Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
1998-07-07
2003-03-25
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S170000, C430S171000, C430S176000, C430S944000, C430S964000
Reexamination Certificate
active
06537722
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to infrared-imageable recording materials for producing offset printing plates, as well as to processes for producing offset printing plates from the recording material.
2. Description of Related Art
In conventional processes for producing offset printing forms, a film original is placed on a radiation-sensitive layer. The layer is then irradiated through the original with ultraviolet and/or visible light. More recent developments enable laser beams to act on the radiation-sensitive layer under computer control. In this way direct transfer of digital data to an imagewise-irridated recording material is possible; that is, a film original is no longer required.
Recording materials whose radiation-sensitive layer is sensitized for laser beams with wavelengths in the visible light range generally are known, as disclosed, for example, in EP-A 0 704 764. Lasers which emit in the visible range, however, are relatively expensive. Thus such recording materials have not been widely established.
Infrared lasers, on the other hand, especially laser diodes, are substantially less expensive. However, the use of infrared lasers requires the availability of recording materials which are “thermally imageable”, i.e., materials which are sensitized in a wavelength range from about 700 to 1100 nm.
Some thermally imageable materials for producing printing plates are likewise already known. For example, EP-A 0 625 723 discloses a material whose radiation-sensitive layer comprises a mixture of a resol, a novolak, an infrared absorber and a compound which gives a Brönsted acid upon dissociation. The infrared absorbers are typically dyes or pigments which fragment on exposure to radiation, thereby producing an exposure contrast, especially cyanines, merocyanines, indolizines, pyrylium compounds or squarylium compounds. Onium salts, in particular, are often used as acid donor compounds. On irradiation, the infrared absorber and acid former are broken down, as a result of which the irradiated areas become soluble in an aqueous-alkaline developer.
However, the above-described solubility behavior is reversed if the material is heated following irradiation, in which case the Brönsted acid then catalyzes a crosslinking reaction between the resol and the novolak. Under these conditions, it is the irradiated areas of the layer that are insoluble in an aqueous-alkaline developer solution. In this embodiment, therefore, the material is negative-working. EP-A 0 672 954 describes a very similar recording material. Instead of the onium salt, it employs a haloalkyl-substituted s-triazine as acid former.
EP-A 0 652 483 describes a material which is thermally imageable with IR lasers and from which a printing plate is produced without development. The radiation-sensitive layer of this material is hydrophobic in the nonirradatiated state and accepts offset printing ink. It includes a substance which absorbs infrared radiation and converts it to heat. It also includes a polymer having groups which on exposure to heat and/or acid, are transformed into more hydrophilic and more strongly polar groups. Reactive groups mentioned are tert-alkyl carboxylates, tert-alkyl carbonates, benzyl carboxylates, cyanobenzyl esters, dimethylbenzyl esters and alkoxyalkyl esters (especially tetrahydropyran-2-yl esters). The layer preferably also includes a compound which produces an acid on exposure to heat. In the irradiated areas, the layer accepts an aqueous dampening solution and repels the printing ink. Overall, however, the layer is retained. Printing (hydrophobic) and nonprinting (hydrophilic) areas differ only in their wettability with the dampening solution/ink mixture in the printing press. With a printing plate of this type, however, it is possible only to achieve a very low print run (a few thousand prints). Moreover, the hydrophilic areas of the layer exhibit much poorer acceptance of the dampening solution than an aluminum oxide surface, resulting in a poorer water supply. Since exposed and unexposed (still heat-sensitive) layer areas are present alongside one another, generally it is also not possible to render such a plate more mechanically resistant by baking (which would lead to a higher print run). As IR absorbers, preference is given to those substances which on exposure to IR radiation generate heat and acid. Examples given are aromatic diammonium salts of strong acids. Trihalomethyl-s-triazines are preferred as additional acid donors.
WO 90/12342 describes a thermally imageable material for colorproofing techniques which uses polymeric binders that break down on exposure to heat and/or acid. The expansion of the gaseous breakdown products evolved in the IR-exposed areas of the material leads to an ablation. By transferring the ablated material from an IR-transparent support film to a receiver sheet it is possible to obtain the desired color image. This process, of course, operates without wet development. As IR absorbers preference is given to substances which give off heat and acid under the influence of IR radiation (examples being diammonium salts of strong acids). Also mentioned as an IR absorber is carbon black, whose disadvantage, however, is seen in its usefulness only for black progressives. Binders used are those which are broken down rapidly by acid, preferably at temperatures below 100° C.; for example, nitrocellulose, polycarbonates and certain polyurethanes, and also other binders which can be thermolized to form tertiary carbonium ions with subsequent proton elimination (autocatalysis of the breakdown reaction). As a model of the latter, mention is made of the known thermally and/or acid-catalyzed decomposition of phenols with tert-butyloxycarbonyl groups to form carbon dioxide, isobutene and protons (see also Frèchet et: al., J. Imaging Sci. 30 (2) [1986] 59). The formation of the gaseous decomposition products brings about the ablation of the material on IR irradiation.
Materials which are subjected to wet development following acid-catalyzed thermolysis are disclosed, for example, in EP-A 0 366 590 and in Ito et al., Polym. Eng. Sci. 23 [1983] 1012. They form the basis of highly sensitive photoresists that are sensitized for the UV range. A preferred polymer is poly(tert-butoxycarbonyloxystyrene). A disadvantage of these materials is their low thermal stability, brought about by autocatalytic decomposition with the participation of acidic phenolic hydroxyl groups that are still present in the polymer (see Ito et al., J. Polym. Sci. 24 [1986] 2971). This Ito reference does not disclose or mention the use of these materials in connection with the production of printing plates which place special requirements on polymers in terms both of the developer resistance and in terms of the print run. There is also no disclosure in this Ito reference regarding the sensitization of such materials for radiation in the IR range.
The use of polymers having acid-labile groups as binders for printing plates is described, for example in EP-A 0 683 435. The polymers preferably include monomer units comprising 4-tert-butoxycarbonyloxystyrene (BOCST). Utmost sensitivity for UV/VIS, extremely low removal of material from unexposed areas and very high thermal stability are achieved if the binder includes not only the monomer units (I) having acid-labile groups, e.g., BOCST, but also monomer units (II) having phenolic hydroxyl groups and monomer units (III) having at least one aliphatic hydroxyl group. The content of (II+III) is responsible for good developability in aqueous-alkaline solution. If the content of (II) is too high, the auto-catalytic effect described above reduces the thermal stability of the binder and increases the removal of material from unexposed areas. If some of the monomer units having phenolic hydroxyl groups are replaced by those having aliphatic hydroxyl groups, the thermal stability is improved. The thermal stability is also of particular importance because printing plates produced with th
Eichhorn Mathias
Grabley Fritz-Feo
Maerz Karin
Afga-Gevaert AG
Baxter Janet
Gilliam Barbara
LandOfFree
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