US3198110A - Lithographic printing plate with polymer coated metal image - Google Patents

Description

Aug- 3, 1965 B. w. Nr-:HER 3,198,110

LITHOGRAPHIG PRINTING PLATE WITH POLYMER COATED METAL IMAGE Original Filed June l5, 1959 I NVEN TOR.

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3,193,110 v MTHUGRAPH C PRHNTENG PLATE tt/ETH POLYMER CATED METAL lli/HAGE Byron W. Naher, Hudson, Wis., assigner to Minnesota Mining and Manufacturing Company, St. Pani, Minn., a corporation of Delaware Original application .inne l5, 1959, Ser. No. 820,254, new Patent No. 3,127,331, dated Mar. 31, 1964. Bivided and this application Apr. 19, 1963, Ser. No. 279,349

2 Claims. (Ci. lili-449.2)

This application is a division of U.S. Serial No. 820,- 254, now U.S. Patent No. 3,127,331.

This invention relates to a novel process for reproducing visible images. In one aspect, this invention relates to a method for the electrolytic destruction ofthe rectification effect of selected areas of photoconductive material bonded to an electrically conductive backing. In another aspect, this invention relates to a nevel process for reproducing visible images on a photosensitive sheet. Still y quired, and the copy produced needs no further heating or other processing to render the image permanent. Brietiy, the copy sheet comprises a photoccnductive powder, such as zinc oxide, bonded to a contiguous electrically conductive backing. Since the photoconductive coating shows a rectification effect, i.e. allows current to pass essentially in only one direction, it is more effective to use the electrically conductive backing sheet as the cathode in the electrolytic development process. With thebacking sheet as anode, the electrical resistance of the photoconductive material is high, even in wet light rstruck areas.

Because of the rectification effect of photoconductors such as zinc oxide, on a conductive backing sheet, the electrolytic development of the light struck photoconductive areas has been generally restricted to the use of those materials which are reduced at the cathode to produce a visible image. Many oxidizable developers and materials bearing a negative charge have not heretofore been efliciently useable in the above-described process.

It is therefore an object of this invention to provide a reverse current process for the development or" an eX- posed photoconductive image.

Itis another object of this invention to provide a process for developing photoconductive images by electrolytic oxidation.

It is a further object of this invention to provide an electrolytic method for preparing litho-graphic plates.

Still another object of this invention is to provide novel lithographie plates.

In accordance with this invention, a photosensitive sheet is made of photoconductive material, usually in powdered form, bonded to an electrically conductive backing, preferably by means of a sui-table nonconductive binder material. After exposure of the photoconductive layer to a light image, the electrically conductive backing of the photosensitive sheet is connected to the negative pole of a direct current source and electrolyzed in the presence of a conductive salt of a reducible metal. During electrolysis, the metal of the conductive salt is plated out as a conductive metal-lic layer or coating on the light struck areas of the photoconductive surface. If

lUnited States Patent O iddi@ Patented Aug. 3, 1965 this electrolysis is carried out with the electrically conductive backing, connected as the anode, i.e. if the electrodes are reversed, the rectification effect or selective resistance of the photoconductive layer greatly hinders and substantially prevents free current passage in the opposite direction. However, it has now been found that the presence of free metal, which is plated out on the photo-'conductive surface in a differential pattern corresponding to the original light image, unexpectedly destroys the rectification effect of the photosensitive sheet in those areas containing the free metal and permits current passage in both directions. Upon reversing the electrodes and making the electrically conductive backing the anode, the conductive image can be developed electrolytically by an anodic process or reverse current process.

Among the developers which may be used in this anodic process are substances capable of changing color Value on oxidation, such as the leuco form of vat dyestuffs used in the dyeing of various commercial fibers. For example, if the anodic process is carried out with Indigo white in con-tact with the metal plated photoconductive surfaces, the anodic reaction oxidizes Indigo white from its colorless leuco form to insoluble colored Indigo in the conductive surface areas. The final visible image is found to be stable except for the tendency to fade slowly, probably because of the oxidation of the leuco dye on exposure to air. These dyestufis can be incorporated into the electrolyte or may be coated on metal plated photoconductive surface prior to electrolysis.

Still another developer material that may be employed in the anodic development process is lthe colored anion, as exemplified by the acid type dyestuffs. By carrying out the electrolysis with the metal plated photosensitive sheet as anode and with an acid type dyestuff in the electrolyte, the colored anions of the acid type dye migrate selectively to the conductive metal plated image areas and are deposited thereon, thereby coloring the light exposed and metal plated surface areas. These dyes are commonly marketed in the form of a salt of their sulphonic acid, usually the sodium salt. Illustrative of such developers are the nitro dyestuffs, such as Naphthol Yellow ((11.9), the mono-alo dyestuffs, such as Fast Red (C.I.176), the dis-azo dyestus, such as Crocein Scarlet (C1277), the nitro dyestufs, such as Naphthol Green (C.I.5), the triphenylmethane dyestuffs, such as Wool Green (C1. '7 37), the xanthene dyestuffs, such as Brio Fast Fuchsine BL (C1758), the orthraquinone dyestufts, such as Solway Blue SES (01.1053), the azine dyestuffs, such as Azocarmine (CLSS) and the quinoline dyestufis, such as Quinoline Yellow (C1801). Although some color is often deposited in the background areas when the colored anion containing electrolyte is brought into contact with the metal plated photosensitive sheet surface, the depth of color is significantly greater in the light struck, metal plated areas, and the contrast can be controlled by selection of the colored anion, concentration of colored anion in the electrolyte, duration and conditions of the electrolysis, etc.

in some cases, during the above reverse current development some of the metal plated on the light struck areas may be reoxidized by the anode reaction, with a resultant decrease in current flow. However, this does not significantly affect the color development, and for certain purposes is desirable in order to improve the quality or intensity of the final color.

It is also within the scope of this invention to incorporate colored or uncolored negatively charged particles into the electrolyte and carry out electrolysis using the metal plated phot-osensitive sheet as anode, as earlier described. These negatively charged particles are preferably in suspension or dispersion and may constitute a latex, such as a latex of polyethylene, polypropylene, etc.,

etc. Generally, those polymer latices stable in alkaline -V media contain negatively charged particles and are therefore operable in the instant electrolysis. The charge on such particles is readily'determined by well knownV methods.` Y f During the` electrolysis the -negatively charged particles are deposited selectively on the metal plated or conductive areas of the photoconductive layer. When such negatively charged particles constitute materials which are hydrophobic in relation to the photoconductive surface Aand are therefore selectively ink receptive and water rejecting, e.g. polyethylene, polypropylene, etc., the surface of the resulting polymer coated sheet displays ink receptive, hydrophobic properties in the polymer coated, light struck lareas, and relative ink rejecting,jhydrophilic background areas. Other negatively charged particles or ions which Kdevelop -a layer or film on the light struck image areas that display ink receptive properties different Y from, i.e. relative'to, the background or non-light struck areas, wouldalso produce a similar result. The selectively coated photosensitive sheet having varying degrees of kink receptivity,` corresponding to theil original light image, provides a simple, inexpensive and effective lithographic plate and produces outstanding reproductions when used in conventional lithographie processes, e.g. the Multilithprocess In certain instances itmay also be desirable to incorporate certain iller materials in the negativelycharged particles, thereby to alter the physical properties of the deposited polymer layer. These fillers need not carry an electrical charge, since the negatively charged particles of polymer serve as carrier for these ller materials. Suitable fillers include colored pigments, carbon black, silica, etc. Y

FIGURE 1 illustrates the above-described novel ,lithographic sheet-obtained in accordancewith this invention'.

In the above process, the deposition'of freefmetal can be effected by electrolysis of an organic or inorganic conducting salt of a reducible metal, such as silver nitrate,

nickelous chloride, copper sulfate, etc. Any salt of an electrolytically reducible metal, which metal is electricalily conductive, may be used. Moreover, the unreduced metal salt can be incorporated in'or'on the photoconductive material and thereafter be reduced in' situ byV eleci trolysis. The actual mechanism by-which this deposited freemetal destroys the rectification effect ofthe photoconductive material. is not fully understood;l Y

The receptor sheet which is exposed to the light image and on which the reduced metal is differentially deposited t contains an electrically .conductive base, such as Ymetal foil, upon which a photoconductive layer, having a photoconductivity value of at least about 10-7 -mho/cm. and a dark conductivity value not greater than about onetwentieth of the photoconductivity value, is placed or bonded. Such sheets are described in greater detail in SN. 692,529, led October 28, 1957. In bondingthe photoconductive material to the electrically conductive base, various water resistant, flexible adherent film form- W king polymersrcan be used, provided the polymer isV light in color and does'notadversely affect the light sensitivity of the photoconductive material. Y Such polymers include, for example, a 30:70 copolymer of butadiene and styrene (Pliolit'e S-7 solution, 340% solution in toluene),Y polystyrene, chlorinated rubber, rubberhydrochloride,poly-y vnylidene chloride, nitrocellulose, polyvinyl butyral, silicones (e.g., D-C 803 silicone solution, 50% solution in xylol of alkyl amyl silicone resin capable of curing one hour at 480 F.- to a hard .and somewhat brittle polymer), etc. Polymers which are dissolved or softened by water, or which are dark in color, or insoluble in commercial solvents, or reactive with the photoconductive material, or which readily wet the photoconductive particles, are

found to be less desirable. Thus, polyvinyl alcohol,y

polyacrylie acid, shellac and sodium carboxymethyl'cellulose are generally not employed as binders for the light- Y values in coated rilrn form and to produce receptor sheets suitable forr use in the instant invention. Mixtures of these photoconductive materials may also be employed. Generally, lzinc oxide is preferred. To improve or enhance the sensitivity ,ofthese photoconductive materials in certain visible and non-visible areas of the light spectrum, dyesensitizers such' as Acridine Orange, Fluorescein,. Eosin jY, Rose Bengal, MethyleneV Blue, etc., are preferably admixedin smallquantitieswith the photoconductive powder.

Metal foil or sheet providesa suitable electrolytically conductive backing. Metal' conductors, such as aluminum, chromium, nickel and copper, are suitable for the electrically conductivebacking and may additionally be placed on the surface of a nonconductive `supporting sheet, c g., 'by vapor deposition, lamination,` etc. When the photosensitive sheet is immersed in the electrolyte during electrolysis the exposed conductive backing must beinsulated from the electrolyte, e.g. by anonconductive surface coating such as plastic, etc. s f The following examples are illustrative and are not to be construedas limiting the` scope of theV instant invention. f v

Examplel `A suitable light-sensitive sheet material was first prepared. A flexible film of transparent cellulose acetate having a thickness of about l0 mils (0.010 inch) was first metallized on one surface, by vapor deposition ina vacuum, with an extremely thin 'coating ofY aluminum.V The coating was found to have arsurface resistivity of labout 200 ohms per square, and transmitted-about 55% of incident light in the visible range. Over this metal layer was then applied a suspension of 48 parts byweightV of Merck & Companys Reagent-Grade zinc oxide microcrystals in a solution, in 48 parts toluene, of 4 parts of Pliolite S-7 resin, la resinous copolymer of about 30 parts butadiene and correspondingly about 70 Yparts styrene, serving as a binder, the mixturehaving been ground in a ball mill until smooth. After drying, the firmly bonded smooth lwhite coating was'found to be between 0,3 and 0.6 mil in thickness. The sheet material was highly water-resistant. Y l Y Sheet material prepared asjust described was suspended in -a transparent glass cell containing a solution of 28 grams of copper sulfate m200 ml. of water. A at electrode of slightly larger area, in this case a copper plate, was suspended in the solution facing and somewhat removed from the coated surface of the sheet material. A light-image was focused on the uncoated surface of the sheet through the -glass wall of the cell, the source of the light'being a 10G-watt bulb and providing an intensity of aboutr 70 foot-lamberts.v Exposure was maintained for about 5l seconds. A source of potential was then connected across the copper plate and the conductive aluminum layer of the sensitive sheet, the latter being connected to the negative pole, and a lcurrent of `about 15 milliameperes was passed through the system for about 3 seconds. vThe sheet was withdrawny and rinsed, and was found to have a negative reproduction of the light-image on the sensitive coating. Non-illuminated 'areas of the sensitive coating remained white, while the exposed areas were darkened by deposition of metallic copper thereon. Equally effective copy was obtained by exposing the coated sheet to thelight-image underldry conditions, and then promptly immersing the rsheet in the `electrolytic cell and electrolytically developing the image in the manner described. Y

Silver nitrate solution was substituted for the copper sulfate to provi-de equally effective ivm-age development. Nic-kelous chloride is also eiiective, and is improved by the addition of sodium thiosulfate. A particularly effective developing solution contains nickelous chloride and 5% sodium thiosulfate,

'Ilhe ratio of pigment to binder in the light-sensitive coating was eiectivelly varied over wide ranges. At 12 parts of Zinc oxide to one of resin, as in the specific formula just given, the white areas of the print are sometimes found to contain dark spots, indica-ting non-uniform or insufficient resistivity. Excellent prints are obtained at lower ratios, for example at 8:1 and at 4:1. Somewhat less effective prints are obtained a-t 3:1 ratios of zinc oxide and Pliolite, resin, and at 2:1 the lightsensitivity is inadequate and the results are decidedly inferior. These ratios may be specifically difiere-nt with other specili-c oxides and resins but will serve to illustrate p a generally desirable range.

Electrically conductive glass plates have been substituted for the partially transparent metallized cellulose acetate film as a `carrier or base for the light-sensitive coating. A glass having a surface layer high in stannic oxide, h-aving a surface resistivity of about 600 ohms per square and a lig-ht transmission of at least about 90%, has proven useful, although somewhat lower resistivity is preferred.

The sensitive surface of such transparent photosensitive coated pla-tes is effectively exposed to the light-image through the transparent plate and simultaneously electrolytically developed, as described in the foregoing example. rThese plates may ,alternatively be first exposed to the light-image and then, without further irradiation, transferred to the developing station and separately developed, lthe iight-memory of the zinc oxide coating being sufficient to maintain the necessary conductivity at the irradiated areas. The latter procedure is equally effective on fully opaque plates such as metal plates coated with the sensitive zinc oxide coating.

Opaque plates have been simultaneously exposed and developed by substituting a copper wire frame for the Icopper plate of Example 1 and then exposing the sensitive surface of a coated metal plate to a light-image through the trame while carrying out the elect-rolysis as before. Where the plate area is too large for uniform electrolysis in this manner, a screen is provided Iin place of the frame, and the screen is moved steadily during electrolysis so as to avoid producing a visible shadow pattern on the sensitive sheet.

Example 2 A photosensitive sheet having a 5 mil conductive aluminum backing and a coating thereon of zinc oxide and iPliolite S-7 resin (about 10:1 respective weight ratio) was exposed to a light image for a period of seconds, using .a 100 .Watt light source to project the image from a positive mircoiilm tra-me to the pho-toconductive surface of the sheet. The exposed surface was then brought into contact with a solution of 10% by weight of nickelous chloride and 5% by Weight of sodium thiosulfate and electrolytically developed at an impressed potential of between about 30-40 volts D.C., the conductive .aluminum .backing being connected to the negative pole of the current source and being masked on its exposed face with a plastic covering, .as described in Example 1.

A-fter a period of 15 seconds the electrolysis was stopped and the exposed surface Was rinsed with cold water to remove residual metal salts, A negative reproduction of the light .image was formed, the nickel having been reduced and deposited as the free metal on the light struck areas of the photosensit-ive surface. Conversely, if a negative light-image is used, a positive reproduction iS formed.

The sheet was then suspended in a 1% solids aqueous polyethylene latex [A-C Polyethylene 629, Allied Chemical and Dye Company, MP. Z13-221 F., penetration gm., 5 sec., 77 F.) of 3-6, aoid No. 1-417, color less than l NPA]. Using the conductive aluminum backing sheet as the positive pole and masking the exposed aluminum backing with a plastic covering, the polyethylene latex was electrolyzed at a potential of labout 30-40 volts for about l5 seconds. Polyethylene was deposited selectively on those light struck areas having a free meta-l deposit, producing a polyethylene image which had greater .ink receptivity relative to the zinc oxide background areas. When used as a lithographc plate negative prints were obtained. If the original light-image is negative, the iinal lithographie prints are positive.

Other polymeric latices which are stable in alkaline media and which contain negatively charged particles can be similarly employed, including polytetraltuoroethylene, `synthetic rubber, eg. Ohemigum latex 24S-B (butadieneacrylonitni-le, non-staining, oil resistant, vulcanizable synthetic rubber latex supplied by the Goodyear Tire and Rubber Company), polyvinyl acetate, polystyrene, Pliolite, rubber, polyvinylidene chloride (Saran), etc- The above described lithographie` plate was installed on a Multilith lithographie machine and provided over 250 copies with excellent contrast and definition. An acid fountain solution containing canboxymethyl cellulose and various inks were used. Final prints were dried at 200 F. for 15 minutes.

Various other embodiments and variations within the scope of this invention will be readily apparent to persons skilled in the art.

Having thus described my invention, I claim:

1. A novel lithographic sheet which comprises .an electrical-1y conductive backing, a photoconductive layer bonded to one contiguous electrically conductive surface Ithereof, a free metal image on selected surface portions of said photocond-uctive layer, and a polymer coating on the surface of and coextensive with `said free metal image, the polymer off said polymer layer being capable of forming a latex in which the polymer particles bear a negative charge and being selected to provide a polymer coating having different wetting characteristics with respect lto lithographie ink and fountain solution than the other portions ofthe sheet surface,

2. The lithographie sheet of claim 1 in which said polymer is polyethylene.

References Cited by the Examiner UNITED STATES PATENTS 2,957,765 10/60 Resetich lOl-149.2 X 2,993,787 7/ 611 Sugarman. 3,107,169 10/63 Bornarth lOl-149.2 X

WILLIAM B. PENN, Primary Examiner. DAVID KLEIN, Examiner.

Claims (1)

1. A NOVEL LITHOGRAPHIC SHEET WHICH COMPRISES AN ELECTRICALLY CONDUCTIVE BACKING, A PHOTOCONDUCTIVE LAYER BONDED TO ONE CONTIGUOUS ELECTRICALLY CONDUCTIVE SURFACE THEREOF, A FREE METAL IMAGE ON SELECTED SURFACE PORTIONS OF SAID PHOTOCONDUCTIVE LAYER, AND A POLYMER COATING AN THE SURFACE OF AND COEXTENSIVE WITH SAID FREE METAL IMAGE, THE POLYMER OF SAID POLYMER LAYER BEING CAPABLE OF FORMING A LATEX IN WHICH THE POLYMER PARTICLES BEAR A NEGATIVE CHARGE AND BEING SELECTED TO PROVIDE A POLYMER COATING HAVING DIFFERENT WETTING CHARACTERISTICS WITH RESPECT TO LITHOGRAPHIC INK AND FOUNTAIN SOLUTION THAN THE OTHER PORTIONS OF THE SHEET SURFACE.

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