Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
1999-12-15
2001-06-05
Chu, John S. (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S157000, C430S163000, C430S175000
Reexamination Certificate
active
06242159
ABSTRACT:
This invention relates to the production of so-called waterless lithographic printing forms, especially to a method for preparing an imaged water-less lithographic printing form on press using a digitally controlled laser output and to imageable compositions for use thereon.
Currently the commonest method of preparing a lithographic printing form is to image a photosensitive lithographic printing form precursor using an image mask, such as a photographic negative, and to prepare the printing form therefrom using aqueous developing solution. This procedure is time consuming and requires facilities and equipment to support the necessary chemistry.
Thus recently, various methods have been proposed for preparing lithographic printing forms on the press which is to be used to produce prints from the printing form. These methods prepare the image using a digitally controlled laser image head. As described in E.P.A. 580393 such methods include ink-jet methods digitally controlled, spark-discharge methods and the production of electromagnetic radiation pulses that create chemicals changes of the printing form blank. Also etching methods have been described as well as blank printing forms which are ablated by the laser to form an ink-receptive image.
In addition, lithography may divided into two classes:
1) a process which requires dampening water which is fed to the non-image areas of a printing form on a press and forms a water film that acts as an ink repellent layer (this is the so-called fount solution), and
2) a process which requires no fount solution, so-called water-less lithography or driography. Most lithographic printing form presses at present are of the first type and require a fount-solution during the printing process. However, lithographic printing forms of this type suffer from a number of disadvantages. Some of these are:-
a) Adjustment of the proper ink-water balance during press operation is difficult and requires great experience. If the correct ink-water balance is not achieved, scumming is occasioned when the printed ink image extends into the non-image areas ruining the printed image.
b) Adjustment of the ink-water balance at start-up or re-start is particularly difficult and cannot be stabilised until a large number of sheets have been printed, thus incurring waste.
c) The ink tends to become emulsified which leads to poor adherence of the ink on to the printing form which causes problems in colour reproduction and dot reproduction.
d) The printing process has to be provided with a dampening system, thus increasing its size and complexity. These dampening solutions contain volatile organic compounds.
e) The printing form care chemistry and fount solutions require careful control and selection Further, printing form cleaners contain significant levels of solvent which is not desirable.
However with water-less printing (or driographic printing forms) in which the ink-releasing layer is for example, a cured silicone layer, there is no scumming and clearer images can be produced. Very often water-less printing forms comprise a base material, for example aluminium plate, on which a photosensitive layer is coated, on this photosensitive layer is coated a silicone layer. After imagewise exposure and aqueous development in which selected areas of the photosensitive composition are altered, the overlying silicone layer is removed and the printing forms inked up. The ink adheres only to the those areas of the printing forms not covered by the silicone remaining after development Thus the printing form can be used without the need to employ a fount solution.
However, water-less printing forms are often of complex design (in practice it has proved difficult to get the silicone layer composition to adhere to the photosensitive layer), which increases their cost. Moreover they require a dedicated processing system.
In spite of the idea of water-less printing forms having been described in patent specifications for at least fifteen years, very little has been done to commercialise the idea and water-less printing forms which have been and are being sold are more expensive than the conventional printing forms which require a fount solution.
In U.S. Pat. No. 3,997,349 there is described a novel type of water-less lithographic printing form which requires no post-exposure processing. In this patent specification there is described a lithographic printing form precursor which comprises coated on a support a diazonium salt formed from an aromatic diazonium compound and an abhesive perfluoroaliphatic acid or salt thereof, said acid containing at least three carbon atoms, said coating being capable of conversion from an ink abhesive surface to an ink adhesive surface merely upon exposure of said coating to an imagewise pattern of actinic radiation without further treatment thereof the dry coating weight of said coating being sufficient to provide free abhesive acid only in exposed areas which can be displaced by driographic ink whereby said ink deposits in only exposed areas in a negative acting driographic system.
It is suggested in U.S. Pat. No. 3,997,349 that upon light exposure of the light-sensitive diazonium salt to actinic radiation, the diazonium bond between the anionic abhesive moiety and the aromatic moiety is broken. There is probably some nitrogen gas produced, along with some complex organic matter and free abhesive acid. While there may be a minor amount of chemical coupling between the free abhesive acid and the ring structure of the aromatic moiety, the major effect of exposure is to change the composition from a hard crystalline solid with a high melting point to a two-phase material containing a low molecular weight abhesive component overlying a hard decomposition product or organic cap of the aromatic diazonium nuclei. This hard decomposition product or organic cap from the diazonium nuclei is adhesive, i.e. ink receptive. The free abhesive acid thought to be formed in accordance with this exposure mechanism overlies the ink receptive organic cap and can be easily displaced by driographic ink upon inking of the surface.
We have now discovered that the preferred diazorium compounds as set forth in U.S. Pat. No. 3,997,349 are useful in a heat mode imaging process to prepare a waterless lithographic printing form.
In addition, we have discovered novel methods of preparing a water-less printing form using the preferred diazonium compounds as set forth in U.S. Pat. No. 3,997,349, which are exposed in a heat mode imaging process using an imaging head which is attached to the press.
According to one aspect of the present invention, there is provided a method of preparing a water-less lithographic printing form using a precursor which includes a support having a coating thereon comprising a diazo salt formed from an aromatic diazonium compound and an abhesive counter anionic moiety, the method comprising heat mode imaging the precursor and processing the imaged precursor on press by the application of printing ink to remove the abhesive products of the imaging so that areas of the precursor which have been imaged are ink-accepting.
Preferably the abhesive counter anionic moiety is a perfluoroaliphatic acid. Another useful abhesive counter anionic moiety is a trimethylsilanolate.
The preferred aromatic diazonium compound of use in the present invention has the formula:-
The preferred diazo salt of use in the present invention and which is used in the examples has the formula:-
It is hereinafter referred to as Diazo A.
The support may be a material suitable for use on a waterless lithographic press. The base which can be used as the support is usefully an aluminium base which has undergone the usual treatments well known in the lithographic art for enabling a radiation sensitive composition to be coated thereon. Other suitable metal supports are chromium or steel.
Another base material which may be used in the method of the present invention is a plastics material base or a treated paper base as used in the photographic industry. A particularly useful plastics materia
Firth Deborah Jane
Firth Herbert Stuart
McCullough Christopher David
Ray Kevin Barry
Spowage Mark John
Baker & Botts L.L.P.
Chu John S.
Firth Herbert Stuart
Kodak Polychrome Graphics LLC
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