US 3161517 A
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Dec. 15, 1964 'r. DOGGETT 3,161,517
PRESENSITIZED LITHOPLATE WITH comma METAL BASE AND METHOD OF PREPARING SAME Filed May 4, 1961 l) PHOTOSENSITIVE DIAZO WASH 2) PIGMENTED HYDROPHILIC TOP COAT COAT DERIVED FROM WATER gg'fifigK 3) RESINOUS BINDER HEAT-FUSEDTO wxxgix iiififiyiii TOP COAT 5) ADHESIVE LAYER 4) METAL FOIL 6) REINFORCING PAPER WEB lNVE/WUR:
TOWERS DOGGETT ATTORNEYS United States Patent ice 3,161,517 PRESENSITIZED LITHQPLATE WTTH COATED METAL BASE AND METHGD 0F PREPAR- ING SAME Towers Doggett, West-brook, Maine, assignor to S. 1). Warren Company, Boston, Mass, a corporation of Massachusetts Filed May 4, 1961, Ser. No. 167,657 5 tClairns. (Cl. 95- 75) This invention relates to planographic printing and more particularly to the production of dimensionally stable coated plates or masters for use in planographic printing.
In planographic printing the design or image to be reproduced or printed is substantially level with the surface of the printing plate and is water-repellent and grease-receptive in nature. Aqueous lithographic solution is applied to the imaged plate and wets the unimaged or blank areas, making them repellent to greasy printingink, but it does not wet the water-repellent image. An inking-roller now passed over the surface of the wetted plate leaves no ink on the water-wet background areas; but it does leave ink on the imaged areas, which can thereafter print the image in reverse on any ink-receptive surface brought into contact therewith. In practice the image is usually first printed from the plate upon an offset blanket from which it is transferred to paper so that the final print is identical with the design on the plate.
If the surface of a plate is sensitized with photosensitive material, an image may be formed on the plate by exposure of the plate to actinic radiation through a transparency or stencil or by means of lenses as in photography. The preferred modern practice is to purchase plates which have already been sensitized with a photosensitive diazo compound. .Such presensitized plates when removed from the package are ready to be exposed without any preliminary treatment. After the plate is exposed to actinic radiation through a transparency, the image areas are water-repellent and grease-receptive and the diazo material on the background areas is hydrophilic and/or so soluble that it can be readily washed off by water or aqueous liquid.
Prior art presensitized planographic printing plates, in spite of their unquestioned desirability from the standpoint of convenience, have had some undesirable qualities. lfresensitized coated paper plates have been produced which are excellent in almost all respects, except that they are not entirely dimensionally stable in the presence of moisture or when wet. In cases where absolutely exact register is essential metal plates give the best dimensional stability. The usual diazonium sensitizers will adhere to aluminum sheet or foil which has been treated with sodium silicate solution. A sensitized aluminum plate of this character is stable dimensionally and is capable of giving excellent printing results at the hands of a skillful pressinan. Such a plate, however, has a flat and non-absorptive surface. Consequently the film of lithographic solution which can be carried thereon is necessarily very thin. Expert adjustment of the lithographic press is required to attain the critical water-balance necessary to maintain the water film on the surface of the plate at the proper thickness. If too little aqueous solution is supplied, even momentarily, the background areas become soiled with ink and produce toning in the background of prints made therefrom. On
the other hand, as is well known, presence ofexcess aqueous solution tends to cause emulsification' of the printing-inkwith consequent decrease both in distinctness of the print and in the thickness of the ink film deposited. V
3,161,517 Patented Dec..15, 1964 The present invention results in a presensitized lithographic printing plate which is dimensionally stable and which possesses considerable latitude in regard to waterbalance. This is accomplished by providing a metal base with a water-absorptive surface of appreciable thickness and which comprises hydrophilic colloid material and pigment. The usual hydrophilic colloids useful in planographic printing plates such as the water-soluble cellulose ethers, polyvinyl alcohol, starch adhesives, and the like do not adhere well to metal. It is found, however, that a thin layer of a particular form of anchoring matter between the metal base and the hydrophilic or water absorptive surface layer will cause the latter to adhere well to the base. Wash coats of certain resinous thermoplastic polymers containing carboxyl groups in their molecule are very satisfactory for this purpose. Somewhat less satisfactory, but still usable is an aqueous dispersion or solution of such resinous polymers. Such a coating will adhere strongly to metal such as aluminum sheet or foil, and at the same time the commonly used Water-dispersible hydrophilic adhesives can be made to adhere well to said resinous coating by heat sealing. From 4 to 6 grams per square meter of dry weight of the resinous polymer applied from organic solvent, or somewhat more if applied from aqueous dispersion, is normally adequate to ensure complete coverage of the metal base and to afford a secure anchorage for the hydrophilic layer subsequently deposited thereon.
Over the dry anchoring layer of resinous polymeric substance there is applied a layer of aqueous coating composition comprising finely divided pigment and hydrophilic adhesive. Normally about 15 to 20 grams per square meter, dry weight, of such coating is a satisfactory quantity to use. The hydrophilic coating is dried and heat sealed to the anchoring layer by heat. After it has dried the hydrophilic coating should be substantially insoluble in water. In some cases, it is desirable to include in the coating composition an ingredient to insure insolubilization of the coating when it has once been dried.
The dry hydrophilic coating is finally washed over with a solution of diazo sensitizing compound to make the finished product photosensitive. Normally the diazo compound selected for use will be one which is not unduly sensitive to diffused light or light from ordinary incandescent lamps. It is of course desirable that it shall be possible to prepare and handle the plate under ordinary illumination without it being affected appreciably by such illumination. Care should be exercised during such preparation not to prolong such illumination unduly and to avoid exposure to direct sunlight or other intense actinic radiation.
As is well known, for negative-acting sensitizers in which the surface is illuminated by actinic radiation through a negative transparency, the diazo compound used is one which becomes oleophilic and water-insoluble in the illuminated or exposed areas but remains hydrophilic and/ or water-soluble in the unexposed areas so that it may be readily washed off the surface. On the other hand, for positive-acting coatings in which the surface is illuminated through a positive transparency, the diazo compound used is one which is originally water-insoluble and oleophiilic but which upon exposure to actinic radiation becomes hydrophilic and is washed off by water or aqueous alkaline solution. Thus in either case the image areas on the surface are left oleophilic and the background areas are left hydrophilic in character. Before printing from the plate so made it is frequently desirable to strengthen the image thereon by application of developing ink, oleic acid, or metallic soap according to known practice.
The drawing attached to and forming a part of this specification illustrates the photolithographic printing plate of this invention, wherein 1 indicates the photosensitive coating, 2 the hydrophilic coating, 3 the resinous adhesive binder coating, 4 the metal foil, 5 the adhesive layer for anchoring a reinforcing paper web, and 6 .the paper web.
Since it is the function of the metal surface to provide for the sensitized lithographic surface a base which is not subject to dimensional variation by reason of moisture or Water absorption, any metal sheet material is operably possible for use. From a practical viewpoint aluminum sheet or foil has many obvious points of superiority over other metals. Its ready availability in foil thin enough to be easily wound into coils or rolls makes it the first choice. Actual thickness or unit weight of the aluminum sheet material is of little consequence insofar as end results are concerned. If the sheet is a thin foil of too little strength by itself it may suitably be backed-up or strengthened by being laminated or adhered to a supporting sheet of kraft paper or the like. Thus is produced a sheet of adequate strength with a surface dimensionally unaffected by water. A typical sheet which has been found satisfactory consists of a sheet of hard aluminum foil 2 mils in thickness and having a weight of about 110 grams per square meter which is laminated or heat-sealed by means of an interposed sheet of half-mil polyethylene to a sheet of kraft paper having a weight of the order of 150 to 300 grams per square meter.
The anchoring coat applied to the metal surface serves the purpose of forming a bond between the metal and the subsequently applied hydrophilic coating layer and is particularly selected with this purpose and the problems posed thereby in mind. It is advantageous that the resinous polymer shall contain carboXyl radicals to ensure good adhesion to the metal surface. Hydrophilic radicals such as carboxyl, hydroxyl, or amino radicals in the polymer are useful in promoting good adhesion to the superposed hydrophilic coating layer, and preferably the polymer should be thermoplastic to facilitate heat sealing it to the hydrophilic coating. It is found that while some resinous polymers make good anchoring coats for various hydrophilic top coats, certain other resinous polymers are more satisfactory with specific hydrophilic top-coats than with others. In such cases for best results the resinous polymer to use will naturally be selected in relation to the particular top-coat expected to be used therewith. A resinous polymer that appears to form a satisfactory anchoring layer for many hydrophilic top-coats is a tripolymer of 86 parts of vinyl chloride, 13 parts of vinyl acetate, and one part of maleic acid.
The layer of hydrophilic coating applied over the anchoring layer may be merely a film of hydrophilic colloid material. It is found, however, that lithographic prints made from such a layer may be somewhat lacking in definition and/or cleanliness of background. Consequently it is preferred to have the hydrophilic coating also contain mineral filler or pigment, which normally may be present in amount from a quantity roughly equal to the weight of hydrophilic colloid to a quantity about four times the weight of said hydrophilic colloid. While any mineral filler or pigment may be used which is nonreactive with the other components of the coating composition or with lithographic solutions, it is well known that non-hydrous pigments such as barium sulfate or titanium dioxide are to be preferred for use in lithographic printing surfaces.
It is known that water-soluble cellulosic or the like compounds are suitable for use in lithographic printing surfaces. Such compounds include methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, carboxymethylhydroxyethyl cellulose, starch and the like. These are all usable as adhesive material to bind mineral pigment in the hydrophilic coating according to the invention,
especially if an agent is added to promote insolubility of the adhesive in the finished coating layer. Amino resinous condensates such as melamine-formaldehyde condensate are very suitable for use with the cellulosic compounds to promote insolubility thereof.
In general the hydrophilic coating should have several specific properties of particular importance in the context of photosensitized planographic masters or plates. First, the coating should present an essentially continuous film at its surface. If the film is not continuous, the sensitizing compound is absorbed by the hydrophilic coating and this in turn results in a reduction in the quality of lithographic printing from the plate. Second, the hydrophilic surface must be capable of receiving and securely anchoring the sensitizing compound. This feature is important for the purpose of preserving the strength of adhesion of the image to the sheet during printing. Third, the coating must be hydrophilic. This is necessary for lithographic printing. Fourth, it must be water insoluble in order to prevent the lithographic fountain solution from washing it off. And fifth, the coating should not be absolutely smooth. A slight graininess or matte finish is important not only for promoting adhesion of the sensitizer to the coating, but also to ensure a proper thickness of water layer covering the background areas during printing. Also the matte finish improves the intensity of the image. It will be appreciated that these features impose conflicting conditions on the problem. Thus it is difficult to obtain, at one and the same time, a film which is both continuous and grainy, and it is equally difficult to adhere hydrophilic materials to resinous polymers or metal.
Certain properties possessed by polyvinyl alcohol make that substance a preferred hydrophilic adhesive binder for the top hydrophilic coating. In general polyvinyl alcohol appears to have more inherent flexibility than the cellulosic adhesives; so coating layers in which it is used are less liable to develop cracks than are less flexible coating layers.
Grades of polyvinyl alcohol which are appreciably soluble in cold water are usable as adhesive in the hydrophilic top-coat provided steps are taken to render the coating water-insoluble. The addition of an amino-resin, such as a melamine-formaldehyde condensate, to a coating containing such grades of polyvinyl alcohol will render the finished coating layer adequately water-insoluble without significantly decreasing its hydrophilic properties. The preferred grades of polyvinyl alcohol to use, however, are those which are soluble in hot water but which are not readily soluble in cold water. One such desirable grade is Vinol 125. Very satisfactory results are given by use of a hydrophilic top-coat of about 15 grams dry weight per square meter applied in a uniform layer in the form of an aqueous composition of the following formula:
100 parts by weight of titanium dioxide pigment 33 parts by weight of Vinol 125, polyvinyl alcohol adhesive 7 parts by Weight of UP Concentrate 0.2 part by weight of tributyl phosphate defoamer 0.4 part by weight of Tergitol 4, Wetting agent Water to make solids content approximately 30%.
UP Concentrate 85 is an approximately 85% aqueous solution of a urea-formaldehyde condensate.
Tergitol 4 is the sodium sulfate derivative of 7- ethyl-Z-methyl-undecanol-4.
The hydrophilic coating made according to the preceding formula adheres well to a resinous copolymer of vinyl chloride, vinyl acetate, and maleic acid, such as Vinylite Resin VMCH. Such a resinous layer may be deposited from solution in methyl isobutyl ketone in amount equal to about 4 grams dry weight per square meter. The hydrophilic coating also adheres well to a layer of copolymer of vinyl acetate and crotonic acid, such as Gelva C3 or Lemac 541. Such a layer may be deposited from solution in alcohol or from aqueous ammoniacal solution in amount equivalent to about 6 grams dry weight per square meter. When ammoniacal solutions are used the film should be well heated to ensure insolubilization of the resin by removal or" all the ammonia which may have combined therewith.
The sheet so made, having a surface of aluminum foil bearing a layer of resinous anchoring coating and a top layer of hydrophilic coating containing hydrophilic adhesvie material and pigment can be photosensitized by application of a thin wash-coat of known diazo sensitizers in solution. For negative-working plates it is known that satisfactory sensitization can be obtained by treatment with an aqueous solution of the zinc chloride double salt of the equi-molar condensation product of diazotized para-amino-diphenylamine with formaldehyde, or with an aqueous solution of the sulfonate of that product. As little as 0.15 gram of the sensitizer per square meter of surface sensitized is adequate. More can be used if desired, of course. When the sensitized sheet [is exposed to actinic radiation through a negative transparency the exposed areas become water-insoluble and somewhat oleophilic, while the unreacted matter on the unexposed areas remains water-soluble and is washed ofl before the plate is used.
For positive-working plates a known sensitizer is 6- benzoyl 3 (1,2 naphthoquinone-Z-diazide-S-sulfonyl) pyrogallol. This sensitizer may be applied from solution in methyl isobutyl ketone in'amount equivalent to about 0.25 gram dry weight per square meter of surface sensitized. To harden the image area and make it more resistant to wear during the printing operation it is desirable to use with the sensitizer in the ketone solution a compatible resinous material, such as, for example, a cresol-formaldehyde condensate. If desired, the resin may be used in quantity 2 or 3 times as great as that of the sensitizer itself. When such a sensitized plate is exposed to actinic radiation through a positive transparency, the exposed areas become sufliciently hydrophilic so that the exposed sensitized layer thereon can be washed oil with aqueous alkaline solution such as a 5% solution of trisodium phosphate in water. The unexposed image area remains oleophilic and is not removed by the alkaline wash.
The sensitized web material so produced is cut into sheets and covered by a light proof and moisture-proof wrapper. In such condition it can be stored for considerable periods of time without deterioration. The user merely needs to remove a sheet from the package, expose it to actinic radiation, such as an arc-light, and then wash away the material on the unimaged areas. The plate 1 is then ready to be used in lithographic printing.
The finished product has the desirable properties of the known coated paper planographic printing plates in regard to ease'of handling and using and possesses the same high degree of 'foolproofness shown by the paper plates. 'At the same time the product is not affected dimensionally by the presence of water or by changes in humidity to which it may be subjected. In other words it has the stability of a metal plate but at the same time has the tolerance of the best coated paper plates in regard to wide variations in operating conditions.
Since minor variations in the preferred embodiments herein described Will be readily apparent to those skilled in the art, it is not my intention to confine the invention to the precise forms herein shown, but rather to limit it in terms of the appended claims.
Having thus described and disclosed preferred embodiments of my invention, what I claim as new and desire to secure by Letters Patent of the United States is:
1. A method of preparing a photolithographic printing plate comprising the steps of depositing a thin anchor coating of a solvent solution of a thermoplastic carboxylated polymer adhesive onto a supporting substrate of an impervious flexible metal film; drying said thin anchor coating to remove solvent therefrom and to secure adhesion of said adhesive to said metal film; depositing a continuous and uniform hydrophilic coating of a water dispersion of a water dispersible hydrophilic adhesive and mineral pigment that upon drying yields a hydrophilic, water insoluble coating with a matte finish, the weight ratio of pigment to adhesive therein being in the range of 1 to 4 and said hydrophilic adhesive being selected from the group consisting of cellulose ethers, polyvinyl alcohols, and starches; drying said hydrophilic coating to remove the water from said water dispersion; thereafter heat sealing the thus dried hydrophilicv coating to said anchor coating; and sensitizing said hydrophilic coating with a wash solution of a photosensitive organic diazo compound.
2. The product of the process of claim 1.
3. A method of preparing a photolithographic printing plate comprising the steps of: depositing in the range of 4 to 6 grams per square meter (dry weight) of a thin anchor coating consisting of solvent solution of a thermoplastic adhesive onto a supporting substrate of a flexible impervious metal film reinforced on the reverse side with an adherent paper web, said thermoplastic adhesive being a flexible tripolyrner of vinyl chloride, a minor proportion of vinyl acetate and a small amount of maleic acid; drying said thin anchor coating to remove solvent therefrom and leave a smooth. adherent film of said thermoplastic adhesive on said flexible metal film; depositing in the range of 15 to 20 grams per square meter (dry weight) of a continuous and uniform hydrophilic coating of a water dispersion of a hydrophilic adhesive binder and a nonhydrous mineral pigment, the weight ratio of pigment to binder being in the range of 1 to 4 and said hydrophilic adhesive binder being selected from the group consisting of cellulose ethers, polyvinyl alcohols and starches, said water dispersion on drying leaving a flexible hydrophilic Water insoluble coating with a matte finish; drying said hydrophilic coating to remove the water from said dispersion; heat sealing the thus dried hydrophilic coating to said anchor coating; and sensitizing said hydrophilic coating with a thin wash coating of a photosensitive diazo compound.
4. The process of claim 3 wherein said thermoplastic adhesive is a t'ripolymer of 86 parts by weight vinyl chloride, 13 parts vinyl acetate and one part maleic acid.
5. The process of claim 1 wherein said water dispersible hydrophilic adhesive consists essentially of a polyvinyl alcohol.
References Qited in the file of this patent UNITEDSTATES PATENTS 2,685,511 Nadeau et a1. Aug. 3, 1 954 2,692,826 Neugebauer Oct.'26, 1954 2,760,863 Plambeck Aug. 28, 1956 2,778,735 Brinnick Jan. 22, 1957 2,983,220 Dalton et al May 9', 1961 3,073,723 Deal et al Jan. 15, 1963 Buskes May 28, 1963