SOLVENT-FREE FLEXOGRAPHIC IMAGING AND PRINTING WITH PHOTORESPONSIVE PRINTING MEMBERS

Abstract

Flexographic printing members amenable to aqueous (or organic) development do not exhibit the deleterious effects on printing performance characteristic of some conventional alternatives. Embodiments of the invention utilize a photopolymerizable layer comprising, consisting of, or consisting essentially of a photopolymerization initiator and a water-dilutable (but not water-soluble) monomer.

Claims

1.-20. (canceled)

21. A method of preparing a printing member, the method comprising the steps of: providing a printing member comprising (i) an imaged photomask having an imagewise pattern of opaque and non-opaque regions, the opaque regions substantially blocking passage therethrough of actinic radiation, (ii) a support, and (iii) disposed between the photomask and the support and in contact with opposed surfaces thereof, a layer of photopolymer polymerizable by actinic radiation, the photopolymer layer consisting essentially of an initiator, a water-dilutable but not water-soluble oligomer and a butadiene maleic anhydride copolymer; exposing the precursor to actinic radiation through the photomask to cause polymerization of the photopolymer; and developing the printing member with an aqueous medium to remove only unexposed portions of the photopolymer layer.

22. The method of claim 21, wherein the photopolymer is capable of emulsification in water.

23. The method of claim 21, wherein the water-dilutable but not water-soluble oligomer is an aliphatic urethane acrylic oligomer.

24. The method of claim 21, wherein the photopolymer layer further comprises a dye.

25. The method of claim 21, wherein the photopolymer layer further comprises an anti-oxidant.

26. The method of claim 21, wherein the printing member is a flexographic printing member.

27. The method of claim 21, wherein the printing member is a letterpress printing member.

28. The method of claim 21, wherein the oligomer is an aliphatic urethane acrylic oligomer.

29. A printing member comprising: a laser-ablatable layer comprising a material that absorbs laser radiation; a support; and disposed between the laser-ablatable layer and the support and in contact with opposed surfaces thereof, a layer of photopolymer polymerizable by actinic radiation, the photopolymer layer comprising an initiator and a water-dilutable but not water-soluble oligomer and a butadiene maleic anhydride copolymer, wherein the photopolymer is developable by aqueous solvents.

30. The printing member of claim 29, wherein the laser-ablatable layer is ablatable by IR radiation.

31. The printing member of claim 30, wherein the laser-ablatable layer consists essentially of a polymer and carbon black dispersed therein.

32. The printing member of claim 29, wherein the butadiene maleic anhydride copolymer is a butadiene/styrene/maleic anhydride copolymer.

33. The printing member of claim 29, wherein the photopolymer layer further comprises a dye.

34. The printing member of claim 29, wherein the photopolymer layer further comprises an anti-oxidant.

35. The printing member of claim 29, wherein the oligomer is an aliphatic urethane acrylic oligomer.

Description

DESCRIPTION OF DRAWINGS

[0018] The foregoing discussion will be understood more readily from the following detailed description of the disclosed technology, when taken in conjunction with the following drawings, in which:

[0019] FIG. 1 schematically illustrates preparation of a flexographic printing plate according to embodiments of the invention.

[0020] FIG. 2 is a sectional side elevation illustrating the structure of a flexographic printing plate according to embodiments of the invention.

DETAILED DESCRIPTION

1. Preparation and Use of Flexographic Plates

[0021] Refer to FIG. 1, which illustrates a representative process for imaging a flexographic printing plate 100. The plate 100 includes a laser-ablation layer 105, a photopolymerizable layer 110, an adhesive and anti-halation layer 115, and a polymer base 120. First, the base 120 of the plate 100 is exposed to actinic radiation from, e.g., a source 125 of UV radiation in order to harden the base 120. The orientation of the plate 100 is then reversed and the ablation layer 105 is selectively removed by a laser 130 emitting, for example, in the near-IR region. The ablation layer contains a material, such as carbon black in an acrylic binder, that absorbs laser radiation and catastrophically overheats, removing the layer 105 where exposed or rendering it easily removed. The output of the laser 130 can be provided directly to the plate surface via lenses or other beam-guiding components, or transmitted to the surface of a blank printing plate from a remotely sited laser using a fiber-optic cable. A controller 135 and associated positioning hardware maintain the beam output at a precise orientation with respect to the plate surface, scan the output over the surface, and activate the laser at positions adjacent selected points or areas of the plate 100. The controller 135 responds to incoming image signals corresponding to the original document or picture being copied onto the plate 100, activing the laser 130 to ablate plate regions that will not receive ink during printing. The image signals are stored as a bitmap data file on a computer. Such files may be generated by a raster image processor (“RIP”) or other suitable means.

[0022] Following imaging, the ablation layer 105 functions as a mask when the now-reversed plate 100 is again exposed to the source 125 of actinic radiation. Radiation admitted through removed regions of the ablation layer 105 polymerizes the underlying layer 110, hardening it. The effects of the radiation expand with depth in the photopolymerizable layer 110 so that a floor is formed beneath the unexposed regions, thereby creating a series of raised regions 130 and recesses 135. Remaining portions of the imaged ablation layer 105 and unpolymerized regions of layer 110 are then removed by brushing and washing in a “washout” step, leaving a raised pattern of features 140 that will accept and transfer ink during printing. The plate may now be dried, e.g., under a hot-air dryer 145, and subjected again to the source 125 of actinic radiation to complete the photo-curing process.

[0023] Different types of UV light may be employed at different points in the process. The first and second exposures may involve a combination (typically the same combination) of UVA or UVB, for example, and the final exposure may be UVC light (“germicidal UV,” 100 to 280 nm, usually 280 nm).

[0024] Optionally, a very thin additional layer may be included above the photopolymer layer 110 but below the ablative layer 105. This “anti-slip” layer may protect the user and equipment from encountering the photopolymer layer 110, which may be tacky even after the second UV light exposure. This layer, if included, is removed in the washout step. Another optional layer is a plastic coversheet over the ablative layer 105, which provides protection prior to use and its peeled off before or after the ablative layer 105 is imaged.

[0025] A representative flexographic printing plate 200 in accordance with embodiments of the invention is shown in FIG. 2 and includes a polymer base layer 220, an anti-halation layer 215, a polymeric ablation layer 205 and, sandwiched therebetween, a photosensitive layer 210 comprising a water-dilutable monomer or oligomer. The polymer base layer 220 may be, for example, polyethyelene terephthalate (PET), polycarbonate, polyethylene napthalate, polybutylene terephalate, polyimide, and polyetherketone with a thickness ranging from 0.05 to 0.3 mm. The polymeric ablation layer 205 may be a carbon-loaded acrylic, polyvinyl alcohol or cellulosic resin having a thickness of 0.1 to 5 microns and a loading level of 1 to 75%, and may be coated with an optional anti-slip layer 225. Alternatively, a photomask may be employed.

[0026] The photosensitive layer 210, described in greater detail in the examples below, may have a thickness of 0.1 to 10.0 mm. In addition to the water-dilutable (but not water-soluble) monomer, the photosensitive layer may include a hydrolyzable binder such as a natural or synthetic rubber, e.g., butadiene maleic anhydride copolymer or polyisoprene maleic anhydride rubber. In this case, other monomers may be part of the copolymer; for example, the synthetic rubber may be a butadiene/styrene/maleic anhydride copolymer. In some cases, the photopolymer layer 210 can be a stacked series of different layers. The reflection-preventing anti-halation layer 215 may also promote adhesion between the base 220 and the photopolymer layer(s) 210. An anti-slip layer 230 may optionally be included or, in some embodiments, the ablation layer 205 may perform the anti-slip barrier function. In still other embodiments, the ablation layer 205 may be omitted altogether.

[0027] The foregoing approach may be applied to lithographic or letterpress plates. A letterpress plate may be structurally similar to a flexographic plate but the support 220 is typically magnetic steel rather than polyester and the exposed photopolymer 210 may be harder than that of a flexographic plate; whereas a typical flexographic plate may have a hardness of about 65-75 Shore A, a letterpress plate may have a hardness of about 65 Shore D.

2. Examples

[0028] A series of photopolymer compositions were prepared using the following formulations listed in Table 1 (with all units in grams). Component descriptions and sources appear in Table 2.

TABLE-US-00001 TABLE 1 Ingredient Composition 1 2 3 4 5 6 7 8 9 10 1 76.28 5.00 7.50 0.50 0.50 0.005 0.00 7.50 0.00 52.72 2 77.25 0.00 15.00 0.75 0.75 0.008 22.73 0.00 11.25 72.27 3 92.48 0.00 9.75 0.75 0.75 0.004 0.00 15.00 9.00 72.27

TABLE-US-00002 TABLE 2 Ingredient No. Material Source 1 Tetrahydrofuran Sigma Aldrich, Milwaukee, WI 2 Trimethylolpropane Sigma Aldrich, ethoxylate tri acrylate Milwaukee, WI 3 SARTOMER CN307 Sartomer, Exton, PA (hydrophobic acrylate ester) 4 IRGACURE 651 Ciba, Newport, DE photoinitiator (2,2-dimethoxy- 2-phenyl acetophenone) 5 Butylated hydroxytoluene Sigma Aldrich, Milwaukee, WI 6 ValifastBlue 1605 Orient Corporation of (spirit color) America, Seaford, DE 7 NEOREZ 1391 TCI Materials, (polyurethane supplied at Shrewsbury, MA 35% solids in water) 8 POLYVEST EPMA120 Evonik, Parsippany- (polybutadiene resin) Troy Hills, NJ 9 BOMAR XR9416 (oligomer Dymax, Torrington, capable of emulsification in CT water) 10 NIPOL LX11 INF (latex Zeon Corporation, supplied at 55% solids in Tokyo, Japan water)

Example C1

[0029] Ingredients 1-9 of Composition 1 were dissolved/dispersed in a beaker. Ingredient 10 was then added to the solution slowly while stirring with a Silverson L5 high shear lab mixer. The resulting slurry was poured into a glass dish and placed in an oven for 24 hours at 75° C. to dry.

[0030] The resulting composition was compounded into sheet form using a two-roll mill available from Reliable Rubber and Plastic Manufacturing Co, North Bergen, N.J. The roll nip was set to produce a photosensitive sheet with a thickness of approximately 70 mils (0.070 inch). The resulting sheet was laminated to transparent polyester sheets to provide a flexographic printing construction (lacking the ablation mask shown in FIG. 1), with the 70-mil photosensitive layer sandwiched between a PET base layer of 7 mils thickness and a 2-mil PET cover film coated with a release layer.

[0031] Imaging was carried out on a Thiemer COPYMAT exposure unit equipped with a THS3007 3000W metal halide UV lamp, available from Thiemer GMBH, Birstein, Germany.

[0032] Back exposure: the imageable composition was placed cover sheet down in the light exposure unit and exposed under vacuum for 90 seconds with light intensity set at 50%.

[0033] Main exposure: the composition was then placed base sheet down in the exposure unit. A Stouffer graphic-arts test strip was placed on top of the cover sheet, as was a photographic negative imaged with a 65-line screen calibrated screen tint. The photopolymer was then exposed under vacuum for 18 minutes with light intensity set at 50%.

[0034] Development: this refers to removal of unexposed (and therefore unpolymerized) photopolymer. The cover sheet was removed, and the patterned photopolymer was placed in a glass dish containing heptane. It was scrubbed with a stiff brush until a relief image formed. The sheet was then dried in an oven at 75° C. for 15 minutes.

[0035] Post-exposure: the integrity of the imaged polymer was further enhanced by post-exposure to UV radiation. The sheet was placed base sheet down in the exposure unit and exposed for 6 minutes under vacuum with light intensity set at 50%.

Example C2

[0036] Another sheet using Composition 1 ingredients was produced using the same procedures as in Example 1, except the patterned sheet was developed in water containing 5% by weight Stop and Shop brand (54 Hazard Avenue, Enfield, Conn. 06082) dishwasher detergent warmed to 50° C.

Example 3

[0037] A sheet using Composition 2 was produced according to the procedures set forth in Example 1.

Example 4

[0038] A sheet using Composition 2 was produced according to the procedures set forth in Example 2.

Example 5

[0039] A sheet using Composition 3 was produced according to the procedures set forth in Example 1, except the patterned sheet was developed in water containing 2% by weight Stop and Shop brand dishwasher detergent and 2% AJAX Ultra Triple Action liquid dish soap (Colgate-Palmolive, New York City, N.Y. 10022) warmed to 50° C.

Example 6

[0040] Another sheet using Composition 3 was produced according to the procedures set forth in Example 1, except the patterned sheet was developed in plain water warmed to 50° C.

[0041] Evaluation

[0042] The photosensitivity of the sheet was determined by reading the Stouffer test strip. The shore hardness of the imaged photosensitive sheet was measured with a durometer. The ease of development of the composition was graded on a scale of 1 to 5 with 1 corresponding to excellent development (the unexposed material was removed quickly and easily) and 5 representing no development at all. The quality of the image was assessed under magnification and also rated on a scale of 1 to 5 with 1 being full reproduction of the photographic mask and 5 being no image.

TABLE-US-00003 Results for Examples 1-6 shore A ease of Ex- Stouffer hard- develop- image ample reading ness ment guality Comments 1 solid 8, 70 2 2 clear 14 2 — 70 5 — Does not develop in detergent 3 solid 8, 75 2 2 clear 15 4 solid 8, 75 2 3 develops (rather slowly) in clear 16 5 solid 6, 68 1 1 detergent easy to develop and clear 17 excellent image quality 6 solid 10, 68 2 2 clear 18

[0043] Printing with Example 6

[0044] In one experiment, a rubber roller was coated in Titan black ink, supplied by Spinks Ink Company, Addison, Ill. The ink was rolled across the surface of the patterned sheet obtained in Example 6. The sheet was placed ink-side-down onto a piece of paper and pressure was applied to reveal a print with good image fidelity.

[0045] In another experiment, one part Smooth Lith, available from Van Son, Islandia, N.Y., was added to 9 parts Titan black ink to reduce the tackiness. A Heidelberg GTO printing press was equipped with the ink and the patterned sheet obtained in Example 6 was attached to the plate cylinder using double-sided tape. All packing was removed from the blanket cylinder. The press was run dry (no fountain solution). On the first revolution a good image was transferred from the plate to the blanket. When the blanket was contacted with paper, an image with good fidelity was formed. In this way, multiple copies were made.

[0046] Discussion of Results

[0047] The formulations of Examples 1 and 2 do not contain a monomer capable of emulsification in water. The composition will develop in organic solvent but will not develop aqueously. Examples 3 and 4 contain a monomer capable of emulsification in water, specifically, an aliphatic urethane acrylic oligomer, BOMAR XR9416. The composition will develop in solvent or aqueous developer. Examples 5 and 6 contain a monomer capable of emulsification in water and a binder capable of hydrolysis in water (XR9416 and Polyvest MA75). The composition will develop easily in aqueous developer or even pure water.

[0048] Examples 7 and 8 demonstrate the composition can function as a printing plate.

[0049] Although the present invention has been described with reference to specific details, it is not intended that such details should be regarded as limitations upon the scope of the invention, except as and to the extent that they are included in the accompanying claims.