Printing – Planographic – Lithographic printing plates
Reexamination Certificate
1997-12-12
2002-08-06
Funk, Stephen R. (Department: 2854)
Printing
Planographic
Lithographic printing plates
C101S462000, C101S467000
Reexamination Certificate
active
06427595
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a heat-sensitive imaging element for making a lithographic printing plate. More in particular the present invention relates to a heat-sensitive imaging element comprising an image-forming layer comprising hydrophobic thermoplastic polymer particles having a specific particle size and polydispersity.
BACKGROUND OF THE INVENTION
Lithographic printing is the process of printing from specially prepared surfaces, some areas of which are capable of accepting ink, whereas other areas will not accept ink.
In the art of photolithography, a photographic material is made. imagewise receptive to oily or greasy ink in the photo-exposed (negative working) or in the non-exposed areas (positive working) on an ink-repelling background.
In the production of common lithographic plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used.
Upon imagewise exposure of such light-sensitive layer the exposed image areas become insoluble and the unexposed areas remain soluble. The plate is then developed with a suitable liquid to remove the diazonium salt or diazo resin in the unexposed areas.
On the other hand, methods are known for making printing plates involving the use of imaging elements that are heat-sensitive rather than photosensitive. A particular disadvantage of photosensitive imaging elements such as described above for making a printing plate is that they have to be shielded from the light. Furthermore they have a problem of stability of sensitivity in view of the storage time and they show a lower resolution. The trend towards heat-sensitive printing plate precursors is clearly seen on the market.
For example, Research Disclosure no. 33303 of January 1992 discloses a heat-sensitive imaging element comprising on a support a cross-linked hydrophilic layer containing thermoplastic polymer particles and an infrared absorbing pigment such as e.g. carbon black. By image-wise exposure to an infrared laser, the thermoplastic polymer particles are image-wise coagulated thereby rendering the surface of the imaging element at these areas ink acceptant without any further development. A disadvantage of this method is that the printing plate obtained is easily damaged since the non-printing areas may become ink-accepting when some pressure is applied thereto. Moreover, under critical conditions, the lithographic performance of such a printing plate may be poor and accordingly such printing plate has little lithographic printing latitude.
EP-A-514145 discloses a heat-sensitive imaging element including a coating comprising core-shell particles having a water insoluble heat softenable core component and a shell component which is soluble or swellable in an aqueous alkaline medium. Red or infrared laser light directed image-wise at said imaging element causes selected particles to coalesce, at least partially, to form an image and the non-coalesced particles are then selectively removed by means of an aqueous alkaline developer. Afterwards a baking step is performed. However the printing endurance of a so obtained printing plate is low.
EP-A-599510 discloses a heat-sensitive imaging element which comprises a substrate coated with (i) a layer which comprises (1) a disperse phase comprising a water-insoluble heat softenable component A and (2) a binder or continuous phase consisting of a component B which is soluble or swellable in aqueous, preferably aqueous alkaline medium, at least one of components A and B including a reactive group or precursor therefor, such that insolubilization of the layer occurs at elevated temperature and/or on exposure to actinic radiation , and (ii) a substance capable of strongly absorbing radiation and transferring the energy thus obtained as heat to the disperse phase so that at least partial coalescence of the coating occurs. After image-wise irradiation of the imaging element and developing the image-wise irradiated plate, said plate is heated and/or subjected to actinic irradiation to effect insolubilization. However the printing endurance of a so obtained printing plate is low.
Furthermore EP-A 952022871.0, 952022872.8, 952022873.6 and 952022874.4 disclose a method for making a lithographic printing plate comprising the steps of (1) image-wise exposing to light a heat-sensitive imaging element comprising (i) on a hydrophilic surface of a lithographic base an image-forming layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and (ii) a compound capable of converting light to heat, said compound being comprised in said image-forming layer or a layer adjacent thereto; (2) and developing a thus obtained image-wise exposed element by rinsing it with plain water.
The above mentioned heat-sensitive imaging elements for making lithographic printing plates are not optimal regarding sensitivity and developablity.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a heat-sensitive imaging element for making lithographic printing plates having excellent printing properties, developable in a convenient ecological way.
It is further an object of the present invention to provide a heat-sensitive imaging element for making lithographic printing plates with an improved sensitivity, a high throughput and less scumming.
Further objects of the present invention will become clear from the description hereinafter:
According to the present invention there is provided a heat-sensitive imaging element for making lithographic printing plates comprising on a hydrophilic surface of a lithographic base an image-forming layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a compound capable of converting light into heat present in said image-forming layer or a layer adjacent thereto, characterized in that said hydrophobic thermoplastic polymer particles have an average particle size of 40 nm to 150 nm based on the intensity-weighted size distribution and a polydispersity of less than 0.2.
DETAILED DESCRIPTION OF THE INVENTION
It has been found that according to the present invention, using an imaging element as described above, lithographic printing plates with improved sensitivity, excellent developability, high throughput and less scumming can be obtained when said imaging element comprises hydrophobic thermoplastic polymer particles that have an average particle size of 40 nm to 150 nm based on the intensity-weighted size distribution measured with a BI-90 particle Sizer from Brookhaven Instruments Corporation and a polydispersity of less than 0.2. Polydispersity has no units. It is small or close to zero (0.0-0.2) for nearly monodisperse samples or narrow distributions and it is larger for very broad distributions (>0.2). The polydispersity and the average particle size are calculated as described in the manual of the BI-90 Particle Sizer (Catalog Number: BI-9KATMAN, Ver 2.2). More information on laser light scattering and cumulant analysis can be found in the following references: ‘Laser Light Scattering’, Academic Press, N.Y., 1974; ‘Dynamic Light Scattering with Applications to Chemistry, Biology and Physics’, Wiley-Interscience, N.Y., 1976; ‘Uses and-Abuses of PCS in Particle Sizing’ in ‘Particle Size Distribution, Assessment and Characterization’ ACS Symposium Series 332, 1987; Rev.Sci. Instrum., .62(12), 1991, page 2963; ‘Proc. S.P.I.E.’, 16, 1993, page 1884; Journal of Chemical Physics, 62, 1975, page 1136.
In the present invention a heat-sensitive imaging element is used comprising on a hydrophilic surface of a lithographic base an image-forming layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydroph
Damme Marc Van
Louwet Frank
Samijn Raf
Vermeersch Joan
Agfa-Gevaert
Breiner & Breiner L.L.C.
Funk Stephen R.
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