US3115829A - Planographic printing plates - Google Patents

Description

United States Patent Ofilice $513212 3,115,829 PLAN ()GRAPHIC PRINTING PLATES John B. Mazanek and Leon J. Paquin, Glens Falls, N.Y., assignors to International Paper Company, New York, N.Y., a corporation of New York No Drawing. Filed July 24, 1962, Ser. No. 215,233 7 Claims. (Cl. 101149.2)

This invention relates to planographic printing plates or masters formed of coated paper for use in copying machines in which a graphic or printed image placed on the printing plate is imparted to copies, and constitutes an improvement in the article and process covered in copending application Serial No. 769,984, filed October 28, 1958, now abandoned. This application is a continuationin-part of co-pending application Serial No. 777,853, filed December 3, 1958, now abandoned.

Planographic reproduction or printing is a modern form of lithography in which an inked image is placed on a master plate having a plane surface, i.e. neither intaglio nor relief, and transferred to copies. In the early art, smooth stones were used as plates or masters but in modern work the plates may be metal and in recent years have been made of paper suitably coated.

Office copying machines operating on this principle may be used to make ten or more copies known as short run work, and several hundred to several thousand reproductions are known as medium run and long run work.

A greasy or waxy imprint capable of receiving ink is imprinted, as by a typewriter, on the printing plate surface. The imprint receives ink but repels water. The surrounding area, when Wetted, repels ink. Thus after the imprinted printing plate is Wetted with aqueous solution, an inking roll may be run over it and the ink is received and held for one reproduction on the imprinted image but repelled by the water on the remainder of the plate surface. This inked image is then brought in contact with an offset blanket or roll which receives the image in reverse from the printing plate. The offset blanket is then brought in contact with a copy sheet and the image is transferred in its original aspect to the copy sheet. The master is re-wetted and re-inked and the process repeated to make each successive copy.

The printing plate has to have a surface which receives the imprint and which readily receives and retains a film of water or substantially aqueous lithographic solution known as etching solution and fountain solution on the un-imprinted part; in other Words, it has to be hydrophilic.

The printing plates must take a good imprint for receiving ink and reproducing the image, and the surrounding surface must receive and retain a good aqueous film and must completely repel ink. At the same time the coating which provides these characteristics must have good rub resistance. That is, it must remain insoluble when wet so that it will not rub off. The surface must also repel pick up of dirt and fingerprint marks which would otherwise receive and reproduce ink and dirt in the form of smudges.

The printing plates must be inexpensive to make and must be capable of retaining their desired above described characteristics during storage periods before use and must maintain them while reproducing the desired number of copies.

It is accordingly an object of the present invention to provide a paper base printing plate which has the above described characteristics and which is economical to manufacture on a quantity and quality basis.

These and other objectives have been accomplished with the use of essential but inexpensive and commonly available and easily applied coating ingredients, which have heretofore been considered unworkable in forming printing plates.

A suitable paper base for use as a printing plate must have stiifness and body so that it can be handled in the printing press, wet strength so that it stands up under the wetting of the etching and fountain solutions, good dimensional stability for accurate registry and should lie flat, i.e. avoid curling when wetted. These characteristics may be embodied by means known to those skilled in the paper making art.

It has been previously found that printing plates may be made, using a suitable paper base, by coating the paper with an aqueous slurry having as essential components finely divided mineral pigment such as paper coating clay, and a Water insoluble adhesive or binder for the pigment. Certain starches have been found to be satisfactory as adhesive when converted and treated so as to reduce viscosity. The untreated starch is so heavy bodied that when introduced into a coating slurry, the slurry will not flow freely enough to be applied as a coating film. While some converted starches have been found to work it has been stated, for instance in Patent No. 2,724,665 that other converted starches including enzyme-converted starch and oxidized or chlorinated starch will not form a coating useful for a planographic printing plate.

We have found, to the contrary, that oxidized starch, starch acetate esters, enzyme-converted starch, dextrins and acid modified starches, when incorporated in a coated slurry according to our present invention, provide an entirely satisfactory and desirable printing plate.

As a principal constituent of the hydrophilic coating, we prefer to use a mixture of paper-coating clay and a mineral ingredient, preferably Zinc oxide, which mixes with one of the above-mentioned starches.

The Zinc oxide, when added to the clay, improves the hydrophilic characteristics of the coating and causes the plate to resist smudging and toning which is ink pickup outside of the image.

Suitable substituents for the zinc oxide are blanc fixe, talc (coating grade), barytes, titanium dioxide and cal cium carbonate pigments.

An insolubilizing agent for the starch is provided in the form of an amino-aldehyde condensate of the water soluble type, preferably an urea-formaldehyde concentrate consisting of a Water solution of about 26% urea, about 59% formaldehyde and about 15% water as manufactured by the Nitrogen Division of Allied Chemical and Dye Corporation under the name U.F. Concentrate 85.

Suitable substitutes for the urea-formaldehyde water soluble condensate are dimethylol-urea and melamineformaldehyde water soluble condensate.

These condensates are partially reacted resin forming components which go into the solution with the starch in the slurry prior to reaction with the starch to form a water insoluble adhesive or binder to hold the finely divided pigment material as a homogeneous film coating on the paper.

A catalyst for the insolubilizing reaction between the starch adhesive and the water soluble type condensate is added directly to the coating composition in the form of sodium aluminate. Contrary to the teaching of the prior art that a satisfactory insolubilizing reaction should take place at a low pH, this reaction takes place satisfactorily at a high pH. The paper is then force dried as by passing it through conventional drying equipment and the elevated drying temperature allows the sodium aluminate to catalyze the insolubilizing reaction, still at the elevated pH.

The product of this first coating gives a surface which has greatly improved resistance to fingerprint reproduction, particularly after ageing. But at this point, although the coating has been completely insolubilized, the surface has low ink receptivity because of the high alkalinity of the coating.

This condition is cured by applying an acidic overwash coating in a minimal amount just sufiicient to lower the pH of the surface of the base or prime coating to the acid side. Such an over-wash coating can be comprised of an acid solution made up of acid in water or a salt in water. Specifically, it can be made up of a weak acid, an acid salt, or a normal salt (e.g., made from a strong acid and a weak base) in water. In the category of normal salts, it has been found that both monovalent and bivalent salts are useful and that, preferably, ammonium chloride or zinc chloride should be used in concentrations of about 5% to A preferred example of a paper coated according to this invention is a starch surface-sized sheet first coated with an aqueous coating slurry of the following composition in which the ingredients are expressed as approximate parts by weight:

Paper coating clay 250 Zinc oxide (Green Seal) 100 Oxidized starch (as a solution) 105 U.F. concentrate 85 37.80 Sodium aluminate 18.90

Water, to make a solids content of approximately (depending on type of application) The above slurry may be applied to the paper by conventional means such as an air knife and applied in an amount between 6 and l2# per ream based on a ream of 500 sheets 24 x 36". The above coating is force-dried as by passing it through conventional drying equipment.

As a second or overwash coating, ammonium chloride or zinc chloride at 10% solution is coated on the paper over the first coating in an amount just sufiicient to wet the paper and to reduce the pH of the first coating below 7.0.

If there is any reaction between the chloride overwash and the adhesive of the first coating, it is ineffectual with respect to producing the insolubility of the coating because the insolubilizing reaction has taken place prior to application of the chloride solution and because the chloride applied is insufficient to provide an effective rev action. The sole purpose of the chloride is to reduce the pH of the coating and probably any reaction which takes place in doing so is with the sodium aluminate.

The clay to zinc oxide ratio in the above formulation may be varied from 1:1 to 1:0. The starch content may be varied from 20 to 100% based on the total pigment (clay and zinc oxide). The water soluble amino-aldehyde condensate may be varied from 10 to 200% based on the starch content.

Suitable substitutes for the oxidized starch are starch acetate esters such as the National Starch Ko-Films, enzyme-converted corn, tapioca or potato starches, dextrins and acid modified starches (the so-called thin boiling starches).

The coated paper base printing plates made according to this invention are particularly well suited for short run work but are capable, when made according to the above description, for use in longer run work. Of particular interest in this regard are comparative tests run with the plates of the present invention made with the zinc chloride overwash and commercially available plates manufactured under US. Pat. No. 2,534,650 to Worthen. Using a standard duplicating machine, commercially available offset ink (M-36), commercially available fountain solution concentrate A/ M Repelex) diluted with water in a ratio of about 1 part of concentrate to 7 parts of water, and two different commercially available etch solution (A. B. Dick Company Blue and Platex), it was found that the plates performed equally well with either etch and that the resulting copy quality could be rated as good overall. Moreover, it was found that the plates of the present invention performed especially satisfactorily without re-etching, despite short interruptions in running them on the machine.

More particularly, it was found that, when a cotton ribbon was used on the typewriter which imaged or inscribed the plates, the plates of the present invention yielded 1750 satisfactory copies, while the Worthen plates yielded 2000 satisfactory copies; when a silk ribbon was used, the present plates yielded 1750 satisfactory copies, while the Worthen plates yielded 900 satisfactory copies; and, when a carbon ribbon was used, the present plates yielded better than 3000 satisfactory copies, while the Worthen plates yielded 5000 satisfactory copies. Again, when a reproducing ball point pen was used for the imaging, rather than a typewriter, the present plates picked up a weak image, while the Worthen plates picked up none. When a reproducing pencil was used, the present plates yielded 2000 satisfactory copies and the Worthen plates yielded 900 satisfactory copies. And, when a reproducing crayon was used, the present plates yielded 1500 satisfactory copies and the Worthen plates yielded 800 satisfactory copies.

Upon correction of a carbon ribbon image, it was found that the present plates yielded good correction, while Worthen plates retained a very slight old image.

Using a commercially available machine which utilized electrostatic imaging means, it was found that the plates of the present invention fed into the machine better than competitive masters and could be imaged at the lowest fuser settings. The present plates did not curl, jam, or scorch, gave sharper images, and gave a minimum background tone.

Otherwise, it has been found that the coating of this invention can be applied after first treating the paper base with a barrier coat for particular suitability to long run work.

What is claimed is:

1. Process of making a planographic printing plate having a water-insoluble hydrophilic surface which comprises applying to a paper base an aqueous slurry coating consisting essentially of finely divided mineral pigment, a starch selected from the group consisting of oxidized starch, starch acetate esters, enzyme converted starch, dextrins and acid modified starches, an amino-aldehyde condensate of the water-soluble type and sodium aluminate as a catalyst, then drying the coating thereby producing a reaction between the starch and the condensate at a pH above 7.0 with the sodium aluminate acting as a catalyst to produce a water-insoluble adhesive component for the mineral pigment, and then, the further step, after producing said insolubilizing reaction, of applying as an over-wash coating an acid solution, said over wash coating being applied in a minimal amount and strength just sufficient to reduce the pH of the coating to a point below 7.0.

2. The process as set forth in claim 1 in which the acid solution consists of a solution of a weak acid.

3. The process as set forth in claim 1 in which the acid solution consists of a solution of an acid salt.

4. The process as set forth in claim 1 in which the acid solution consists of a solution of a normal salt.

5. The process as set forth in claim 1 in which the 5 6 acid solution consists of a solution of a normal salt of References Cited in the file of this patent a blvalemmetal- UNITED STATES PATENTS 6. The process as set forth in claim 4 in which the acid solution consists of a solution of ammonium chloride. 2,156,100 Slmons P 1939 7. The process as set forth in claim 5 in which the 5 2,534,650 W Orthen 1950 acid solution consists of a solution of zinc chloride. FOREIGN PATENTS 772,112 Great Britain Apr. 10, 1957

Claims (1)

1. PROCESS OF MAKING A PLANOGRAPHIC PRINTING PLATE HAVING A WATER-INSOLUBLE HYDROPHILIC SURFACE WHICH COMPRISES APPLYING TO A PAPER VASE AN AQUEOUS SLURRY COATING CONSISTING ESSENTIALLY OF FINELY DIVIDED MENERAL PIGMENT, A STARCH SELECTED FROM THE GROUP CONSISTING OF OXIDIZED STARCH, STARCH ACETATE ESTERS, ENZYME CONVERTED STARCH, DEXTRINS AND ACID MODIFIED STARCHES, AN AMINO-ALDEHYDE CONDENSATE OF THE WATER-SOLUBLE TYPE AND SODIUM ALUMINATE AS A CATALYST, THEN DRYING THE COATING THEREBY PRODUCING A REACTION BETWEEN THE STARCH AND THE CONDENSATE AT A PH ABOVE 7.0 WITH THE SODIUM ALUMINATE ACTING AS A CATALYST TO PRODUCE A WATER-INSOLUBLE ADHESIVE COMPONENT FOR THE MINERAL PIGMENT, AND THEN, THE FURTHER STEP, AFTER PRODUCING SAID INSOLUBILIZING REACTION, OF APPLYING AS AN OVER-WASH COATING AN ACID SOLUTION, SAID OVERWASH COATING BEING APPLIED IN A MINIMAL AMOUNT AND STRENGTH JUST SUFFICIENT TO REDUCE THE PH OF THE COATING TO A POINT BELOW 7.0.