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
2000-08-23
2002-09-17
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
C430S303000, C430S348000, C101S463100, C101S467000
Reexamination Certificate
active
06451500
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to lithographic printing plates and specifically to lithographic printing plates that require no wet processing after imaging. The invention also relates to a method of digitally 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 negative working printing plate 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 reverse holds true for positive working plates, in which the background is imaged. 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 less common, yet represent a steadily growing market. Currently, most of these plates utilize similar materials and similar imaging mechanisms as UV-imageable plates. For example, a thermal acid generator might be used in lieu of a photoacid generator and the same series of preheat and development steps might be employed. The main advantage of these digital plates is that the thermal imaging process is rapid and inexpensive compared to the analog process involving the creation of a mask and blanket UV exposure. 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. 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. Other publications describing ablatable printing plates include U.S. Pat. No. 5,385,092 (Lewis et al), U.S. Pat. No. 5,339,737 (Lewis et al), U.S. Pat. No. 5,353,705 (Lewis et al), U.S. Pat. No. Reissue 35,512 (Nowak et al) and U.S. Pat. No. 5,378,580 (Leenders).
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. Such plates generally require at least two coated layers on a support.
One approach toward non-process, non-ablation printing plates involves the use of “switchable polymers.” These polymers will undergo thermally driven chemical reactions in which highly polar moieties are either created or destroyed under imaging conditions. This results in the storage of the imaging data as hydrophilic and hydrophobic regions of a continuous polymer surface. In addition to not needing wet processing, such plates have the advantage of not needing any type of material collection devices which ablation-based plates require. Also unlike ablation plates, a switchable polymer plate in its ideal form would consist of one layer and can be manufactured on a single pass through a coating machine.
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 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. The imaged materials also require wet processing after imaging.
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), but wet processing is required after imaging. In addition, the polyamic acid switchable polymers in this invention show low discrimination magnitude and the quaternary ammonium-based examples suffer from wash-off problems of both the foreground and the background.
U.S. Pat. No. 5,512,418 (Ma) describes the use of polymers having cationic quaternary ammonium groups that are heat-sensitive. However, like most of the materials described in the art, wet processing is required 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 and the imaged areas are prone to scumming.
In addition, 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 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. As with the plates described in WO 92/09934, the plates described in Ellis et al are also prone to scumming.
Although a number of switchable polymer-based printing plates are known, there remain technical barriers towar
Ashton Rosemary
Baker & Botts L.L.P.
Gilmore Barbara
Kodak Polychrome Graphics LLC
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