Radiation imagery chemistry: process – composition – or product th – Imaged product – Including resin or synthetic polymer
Reexamination Certificate
1996-10-18
2002-06-11
Hamilton, Cynthia (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaged product
Including resin or synthetic polymer
C430S300000, C430S284100
Reexamination Certificate
active
06403269
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a photosensitive resin composition, to printing plates made therefrom, a method of making the printing plate, and to a method of printing therewith.
BACKGROUND OF THE INVENTION
In flexographic printing, also known as relief printing, ink is transferred from a pool of ink to a substrate by way of a printing plate. The surface of the plate is shaped so that the image to be printed appears in relief, in the same way that rubber stamps are cut so as to have the printed image appear in relief on the surface of the rubber. Typically, the plate is mounted on a cylinder, and the cylinder rotates at high speed such that the raised surface of the printing plate contacts a pool of ink, is slightly wetted by the ink, then exits the ink pool and contacts a substrate web, thereby transferring ink from the raised surface of the plate to the substrate to form a printed substrate.
Flexographic printing competes with other forms of printing, e.g., lithography, gravure and letterpress printing. Those involved in the flexographic printing industry are constantly striving to improve the flexographic printing process in order to more effectively compete with other printing methods. One area which has received much attention from researchers is the development of improved plates for flexographic printing.
The demands placed on flexographic printing plates are numerous. For instance, a flexographic printing plate must have sufficient flexibility (a mechanical property) to wrap around a printing cylinder, yet be strong enough to withstand the rigors experienced during typical printing processes. Further, the printing plate should possess a low hardness to facilitate ink transfer during printing. It is also important that the surface of the printing plate be dimensionally stable during storage. For example, some compositions used for making plates have shown inferior stability properties in that they become tacky and pasty during storage.
It is also required that the printing plate have a relief image that has a chemical resistance against the aqueous-based ink or alcohol-based ink which is usually used in flexographic printing. It is further desired that the physical and printing properties of the printing plate are stable and do not change during printing. In order to maintain high quality, clear printing during a run, it is highly desirable that a printing plate not pick up deposits of paper fibers and dried ink which would fill in reverse areas of the plate and deposit at the edges of the printing areas of the plate. When plates pickup excessive deposits during printing, the printing press must be shut down periodically during the run to clean the plates, resulting in a loss of productivity.
In the early days of flexographic printing, a rubber printing plate was produced by direct engraving on a rubber plate. Another early mode of preparing flexographic plates was to first produce an original engraved plate by etching of a metallic plate, then form a plastic matrix in the form of the metallic plate, followed by pouring rubber into the matrix plate and vulcanizing the rubber. These methods for preparing what may be termed a conventional printing plate, are complex and require considerable expertise to provide a satisfactory product. Since these manufacturing processes require many steps, they are expensive and very time-consuming.
In order to overcome the shortcomings of conventional plates, there has recently been proposed a method of producing a flexographic printing plate from a photosensitive, also known as photopolymerizable or photocurable, resin composition. These so-called photopolymer plates are rapidly coming into general use in various fields of printing because of their better handleability, higher productivity, lower price, and improved printing performance as compared with a conventional printing plate.
Photopolymerizable resin compositions generally comprise an elastomeric binder, herein sometimes referred to as a prepolymer or an oligomer, at least one monomer and a photoinitiator. To prepare the plates, there is generally formed a photopolymerizable layer interposed between a support and one or more cover sheets that may include slip and release films to protect the photosensitive surface. Prior to processing the plate, the cover sheets may be removed, and the photosensitive surface is exposed to actinic radiation in an imagewise fashion. Upon imagewise exposure to actinic radiation, polymerization, and hence, insolubilization of the photopolymerizable layer occurs in the exposed areas. Treatment with a suitable developer removes the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing.
Many different elastomeric materials have been investigated for the preparation of the photopolymer plates. These include polyamide-based photopolymer (containing a polyamide as an essential component which dissolves or swells in a washout solution (typically, water, alkaline aqueous solution, or alcohol), a polyvinyl alcohol-based photopolymer (containing polyvinyl alcohol as an essential component), a polyester-based photopolymer (containing a low-molecular weight unsaturated polyester as an essential component), an acrylic-based photopolymer (containing a low-molecular weight acrylic polymer as an essential component), a butadiene copolymer-based photopolymer (containing a butadiene or isoprene/styrene copolymer as an essential component), or a polyurethane-based photopolymer (containing polyurethane as an essential component).
Methacrylate- or acrylate-terminated polyurethane oligomers diluted with various acrylate or methacrylate monomers, along with a photoinitiator, are described in U.S. Pat. Nos. 4,006,024 and 3,960,572. The entire disclosures of U.S. Pat. Nos. 4,006,024 and 3,960,572 are incorporated herein by reference. The polyurethane oligomers of the '024 and '572 patents are derived from a diisocyanate such as toluene diisocyanate (TDI) and various polyester polyols or polyether polyols such as polypropylene glycol adipate, polyethylene oxide/propylene oxide copolymer, or a mixture thereof. The resultant printing plate can be used for printing on a wide variety of substrates, including corrugated board, various types of paper bags, and various types of cardboard packaging. Unfortunately, the photopolymer products described in the '024 and '572 patents can pick up significant amounts of paper fibers, dust and dried inks, and thus do not print cleanly and must be cleaned often during a print run.
U.S. Pat. Nos. 4,057,431, 4,139,436, 4,221,646 and 3,850,770, which are all incorporated in their entirety by reference, all teach the use of photosensitive ethylenically unsaturated polyether urethanes for the production of printing plates. In each of these patents, the polyether is either polyethylene oxide, polypropylene oxide or a copolymer of the two.
The use of ethylene oxide/propylene oxide copolymer as the sole diol to prepare a polyurethane oligomer causes a photopolymer printing plate formed therefrom to be swelled by contact with the aqueous inks used in flexographic printing. While a small amount of swelling can be tolerated, photopolymer plates made from urethane oligomers incorporating only ethylene oxide/propylene oxide copolymers as diols demonstrate a degree of swelling upon contact with water that characters, lines and halftone dots become unacceptably large, thus distorting the print beyond tolerable levels.
In general, the maximum allowable amount of swell of a printing plate upon exposure to water, or a water-based flexographic ink, for a 24-hour period at 25° C. is a 10% gain in either the weight of the plate sample or a 10% gain in its thickness. More preferably, the maximum allowable increase in either weight or thickness due to swelling by water, or a water-based flexographic ink is 6% after a 24-hour exposure. An average percent swell in water of a flexographic printing plate made from a photosensitive resin comprising a polyuretha
Carmody & Torrance LLP
Hamilton Cynthia
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