US2448861A

US2448861A - Cellulose ester lithographic printing process

Info

Publication number: US2448861A
Application number: US612247A
Authority: US
Inventors: Ralph S Colt
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1945-08-23
Filing date: 1945-08-23
Publication date: 1948-09-07
Anticipated expiration: 1965-09-07
Also published as: GB617227A

Description

P 1948. R. s. COLT 2,448,861

CELLULOSE ESTER LITHOGRAPHIC PRINTING PROCESS Filed Aug. 23, 1945 FIGJA.

HYDROLYZED sun/54c:

FIG. I B. 12 EXPOSED AREAS 11-1 FIG. lc.

,2 uusxrox'p HYDROLYZED LAYER J REMOVED WITH ZnCIz IO FIG ZA. I HYDROLYZED SURFACE co/vmuvma BK'HROMATE l 2ZLCELLULOSE ACETATE FIG. 2B- arms AREAS H1 [0'1 A FIG. 2C

. EXPOSED HYDROLYZED LAYER II I Rg'Ma/w WITH 500m urrocmolre RALPH $.COLT

Patented Sept. 7, 1948 2,440,001 cmumsa as'raa urnoammc PRINTING raocass Ralph S. Colt. Rochester, N. Y., alllgnor to Eastman Kodak Company. Rochester,

poratlon of New Jersey Application sum 2:, ms. Serial No. 012,241

80mins.

This invention relates to the preparation of lithographic plates for printing purposes.

Lithographic printing plates are generally made of metal having a bichromate sensitized coating. Lithographic printing plates of cellulose ester are known and have usually been made similarly to the metal plates by coating a cellulose ester sheet with a sensitive resist. Loening and Mitson British Patent 565,752 describes a method for the preparation of cellulose ester lithographic printing plates according to which a sheet of surface hydrolyzed cellulose ester is coated with a silver halide or bichrcmate sensitized colloid, an image formed in the sensitive layer, the unexposed parts of the stencil and hydrolyzed surface removed with zinc chloride and the remaining portions of the stencil then removed with sodium hypochlorite. The unexposed parts of the cellulose acetate accept fatty inks, and the exposed parts remain hydrophilic and repel the ink.

It has also been proposed to impregnate the surface of a regenerated cellulose sheet with bichromate and to depend upon the diil'erence in hydrophilic properties of the exposed and unexposed areas to render the sheet useable as a printing plate. A sumcient exposure to light of the sensitized surface makes it non-wettable so that differential inking canbe obtained in the exposed and unexposed areas. This method requires a long exposure time and often gives inferior differentiation between ink and waterreceptive areas.

It is an object of the present invention to provide a novel lithographic printing plate. A further object is to provide a novel method of forming a design for printing purposes in a cellulose ester sheet. Other objects will appear from-.the following description of my invention.

These objects are accomplished by more or less hydrolyzing a shallow region, including the surface, of a cellulose ester sheet, which, when N. Y., a ourlyzed surface layer which is hydrophilic and repels penetrates entirely through the face layer the supporting layer must provide the ink-receptive areas of the final surface although it may or may not be hydrolyzable. 1

According to one modification of my invention the hydrolyzed surface layer of the cellulose ester sheet is impregnated with a blchromate ,or chromate sensitizer such as potassium bichromate, or chromate, exposed under a halftone or line positive and the unexposed hydrolyzed skin etched away with zinc chloride. The exposed hydrolyzed layer is insoluble in zinc chloride al--.

though it remains wettable with water when the exposure is of relatively short duration, i. e., when it is a fraction of the exposure required to render the exposed regions water repellent. After washing off the zincchloride, the plate is .ready for printing. a

This method gives a more positive differentia- 1 ,tion between the wettable and non-wettable areas properly carried out, produces a tough layer that may be regenerated cellulose, or a mixture thereof with some unchanged cellulose ester, impregnating the hydrolyzed surface layer with bichromate or chromate, exposing it under a design and selectively removing a portion of the hydrolyzed surface layer, either the exposed or the unexposed portion, with a substantiallyaqueous solution of an inorganic etching agent.

In the accompanying drawing, Figs. 1A, 1B and 1C are sectional views showing one method of forming a cellulose ester printing plate according to my invention and Figs. 2A, 2B and 2C are sectional views of an alternative method of forming a cellulose ester printing plate accordingto my invention.

I use as the printing plate a sheet of cellulose ester, such as cellulose acetate, having a hydrothan does simple exposure without etching. A much shorter exposure can be used than is required to' make the bichromated or chromated hydrolyzed surface unwettable.

My invention will now be described by reference to the following example which is illustrative only.

Example 1 v A sheet of cellulose acetate propionate film base is hydrolyzed for five minutes in the following solution at room temperature:

Sodium hydroxide "ma.-. sov Ethyl alcohol cc 400 Water to 1 liter The film is then washed with water and immersed in the following sensitizing bath for three minutes.

Potassium bichromate grams Ammonium hydroxide (28% solution) ..cc .25

Condensation product of an aldehyde with naphthalene sulfonic acid. -grams Water to .1 liter The sensitized coating is then drained without rinsing and dried, exposed under a positive transparency for 30 seconds to 5 minutes at 30 inchesfrom an are light and developedby swabbing for 3 minutes with a Be. solution of zinc chloride.

This modification of my invention is illustrated H 3 in the accompanying drawing in which Fig. 1A is a sectional view of a cellulose acetate sheet l having a hydrolyzed surface layer H which is sensitized by bichromate or chromate. This material is exposed under a line positive to form exposed areas I! in the hydrolyzed surface layer II, as shown in Fig. 1B. The unexposed portions of the hydrolyzed surface layer II are then removed in zinc chloride to leave an intaglio surface as shown in Fig. 1C. The drawing does not correctly represent the thickness of the hydrolyzed and unhydrolyzed portions ofthe film but the thickness of the hydrolyzed portion is exaggerated for purpose of clarity of illustration.

According to another modification of my invention, the bichromated hydrolyzed cellulose acetate is softened, disintegrated, or dissolved, in a diflerent type of reagent after expose to light. The hydrolyzed surface layer of the cellulose ester film is impregnated with a bichromate or chromate sensitized, dried and exposed under a halftone or line image as before. In this case a negative halftone or line image is preferably used and the exposed material is etched with a reagent suchas sodium hypochlorite, sodium chlorite, or sulfuric acid which disintegrates the exposed hydrolyzed layer without affecting the unexposed areas.

This method afiords a better differentiation between wettabl and non-wettable areas than does exposing a bichromated hydrolyzed area without etching. It can be used to convert a camera negative directly into a printing plate and does not depend upon the use of a wash-oil relief emulsion, which must be developed in a tanning developer, or on any other type of-protective stencil. The use of a tanning developer results in some loss of sharpness caused by a spreading of the tanning action beyond the edges of the developed silver image, and requires more processing operations.

This modification of my invention will be further illustrated by reference to the following example which is illustrative only.

Erample 2 A film having thereon a slow-speed, high-contrast silver halide emulsion is exposed in a camera through a half-tone screen and developed to a halftone negative. After fixing and washing. it is immersed in the following hydrolyzing solution for five minutes at room temperature:

Sodium hydroxide -grams- Ethyl 3101101 on Water to 1 liter.

This treatment hydrolyzes the rear surface layer of the film which already carries a developed silver image on the opposite surface. The film is then washed in running water for three minutes and the sensitizing bath, whose formula was disclosed in Example 1, is applied to the rear surface with a cotton swab, a brush or by spraying. The film is then drained without rinsing and dried, and exposed through the emulsion for 4 20% aqueous sulfuric acid or for a longer time with sodium chlorite solution produces the same effect. The last traces of the hydrolyzed cellulose ester softened by the sodium hypochlorite in the exposed areas are removed with cotton or a piece of carpet and the plate is ready for printing.

It will be understood that the printing plate can be made from a sheet of cellulose ester not carrying a silver halide emulsion, by exposure under an image on a separate support, but the method described above, provides for exposure in the camera and use of the same support for forming the printing plate, the negative image on the support being removed during processing of the printing plate.

This modification of my invention is illustrated in Figs. 2A, 2B and 20 of the drawing. As shown in Fig. 2A a cellulose acetate support In having a hydrolyzed surface layer ll containing a chromate or bichromate sensitizer is exposed under a line image to produce exposed areas i3 as shown in Fig. 23. On treatment of the film with sodium hypochlorite, sulfuric acid or exposed sodium chlorite areas are removed as shown at It in Fig. 20 while leaving the unexposed hydrolyzed portions I l on the printing surface.

In a further modification of my invention, surface-hydrolyzed cellulose ester film is coated with a resist containing a water-insoluble light-sensitive compound such as dicinnimal acetone, or compositions containing cinnamal ketones and resins. After exposure through a screen positive which decreases the solubility of the exposed parts, the unexposed parts are washed away in a solvent such as formamide or triethanolamin-e. The film is then treated with zinc chloride which removes the formamide or triethanolamine and simultaneously etches the hydrolyzed surface layer of the cellulose ester where it is unprotected by the resist. Upon removal of the resist a lithographic printing plate is formed which repels ink in the areas where the hydrolyzed areas remain.

It will be understood that in all modifications of my invention the hydrolyzed layer which remains on the surface of the final printing plate is hydrophilic and repels greasy inks. The unhydrolyzed material is hydrophobic and carries the ink used for printing. Thus in the modification in which zinc chloride or a similar agent is used for etching the hydrolyzed layer, the unexposed areas which are etched with the zinc chloride carry the printing ink. In the modification in which sodium hypochlorite, sodium chlorite, sulfuricacid, or a similar material is used, the exposed areas of the layer are removed and these parts carry the printing ink.

The cellulose ester used in the formation of lithographic printing plates, according to my invention, must be one which is hydrolyzable and I prefer to use an ester of cellulose with a carboxylic acid or acids such as cellulose acetate. cellulose acetate propionate, cellulose acetate butyrate, etc. A layer of a cellulose ester such as cellulose nitrate which can be hydrolyzed only with difliculty may be covered with a layer of cellulose acetate or other hydrolyzable material and this layer may then be hydrolyzed throughout. This method has the characteristic that the hydrolysis is more easily controlled in that it cannot penetrate further than through the layer of cellulose acetate and the back of the sheet is not affected by the hydrolysis. Alternatively, a protective lacquer may be used on the reverse side of the cellulose acetate or other hydrolyzable sheet for the same purpose.

Sensitizers' used may consist of any watersoluble bichromate or chromate such as calcium bichromate or alkali metal salts such as sodium bichromate, potassium bichromate or ammonium bichromate or chromate. alkaline bichromates or chromates appear to be preferable.

Where I refer to the formation of a design," it will be understood that this term is used in its broader sense to include line and halftone images, lettering,'maps, pictures and any other matter normally capable of representation on lithographic plates.

Various types of well-known greasy inks used in lithographic printing which are repelled by hydrophilic 'colloid's,- such as bodied linseed oil inks, may be used for the printing operation using the printing plates of my invention.

It will-be understood that the examples and modifications included herein are illustrative only and that my invention is to be taken as limited only by the. scope of the appended claims.

I claim:

1. The method of forming a design in a cellulosic sheet for printing purposes, which comprises impregnating a hydrolyzed surface layer of a surface hydrolyzed cellulose organic acid ester sheet with ammonium bichromate, exposing said surface to light under a design and selectively removing a portion of the exposed hydrolyzed surface with a solution selected from the class consisting of water-soluble chlorites and water-soluble hypochlorites.

2. The method of forming a design in a cellulosic sheet for printing purposes, which comprises impregnating a hydrolyzed surface layer of a surfacerhydrolyzed cellulose ester sheet with a water-soluble salt of chromic acid, exposing said surface to light under a design, and treating said exposed surface with a solution of a water-soluble hypochlorite to disintegrate the exposed portions of the hydrolyzed. surface layer.

3. The method of forming a design in a cel-' lulosic sheet for printing purposes, which comprises impregnating a hydrolyzed surface layer of a surface hydrolyzed cellulose ester sheet with a water-soluble salt of chromlc acid, exposing said surface under a design to less than about 50% of the quantity of light required to make the exposed regions water-repellent, and treating said exposed surface with a solution of a water-soluble hypochlorite to disintegrate the exposed portions of the hydrolyzed surface layer.

4. The method of forming a design in a cellulosic sheet for printing purposes, which comprises impregnating a hydrolyzed surface layer of a surface hydrolyzed cellulose organic acid Neutral or slightly ester sheet with ammonium bichromate, ex-

posing said surface to light under a design and selectively disintegrating a portion of the exposed hydrolyzed layer with a solution of a watersoluble chlorite.

5. The method of forming a design in a cellulosic sheet for printing purposes, which comprises impregnating a hydrolyzed surface layer of a surface hydrolyzed cellulose organic acid ester sheet with ammonium 'bichromate, ex-

posing said surface to light under a design. se-

lectively disintegrating a portion of said exposed hydrolyzed surface with a solution of a water-soluble chlorite and selectively removing the disintegrated portions by rubbing with a moist pad.

6. The method of forming a design in a cellulosic sheet for printing purposes, which comprises impregnating a hydrolyzed surface layer of a surface-hydrolyzed cellulose ester of a carboxylic acid sheet with ammonium chromate. exposing said surface to light under a design. and removing only the exposed portions of said hydrolyzed surface by treatment with a solution of sodium hypochlorite.

7. The method of forming a design in a cellulosic sheet for printing purposes, which comprises impregnating a hydrolyzed surface layer of a surface hydrolyzed sheet of cellulose ester of a carboxylic acid with ammonium bichromate, exposing said surface to light under a design and removing only the exposed portions of the hydrolyzed surface by treatment with a solution of sodium chlorite.

8. The method of forming a design in a eellulosic sheet for printing purposes which comprises impregnating a hydrolyzed surface layer of a surface hydrolyzed sheet of cellulose ester of a carboxylic acid with an alkali-metal salt 40 of a chromic acid, exposing said surface to light under a design, and removing only the exposed portions of the hydrolyzed surface by treatment with a solution of sodium hypochlorite.

45 RALPH S. COLT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,943,388 Ostwald Jan. 16, 1934 2,020,901 Miller Nov. 12, 1935 56 2,043,905 Seymour June 9, 1936 FOREIGN PATENTS Number Country 7 Date 509,010 Germany Oct. 4, 1930 337,888 Great Britain Nov. 13, 1930 60 505,752 Great Britain Nov. 27, 1044