Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
2000-06-07
2002-10-01
Chu, John S. (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S011000, C430S018000, C430S175000, C101S456000, C101S457000, C101S467000
Reexamination Certificate
active
06458511
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a radiation-sensitive composition, a thermally imageable positive-working lithographic printing plate precursor having a radiation-sensitive layer obtained by coating a substrate with the radiation-sensitive coating, a method for imaging such a precursor and a lithographic printing plate obtained therefrom. More particularly, the radiation-sensitive composition comprises at least one component capable of absorbing IR radiation, and a diazonium polycondensation product soluble in an organic solvent, with the proviso that the composition does not contain compounds capable of reacting with decomposition products produced from IR radiation exposure of the diazonium polycondensation product. The radiation-sensitive composition may optionally include a polymeric binder of the type which are well known to be useful for conventional negative-working printing plates.
2. Background Information
Today, the demands are quite high on radiation-sensitive compositions which are particularly useable for high-capacity printing plates.
For improving the properties of radiation-sensitive compositions and thus also the corresponding printing plates, basically two methods have been pursued. One of these methods relates to the improvement of the properties of the radiation-sensitive components in the compositions (e.g. diazonium polycondensation products, photopolymers etc.). The other method relates to the detection of new polymeric compounds (“binders”), which are intended to control the physical properties of the radiation-sensitive layers. Particularly the latter method is of crucial importance for printing plates since behavior in the developing and printing processes (such as developability, ink accepting capacity, scratch resistance, and print run stability) is decisively influenced by the polymeric binders. Moreover, such polymeric compounds have a strong influence on the storage stability and radiation-sensitivity of the materials.
In negative-working printing plates, radiation-sensitive compositions are used such that, after image-wise exposure, the exposed areas are cured. During the developing step, only the non-exposed areas are separated from the substrate. In positive-working printing plates, on the other hand, photosensitive compositions are used, the exposed areas of which dissolve faster in a given developer than the non-exposed areas. This process is called photosolubilization. A great number of positive-working radiation-sensitive compositions having quinone diazides and phenolic resins as main components exist.
The latest developments in the field of printing plates deal with radiation-sensitive compositions which can be imaged by lasers. In this type of imaging, films as intermediate information carrier may be omitted since lasers can be controlled by computers.
Plates which are imageable by IR lasers are known from EP-A-0 672 544, EP-A-0 672 954, U.S. Pat. Nos. 5,491,046 and 5,919,601 and EP-A-0 819 985. These plates are negative-working and require a preheating step after imaging whereby the image area of the layer, however, is cross-linked only to a minor extent. To comply with the highest print run demands and to exhibit sufficient resistance to pressroom chemicals, however, a further heating step (i.e. a so-called baking step) is required wherein these layers are further cross-linked. Moreover, these printing plates require the use of highly alkaline developers which are prone to reactions with atmospheric carbon dioxide, thereby necessitating the use of sealed, specialized processing equipment.
U.S. Pat. No. 5,741,619 is an example of a negative-working printing plate containing an IR sensitive composition of an acrylic resin, a diazonium compound and carbon black. However, such formulations require high energy of the IR exposure to yield an image. High energies are also needed for those compositions described in WO 98/31545.
A general disadvantage of these negative-working prior art printing plates is that the image areas are generated by IR exposure, i.e. the curing of the layer depends on the laser exposure. Therefore, variations in the IR exposure such as its intensity directly affect the image areas.
In positive-working systems, this is not the case since the laser only “writes” the background (i.e. non-image area). Regarding the radiation-sensitive compositions of such positive systems, however, there is a certain dilemma. To obtain a high number of copies, cross-linked polymers are required. However, such products are insoluble in the solvents or solvent mixtures usable for coating the plates. Thus, it may be necessary to revert to non-cross-linked or only slightly cross-linked starting products. The required cross-linking is then obtained by preheating steps which can be carried out at different stages of the plate treatment.
A positive-working system has recently been described in EP-A-0 819 980 where it is assumed that the non-image areas are formed by reacting the acid formed with carbon black. The image areas only form during a preheating step. Here, the same applies as above, i.e. the image areas have to be baked to obtain the highest possible number of copies.
Another positive-working system is described in U.S. Pat. No. 5,658,708. The required cross-linking of the layer is carried out during the drying step of the coating. For this, however, the system has to be treated for 10 minutes at 120° C. with chemical processes taking place which finally lead to cross-linking. However, the relatively long dwell times at such high temperatures are an unacceptable expenditure of time in the common fully automatic plate production line. Here, baking does not lead to an increase in the number of copies since the cross-linking is partly reversed.
Further positive-working systems are described in WO 99/11456. The imaged areas of such systems have transient solubility in aqueous alkaline developing systems.
In spite of this intensive research in the field of radiation-sensitive compositions for printing plates, all existing compositions call for improvement. Positive-working printing plates with known compositions usually require a baking step at about 250° C. for obtaining high print run stability. This is a complicated additional process step which sometimes results in warping of the aluminium substrate, “blooming out” of the image or toning problems due to the decomposition of the baking rubber necessary for this process step. Negative-working plates known in the art require a preheat step at about 135° C., where the temperature has to be maintained within a small range of about +/−5° C.
Thus, it is an object of this invention to provide thermally imageable lithographic printing plate precursors, which require less critical steps when preparing an imaged printing plate than methods known in the art and have good storage stability as well as result in imaged plates with high print run stability. It is another object of this invention to provide a radiation-sensitive composition useable on such precursors. Furthermore, it is an object of this invention to provide a method for producing imaged printing plates from such plate precursors. Still another object of this invention is to provide a lithographic printing plate obtained from such plate precursors.
SUMMARY OF THE INVENTION
The thermally imageable positive-working printing plate precursor of this invention comprises an optionally pre-treated substrate and a radiation-sensitive layer applied thereon. The radiation-sensitive layer is obtained by coating the substrate with a radiation-sensitive composition which comprises:
(i) at least one component capable of absorbing IR radiation; and
(ii) a diazonium polycondensation product soluble in at least one organic solvent and useful for conventional lithographic printing plates, with the proviso that the radiation-sensitive composition does not contain components capable of reacting with the decomposition products produced from the diazonium polycondensation product by exposure to IR
Savariar-Hauck Celin
Timpe Hans-Joachim
Wittig Tobias
Baker & Botts L.L.P.
Chu John S.
Kodak Polychrome Graphics LLC
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