Radiation imagery chemistry: process – composition – or product th – Transfer procedure between image and image layer – image... – Imagewise heating – element or image receiving layers...
Reexamination Certificate
2002-02-27
2004-08-10
Schilling, Richard L. (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Transfer procedure between image and image layer, image...
Imagewise heating, element or image receiving layers...
C430S254000, C430S273100, C430S306000, C430S309000, C430S327000, C430S330000
Reexamination Certificate
active
06773859
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for making a flexographic printing plate. In particular, the process forms the plate using a photosensitive element having at least one thermally-removable layer on at least one photopolymerizable layer and heating the element to form a relief surface suitable for flexographic printing. The invention also relates to a photosensitive element having a thermally-removable layer, which after undergoing thermally processing, is suitable for use as a flexographic printing plate.
2. Description of Related Art
Flexographic printing plates are widely used for printing of packaging materials ranging from corrugated carton boxes to card boxes and to continuous web of plastic films. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a layer of the photopolymerizable composition interposed between a support and a coversheet or a multilayer cover element. Flexographic printing plates are characterized by their ability to crosslink or cure upon exposure to actinic radiation. Typically the plate is uniformly exposed through the backside of the plate to a specified amount of actinic radiation. Next, an imagewise exposure of the front-side of the plate is made through an image-bearing artwork or a template, such as a photographic negative or transparency (e.g. silver halide film) inside a vacuum frame to ensure intimate contact of the artwork to the plate surface. The plate is exposed to actinic radiation, such as an ultraviolet (UV) or black light. The actinic radiation enters the photosensitive material through the clear areas of the transparency and is blocked from entering the black or opaque areas. The exposed material crosslinks and becomes insoluble to solvents used during image development. The unexposed, uncrosslinked photopolymer areas under the opaque regions of the transparency remain soluble and are washed away with a suitable solvent leaving a relief image suitable for printing. Then the plate is dried. The printing plate can be further treated to remove surface tackiness. After all desired processing steps, the plate is mounted on a cylinder and used for printing.
However, developing systems that treat the element with a solution are time consuming since drying for extended period (0.5 to 24 hours) is necessary to remove entrained developer solution. In addition, these developing systems produce potentially toxic by-product wastes (both the solvent and any material carried off by the solvent) during the development process.
To avoid the problems with solution development, a “dry” thermal development process may be used. In a thermal development process, the photosensitive layer, which has been imagewise exposed to actinic radiation, is contacted with an absorbent material at a temperature sufficient to cause the composition in the unexposed portions of the photosensitive layer to soften or melt and flow into an absorbent material. See U.S. Pat. Nos. 3,264,103 (Cohen et al.); U.S. Pat. No. 5,015,556 (Martens); U.S. Pat. No. 5,175,072 (Martens); U.S. Pat. No. 5,215,859 (Martens); and U.S. Pat. No. 5,279,697 (Peterson et al.). In all these cited patents imagewise exposure is conducted with a silver halide film target in a vacuum frame. The exposed portions of the photosensitive layer remain hard, that is, do not soften or melt, at the softening temperature for the unexposed portions. The absorbent material collects the softened un-irradiated material and then is separated and/or removed from the photosensitive layer. The cycle of heating and contacting the photosensitive layer may need to be repeated several times in order to sufficiently remove the flowable composition from the un-irradiated areas and form a relief structure suitable for printing. Thus remains a raised relief structure of irradiated, hardened composition that represents the desired printing image.
The photosensitive element for flexographic use typically includes one or more additional layers on the photopolymerizable layer on the side opposite the support, that is, sandwiched between the photopolymerizable layer and the coversheet. The one or more additional layers can be, for instance, a release layer for ease in removing the image-bearing transparency (mask film) after exposure under vacuum, an elastomeric capping layer, or a radiation opaque layer. The radiation opaque layer is used to form a mask in situ on or above the photopolymerizable layer. Since the radiation opaque layer typically is also sensitive to infrared-radiation, the radiation opaque layer is exposed imagewise to infrared laser radiation to form an image on the photopolymerizable layer, i.e., the in situ mask. The infrared laser radiation can selectively remove or ablate the infrared sensitive layer (i.e., radiation opaque layer) from the photosensitive element, as disclosed by Fan in U.S. Pat. Nos. 5,262,275 and 5,719,009; Fan in EP 0 741 330 A; and Van Zoeren in U.S. Pat. No. 5,506,086; or can selectively transfer the infrared sensitive material onto the photopolymerizable layer of the element as disclosed by Fan et al. in U.S. Pat. No. 5,607,814; and Blanchett in U.S. Pat. Nos. 5,766,819; 5,840,463; and EP 0 891 877 A. One or more barrier layers may also be present between the photopolymerizable layer and the radiation opaque layer to maintain the integrity of the radiation opaque layer on the photopolymerizable layer. Thereafter, the element is overall exposed with actinic radiation through the mask and then wet processed, that is, in a solvent or aqueous solution, to form the relief as described above.
U.S. Pat. No. 5,888,697 discloses a photosensitive element having an overall layer of powder material on a photopolymerizable layer. The layer of particulate material can be opaque or transparent depending on desired use. A transparent powder layer can function as a release layer for the photosensitive element. The opaque powder layer can function as the radiation opaque layer for the photosensitive element as explained above. The powder material can be organic, inorganic, mixtures of organic and inorganic compounds, or multicomponent. The particle size of the powder material is less than 50 microns. After imagewise exposure, the relief is formed in the photosensitive element by washing with a suitable developer selected from organic solutions, water, aqueous, or semi-aqueous solutions. Thus, only conventional wet processing to form the relief image was disclosed.
EP 0 665 471 A2 discloses thermally developable flexographic printing plates having a release layer on a photosensitive elastomeric layer. The release layer is composed of a cellulose-based polymer, such as hydroxyalkyl cellulose, which has a softening point and melt viscosity compatible with the softening temperature of the photosensitive elastomeric layer forming the relief layer. The preferred developing temperatures range from 95° C. to about 150° C. The cellulose-based polymer of the release layer has a softening temperature less than about 130° C. However, not all hydroxyalkyl cellulose compounds were appropriate as the release layer for thermally developed plates. Hydroxyalkyl cellulose having too high a molecular weight was too viscous at the desired thermal developing temperatures. EP 0 665 471 A2 also discloses that a layer of polyvinyl alcohol on a photosensitive elastomeric layer does not melt sufficiently for thermal development.
U.S. Pat. No. 3,060,024 discloses a photopolymerization process for reproducing images using a thermoplastic element composed of a thermoplastic organic polymer, an ethylenically unsaturated compound with a terminal ethylenic group and a photoinitiator. In the process, the thermoplastic element is exposed imagewise to actinic light through a target in a vacuum frame, heated to the tack temperature, an
Fan Roxy Ni
Hackler Mark A.
Kannurpatti Anandkumar R.
Lungu Adrian
Taylor Bradley K.
E. I. du Pont de Nemours and Company
Magee Thomas H.
Schilling Richard L.
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