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
2001-09-05
2003-06-17
Ashton, Rosemary (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, C430S286100, C430S302000, C430S348000, C430S944000, C430S945000, C430S964000, C101S453000, C101S457000, C101S465000, C101S467000
Reexamination Certificate
active
06579662
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to thermal imaging compositions and to imaging members (particularly lithographic printing plates) prepared therefrom. The invention also relates to a method of imaging such imaging members, and to a method of printing using them.
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 by an imaged area and the water or fountain solution is preferentially retained by the non-imaged areas. When a suitably prepared surface is moistened with water and ink is then applied, the background or non-imaged areas retain the water and repel the ink while the imaged areas accept the ink and repel the water. The ink is then 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 becoming more 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.
It has been recognized that a lithographic printing plate could be created by ablating an IR absorbing layer. For example, 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). 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.
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 for resulting printing quality. Such plates generally require at least two coated layers on a support.
Thermally switchable polymers have been described for use as imaging materials in printing plates. By “switchable” is meant that the polymer is rendered from hydrophobic to relatively more hydrophilic or, conversely from hydrophilic to relatively more hydrophobic, upon exposure to heat.
U.S. Pat. No. 4,034,183 (Uhlig) describes the use of high-powered lasers to convert hydrophilic surface layers to hydrophobic surfaces. A similar process is described for converting polyamic acids into polyimides in U.S. Pat. No. 4,081,572 (Pacansky). The use of high-powered lasers is undesirable in the industry because of their high electrical power requirements and because of their need for cooling and frequent maintenance.
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), and the plate's use is in a positive-working mode only.
U.S. Pat. No. 4,405,705 (Etoh et al.) and U.S. Pat. No. 4,548,893 (Lee et al.) describe amine-containing polymers for photosensitive materials used in non-thermal processes. Thermal processes using polyamic acids and vinyl polymers with pendant quaternary ammonium groups are described in U.S. Pat. No. 4,693,958 (Schwartz et al.). U.S. Pat. No. 5,512,418 (Ma) describes the use of polymers having cationic quaternary ammonium groups that are heat-sensitive. However, the materials described in this art require wet processing after imaging.
WO 92/09934 (Vogel et al.) describes photosensitive compositions containing a photoacid generator and a polymer with acid labile tetrahydropyranyl or activated ester groups. However, imaging of these compositions converts the imaged areas from hydrophobic to hydrophilic in nature.
In addition, EP-A 0 652 483 (Ellis et al.) describes lithographic printing plates imageable using infrared radiation (“IR”) lasers, and which do not require wet processing. These plates comprise an imaging layer that becomes more hydrophilic upon 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. Imaging such compositions converts the imaged areas from hydrophobic to relatively more hydrophilic in nature, and thus requires imaging the background of the plate, which is generally a larger area. This can be a problem when imaging to the edge of the printing plate is desired.
U.S. Pat. No. 5,985,514 (Zheng et al.) is directed to processless direct write printing plates that include an imaging layer containing heat sensitive polymers. The polymer coatings are sensitized to infrared radiation by the incorporation of an infrared absorbing material such as an organic dye or a fine dispersion of carbon black. Upon exposure to a high intensity infrared laser, light absorbed by the organic dye or carbon black is converted to heat, thereby promoting a physical and/or chemical change in the polymer (usually a change in hydrophilicity or hydrophobicity). The resulting printing plates can be used on conventional printing presses to provide, for example, negative images. Such printing plates have utility in the evolving “computer-to-plate” or “direct-write” printing market.
Other imaging members having “switchable” imaging layers are described in U.S. Pat. No. 6,146,812 (Leon et al.) wherein switching occurs rapidly yet the heat-sensitive polymers have improved shelf life stability.
Some of the heat-sensitive polymers described in the copending applications, particularly the polymers containing organoonium or other charged groups, have a tendency to undergo physical interactions or chemical reactions with the organic dye or carbon black, thus compromising the effectiveness of both polymers and heat-absorbing materials.
Organic dye salts, by nature, are often partially soluble in water or alcoholic coating solvents and are thus preferred as IR dye sensitizers. However, many such salts have been found to be unacceptable because of insufficient solubility, because they react with the charged polymer to form hydrophobic products that can result in scummed or toned images, or because they offer insufficient thermal sensitization in imaging members. In particular, there is a need to have IR dye sensitizers that are compatible with thiosulfate polymers, such as those described in U.S. Pat. No. 5,985,514 (noted above).
Other imaging members comprise cationic heat-sensitive ionomers that are used in combination with IR-sensitive dyes or carbon. Representative cationic ionomers are described for example in U.S. Pat. No. 6,190,830 (Leon et al.) and U.S. Pat. No. 6,190,831 (Leon et al.).
Improved thermally sensitive compositions and imaging members are also described in GB 2,358,710 (DoMinh et al.). These compositions comprise IR sensitive oxonol dyes that are described in U.S. Pat. No. 6,248,886 (Williams et al.) and U.S. Pat. No. 6,248,893 (Williams et al.).
There is a need for direct-write lithographic im
Williams Kevin W.
Zheng Shiying
Ashton Rosemary
Gilliam Barbara
Tucker J. Lanny
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