Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
1998-02-11
2001-02-13
Chu, John S. (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S281100, C430S907000, C430S922000
Reexamination Certificate
active
06187511
ABSTRACT:
This invention relates to the production of so-called water-less lithographic plates.
Lithographic plates may be divided into two classes. Those which require dampening water which is fed to the non-image areas of the plate, forms a water film and acts as an ink-repellant layer; this is the so-called fount solution and those which require no fount solution are called driographs or water-less lithographic plates. Most lithographic plates at present in use are of the first type and require a fount-solution during printing. However, lithographic plates of this type suffer from a number of disadvantages. Some of these are:
a) adjustment of the proper ink-water balance during press operation is difficult and requires great experience. If the correct ink-water balance is not achieved scumming is occasioned when the printed ink image extends into the non-image areas ruining the printed image.
b) adjustment of the ink-water balance at start-up or re-start up is particularly difficult and can not be stabilised until a large number of sheets have been printed, thus incurring waste,
c) the ink tends to become emulsified which leads to poor adherence of the ink on to the plate which causes problems in colour reproduction and in dot reproduction,
d) the printing press has to be provided with a dampening system, thus increasing its size and complexity. These dampening solutions contain volatile organic compounds.
e) The plate care chemistry and fount solutions require careful control and selection. Further plate cleaners contain significant levels of solvent which is not desirable.
However, with water-less plates in which the ink-releasing layer is, for example, a cured silicone layer there is no scumming and clearer images can be produced. Very often water-less plates comprise a base material, for example aluminium plate. on which a photosensitive layer is coated, on this photosensitive layer there is coated a silicone layer. After imagewise exposure and development in which selected areas of the photosensitive composition are altered, the overlying silicone layer is removed and the plate is inked up. The ink adheres only to those areas of the plate not covered by the silicone remaining after development. Thus the plate can be printed without the need to use a fount solution. In practice it is difficult and costly to formulate and manufacture the silicone layer composition with sufficient adhesion to the photosensitive composition in these multilayer assemblies. Thus the only commercially available water-less lithographic plates are expensive and of complex design.
There exists in patent literature water-less lithographic plate designs which do not exhibit these disadvantages. These inventions disclose photosensitive water-less lithographic plate precursors comprising a support with an oleophilic surface and a single layer, photosensitive, ink-releasing composition such that imagewise exposure causes changes in developer solubility of the composition where development produces an ink accepting image pattern on the uncovered support surface and an ink-releasing non-image area corresponding to unremoved composition. Compositions which comprise a photosolubilising substance provide a negative working, waterless printing plate as the the unimaged area remains on the plate post-development and acts as a printing background due to the properties of the release materials. Conversely, a photoinsolubilising composition will provide for a positive working waterless printing plate.
U.S. Pat. No. 3,953,212 discloses a plate comprising a support and a coating layer of a mixture of a photosensitive material and a silicone rubber. The photosensitive materials utilised in the invention are typical of standard pre-sensitised wet, lithographic printing plates. These include photosolubilising napthoquinonediazide systems and photoinsolubilising systems including diazo compounds and photopolymer systems such as poly vinyl cinnamates and copolymers of acrylates and methacrylates.
U.S. Pat. No. 4,724,195, U.S. Pat. No. 5,006,443 and U.S. Pat. No. 5,104,961 disclose a single layer waterless plate precursor which comprises a mixture of radiation sensitive material and a class of fluorinated polymers. These fluorinated polymers represent novel release materials over U.S. Pat. No. 3,953,212. A wide range of photosolubilising and photoinsolubilising composition types which are sensitive to UV or Visible light are disclosed as useful in the invention.
EP 0,069,978 discloses a single layer photosensitive composition which comprises an emulsion polymerisation product of an acrylic or methacrylic monomer having a perfluoroalkyl group in the side chain and a photoinsolubilising photosensitive substance to provide a positive working waterless plate precursor.
The differentiation between image and non-image areas is made in the exposure process. In conventional waterless lithography a film original is applied to the plate with a vacuum to ensure good contact and the plate is then blanket exposed to a light source, a portion of which is composed of UV radiation. The aforementioned prior art all disclose this imaging process for their plate precursors.
More recent developments in the field of lithographic printing plates have provided imaging devices which allow for the preparation of direct laser written printing forms. Digital imaging information is used to image the plate directly by laser radiation without the need to utilise an imaging master such as a photographic transparency.
In addition to UV sensitivity, U.S. Pat. No. 4,724,195, U.S. Pat. No. 5,006,443 and U.S. Pat. No. 5,104,961 additionally disclose the use of visible light to image the plate, including the use of a laser emitting visible radiation. Such precursors would be imageble on commercially available laser imaging devices which use visible light, e.g 488 nm from an Argon ion laser or 532 nm from a Frequency Doubled YAG laser.
However, the imaging devices utilising such laser technology are limited commercially due to cost of the lasers used. Most recently there has been a trend to utilise lasers which emit infra-red radiation.
We have discovered a method of producing positive working waterless lithographic printing plate precursors which are imageable on exposure to infra-red radiation and/or heat and are therefore suitable for use with these new imaging systems.
According to the present invention there is provided a method of preparing a positive working water-less lithographic plate which comprises a support having an oleophilic surface, there being coated on the support a mixture which comprises as one component an ink-repellant and water-repellant polymer or a mixture of such polymers or polymer precursors, and as the other essential component of the mixture an infra-red or heat sensitive composition selected from (a) an organic solvent soluble diazo compound and an infra-red absorbing compound, (b) a photopolymer and an infra-red absorbing dye, or (c) a mixture of a free-radically polymerisable ethylenically unsaturated compound or compounds together with either a photoinitiator which is infra-red sensitive or an initiator which is heat sensitive together with an infra-red absorbing compound, the ratio of ink-repellant polymer to infra-red or heat sensitive composition (a), (b) or (c) in the mixture being from 20-80 ink repellant polymer to 80-20 infra-red or heat sensitive composition by weight, imagewise acting on the coated mixture, developing the acted on mixture with the appropriate developing solution depending on the composition (a), (b) or (c) used, to remove the composition and the water-repellant polymer in the unacted on areas to reveal the oleophilic surface of the support in the unacted on areas of the plate and leaving the coated mixture in the acted on areas of the plate.
Preferably the composition (a), (b) or (c) is imagewise exposed by a laser emitting radiation above 600 nm which is controlled by an image storage means which carries in digital form the required image.
The laser radiation absorbing compound may be carbon such as car
Bennett Peter Andrew Reath
Smith Carole-Anne
Chu John S.
Kodak Polychrome Graphics LLC
Ratner & Prestia
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