US2129071A

US2129071A - Planographic printing plate

Info

Publication number: US2129071A
Application number: US47595A
Authority: US
Inventors: George S Rowell
Original assignee: Multigraphics Inc
Current assignee: AB Dick Co
Priority date: 1935-10-31
Filing date: 1935-10-31
Publication date: 1938-09-06
Anticipated expiration: 1955-09-06

Description

p 1938. G. s. ROWELL 2,129,071

PLANOGRAPHI C PRINTING PLAT E Filed 001;. 31, 1935 j g'y' @6671532'07 age 51 Z0622 Patented Sept. 6, 1938 UNITED STATES PATENT orrlcr rmnoonarmc PRINTING PLATE tion of Delaware Application October 31, 1935, Serial No. 47,595

9 Claims.

bonate, and thereafter washing said so-treated sheet aluminum to remove a superficial scum incidental to said treatment, and then drying the product, all as more fully hereinafter described and claimed.

The art of planographic printing from metal plates depends, as does the older lithographic art, upon the immiscibility of oil-and water and the preferential retention of the greasy imageforming substance by the image areas, and a similar retention of an aqueous dampening fluid by the non-image areas. In order to condition a metal printing plate for these preferential retentions, it has heretofore been found customary to remove all traces of grease and then to counter-etch the plate to provide ,a basic surface wherewith the fatty acid component of the greasy image-forming material may react. A so-treated metal plate is said to be sensitized" to ink. After the greasy image has been delineated in known manner, the non-image areas are customarily etched or desensitized by treatment with an aqueous solution adapted to at least neutralize the basic condition previously produced, and preferably to render the surface lyophilic. In

' general it has been the custom to desensitize" by the use of a weak acid and to obtain a lyophilic condition by means of an adsorbable lyophilic colloid, of which gum arabic is the most commonly used example. It is also known that certain salts of the metal are preferentially waterwettable in themselves and hence do not require the adsorption of a lyophilic colloid. Metal plates are customarily roughened or grained for the purpose of minimizing the attritional effect of the ink rollers on the image and, perhaps more importantly, for the purpose of preventing these rollers from too greatly reducing the film of moisture which must be retained to prevent ink contamination of the non-printing image. This graining operation is customarily performed by gyrating marbles over fine sand on the plate; and it is a costly, time-consuming operation requiring much skill. When the image to be printed has been directly delineated upon the printing surface by means of crayon or the (Cl. 414l.5)

like, it is customary to then desensitize the nonprinting areas and thereafter wash out the image with turpentine and replace it with asphaltum or some other image-forming material which will take a firmer grip on the plate and hence better withstand the attritional effect of the inking rolls during printing, thus assuring the maintenance of the integrity of the image throughout editions of many thousands of copies.

It is an object of this invention to obtain a requisite grain and sensitization to ink by a single chemical operation. It is a further object of this invention to prepare aluminum plates for planographic printing by a single chemical treatment whereby a plate is cleaned, i. e., freed from that contamination by grease which is inevitable in the production of sheet aluminum, and is provided with an adequate grain and an overlying highly grease-sensitive surface layer. It is another object of this invention ,to provide aluminum planographic printing plates surfaced with a layer which is found to be alkaline reacting and to contain a carbonate.

It is an object of this invention to provide a process for treating sheet aluminum whereby both major surfaces are simultaneously conditioned for the reception and for the retention of pianographic printing images and in particular for such images when they are directly delineated upon the said surfaces, as by pen, crayon or typewriter. It is yet another object of this invention to provide planographic printing plates of aluminum of indented or pitted surface, coated with a layer of porous crystalline material comprising an alkali carbonate and capable of adsorbing image-forming material throughout the depth of said layer, whereby an image formed of a greasy substance comprising a fatty acid is firmly retained by said layer against the attrition incident to the printing of long editions therefrom. Another object of this invention is to provide a process whereby sheet aluminum may be conditioned to receive and retain a planographic printing image by a single chemical treatment which simultaneously removes all traces of grease (both superficial and that ground into the metal during the rolling process to which the sheet aluminum is subjected in manufacture) and pits or grains the surfaces and coats them with a layer of crystalline material, comprising an alkali carbonate; which material is porous and highly adsorptive and retentive of original greasy imageforming compositions as lithographic crayons, transferring drawing inks, tracing carbons, and typewriter ribbon inks containing a fatty acid,

without the necessity for washing out and reforming the image as has been the usual practice.

Among the advantages of the process of this invention may be mentioned the fact that the aluminum is by a single short chemical treatment both freed from, and highly sensitized to, grease and particularly to grease containing a fatty acid. A further advantage of the pianographic printing plate of this invention is that by reason of the relatively thick and relatively soft image-retaining layer with which the surfaces of the said plate are coated, corrections may be readily made and inadvertent greasy contaminations of the surface may be removed with a soft grit-free rubber eraser and without resort to the usual stoning operation.

Chemically pure aluminum is not particularly suited to the purposes of this invention and while any commercial sheet aluminum may be used the best results are obtained from, and I prefer to use, that alloy of aluminum (of commercialpurity) and manganese containing 1.25% manganese which is designated by its manufacturers and known to the trade as 38H. Other objects and advantages will become apparent as the description proceeds.

An alkali carbonate. bath suitable for the practice of this invention may be made by dissolving potassium or sodium carbonate or a mixture of the two in ordinary tap water. Sodium carbonate alone is preferred. It has been observed that the temperature of the bath, the concentration of the carbonate in the bath, and the time of treatment are to some extent compensating variables. Thus, small differences in coneentration can be offset and substantially the same end result obtained by inverse adjustment of the temperature or time of treatment, or both. In general, I prefer a bath containing one gram of sodium carbonate (58% soda ash) for every cc. of water. And I obtain the best results with this bath when it is maintained at a temperature of about 200 F. when first made up, hot solutions of sodium carbonate, as above described, are too vigorous and do not produce plates which are suitably coated for the purposes of this invention. To obviate this difficulty I permit the bath to act, preferably, on scrap aluminum until it is sufllciently quiescent to produce the desired coating on sheet aluminum when immersed therein forabout ten minutes. Alternatively I sometimes, and preferably when operating by the batch method, start with a fresh bath containing about t; of a gram of sodium carbonate per 100 cc. of water at about 200 F. and then immerse about one square foot of total aluminum surface per gallon for about fifteen minutes. After removal of the first treated aluminum, I add about $6 of a gram of carbonate per 100 cc. of bath and treat another batch of aluminum ofthe same area for ten minutes; and repeat this cycle twice more until the bath contains one gram of carbonate per 100 cc; of bath solution. If sheet aluminum in insufficient amount is fed to the bath, it regains some of its early undesirable vigor. This regain in vigor is particularly marked when the bath is allowed to cool and/or stand overnight. After such standing or cooling, the bath is best rehabilitated by allowing it toact on scrap aluminum until it is found by test to yield satisfactorily coated plates. To maintain the bath at the optimum,

that is, in that condition which will yield a coatabout one square foot of total aluminum surface to every gallon of solution when the solution is held at 200' 1". and contains 10 grams of sodium carbonate per liter of water and the treatment is of about ten minutes duration. This ratio is readily maintained by controlling the speed with which the aluminum is drawn through the bath, if the process is operated continuously, or by immersing the requisite surface of aluminum at ten minute intervals if the process is operated discontinuously, that is, by the batch method. When operating by the batch method, the batch of plates should be held vertically or nearly so in the treating tank and should be spaced about two inches apart. Less spacing than the stated two inches tends to intensify convection currents in the bath and to make for irregularities in the thickness of the coating, which are obsome solution is removed with them, thus lowering the solution level without, however, changing the concentration; but there is also a continuing loss of water through evaporation. When the level is maintained by addition to the bath of a solution of sodium carbonate of about onehalf the concentration of the normal bath, that is, of about 0.5 gram per 100 cc. of water; and further when the treating solution is well agitatedbetween batches if the process is operated discontinuously, or constantly agitated, if the process is operated continuously, it has been found in practice that the product is satisfac torily constant. The solution is preferably heated by water Jacketlng in known manner. A coating of the desired character, obtainable by controlling the bath as above mentioned will be more fully hereinafter described.

After the aluminum plate is removed from the bath, it is swabbed in running water to free it from superficially entrained solution and to remove a blackish scum incidental to the treatment, which scum is readily displacedif the swabblng is done immediately after the plate is removed from the treating bath. Pressure exerted against the scum, or long standing and drying, all tend to make its removal more difflcult and also tend to cause a brownish stain which is undesirable because, while not appreciably affecting the performance of the plate, it is unsightly and lowers the visual contrast between the plate background and a printing image on the plate.

The described treatment has three efiects: One is to remove all traces of that grease which is inevitably associated with rolled sheet aluminum. some of this grease appears not to be merely superficial but to be embedded in the metal itself, and it is one of the advantages of the process of this invention, as stated, that all such grease is freed from the plate simultaneously with the chemical changegwhlch takes place at the plate surface. Another effect is to minutely roughen all surfaces of the plate by the pitting action of the bath. The third eflect is to coat the plate with a thick layer of relatively soft,

V porous and apparently crystalline material com- (hardness) characteristic of the softest anodic coatings of aluminum oxide which have been heretofore prepared for use on aluminum planographic printing plates. The softness of the coating of this invention constitutes a major improvement over such prior art coatings in that it permits of ready erasure and this without scratching or other substantial damage to the plate. While the said soft coating is sumciently firm and adherent to withstand ordinary handling and to retain a printing image throughout the printing of long editions, it is nevertheless soft and friable enough to be readily displaced by means of an ordinary rubber pencil eraser. Under the microscope it appears, when scratched with a needle point, as a soft whitish apparently crystalline mass rather than a hard pulverulent amorphous solid. While prior art anodically-coated aluminum plates are suitable for photo-lithographic printing because of their relatively hard surfaces, they do not retain direct images well because of their acid nature; and while per contra the coatings of the plates of this invention are basic in character, they are not well adapted to photolithography.

Reference is had to the drawing in which Fig. 1 is a plan view of a portion of a lith'ographic printing plate prepared. in the manner set forth above and having an image thereon;

Fig. 2 is a sectional view thereof taken along the line 2-2 in Fig. 1; and

Fig. 3 is a plan view taken microscopically of a portion of the image shown in Fig. 1. It will be understood that the sizes and proportional dimensions employed in the drawing are much greater than they would be in a commercial plate and that such increase is necessary in order to bring out the invention with suflicient clarity.

In the drawing is shown a printing plate l having a plurality of pits ll formed in both surfaces thereof by the alkali carbonate bath. Such pitted surfaces are covered by a coating l2 which receives and holds the pigment forming the image I5 (see'Fig. 2).- The pits ll vary in depth, contour and extent, as shown in Figs. 2 and 3. The coating I2 is applied to the surfaces of the plate at the same time that the plates are pitted, i. e. grained by the action of the alkali carbonate bath described above. It is well known that an alkali carbonate reacts with aluminum to form an aluminum oxide which is initially hydrous but becomes anhydrous upon drying. The coating thus provided is open-texture and crystalline or of crystalline material", and possesses appreciable thickness and softness. The non-printing areas of the image bearing surface are desensitized by the use of a suitable etch as, for example, that disclosed in my Patent No. 1,977,646, dated October 23, 1934.

The appreciable thickness of the coating, as compared to that of anodic coatings heretofore suggested, is a feature of this invention. The coating I2 is thickest over the deepest pits and thinnest over the areas between adjacent pits. Best results are obtained when the coating, in its thinnest portions, has a thickness of about one ten-thousandths' of an inch. In any event, the thinnest portions of the coating should be thick enough to retain, over the entire image area, absorbed pigment such for instance as carbon black or blue toner, when the oily vehicle of the image-forming material has been leached out with an appropriate solvent.' To test the continuity and sufliciency of the coating, it is merely necessary to form an image on the plate, as by typewriting or writing with a stylus through a soft-to-medium carbon paper or through a medium-to-dry typewriter ribbon, and then to dissolve out as-much of the so-produced image as is possible with repeated applications of a suitable solvent, as for instance carbon tetrachloride. If the coating is continuous and sufficiently thick for the purposes of the present invention, the image will remain microscopically discernible although -somewhat weaker than before. Viewed microscopically, the entire image area will appear distinctly darker in color than will the surrounding plate surface, and will comprise at least two distinct shades: the darker of these shades will appear as isolated small patches against a continuous background of a lighter and sometimes varying shade. In Fig. 3 of the drawing the darker areas are indicated by the numeral l6 and the lighter areas by the numeral ll. The apparent density of the image, residual after the above-described solvent treatment, is believed to be proportional to the thickness of the coating, and hence is greatest in the isolated areas overlying deep pits and is least in the greater expanses of unpitted or very slightly pitted areas lying therebetween. When the coating is not continuous or is of insufficient thickness, the image which is residual after the solvent treatment as above described, will consist of isolated dark spots on a background of substantially the same shade as the normal surface or non-image area of the plate.

The expression "apparently crystalline, as hereinabove used in reference to the coating obtained by the process of this invention, is employed without prejudice for the reason that the exact nature of the coating is not understood. It may, however, be distinguished from anodic aluminum oxide by two distinct characteristics. When scratched with a needle point, my coating material appears under the miscroscope and at a magnification of 100 diameters or so to consist of whitish particles suggestive of crystals, as stated, and distinctly not the powdery product of a hard amorphous mass such as anodic aluminum oxide. For the purpose of distinguishing from .anodic aluminum oxide or atmospheric aluminum oxide, the expression "crystalline or "crystalline material" is thought useful in the circumstances. The other distinguishing characteristic is that the coating may be shown by appropriate indicator to be alkaline reacting and may be shown by microanalysis to comprise an alkali carbonate. Whether the alkali carbonate is merely taken from the bath and entrained by or occluded in the micropores of the crystalline material in the aluminum oxide coating, or whether it is a component of a compound of the metal constituting said crystalline material, is not known; but the presence of the alkali carbonate in the coating material is of advantage. This advantage is manifested by the substantially greater life, under printing conditions, of an image derived from a greasy image-forming substance containinga fatty acid reactive with said alkali carbonate than the life of an image obtainable in the absence of such fatty acid. This advantage may also be demonstrated by comparing the life of an image comprising a fatty acid on a coating of this invention comprising an alkali carbonate, with the substantially shorter life of a similar image on a like coating in which the alkali carbonate has been neutralized by treatment with a weak acid.

Because of the highly grease-sensitive nature of the described coating of this invention, it is necessary to protect the reverse side of the plate while applying an image to and while printing from the obverse side. This may be accomplished by coating the reverse side of the plate with a water soluble protective film, none of the constituents of which are adsorbed by the crystalline coating. The protectives customarily used in planography, as gum arabic or gum tragacanth for instance, may not be used to temporarily protect the reverse side of the plates of this invention because these gums are adsorbed and may not be completely washed away. Neutral glue, or gelatin, with suflicient water and glycerin to form a soft but non-tacky gel, such as the commonly known hectographic compositions, may be used and are particularly desirable if the plates protected therewith are intended for long storage or if, in the locality of use, the atmosphere is subject to wide fluctuations in humidity. Under ordinary conditions I have found glycerin itself to be a satisfactory protective for the reverse sides of the plates of this invention. For this purpose, glycerin of the usual commercial grade may be somewhat diluted with water and then swabbed on the surface it is desired to protect; and, after the surface is thoroughly wetted with the glycerin solution, the superficial solution may and preferably should be rubbed off with a swab of cotton or vsoft cloth until the surface is free from mobile fluid. To remove the protective and recondition the protected side of the plate for use, it is merely necessary to wash away the protective material and dry the plate.

While, as has been stated, the coated plates of this invention are particularly adapted to take and retain for long editions direct images formed from usual lithography image-forming compositions containing fatty acids, it has been found in practice that even when the image is formed by greasy materials low in, or devoid of, free fatty acid, the said plates will retain such images satisfactorily for editions of from two to five thousand copies, or more; whereas the images, formed of the same compositions on prior planographic plates, will "walk off the plate in at most a few hundred copies.

I claim:

1. A process of preparing aluminum planographic printing plates which includes the steps of reacting sheet aluminum with an alkali carbonate by immersing said aluminum in a bath maintained at between F. and the boiling point and consisting of an aqueous solution of said carbonate of between 0.3 per centum and 1 per centum concentration for from 15 to 8 minutes, whereby said aluminum is simultaneously freed from grease, is grained and is coated with a porous crystalline layer comprising said alkali carbonate and thereafter washing and drying the so-treated sheet aluminum.

2. An aluminum planographic printing plate characterized by a surface which is grease free, chemically grained and coated with a porous crystalline layer comprising an alkali carbonate, said crystalline layer being thickest over the deepest pits and thinnest over the areas between adjacent pits.

3. An aluminum planographic printing plate characterized by a surface which is grease free, chemically grained and coated with a porous crystalline layer comprising an alkali carbonate, said layer having a mean thickness of the order of one ten-thousandths of an inch, said crystalline layer being thickest over the deepest pits and thinnest over the areas between adjacent pits.

4. An aluminum planographic printing plate characterized by surfaces which are grease free, chemically grained and coated with a porous crystalline layer comprising an alkali carbonate, one at least of said surfaces being further coated with a water removable protective layer, said crystalline layer being thickest over the deepest pits and thinnest over the areas between adjacent pits.

5. Analuminum planographic printing plate characterized by surfaces which are grease free,

chemically grained and coated with a porous crystalline layer comprising an alkali carbonate, one at least of said surfaces being further coated with a grease repellent layer comprising glycerin, said crystalline layer being thickest over the deepest pits and thinnest over the areas between adjacent pits.

6. An aluminum planographic printing plate carrying a greasy printing image characterized by surfaces which are grease free, chemically grained and coated with a porous crystalline layer comprising an alkali carbonate, and by the fact that said image extends throughout the thickness of said layer, said crystalline layer being thickest over the deepest pits and thinnest over the areas between adjacent pits.

'7. An aluminum planographic printing plate carrying upon one surface a greasy printing image characterized by surfaces which are grease free, chemically grained and coated with a porous crystalline layer comprising an alkali carbonate, the non-image carrying surface being further coated with a grease repellent layer comprising glycerin and further characterized by the fact that said image extends throughout the thickness of said first-named layer on the image bearing surface, said crystalline layer being thickest over the deepest pits and thinnest over the areas between adjacent pits.

8. A process of preparing aluminum planographic printing plates which includes the steps of reacting sheet aluminum with an alkali carbonate by immersing said aluminum in a bath maintained at between 170 F. and the boiling point and consisting of an aqueous solution of said carbonate of between 0.3 per centum and 1% per centum concentration whereby said aluminum is freed from grease, is grained and is coated with a porous crystalline layer comprising said alkali carbonate and thereafter washing and drying the so-treated sheet aluminum.

9. An aluminum planographic printing plate characterized by a surface which is grease free, chemically grained and coated with a porous crystalline layer comprising an alkali carbonate, said layer forming a porous and soft surface adapted to directly receive a planographic printing image.

GEORGE S. ROWELL.