Abstract:
An offset plate sensitive to UV or visible light, prepared simply and imaged digitally therewith in a CTP method, and also processed in a simple manner. The plate is provided as a photopolymer plate with increased sensitivity, and is used in a simple imaging-on-press system, using the surface of a lithographic printing cylinder itself as a plate substrate. The method of preparing the printing plate begins by applying a photosensitive liquid coating to the substrate, digitally imaging the coated substrate, using an actinic light source to polymerize portions of the liquid coating in accordance with the imaging, and washing the liquid.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to computer-to-plate (CTP) offset lithographic printing, and more particularly, to a novel method for producing an inexpensive and simple, offset printing plate which is digitally-imaged using actinic light. 
     BACKGROUND OF THE INVENTION 
     Offset lithographic printing has remained a most popular method of printing for many years. An important reason for this is the relative ease with which offset lithographic printing plates can be produced. Currently, the most widely-used method for plate preparation has remained that which utilizes specially-prepared masking films through which pre-sensitized printing blanks are selectively hardened or softened (according to the chemistry of the plate) by exposure to ultra-violet light. The plate then undergoes a development process, during which the more soluble regions of the plate (which may be the exposed or the unexposed areas) are washed away. A detailed description of the system and the plates used can be found in Chapter 20 of the book Printing  Materials: Science and Technology  by Bob Thompson, published by Pira (Leathershead, Sussex, UK), 1998. 
     In recent years, various considerations have arisen that point to advantages for modification of hitherto generally-accepted practices. With the advent of computers, information for printing is prepared digitally and it has become preferable to use this digital information as directly as possible in plate preparation. One obvious way would be to eliminate the masking film. Not only are these films a source of expense, but the most widely-used films are based on silver chemistry whereby the exposure and handling of the film must be in a light-excluding environment. In addition, the exposed film must be processed with chemical solutions which are unstable, messy and environmentally problematic. 
     One answer is to be found in computer-to-plate (CTP) systems whereby the offset lithographic plates are directly imaged with a light source which is modulated to correspond to the digital information from the computer. Thus the film intermediate is completely eliminated. It would have been easy and convenient if the UV sensitive pre-sensitized plates previously used for imaging with an intermediate film could have been used for direct digital imaging. However, it has been found necessary to either sensitize plates for imaging with visible light or to develop plates that are sensitive to radiation in the near infrared. The reason for this is that UV lasers are very expensive and difficult to modulate. Visible and infrared lasers are more readily available and less costly and easier to modulate to produce the digital signal necessary for imaging. 
     U.S. Pat. No. 4,486,529 describes the sensitizing of a negative working diazo system to the 450 nm to 530 nm region for imaging with, for example, argon-ion lasers with most of its energy being emitted in the 488 nm to 514.5 nm region. Such systems require a cover layer of, for example, polyvinyl alcohol to protect against oxygen inhibition and may also require a post-image heating stage. Sensitivity of diazo systems which are used in the coatings of pre-sensitized plates is generally in the region of 150 to 600 mJ/cm 2 . This type of plate, like most pre-sensitized plates, needs processing with a strong alkali, although attempts are being made to develop CTP systems that are processless. The subject of CTP systems can be found in Chapter 21 of the book (Thompson, 1998) cited above. 
     U.S. Pat. No. 5,339,737 Lewis et al describes the processless preparation of offset lithographic printing plates, wherein the upper layer or layers of the plate are ablated away. The upper layer is either oleophobic for waterless plates or hydrophilic for conventional wet-process plates. The substrate is oleophilic in both cases. U.S. Pat. No. 5,353,705 Lewis et al is similar to the previous patent, but describes additional layers for secondary partial ablation. U.S. Pat. No. 5,487,338 is similar, but includes reflective layers. 
     All of these inventions involve multi-layered plates which are expensive to produce. Also, for such multi-coated systems, it is more difficult to maintain a consistent standard of quality from plate to plate. So called processless plates that are imaged by laser ablation either involve a scrubbing stage to remove ablated debris from the surface, or they need extraction systems to remove debris ‘on the fly’ as it is being produced during imaging. 
     Recently, it has been found that there are ways of imaging UV-sensitive plates by digital means, permitting the development of machines for use in CTP systems. Examples of this are the UV-Setter® 710 and ProSetter®, CTPs produced by basysPrint GmbH of Boizenburg, Germany. These are flatbed image setters based on a Spatial Light Modulator device working in transmission and a non-coherent UV light source. U.S. patent application Ser. No. 09/312,763 assigned to Scitex Corporation, describes the invention of a digital image-setter utilizing a high-resolution micro-display which can be used inter alia to image pre-sensitized offset lithographic plates which are sensitive to either UV or visible light. 
     With the development of these technologies, there is a need for inexpensive plates highly-sensitive to either UV or visible light. It would also be beneficial to be able to process the plate without recourse to highly-alkaline solutions that, in many countries, is prohibited by law from being disposed of in sewage systems due to environmental hazards. 
     A reason for simplicity of processing can be found in the development of imaging-on-press. Although the concept of imaging-on-press has been described in, for example, U.S. Pat. No. 3,654,864 (Ovshinski), U.S. Pat. No. 3,741,118 (Carley), and U.S. Pat. No. 4,718,340 (Love III), printing presses incorporating this principle, such as the GTODI by Heidleberg, have only appeared within the past few years. Such presses utilize waterless plates imaged by infrared ablation where post-imaging processing is by a cleaning method to remove ablated material, rather than by image development. As the processing has to be on press, it has to be relatively simple. 
     Further developments in plate design have followed the path of elimination of a plate substrate and, in its stead, the press cylinder which traditionally holds the plate becomes a reusable lithographic ‘master’ surface. U.S. Pat. No. 4,718,340 (Love III) describes such a process which, in one embodiment, spreads an oleophilic material onto a hydrophilic surface and then removes oleophilic material from the non-image areas. As is stated in the Abstract of this prior-art patent, no photo-induced chemical reaction or latent imaging development steps are required at any time. 
     All present pre-sensitized UV sensitive offset printing plates have a common constraint. The manufactured plates are coated in continuous rolls in a factory where the plates are then cut to size, boxed and sent to the customer. The plate is removed from the box by the customer and placed on a machine for imaging. In the case of UV sensitive plates, it has been the general rule that they are placed in emulsion-to-emulsion contact with an imaged masking film, imaged by flood UV and then the film is peeled off before processing the plate in a bath of aqueous alkali. It is an obvious constraint of the plate that the surface must be resistant to damage. The plate must also be dry to the touch. Although this latter constraint has been removed if non-contact imaging as used in CTP is involved, it is a constraint that hitherto has not been appreciated as restrictive in certain areas of performance. In order to achieve dry coatings, the coating formulation is either confined to substances that produce such a dry layer or have added polymers that must be present in quantities that produce the dry layer. The presence of such polymers, and limitations of the kind of photosensitive materials used to produce the dry layers, reduces opportunities to optimize coating sensitivity and simplicity of processing. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a principal object of the present invention to overcome the disadvantages associated with prior art digitally-imaged offset plates by providing an inexpensive and simple offset plate sensitive to UV or visible light that can then be imaged digitally therewith and also processed in a simple manner. 
     It is another object of this invention to provide photopolymer plates with increased sensitivity over previous photopolymer printing plates. 
     It is still a further object of this invention to provide a CTP method that can utilize an inexpensive UV or visible light source. 
     It is still another object of this invention to provide a simple imaging-on-press system that can utilize inexpensive plates as well as an inexpensive imaging system in the UV or visible region. 
     Yet a further object of this invention is to provide a plateless process for offset lithographic printing using an inexpensive imaging system and using the surface of a lithographic printing cylinder itself as a plate substrate. 
     In accordance with a preferred method of the present invention there is provided a simple and inexpensive method of preparing an offset printing plate usable in a lithographic printing system, said method comprising the steps of: 
     a) providing a printing plate and a quantity of photosensitive liquid coating; 
     b) applying said liquid coating to the substrate of said printing plate; 
     c) digitally imaging said coated substrate, in a non-contain fashion, using an actinic light source to polymerize portions of said liquid coating in accordance with said imaging; and 
     d) washing said liquid coating with an aqueous solution after said imaging to remove unpolymerized portions of said liquid coating. 
     The present invention seeks to remove such constraints on coatings for offset lithographic plates as are found in the prior art, and utilizes sticky or wet layers as coatings so that little or no binder is present to interfere with the sensitivity or ease of processing. As the image processes used with this invention are non-contact and do not require contact of a film mask, having a sticky or liquid surface is not a problem. 
     Also, in the preferred embodiments described, since the coating process is done at the customer&#39;s location, and not provided to the customer as a finished and packed plate, handling a sticky or liquid surface is not a problem. Where the plate is provided with a sticky surface, such surface may be protected from sticking by use of a release layer bonded to a cover film, rather like that provided to sticky labels. It has been found that such coatings do not need processing by strong alkalis, but can be processed more simply as will be described in the various embodiments of the invention. 
     Other features and advantages of the present invention will become clear from the further detailed description and examples. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention with regard to the embodiments thereof, reference is made to the accompanying drawings, (shown not to scale), in which like numerals designate corresponding elements or sections throughout, and in which: 
         FIG. 1  shows three consecutive, enlarged, sectional views from the prior art of a conventional negative printing plate as it undergoes changes while being processed; 
         FIGS. 2A through 2C  show consecutive, enlarged sectional views from the prior art of a typical, thermally-ablated plate in various stages of processing; 
         FIGS. 3A through 3D  show consecutive, enlarged sectional views of an embodiment of the invention and a preferred method for the use thereof; 
         FIG. 4  shows another embodiment of the invention as used in a plateless, cylinder system; and 
         FIGS. 5A through 5C  show enlarged sectional views of a further embodiment of the invention and a preferred method for the use thereof 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings,  FIG. 1  shows, in a series of enlarged sectional views, an example of the widely-used prior art process of platemaking with pre-sensitized plates. Printing plate  10  comprises a grained, anodized aluminum substrate  12  and an emulsion coating  14 , containing a prepolymer, photoinitiator, binder, resin, and dyes or pigments. A film negative  16  acts as a negative mask in emulsion-to-emulsion contact with plate  10  and is flood-exposed with UV light  20 . The path of UV light  20  to the photosensitive, prepolymeric emulsion coating  14  is blocked by deposits of blackened, light-opaque silver  18 . Thus, UV light  20  reaches emulsion coating  14  only in transparent area  22  of the negative image. Exposure of emulsion coating  14  under transparent area  22  activates the photoinitiator component of emulsion coating  14  and initiates polymerization. Negative masking film  16  is then removed from plate  10  which is then passed through a plate processor (not shown) usually providing a strong aqueous alkali for washing away the unexposed portions of emulsion coating  14 . 
     The binder resin contained in emulsion coating  14  is usually a phenol-formaldehyde type known as Novalak. The resin is not photosensitive and its very presence acts as a diluent for the photosensitive material and consequently reduces the sensitivity of emulsion coating  14  to UV radiation  20 . Emulsion coating  14  is an alkali-soluble resin except that in the polymerized areas, such as under transparent area  22 , emulsion coating  14  is no longer alkali-soluble and remains hardened in place after development. Under image areas corresponding to light-opaque areas  18 , however, the alkali dissolves out the unexposed, unpolymerized areas of emulsion coating  14 , leaving exposed surface  24  which is hydrophilic. 
     Image area  26  of emulsion coating  14  which remains is an oleophilic and hydrophobic surface. In the course of the offset lithographic process, plate  10  is first damped with fountain solution which remains only on exposed, hydrophilic surface  24  and is then inked up, the ink remaining solely on image area  26  since it is oleophilic. Multiple impressions are made by the offset lithographic printing process by successively transferring ink from inked image area  26  of plate  10  to an offset press blanket cylinder (not shown) and then onto the required substrate (not shown) which is usually paper. 
       FIG. 2A  shows, in an enlarged sectional view, a typical structure for a thermally-ablated, waterless, offset lithographic plate  13  as known in the prior art. Plate  13  comprises, in its simplest form, a substrate  122 , a thermally-ablatable layer  28 , and a silicone rubber top layer  30 . Substrate  122  often is made up of a think, polyester sheet which is bonded to a grained, anodized aluminum base. 
       FIG. 2B  shows, in an enlarged sectional view, plate  13  during exposure to infrared radiation  32 . Ablative layer  28  is heated to a high temperature in localized areas  33  in accordance with a digitized image and undergoes physical and/or chemical changes in these areas ablating portions of top layer  30  which lie above said localized areas  33  in ablative layer  28  which are detached together with said localized areas  33 . 
       FIG. 2C , in an enlarged sectional view, shows the exposed image areas  36  on substrate  122 . The remaining portions  34  of top layer  30  act as ink-repellant areas and exposed image areas  36  on substrate  122  provide the ink-receptive areas of said plate  13 . 
     In order to ensure that exposed image areas  36  are completely free of silicone residue and ready to receive ink, it is necessary to clean plate  13  after ablation, either by dry rubbing or by washing with a liquid. In the course of the waterless, offset lithographic process, plate  13  is inked up, the ink remaining solely on the oleophilic substrate image areas  36 . Multiple impressions are made by the offset lithographic printing process by successively transferring ink from the inked image areas  36  of plate  13  to an offset blanket (not shown) and then onto the required printing substrate (not shown) which is usually paper. 
     Referring now to  FIGS. 3A through 3D , there are depicted various enlarged sectional representations of a preferred embodiment of the present invention operated in accordance with the principles of the present invention. 
     As shown in  FIG. 3A , substrate  222  is coated in proximity to, or at the imaging stage with photosensitive material  40  which is deposited from container  38 . Thus the cost of making a plate becomes relatively low as the customer purchases an uncoated plate plus a photosensitive coating solution. This can be either as CTP or for imaging-on-press, or for a plateless system. 
     Substrate  222  is preferably grained, anodized aluminum, but could be any hydrophilic substrate known to the art. Photosensitive material  40  is held in a liquid state in container  38  which can advantageously serve as an entire coating applicator device by moving across substrate  222 , applying an exact measure of material  40  uniformly over the surface. 
     In yet another embodiment, material  40  is applied by applicator  42  which could be, for instance, a wire-would rod held in contact with substrate  222  so that the thickness of the coating applied to substrate  222  is controlled by the thickness of the wire used on applicator  42 . The combination of devices  38  and  42  results in a method of application of a uniform liquid layer of material  40  of a controlled thickness laid down upon substrate  222 . 
     In still another embodiment of the present invention (not shown), container  38  (as shown in  FIG. 3   a ) may consist of two or more compartments, each holding one component of material  40  which are mixed shortly before being deposited on substrate  222 . This allows the use of materials which may have a limited pot life after mixing. 
     As shown in  FIG. 3B , after substrate  222  is coated with material  40 , it forms a photosensitive layer  216  which is not subject to any drying process as it contains little or no volatile material. In accordance with digital-imaging control signals pre-programmed by a user, layer  216  is exposed to a UV or visible light source  220 , so as to produce polymerized areas  226  in layer  216 , where exposed to the light source, and unpolymerized areas  46 , where layer  216  remains unexposed. 
       FIG. 3C  shows exposed plate  48  being washed, preferably with tap water  50 , or with a dilute aqueous solution to remove unpolymerized areas  46  (shown in  FIG. 3B ) of photosensitive material  40  and plate  48  is then ready for printing by a known offset lithographic process. 
       FIG. 3D  shows a further step whereby plate  48 , after washing, is flood-exposed with UV or visible light to further harden the polymerized areas  226  prior to printing by a known offset lithographic process. 
     Although all of the steps are depicted here as flat-bed, either the entire process can be arranged around a cylinder, or the coating can be flat-bed, as shown, and exposure to UV or visible light source can also be done either on a cylinder or a flat-bed. 
     The method of the present invention lends itself to application as a technique of CTP. The information to be printed may be provided as a digital signal in combination with a UV or visible light source, producing an image on an offset printing plate which may subsequently be printed. 
     Alternatively, in a further embodiment (not shown), the method can be adapted for imaging-on-press, the coating devices  38  and  42  (shown in  FIG. 3A ) replacing the automatic plate feed devices now available on offset printing machines, and the processing may be done by automatic plate cleaning devices (not shown), also currently available. In accordance with this preferred embodiment, the imaging device is attached to each plate cylinder. 
       FIG. 4  shows yet another embodiment of the present invention, whereby, photosensitive coating  40  is applied directly onto surface  54  of cylinder  52  itself instead of using a cylinder as in an offset machine which normally holds a plate. Thus, in the plateless process depicted here, surface  54  of cylinder  52  provides a reusable plate substrate which is cleaned after the completion of each printing job. Applicator  56  coats surface  54  of cylinder  52  with photosensitive material  40 . The prepolymer coating formed is then digitally-imaged with UV or visible light source  220  which polymerizes the image areas  326 . Any unpolymerized material  46  remaining on surface  54  of cylinder  52  is washed away by washing means  62  which contains an aqueous liquid. 
     The imaged and inked cylinder  52  is then printed by applying a fount from a conventional fountain system  64  and inking by means of an ink train  66 . The ink is transferred from cylinder surface  54  to blanket cylinder  68  and thereafter printed in a known offset lithographic process. After the required number of copies have been produced, the entire layer of imaged material  326  is removed, either by abrasion or by means of a non-volatile solvent oil, thus erasing the image. An example of such an oil is ethyl lactate. The cycle can then be repeated by reapplying the photosensitive prepolymer material  40 . 
     Another preferred embodiment of the present invention is depicted in  FIGS. 5A through 5C . A manufactured plate  70  is supplied directly to the customer and comprises substrate  322 , which, in a preferred embodiment, is comprised of grained, anodized aluminum, but generally characterized by a hydrophilic surface and a photosensitive coating  314 , which also contains a small amount of non-light-sensitive binder. Such a coating would be sticky to the touch and therefore, film  17 , comprising polypropylene, polyester or the like, with a release coating  316 , such as silicone, is also provided. A sticky layer is applied to the hydrophilic anodized and grained aluminum substrate  322  in the manufacturing process, during which a release film is laminated onto the coating. This permits easy handling of the film during packaging and loading into the CTP system. The imaging can be done with or without the film in place. Generally, for UV imaging, any film between the masking image intermediate and the sensitive coating of the offset plate could reduce the image quality. But for CTP imaging without a masking film, this not the case. Moreover, if the release film remains, it provides an oxygen barrier and thus enhances the sensitivity of oxygen-inhibited photosensitive coatings. 
     During manufacture, film  17  with release coating  316  is laid upon the sticky surface of coating  314 . Manufacturing is accomplished in on continuous process involving coating substrate  322  with a solution comprising a volatile solvent; evaporating the solvent; and then laminating film  17  with release coating  316  onto plate  70 . 
     The customer receives a box of plates manufactured as described above and shown in FIG.  5 A. These are for use in a CTP system in combination with a digitally-controlled UV or visible light source. Film  17  acts as a cover and provides a means of oxygen exclusion as well as giving optimum sensitivity and ease of development with aqueous solutions to the processing of plate  70 . 
     As shown in  FIG. 5B , photosensitive coating  314  is polymerized in imaged areas  326  after digital exposure to UV light  320 . Film  17 , together with its attached release coating  316 , is then peeled away and the unexposed, unpolymerized material of photosensitive coating  314  is washed away with water or water containing minimal additives, leaving polymerized imaged areas  326  on substrate  322 , as shown in FIG.  5 C. Plate  70  is then ready for printing by a known offset lithographic process. 
     Although in the embodiment provided with a release film, a minimal amount of non-active binder is beneficial in providing some pre-polymerization bonding to substrate  322  to prevent distortion of layer uniformity caused by pressure from the release film, this is not the case in the embodiment where coating material  40  is supplied separately for application by the user. In this case, use of a non-active binder is not necessary. In both cases, the processing is done by washing in water or by use of a very dilute aqueous solution containing additives to aid washing. The inventor has found that even though the composition used may be insoluble and immiscible with water, nevertheless, they are removed by water washing. 
     However, it is preferable that part or all of the photopolymer precursors which may be oligomers or monomers are themselves water-soluble or miscible. In the embodiment providing a manufactured plate, whatever binder resin is present is water-soluble, while in the case of the embodiment where coating material  40  is applied by the user, water washability is achieved because material  40  is unpolymerized and in the form of a sticky liquid or semi-solid. 
     In defining the composition of the layer, there is a large range of suitable pre-polymer mixtures that may be used. The mixture may consist of oligomers, monomers and diluents together with photoinitiators and synergists and dye colorants. 
     Optionally, in the embodiment where the user is provided with an uncoated plate and some coating material, and preferably, in the embodiment providing a manufactured plate, as shown in  FIG. 5 , a small amount of binder should be added. This binder should be soluble in water or may be soluble in a dilute aqueous alkali such as provided by small quantities of sodium carbonate or sodium borate. Examples of suitable binders are hydroxypropyl cellulose, poly(2-ethyl-2-oxazoline), polymethylvinyl ether alt maleic acid, and styrene maleic anhydride copolymers and derivatives. The total amount of this polymer does not exceed 15% by weight of the solids content of the coating. 
     In the case of the embodiment where a finished, manufactured plate is sold to the customer, the coating may comprise volatile solvents or solvent mixtures such as methyl ethyl ketone, ethyl alcohol, toluene, ethyl acetate or butyl acetate. Such solvents are termed VOC&#39;s (volatile organic compounds) and are subject to strict control. They are more suitable for use and more manageable in a manufacturing plant where the solvent can be either recovered or incinerated. Where the coating is done in a customer environment, this is less appropriate and where a small amount of volatile solvent (less than 5% of total formulation) may be tolerated, it is most preferable to utilize solventless coatings characterized as 100% solids whereby, even though the coating material is a liquid under the conditions of coating, the entire coating is capable of polymerization without any material evaporating into the atmosphere. 
     There are a large variety of photopolymerizable oligomers and monomers as well as diluents that have been found to be advantageous for use in the present invention. It is preferable to have water-soluble oligomers, monomers and diluents present because this makes the washing off of the unpolymerized coating extremely easy. Examples of such oligomers are polyethylene glycol diacrylates, ethoxylated trimethylol propane acrylate and polyether acrylates. 
     Examples of monomers are 2 hydroxy-3-methylacryloxy propyltrimethylammonium chloride, hydroxyalkyl acrylate and dimethylaminoethyl acrylate. Examples of water-soluble diluents are N-methyl pyrrolidone, 2-amino ethanol, ethyl lactate and morpholine. Not all of the oligomer and monomer content needs to be water-soluble, and materials can also be chosen for their high reactivity and good adhesion to the aluminum base, once polymerized. Examples of oligomers and monomers found useful are tris(2-hydroxyethyl) isocyanurate triacrylate, carboxyl functional multifunctional methacrylate oligomers, and polyurethane acrylates. 
     In addition, such materials as amine synergists and surfactants to improve coating properties may also be present. 
     The following is an example of a formulation suitable for the preferred embodiment (all parts by weight): 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 Tris (2-hydroxyethyl) isocyanurate triacrylate 
                 50 
               
               
                 2-methyl-1-[4-(methylthio)phenyl]-2-morpholino propan-1-one 
                 4 
               
               
                 2-benzyl-2-N,N-dimethylamino-1-(4-morpholinophenyl)-1- 
                 4.3 
               
               
                 butanone 
               
               
                 Isopropylthioxanthone 
                 5.3 
               
               
                 CN 435 (polyether trifunctional acrylate, sold by Cray Valley 
                 30 
               
               
                 Products of Exton, Pa, USA) 
               
               
                 BYK 306 (silicone surface active agent sold by BYK-Cera, 
                 1 
               
               
                 AM Deventer, Holland) 
               
               
                 Rose Bengal 
                 1.8 
               
               
                 Triisopropanolamine 
                 1.9 
               
               
                   
               
             
          
         
       
     
     The above-described formulation was mixed and then heated to 50° C. and maintained at that temperature until a clear solution was obtained. The mixture was coated onto a grained, anodized aluminum plate with a wire-wound rod to produce a minimal continuous layer. The layer was selectively exposed to a UV source of 300 nm to 400 nm to an extent of 200 micro-joules/cm 2 . The unexposed material was washed away under running water drawn from a tap. The resulting image was inked up with offset lithographic ink and printed in an offset lithographic process known to the art. 
     Having described the present invention with regard to certain specific embodiments thereof, it is to be understood that the description is not meant as a limitation, since further modifications may now suggest themselves to those skilled in the art, and it is intended to cover such modifications as fall within the scope of the appended claims.