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
2000-09-12
2002-06-25
Hamilton, Cynthia (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
C430S300000, C524S127000, C524S128000, C524S243000
Reexamination Certificate
active
06410205
ABSTRACT:
The present invention relates to a photosensitive recording material comprising a base, a photopolymerisable layer and on top of that a covering layer being transparent to actinic radiation. The present invention also relates to a process for preparing an offset printing plate with this recording material.
Recording materials of this kind are already known. For example U.S. Pat. No. 3,458,311 discloses a recording material provided with a relatively thin covering layer (“overcoat”) being transparent to actinic radiation. The covering layer is water-permeable, but only slightly permeable to (atmospheric) oxygen. The layer has a weight of approximately 0.2 to 3 g/m
2
and is made of a polymeric material being soluble in the conventional aqueous-alkaline developers. However the covering layer is not easily removable by simply stripping it off. As its essential function, the covering layer keeps atmospheric oxygen away from the photopolymerisable layer, atmospheric oxygen having an inhibiting effect on the photopolymerisation reaction. The covering layer is prepared by applying an aqueous solution of the polymeric material onto the photopolymerisable layers and drying it. The solutions used therefore contain at least one polymer being soluble at 20° C. in water or a mixture of water and an organic water-miscible solvent. Preferred polymers are polyvinyl alcohol, polyvinylpyrrolidone, gum arabic, copolymers of methyl vinyl ether and maleic anhydride and polyethylene glycol. The covering layer may additionally comprise wetting agents, especially sodium alkylsulphates, N-cetyl or C-cetyl betaine, alkyl aminocarboxylates or -dicarboxylates or polyethylene glycols having an average molecular weight of less than 400. Wetting agents strongly affect the coating properties. The covering layers described in U.S. Pat. No. 3,458,311 are thinner and adhere more uniformly to the photopolymer layer, being however less permeable to oxygen than strippable overcoats.
Printing forms prepared from this recording material have an improved contrast and sensitivity and allow longer print runs than printing plates made of a comparable recording material provided with a strippable overcoat.
Recording materials of a fully similar kind are described in U.S. Pat. No. 4,072,527 and U.S. Pat. No. 4,072,528. These recording materials comprise a covering layer being virtually completely impermeable to oxygen and having a thickness varying between 0.2 and 25 &mgr;m, being here too not mechanically peelable, but removable by using an aqueous developer. The covering layer additionally contains solid particles having a diameter of at least 0.05 &mgr;m composed of a water-insoluble acrylic or alkacrylic (co)polymer or a water-insoluble copolymer of vinylpyrrolidone and vinyl acetate (U.S. Pat. No. 4,072,527) or a water-insoluble chlorinated vinyl polymer or vinyl copolymer (U.S. Pat. No. 4,072,528). Anionic or non-ionic surfactants are recommended as additives, in order to achieve a more uniform coating. Cationic surfactants however should be avoided, as they cause unwanted flocculation of the oppositely charged dispersion particles. The covering layer should make the recording materials insensitive to changes of temperature or atmospheric humidity and in addition thereto improve photosensitivity and image resolution.
EP-A 275 147 discloses a recording material for preparing planographic or flexographic printing plates. This recording material comprises a covering layer keeping atmospheric oxygen away from the underlying photopolymerisable layer. The covering layer contains a polymer being transparent to actinic radiation (preferably polyvinyl alcohol) and an amphoteric compound. By amphoteric compounds are to be understood compounds comprising both basic groups and acid groups. Suitable amphoteric compounds are for example amino acids which may be N-substituted. The amphoteric compound can also be in the form of a polymer or copolymer. Preferred amphoteric copolymers are copolymers comprising units of acrylic amides, aminoalkylacrylates or similar monomers and units of acid group-containing monomers as there are vinylphosphonic acid or (meth)acrylic acid. Amphoteric compounds of the above-mentioned nature often cause turbidity or flocculation in the coating solutions used for preparing the covering layer. Possible foaming on the coating solutions should also be taken into account.
EP-A 352 630 discloses a recording material comprising a layer base, a photopolymerisable layer and a covering layer containing a water-soluble polymer with low permeability to atmospheric oxygen (for example polyvinyl alcohol) and a water-soluble polymer binding atmospheric oxygen (for example polyalkylene imine). In this way also relatively thin covering layers having a thickness between 0.5 and 2.5 &mgr;m will be able to ensure an efficient barrier effect to oxygen. A complete elimination of oxygen however would deteriorate the storage stability of the recording material as a defined oxygen concentration is indeed needed, to scavenge radicals thermally produced in a dark reaction, said radicals would induce an uncontrolled crosslinking of the photosensitive layer (see J. Streeter in “Photopolymer Plate Technology Developments and Trends”, 2nd Annual CtP Technology Conference Orlando, Fla. 31.1.1997). Moreover, polyalkylene imines increase the viscosity of the coating solutions and ensure a highly stable foaming. Foaming causes coating problems, leading to air bubbles in the covering layer and defects in the coating process.
EP-A 403 096 describes a photopolymerisable recording material, the covering layer of which contains a photoinitiator and optionally also a sensitising agent. The covering layer acts here again as oxygen barrier layer, thus ameliorating the speed and storage stability of the recording material. The addition of N,N-disubstituted aniline compounds, being simultaneously also substituted in para-position, to the photopolymerisable layer gives a further increase in storage stability. The covering layer may contain the same photoinitiator as the photopolymerisable layer or another one. The coating weight of the covering layer generally ranges from 0.5 to 3.3 g/m
2
. Preferred photoinitiators are aromatic iodonium salts, trichloromethyl-s-triazines and other halogenated hydrocarbon compounds. The polymer in the covering layer preferably is polyvinyl alcohol or carboxymethyl cellulose. The storage stability of the recording material is increased by using a covering layer of this nature, the speed however decreases generally by up to two wedge steps on the 21-step Stouffer exposure gauge. It should be noted, that a loss of two steps implicates a decrease of the speed by half.
EP-A 487 343 relates to a process for preparing a lithographic printing plate. The starting material is a recording material, the photosensitive layer of which comprises an aromatic diazonium compound containing a carboxyl, a phenolic hydroxyl, a sulphonic, a sulphinic and/or a phosphorus oxy acid group. This photosensitive layer is coated in a separate step with a matting layer by melt-coating or spray coating. Although this process solves the air bubble problem encountered upon applying the layer by known flowing or roll coating methods, the above-mentioned formation of a homogeneous layer is however not yet ensured.
EP-A 509 514 describes a recording material comprising a base, a photopolymerisable layer and a protective layer being permeable to atmospheric oxygen. The protective layer has an oxygen permeability coefficient comprised between 10 and 13 cm
2
/s or more and is made preferably of hydroxyalkyl cellulose and/or polyethylene glycol. The protective layer is coated by ink-jet printing with an image layer composed of a material being transparent to light, but only weakly permeable to oxygen. This layer contains 0.2 to 15% by weight of a water-soluble or water-dispersible polymer (preferably polyvinyl alcohol, polyvinylpyrrolidone or a cellulose derivative), 0.5 to 40% by weight of a polyhydric alcohol (preferably a glycol) and 0.1 to 5% by wei
Boxhorn Mario
Leichsenring Thomas
Lifka Thorsten
Agfa-Gevaert
Connolly Bove & Lodge & Hutz LLP
Hamilton Cynthia
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