CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of priority and is a Divisional of U.S. Non-Provisional Application Patent Application Ser. No. 12/356,330, filed Jan. 20, 2009, by Kanga and entitled “Laser Engravable Flexographic Printing Articles Based on Millable Polyurethanes, and Method”. The Non-Provisional Patent Application Ser. No. 12/356,330 is a Continuation-in-Part (CIP) of U.S. Non-Provisional Patent Application Ser. No. 11/813,612, filed Jul. 10, 2007, by Kanga and entitled “Laser Engravable Flexographic Printing Article.” The Non-Provisional Patent Application Ser. No. 11/813,612 is a U.S. national stage application of International Application No. PCT/US07/72246, filed Jun. 27, 2007, by Kanga and entitled “Laser Engraveable Flexographic Printing Article.” The International Application No. PCT/US2007/072246 claims the benefit of priority of U.S. Provisional Patent Application No. 60/816,786, filed Jul. 27, 2006, by Kanga. This application also claims priority to U.S. Provisional Patent Application No. 61/083,327, filed Jul. 24, 2008, by Kanga and entitled “Laser Engravable Flexographic Printing Article Based on Millable Polyurethanes.” The contents of the above-referenced applications are incorporated by reference herein.
FIELD OF THE INVENTION
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The invention relates to an article for use in flexographic printing, such as a plate or sleeve, and a method for laser engraving the printing article to form a relief such that the article can be used in flexographic printing. The present invention also provides a method of crosslinking a Polyurethane Elastomer for making a directly laser engravable flexographic printing article by the use of commercially available Millable Polyurethanes (MPU). The printing article could be either a flat printing plate or a continuous in-the-round printing sleeve. Commercially available MPUs can be compounded either in an extruder or a compounder such as a Brabender using various crosslinking and laser sensitive additives. The compounded MPU is then extruded either on a flat carrier or a round sleeve, crosslinked either during extrusion or thereafter using thermal energy. The extruded and crosslinked MPU is ground or machined to the dimension required for the printing process and is ready for laser engraving. In one embodiment of the invention, the article does not require further processing, and as such can be used in a “direct-to-plate” laser engraving system.
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OF THE INVENTION
Printing plates are well known for use in flexographic printing, particularly on surfaces which are corrugated or smooth, such as packaging materials like cardboard, plastic films, etc. Typically, flexographic printing plates are manufactured using photopolymers which are exposed through a negative, processed using a solvent to remove the non-crosslinked areas to create a relief, which is post-crosslinked and detackified. This is typically a very lengthy and involved process. Recently, flexographic plates have been manufactured using digital imaging of an in situ mask layer which obviates the need for a negative or a photomask to make the plate, and which has other performance benefits as well.
Recently, it has been possible to laser engrave a rubber element directly to provide the desired relief surface necessary for flexographic printing. Laser engraving has provided a wide variety of opportunities for rubber printing plates. Highly concentrated and controllable energy lasers can engrave very fine details in rubber. The relief of the printing plate can be varied in many ways. Very steep as well as gently decreasing relief slopes can be engraved so as to influence the dot gain of such plates. Ethylene propylene diene monomer (EPDM) rubber can be laser engraved to form flexographic printing plates.
The directly engraved type of flexographic printing plate is made from vulcanized rubber. Commercial rubbers can be natural or synthetic, such as EPDM elastomers. Lasers can develop sufficient power densities to ablate certain materials. For example, high-power carbon dioxide (CO2) lasers can ablate many materials such as wood, plastic and rubber and even metals and ceramics. Once the output from a laser is focused at a particular point on a substrate with a suitable power density, it is possible to remove material to a desired depth to create a relief. Areas not struck by the laser beam are not removed. Thus, the use of the laser offers the potential of producing very intricate engravings in a desired material with substantial savings.
U.S. Pat. No. 3,459,733 to Caddell describes a method for producing polymer printing plates. The printing plate is made by exposing a layer of the polymeric material to a controlled laser beam of sufficient intensity to ablate the polymer and form depressions on the surface.
U.S. Pat. Nos. 5,798,202 and 5,804,353 to Cushner et al. disclose processes for making a flexographic printing plate by laser engraving a reinforced elastomeric layer on a flexible support. The process disclosed in U.S. Pat. No. 5,798,202 involves first reinforcing and then laser engraving a single-layer flexographic printing element having a reinforced elastomeric layer on a flexible support. The elastomeric layer may be reinforced mechanically, thermochemically, photochemically or with combinations of these processes. Mechanical reinforcement is provided by incorporating reinforcing agents, such as finely divided particulate material, into the elastomeric layer. Photochemical reinforcement is accomplished by incorporating photohardenable materials into the elastomeric layer and exposing the layer to actinic radiation. Photohardenable materials include photo-crosslinkable and photo-polymerizable systems having a photo-initiator or photo-initiator system.
The process disclosed in U.S. Pat. No. 5,804,353 is similar to U.S. Pat. No. 5,798,202, except that the process involves reinforcing and laser engraving a multilayer flexographic printing element having a reinforced elastomeric top layer, and an intermediate elastomeric layer on a flexible support. The elastomeric layer is reinforced mechanically, thermochemically, photochemically or combinations thereof. Mechanical and photochemical reinforcement is accomplished in the same manner as described by U.S. Pat. No. 5,798,202. The intermediate elastomeric layer may be reinforced as well.
A problem associated with elastomeric elements that are reinforced both mechanically and photochemically is that laser engraving does not efficiently remove the elastomeric material to provide desired relief quality, and ultimately, printing quality. It is desirable to use an additive in the elastomeric layer that is sensitive to infrared light in ordeff to enhance the engraving efficiency of the element. Photo-chemically reinforcing the element provides the desired properties for engraving as well as in its end-use as a printing plate. However, the presence of the additive as particulate or other absorbing material tends to reduce the penetration of the ultraviolet radiation required to photo-chemically reinforce the element. If the elastomeric layer is insufficiently crosslinked during photochemical reinforcement, the laser radiation cannot effectively remove the material and poor relief quality of the engraved area results. Further, the debris resulting from laser engraving tends to be tacky and difficult to completely remove from the engraved element. Additionally, if the element is not sufficiently photo-chemically reinforced, the required end-use properties as a printing plate are not properly achieved. These problems tend to be exacerbated with increasing concentration of the additive that enhances engraving efficacy.
U.S. Pat. No. 6,627,385 teaches the use of graft copolymers for laser engraving. U.S. Pat. No. 6,511,784, U.S. Pat. No. 6,737,216 and U.S. Pat. No. 6,935,236 teach the use of elastomeric copolymers for laser engraving using various infrared (IR) additives.
Many patents in the field teach the use of typical styrenic thermoplastic elastomers (TPEs) that have been used for photo-crosslinking applications. One problem associated with these non-polar TPEs is that they have limited sensitivity to laser engraving because of their hydrocarbon backbone nature. The use of polar TPEs such as thermoplastic polyurethanes (TPUs) thermoplastic polyester elastomers (TPPE) and thermoplastic polyamide elastomers (TPAE) as both laser engravable systems and as printing elements would be desirable. However, most of the above polar TPEs on the market would not be effective either as laser engravable systems, or as printing plates because they are not crosslinked.
The crosslinking of the above TPEs and especially TPUs has not been done before in flexography, and thus, TPUs have not been used in flexography. However, polyurethanes for flexography have been well known, particularly for liquid photopolymers. By definition, a TPU is solid at room temperature and can be extruded, and is workable at higher temperatures. This characteristic is due to the presence of hard and soft segments that form a network at room temperature, and is thus solid.
This network structure also differentiates TPUs from traditional polyurethanes in its outstanding physical attributes and thus offers an attractive system to be used in flexo applications. However, most elastomers used in Flexo need to be crosslinked to withstand the rigors of the printing process and to minimize swells in the inks used for printing. Additionally, the elastomers used in laser engraving have to be crosslinked. Traditional flexo photopolymers have unsaturation in the backbone, which allows the crosslinking with acrylate monomers and UV photo-initiators. The TPUs on the market today do not have unsaturation. Hence, the difficulty in UV crosslinking these for flexo applications. Additionally, laser engraving of elastomers with lasers lasing in the Near IR wavelengths need to be doped with highly absorptive laser additives. This does not allow UV crosslinking as a viable option to crosslink such elastomers. Thermal crosslinking or vulcanization is the only feasible approach in such applications. Millable Polyurethanes (MPUs) are a special category of TPUs. Millable Polyurethanes, as the name suggests, could be processed in the same way as rubber elastomers, including the use of compounding and extrusion methods. MPUs can be thermally crosslinked in a subsequent crosslink and post-crosslink step.
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OF THE INVENTION
Therefore, an object of the present invention is to provide a method for making a laser engravable flexographic printing article.
Another object of the present invention is to provide a reliable method for making a printing plate from crosslinking of Millable Polyurethanes (MPUs).
These and other objects of the present invention can be achieved in the preferred embodiments of the invention described below.
One preferred embodiment of the invention includes a method for making a flexographic printing article including the steps of providing a millable polyurethane, and crosslinking the polyurethane whereby the article can be used in a direct laser engraving flexographic process.
According to another preferred embodiment of the invention, the crosslinked millable polyurethane can be used in the direct laser engraving flexographic process and in flexographic printing without further processing.
According to another preferred embodiment of the invention, the printing article is laser engraved by infrared laser radiation to form a relief such that the article can be used in flexographic printing.
According to another preferred embodiment of the invention, the printing article can be a plate or a sleeve.
According to another preferred embodiment of the invention, the binder is a high performance polyester-based polyurethane processed as a millable polyurethane.
According to another preferred embodiment of the invention, the binder is a high performance polyether-based polyurethane processed as a millable polyurethane.
According to another preferred embodiment of the invention, the millable polyurethane is extruded and thermally crosslinked during extrusion.
According to another preferred embodiment of the invention, the millable polyurethane is compounded in a compounder and thermally crosslinked in a hot press.
According to another preferred embodiment of the invention, the millable polyurethane is milled on a 2-roll mill and thermally crosslinked in a hot press.
According to another preferred embodiment of the invention, at least one crosslinking additive for inducing the thermal crosslinking of the millable polyurethane is provided.
According to another preferred embodiment of the invention, at least one laser additive comprising such as carbon black, kaolin clay, mica, antimony tin oxide, or copper oxide is provided.
According to another preferred embodiment of the invention, the millable polyurethane is thermally crosslinked after extrusion.
According to another preferred embodiment of the invention, the millable polyurethane is crosslinked for about 15-30 minutes at about 240 to 350° F., and the polyurethane is crosslinked during the crosslinking.
According to another preferred embodiment of the invention, the mllable polyurethane is post-crosslinked for about 8 to 12 hours at about 180-240° F.
According to another preferred embodiment of the invention, the millable polyurethane is crosslinked during crosslinking with electron beam radiation.
According to another preferred embodiment of the invention, the printing article is hot-pressed to a desired dimension.