WO2013134380A1

WO2013134380A1 - Method of developing a lithographic printing plate including post treatment

Info

Publication number: WO2013134380A1
Application number: PCT/US2013/029363
Authority: WO
Inventors: Howard A. Fromson; William J. Ryan; Brian L. Anderson; Anthony Paul Kitson
Original assignee: Anocoil Corporation
Priority date: 2012-03-06
Filing date: 2013-03-06
Publication date: 2013-09-12

Abstract

A method for producing a lithographic plate from a negative working, radiation imageable plate having an oleophilic resin coating that reacts to radiation by cross linking and is non-ionically adhered to a hydrophiiic substrate. The steps include imagewise radiation exposing the coating to produce an imaged plate having partially reacted image areas including unreacted coating material, and completely unreacted nonimage areas; developing the plate without solubilization by removing only the unreacted, nonimage areas from the substrate while retaining unreacted material in the image areas; and blanket exposing the developed plate with a source of energy which further reacts the retained unreacted material in the image areas. Development is preferably by mechanical removal of the non-image areas.

Description

METHOD OF DEVELOPING A LITHOGRAPHIC PRINTING PLATE INCLUDING POST TREATMENT

Background

[0001] The present invention relates to lithographic printing plates.

[0002] Plates of interest have a solvent-soluble, radiation- polymerizable, oleophilic resin coating on a hydrophilic substrate. In conventional practice, after image-wise exposure at ultraviolet (UV), visible (violet), or infrared (IR) wavelengths, the plates are developed with solvent to remove the unexposed areas of the coating by dissolution, thereby producing a substantially planographic pattern of oleophilic and hydrophilic areas. The developed plates are then ready for mounting on a cylinder of a printing press, where the plates are subjected to fountain fluid and ink for transfer of ink to a target surface according to the pattern of oleophilic and hydrophilic areas on the plate.

[0003] The imaging radiation produces a cross-linking reaction in the imaged areas, which increases the mechanical adhesion of the image areas to the grained surface of the substrate, and also increases the cohesion (hardening) of the image area so that it can withstand the abrasive effect of receiving and transferring ink during the production run on-press.

[0004] Although thermally imageable plates that require no preheat are commercially available, they usually have relatively low resolution and short press lives. Since the dot perimeters and the cross- linking are achieved with the same imaging source, it is very difficult to use very high energy without considerable "dot" growth. An image created in this manner creates "dots" or "pixels" substantially larger in order to establish image integrity. This phenomenon is called "dot gain" and results in lower plate resolution.

[0005] As an alternative, the plate can be exposed at lower imaging energy and then pre-heated before development, but "dot gain" still occurs. However, in this case it is the excess energy of the heater that causes the "dot gain". This energy (in the form of heat) causes the polymerization to continue not only in the center of the dots (which is needed for longer press life) but it also causes the dots to grow out from the edges.

[0006] Another alternative for increasing press life while maintaining acceptable resolution, is to "post cure" the plate after development. By applying energy after the non imaged area has been removed during development, "dot gain" cannot occur. At present, the most widely used method of producing plates with high resolution and long run length is with using a post development baking oven. With this method the plate is imaged at a moderate exposure level and then developed. The development not only removes the non image areas, but also dissolves unreacted ingredients in the image areas. After development the plate is run through a baking oven where it is heated to approximately 450 deg. F and held there for up two minutes. This reacts any residual active component and fuses the image areas. Although this produces an extremely long running image with good resolution, this procedure has two problems. By reacting and fusing the image to such a high degree, the dots shrink. Such "dot loss" causes unwanted sharpening of the image. Also, by heating the aluminum substrate to such a high temperature the plate warps and buckle, causing registration and lock up problems on press.

[0007] Regardless of how plate manufacturers and end users make this tradeoff, in conventional solvent based development of negative, actinically imageable lithographic plates, no substantial further cross-linking can be achieved in the image areas after development of the plate in the solvent. Any coating material in the image areas that did not react with the radiation, is dissolved and therefore removed from the image areas during the development step. Summary

[0008] The present invention addresses and minimizes the necessity of such tradeoff. The disclosed method achieves the remarkable combination of significantly reducing the imaging time, increasing the resolution, and increasing the molecular weight of the imaging, and thus on-press life of the printable plate. The method produces a plate with high resolution, long press life, using low power imaging, and low energy post treatment.

[0009] This method is based on the combination of (i) a coating formulation that yields an image of high resolution when image-wise exposed to one or more of IR, visible, or UV energy; (ii) a coating formulation that when exposed to such imaging has sufficient image integrity to survive the development step with negligible loss of the active ingredients; (iii) a developer that does not leach out or destroy the active ingredients of the image areas; and (iv) a low power post treatment that supplies UV and/or IR energy to the surface of the plate to further react the active ingredients in the image areas at a low cost.

[0010] These advantages are achieved by shifting a large fraction of the cross-linking, from the imaging step to the post-treating step. Because no nonimage coating material remains on the substrate after development, while the imaged areas on the substrate contain a significant unreacted content, there is practically no limit to the intensity of energy that can be beneficially applied to the developed plate.

[0011] Preferably, the degree of imaging radiation is slightly above the minimum level that provides sufficient cross-linking to prevent removal of the imaged areas during mechanical development. Post- treating is then relied on to maximize the cross-linking and thereby achieve improved plate life on-press. For example, conventional infrared (IR) imaging energy is about 125 mj/cm², before preheating at 102° C. For a commercial implementation of the present method, imaging can be achieved at up to three or more times the speed, i.e., in the range of about 80-40 mj/cm², with post-heating only so long as is needed to raise the temperature of the coating to at least 125° C, and preferably to the transition temperature of the resin, such as 160° C. The per cent of cross linking resulting from the post-heating can be greater than the percentage of cross linking from the imaging radiation.

[0012] Imaging at this much lower energy level has another advantage beyond increased production speed. Imaging at a relatively high but common resolution of 2400 dpi at 200 lines per inch requires that each "dot" or "pixel" of imaged coating have the desired area as imaged and that the surrounding unimaged material be cleaned out. The use of the energy level of 125 mj/cm², can produce dot gain in which coating material surrounding the nominal area of the dot is exposed to radiation, and experiences residual or ancillary cross-linking at the edge of the dot, thereby degrading the resolution. At less than 100 mj/cm², especially at 70 mj/cm², resolution degradation due to dot gain is negligible, if not avoided all together.

[0013] Whereas conventional negative working plates developed via solubilization or dispersion provide up to several hundred thousand impressions on-press, the plates manufactured according to the presently disclosed method easily achieve in excess of 500,000 impressions on- press. This combination of high resolution and long run length, permits the present method to compete with positive working plates.

[0014] The inventors have recognized the especially suitable applicability of mechanical development for implementing the present invention. With mechanical development all unreacted material remains in the image areas even after development is complete, so that post- treating produces additional cross-linking. Development using only mechanical forces, is described in U.S. Pat. No. 8,137,897 "Processless Development of Printing Plate" (the disclosure of which is hereby incorporated by reference). The mechanical development of imaged plates has numerous advantages over known techniques that rely on solubilization or dispersion for removal of the unimaged areas. These advantages include retention of the full integrity of the imaged areas, in addition to avoidance of the handling of chemical waste product.

[0015] In one general aspect, the invention is directed to a method for producing a lithographic printing plate from a negative working, radiation imageable plate having an oleophilic resin coating that reacts to radiation by cross linking, is non-ionically adhered to a hydrophilic substrate. In one embodiment, the method comprises imagewise radiation exposing the coating to produce an imaged plate having partially reacted image areas including unreacted coating material, and completely unreacted nonimage areas Directly after imaging, the plate is developed without solubilization by removing the unreacted, nonimage areas from the substrate while retaining unreacted material in the image areas. The developed plate is blanket exposed with a source of energy, which further reacts the image areas.

[0016] The plate is preferably developed by removing the nonimage areas from the substrate without dissolution or dispersion of any of the nonimage and image areas, but the advantages over previously known techniques are achievable in a practical implementation even if the coating experiences minor, incidental dissolution or dispersion.

[0017] Preferably the plate is developed by application of mechanical force on all the coating to mechanically dislodge only the nonimage areas as particles from the substrate without dissolution or dispersion of any coating material, such that the integrity of the image areas remains intact. With this preference method, the nominal (initial) coating weight is retained in the image areas through completion of the blanket exposure.

[0018] The preferred plate as imaged comprises (i) a substrate with a grained, anodized, hydrophilic surface and (ii) a negative working, organic, polymerizable coating in which all active components for polymerization are not soluble or dispersible in any of water, fountain solution, ink or similar fluids. The coating is non-ionically bonded to the substrate and is cross-linked by polymerization in areas exposed to radiation. In the plate as manufactured, the cohesion of the unimaged areas of the coating exceeds the adhesion of the unimaged areas of the coating to the substrate. After imaging, the entire coating is subjected to mechanical forces, preferably while the coating is in contact with a flushing fluid such as water, which forces disrupt and remove only the unimaged areas of the coating from the substrate surface in the form of particulate material, without dissolving or dispersing the unimaged areas of the coating into the fluid at the substrate surface. The fracturing phenomenon of the imaged organic film is possible because the adhesion of the as-coated unimaged organic film to the substrate is less than the internal cohesion of the imaged film.

[0019] During the development process, none of the active ingredients in the coating are lost from the image areas. In this context, "active" means an ingredient that participates in the radiation induced polymerization in the imaged areas. This generally means the active ingredients are a polymer, a monomer and/or oligomer, at least one polymerization or cross link initiator, and a dye.

[0020] Practitioners in this field had no reason to investigate or optimize the difference in adhesion of solvent-soluble resins as a basis for non-chemical, and especially mechanical, removal of the nonimage areas. Because it was the established practice that nonimage areas of the imaged plate could be substantially completely dissolved by the chemical reactions with the developer solution, the main objective for improving coatings has been to increase the adhesion, cohesion, and durability of the imaged areas and thereby enable the plate to better withstand the rigors of the printing press. Any desired relationship between the imaged and unimaged areas was based on relative solubility, not relative mechanical adhesion, to minimize incidental dissolution of any of the exposed surface the imaged areas while the developer solution dissolved substantially all of the non image areas.

[0021] In general, imaging produces no cross linking in the nonimage areas and as little as half of the ultimate cross linking in the image areas. Development removes at least 95 per cent and preferably at least 98 per cent of nonimage material on the plate, while retaining at least about 98 per cent of the basis weight in the image areas. Although in some embodiments a small degree of dissolution or dispersion of the binder or stabilizer might occur in image areas, none of the active ingredients in the image areas are dissolved or dispersed. [0022] In the present context, mechanical removal means that the coating is not water soluble and is at least about 95 per cent removable with brushes and flushing water. To achieve better than 98 per cent removal of the non image material with mechanical development by means of brushing in an aqueous fluid, the developing fluid can include a component, such as a surfactant, that is not necessary for the brushes to mechanically remove most of the nonimage material but which helps the brushes pull off remnants adjacent to imaged pixels or dots. In some embodiments, the composition of the coating can include a component that interacts with an agent in the aqueous developing fluid, such as a surfactant, that is necessary for achieving removal of at least about 98 per cent. But under all circumstances, the active ingredients in the imaged areas are unaffected and can be further cross linked. The combined advantages are that (i) plates can be imaged with low intensity radiation; (ii) pre-heating of the imaged plates can be avoided; (iii) development is with an aqueous fluid rather than a solvent, whereby at least 98 per cent of the nonimage material can be removed while at least about 98 per cent of the imaged material is retained; and (iv) unrestricted intensity of post-treatment blanket energy exposure produces maximum cross linking.

Brief Description of the Drawings

[0023] Figure 1 schematically shows a printing system comprising plate stack, imager, and press;

[0024] Figure 2 is a schematic plate cross section showing an imageable coating directly supported on a substrate;

[0025] Figure 3 is a schematic plate cross section showing an imageable plate with a subcoat and top coat;

[0026] Figure 4 is a schematic plate cross section upon exposure to radiation;

[0027] Figure 5 is a schematic plate cross section showing the pattern of remaining oleophilic imaged areas of the coating and the hydrophilic substrate surface areas where the unimaged areas have been removed in solidus; and

[0028] Figure 6 is a schematic of one embodiment of a pre-press water processor.

Description of the Preferred Embodiments

[0029] Figure 1 shows a schematic of a printing plant 10, such as for newspaper printing, in which a stack of radiation imageable plates 12 is situated upstream of an imager 14, where the coating on the plates is selectively cross linked by selective exposure to radiation to form a pattern of highly cohesive and adhesive areas, and areas that exhibit less cohesion and adhesion. The plate substrate is hydrophilic, whereas the coating is oleophilic. The radiation exposure produces high internal cohesion, and high adhesion to the plate. In a conventional negative working system, the original (unimaged) coating is soluble in a specified developer solvent, so the imaged plate must be developed with such solvent to remove the non-exposed areas and thus produce a plate usable in the press. The developer solutions most frequently used contain either some amount of an organic solvent (typically benzyl alcohol) or have an elevated pH (alkaline).

[0030] In the present preferred embodiment, the imaged plates are transferred to a mechanical processor 16, in which the non-imaged areas are removed by mechanical impingement of the coating with resulting dislodgment and removal. The energy level of the imaging at 14 is selected such that the imaged areas are only partially reacted, i.e., the imaged plate has partially reacted image areas including unreacted coating material, whereas the non-image areas are completely unreacted, i.e., they have not been affected by the radiation. The mechanical impingement removes only the unreacted non-image areas from the substrate while retaining all of the unreacted material in the image areas. The mechanically developed plates are then delivered to a post-treatment unit 18 where blanket exposure of the plate from an external source of energy further reacts the image areas, thereby increasing the cross linking within the image areas.

[0031] In one embodiment, completely imaged plates can be mounted directly on press 20 where the ink 22 develops the plates and after cleaning out produces printed product 24.

[0032] Tables A and B show data that was obtained for IR imaging with thermal post treatment by varying the imaging energy, the blanket heating energy temperature after imaging, and the rate of travel of the plate at a given temperature.

TABLE A

TABLE B

[0033] In Table A, both samples were imaged at 50 mj/cm², one with post-heating at 160° at a throughput of 4 feet per minute, whereas the other was not post-heated. The plates as imaged and post-heated were rubbed with a cotton swab with benzyl alcohol for 100 double rubs. For imaging at a dot density of 80, 90 and 100, the average percent color value loss for the post-heated plate was 21 % whereas the average loss for the non-post heated plate was 74%. Table A shows that post heating increases durability significantly relative to no post-heating for plates that were imaged at low energy and developed mechanically.

[0034] Table B shows that at the relatively low heating temperature (105° C) used in conventional preheat of negative working plates, the higher the imaging energy level the lower the color value loss (Plate #4). The improvement in the average color change relative to no pre or post heating (Plate #3) only went from 26% without post-heat to 17% with post-heat over the range of 100-160 mj/cm². However, with a post-heat temperature of 160° (Plate #5), the average percent loss over the same energy exposure range is only 6%. Moreover, energy of only 100 mj/cm² and 160° post heat temperature produces the same loss as energy exposure of 160 mj/cm², at 105° preheat temperature (i.e., 9%).

[0035] The conclusion to be drawn is that the capability of post- heating at high temperature produces significantly enhanced cross- linking. The average at 60° post-heat verses no post-heating is a fourfold improvement from 26% loss to 6% loss. Relative to post-heating at conventional temperature of 105°, post heating at 160° shows approximately a three-fold improvement, from 17% to 6% loss.

[0036] Figures 2-5 illustrate schematically, the physical attributes of a plate according to an aspect of the present invention. Figure 2 is a schematic section view of the basic embodiment 26, consisting of a substrate or carrier S on which an organic, non-aqueous solvent-based coating C has been applied and dried. The substrate S is preferably a grained, anodized aluminum sheet. The substrate is preferably treated with a hydrophilizing agent prior to coating. Such treatments are well known in the art, and include silicate solutions, polyvinylphosphonic acid (PVPA) or amino trimethylenephosphonic acid (ATMPA). The coating C is applied from a solvent soluble composition comprising one or more components capable of cross linking by free radical polymerization. The polymerization arises as a result of imaging with ultraviolet, visible or infrared radiation. As such, the coating may further comprise radiation absorbers and/or initiators to facilitate the cross linking efficiency. None of these active components is soluble in water. Preferred coating compositions further comprise a polymeric binder material to enhance the oleophilicity and durability of the coating in the ink receptive printing areas.

[0037] Figure 3 is a schematic section view of a plate according to an alternative embodiment where a subcoat SC has been applied to the substrate S, the imageable coating C is applied over the subcoat, and a topcoat TC is applied over the imageable coating. The top coat TC is typically a water soluble film forming layer such as polyvinyl alcohol (PVOH) that serves to prevent atmospheric oxygen from diffusing into the coating and quenching the free radicals. Without the topcoat, the polymerization efficiency is dramatically decreased. The subcoat SC must not adversely impact the adhesion of the coating to the substrate in the imaged areas of the coating. 4-hydroxybenzene sulfonic acid, sodium salt has been found to be particularly suitable as a subcoat.

[0038] Figure 4 corresponds to Figure 2, and illustrates the effect on the coating of exposure to imaging radiation. The radiation source is preferably a digitally controlled laser, which produces exposure pixels such that a pattern of unexposed coating 38a, 30b, and 30c and exposed coating 32a and 32b covers substantially all of the plate. However, any of the sources of incident imaging radiation used in the art to form selectively written surfaces can be used. The selective imaging results in relatively distinct boundaries 34 at the interface between the imaged and unimaged areas. It should be appreciated that the Figures are not to scale, especially as to relative thickness of the coating and substrate, but are merely illustrative. For the illustrated negative working plate, the exposed coating in areas 32a, 32b becomes highly but still only partially cross linked, thereby creating areas that have sufficient cohesion and adhesion such that they are not removable by subjecting these areas to substantial mechanical forces or pressure. The unexposed areas 30a, 30b, and 30c retain the original characteristics and properties of the dried coating before imaging. This material is not highly cross linked, and lacks the adhesion to withstand substantial mechanical forces or pressure.

[0039] Figure 5 shows a portion of the resulting plate 26 ready for post-treatment with areas 32a and 32b representing the partially reacted oleophilic coating areas and 42a, 42b, and 42c representing the hydrophilic substrate surfaces. It is to be understood that the plates and process described herein are essentially planographic and, the relative thickness of the areas and surfaces shown in the figures should not be considered as in scale.

[0040] Figure 6 is a schematic of the operative components of one possible processor 200 for the pre-press mechanical development of an imaged plate in a system as depicted in Figure 1 (where the mechanical processor is indicated at 16). The imaged plate 202 is conveyed over a basin or tank 204 onto a platen 206 or the like. Water, but preferably an aqueous wash fluid, is sprayed or otherwise deposited 208 on or near a coarse rotary brush 210 which impinges on the coating surface to dislodge and remove the unimaged areas as particulates. The overflowing fluid with removed particles is captured in the basin or sump 204 and continuously drained and delivered via line 212 to particle filter 214. The filtered fluid is recirculated back to the spray nozzle 218 by pump 216 and return line 218. The resinous material removed as particles is trapped in the filter, so there is little or no chemical treatment required of the waste stream associated with developing the plate.

[0041] Significant advantage of the present invention is that the integrity of the imaged coating is not adversely affected by the developing fluid. For conventional plates, the imaging process causes a change in the solubility of the coating in the developer. The change is never 100% efficient; that is, even the imaged coating will have some level of solubility in the developer. This residua! solubility may significantly alter the adhesive and/or cohesive integrity of the coating. Mechanical development does not suffer from this problem. The coating weight of the imaged areas is not affected by such development.

[0042] In one particular embodiment of the invention having the basic configuration shown in Figure 2, the coating comprises from about 5 to about 30 wt% based on solids content, of a polymer that is generally considered, as insoluble in water by practitioners of applied chemistry The polymer material may be selected from a wide range of types such as but not limited to acrylates (especially urethane acylates), siloxanes, and styrene maleic anhydrides.

[0043] Advantageously, the coating comprises from about 35 to about 75 wt% based on solids content, of a polymerizable monomer, a polymerizable oligomer, or combination thereof that is similarly insoluble in water. Some suitable radically polymerizable (cross linkable) materials are a multifunctional acrylate such as Sartomer 399 and Sartomer 295 commercially available from Sartomer Co.

[0044] The coating comprises a non-water-soluble initiator system capable of initiating a polymerization reaction upon exposure to imaging radiation. Some suitable initiator systems comprise a free radical generator such as a triazine or an onium salt.

[0045] The coating could include from about 5 to about 15 wt% based on solids content of an organic compound that is soluble in organic solvents and only partially soluble in water. Some suitable compounds include a substituted aromatic compound, such as DTTDA (an allyl amide derived from tartaric acid) and tetra methyl tartaramide. The water solubility must not be so great as to overcome the hardening of the imaged areas and compromise the ability of these areas to remain on the plate without loss of active ingredients.

[0046] Additional optional components include dyes that absorb the imaging radiation (e.g. infrared absorbing dyes) and pigments or dyes that serve as colorants in the coating.

[0047] With the preferred implementation of the invention whereby imaging is performed at a relatively low energy, e.g., below 100 mj/cm², mechanical development can clean out non-imaged material to a level approaching 100%, because less than about 50% of the ultimate (post heat) cross linking can be performed during imaging. Even relatively coarse brushes with flushing water can remove unimaged material at the edges of the dots and, furthermore, there is little if any undesirable cross linking of coating material immediately surrounding the nominally exposed pixel due to avoidance of the "dot gain" effect.

[0048] There are many types of resins, oligomers and monomers that can be used to produce coatings that would have properties suitable for use in the present invention. It is believed that the monomer to polymer ratio in the range of 2-4 and the use of an organo-borate catalyst with an onium salt catalyst are important preferences. A wide mixture of functionalities can be used but dried coatings with better adhesion and cohesion are achieved with multi functional monomers and oligomers (functionality of 3 or higher). It is not necessary to use a resin which contains unsaturated groups but in the majority of the cases the cured film will exhibit better adhesion and integrity. Types of resins can include poly vinyls (poly vinyl acetate, poly vinyl butyral, etc), cellulosic, epoxies, acrylics and others as long as the resin does not produce a strong adhesive bond with the substrate. Monomers and oligomers should be somewhat viscous liquids and can be polyester/polyether, epoxy, urethane acrylates or methacrylates (such as polyether acrylate, polyester acrylate, modified epoxy acrylate, aliphatic urethane methacrylate, aliphatic urethane acrylate oligomers, polyester acrylate oligomers, aromatic urethane acrylate, dipentaerythritol pentaacrylate, pentaacrylate ester, etc.).

Radiation Sensitive Coatings

Formulations

Inqrediente #1 #2 #3

PGME 94.990 94.990 94.990

Poly 123 1.500

Bayhydrol 2280 1.500

ACA Z250 1.500

Sartomer 399 1.750 1.500 2.000

Sartomer 454 0.250 0.250

Sartomer 355 0.250

IRT thermal Dye 0.150 0.150 0.150

Penn Color 0.350 0.350 0.350

HOINP02 0.400 0.350 0.050

Showa-Denko P3B 0.050 0.350

Phenothiazine 0.010 0.010 0.010

Showa-Denko 2074 0.600 0.600 0.600

TOTAL 100.00 100.00 100.00

[0049] Formulations #1-3 are consistent with the preferred implementation of the present invention, to the effect that a wide range of ingredients can be used in order to produce a lithographic printing that can be developed using only a mechanical force applied to the coating, without reliance on dissolution or dispersion of the coating in water.

[0050] All plates having coating formulations #1 -3 are comprised of a substrate with a hydrophilic surface and a very oleophilic radiation sensitive layer, but the mode of development of coating formulations #1 -3 relies strictly on the adhesive and cohesive properties of the coating. These coatings as applied and prior to imaging exposure have better cohesive strength than adhesive strength. When the coating is exposed to radiation it undergoes polymerization which greatly amplifies its adhesive and cohesive strengths.

[0051] The following list of representative ingredients will enable practitioners in this field to formulate coating compositions that are adapted to a meet targeted performance that balance cost of ingredients, coating process control, shelf life, range of imaging radiation wavelength, type or types of mechanical forces to be used for development, type of fountain and ink on press, and ease of achieving target resolution. For commercial purposes additional, non-active water insoluble ingredients can be included such as viscosity agents for facilitating coating of the plate, shelf life stabilizers, and agents for reducing any tendency for removed coating particles to build up in, e.g., a water and rotary brush processor. In variations not shown in Table D, the solvent can be Arcosolve PM, DMF, and MEK; non-active stabilizers, pigments and the like can include Karenz PE1 and 29S1657 as well as the ACA Z 250. Urethane acrylate resins with active ingredients similar to formulation #2 and various water-insoluble inactive ingredients are presently preferred.

[0052] The currently favored prototype coating is shown in Table D.

TABLE D

[0053] According to the invention, only a partial cross-linking of the photosensitive layer is desired during the imaging step with the balance of cross-linking occurring during post treatment. With thermal post treatment, maximum cross linking can sometimes be further enhanced if the temperature exceeds the glass transition temperature of particles of resin that may not have dissolved in the monomer. If such particles are closely enough distributed in the matrix they can fuse with one another, creating a network or web which further enhances the strength of the oleophilic areas that will perform the print image on press. Because if such fusion occurs it would only be in the image areas after the non- image areas have been removed, the fusion would not increase the dot or pixel size.

[0054] Mechanical development is preferably achieved with relatively stiff, coarse, rotating brushes in an aqueous environment such as in the Agfa Azura wash out unit or the Proteck XPH 85 HD processor. Both machines use two relatively stiff, coarse brushes supported by a platen and have spray bars that deliver an aqueous wash out solution to the plate. The wash out solution is allowed to flow over the plate and then run back into the sump that is located below the machines. The solution is kept at about 70-100 deg. F in the sump. The basic wash out solution contains anionic surfactants, nonionic surfactants and silica. The components of the wash out solution should be selected to serve three basic purposes. First, they help prevent the particles of coating that are removed by the brushes from sticking to each other or any surfaces that they encounter. Second, they serve as a finisher on the plates to protect against fingerprints and heat. Third, they increase the hydrophilicity of the substrate.

[0055] Mechanical development for plates imaged above 100 mj/cm² with brushes and this basic wash out solution will clean out up to about 97% or 98% of unimaged material, which is quite adequate for newspaper printing. However, if the plates are to be used for commercial or other high quality jobs, cleanout should approach 100% before post heating.

[0056] To achieve this level of cleanout, residual unimaged material at the base of the image dots can be removed by the action of one or two additional, non ionic surfactants that have high HLB values. As a practical matter, the surfactant molecule has one end that has an affinity to water and another end that has an affinity to the oleophilic coating, so the action of the brushes and water turbulence removes the residual coating as if by pulling it off the substrate (as distinguished from dissolving the residual coating).

[0057] Increased cleanout can also be achieved only with brushes and tap water if the brush impact duration is extended by decreasing the throughput rate. [0058] If the coating includes a partially water soluble compound, the water penetrates the unimaged coating to the substrate whereby the coating separates from the substrate in particulate form with less mechanical action than in the preferred embodiment. As in all embodiments, the imaged areas have been exposed to sufficient energy to enhance the adhesion to the substrate and the internal cohesion and thereby resist removal during development. This enhancement in the image areas minimizes the penetration of water due to the presence of the partially water soluble compound. Even if some of the material in the image areas is lost during development, enough partially cross linked material remains such that the additional cross linking reactions during post heating provide the desired advantages.

[0059] Table E shows that over a wide range of imaging energy, the hardening of the imaged areas is predominantly dependent on the post heating energy. Thus, one can obtain the advantage of imaging at a low energy/high speed (e.g. 40 to 80- mj), while easily achieving higher durability using post heat temperatures (e.g., 160 C) well above the practical pre-heat limit of 105 deg. C. The table shows that 40 mj imaging with 160 C post heat produces higher cross linking (50% vs. 40%) and much more plate life (1.76% vs. 5.08% color loss) than imaging at 200 mj without pre or post heat. The table also shows that initial radiation imaging at 40 mj or 80 mj, produces 16% and 24% cross linking, respectively. Post heating increases the cross linking to 50% and 52% respectively. It can thus be appreciated that in the preferred embodiment, I imaging at under 100 mj followed by post heating at 160 deg. C is very effective.

TABLE E

[0060] The following Table F shows the additional advantage that imaging with low energy and high post heating temperature also achieves higher resolution.

TABLE F

[0061] With imaging at 200 mj the measured resolution does match the target resolution to commercially acceptable standards, whether or not the plate is pre or post heated. Such high imaging energy polymerizes coating material outside the footprint of radiation as it penetrates the coating, producing unwanted hardening outside the desired pixel boundary. With imaging at 40 mj, and no pre or post heating, the resolution is within commercially acceptable standards, but as discussed with respect to Table, the plate would have unacceptably low life on press. With the known method of imaging at 40 mj after pre heat at 105 deg. C, the resolution is still acceptable and the plate life would be improved relative to no heating, but not up to commercial standards. With imaging at 40 mj and post heating at 160 deg. C, the resolution is overall at least as good if not better than with either no or pre heating.

[0062] The following Tables G-K demonstrate either the amount of double bond conversion and/or the resolution of the images when exposed to the various types and amounts of energies. In all examples, the plates were blanket exposed to imaging radiation, development was performed with rotating brushes in an aqueous wash out solution, as described above, and then post-treated.

TABLE G

IR Imaging With Mechanical Development

(Varied Post Exposure Treatments for % conversion)

(All exposures at 130 mj)

Exposure only 36% conversion

Exposure + Pre Heat 44% conversion (+22%)

Exposure + UV Post Cure 46% conversion (+28%) Exposure + IR Post Cure - ^■ 54% conversion (+50%)

Exposure + UV + IR Post Cure— 66% conversion (+83%)

The numbers in parentheses show the per cent increase in double bond conversion with pre-heat or post treatment, relative to the conversion due to imaging exposure only. Whereas preheating improves the conversion by 22%, all the post-treatment techniques improve the conversion by at least 28% (with UV only) up to 83% (with a combination of UV on an IR heated plate). Stated differently, at least 20% and up to 45% of the final double bond conversion is achieved in the post-treatment. IR post- treatment alone achieves over 30% of the total conversion. TABLE H

IR Imaqinq With Mechanical Development and UV+IR Post Cure (Varied exposures for % conversion)

Exposure too low to obtain

66.5% conversion

65.7% conversion

68.3% conversion

67.0% conversion

65.3% conversion

67.5% conversion

68.0% conversion

TABLE I

IR Imaqinq with Mechanical Development and UV+IR Post Cure (Varied exposures for resolution measurements (j¾ 175 lpi/2400dpi)

Exposure 1 pixel 2 pixel 3 pixel 4 pixel

30 mj N/A N/A N/A N/A

50 mj N/A N/A N/A N/A

70 mj 93% 61 % 57% 55%

90 mj 97% 68% 60% 58%

110 mj 99% 75% 67% 60%

130 mj 100% 76% 68% 61 %

150 mj 100% 78% 69% 62%

170 mj 100% 81 % 73% 66%

190 mj 100% 99% 93% 90% TABLE J

Violet Imaging With Modified Mechanical Development

(Varied Post Exposure Treatments for % Conversion)

(All exposures were done at 40 micro joules)

Exposure only 66%

Exposure + Pre Heat 69% (+5%)

Exposure + Post UV Cure 70% (+6%)

Exposure + Post IR Cure 72% (+9%)

Exposure + Post UV&IR Cure 80% (+21 %)

The numbers in parentheses show the per cent increase in double bond conversion with pre-heat or post treatment, relative to the conversion due to imaging exposure only.

TABLE K

Performed under same conditions as for Table J on AGFA - N94 Violet Plate

Exposure only

Exposure + Pre Heat

Exposure + Post UV Cure

Exposure + Post IR Cure

Exposure + Post UV&IR Cure

The asterisk (^*) in Table L indicates that none of the post energy tests could be performed because the N-94 plate lost 90% of its image when developed without a pre-heat treatment.

Claims

1. A method for producing a lithographic printing plate from a negative working, radiation imageable plate having an oleophilic resin coating material that reacts to radiation by cross linking and is non- ionically adhered to a hydrophilic substrate, comprising:

imagewise radiation exposing the coating to produce an imaged plate having partially reacted image areas including unreacted coating material, and completely unreacted nonimage areas;

directly after imaging, developing the plate without solubilization to remove only the unreacted, nonimage areas from the substrate while retaining unreacted material in the image areas; and

blanket exposing the developed plate with an external source of energy which further reacts the retained unreacted material in the image areas.

2. The method of claim 1 , wherein the plate is developed by removing only the nonimage areas from the substrate without solubilization or dispersion of any of the material in the nonimage and image areas.

3. The method of claim 1 , wherein the plate has a nominal coating weight before developing and the image areas substantially retain the nominal coating weight through completion of the blanket exposure.

4. The method of claim 1 , wherein the plate is developed by application of mechanical force on all the coating to mechanically dislodge only the nonimage areas as particles from the substrate while the image areas remain intact.

5. The method of claim 4, wherein

the coating is solvent soluble but not soluble or dispersible in any of water, press ink, or press fountain solution; and the application of mechanical force dislodges the nonimage areas as particles from the substrate without solubilization or dispersion of any coating material.

6. The method of claim 1 , wherein developing removes at least 98% of the coating material in the unreacted, nonimage areas from the substrate while none of the coating material in the image areas is removed.

7. The method of claim 1 , wherein

the imagewise radiation exposure produces dot image areas in the imaged plate that have increased adhesion to the substrate and increased internal cohesion relative to the unimaged areas;

the dot image areas have boundaries defined by the imagewise radiation exposure; and

the increased adhesion and cohesion from the radiation exposure is sufficient to maintain the dot boundaries throughout development of the plate.

8. The method of claim 1 , wherein the plate is developed in an aqueous wash including anionic surfactants, nonionic surfactants and silica.

9. The method of claim 1 , wherein no ingredient in the coating that participates in cross linking is soluble in water and development is by mechanical impingement in an aqueous fluid.

10. The method of claim 9, wherein no ingredient in the coating is soluble in water.

11. The method of claim 1 , wherein the coating is sensitive to infra-red radiation, the initial exposure is with infra-red radiation at an energy level in the range of 50-150 mj/cm², and the blanket exposure is with thermal energy at a temperature above 120 deg. C, preferably about 160 deg. C.

12. The method of claim 1 1 , wherein the plate is developed in an aqueous wash including anionic surfactants, nonionic surfactants and silica.

13. The method of claim 1 , wherein the plate is imaged with violet radiation, developed with brushes in an aqueous wash including surfactants, and the blanket exposure comprises UV radiation.

14. A method for producing a lithographic printing plate from a negative working, radiation imageable plate having an oleophilic resin coating that reacts to radiation by cross linking and is non-ionically adhered to a hydrophilic substrate, comprising:

imagewise radiation exposing the coating with IR energy to produce an imaged plate having partially reacted image areas including unreacted coating material, and completely unreacted nonimage areas; developing the plate to remove only unreacted, nonimage areas from the substrate while retaining all the unreacted material in the image areas; and

15. The method of claim 14, wherein the blanket exposure comprises IR radiation.

16. The method of claim 14, wherein the blanket exposure comprises UV radiation.

17. The method of claim 14, wherein the plate is developed by application of mechanical force on all the coating to mechanically dislodge only the nonimage areas as particles from the substrate while the image areas remain intact.

18. The method of claim 14, wherein

the coating is non-ionically adhered to the substrate and has a cohesion that is greater than the adhesion to the substrate;

the developing is by application of mechanical force to disrupt and dislodge the nonimage areas as particles from the substrate without solubilization or dispersion; and

the imagewise exposure increases the adhesion of the image areas to the substrate to resist removal by the mechanical force that disrupts and dislodges the nonimage areas from the substrate.

19. The method of claim 14, wherein before imaging the resin coating has a nominal coating weight and after imaging the image areas retain the nominal coating weight throughout development, and blanket exposure.

20. The method of claim 14, wherein the plate is developed in an aqueous wash including anionic surfactants, nonionic surfactants and silica.

21. The method of claim 14, wherein the nonimage areas of the coating are removed from the substrate without dissolution or dispersion of any of the coating.

22. The method of claim 14, wherein upon completion of said further reacting, at least 20% of the total cross linking was achieved during said blanket exposure.

23. The method of claim 14, wherein at least about 30% of the total cross linking was achieved during said blanket exposure.

24. The method of claim 18, wherein the mechanical force is applied by a rotating brush in an aqueous fluid.

25. The method of claim 24, including distributing the dislodged particles in a flow of said aqueous fluid; filtering the dislodged particles from the flow; and recirculating the filtered fluid back to the brushes.

26. The method of claim 18, wherein the unimaged areas of the coating are removed from the substrate without dissolution or dispersion of any of the coating.

27. The method of claim 18, wherein

the coating is solvent soluble but not soluble or dispersible in any of water, press ink, or press fountain solution;

the plate is developed in an aqueous wash including anionic surfactants, nonionic surfactants and silica; and

the application of mechanical force disrupts and dislodges substantially all of the nonimage areas as particles from the substrate without dissolution or dispersion of any coating material.

28. A method for producing a lithographic printing plate from a negative working, radiation imageable plate having a hydrophilic substrate covered at an initial coating weight with an oleophilic resin coating material having active ingredients that react to radiation to produce cross linking, comprising:

(a) imagewise radiation exposing the coating to produce an imaged plate having partially reacted image areas and completely unreacted nonimage areas;

(b) without pre-heating, developing the imaged plate to remove at least 98 per cent of the nonimage areas from the substrate while retaining the image areas at a coating weight of at least about 98 per cent of the initial coating weight without loss of active ingredients; and

(c) subjecting the upper surface of the plate to blanket exposure with an external source of energy which further reacts the retained unreacted material in the image areas.

29. A method for producing a lithographic plate from a negative working, radiation imageable plate having an oleophilic resin coating that reacts to radiation by cross linking and is non-ionically adhered to a hydrophilic substrate, wherein no active ingredient that participates in a cross linking reaction is soluble in water, said method comprising:

without pre-heating, developing the plate with brushes in an aqueous developer by substantially completely removing only the unreacted, nonimage areas from the substrate while retaining all the active ingredients in the image areas; and