Patent Publication Number: US-6906019-B2

Title: Pre-treatment liquid for use in preparation of an offset printing plate using direct inkjet CTP

Description:
FIELD OF THE INVENTION 
     The present invention relates to a plate making method, and more particularly, to a surface treatment of the plate surface, prior to the deposition of ink on the media. 
     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 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 Thomson 1998, published by Pira. 
     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 method which avoids these problems is found in computer-to-plate (CTP) systems where the offset litho plates are directly imaged with a light source that is modulated to correspond to the digital information from the computer. Thus the film intermediate is completely eliminated. In general, such plates still need processing by solution although attempts are being made to develop computer-to-plate systems that are processless. The subject of computer-to-plate can be found in the above quoted book in Chapter 21. 
     U.S. Pat. No. 5,339,737 to Lewis et al describes the processless preparation of offset litho 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 to 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 multiple layered plates that are expensive to produce. Also, it is difficult to maintain a consistent standard of quality from plate to plate. Moreover, they utilize laser imaging systems which are in themselves costly. 
     Therefore, it would be desirable to have a simplified, quick plate making process with elimination of all chemical processing and a minimal cost for the equipment for plate production and for the plate production itself. 
     A technology that has been developed in recent years may be seen to offer solutions to easy and inexpensive plate production. This is the technology of inkjet printing. 
     Inkjet is a non-impact printing process whereby ink is sprayed through very fine nozzles and the resultant ink droplets form an image directly on a substrate. There are two main types of ink jet processes. In one process, usually termed continuous ink-jet printing, a stream of ink drops are electrically charged and then are deflected by an electric field either directly or indirectly onto the substrate. The viscosity of inks used in such systems is typically 2 or 3 centipoise. In the second process, usually called Drop on Demand (DOD) inkjet printing, the ink supply is regulated by an actuator such as a piezoelectric actuator. The pressure produced during the actuation forces a droplet through a nozzle onto the substrate. Inks for DOD ink-jet printing do not need to be conductive and their viscosity is typically between 2 and 40 centipoise. 
     Several inventors have tried to apply the principles of ink-jet printing to offset plate making. U.S. Pat. No. 4,003,312 was one of the first patents to recognize the advantages of using inkjet printing technology in a process for preparing a waterless lithographic printing plate. This patent discloses the use of an inkjet printing apparatus to deposit a background coating of silicone being curable. 
     U.S. Pat. No. 4,718,340 discloses a method for preparing a reusable planographic plate for lithography printing wherein a hydrophilic substrate is provided with a thin hydrophobic layer which is selectively removed. This process involves a multi-step plate preparation using hydrophobic organic acids and derivatives thereof. The treated surface is then selectively imaged with a spark discharge, or laser ablation technique. This has limited run length capability, similar to other spark discharge and laser ablation techniques. 
     U.S. Pat. No. 5,312,654 discloses a method for making a printing plate wherein an ink absorbing layer is selectively imaged with photopolymerizable composition using an inkjet printer. The ink absorbing layer prevents the ink from spreading and is removed after the ink is cured by exposure to actinic light, thereby exposing a hydrophilic surface where photopolymer has not been deposited. This process is impractical because the water-soluble or alkali-soluble coating used in the ink-absorbing layer has serious disadvantages since the ink imaged photopolymer sits on top of this layer. On typical offset press, the use of an aqueous fountain solution would be disastrous for this plate. Additionally, the ink absorbing properties of this film limit control of dot or image formation and the resolution of fine details is still problematic. 
     U.S. Pat. No. 5,750,314 to Fromson et al. describes selective positive imaging on a substrate coated with a first continuous layer of a material, and further coated by inkjet with a second material, which is adhesive to the first layer. The difference in solubility of each material in a specific solvent enables developing and removing the non-imaged layer of the first material. In this method the substrate is made hydrophilic. The first material is preferably a negative working photopolymer. The disadvantage of this method is that after masking the imaged area, the plate needs to be exposed and developed. 
     Japanese Patents JP10,157,053, JP 10,076,624 and JP 10,024,549 describe a method which involves injecting oily ink through a nozzle and forming an image on an image forming layer of an original plate. Resin is impregnated in the non-image formation area of the image-forming layer. The non-image formation area is desensitized by subjecting the resin to chemical reaction, to form a flat printing plate. 
     JP 10,076,625 involves performing thermofusion of solid ink composition at normal temperature. Ink is injected from a nozzle on to an intermediate transfer object and an image is formed. Then, contact transfer of the image from the intermediate transfer object to the image acceptance layer of an original plate is carried out. The area where the image is not formed in the image acceptance layer is processed by chemical reaction and it is formed on the waterproof support body that has resin. 
     U.S. Pat. No. 4,833,486 to Zerillo (assigned to Dataproducts) utilizes a hydrophobic solid inkjet ink (containing waxes) which is held at a sufficiently high temperature to jet it through a DOD head. (This solid ink technology is more fully described in U.S. Pat. Nos. 4,390,369, 4,484,948, and 4,593,292.) The substrate is a hydrophilic offset plate—either paper or aluminum onto which the image is jetted. When the ink hits the plate it immediately cools and solidifies. One problem of such an approach is the difficulty of obtaining sufficiently good adhesion of the waxes of the ink to the plate to run multiple impressions during lithographic printing. 
     European Patent EP503621 (Applicant NIPPON PAINT CO) describes two approaches. One approach describes jetting inks onto a pre-sensitized plate, which then needs further treatment, including a developing stage with a liquid developer. The other approach uses a non-presensitized plate and the inkjet ink is photosensitive so that it can be hardened on the plate. 
     EP533168 to Nippon describes the use of a photopolymeric based inkjet ink together with an ink absorbing layer on the litho plate surface. 
     EP697282 to Leanders (Agfa) describes a two component system whereby one reactive component is in the ink and the other in the litho plate surface, so that when the ink is deposited on the plate it produces an oleophilic reduced silver image that can be used in the offset printing process. 
     U.S. Pat. No. 5,495,803 to Gerber describes imaging a coated, presensitised plate with a UV opaque hot-melt inkjet ink and using the ink as a photomask to expose the plate. The unexposed presensitised polymer and the ink are subsequently removed by washing. 
     U.S. Pat. No. 5,738,013 to Kellet describes an ink-jet plate-making process involving the use of a reactive inkjet ink which is bonded to the litho plate by a chemical reaction activated by radiant energy. This assumes that such inks have very good stability at room temperature so that no jet blocking will occur, yet have good reactivity at high temperatures so that the ink becomes insoluble with good adhesion to the offset plate and with good oleophilic properties. 
     Another option is coating the substrate with solution containing cationic surfactants, as described in U.S. patent application No. 60/174,713, assigned to the owners of the present invention. According to this method the surface of a substrate, bare anodized aluminum with no pre-coating (as polymeric binder that should be washed away), is coated with a very thin layer (almost mono-molecular) of cationic surfactants. The coating is water repellent and insoluble in the CTP liquid. The plate is then imaged using an inkjet printing head providing an excellent image quality and a strong stable oleophilic image from which to print high numbers of good quality impressions. 
     However, these prior arts attempts to use the inkjet process for imaging plates, remain with difficulties in producing satisfactory quality, run length and plate-making speed, because of problems of spreading and clustering. 
     In order to produce high-resolution plates at high speed it is necessary to position large number of droplets in rapid succession very close together. Creating an image on a highly hydrophilic, water receptive surface of an anodized aluminum offset plate with water-based liquid by means of an ink jet process, is very problematic. The high surface energy of the anodized grained aluminum causes an intensified spreading of the liquid drops. Therefore, it is almost impossible to create a sharp image on the plate. Both water-based and solvent-based inks have problems of spreading of the liquid on the high surface energy hydrophilic plate surface due to the properties needed to jet the ink. 
     In addition, water-based inks, due to the hydrophobic nature of the plate&#39;s coating, tend to create a clustering phenomenon at the dark tone areas of the image (shadows). Prior art methods, in order to contend with the problem of clustering, have required multiple passes of the inkjet head with a drying step between the passes. This makes the plate preparation time quite long. 
     One option of controlling the spreading of drops is by controlling the viscosity of the CTP liquid, as described in Israel patent application No. 132789 and the parallel PCT application PCT/IL 00/00722. This application describes the use of polymer emulsion in water to produce good quality long run lithographic printing plates. Yet the resolution received is plate dependent, i.e. with certain substrates this is more successful than with others. 
     U.S. Pat. No. 4,381,85 discloses a simple process to obtain a water-fast print on paper using a colorant solution containing water-soluble polymeric dyes. This is accomplished by using a paper employing substantial cation content, especially a substantial polyvalent metal ion content for example, Fe 2+ , Fe 3+ , Cu 2+ , Zn 2+ , Al 3+ , Mg 2+ , Ca 2+ and Ba 2+ , and applying a solution containing one or more polymeric colorants possessing anionic net charge, for example, anionic groups. The reaction between the cations and anions immobilize the ink drops on the surface where they are applied so that spreading does not occur. 
     It would be desirable to provide an offset printing plate for direct inkjet CTP which would not have the problems of spreading and clustering and which would provide an easy, economical method for plate preparation. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a broad object of the present invention to overcome the problems of the prior art and provide a method for selective positive imaging of a suitable coated anodized aluminum offset plate using a water based CTP liquid (as described in Israel patent application No. 132789 (and the parallel PCT application PCT/IL 00/00722), and U.S. application No. 60/174713 by means of inkjet process. 
     The inventors have found that the image quality on the plate can be further improved, and the speed of plate preparation can be elevated by treating the anodized aluminum plate with a pre-treatment liquid that interacts immediately, both chemically and physically with the CTP liquid to form a stable image with no clustering phenomenon. According to the present invention the surface of a substrate such as, bare anodized aluminum with no pre-coating, is treated with a pre-treatment solution prior to the deposition of the ink on the surface. 
     The inventive pre-treatment liquid is an aqueous and/or alcoholic solution or an oil in water emulsion (where the oil is a non-miscible swelling reagent) containing a polyvalent metal salt, and at least one of an organic swelling reagent and/or a coalescence reagent. The pre-treatment liquid is applied to form a thin, homogenous layer of approximately 4 μm to the entire upper surface of the recording plate. The swelling reagent and/or the coalescence reagent and the polyvalent metal cations, in the pre-treatment liquid, are physically well localized in the porous structure of the plate&#39;s surface. After partial drying of the pre-treated anodized aluminum plate, CTP liquid is deposited onto the surface to form an image. The CTP liquid solids react with the pre-treatment liquid and are, therefore, chemically bound to the surface. This allows all data to be deposited in a single pass of the inkjet head without the problem of clustering. The solids in the CTP liquid precipitate in response to the localized cations deposited in the pre-treatment, and form interactions above and in the pores of the surface of the plate to give a mechanically stable ink dot. After post-print drying at temperatures high enough to evaporate the swelling reagent and/or the coalescence reagent, the ink dot remains mechanically bound to the surface of the media. 
     Other features and advantages of the invention will become apparent from the following drawings and description. 
    
    
     
       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, in which like numerals designate corresponding elements or sections throughout, and in which: 
         FIGS. 1A-C  show an enlarged sectional view of a negative working printing plate as known in the prior art; 
         FIGS. 2A-C  show a prior art laser ablation process used in imaging infrared sensitive computer-to-plate litho plates; 
         FIGS. 3A-B  show the drop-wise deposition of the inkjet fluid on an uncoated plate according to a prior art method; 
         FIGS. 3C-E  show the drop-wise deposition of the ink jet fluid on a plate coated with solution containing cationic surfactants according to a prior art method; and 
         FIGS. 4A-D  illustrate the anodized aluminum plate surface treatment, of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Referring now to  FIGS. 1A-C , there is shown an example of the widely used prior art process of platemaking with pre-sensitized plates. As shown in  FIG. 1A , film  20  contains in negative form the image to be printed. Film  20  is used to image pre-sensitized printing plate  22 . Plate  22  consists of a grained anodized aluminum substrate  24  which has been coated with coating  26  which contains a photosensitive pre-polymer with a carrier resin. Film  20  is placed in emulsion-to-emulsion contact with the pre-sensitized plate  22  and flood-irradiated with ultraviolet light (UV)  28  as shown in FIG.  1 B. Transparent areas  30  of negative film  20  represent the image areas to be printed and permit the penetration of UV light  28  causing photopolymer coating  26  to form hard, insoluble oleophilic area  32 . Black areas  34  of film  20  corresponding to the background areas of the print, prevent UV light  28  from penetrating and photopolymer coating  26  remains in the prepolymer state. Negative film  20  is then removed and the plate is processed—usually with a high pH aqueous solution in which the unpolymerized portions of coating  26  are readily soluble. This exposes the grained anodized surface of aluminum substrate  24  and provides the hydrophilic background areas for the printing plate, as shown in FIG.  1 C. 
       FIGS. 2A-C  show a simplified infrared ablatable computer-to-plate process as known in the prior art. In  FIG. 2A , plate substrate  36 , which may be, by way of example, aluminum, is coated with an infrared (IR) absorbing coating  38 . Another possible plate substrate is polyester. Layered on top of coating  38  is hydrophilic coating  40 . Plate substrate  36  is imaged by digitally modulated IR radiation  39  that is absorbed by layer  38  as shown in FIG.  2 B. The energy absorbed causes an extremely fast rise in temperature, resulting in ablation of IR absorbing coating  38 , which causes hydrophilic coating  40  to also be removed.  FIG. 2   c  shows the resulting plate substrate  36  with coating  40  providing the hydrophilic background areas of the plate and the exposed parts of the surface of plate substrate  36  providing the oleophilic image parts of the plate. 
       FIG. 3A  shows the prior art method of ink jet system  42  jetting fluid onto the surface of uncoated plate substrate  44 . This plate substrate may be any type of substrate known to the art from which offset litho plates are fabricated. It must have a hydrophilic surface with no pre-coating on it. The preferred substrate is aluminum-based with grained, anodized surface  46 . Although any type of ink jet system is useful in this invention, the figure shows a generic impulse (drop-on-demand) system  42  as this is the preferred system. The inkjet fluid is deposited in a pattern that is digitally determined to provide the information that will be contained in the plate directly from a computer. 
     Due to the high surface energy of anodized grained aluminum surface  46 , spreading of the water-based liquid drops  48  occurs. Spreading can be restrained by the viscosity of the CTP liquid, as described in Israeli Patent Application No. 132789 (and the parallel PCT application PCT/IL 00/00,722). This application describes a method whereby the surface area of the drop can be limited by a change in the liquid&#39;s viscosity, yet the viscosity change mechanism for drop-restraining is plate-dependent, i.e. the same ink will give different results on different substrates or substrate finishes. 
     After fluid deposition on plate substrate  44 , plate  44  is heated to evaporate the water in the fluid and to fuse the resins to the substrate&#39;s grained anodized surface  46  to create a hydrophobic ink receptive image as shown in FIG.  3 B. 
     Quality can be further improved by coating substrate  44  with a solution containing cationic surfactants as described in U.S. patent application No. 60/174713.  FIG. 3C  shows an inkjet system  42  depositing CTP liquid on a plate substrate  44  with grained anodized surface  46  which is coated with a very thin layer (almost monomolecular) of cationic surfactants  50 . Due to the low surface energy of coated surface  50  of anodized grained aluminum surface  46 , it is water repellent. Therefore, the spreading of water-based CTP liquid drops  48  is limited, meaning that the contact angle of the CTP liquid drop  48  with the interface of grained anodized aluminum  46  becomes high, as a result the spot size becomes very small and the image quality is further improved. The coating mechanism of controlling the drop spreading is not plate-dependent, so that the plate effect is cancelled. After fluid deposition, on the plate substrate  44 , plate substrate  44  is heated to evaporate the water in the fluid and to fuse the resins to the substrate&#39;s coated grained anodized interface  46  to create an excellent image quality and a strong stable oleophilic image as shown in FIG.  3 D. 
     After drops  48  are fused, arabic gum solution is applied to plate  44 , as known in the art. Plate  44  is then placed on an offset printing machine and surfactants  50  are washed by the fountain solution prior to the printing procedure, so as to expose the grained anodized water-receptive surface  46  without causing damage to the image (FIG.  3 E). 
     The present invention is described in  FIGS. 4A-D .  FIG. 4A  shows an ink jet system  42  which is jetting fluid drops  48  onto the surface of standard anodized grained aluminum plate  44  having a high surface energy, which has been pretreated with pre-treatment liquid  52  of the present invention. Pre-treatment liquid deposition onto offset plate surface  46  may be carried out by applying a thin layer (not more than 4 μm, wet). CTP pretreatment liquid  52  comprises a polyvalent metallic salt or an inorganic acid, and a water-soluble polymer swelling reagent and/or a coalescence agent. 
     The polyvalent metal salt in pre-treatment liquid  52  is comprised of divalent or higher polyvalent metallic ions and anions bonded to the polyvalent metallic ions and is soluble in water, alcohol or a mixture of both. Specific examples of polyvalent metallic ions include divalent metallic ions, such as a Ca +2 , Zn +2 , Ba +2 , Mg +2 , and trivalent metallic ions such as Al +3 , Fe +3  and Cr +3 . Anions include Cl − , I − , Br − , NO 3   − , HCOO − , CH 3 CH 2 COO − and CH 3 COO − . A metal salt comprised of the metallic ions Zn +2  and Ca +2 , provides especially favorable results in terms of image mechanical stability. The concentration of the polyvalent metal salt in pre-treatment liquid  52  may be suitably determined so as to attain the effect of providing a good print and a high speed of plate production, e.g. prevention of clustering phenomenon at the shadows. It is, however, preferably approximately 0.1% to 30% by weight, more preferably approximately 2% to 25% by weight. 
     An inorganic acid such as phosphoric, sulfuric, nitric or hydrochloric acid can also be used instead of the polyvalent metal salt as the cation donor. The pH of this acidic pre-treatment solution can vary between 1-4, preferable pH is between 1-2. 
     According to the present invention, the polyvalent metal salt anions are preferably a chloride (Cl − ) or an acetate (CH 3 COO − ), for optimal thermal stability. 
     According to a first embodiment of the present invention, pre-treatment liquid  52  comprises a swelling and/or coalescence reagent. The presence of a swelling/coalescence reagent, which diffuses within the CTP liquid, enables the creation of stable dot shape  54  (as seen in  FIG. 4   b ) with good film properties with very good adhesion to anodized aluminum surface  46 . These good film properties promote the mechanical stability of dot  54  and consequently of the entire image. Preferred examples of the swelling reagent are N-methyl pyrrolidone, esters such as ethyl acetate, propyl acetate, butyl acetate, ethyl lactate, butyl lactate etc, ketones such as acetone and methyl-ethyl-ketone (MEK) and cyclic ethers such as THF. Butyl glycol, Butyl carbitol, di(propylene glycol) methyl ether (DPM), tripropylene glycol mono methyl ether (TPM), propylene glycol mono methyl ether, propylene glycol mono propyl ether, dipropylene glycol dimethyl ether. The concentration of the coalescence reagent in the pre-treatment liquid may be suitably determined so as to attain the effect of providing a good print quality with a highly mechanical stable image and no negative effect on the recording media. It is preferably approximately 0.1% to 15% by weight, more preferably approximately 0.5% to 12% by weight. Swelling reagents and coalescence reagents may be used together. After the pre-treatment liquid is applied, the surface is allowed to partially dry. 
       FIG. 4B  demonstrates the formation of ink dot  54  by a phase separation mechanism. Dot  54  is composed of the resin and colorant found in the CTP liquid and is attached by high adhesion forces onto the porous surface  46 , of the grained anodized aluminum plate,  44 . The presence of the coalescence reagent and polyvalent metal salt of the pre-treatment liquid  52  are attached (in and on top) to the porous surface  46 , causing a fast phase separation of the resin and colorant from the CTP liquid  52 , and creating stable dot shape  54  with good film properties. 
       FIG. 4C  illustrates printed ink dot  54  on dried surface  46 . After dot formation the plate is dried at high temperatures to evaporate all liquids, including CTP liquid  52  swelling reagent and/or coalescence reagent leaving a thin layer of metal salt ions  56 , on the surface. Dot  54  is trapped in porous surface  46 , of grained anodized aluminum plate  44 , to provide ink dot  54  with strong adhesion and strong mechanical stability. 
     After dot  54  is fused, arabic gum solution is applied to plate  44  as known in the art. Plate  44  is then placed on an offset printing machine, prior to the printing procedure, to expose the grained anodized water-receptive surface  46  with no damage to the image, as illustrated in FIG.  4 D. 
     EXAMPLES 
     All the examples were made under constant conditions of commercially available uncoated, post-anodized, brushed and electrochemically grained aluminum plates and the CTP liquid as described in Israel patent application no. 132789 with a viscosity of 7.8 centipoise using the inkjet print head described in EP640481. Component concentrations are expressed by [% w/w]. The dot sizes measured on the plate are presented in Table 1 (see Isr. pat. appln. per above). 
     Example 1 
     An offset plate was coated using a rubbing motion with a coating solution consisting of 5% butyl carbitol, 3% N-methyl pyrrolidone, 92% deionized water and phosphoric acid, which was added to provide a pH between 1 to 2, and dried at 50° C. for 30 sec. The plate was then placed on an XY bed where it was imaged, in 600 dpi resolution, (using the inkjet print head described in EP640481 filled with CTP liquid described in Israeli patent application no. 132789) to produce a very sharp 600 dpi quality image by single pass of the head, with no clustering phenomenon. The plate was then coated with acidified gum arabic. 
     Example 2 
     An offset plate was coated using a rubbing motion with a coating solution consisting of 13% Zinc acetate, 3% Calcium chloride, 5% Propyl acetate, 5% Butyl carbitol, 40% ethanol, 34% deionized water, and dried at 50° C. for 30 sec. The plate was then placed on an XY bed where it was imaged, in 600 dpi resolution, (using the inkjet print head described in EP640481 filled with CTP liquid described in Israeli patent application no. 132789) to produce a very sharp 600 dpi quality image by single pass of the head, with no clustering phenomenon. The plate was then coated with acidified gum arabic. 
     Example 3 
     An offset plate was coated using a rubbing motion with a coating solution consisting of 13% Zinc acetate, 3% Calcium chloride, 5% Butyl lactate, 5% Butyl carbitol, 32% Ethanol and 42% of deionized water, and dried at 50° C. for 30 sec. The plate was then placed on an XY bed where it was imaged, Dpi 600 dpi resolution, (using the inkjet print head described in EP640481 filled with CTP liquid described in Israeli patent application no. 132789) to produce a very sharp 600 dpi quality image by single pass of the head, with no clustering phenomenon. The plate was then coated with acidified gum arabic. 
     Example 4 
     An offset plate was coated using a rubbing motion with a coating solution consisting of 13% Zinc acetate, 3% Calcium chloride of 5% Ethyl acetate, 5% Butyl carbitol, 12.5% Ethanol and 61.5% of deionized water, and dried at 50° C. for 30 sec. The plate was then placed on an XY bed where it was imaged, in 600 dpi resolution, (using the inkjet print head described in EP640481 filled with CTP liquid described in Israeli patent application no. 132789) to produce a very sharp 600 dpi quality image by single pass of the head, with no clustering phenomenon. The plate was then coated with acidified gum arabic. 
     Example 5 
     An offset plate was coated using a rubbing motion with a coating solution consisting of 13% Zinc acetate, 3% Calcium chloride 5% Butyl acetate, 5% Butyl glycol and 32% Ethanol and 42% deionized water, and dried at 50° C. for 30 sec. The plate was then placed on an XY bed where it was imaged, in 600 dpi resolution, (using the inkjet print head described in EP640481 filled with CTP liquid described in Israeli patent application no. 132789) to produce a very sharp 600 dpi quality image by single pass of the head, with no clustering phenomenon. The plate was then coated with acidified gum arabic. 
     Example 6 
     An offset plate was coated using a rubbing motion with a coating solution consisting of 13% Zinc acetate, 3% Calcium chloride 5% Propyl acetate, 5% Di(propylene glycol) methyl ether, 40% Ethanol and 34% deionized water, and dried at 50° C. for 30 sec. The plate was then placed on an XY bed where it was imaged, in 600 dpi resolution, (using the inkjet print head described in EP640481 filled with CTP liquid described in Israeli patent application no. 132789) to produce a very sharp 600 dpi quality image by single pass of the head, with no clustering phenomenon. The plate was then coated with acidified gum arabic. 
     Example 7 
     An offset plate was coated using a rubbing motion with a coating solution consisting of 13% Zinc acetate, 3% Calcium chloride 5% Butyl acetate, 5% Di(propylene glycol) methyl ether, 32% Ethanol and 42% deionized water, and dried at 50° C. for 30 sec. The plate was then placed on an XY bed where it was imaged, in 600 dpi resolution, (using the inkjet print head described in EP640481 filled with CTP liquid described in Israeli patent application no. 132789) to produce a very sharp 600 dpi quality image by single pass of the head, with no clustering phenomenon. The plate was then coated with acidified gum arabic. 
     Example 8 
     An offset plate was coated using a rubbing motion with a coating solution consisting of 13% Zinc acetate, 3% Calcium chloride 5% Butyl lactate, 5% Di(propylene glycol) methyl ether, 12% Ethanol and 62% of deionized water, and dried under 50° C. for 30 sec. The plate was then placed on an XY bed where it was imaged, in 600 dpi resolution, (using the inkjet print head described in EP640481 filled with CTP liquid described in Israeli patent application no. 132789) to produce a very sharp 600 dpi quality image by single pass of the head, with no clustering phenomenon. The plate was then coated with acidified gum arabic. 
     Having described the 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.