Use of carboxyl group-containing acetal polymers in...

Radiation imagery chemistry: process – composition – or product th – Diazo reproduction – process – composition – or product – Composition or product which contains radiation sensitive...

Reexamination Certificate

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C430S175000, C430S176000, C430S302000

Reexamination Certificate

active

06808858

ABSTRACT:

DESCRIPTION OF INVENTION
Use of acetal polymers containing carboxyl groups in photosensitive compositions and lithographic printing plates
The present invention relates to binders and photosensitive compositions which are suitable for the production of printing plates.
Photosensitive compositions which are used in particular for high-performance printing plates now have to meet high requirements.
Essentially two approaches are adopted for improving the properties of photosensitive compositions and hence also of the corresponding printing plates. One is concerned with improving the properties of the photosensitive components in the mixtures (negative diazo resins, photoinitiators, photopolymers etc.), while the other is concerned with finding novel polymeric compounds (“binders”) which are intended to control the physical and the mechanical properties of the photosensitive layers. Particularly the latter approach is of considerable importance for printing plates since the behavior in the development and printing process, such as, for example, developability, ink acceptance, scratch resistance and press life, is decisively influenced by the polymeric binders. The shelf-life and the photosensitivity of the materials are also influenced by such polymeric compounds.
In order to meet the extensive requirements, the polymeric binders have various structural elements which can affect individual properties differently. Thus, hydrophilic structural elements, such as for example carboxyl groups and hydroxyl groups, generally promote good developability of the photosensitive compositions in aqueous alkaline developers and in some cases ensure sufficient adhesion to polar substrates. Hydrophobic structural elements, on the other hand, complicate developability in said developers but ensure the good acceptance of the inks in the printing process, which is absolutely essential for printing plates.
Because of the wide range of requirements which the polymeric binders have to meet, there has been extensive work for many years on synthesizing, and optimizing the use of, these substances for photosensitive compositions (cf. for example H. Baumann and H.-J. Timpe: “Chemical Aspects of Offset Printing” in J. prakt. Chem. Chemiker-Zeitung 336 (1994), pages 377-389). It was found that polymers which contain vinyl alcohol groups and acetal groups are particularly suitable.
First, acetal polymers of aliphatic aldehydes having an unsubstituted lower alkyl group and vinyl alcohol/vinyl acetate copolymers were described (U.S. Pat. No. 2,179,051, EP-A-0 216 083 and DE-A-14 47 957). However, such binders give rise to problems during the development of printing plates because the proportion of hydrophilic groups in the polymer is too low. If, however, the degree of acetalation is chosen to be low in the case of these polymers, in order to realize sufficient developability through a high proportion of vinyl alcohol, the press life is too short.
In order to achieve an improvement, partly acetalated vinyl alcohol/vinyl acetate copolymers were provided with hydrophilic or alkali-soluble groups by special reactions. In DE-A-36 44 162 and EP-A-0 274 075, aldehydes containing hydroxyl groups are used in addition to aldehydes having hydrophobic groups (e.g. alkyl or aryl groups) for the acetalation. However, this structural change does not lead to a substantial improvement in the developability or the press life.
In DE-A-20 53 363, DE-A-20 53 364 and EP-A-0 048 876, sulfonylurethane groups were introduced into the polyvinyl acetals in order to improve the developability. However, the low acidity of these groups requires developers having a high solvent content. Moreover, micro elements of printing plates produced in this manner have very little adhesion so that they are removed too rapidly by abrasion in the printing process.
DE-A-22 45 433 describes binders which can be prepared by acetalation of saponified copolymers of vinyl acetate and a monomer carrying carboxyl groups, such as crotonic acid or acrylic acid. For the acetalation, aromatic aldehydes are disclosed. However, this type of binder leads to systems having low photosensitivity, poor developability and a short press life when used for printing plates.
In EP-A-0 211 391 and EP-A-0 152 819, the carboxyl groups are introduced by reacting separately prepared acetals of aliphatic aldehydes with polyvinyl alcohol with intramolecular, cyclic anhydrides of dicarboxylic acids. However, the necessary cost of the synthesis is considerable since, in addition to the acetalation, there is the reaction with the anhydrides, which is possible only in aprotic solvents. Furthermore, the photosensitivity of the mixtures prepared from these binders is still too low.
EP-A-0 208 145 describes binders which are prepared in a three-stage synthesis starting from polyvinyl alcohol: acetalation with aliphatic aldehydes, reaction with intramolecular, cyclic anhydrides of dicarboxylic acids and partial esterification of the carboxyl groups with substituted alkyl halides. In spite of the high cost for the binder synthesis, an improvement of the photosensitivity of the layers produced therefrom is desirable.
The cost of the multistage syntheses can be avoided if, as described in WO 93/03068 and U.S. Pat. No. 5,169,897, polyvinyl alcohol is reacted with aliphatic aldehydes and aliphatic aldehydes containing carboxyl groups or aromatic aldehydes containing carboxyl groups. However, the photosensitive mixtures prepared therefrom have an unfavorable relationship with regard to developability and sensitivity. Moreover, the aldehydes containing carboxyl groups give rise to high costs.
EP-A-0 752 430 describes acetal polymers which can be prepared by reacting polyvinyl alcohol with aliphatic aldehydes and aldehydes having a —X—N(R1)CO—Y—COOH side group. The cost for the preparation of such polymers is however high since the special aldehydes are very expensive, the use of organic solvents cannot be dispensed with in the synthesis and a precipitation step using a nonsolvent is required for isolating the binders.
In spite of this intensive research in the area of photosensitive compositions for printing plates, improvements appear to be desirable in the case of all existing compositions, in particular with regard to their developability and the sensitivity to press room chemicals. Furthermore, either many of the compositions have a large number of components, some of which are expensive, or the preparation of their main components is associated with a high synthesis cost, both of which prevent commercial use.
It was therefore the object of the present invention to provide photosensitive compositions which are used in the production of printing plates, containing a binder whose preparation manages with as small a number of synthesis steps as possible compared with the binders described in the prior art, and which nevertheless has the same physical properties, or physical properties improved in individual areas, in photosensitive compositions. In particular, improved acceptance of the printing ink and/or a longer press life of the corresponding printing plates is to be achieved in comparison with the compositions described in the prior art, while retaining the good developability in developers which contain only small amounts of organic solvents, salts and wetting agents, the high photosensitivities and the good image resolutions, and the compositions should moreover be compatible with the other products used in the processing of printing plates.
This object is achieved by a photosensitive composition comprising:
(i) at least one, preferably one, diazonium polycondensate or at least one, preferably one, system capable of free radical polymerization and comprising photoinitiators and unsaturated compounds which are capable of free radical polymerization or at least one, preferably one, hybrid system comprising a diazonium polycondensate and a system capable of free radical polymerization and comprising photoinitiators and unsaturated compounds which are capable of free radical polymerization,
(i

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