Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
2001-02-22
2002-11-12
Hamilton, Cynthia (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S300000, C430S309000, C430S292000
Reexamination Certificate
active
06479217
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for forming selectively colored printing plates using photosensitive recording elements.
Flexographic printing plates are widely used, primarily in the packaging industry, for printing on a wide variety of substrates. Flexographic printing plates can be used to print on soft and easily deformable substrates such as cardboard, plastic films, etc. without sacrificing print quality. Flexographic printing plates are characterized by a relief portion corresponding to the image to be printed. During printing, the relief portion of the plate is inked and subsequently transfers the printed image to the substrate by impression.
Flexographic printing plates can be prepared from a variety of materials including rubber and photosensitive compositions. The conventional process for preparing flexographic plates from rubber involves several laborious steps including art work, photo engraving and formation of the plate by vulcanizing the rubber in a suitable mold. Rubber flexographic plates can also be prepared by laser engraving a rubber ablatable element structure thereby avoiding the molding step in the conventional process.
Flexographic printing plates can also be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,749. The photopolymerizable compositions generally comprise a binder, at least one monomer and a photoiniator. The photopolymerizable composition in the form of a photosensitive recording element is image wise exposed to actinic radiation thereby polymerizing the polymerizable composition in the exposed areas. The unexposed areas of the photopolymerizable layer are removed, typically by washing with a solvent developer, thereby forming a printing relief which can be used for flexographic printing. Image wise exposure of the photosensitive recording element typically involves the use of a negative mask having a predetermined pattern of transparent and opaque areas covering the photopolymerizable layer. Alternatively, the negative can be eliminated by the use of a laser beam capable of directly recording information on the photosensitive element as disclosed in U.S. Pat. No. 5,262,275 to Fan.
Conventional flexographic printing plates are characterized by relief portions corresponding to the exposed portion and non-image portions corresponding to the non-exposed portions wherein the color and appearance of the relief portion and the non-image portion are the same. Therefore, there is little or no contrast between the relief portion and the non-image portion of the plate. The inability to quickly and accurately distinguish between the image and non-image areas of the plate can create quality control problems and interfere with proper positioning of the printing plate onto a print cylinder of a press. Accordingly, it would be desirable to provide a photosensitive recording element useful in producing a flexographic printing plate wherein the areas of the recording element exposed to actinic radiation are cured and colored to provide an obvious contrast between the image and non-image portions of the flexographic printing plate.
U.S. Pat. No. 6,030,749 to Takahashi describes one method for producing a flexographic printing element wherein the image portion has a color which is different from the color of the non-image portion based on the color change produced by an anthraquinone dye exposed to ultraviolet light having a wavelength from 300 to 400 nm. The disclosure of this reference is limited to the use of anthraquinone dyes which exhibit a color change upon being irradiated with an ultraviolet ray of wavelength in the range from 300 to 400 nm in the presence of a radical producer capable of producing a free radical upon irradiation.
Photocurable and photocolorable compositions which can be both cured and colored upon exposure to actinic radiation are known. However, there is no disclosure or suggestion of using such compositions in the production of printing plates. The following references, incorporated herein by reference, disclose various photocurable and photocolorable compositions.
Commonly assigned U.S. Pat. No. 5,942,554, herein incorporated by reference, discloses a curable composition containing a color precursor and an onium salt which is both cured and colored upon exposure to actinic radiation. Color precursors disclosed as being useful are those which are capable of reacting with acid or whose excited states are capable of donating an electron.
Commonly assigned U.S. patent application Ser. No. 09/558,899 filed on Apr. 26, 2000 (Attorney Docket No. 594562-024), herein incorporated by reference, discloses selectively colorable polymerizable compositions comprising a leucobase color former and a leuconitrile color former. Irradiation of the composition cures and selectively colors the composition. The color at any one location depends on the actinic dose and the interaction of the leucobase and leuconitrile color formers. In one embodiment, a selectively colorable, polychromic composition is provided wherein low light exposure creates a polymer of one color, intermediate light exposure changes the color of the polymer and a high dosage of light exposure bleaches the color of the polymer.
Selectively colorable polymerizable compositions are also disclosed in commonly assigned U.S. patent application Ser. No. 09/638,252 filed on Aug. 14, 2000 (Attorney Docket No. 594562-027), the disclosure of which is incorporated herein by reference. A selectively colorable polymerizable composition comprising a leucobase color former and an oxidizing agent which is irradiated with light of a particular wavelength and specific intensity for a specified duration is disclosed. Exposure to actinic radiation cures the composition and activates the color former which can be optionally deactivated or bleached by further exposure. The irradiation dosage can be varied to selectively color the polymeric body whereby the resultant color of any particular area depends on the exposure dose received at that location. By varying the dose, a polymeric body can be prepared having distinctly colored elements at specific locations.
International Publication Number WO 97/09168 to Zeneca discloses a photocurable, photocolorable composition which is irradiated with a low dose of light to cure and a different dose of light, preferably higher, to cause color formation. Color formation or color change occurs as a result of contact between a color former and a photochemically generated developer. The color formers disclosed as being useful include acid sensitive compounds such as lactones, fluorans, etc.
SUMMARY OF THE INVENTION
The present invention relates to selectively colorable photosensitive recording elements used in the production of printing plates and to a method for forming selectively colored printing plates using such recording elements. In accordance with the invention a photosensitive recording element containing a photopolymerizable recording layer is disclosed. The photopolymerizable recording layer of the invention comprises a photopolymerizable monomer, a radical photoinitiator, a color former, and a color photoinitiator. The recording layer may optionally include a polymeric binder. Color formers can be leuco dyes or reducible dyes. The present invention is also directed to a method for producing a printing plate from the photosensitive recording element of the invention by uniformly exposing the back side of the photopolymerizable recording layer to actinic radiation of a first dose thereby forming a cured back surface without coloration or color change, imagewise exposing the front of the photopolymerizable recording layer to a second dose of actinic radiation thereby forming exposed and unexposed areas on the recording layer wherein the exposed areas are cured and undergo a color change by exposure to the actinic radiation and removing the unexposed areas of the imagewise exposed recording layer thereby forming an image in relief wherein the image portion and the
Grinevich Oleg V.
Mejiritski Alexander
Neckers Douglas C.
Serguievski Petr
Hamilton Cynthia
Spectra Group Limited, Inc.
Thompson Hine LLP
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