Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
1999-11-01
2002-07-02
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S278100, C430S281100, C430S302000
Reexamination Certificate
active
06413694
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to direct write, processless imaging members, and particularly to heat-sensitive imaging members, that require no wet processing after imaging. The invention also relates to methods of digital imaging and printing using these imaging members.
BACKGROUND OF THE INVENTION
The art of lithographic printing is based upon the immiscibility of oil and water, wherein an oily material or ink is preferentially retained in certain areas and the water or fountain solution is preferentially retained in other areas, depending upon the type of image produced. When a suitably prepared surface is moistened with water, and ink is then applied, either a positive or negative image is obtained, depending upon whether the imaged or non-imaged areas are ink-accepting. The ink is eventually transferred to the surface of a suitable substrate, such as cloth, paper or metal, thereby reproducing the image.
Very common lithographic printing plates include a metal or polymer support having thereon an imaging layer sensitive to visible or UV light. Both positive- and negative-working printing plates can be prepared in this fashion. Upon exposure, and perhaps post-exposure heating, either imaged or non-imaged areas are removed using wet processing chemistries.
Thermally sensitive printing plates are less common. Examples of such plates are described in U.S. Pat. No. 5,372,915 (Haley et al). They include an imaging layer comprising a mixture of dissolvable polymers and an infrared radiation absorbing compound. While these plates can be imaged using lasers and digital information, they require wet processing using alkaline developer solutions.
Conventional preparation and use of such printing plates generally involves multiple processing steps such as exposure to either light or heat (or both) through a patterned image, and wet processing with an alkaline developer to provide a printing plate image.
Dry planography, or waterless printing, is well known in the art of lithographic offset printing and provides several advantages over conventional offset printing. Dry planography is particularly advantageous for short run and on-press applications. It simplifies press design by eliminating the fountain solution and aqueous delivery train. Careful ink water balance is unnecessary, thus reducing rollup time and material waste. Silicone rubbers [such as poly(dimethylsiloxane) and other derivatives of poly(siloxanes)] have long been recognized as preferred waterless-ink repelling materials. The criteria for waterless lithography and the ink repelling properties of poly(siloxanes) have been extensively reviewed in the TAGA Proceedings 1975 pages 120, 177 and 195 and 1976 page 174. It was concluded that, in addition to low surface energy, the ability to swell in long-chain alkane ink solvents (i.e., its “oleophilic” nature) accounts for silicone's superior ink releasing characteristics. An important consideration is that siloxane polymers repel ink.
It has been recognized that a lithographic printing plate could be created containing an IR absorbing layer. Canadian 1,050,805 (Eames) discloses a dry planographic printing plate comprising an ink receptive substrate, an overlying silicone rubber layer, and an interposed layer comprised of laser energy absorbing particles (such as carbon particles) in a self-oxidizing binder (such as nitrocellulose) and an optional cross-linkable resin. Such plates were exposed to focused near IR radiation with a Nd
++
YAG laser. The absorbing layer converted the infrared energy to heat thus partially loosening, vaporizing or ablating the absorber layer and the overlying silicone rubber. The plate was developed by applying naphtha solvent to remove debris from the exposed image areas. Similar plates are described in
Research Disclosure
19201, 1980 as having vacuum-evaporated metal layers to absorb laser radiation in order to facilitate the removal of a silicone rubber overcoated layer. These plates were developed by wetting with hexane and rubbing. CO
2
lasers are described for ablation of silicone layers by Nechiporenko & Markova, PrePrint 15th International IARIGAI Conference, June 1979, Lillehammer, Norway, Pira Abstract 02-79-02834. Typically, such printing plates require at least two layers on a support, one or more being formed of ablatable materials.
“Direct write” imaging eliminates the use of the pattern of light or heat to generate an image. When a laser is used for this purpose, the laser can be used to heat only small regions at a time. Moreover, a computer can be used to produce the high resolution images pixel by pixel. If the plate is processless, chemical development is also eliminated.
While the noted printing plates used for digital, processless printing have a number of advantages over the more conventional photosensitive printing plates, there are a number of disadvantages with their use. The process of ablation creates debris and vaporized materials that must be collected. The laser power required for ablation can be considerably high, and the components of such printing plates may be expensive, difficult to coat, or unacceptable in resulting printing quality. Typically, such printing plates require at least two layers on a support, one or more being formed of ablatable materials.
Thermally switchable polymers have been described for use as imaging materials in printing plates. By “switchable” is meant that the polymer is rendered either more hydrophilic (or oleophobic) or hydrophobic (or oleophilic) upon exposure to heat.
As an alternative method of preparing printing plates, U.S. Pat. No. 4,634,659 (Esumi et al) describes imagewise irradiating hydrophobic polymer coatings to render exposed regions more hydrophilic in nature. While this concept was one of the early applications of converting surface characteristics in printing plates, it has the disadvantages of requiring long UV light exposure times (up to 60 minutes).
EP-A 0 652 483 (Ellis et al) describes lithographic printing plates imageable using IR lasers, and which do not require wet processing. These plates comprise an imaging layer that becomes more hydrophilic upon the imagewise exposure to heat. This coating contains a polymer having pendant groups (such as t-alkyl carboxylates) that are capable of reacting under heat or acid to form more polar, hydrophilic groups. The problem with such materials is that they are very difficult to manufacture, exhibit poor shelf life, require a photoacid generator for imaging, and are positive-working only. Other lithographic printing plates hydrophilic polymers containing pendant carboxylic acids are described in U.S. Pat. No. 4,081,572 (Pacansky).
Positive-working photoresists and printing plates having crosslinked, UV-sensitive polymers are described in EP-A 0 293 058 (Shirai et al). The polymers contain pendant iminosulfonate groups that are decomposed upon UV exposure, generating a sulfonic group and providing polymer solubility.
U.S. Pat. No. 5,512,418 (Ma) describes the use of cationic polymers containing pendant ammonium groups for thermally induced imaging. However, chemical processing is still required to provide the desired image.
Japanese Kokai 9-197,671 (Aoshima) describes a negative-working printing plate and imaging method in which the imaging layer includes a sulfonate-containing polymer, an IR radiation absorber, a novolak resin and a resole resin. Wet processing with a conventional alkaline developer is required to produce the desired negative image.
Thus, the graphic arts industry is seeking alternative means for providing a processless, direct-write, positive-working lithographic printing plate that can be imaged without ablation and the accompanying problems noted above.
SUMMARY OF THE INVENTION
The problems noted above are overcome with a positive-working imaging member comprising a support having thereon a heat-sensitive surface imageable layer comprising:
a) a heat-sensitive polymer comprising a heat-activatable sulfonate group, and
b) a photothermal conversion mate
Dominh Thap
Zheng Shiying
Baker & Botts L.L.P.
Baxter Janet
Gilmore Barbara
Kodak Polychrome Graphics LLC
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