Compositions: coating or plastic – Coating or plastic compositions – Carbohydrate or derivative containing
Reexamination Certificate
2000-11-29
2002-12-03
Brunsman, David (Department: 2854)
Compositions: coating or plastic
Coating or plastic compositions
Carbohydrate or derivative containing
C106S204010, C106S205010, C106S205900, C106S217900, C101S450100, C101S147000
Reexamination Certificate
active
06488754
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The invention is in the field of offset lithography, and more specifically relates to a lithographic dampening solution and a method for dampening a lithographic plate.
BACKGROUND OF THE INVENTION
Offset lithography is a printing process in which an image carrier takes the form of a plate which includes a photochemically produced ink-receptive image area and an ink-repellant non-image area. Inks used in traditional offset lithography processes are oil-based, and thus the image area on a printing plate typically is hydrophobic and the non-image area is hydrophilic. In an offset lithographic operation, the plate is mounted on a cylinder and is dampened with a dampening solution, which is typically an aqueous solution of chemicals, and which adheres to the non-image area of the plate. The plate is next contacted by inked rollers, which apply ink to the image area of the plate. The inked image is transferred to a rubber-like blanket, on which the image becomes reversed. The inked image on the blanket is then transferred to a printing substrate, typically a sheet of paper, thereby producing an impression of the inked image on the paper.
The dampening solution is used to cause a crisp division between the image and non-image areas of the printing plate, to thereby provide high resolution in the print. Conventional dampening solutions include water and a gum and, typically, an acid or a base, corrosion inhibitor, buffer, wetting agent, drying stimulator, fungicide, anti-foaming agent, and possibly other ingredients of a conventional nature. The gum is used in an amount effective to provide sufficient film formability to enhance the ink repellency of the non-image area of the lithographic plate.
Historically and conventionally, the gum used in dampening solutions is gum arabic, as disclosed, for instance, in U.S. Pat. Nos. 4,030,417; 4,150,996; 5,256,190; and 5,382,298; in EP 066176,249752; and 251,621; and in DE 3,536,485. Although dampening solutions made with gum arabic are satisfactory from a technical standpoint, there are a number of potential and actual problems in obtaining gum arabic. Gum arabic is the exudate of a woody plant (acacia) that is grown in areas of the world that presently are politically unstable, for example, the Sudan. For this reason, and because the supply of gum arabic is largely dependent upon unpredictable climactic conditions, the constancy of the supply of gum arabic is somewhat unstable. In addition, the quality of harvested gum arabic can be variable.
In recognition of these potential and actual problems, the prior art has provided dampening solutions that include a number of chemically synthesized substitutes for gum arabic. For instance, U.S. Pat. No. 5,279,648 discloses a dampening solution that includes hydroxypropyl cellulose. Another reference, DE 2,504,594, discloses a dampening solution that includes an acrylamide-acrylic acid copolymer. These dampening solutions are believed to be somewhat unsatisfactory. A dampening solution that includes a chemically derivatized dextran is taught in EP 517,959, and a dampening solution that includes a chemically derivatized pullulan is disclosed in DE 2,648,805. While these solutions are somewhat satisfactory, the need for derivatization adds cost to the formulation process for these solutions.
It is a general object of the invention to provide a dampening solution that is composed of water and a naturally occurring biopolymer. A related general object of the invention is to provide a method for dampening a lithographic plate.
THE INVENTION
It has now been found that aqueous solutions of hemicellulose, particularly corn-hull-derived hemicellulose, may be used to dampen lithographic plates. Hemicellulose is a naturally occurring component of seed hulls, such as corn seed hulls, and is obtained from the hulls during industrial processing. It has further been found that the enzyme digestion product of hemicellulose, in particular, the xylanase digestion product, may be used in a dampening solution to dampen a lithographic plate. The commercial availability of hemicellulose is more steady and reliable than that of gum arabic, in that the corn crop in the United States is large and stable, and the hemicellulose content of corn seeds is uniform and predictable. For this reason, the use of hemicellulose as a dampening solution gum is advantageous. Moreover, although the use of soybean hemicellulose has been provided in the art (for instance in U.S. Pat. No. 5,615,613), the composition of soybean derived hemicellulose is substantially similar to that of gum arabic, while the composition of corn hull hemicellulose is markedly different. The ability of corn hull derived hemicellulose to function in a dampening solution in a manner as satisfactory as gum arabic is surprising, in light of the substantial differences in composition between corn hull derived hemicellulose and gum arabic.
In one embodiment, the invention generally provides a method for dampening a lithographic plate. The method generally comprises the step of applying a dampening solution to at least the non-image area of the lithographic plate. The dampening solution comprises an aqueous solution of a corn-hull-derived hemicellulose, which is present in the dampening solution in an amount effective to provide sufficient film formability to enhance the ink repellency of the non-image area of the plate. In another embodiment, the method includes the steps of applying a dampening solution that includes water and the enzyme digestion product of a hemicellulose, such as a corn hull derived hemicellulose, or alternatively a hemicellulose derived from another seed source. The invention further provides a dampening solution that includes the enzyme digestion product of a hemicellulose and a surfactant, the surfactant being present in an amount effective to enhance the wettability of the dampening solution.
Other features and embodiments of the invention are set forth in the following description of the preferred embodiment, and in the appended claims.
DESCRIPTION OF PREFERRED EMBODIMENT
The invention generally contemplates a dampening solution that includes hemicellulose, in particular, corn-hull-derived hemicellulose, or that includes the enzyme digestion product of hemicellulose. Hemicellulose is a water-soluble, highly branched polymer of xylose having side-chains composed of arabinose, galactose, and terminal glucuronic acid. The isolation of corn hull hemicellulose from corn hulls is taught in U.S. Pat. Nos. 2,801,955; 2,868,778; 3,716,526; and 4,038,481.
Once isolated, the hemicellulose may be used in an aqueous solution as a dampening solution, or the hemicellulose may be treated with an enzyme to thereby result in an enzyme digestion product that may be used in aqueous solution as a dampening solution. Most preferably, the enzyme is a xylanase enzyme. By “digestion” is contemplated the full or partial, most preferably, partial, digestion by the enzyme to yield a product that is fully or partially depolymerized by the enzyme, and in the case of partial digestion, possibly including some undigested hemicellulose. The hemicellulose used in conjunction with this embodiment of the invention preferably is corn-hull-derived hemicellulose, but alternatively may be a hemicellulose derived from another source, such as rice, wheat, or soybeans. While it is feasible to use hemicellulose without enzymatic digestion, such is less preferred inasmuch as the dampening solution generally will not be satisfactory for commercial purposes. In accordance with the invention, the enzyme digestion should be sufficient to enhance the ability of the hemicellulose to repel ink in the dampening solution. While it is not intended to limit the invention to a particular theory of operation, it is believed that the enzyme will reduce the average molecular size of the xylose polymers in hemicellulose to thereby cause the hemicellulose digestion product to interact more successfully with the surface of the lithographic printing plate to form a hydrophilic
Antrim Richard L.
Bishop Leona E.
McPherson Roger E.
Brunsman David
Grain Processing Corporation
Leydig Voit & Mayer LTD
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