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 is 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 coating on 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.
Offset lithographic plates can be produced using inkjet printing. Inkjet is a non-impact printing process whereby ink is squirted through very fine nozzles and the resultant ink droplets form an image directly on a substrate. There are two main types of inkjet processes. In one process, usually termed continuous inkjet 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 centipoises. 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 inkjet printing do not need to be conductive and their viscosity is typically between 2 and 40 centipoises.
The application of inkjet to produce printing plates is an idea that can be traced back to the origins of the inkjet process. In U.S. Pat. No. 2,512,743 assigned to the Radio Corporation of America, Clarence W. Hansell, the inventor of the DOD method described in this patent, wrote: “The invention may also be used for spraying acids and chemicals for etching of printing plates . . .”. Although the general concept was disclosed here, no further details were given. U.S. Pat. No. 4,003,312 by Gunther details methods of inkjet offset lithographic platemaking, but the plate substrate is confined to silicone surfaces for driographic (waterless) printing which had been at that time recently invented (see U.S. Pat. No. 3,511,178—Curtin). UK Patent 1431462 describes the use of a continuous jet process to image a coated plate by hardening the coating with reactive ink. The unhardened background areas are then washed away. JP56105960-Nakayama et al. assigned to Fuji Photo Film (1981) includes the use of heat activated hardening material in oleophilic inkjet inks, forming the image areas on hydrophilic substrates. The substrate may or may not be coated.
Although the idea of plate making by inkjet has a long history, commercial success in the field has been limited by the lack of maturity of the inkjet process itself. Development of plate making followed the developments in inkjet. U.S. Pat. No. 4,833,486 by 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 in obtaining sufficiently good adhesion of the waxes of the ink to the plate to run multiple impressions during lithographic printing.
EP503621 (Applicant NIPPON PAINT CO) discloses 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-pre-sensitized plate and the inkjet ink is photosensitive so that it can be hardened on the plate.
EP533168 by Nippon describes the use of photopolymeric-based inkjet ink together with an ink absorbing layer on the litho plate surface.
EP697282 by 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 hits 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 by Gerber describes imaging a coated, pre-sensitized plate with a UV opaque hot melt inkjet ink and using the ink as a photo-mask to expose the plate. The unexposed pre-sensitized polymer and the ink are subsequently removed by washing.
U.S. Pat. No. 5,738,013 by Kellet describes an ink-jet plate-making process involving the use of 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.
Kato et al. utilize oil-based inks of a variety of types in plate formation (U.S. Pat. Nos. 6,106,984, 6,174,936, 6,184,267 and 6,197,847).
U.S. Pat. No. 5,820,932—Hallman et al (Sun Chemicals), describes a variety of reactive inks and processes.
Methods of inkjet plate making such as that described in U.S. Pat. No. 6,315,916, whereby a pre-sensitized plate has an inkjet image deposited onto it and whereby this image reacts with the sensitized layer, require a subsequent process of development in a processor. Development is a well-known method used with standard pre-sensitized plates, but it is a method which users would like to avoid. It uses highly alkaline liquids, which may have problems of drainage disposal in many countries. The processing liquid is subject to reaction with the air and must be changed every few weeks; also, the processing liquid gradually becomes contaminated with the material it is removing. This often forms sludge and the processor needs to be thoroughly cleaned out periodically. It is no wonder that the industry is seeking out processless plates. Moreover, pre-sensitized plates used in this inkjet method are light sensitive and have to be handled in subdued or yellow light.
Newington et als. (WO 00/37254) and Nitzan et als (WO 01/49506) both use aqueous emulsions as the inkjet fluids. These emulsions have oleophilic particles dispersed in aqueous media and are suitable for ink jetting. On deposition onto the hydrophilic substrate, which is anodized aluminum, the emulsion particles coalesce, forming an insoluble oleophilic image. Such an image can be hardened by heating to increase the adhesion of the image to the plate surface, thus giving a larger number of acceptable printing impressions.
Methods involving the use of uncoated anodized aluminum suffer from the problem of poor surface stability. The surface of uncoated anodized aluminum is oxidized with time and loses its hydrophilic properties. This is well known in the art and in the case of pre-sensitized plates that have been imaged and developed to expose uncoated anodized aluminum in background areas it is necessary to preserve the plate with a layer of gum or gum substitute.
WO Application No PCT/IL03/01032 (incorporated for reference herein) describes the use of a hydrophilic coating on aluminum which does not suffer from the stability problems of anodized aluminum and which can be imaged with an aqueous pigmented inkjet ink containing polymer binder. In order to increase the run length of the plate on the printing press it is heated after imaging to obtain good anchorage of the inkjet ink to the polymer coating and good resistance of the coating to wear and fount attack in unimaged areas. The coating is formulated to ensure that under the conditions of heating it retains its hydrophilic nature. It is possible to over-heat the coating, in which case it becomes oleophilic and unsuitable for use as a printing plate, because the background as well as the image would then take ink.
The dilemma of ink jetting an aqueous based inkjet ink onto a hydrophilic plate surface for use as a printing plate is that the surface needs to be highly hydrophilic in order to take the printing fount solution that keeps the print background clean by repelling the ink from non image areas. As a consequence of this, aqueous inkjet inks tend to spread when they impact surfaces of high surface energy (which is another way of describing hydrophilic surfaces). This hinders the achievement of high resolution for which the ink jet droplets need to be small and spread needs to be minimized.
Although in general the prior art does not deal with this problem, recently the effect has been recognized by inventors who have attempted to address the issue. Nitzan (WO 01/49506) claims better control of dot size by providing for instance a thin cationic surfactant coating on the surface of the anodized aluminum, which received the aqueous inkjet ink. According to this patent the dot size may be reduced from 100 microns diameter to a minimum of 45 to 50 microns. Such coating need not be removed before printing and the plate may be heated up to 200° C. to increase run length. According to this application, the cationic coating does not interfere with performance. The cationic layer as described by Nitzan for improving resolution will not prevent the surface oxidation effect experienced when uncoated aluminum is stored for any length of time.
Aurenty et als in WO 00/76779 are particularly concerned with the problem of increasing the resolution for images produced on offset lithographic plates using inkjet inks. They use surfactants absorbed on the surface of the plates and then desorbed after imaging, either with gum or fount. The application WO 00/46036 is similar.
Aurenty et als and Nitzan are primarily concerned with jetting onto treated aluminum. This permits the surfactant coatings used to promote enhanced resolution to be removed by one means or another after imaging and even after fusing the image at high temperatures. As has been stated above, such aluminum plates tend to show low shelf life with the appearance of scumming due to oxidation.
Attempts to apply methods of enhanced resolution to the coated aluminum inkjet plates described in WO Application No. PCT/IL03/01032 have been unsuccessful. It has been found that after imaging and subsequent fusing, the layers of Nitzan and Aurenty become embedded into the plate coating and cannot be easily removed. Consequently, the background areas, which should receive fount during printing, giving an oleophobic surface so that a clean background is obtained on the printing impressions, continue to be oleophilic and produce bad background scumming.