Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Patent
1983-08-29
1985-11-26
Brammer, Jack P.
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
430272, 430278, 430302, 430300, 430945, G06F 700
Patent
active
045554757
DESCRIPTION:
BRIEF SUMMARY
Planographic printing involves printing from a member on which ink is distributed imagewise solely or primarily as a result of imagewise differences in the surface properties of the member. Thus the surface of the plate may be absolutely level or there may be some trivial imagewise profiling effect, for example as an unavoidable consequence of the generation of the imagewise differential properties.
In lithography, the most common form of planographic printing, imagewise distribution of ink is achieved by applying an oil-based ink to a member which carries an imagewise distribution of relatively oleophilic image areas on a background that is relatively hydrophilic (oleophilic), the hydrophilicity having been enhanced by wetting the background with water.
Planographic printing members can also be used for the production of deep etch plates, in which the differential imagewise surface properties are utilised to produce differential imagewise etching.
Planographic printing members comprise a substrate carrying an image forming layer. The substrate is often of aluminium, usually having an anodised surface. Generally it is provided also with a coating of an aluminium silicate by treating the aluminium, or anodised aluminium, with sodium silicate, for instance as described in U.S. Pat. No. 3,181,461. An image forming layer is applied to the aluminium, anodised aluminium or aluminium silicate. The photosensitive material in this image forming layer may, for instance, be ammonium bichromate or a diazo resin, as described in U.S. Pat. No. 3,181,461, or a photopolymerisable resin. Commercially the image forming layer may be formed immediately prior to use, for instance by wiping on diazo or other photosensitive material just prior to photoexposure, or the printing member may be a presensitised plate having a preformed coating of photopolymerisable resin.
An image is formed on the planographic printing member by imagewise photoexposure of the image forming layer. The exposure is usually conducted using ultraviolet radiation. It results in imagewise changes in the properties of the image forming layer, for instance with the exposed areas being hardened as a result of exposure. The exposed image forming layer is then developed. Development normally involves removal of the unexposed image forming layer, to reveal the relatively hydrophilic silicate or anodised aluminium substrate. Additionally development may involve strengthening the exposed image, for instance by coupling a resin onto the exposed image forming material to give an imagewise deposition of resin bonded to the substrate. Typical developer compositions comprise a large amount of water, to remove the unexposed image forming layer, and a small amount of an organic phase carrying the resin and other additives such as pigment. It is necessary that the amount of organic solvent should be relatively low as otherwise the solvent in the developer would strip the exposed areas off the substrate.
These systems all suffer from the disadvantage that it is necessary to provide photosensitive coating over the anodised, and often silicated, aluminium substrate, and the cost of this is usually quite considerable relative to the cost of the substrate.
Various detailed modifications of these general methods have been proposed in the literature. For instance in U.S. Pat. No. 4,054,094 it is proposed to expose imagewise by a laser a printing member comprising an aluminium substrate carrying a polymeric composition that is coated by polysilicic acid. Thus this method requires two coating steps over the substrate. The imagewise exposure results in decomposition of the organic resin so as to render the exposed areas oleophilic, while the polysilicic acid in the unexposed areas renders the surface hydrophilic. It is stated that when the polysilicic acid is applied directly to the aluminium plate imagewise exposure by the laser does not transform the surface from a water accepting to a water rejecting surface. Although it is stated in U.S. Pat. No. 4,054,094 that almost any solid
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Gamson Alan R.
Kellner Phillip R.
Brammer Jack P.
Crossfield Electronic Limited
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