Patent Application
20120291645
In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening fluid to the plate prior to inking. The dampening fluid prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
To circumvent the cumbersome photographic development, plate-mounting, and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers.
Current laser-based lithographic systems frequently rely on removal of an energy-absorbing layer from the lithographic plate to create an image. Exposure to laser radiation (typically in the near-infrared (IR) range) may, for example, cause ablation—i.e., catastrophic overheating—of the ablated layer in order to facilitate its removal. Because ablation produces airborne debris, ablation-type plates must be designed with imaging byproducts in mind; for example, the plate may be designed so as to trap ablation debris between layers, at least one of which is not removed until after imaging is complete.
Dry plates, which utilize an oleophobic topmost layer of fluoropolymer or, more commonly, silicone (polydiorganosiloxane), exhibit excellent debris-trapping properties because the topmost layer is tough and rubbery; ablation debris generated thereunder remains confined as the silicone or fluoropolymer does not itself ablate. Where imaged, the underlying layer is destroyed or de-anchored from the topmost layer. A common three-layer plate, for example, is made ready for press use by image-wise exposure to imaging (e.g., infrared or “IR”) radiation that causes ablation of all or part of the central layer, leaving the topmost layer de-anchored in the exposed areas. Subsequently, the de-anchored overlying layer and the central layer are removed (at least partially) by a post-imaging cleaning process—e.g., rubbing of the plate with or without a cleaning liquid—to reveal the third layer (typically an oleophilic polymer, such as polyester).
The commercial viability of any printing system depends critically on the speed at which a printing plate can be imaged, and secondarily on the required laser power. These two parameters are intimately related, as higher laser power results in greater beam fluence, delivering a greater quantity of energy with each imaging pulse. Within limits, higher beam fluence levels increase the rate at which ablation takes place, so that imaging can be carried out at faster speeds—that is, each imaging pulse can be of shorter duration, so the plate can be imaged more quickly.
The relationship between laser power and imaging speed is not strictly inverse, however, and increasing laser power soon leads to diminishing returns, as the responsiveness of the plate imaging layer is constrained by physico-chemical characteristics that limit the rate at which ablation can take place. Moreover, high-power lasers are expensive both to procure and to operate, and can cause damage to the plate beyond the intended results of ablation. Accordingly, increases in imaging speed are desirably realized through improvements in plate characteristics. Such improvements are not easily achieved, however, because increasing exposure sensitivity typically degrades the durability of the plate. For example, sensitivity can be improved by thinning the plate layers or increasing the loading level of an IR-absorbing material, but the result is a more delicate plate structure.
As explained in the '651 parent application, it has been found, surprisingly, that plates having improved exposure sensitivity can be produced using an imaging layer—i.e., the plate layer that absorbs and ablates in response to imaging radiation—whose composition includes a large proportion of melamine resin crosslinker. It has further been discovered, also surprisingly, that the dry coating weight of the imaging layer can strongly affect the plate's printing performance following cleaning (and particularly mechanical cleaning in a machine).
In a typical matrix for a polymeric imaging layer, the “binder” resin predominates (typically at levels in the 70% range) and the crosslinker is present at a much lower level (e.g., in the 10% range). Imaging-layer compositions in accordance with the present invention achieve improved speed with good durability at much higher levels of crosslinker, e.g., on the order of 80% or more of the composition in some embodiments. For example, whereas a prior-art composition based on a resole resin might contain 12 to 25% IR-absorptive dye, 15% melamine crosslinker, 0.7 to 4.8% sulfonic acid catalyst, and 70% resole resin, a corresponding formulation in accordance herewith may contain 12 to 25% IR-absorptive dye, 80% melamine crosslinker, 0.7 to 4.8% sulfonic acid catalyst, less than 25% (and as little as zero) resole resin. The term “resole resin” refers to the reaction of phenol with an aldehyde (usually formaldehyde) under alkali conditions with an excess of formaldehyde. The molar ratio of phenol to aldehyde is typically 1:1.1 to 1:3, and the excess formaldehyde causes the resulting polymer to have many CH2OH (methylol) pendant groups. This distinguishes resoles from other phenolic resins (including phenol formaldehyde resins such as novolaks, which are prepared under acidic conditions with an excess of phenol rather than aldehyde).
Without being bound to any particular theory or mechanism, it is hypothesized that, after exposure, the ablation debris generated in a plate in accordance with the present invention is water compatible or otherwise easier to remove during cleaning, resulting in the ability to tolerate less complete ablation and, consequently, faster imaging at a given fluence level. It is also found that the curing temperature of the imaging layer during plate manufacture can be important to plate performance, since too much heat during curing compromises the sensitivity of the finished plate while inadequate heat leads to incomplete cure and consequent plate instability. Curing temperatures ranging from 220 to 320° F., and especially 240 to 280° F., have been used to advantage.
It is also found that if the coating weight of the imaging layer coat is too low (e.g., 0.5 g/m2), the printing member will fail to clean thoroughly in a processing machine after correct exposure, resulting in poor ink-accepting plate regions on-press. Although this performance deficiency can be rectified to some degree by increasing exposure fluence, this entails the need for higher-power lasers or reduces productivity in the pre-press room. In particular, it is found that imaging layers having dry coating weights from 0.9 to 2.5 g/m2, and especially from 0.9 to 1.5 g/m2, exhibit good performance following mechanical cleaning.
Poor inking performance at low coating weights is not observed when cleaning is performed manually (e.g., dry rubbing the imaged plate with a cotton towel followed by wet rubbing with a cotton towel saturated with isopropanol). A printing member having an imaging layer coated at 0.5 g/m2, for example, will receive ink on-press satisfactorily after correct exposure and subsequent hand cleaning. But hand cleaning requires an experienced practitioner, can damage the non-image oleophobic regions of the plate, and can lead to inconsistent results.
Accordingly, in a first aspect, the invention relates to a method of imaging a printing member. In various embodiments, the method comprises providing a printing member that itself comprises (i) an oleophilic first layer; (ii) disposed over the first layer, an oleophilic imaging layer having (A) a cured resin phase consisting essentially of a melamine resin and a resole resin, the resole resin being present in an amount ranging from 0% to 28% by weight of dry film, (B) dispersed within the cured resin phase, a near-IR absorber, and (C) a dry coating weight of at least 0.9 g/m2; and (iii) disposed over the imaging layer, an oleophobic third layer. The printing member is exposed to imaging radiation in an imagewise pattern, and the imaging radiation at least partially ablates the imaging layer where exposed. Following imaging, the printing member is subjected to machine cleaning (e.g., spray-on cleaning) to remove the third layer and at least a portion of the imaging layer where the printing member received imaging radiation, thereby creating an imagewise pattern on the printing member. Because the imaging layer is oleophilic it need not be fully removed, which permits fast operation with low-power lasers and large imaging-layer thicknesses, which are found to be beneficial to post-cleaning performance. Indeed, there is no maximum coating weight for the imaging layer other than that dictated by plate-production costs. Plate-cleaning improvements are found to plateau when coating weights reach 1.2 to 1.5 g/m2 when machine cleaning is employed. Increasing the coating weight beyond 1.5 g/m2 is found to yield little additional plate performance improvement while adding substantially to cost. It should further be noted that coating weights do not materially affect responsiveness to imaging radiation.
The printing member is usually exposed at a fluence not exceeding 210 mJ/cm2, which is sufficient to resolve high-resolution patterns such as 2×2 screens and single pixel lines. The oleophilic first layer may be an aluminum sheet, and the cured resin phase of the printing member preferably contains no resole resin. The near-IR absorber may be a near-IR absorbing dye, and the third layer is typically silicone.
The cleaning fluid may be an aqueous liquid, e.g., plain tap water. In some embodiments, the aqueous liquid comprises water and a component that eases the removal of silicone. For example, the aqueous liquid may include not more than 20% (or not more than 15%) by weight of an organic solvent, e.g., an alcohol, and the alcohol may be a glycol (e.g., propylene glycol), benzyl alcohol and/or phenoxyethanol. The aqueous liquid may comprise a surfactant. It may be heated to a temperature greater than about 80° F.
As used herein, the term “plate” or “member” refers to any type of printing member or surface capable of recording an image defined by regions exhibiting differential affinities for ink and/or fountain solution. Suitable configurations include the traditional planar or curved lithographic plates that are mounted on the plate cylinder of a printing press, but can also include seamless cylinders (e.g., the roll surface of a plate cylinder), an endless belt, or other arrangement.
“Ablation” of a layer means either rapid phase transformation (e.g., vaporization) or catastrophic thermal overload, resulting in uniform layer decomposition. Typically, decomposition products are primarily gaseous. Optimal ablation involves substantially complete thermal decomposition (or pyrolysis) with limited melting or formation of solid decomposition products.
The terms “substantially” and “approximately” mean±10% (e.g., by weight or by volume), and in some embodiments, ±5%. The term “consists essentially of” means excluding other materials that contribute to function or structure. For example, a resin phase consisting essentially of a melamine resin and a resole resin may include other ingredients, such as a catalyst, that may perform important functions but do not constitute part of the polymer structure of the resin. Percentages refer to weight percentages unless otherwise indicated.
In the following description, various embodiments of the present invention are described with reference to
Most of the films used in the present invention are “continuous” in the sense that the underlying surface is completely covered with a uniform layer of the deposited material. Each of these layers and their functions is described in detail below.
1.1 Layer 102
When serving as a substrate, layer 102 provides dimensionally stable mechanical support to the printing member. The substrate should be strong, stable, and flexible. One or more surfaces (and, in some cases, bulk components) of the substrate may be hydrophilic. The topmost surface, however, is generally oleophilic. Suitable materials include, but are not limited to, polymers, metals and paper, but generally, it is preferred to have a polymeric ink-accepting layer (e.g., applied over a metal or paper support). As used herein, the term “substrate” refers generically to the ink-accepting layer beneath the radiation-sensitive layer 104, although the substrate may, in fact, include multiple layers (e.g., an oleophilic film laminated to an optional metal support, such as an aluminum sheet having a thickness of at least 0.001 inch, or an oleophilic coating over an optional paper support).
Substrate 102 desirably also exhibits high scattering with respect to imaging radiation. This allows full utilization of the radiation transmitted through overlying layers, as the scattering causes back-reflection into layer 104 and consequent increases in thermal efficiency. Polymers suitable for use in substrates according to the invention include, but are not limited to, polyesters (e.g., polyethylene terephthalate and polyethylene naphthalate), polycarbonates, polyurethane, acrylic polymers, polyamide polymers, phenolic polymers, polysulfones, polystyrene, and cellulose acetate. A preferred polymeric substrate is polyethylene terephthalate film, such as the polyester films available from DuPont-Teijin Films, Hopewell, Va. under the trademarks MYLAR and MELINEX, for example. Also suitable are the white polyester products from DuPont-Teijin such as MELINEX 927W, 928W 329, 329S, 331.
Polymeric substrates can be coated with a hard polymer transition layer to improve the mechanical strength and durability of the substrate and/or to alter the hydrophilicity or oleophilicity of the surface of the substrate. Ultraviolet- or EB-cured acrylate coatings, for example, are suitable for this purpose. Polymeric substrates can have thicknesses ranging from about 50 μm to about 500 μm or more, depending on the specific printing member application. For printing members in the form of rolls, thicknesses of about 200 μm are preferred. For printing members that include transition layers, polymer substrates having thicknesses of about 50 μm to about 100 μm are preferred.
Especially suitable substrates include aluminum, polyethylene terephthalate, polyethylene naphthalate and polyester laminated to an aluminum sheet. Substrates may be coated with a subbing layer to improve adhesion to subsequently applied layers.
1.2 Layer 104
Layer 104 ablates in response to imaging radiation, typically near-IR radiation. In general, layer 104 has a cured resin phase consisting essentially of a melamine resin and a resole resin, the latter being present in an amount ranging from 0% to 28% by weight of dry film. A near-IR absorber—typically a dye—is dispersed within the cured resin phase.
Suitable melamine resins include methylated, low-methylol, high-imino melamine materials. For example CYMEL cross-linkers from Cytek Industries, Inc., especially CYMEL 385, CYMEL 328, CYMEL 327, CYMEL 325 and CYMEL 323, may be employed. Melamine self-crosslinking or crosslinking with a resole resin, if present, may be facilitated by a sulfonic acid catalyst, typically a p-toluenesulfonic acid catalyst.
If the melamine component has a solution viscosity of 7000 to 15,000 centipoises at 23° C., and especially 8000 to 10,000 centipoises, and most especially 9000 centipoises, then the p-toluenesulfonic acid catalyst is desirably present at 1.5% or less by weight of dry film, especially 1.2% or less, most especially from about 1.2 to 0.7%, but not lower than 0.35%. If the melamine cross-linker has solution viscosity 1000 to 1600 centipoises at 23° C., especially 1100 to 1300 centipoises, and most especially 1100 centipoise, then the p-toluenesulfonic acid catalyst is desirably present at 6% or less by weight of dry film, especially 4.8% or less, most especially from about 4.8 to 2.8%, but not lower than 1.4%.
It appears that the polymeric matrix of layer 104 will not tolerate addition of co-resin together with the melamine, other than the limited amount of resole resin described above. For example, when polyvinylbutyral, phenolic resin or resole resin (in this case, at amounts greater than 28% by weight of dry film) is added into the composition, poor printing-plate durability and/or poor sensitivity result. In addition, the amount of resole added as a co-resin limits the amount of catalyst that can be used to make successful plates. For example, when the melamine resin has viscosity of 9000 centipoises and the matrix includes no resole, then the amount x of catalyst may be in the range 0.35%<x<1.5% by dry weight of film. If resole is added at 5%, however, then the acceptable range of catalyst level narrows to 0.35%<x<1.2%. If resole is used at 15%, then the range narrows to 0.35%<x<1%. Finally, if resole is used at 25%, then the range narrows to 0.35%<x<0.7%. In addition, when the melamine resin has a viscosity of 1100 centipoises and the matrix includes no resole, then the amount x of catalyst may be in the range 1.4%<x<6% by weight of dry film. If resole is added at 5%, then the acceptable range of catalyst narrows to 1.4%<x<4.8%. If resole is used at 15%, then the acceptable range of catalyst narrows to 1.4%<x<4%. Finally, if resole is used at 25%, then the acceptable range of catalyst narrows to 1.4%<x<2.8%.
Layer 104 desirably exhibits water compatibility following ablation. When layer 104 is only partially ablated, it is either (a) sufficiently water-compatible to be fully removed during cleaning, or (b) oleophilic if some of the layer remains even after cleaning. This layer should exhibit good adhesion to substrate 102, and resistance to age-related degradation is also desirable. Typically, layer 104 is cured and dried at 220 to 320° F., and especially 240 to 280° F. (i.e., approximately 104 to 160° C., especially 115 to 137° C.).
For proper printing performance following mechanical cleaning, imaging layers having dry coating weights from 0.9 to 2.5 g/m2, and especially from 0.9 to 1.5 g/m2, are preferred. Because the imaging layer is oleophilic it need not be fully removed, so small imaging-layer thicknesses are not necessary in order to image at low power. Indeed, there is no maximum coating weight for the imaging layer other than that dictated by plate-production costs. Plate-cleaning improvements are found to plateau when coating weights reach 1.2 to 1.5 g/m2 when machine cleaning is employed. Increasing the coating weight beyond 1.5 g/m2 is found to yield little additional plate performance improvement while adding substantially to cost. It should further be noted that coating weights do not materially affect responsiveness to imaging radiation.
In various embodiments, ablatability is achieved at a fluence of 210 mJ/cm2 or less, and more preferably at a fluence of 150 mJ/cm2 or less. The ablation threshold is dictated primarily by layer thickness and the loading level and efficiency of the absorber. Typically the absorbing dye is present at a loading level no more than 25%, although in various embodiments, it is no more than 18, 15 or even 12%. Furthermore, in some embodiments, an IR-absorptive pigment (e.g., carbon black) is added along with the dye at a loading level up to 20%, and in some implementations up to 25%.
1.3 Silicone Layer 106
The topmost layer participates in printing and provides the requisite lithographic affinity difference with respect to substrate 102; in particular, layer 106 is oleophobic and suitable for dry printing. In addition, the topmost layer 106 may help to control the imaging process by modifying the heat dissipation characteristics of the printing member at the air-imaging layer interface.
Typically, layer 106 is a silicone or fluoropolymer. Silicones are based on the repeating diorganosiloxane unit (R2SiO)n, where R is an organic radical or hydrogen and n denotes the number of units in the polymer chain. Fluorosilicone polymers are a particular type of silicone polymer wherein at least a portion of the R groups contain one or more fluorine atoms. The physical properties of a particular silicone polymer depend upon the length of its polymer chain, the nature of its R groups, and the terminal groups on the end of its polymer chain. Any suitable silicone polymer known in the art may be incorporated into or used for the surface layer. Silicone polymers are typically prepared by cross-linking (or “curing”) diorganosiloxane units to form polymer chains. The resulting silicone polymers can be linear or branched. A number of curing techniques are well known in the art, including condensation curing, addition curing, moisture curing. In addition, silicone polymers can include one or more additives, such as adhesion modifiers, rheology modifiers, colorants, and radiation-absorbing pigments, for example. Other options include silicone acrylate monomers, i.e., modified silicone molecules that incorporate “free radical” reactive acrylate groups or “cationic acid” reactive epoxy groups along and/or at the ends of the silicone polymer backbone. These are cured by exposure to UV and electron radiation sources. This type of silicone polymer can also include additives such as adhesion promoters, acrylate diluents, and multifunctional acrylate monomer to promote abrasion resistance, for example.
The silicone layer may have a dry coating weight of, for example, 0.5 to 2.5 g/m2, with the range 1 to 2.5 g/m2 being particularly preferred for typical commercial applications.
1.4 Optional Secondary Imaging Layer 108
With reference to
In general, pigment loading levels are no greater than 20% or 25%, and the coating is applied at a dry weight of about 0.3 g/m2. A typical composition for layer 108 includes or consists essentially of up to 25% carbon black, 60 to 90% resole resin (especially 70 to 80%), up to 20% melamine resin (usually about 10%), less than 5% catalyst and less than 2% surfactant/leveling agent.
Imaging of the printing member 100, 100′ may take place directly on a press, or on a platemaker. In general, the imaging apparatus will include at least one laser device that emits in the region of maximum plate responsiveness, i.e., whose λmax closely approximates the wavelength region where the plate absorbs most strongly. Specifications for lasers that emit in the near-IR region are fully described in U.S. Pat. Nos. Re. 33,512 (“the '512 patent”) and 5,385,092 (“the '092 patent”), the entire disclosures of which are hereby incorporated by reference. Lasers emitting in other regions of the electromagnetic spectrum are well-known to those skilled in the art.
Suitable imaging configurations are also set forth in detail in the '512 and '092 patents. Briefly, laser output 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 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. The controller responds to incoming image signals corresponding to the original document or picture being copied onto the plate to produce a precise negative or positive image of that original. 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. For example, a RIP can accept input data in page-description language, which defines all of the features required to be transferred onto the printing plate, or as a combination of page-description language and one or more image data files. The bitmaps are constructed to define the hue of the color as well as screen frequencies and angles.
Other imaging systems, such as those involving light valving and similar arrangements, can also be employed; see, e.g., U.S. Pat. Nos. 4,577,932; 5,517,359; 5,802,034; and 5,861,992, the entire disclosures of which are hereby incorporated by reference. Moreover, it should also be noted that image dots may be applied in an adjacent or in an overlapping fashion. The imaging apparatus can be configured as a flatbed recorder or as a drum recorder, with the lithographic plate blank mounted to the interior or exterior cylindrical surface of the drum.
In the drum configuration, the requisite relative motion between the laser beam and the plate is achieved by rotating the drum (and the plate mounted thereon) about its axis and moving the beam parallel to the rotation axis, thereby scanning the plate circumferentially so the image “grows” in the axial direction. Alternatively, the beam can move parallel to the drum axis and, after each pass across the plate, increment angularly so that the image on the plate “grows” circumferentially. In both cases, after a complete scan by the beam, an image corresponding (positively or negatively) to the original document or picture will have been applied to the surface of the plate. In the flatbed configuration, the beam is drawn across either axis of the plate, and is indexed along the other axis after each pass. Of course, the requisite relative motion between the beam and the plate may be produced by movement of the plate rather than (or in addition to) movement of the beam.
Examples of useful imaging devices include models of the MAGNUS and TRENDSETTER imagesetters (available from Eastman Kodak Company) that utilize laser diodes emitting near-IR radiation at a wavelength of about 830 nm. Other suitable exposure units include the CRESCENT 42T Platesetter (operating at a wavelength of 1064 nm, available from Gerber Scientific, Chicago, Ill.) and the SCREEN PLATERITE 4300 series or 8600 series plate-setter (available from Screen, Chicago, Ill.).
Following imaging, the printing member is subjected to an aqueous liquid to remove debris where the printing member received imaging radiation, thereby creating an imagewise pattern on the printing member. The aqueous liquid may consist essentially of water, e.g., it may be plain tap water. Alternatively, the aqueous liquid may comprise water and a component that eases the removal of silicone and ablation debris, facilitating faster and more efficient cleaning. The aqueous liquid may include not more than 20% (or not more than 15%) by weight of an organic solvent, e.g., an alcohol, and the alcohol may be a glycol (e.g., propylene glycol), benzyl alcohol and/or phenoxyethanol. The aqueous liquid may comprise a surfactant and/or may be heated to a temperature greater than about 80° F.
In accordance with the present invention, machine cleaning takes advantage of the preferred imaging-layer coating weights. Preferred processing machines utilize warm water as a cleaning agent applied by spraying onto the plate (as opposed to immersion). Suitable examples include the Konings Plate Washer, type KP 650/860 S-CH (Konings GmbH, D-41751, Viersen, Germany) which has two rotary, oscillating brush rollers in the cleaning section), the AS-34 Plate Processor (NES Worldwide Inc., Westfield, Mass., which has three rotary, oscillating brush rollers in the cleaner section), and the Presstek WPP85/SC850 Plate Washer (NES Worldwide Inc., which has two rotary brush rollers).
These examples involve negative-working waterless printing plates that include an oleophobic silicone layer, disposed on an imaging layer comprising an IR-absorbing dye and a polymer disposed on a polyester substrate. A preferred substrate is a 175 μm white polyester film sold by DuPont Teijin Films (Hopewell, Va.) labeled MELINEX 331. This is an opaque white film pretreated on one side to promote adhesion to solvent-based coatings.
Examples of formulations used for the IR-absorbing imaging layer are as follows:
CYMEL 385 is a methylated, low-methylol, high-imino melamine resin supplied as an 80% solids mix with water by Cytek industries, Inc. (West Paterson, N.J.). This sample has viscosity of 9000 centipoises at 23° C. The HRJ-12362 is a phenol formaldehyde thermosetting resin supplied in a 60% n-butanol solution by the SI Group, Inc (Schenedtady, N.Y.). CYCAT 4040 is a general purpose, p-toluenesulfonic acid catalyst supplied as a 40% solution in isopropanol by Cytek Industries, Inc. BYK 307 is a polyether modified polydimethylsiloxane surfactant supplied by BYK Chemie (Wallingford, Conn.). The solvent, DOWANOL PM, is propylene glycol methyl ether available from the Dow Chemical Company (Midland, Mich.). Butvar B98 is polyvinylbutyral from Brenntag Specialties, Inc., Philadelphia, Pa. 50094 is a cyanine near IR dye manufactured by FEW Chemicals GmbH (Bitterfeld-Wolfen, Germany), which has a reported coefficient of absorption of 2.4×105 L/mol-cm at the maximum absorption wavelength, λmax, of about 813 nm (measured in methyl ethyl ketone (MEK) solution). This dye exhibits very good solubility in the preferred solvent, DOWANOL PM, used in the formulations described herein.
The coating solutions were applied to the polyester substrate using a wire-round rod and then dried and cured at 150° C. (measured on the web) to produce dried coatings of about 0.5 g/m2. Drying and curing were carried out on a belt conveyor oven, SPC Mini EV 48/121, manufactured by Wisconsin Oven Corporation (East Troy, Wis.). The conveyor was operated at a speed of 3.2 feet/minute, which gives a dwell time of about 40 seconds in the air-heated zone of the oven. The actual temperatures on the polymer substrate were measured with calibrated temperature strips. In this oven, the temperature dial was set to 160° C. to bring the polymer web to the preferred curing temperature of 150° C.
The oleophobic silicone top layer of the plate members was subsequently disposed on the dried and cured imaging layer using the formulation given below. The silicone layer exhibits a highly crosslinked network structure produced by the addition or hydrosilylation reaction between the vinyl groups (SiVi) of vinyl-terminated functional silicones and the silyl (SiH) groups of trimethylsiloxy-terminated poly(hydrogen methyl siloxane) crosslinker, in the presence of a Pt catalyst complex and an inhibitor.
The PLY-3 7500P is an end-terminated vinyl functional silicone resin, with average molecular weight 62,700 g/mol, supplied by Nusil Silicone Technologies (Charlotte, N.C.). The DC SYL OFF 7367 is a trimethylsiloxy-terminated poly(hydrogen methylsiloxane) crosslinker manufactured by Dow Corning Silicones (Midland, Mich.) which is supplied as a 100% solids solution containing about 30% 1-ethynylcyclohexane [CH≡CH—CH(CH2)5], which functions as catalyst inhibitor. The CPC 072 is a 1,3 diethyenyl-1,1,3,3-tetramethyldisiloxane Pt complex catalyst, manufactured by Umicore Precious Metals (South Plainfield, N.J.), which is supplied as a 3% xylene solution. The formulation solvent, heptane, is supplied by Houghton Chemicals (Allston, Mass.).
The silicone formulation was applied to the polymer imaging layers with a wire-round rod, then dried and cured at 150° C. (measured on the web) to produce uniform silicone coatings of 2 g/m2 using the same oven and conditions above. The printing members were evaluated as follows to assess solvent resistance, environmental stability, and imaging sensitivity.
1. Plates stored at ambient conditions are tested by assessing solvent resistance with MEK. An MEK resistance test is conducted on pieces (˜20 cm length) of the plate samples by applying, in a reciprocating mode at a five-pound load, double-rubs with a cotton towel saturated with MEK. The cycle is repeated to the point of visual evidence failure: marring of the surface or loss of silicone adhesion. To pass this test, the plates should resist more than 10 cycles of the test without showing signs of failure.
2. Fresh plate samples that pass the MEK resistance test (more than 10 MEK rubs) are exposed to accelerated aging conditions to determine their environmental stability. For this purpose, the MEK resistance test is repeated on samples that have been exposed to high temperature and humidity conditions (18 hours in an environmental chamber operated at 80° C. and 75% relative humidity.) To pass this test, aged samples should withstand more than five cycles of the MEK resistance test (more than five MEK rubs) without showing signs of failure.
3. Plate precursors are imaged on a KODAK TRENDSETTER image-setter (operating at a wavelength of 830 nm, available from Eastman Kodak Company). Sensitivity information is obtained from the evaluation of different imaging patterns (solid screen, 3×3, and 2×2 patterns) run at increasing power levels (15 mJ/cm2 steps) at a constant drum speed of 150 rpm. The imaged plates are manually cleaned to remove the loosened silicone debris left on the plate after imaging. Cleaning comprises a two-step procedure: first, dry rubbing the surface with a cotton towel, and second, wet rubbing with a cotton towel saturated with isopropanol.
The degree of plate sensitivity is ascertained from print sheets obtained by running the cleaned plates on a GTO Heidelberg press using black ink (Aqualess Ultra Black MZ waterless ink, Toyo Ink America LLC, Addison, Ill.) and uncoated stock (Williamsburg Plus Offset Smooth, 60 lb white, item no. 05327, International Paper, Memphis, Tenn.). The samples are run for at least 200 impressions. For purposes hereof, a high-speed plate embodiment is defined as one that produces print sheets showing well-defined high resolution patterns (2×2 and 3×3) at power levels below or equal to 200 mJ/cm2. Plates requiring power levels higher than 200 mJ/cm2 to produce prints with high-resolution patterns are classified as not passing this test.
The following table presents results of the evaluation procedures for Example 1 versus comparative examples C1 and C2:
The results show that the composition of Example 1 produces a durable and high-sensitivity waterless printing member. The fresh and aged plate embodiments of this example display very good solvent resistance and the power requirement for imaging is well below the established limit of 200 mJ/cm2. Comparative Examples C1 and C2 do not satisfy our performance criteria: C1 displays excellent durability but fails due to the very poor imaging sensitivity and Example C2 displays very poor durability and imaging sensitivity.
These examples involve waterless printing plates having resole-melamine imaging layers with variable resole/melamine resin ratios. Formulation examples are given in the following table for imaging layers made with lower resole resin content than those of Example C1.
These imaging-layer formulations were applied with a wire-round rod to polyester and dried and cured at 150° C. using the same oven and conditions described above to provide a coating of about 0.5 g/m2. Next, they were coated with the silicone layer described in the previous examples.
The plates were evaluated using the same procedure as above, and the results are summarized in the following table:
The printing plate of Example 2 exhibits good sensitivity and durability. Examples C3 and C4, by contrast, display very good durability but poor sensitivity (power requirements >200 mJ/cm2). The addition of the resole to the imaging layer helps improve durability but also causes considerable loss of plate sensitivity. The resole levels of the imaging layer should be kept below 28% to produce durable waterless printing plates having adequate sensitivity.
In Example 3, a melamine resin imaging layer and topmost silicone layer were disposed on an aluminum substrate. The melamine imaging layer was similar to that of Example 1, but included a visible dye (added to enhance image/non-image contrast of the plate). A preferred substrate is a 200 μm (0.008 inch) anodized aluminum (1052 aluminum alloy, electrochemically etched and anodized to give an anodic layer with Ra values in the order of 0.300 μm.) In Example 4, the imaging layer formulation was disposed on the same polyester substrate used in Example 1 and subsequently coated with the same silicone layer.
The melamine formulation used for these examples is given below:
Victoria Blue R is a visible dye with an absorption maximum λmax at 615 nm, and is supplied by Sigma-Aldrich (Saint Louis, Mo.) as a solid mixture with 85% dye content.
The imaging formulation was applied with a wire-round rod and dried/cured at 150° C. (measured on the web) to provide a coating of about 0.5 g/m2. The oven conveyor was run at 3.2 ft/min. For the aluminum substrate, the temperature set on the oven dial was 150° C. and agrees with the temperature measured on the metal substrate with the temperature strips. For polyester, conditions are as described in Example 1.
The plate samples were evaluated according to the procedures described above and the results are summarized in the following table:
In these examples, the melamine resin imaging layer yields durable and fast waterless printing members on both polyester and aluminum substrates.
Plates were made with melamine resin imaging layers having varying catalyst concentrations. The below table includes a series of melamine resin layers made with catalyst levels higher and lower than that of Example 1.
The formulations were applied with a wire-round rod to polyester, as described in Example 1, and dried and cured at 150° C. to provide a coating of about 0.5 g/m2. Furthermore, the silicone formulation given in the previous examples was disposed on the dried and cured imaging layer.
The following table summarizes the properties measured for these examples:
The results show that printing members incorporating a melamine imaging layer having very low (i.e., lower than 0.35%, including 0 in the dry coating) and high (i.e., higher than 1.5% in the dry coating) levels of catalyst do not exhibit sufficient environmental stability. In addition, there is also a noticeable decrease of imaging sensitivity for samples with melamine layers having excessive catalyst levels (>1.5% in the dry coating). The catalyst level of the dried melamine imaging layer should be higher than 0.35% but lower than 1.5% to produce durable, stable, and highly sensitive waterless plates.
These examples pertain to waterless printing plates made with melamine imaging layers having lower IR-absorbing dye concentrations than that of Example 1. Plate samples utilized the 175 μm white polyester MELINEX 331 film supplied by DuPont Teijin Films (Hopewell, Va.). The following table sets forth the formulations:
The imaging-layer formulations were applied with a wire-round rod and dried and cured at 150° C. to provide a coating of about 0.5 g/m2 as previously described. The silicone formulation given in the previous examples was disposed on the dried and cured imaging layer. The following table summarizes the results:
These examples provide durable and fast-imaging waterless printing plates. Plate samples with melamine imaging layers having dye contents as low as 12% (dry coating) are within the scope of the present invention.
In Examples 9 and 10, the imaging layers have formulations similar to that of Example 1 but use alternate IR-absorptive dyes (having absorption maxima higher than 820 nm and/or broader absorption bands covering a wider portion of the near-IR spectrum). Example 11 is a plate member with a melamine resin layer containing a blend of two dyes.
Formulations of imaging layers for these examples are set forth in the following table:
S 2058 is a cyanine dye supplied by FEW Chemical GmbH with a reported absorption maximum, λmax, at about 970 nm. EPOLIGHT 5588 is a cyanine dye, manufactured by Epolin, Inc. (Newark, N.J.), with an absorption maximum at about 860 nm. These are high-absorptivity dyes with coefficients of extinction (at λmax) comparable to that of the S0094 dye used in previous examples.
The examples were imaged (GATF target) on plate-setters operating at different wavelengths using a nominal power of ˜270 mJ/cm2. Three different image-setters were employed:
Examples 9, 10 and 11 image well on all these devices at 270 mJ/cm2. After cleaning, as in example 1, solid patterns and 2 to 98% dots were well resolved. Example 11 was imaged again on the Kodak Trendsetter, this time at 195 mJ/cm2 (150 rpm drum speed, 12 watts laser power, plot 0 test target). After cleaning, as described in example 1, the sample was mounted on the GTO press and printed at least 200 impressions showing well defined 2×2 and 3×3 pixel patterns. Examples 9 and 10 are imaged on the Kodak Trendsetter at 195 mJ/cm2. After cleaning and evaluating on the GTO press, well-defined high-resolution patterns (2×2 and 3×3 pixel patches) are expected in all cases.
This example involves a waterless printing plate in which the melamine imaging layer and the silicone layer described in Example 1 are disposed on a black polyester substrate. Carbon-filled polyester Hostaphan BSAC, supplied by Mitsubishi Polyester Film (Greer, S.C.), was used for this work. This polyester film is treated on both sides to promote adhesion to silicone adhesives. It was verified that this example displays good durability. On an MEK rub test, the sample measured 25 to 30 rubs when fresh. A sample was imaged on the Kodak Trendsetter at 180 mJ/cm2 (150 rpm drum speed, 11 watts laser power). After cleaning, as described in Example 1, the sample exhibited well defined 2×2 and 3×3 pixel patches.
Example 13 involves three-layer plates in which the silicone and imaging layers of Example 1 are disposed on a thin carbon-polymer matrix layer that is itself disposed on a polyester substrate pretreated with an adhesion-promotion layer. A preferred substrate is the pretreated 175 μm white polyester film sold by DuPont Teijin Films (Hopewell, Va.) used in Example 1. Example C9 involves a two-layer structure in which the silicone layer is directly disposed on the carbon layer disposed on the same polyester substrate.
The formulation of the carbon layer of these examples is as follows:
The Micropigmo AMBK-2 is a 20% solids proprietary carbon dispersion supplied by Orient Corporation of America (Kenilworth, N.H.). The dispersion has a 10% content of carbon in a polyvinyl butyral resin matrix.
This layer was applied with a wire-round rod and dried and cured 150° C. using the same oven and conditions described above (for polyester) to yield a coating of about 0.3 g/m2. The melamine imaging layer and the silicone layer described in Example 1 were subsequently disposed on the carbon layer.
The following table presents a comparison of the properties measured on these two examples using the procedures previously described:
Example 13 exhibits good sensitivity and durability. The two-layer structure of Example C9, which does not include the melamine-dye interlayer, displays good durability but very poor imaging performance.
In Example 14, a melamine resin imaging layer and topmost silicone layer were disposed on an aluminum substrate. The melamine imaging layer was similar to that of Example 1, but included a visible dye (added to enhance image/non-image contrast of the plate), and the CYMEL 385 has viscosity of 1100 centipoises at 23° C. A preferred substrate is a 200 μm (0.008 inch) anodized aluminum. In Example 15, the imaging-layer formulation was applied to the polyester substrate used in Example 1 and subsequently coated with the same silicone layer.
The melamine formulation used for these examples is given below:
The imaging layer formulation was applied with a wire-round rod and dried and cured at 138° C. on the aluminum substrate and 127° C. on the polyester substrate (temperatures set on the oven dial) to provide coatings of about 0.5 g/m2. The conveyor of the Wisconsin oven, described in Example 1, was run at 3.2 ft/min.
The plate samples were evaluated according to the procedures described above and the results are summarized in the following table:
The melamine resin imaging layer yields durable and fast waterless printing members on both polyester and aluminum substrates.
These examples describe negative-working waterless printing plates that comprise an oleophobic silicone layer disposed on an imaging layer including infrared-absorbing dye and polymer disposed on an aluminum substrate. A preferred substrate is a 200 μm (8 mil) anodized aluminum used, for example, in the AURORA EXP plate (1052 aluminum alloy, electrochemically etched and anodized to give an anodic layer with Ra values in the order of 0.300 μm) supplied by Presstek, Inc., Hudson, N.H.
The formulation used for the infrared-absorbing imaging layer of the plate precursor is described below:
CYMEL 385 is a methylated, low-methylol and high imino, melamine resin supplied as an 80% solids mix with water by Cytek industries, Inc. (West Paterson, N.J.), which has viscosity of about 1200 centipoises at 23° C.
The coating solution was applied to the aluminum substrate using a wire-round metering rod and then dried and cured at 138° C. (measured on the substrate) to produce a dried coating of coat weight of 0.5 g/m2. The coat weight was measured gravimetrically on samples prepared with a formulation without catalyst. Drying and curing were carried out on a belt-conveyor oven, SPC Mini EV 48/121, manufactured by Wisconsin Oven Corporation (East Troy, Wis.). The conveyor was operated at a speed of 3.2 feet/minute, which gives a dwell time of about 40 seconds in the air-heated zone of the oven. The actual temperatures on the aluminum substrate were measured with calibrated temperature strips. The temperature set on the oven dial (138° C.) agrees with the temperature measured on the metal substrate with the temperature strips. The oleophobic silicone top layer of the plate member was subsequently disposed on the dried/cured imaging layer using the formulation given below.
The silicone formulation was applied to the dried imaging layer with a wire-round rod, then dried and cured at 150° C. (measured on the substrate) in the same oven described above to produce uniform silicone coatings of 2 g/m2 (gravimetric determination). The printing member precursor of this example was first evaluated in the laboratory to determine durability and environmental stability. These properties were tested in the laboratory by assessing adhesion (X-hatch adhesive test) and solvent resistance (MEK rubs) of fresh plates stored at ambient conditions and plates aged in an environmental chamber at 80° C. and 75% RH for 18 hours. In the X-hatch adhesive test, adhesions to the metal substrate are evaluated on the plate precursor, visually and optical microscopy inspection, by removing the silicone and/or melamine layer with an adhesive silicone tape (e.g., polyester silicone adhesive tape 8402 manufactured by 3M, St. Paul, Minn.) that displays strong adhesion to the top silicone layer. MEK double rubs to test solvent resistance are applied in a reciprocating mode with a five-pound load on pieces of the plate precursor about 20 cm in length. The cycle is repeated to the point of visual evidence failure: marring of the surface or loss of silicone adhesion. To pass this test, the plate examples should resist more than 10 cycles of the test without showing signs of failure.
These and the plate-precursor examples set forth below pass the tape adhesion test and display very good MEK resistance (MEK rubs between 25 and 50 cycles) after being exposed to the accelerated aging conditions.
The resulting printing plate precursor was then imaged on a KODAK TRENDSETTER image setter, available from Eastman Kodak Company (Rochester, N.Y.), operating at a wavelength of 830 nm. An imaging file including a solid screen, a 50% screen, and high-resolution patterns (3×3, and 2×2 patterns) was run at increasing power levels at a constant drum speed of 150 rpm. The output power of the laser was varied from 8 W up to 15 W at increments of one watt, which corresponds to infrared imaging radiation having fluences of 130, 147, 163, 179, 195, 210, 228, up to 240 mJ/cm2 at the plane of the plate, respectively.
Final printing members were produced by processing or cleaning the imaged plate precursor to remove the loosened silicone debris left on the exposed areas of the imaged plate. Printing plates were produced using the following cleaning methods:
Plate sensitivity and cleanability were ascertained from print sheets obtained by running the printing members on a GTO Heidelberg press using black ink (Aqualess Ultra Black MZ waterless ink, Toyo Ink America LLC, Addison, Ill.) and uncoated stock (Williamsburg Plus Offset Smooth, 60 lb white, item number: 05327, International Paper, Memphis, Tenn.). The plates were run for at least 200 impressions. As understood herein, the sensitivity of the plate embodiments is defined as the power required to yield print sheets with well-defined high-resolution patterns (2×2 and 3×3). Machine-processed plate examples requiring power levels higher than 210 mJ/cm2 to print high-resolution patterns are considered uncleanable. The following table gives information on the imaging and cleaning performance of the printing plates produced by different cleaning procedures:
Note that the printing member produced by manual cleaning gives high-resolution patterns at exposures levels below 210 mJ/cm2. However, machine processing fails to yield printing plates with acceptable sensitivity and/or cleaning performance.
These examples describe waterless printing plates that use thick melamine imaging layers of the same composition as in Examples C10-C12. The imaging-layer formulation was disposed on the same anodized aluminum substrate used in previous examples and subsequently coated with the same silicone layer. In particular, the imaging-layer formulation was applied to the aluminum substrate using wire-round rods of wider wire diameter than that used in the earlier examples. The wet coatings were dried and cured at 138° C. (measured on the substrate) using the oven and conditions described above to produce dried coatings having weights between 0.7 g/m2 and 1.3 g/m2. Samples coated with wire-round rods 8, 10, 12, and 14 gave the dry coating weight values set forth below, which were determined on the base of gravimetric measurements of dry coatings made with the formulation without catalyst. Automatic cleaning on the AS34 plate washer described in Example C12 produced final printing members with the imaging sensitivities given in the following table:
It was verified that the sensitivity and/or cleaning of these plate embodiments is improved as a function of the thickness of the melamine imaging layer. Increasing the imaging layer coat weight up to 0.7 g/m2 give some improvement in terms of machine-cleanablility but not enough to produce printing members imageable at fluence levels of around 210 mJ/cm2. Furthermore, Examples 16 to 18, which utilize imaging layers with coating weights ≧0.9 g/m2 exhibit adequate imaging sensitivity and machine-cleaning performance.
The precursor plate of Example 18 was imaged at a fluence of 180 mJ/cm2 using an imaging file with various GATF targets. The final plate, produced after cleaning on the AS34 processor, was then run on the GTO Heidelberg press under the same conditions for up to 3500 impressions. The plate yielded prints with well-resolved high-resolution screens and showed consistent ink receptivity and no signs of plate wear.
Printing plate precursors with thick melamine imaging layers were imaged on the Kodak TRENDSETTER image setter and automatically cleaned on the Konings plate washer utilized in Example C11. The imaging sensitivity measured of the final printing members is given in the below table.
These examples show the same trend observed for plates processed on the AS34 plate washer. The sensitivity and/or cleaning performance improve with increasing thickness of the imaging layer. A melamine imaging layer of thickness higher or equal to 0.9 g/m2 is required to produce fast, machine-cleanable waterless printing plates.
This example describes a waterless printing plate with a top silicone layer thinner than that used in previous examples. The silicone layer is disposed on a melamine imaging layer of same composition and thickness as that of Example 18 disposed on the 200 μm (8 mil) anodized aluminum used in Presstek's AURORA EXP plate.
The same silicone formulation given above was applied to the dried imaging layer with a wire-round rod, then dried and cured at 150° C. (measured on the substrate) in the same belt conveyor oven to produce a uniform silicone coating of 1.5 g/m2 (gravimetric determination). The imaging sensitivity of the final plate produced by cleaning on the AS34 plate washer was determined according to the procedures described above. The sensitivity of this printing member is slightly better than that of Example 18, which uses the higher-coating-weight silicone layer. The 2×2 patterns were fully imaged and cleaned at a fluence of 163-179 mJ/cm2.
This example describes waterless printing plate precursors similar to Examples C10 and Example 18 that were prepared by imaging on the Platerite 4300 B image setter manufactured by Screen USA (Rolling Meadows, Ill.). This device has a 16-channel laser diode imaging head emitting near-IR radiation at a wavelength of 830 nm.
The image setter was run at 95% of total power for both plate examples. The precursor of Example 22 (with a melamine layer coat weight of 1.3 g/m2) was obtained at a drum speed of 550 rpm, which yields the calculated fluence of about 209 mJ/cm2 on the plate, while Example C15 (with a melamine layer coat weight of 0.5 g/m2) was run at a drum speed of 400 rpm (283 mJ/cm2). An imaging file containing a series of UGRA and GATF targets was used to evaluate the imaging and cleaning performance for high and low resolution screens.
The final printing members were obtained by machine cleaning on the Presstek's AS34 plate washer, and information on plate speed was determined from print sheets ran at the same conditions on the GTO Heidelberg press described in Examples C10-C12. Evaluation of the print data showed that Example 22 gives high-resolution prints. The energy requirement of the Screen image setter is higher than that of Example 18 obtained on the Kodak TRENDSETTER, but still within the boundaries of the present disclosure. On the other hand, the printing plate of Example C15, with a thin 0.5 g/m2 imaging layer, does not yield high-resolution prints
In these examples, waterless printing-plate precursors were made with a melamine-resin imaging layer and a topmost silicone layer disposed on a polyester substrate. A preferred substrate is the 175 μm white polyester film sold by DuPont Teijin Films (Hopewell, Va.) under the name MELINEX 331. This is an opaque white film pretreated on one side to promote adhesion to solvent-based coatings. These examples use the modified imaging layer formulation given in the attached table, which has lower catalyst concentration and does not include the visible dye.
The imaging layer formulation was applied to the polyester substrate using wire-round rods of various wire diameters. The wet coating were dried and cured at 121° C. (measured on the substrate) using the conveyor oven and conditions described above to produce dried coatings having weights between 0.5 g/m2 and 1.3 g/m2. Samples coated with wire-round rods 6, 10, and 14 yielded the dry coat weight values given in the below table, determined from gravimetric measurements of dry coatings made with the formulation without catalyst.
A silicone layer of same composition and thickness as in Example C10 was disposed on the dried/cured imaging layer and dried cured at 150° C. (measured on the substrate) on the oven used in earlier examples. The imaging sensitivity of printing plates produced by imaging on the KODAK TRENDSETTER image setter and machine cleaning on the Presstek AS34 plate washer was determined using the same procedure described in Examples C10-C12. An additional comparative example, Example C16, was obtained by manual cleaning. The following table summarizes the estimated plate sensitivities:
These printing members, which utilize polyester substrates, show improvement in terms of sensitivity and/or machine cleaning as a function of the thickness of the melamine imaging layer. Similar to the examples that utilize aluminum substrates, increasing the coating weight of the imaging layer to 0.9 g/m2 produces printing members that exhibit acceptable sensitivity. Further improvement in terms of machine cleanability is observed on Example 24, whose imaging layer has a coating weight of about 1.3 g/m2 and which exhibits an imaging speed close to that of the manually cleaned plate using the thin imaging layer.
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.
This is a continuation-in-part of U.S. Ser. No. 13/109,651, filed on May 17, 2011, the entire disclosure of which is hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13109651 | May 2011 | US |
| Child | 13214475 | US |
