Radiation imagery chemistry: process – composition – or product th – Diazo reproduction – process – composition – or product – Composition or product which contains radiation sensitive...
Reexamination Certificate
2001-10-16
2003-12-23
Chu, John S. (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Diazo reproduction, process, composition, or product
Composition or product which contains radiation sensitive...
C430S176000, C430S302000, C430S281100
Reexamination Certificate
active
06667142
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a recording material having a substrate and a negative-working, radiation-sensitive layer which contains organic coloured pigments, a diazonium salt and at least one film-forming, polymeric binder.
BACKGROUND OF THE INVENTION
Recording materials of this or a similar type are already known. Thus, EP-A 152 819 (=U.S. Pat. No. 4,631,245) describes a negative-working recording material for the production of offset printing plates, the photosensitive layer of which recording material contains a diazonium salt polycondensate, a polymeric binder containing carboxyl groups, and optionally a dye and/or coloured pigments. For example, a paste of a blue copper phthalocyanine pigment (®Hostapermblau B2G), obtainable under the name ®Renolblau B2G-HW from Clariant Deutschland GmbH, in a modified polyvinylbutyral (®Mowital B30H) is used for colouring the layer. However, the pigments produced in this manner dissolve only slowly and to a limited extent in the coating solution, which limits the pigment content and hence the subsequently achievable image contrast. By prolonging the stirring time in the mixing vessel, it is possible to achieve better mixing but undesired sludge formation may occur in the case of larger solution batches. Copper-containing pigments moreover lead to an equally undesired enrichment of copper in the aqueous baths in which the recording materials are developed after the imagewise exposure. Copper phthalocyanine pigments as well as polyvinylbutyrals also dissolve only to a limited extent in aqueous alkaline developers, so that precipitates of layer components frequently form in the case of high developer loads. The precipitates in turn can impair the operation of the developing machines as well as that of the developed plates (by the formation of redeposits).
EP-A 778 497 relates to a negative-working recording material which is suitable in particular for the production of offset printing plates. The unexposed parts of the radiation-sensitive layer of the recording material can be removed using neutral or alkaline aqueous solutions. The layer contains predispersed pigments, preferably predispersed copper phthalocyanine pigments, a diazonium compound or a combination of a photopolymerizable compound and a photoinitiator, and a polymeric binder which is soluble but at least swellable in aqueous alkaline solutions. The binder is prepared by reacting a polymer containing hydroxyl groups with an anhydride, e.g. maleic anhydride or succinic anhydride, in the presence of a catalytically active amount of a tertiary amine. Anhydride-modified polyvinylacetals are disclosed as particularly suitable binders. The acid number of the binder is in general from 5 to 80. The binder also serves for dispersing the coloured pigments. In the pigment dispersion, the binder content is from 2 to 20% by weight. In addition, the pigment dispersion comprises from 60 to 96% by weight of an aliphatic (C
1
-C
8
)alcohol, of a (C
1
-C
8
)alkylene glycol mono- or di-(C
1
-C
8
)alkyl ether (in particular 1-methoxypropan-2-ol) or of a cyclic ketone. The predispersing makes it possible to increase the content of the coloured pigments in the layer, which increases the image contrast in the developed recording material and in this way improves the visibility of the image. This measure simultaneously ensures that the layer is completely removed in the unexposed parts and already detached layer components are not redeposited, which would lead to background fogging or to scumming during subsequent printing. The predispersing is usually carried out in a ball mill. There, if an organic solvent having a relatively low boiling point is used (such as 1-methoxypropan-2-ol, which has a boiling point of from 118 to 119° C. at atmospheric pressure), then thorough cooling of the mill is therefore essential.
Attempts have also already been made to eliminate the described deficiencies of the recording materials known in the prior art. Thus, EP-A 738 931 discloses the colouring of a negative-working layer with heavy metal-free, in particular copper-free, predispersed phthalocyanine pigments. The predispersing is effected using a polyvinyl alcohol in water in the presence of a cationic surfactant. In this way, it is possible to produce stable dispersions which however can be used only in aqueous media. The pigment dispersion according to the EP-A is accordingly used in an aqueous photosensitive composition which contains a polyvinyl alcohol as a binder in addition to a water-soluble diazo resin or diazonium salt. The high content of polyvinyl alcohol in the layers thus prepared frequently leads to problems with ink acceptance during printing. Furthermore, such layers are not sufficiently resistant to the damping solution used during printing, so that only relatively short print runs can be achieved. In photosensitive mixtures which contain water-insoluble diazo or diazonium components and/or water-insoluble polymeric binders, the pigment dispersions practically cannot be used.
The non-prior-published DE-A 199 15 717 describes a recording material which is capable of substantially solving the problems described by using a coloured pigment dispersion. A copper-free phthalocyanine in an organic polymeric binder is dispersed therein. However, it has been found that the freedom from fogging which is mentioned there can be ensured only to a limited extent during the development process since in particular the developability of the coloured pigment particles of the recording material described there depends to a great extent on the drying temperatures used. This is due to the processes which take place in the wet film during the drying. At a high initial drying temperature, the kinetics of initial drying of the wet film change so that a layer having a different morphological structure forms. In particular, the coloured pigment particles are then no longer homogeneously distributed in the dried layer, which in turn means that completely fog-free development under critical conditions (less than 15 s development time, no mechanical support, for example by brushes, during development) is no longer possible. The instability of the dispersion in the coating solution during the drying process is presumably the main cause here. It is assumed that pigment particles agglomerate in this stage and form aggregates which can no longer be completely removed during the subsequent development and then give rise to fogging in the nonimage parts.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a negative-working recording material which is coloured with a coloured pigment dispersion and in which the image formed from the radiation-sensitive layer by imagewise exposure and subsequent development is intensely coloured and is distinguished from the substrate by high contrast. In particular, it should be possible to colour a nonaqueous negative-working photosensitive composition homogeneously with the coloured pigment dispersion. The dispersion should also be capable of being uniformly distributed in the nonaqueous photosensitive composition in a short time, and also in relatively large amounts, in order to achieve intense colouring. Moreover, the dispersion should solve the problems described and involving residual fogging with the use of high initial drying temperatures. All measures to improve the dispersion in this respect should have no adverse effects on the printing properties.
Even if the developer is already highly contaminated with layer components from the recording material, the coloured pigments should not be precipitated from the developer mixture or form deposits in the developing machine or on the developed plates. After development, the printing parts of the offset plate obtained from the recording material should be so clearly detectable that, if required, the plate can be corrected. The clear detectability is also important to enable the plate to be satisfactorily scanned by a scanner during preparation for printing.
SUMMARY OF THE INVENTION
T
Dörr Michael
Elsässer Andreas
AGFA-Gevaert
Chu John S.
Connolly Bove & Lodge & Hutz LLP
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