US2302817A - Improvement in printing plates - Google Patents

Description

Nov. 24,1942. w, C. mgm Em 2,302,817

IMPROVEMENT IE PRINTING PILATES Fi'ledauly 2. 1941 Patented'Nov. 24, 1942 yIMPROVEMENT IN PRINTING PLATES William Craig Toland, Brookline, Mass., and Benjamin B. Burbank, Brunswick, Maine, assignors to said Toland, trustee Application July 2, 1941, serial No. 400,744 s claims. l(o1. 41-41.5)

This invention relates to printing and more especially to printing plates of the planographic type and to methods of preparing these plates.

In making planographic printing plates, it is customary to apply, on a surface which can be rendered water-receptive, a coating material which includes a light-sensitive salt and which,

4 upon exposure to photographic light, may be selectively, hardenedY to form printing areas.

plied.

It has been proposed to use vinyl compounds, and in particular polyvinyl alcohol, in aqueous suspension with a light-sensitive salt suchas ammonium bichromate, as a substitute for convventional colloids now employed, to form such coatings. In practice the use of polyvinyl alcohol, as a substitute for a, colloid printing coating, has not been entirelyr satisfactory. Printing portions of polyvinyl alcohol, when subjected to the action of Water rollers of a lithographie printing press, tend to disintegrate \toa very Islight extent, which allows small `,amounts of moisture to get in and repel greasy ink, with the result that unsatisfactory ink impressions are obtained. Also the polyvinyl., alcohol is not as satisfactorily hardened by light-sensitive salts to a grease-receptive state as is the case with other colloids, and the printing portions refuse to adhere to a water-receptive lithographie base sufficiently firmly to stand up under any substantial period of operations.

An object of the invention, therefore, is to improve printing plates and methods of preparing printing plates of the'planographic type, and to devise a printing coatinglm'aterial of a special type of polyvinyl alcohol with a viewto overcoming the above noted dilculties. It ls a further object of the invention to provide a simple, cheap, and eicient printing plate.

The nature of the invention, and its objects, will bemore fully understood from lthe descrip- .tion of the drawing and discussion relating there- In thev accompanying drawing:

Fig. 1 is an enlarged fragmentary cross section of a planographic plate of the invention.

Fig. 2 is anv enlarged fragmentary cross section of a. modified printing Plate.

Fig. 3 is an enlarged fragmentary cross sec- Fig. 4 is an enlarged fragmentary cross section of still another form of printing plate.

According to the invention, vinyl compounds and especially polyvinyl alcohols are employed to form planographic printing surfaces. Polyvinyl alcohol is prepared from its. lymeric esters by hydrolysis with an acid or a ali. Forexample, hydrolysis of polyvinyl acetate is carried out by passing hydrogen chloride into a boiling alcoholic solution of the ester or by boiling the alcoholic solution for several hours with a light sulphuric acid. As a result of a synthesis of this character, there is obtained a homologous series of polyvinyl alcohols, the simpler alcohols of which are soluble in 'cold water, and the more complex alcohols' of which are insoluble in cold water although tending to-swell and being readily soluble in hot water.

We have found` that a mixture of these coldwater-soluble and cold-water-insoluble alcohols is not suitable for some printing purposes and soluble alcohols cause difficulty.

that in using the mixture to form printing portions 4 washing. in cold water, and the remaining prodtion of still another type of printing plate; and55 uct is Iadapted to form a coating and eventually a solid or film. body, which is'more nearly`r impervious to the action of cold water in making and usingplanographic printing plates.

In carrying out the method of the invention, i

therefore, we provide a solution of polyvinyl alcohol, or a Anumber of polymers of polyvinyl alcohol,

of a relatively complex chain structure insoluble in cold Water. This. may be done for' example by washing a series of the polymers, in a finely divided state as obtained by the above not'ed synthesis, in cold water, and thereafter dissolving the residue in hot water, as for example water heated to C. or higher. Other procedures directed to controlling the synthesis of polyvinyl alcohol so as to produce only the cold-fwater insoluble polymers may alsolbe resorted to.

The solution of polyvinyl alcohols, free from cold-water-solubles, is then mixed with a lightsensitive salt in some convenient manner, as for example by mixing vwith a solution of ammonium bichromate. One suitable suspension consistsin 'Z oz. of a 5% suspension of polyvinyl alcohol in water mixed with 1 oz. of ammonium bichromate.

This light-sensitive mixture is then applied in the form of a coating o ver a surface which ls Water-receptive and adapted to constitute the non-printing portions of a planographic printing plate. Fig. 1 illustrates one form vof such a structure and includes a base I having a layer 2 of polyvinyl alcohol which, While insoluble in cold water, will absorb small amounts of moisture. The coating is applied over the layer 2, dried, exposed to photographic light, covered with a greasy developing ink and washed under running water. Washing removes unexposed portions of the coating and leaves grease-receptive printing portions 3. The use of polyvinyl alcohol to constitute the water-receptive layer 2 is described and claimed in an earlier Patent No. 2,230,981, issued February 4, 1941.

The combination of a water-receptive layer 2, having superimposed thereon printing portions 3 of a polyvinyl alcohol which is free from coldwater-solubles, may be associated with various other types of supporting means, as for example that illustrated in Fig. 2 in which sheets of paper 4, 5 and 6 are held together by waterproofing materials to provide a laminated and substantially non-stretchable base. 1

Fig. 3 illustrates another method of mounting printing portions of a polyvinyl alcohol free from cold-water-solubles on -a water-receptive surface. Numeral 1 refers to a metal base of zinc or aluminum, having a roughened surface 8 which, by conventional treatment with water and acid mixtures, may be rendered grease-receptive.

In Fig. 4 a further modified form of plate having printing portions composed of polyvinyl alcohol free from cold-Water-solubles is setforth.

In this modification, a printing plate of a continuous tone type is provided by utilizing a single coating obtained from a solution of polyvinyl a1- cohol free from cold-water-solubles, and containing a light-sensitive salt, to form both the printing portions and the non-printing portions. The coating may be applied on any suitable base as 9, and is exposed to photographic light passed through a negative to form hardenedv portions and unhardened portions. Thereafter the exposed portions which have been hardened are rendered grease-receptive to form printing portions I, while the unhardened portions are allowed to absorb small amounts of moisture yand remain in a water-receptive state, and constitute non-printing portions I l.

In making a plate of this continuous tone type, polyvinyl alcohols are employed which are of a polymerized character, which provides for limited Water-receptivity when not exposed to the hardening action of a light-sensitive salt therein. The developing procedure with a layer or coating are enabled to withstand additional application of moisture by the water rolls of a lithographie printing press without any of these printing portions becoming dissolved or disintegrated and washed away, with the result that scumming by such printing portions is substantially reduced of alcohol of this character provides for renderv ing the exposed or hardened portions greasereceptive by treatment with a greasy development ink in the usual way. Subsequent washing away of the developing ink in cold water allows moisture to become absorbed in the unexposed portions, thus loosening greasyfink material and providing for its being washed away. Washing also removes unexposed .light-sensitive material in the unexposed parts of the coating. In making the continuous tone type of plate, gelatin may also be mixed with the polyvinyl alcohol. This is especially desirable vas the gelatin tends to provide a reticulated surface which is helpful in continuous tone printing. f

An important feature of the invention is a printing plate of the planographic type containing printing portions of a polyvinyl alcohol which is free from cold-Water-solubles. By employing o1- eliminated. At the same time, the printing portions themselves are of a relatively tougher, harder character, and are more firmly attached to the base on which they are supported, which greatly increases the strength of the plate and allows relatively long runs to be carried out.

The relatively greater hardness of the printing portions, compared to the hardness of the printing portions obtained from polyvinyl alcohol containing cold-water-solubles, facilitates the exposing and hardening of the polyvinyl alcoholby the light-sensitive salt such as ammonium bichromate. printing portions containing cold-water-solubles, small amounts of the light-sensitive salt, which acts to harden the printing portions, may be washed away, thus decreasing the strength of the portions. Exposure of a polyvinyl alcohol coating containing light-sensitive salts is faster and more complete if free of cold-water-solubles. The printing portions obtained by the use of polyvinyl alcohol free from vcold-Water-solubles are also more readily rendered grease-receptive and continue to hold this greasereceptive character for extended periods.

Another important aspect of the method of making printing portions of polyvinyl alcohol which is free from cold-water-solubles consists in the elimination of frothing or foamingl when the coating is applied. Heretofore, upon coating polyvinyl alcohol in the form of a printing coating, considerable difficulty has been encountered, due to frothing or foaming rendering the resulting coating rough and non-uniform, and requiring very careful handling. This frothing action is almost entirely due to the presence of coldwater-solubles and is done away with upon using the relatively insoluble alcohol polymers.

Various other vinyl compounds occurring in a cold-water-insoluble state may be used and other characteristics, as viscosity and the like, improved by extracting a group of polymers with cold water or by isolating a small number of polymers of a generally similar character in other ways. Various other changes may be resorted to.

It will be observed that an improved substitute for the printing portions of a planographic printing plate has been provided, a method of successfully adapting vinyl compounds to serving as both the printing and non-printing portions of a planographic printing plate, has been set forth, improved sensitizing of vinyl compounds has been obtained, frothing or foaming eliminated, and a simple, cheap, and efficient type of printing plate provided.

While we have set forth a preferred embodiment of the invention, it should be understood It is pointed out that in the case of of. polyvinyl alcohol polymers containing no coldwater-solubles.

2. As an article of manufacture, a planographic printing plate comprising printing portions and non-printing portions, said printing portions comprising polyvinyl alcohol polymers free from cold-water-solubles, said polyvinyl alcohol polymers being hardened with a light-sensitive salt.

3. As an article of manufacture, a planographic printing plate comprising a base having a waterreceptive surface, grease-receptive printing portions overlying said water-receptive surface, said vinyl alcohol which is free from cold-'water- 6. A continuous tone printing plate comprising a base material having a coating which comprises polyvinyl alcohol free from cold-watersolubles, selected areas only ofsaid coatingbeing relatively harder than otherareas, the hardened areas being grease-receptive and the other areas being water-receptive, said grease-receptive areas of polyvinyl alcohol being free from cold- Water-solubles.

'7. That improvement in methods of making printing plates which comprises Washing polyvinyl alcohol polymers to remove cold-Watersolubles therefrom, and then forming grease- 'receptive printing portions of a planographic printing plate from the residue.

8. That improvement in methods of making planographic printing plates which comprises removing cold-water-solubles from a group of polyvinyl alcohol polymers, dissolving the residue l in warm water to provide a suspension, coating the suspension upon a lithographie support, and then forming grease-receptive printing portions on the-surface of the coated material.

WILLIAM CRAIG TOLAND. BENJAMIN B. BURBANK.

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