Inkjet-imageable lithographic printing members and methods of preparing and imaging them

This application claims priority to and the benefits of U.S. Provisional Application Ser. No. 60/617,695, filed on Oct. 12, 2004, the entire disclosure of which is hereby incorporated by reference.

The present invention relates in general to lithography, and more particularly to lithographic printing members suitable for inkjet imaging.

In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening fluid to the plate prior to inking. The dampening fluid prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.

Lithographic plates can be fabricated in various ways, ranging from traditional manual techniques involving photoexposure and chemical development to automated procedures involving computer control. Computer-to-plate systems can utilize pulses of electromagnetic radiation, produced by one or more laser or non-laser sources, to create physical or chemical changes at selected points of sensitized plate blanks (which, depending on the system, may be used immediately or following conventional photodevelopment); ink-jet equipment used to selectively deposit ink-repellent or ink-accepting spots on plate blanks; or spark-discharge equipment, in which an electrode in contact with or spaced close to a plate blank produces electrical sparks to alter the characteristics of certain areas on a printing surface, thereby creating “dots” which collectively form a desired image. To circumvent the cumbersome photographic development, plate-mounting, and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate.

Digital imaging of printing plates by inkjet printing is currently widely investigated; see, e.g., U.S. Pat. Nos. 6,526,886 and 6,691,618. The \'618 patent, for example, discloses a method of imaging a lithographic printing plate having an alkaline-soluble polymeric coating with a pH-elevating agent. The need to use alkaline solutions to develop these plates, however, poses environmental and safety problems. It would be desirable to have inkjet-imagable lithographic printing plates that can be solely developed by water, or which do not require a development step.

The present invention relates to lithographic printing members imagable using a combination of inkjet and photopolymerization. An advantage to this approach is the durability of photopolymer plates and the ability to utilize conventional ultraviolet (UV) imaging sources.

Accordingly, in a first aspect, the invention provides a lithographic printing member having a photosensitive top layer, an optional crosslinked intermediate layer, and a substrate thereunder, as well as methods of imaging such a printing member. The top layer contains a photo-polymerizable moiety and the first component of a two-component photo-polymerization initiating system. When reacted with an imaging fluid containing the second component of the two-component photo-polymerization initiating system and thereupon subjected to actinic radiation, the top layer undergoes photopolymerization and becomes crosslinked. In some embodiments, the top layer is oleophilic and the substrate (or intermediate layer) is hydrophilic, whereas in other embodiments, the top layer is hydrophilic and the substrate (or intermediate layer) is oleophilic.

To provide a lithographic image, the imaging fluid is dispensed onto the top layer of the printing member in an imagewise pattern. The imaged plate is then exposed to actinic radiation, allowing the imaging fluid to react with the photosensitive top layer and form a crosslinked imaged area. The imaged printing member is then subjected to a solvent, such as an aqueous fluid, to remove non-image (i.e., uncrosslinked), portions of the top layer, leaving an imagewise lithographic pattern on the printing member ready for inking.

Suitable photo-polymerizable moieties for forming the top layer include, but are not limited to, multifunctional acrylic monomers, oligomers, macromers, and combinations thereof. A suitable two-component photo-polymerization initiating system that can be used to prepare the top layer and the imaging fluid is a hexaarylbiimidazole dimer with a free-radical-producing electron donor agent as a co-catalyst. Although none of these compounds dissolves in water, it is believed that because the functional moieties only weakly adhere to the hydrophilic intermediate layer underneath, water may be applied to the top layer to disrupt the weak adhesion between the top and underlying layers, thereby allowing the removal of the non-image portions of the top layer. On the other hand, adhesion between the imaged areas, i.e., where photopolymerization and crosslinking have occurred, and the underlying layer is sufficiently strong that the imaged portions cannot be removed by water in the development step.

In a second aspect, the invention provides a printing member having a soluble top layer, an optional intermediate layer, and a substrate thereunder, as well as methods of imaging such a printing member. The top layer, which contains a photoreactive reagent, becomes crosslinked and water-insoluble when exposed to actinic radiation. An imaging fluid that contains at least one of a photoreaction-inhibiting agent and a masking agent may be applied to the top layer, reducing or totally inhibiting the reactivity of the photoreactive reagent and thereby preventing the imaged areas from undergoing crosslinking. In some embodiments, the top layer is oleophilic and the substrate (or intermediate layer) is hydrophilic, whereas in other embodiments, the top layer is hydrophilic and the substrate (or intermediate layer) is oleophilic.

To provide a lithographic image, the imaging fluid is dispensed onto the top layer of the printing member in an imagewise pattern. The imaged plate is then exposed to actinic radiation, which induces crosslinking in the non-image areas, causing these areas to become water-insoluble. On the other hand, because the jetted fluid has masked the imaged portions from light and/or reacted within the imaged portions of the top layer to reduce its photoreactivity, crosslinking does not occur in the imaged areas and a water-soluble image is formed in the top layer. The imaged printing member is then subjected to a solvent, such as an aqueous fluid, to remove the imaged portions, leaving an imagewise lithographic pattern on the printing member ready for inking.

The photoreactive reagent found in the top layer can be a photo-crosslinking agent or a photo-polymerization initiator. Suitable photo-crosslinking agents include various bis-azides. Water-soluble polymers that can be crosslinked by bis-azides include, but are not limited to, polyvinyl pyrrolidone, copolymers of vinyl pyrrolidone, acrylamide-diacetone acrylamide copolymer, polyacrylamide, copolymers of acrylamide, gelatin, and mixtures thereof.

An alternative photo-crosslinking agent is a condensation product of an aromatic diazonium salt and a condensation agent. Suitable aromatic diazonium salts include, but are not limited to, 4-diazodiphenylamine sulfate, 3-methoxy-4-diazodiphenylamine sulfate, 4-phenoxydiazobenzene zinc chloride double salt, and 2,5-dimethoxy-4-tolylmercaptodiazobenzene zinc chloride double salt. Water-soluble polymers that can be crosslinked by an aromatic diazonium salt-containing compound include, but are not limited to, polyvinyl alcohol, carboxymethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydropropylpropylcellulose, polyacrylic or methacrylic acid, polyvinyl pyrrolidone, copolymers of vinyl pyrrolidone, polyacrylamide, copolymers of polyacrylamide, gelatin, and mixtures thereof.

Suitable photo-polymerization initiators include those that are water-soluble. Examples include, but are not limited to, ceric ammonium oxalate, manganic ammonium oxalate, and photoreducible dyes and their co-catalysts. Another useful photoreactive reagent is a polyvinyl alcohol comprising a stilbazolium moiety.

The imaging fluid may contain a photoreaction-inhibiting agent such as an antioxidant. Suitable antioxidants include, but are not limited to, hydroquinone, 4-methoxyphenol, 2,6-di-(t-butyl)-4-methylphenol, 2,2′-methylenebis-(4-methyl-6-t-butylphenol), diesters of thiodipropionic acid, and triarylphosphite. Alternatively, the imaging fluid may contain a masking agent that shields the imaged area from exposure to actinic radiation. Suitable masking agents include various dyes and pigments that absorb at wavelengths of photosensitivity of the photosensitive top layer. Examples include, but are not limited to, Acid Yellow 25 (C.I. 18835), Acid Yellow 29 (C.I 0.18900), Acridine Yellow G (C.I 0.46025), Basic Yellow 2 (C.I. 41000), Acid Black 2 (C.I. 50420), and aqueous dispersions of carbon black and yellow pigments. An imaging fluid that contains both a masking agent and a photoreaction-inhibiting agent may also be used according to the invention.

It should be stressed that, as used herein, the term “plate” or “member” refers to any type of printing member or surface capable of recording an image defined by regions exhibiting differential affinities for ink and/or fountain solution. Suitable configurations include the traditional planar or curved lithographic plates that are mounted on the plate cylinder of a printing press, but can also include seamless cylinders (e.g., the roll surface of a plate cylinder), an endless belt, or other arrangement.

Furthermore, the term “hydrophilic” is used in the printing sense to connote a surface affinity for a fluid which prevents ink from adhering thereto. Such fluids include water for conventional ink systems, aqueous and non-aqueous dampening liquids, and the non-ink phase of single-fluid ink systems. Thus a hydrophilic surface in accordance herewith exhibits preferential affinity for any of these materials relative to oil-based materials.