsel Johan
Agfa-Gevaert
Breiner & Breiner L.L.C.
Koslow C. Melissa
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