Lithographic printing typically involves the use of a so-called printing master such as a printing plate which is mounted on a cylinder of a rotary printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master.
A typical photosensitive printing plate precursor for computer-to-film methods comprises a hydrophilic support and an image-recording layer which includes UV-sensitive compositions. Upon image-wise exposure of a negative-working plate, typically by means of a film mask in a UV contact frame, the exposed image areas become insoluble and the unexposed areas remain soluble in an aqueous alkaline developer. The plate is then processed with the developer to remove the diazonium salt or diazo resin in the unexposed areas. So the exposed areas define the image areas (printing areas) of the printing master, and such printing plate precursors are therefore called ‘negative-working’. Also positive-working materials, wherein the exposed areas define the non-printing areas, are known, e.g. plates having a novolac/naphtoquinone-diazide coating which dissolves in the developer only at exposed areas.
In addition to the above- photosensitive materials, also heat-sensitive printing plate precursors have become very popular. Such thermal materials offer the advantage of daylight-stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer or by particle coagulation of a thermoplastic polymer latex, and solubilization by the destruction of intermolecular interactions.
Thermal plates which require no processing are also known; such plates are typically of the so-called ablative type, i.e. the differentiation between hydrophilic and oleophilic areas is produced by heat-induced ablation of one or more layers of the coating, so that at exposed areas a surface is revealed which has a different affinity towards ink or fountain than the surface of the unexposed coating. A major problem associated with ablative plates, however, is the generation of ablation debris which may contaminate the electronics and optics of the exposure device and which needs to be removed from the plate by wiping it with a cleaning solvent, so that ablative plates are often not truly processless. Ablation debris which is deposited onto the plate's surface may also interfere during the printing process.
Other thermal plates that require no processing are described in U.S. Pat. No. 5,855,173, U.S. Pat. Nos. 5,839,369 and 5,839,370 where a method relying on the image-wise hydrophilic-hydrophobic transition of a ceramic such as a zirconia ceramic and the subsequent reverse transition in an image erasure step. This image-wise transition is obtained by exposure to infrared laser irradiation at a wavelength of 1064 nm at high power (the average power is 1 W to 50 W and the peak power lies between 6 kW and 100 kW) which induces local ablation and formation of substoichiometric zirconia. U.S. Pat. No. 5,893,328, U.S. Pat. No. 5,836,248 and U.S. Pat. No. 5,836,249 disclose a printing material comprising a composite of zirconia alloy and α-alumina which can be imaged using similar exposure means to cause localized “melting” of the alloy in the exposed areas and thereby creating hydrophobic/oleophilic surfaces. A similar printing material containing an alloy of zirconium oxide and Yttrium oxide is described in U.S. Pat. No. 5,870,956. The high laser power output required in these prior art methods implies the use of expensive exposure devices.
Another type of processless plates are printing plates based on a so-called “switching” reaction where a hydrophilic surface is irreversibly changed into an oleophilic surface or vice versa by imagewise exposure. EP 652 483 for example, describes a positive working printing plate based on an acid catalyzed cleavage of acid-labile groups pendant from a polymer backbone. EP 200 488 and U.S. Pat. No. 4,081,572 describe negative working plates where a hydrophilic/hydrophobic conversion is obtained by a chemical reaction upon imagewise exposure to heat. Other examples of processless plates are based on the thermally induced rupture of microcapsules and the subsequent reaction of the microencapsulated oleophilic materials (isocyanates) with functional (hydroxyl-)groups on cross-linked hydrophilic binders (U.S. Pat. No. 5,569,573; EP 646 476; WO94/2395; WO98/29258).
U.S. Pat. No. 6,582,882 describes an imaging element comprising a graft copolymer having a hydrophobic backbone and a plurality of pendant hydrophilic groups or a plurality of pendant groups comprising hydrophilic and hydrophobic segments. Upon exposure of the imaging element to thermal energy, the exposed areas become less soluble in a developer than the unexposed areas.
U.S. Pat. No. 6,362,274 describes grafted copolymers comprising three sequences: one sequence for anchoring on solid particles such as pigments and fillers, one hydrophobic sequence and one hydrophilic sequence for using the copolymers in aqueous and/or organic medium. The disclosed copolymers are of particular interest in a wide range of paint formulations; there is no reference in the cited prior art document to lithographic printing plates.
None of the prior art discloses the heat-sensitive copolymer of the present invention in lithographic printing plates.