Printing – Planographic – Lithographic printing plates
Reexamination Certificate
1999-06-24
2001-05-15
Funk, Stephen R. (Department: 2854)
Printing
Planographic
Lithographic printing plates
C101S467000
Reexamination Certificate
active
06230621
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a heat-sensitive material for preparing lithographic printing plates.
More specifically the invention is related to a processless heat-sensitive material which yields lithographic printing plates with a high endurance.
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 daylight. Furthermore they have a problem of unstable sensitivity with regard to 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 accepting 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.
Furthermore EP-A-770 494, 770 495, 770 496 and 770 497 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 staining and scratch resistance.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a processless heat-sensitive imaging material for making lithographic printing plates having excellent printing properties.
It is a further object of the invention to provide a heat sensitive imaging material for making lithographic printing plates with an improved scratch resistance.
Further objects of the present invention will become clear from the description hereinafter.
SUMMARY OF THE INVENTION
According to the present invention there is provided a heat-sensitive material for making lithographic printing plates comprising on a lithographic support an image-forming layer comprising a hydrophilic binder, a cross-linking agent for said hydrophilic binder, metal oxide particles with a mean diameter of at least 100 nm and dispersed hydrophobic thermoplastic polymer particles, characterized in that said image-forming layer has a ratio of specific surface (in m
2
per g) over mean roughness(in &mgr;m) of more than 0.75 and that the mean pore width is less than 15 nm.
DETAILED DESCRIPTION OF THE INVENTION
The specific surface of the coating (in m
2
per g) is measured by a Micromeritics ASAP2400-apparatus. Therefore the material, including the support, is cut in small pieces and introduced into the apparatus, then a sorption/desorption isotherm of the material is measured with nitrogen-gas as adsorbate.
From the obtained sorption/desorption isotherms, the specific surface is calculated, following the sorption/desorption approximation corresponding with BET. Also the mean pore diameter is calculated by the method of Barett, Joyner and Hallender.
The average surface roughness of the plate (in &mgr;m) is measured with a perthometer MAHR PERTHEN S6P containing a measuring head RTK50 (tradename of Feinprüf Perthen GmbH, Goettingen, Germany) equipped with a diamond stylus with a diameter of 50 &mgr;m under a pressure of 1.0 mN according to techniques well known in the art. The sampling length Ls which is the reference length for roughness evaluation measures 0.25 mm. The evaluation length Lm, being that part of the travelling length Lt which is evaluated for acquiring the roughness profile R contains standard 5 consecutive sampling lengths. The traversing length Lt is the overall length travelled by the tracing system when acquiring the roughness profile. The average roughness Ra is the measured roughness averaged over the evaluation length Lm.
Preferably the ratio of specific surface over mean roughness is more than 0.75, more preferably more than 0.85. The mean pore width is preferably less than 10 nm, more preferably less than 7 nm.
According to the present invention to improve sensitivity and throughput and to avoid scumming an imaging element is provided comprising preferably hydrophobic thermoplastic polymer particles with an average particle size between 40 nm and 150 nm. More preferably the hydrophilic thermoplastic polymer particles are used with an average particle size of 40 nm to 80 nm. Furthermore the hydrophobic thermoplastic polymer particles used in connection with the present invention preferably have a coagulation temperature above 50° C and more preferably above 70° C. Coagulation may result from softening or melting of the thermoplastic polymer particles under the influence of heat. There is no specific upper limit to the coagulation temperature of the thermoplastic hydrophobic polymer particles, however the temperature should be sufficiently below the decomposition temperature of the polymer particles. Preferably the coagulation temperature is at least 10° C. below the temperature at which the decomposition of the polymer particles occurs. When said polymer particles are subjected to a temperature above the coagulation temperature they coagulate to form a hydrophobic agglomerate in the hydrophilic layer so that at these parts the hydrophilic layer becomes hydrophobic and oleophilic.
Specific examples of hydrophobic polymer particles for use in connection with the present invention have a Tg above 80° C. Preferably the polymer particles are selected from the group consisting of polyvinyl chloride, polyvinylidene chloride, polyacrylonitrile, polyvinyl carbazole etc., copolymers or mixtures thereof. Most preferably used are polystyrene, polymethylmethacrylate or copolymers thereof.
The weight average molecular weight of the polymers may range from 5,000 to 5,000,000 g/mol.
The polymer particles are present as a dispersion in the aqueous coating liquid of the image-fo
Leenders Luc
Rompuy Ludo Van
Vermeersch Joan
Verschueren Eric
AGFA-GEVAERT
Breiner & Breiner
Funk Stephen R.
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