United States Patent 3,055,295 PLANOGRAPHIC PRINTING PLATES Glenn H. Perkins, West Peru, Maine, assignor to The Oxford Paper Company, Rumford, Maine, a corporation of Maine No Drawing. Filed Sept. 12, 1960, Ser. No. 55,191 Claims. ((11. 101-149.2)

This invention relates to planographic printing plates and the method of making the same. More particularly, this invention relates to a planographic printing plate comprising a base having adhered thereto an insolubilized coating comprising a pigment and the reaction product of a hydrophilic colloid and a cross-linking agent, said pigment containing a colloidal silica having a particle size between about 7 to about 30 millimicrons.

This application is a continuation-in-part of my copending applications, Serial No. 775,685, filed November 24, 1958, now abandoned, and Serial No. 852,211 filed November 12, 1959', now abandoned.

Planographic printing plates are made by applying to a base web or sheet a coating having, among other things, desirable hydrophilic-oleophilic properties, whereby a greasy composition may be retained, to form an inkreceptive image, and an aqueous etch or fountain solution may be retained by the remainder of the plate to form an ink-repellent surface. Such plates may be mounted on a roller of an offset printing press, for example, and caused to pass successively in contact with a fountain solution and a printing ink. Ideally, the ink is totally rejected by the plate, except for the imaged areas thereof to which the ink adheres. The inked image is then transferred to a blanket, for image reversal, and applied to the sheet to be printed.

The making of superior planographic printing plates requires delicate balancing of a variety of coating characteristics, some of which tend to conflict with others. By way of example, the coating must accept and retain a sharp greasy image, While at the same time accepting and retaining the aqueous fountain solution. The fountain solution must be retained in such manner as to avoid undermining of the image in prolonged use. The coating must permit of the making of erasures, when necessary, without causing halos or other undesired images to appear. The image must remain sharp throughout prolonged use, and not tone. Stop-go characteristics, that is, the ability to print satisfactory first copies after a period of machine shut-down, must also be good. These and other characteristics must be properly balanced to provide a superior plate, due regard being had for ease of manufacture and overall cost.

The present invention provides .an improved planographic printing plate, which is superior in the characteristics above mentioned, among others, which is capable of economical manufacture, and which may be varied easily to accommodate a variety of specific end uses.

The use of colloidal silica having a particle size between 7 to about 30 millimicrons accounts for the superior planographic plates prepared according to this invention. Moreover, the colloidal silica greatly assists the insolubilization of the hydrophilic colloids so that a substantiallylower temperature, as described hereinafter, may be used to insolubilize these colloids. One specific advantage of this invention involves the use of a particular adhesive, polyvinyl alcohol, which is at least 88% hydrolyzed. When this polyvinyl alcohol is employed according to this invention, it has been found that the colloidal silica not only assists in the insolubilization of the polyvinyl alcohol, but it also has the eiiect of offsetting the oleophilic properties of the unhydrolized fraction of the polyvinyl acetate that remains in the polyvinyl alcohol. Consequently, a proper oleophilic-hydrophilic balance is obtained in the resultant planographic printing plate.

Another specific advantage of this invention resides in the fact that unmethylolated polyacrylamide may be employed as the adhesive according to this invention. Generally, when polyacrylamide is employed as the adhesive component of a coating composition, it is necessary to methylolate the polyacrylamide in order that an insolubilized coating can be obtained. The presence of the colloidal silica permits the use of the unmethylolated polyacrylamide.

Broadly, the improved planographic printing plates of this invention are prepared by forming an aqueous coating composition comprising a pigment, a hydrophilic colloid and a cross-linking agent, said pigmet containing a colloidal silica having a particle size between about 7 to about 30 millimicrons. This aqueous coating composition is then applied in a conventional manner to the surface desired, usually a wet strength paper 'web, and the coating dried, advantageously at a pH of about 4 to 5, to form an insoluble planographic coating.

The colloidal silica which can be used to form the pigment mixture of this invention must contain silica particles of very small size. The main average size of the silica particles must be within a range of from about 7 millimicrons to about 30 millimicrons. Silica particles greater than about 30 millimicrons are not suitable for use in making planographic printing plates according to this invention. For example, when silica particles of about 65 millimicrons in size are used, the planographic plates so produced are commercially and practicably unacceptable. Although colloidal silicas containing silica particles of less than 7 millimicrons have not been commercially available, it is believed that the coating compositions of this invention may be advantageously prepared from colloidal silicas containing silica particles of a size even less than 7 millimicrons.

The most advantageous colloidal silicas are those which contain silica particles ranging in size from about 7 to 17 millimicrons. Such colloidal silicas are marketed by E. I. du Pont de Nemours and Company under the trade names Ludox S. M., Ludo-X L. S., and Ludox H. S. and also by the Monsanto Chemical Company under the trade name Syton 200. The types of colloidal silicas and methods of making them are described in detail in United States Patents No. 2,244,325 issued June 3, 1941, No. 2,574,902 issued November 13, 1951, and No. 2,597,872 issued May 27, 1952. Such colloidal silicas are generally marketed as a 30% SiO dispersion, but as described in the above patents the colloidal silica can be prepared containing a higher or lower percent silica.

The pigment used in the coating compositions is advantageously a mixture of clay and colloidal silica. The ratio of clay to silica may vary quite widely from about 10:1 to about 1:1 depending upon the properties desired in the finished plate, the particular adhesive employed, the presence of other inert materials or pigments in minor proportions, and so forth.

Various known coating or filler clays can be used to form the pigment mixture according to this invention. Clays of small particle size are preferred since they seem to produce better results. Some examples of clays which can be used include various types of kaolin or china clays, Lustra Clay and so forth. Lustra Clay marketed by the Southern Clay Company has been found to be particwlarly advantageous.

Although it is preferred to use a clay along with the silica in the pigment mixture, other pigments may be used in place of, or in addition to, the clay, such as calcium silicate, barium sulfate, titanium dioxide, etc.

If desired, dispersing agents may be employed along with these pigment mixtures, such as sodium hexametaphosphate, disodium dihydrogen pyrophosphate, tetrapotassium pyrophosphate, sodium silicate, etc. Generally in a particular coating composition, there is an optimum ratio of pigment to adhesive and experiments indicate that, overall, the pigment-adhesive ratio should preferably be between about 7:1 and 3:1.

Various adhesive materials may be used according to this invention including polyacrylamide, methylol polyacrylamide, oxidized corn starch, polyvinyl alcohol, guar gum (ground endosperms of Cyalzopsz's tetragonoloba), Irish moss (dried plant of seaweed Chondrus crispus), chitosan (deacetylated chitin), superlose (100% amylose starch fraction), amioca starch (100% amylopectin), sodium carboxymethyl cellulose, dextrin, dextran, corn hull gum (acidic arabinoxyloglycan, a water soluble hydrocolloid obtained by lime water extraction of the hull of the corn kernel and may be classed as a hemicellulose), gum ghatti (calcium salt of an acidic polysaccharide, ghattic acid), tamarind kernel powder (a polysaccharide obtained from the seed kernels of the tamarind tree, Tamarindzts indica), and other hydrophilic adhesives having active cross-linking points and which are suitable for use in planographic coatings.

When polyvinyl alcohol is used as the adhesive, it should be substantially hydrolyzed, as will be explained in further detail and experimental results indicate that the percentage of hydrolysis should be above 88%. The viscosity of the polyvinyl alcohol solution may be varied quite widely depending on the desired viscosity of the coating composition for proper application to the base stock. Viscosities of from between 4 and 66 have been advantageously used. Unless otherwise indicated herein, polyvinyl alcohol viscosity is referred to in terms of centipoises of a 4% water solution at 20 C., as determined by the Hoeppler falling ball method. Polyvinyl alcohol is generally obtainable and applied in aqueous solution. Various commercial grades are suitable for the purpose such as Elvanol, manufactured by E. I. du Pont de Nemours and Company, and Lemol, manufactured by The Borden Company.

Advantageously, the cross-linking agent used in the composition is a dimethylol urea, and the ratio of the cross-linking agent to the adhesive may advantageously be from approximately 10% to 70% and preferably 50%, based on the adhesive. Other suitable cross-linking agents can also be used, such as trimethylol phenol, urea-formaldehyde, N,N'-ethylene dimethylol cyclic urea marketed by Monsanto Chemical Company under the trademark Scriptite 45, melamine formaldehyde and other aminealdehyde compounds, glyoxal, other dialdehydes, and so forth.

A catalyst is added to the composition, to promote curing, and the catalyst may be a salt, such as zinc phenol sulfonate, zinc sulfamate, zinc silicofluoride, zinc chloride, zinc acetate, alum, ammonium chloride, ammonium sulfate, and ammonium phosphate, as well as salts of other di-valent and tri-valent metals, e.g., lead, calcium, manganese, cerium, lanthanum. In addition, it is frequently desirable to add an appropriate amount of acetic acid, which serves to thin the composition to the desired final consistency, if thinning is necessary, and in some cases is useful in lowering the pH of the composition. If zinc phenol sulfonate is used as a catalyst, it is usually unnecessary to use acetic acid and, for this reason, zinc phenol sulfonate is a particularly desirable catalyst.

Coating compositions according to the invention are advantageously prepared by first preparing the pigment 4 tipoises, Brookfield. The pH can be adjusted by adding acetic acid to bring it down to about 5-6.

The coating composition may be applied to a base web or sheet by conventional apparatus, such as air knives or roll coaters. The amount of coating may vary from about 2 to 10 pounds per ream of 3300 sq. feet, with 6 to 8 pounds being most satisfactory. After coating, the stock is dried and cured at a suitable temperature. Temperature as low as 140 F. and as high as 350 F. have been used, although even lower or higher temperatures may also be used. The curing time may advantageously range from about 2-20 seconds, or more, as may be determined by one skilled in the art. The coating composition is self-insolubilizing, and the components react during the drying and curing cycle to produce a practically water impervious film which possesses excellent properties of accepting and holding both water and greasy ink.

The following examples illustrate the manner of preparing the improved planographic plates of this invention in more detail. The following coating composition mixtures were prepared and coated on a base paper and dried. The resulting planographic printing plates had low water absorbency and possessed an excellent hydrophilic-oleophilic balance permitting good image adherence and good image life while at the same time permitting good water retention. These plates produced clean copy, good toning, and possessed good image, correction of life, and stopgo properties. Parts are by weight. The colloidal silica used was Syton 200 and had a particle size of about 15 millimicrons.

Example 1 Parts Methylol polyacrylamide (6%) Lustra clay (60%) 27 Colloidal silica (30%) 50 Dimethylol urea (100%) 1.5 Alum 2 Example 2 Parts Oxidized corn starch (20%) 100 Lustra clay (60%) 50 Colloidal silica (30%) 100 Dimethylol urea (100%) 4 Alum 2 Example 3 Parts Guar gum (10%) 100 Lustra clay (60%) 33 Colloidal silica (30%) 66 Dimethylol urea (100%) 4 Alum 2 Example 4 Parts Irish moss (5%) 100 Lustra clay (60%) 16 Colloidal silica (30%) 34 Dimethylol urea (100%) 2 Alum 1 Example 5 Parts Chitosan (8% in 5% acetic) 100 Lustra clay (60%) 50 Colloidal silica (30%) 33.3 Dimethylol urea (100%) 4 Zinc acetate 2 Water 50 Example 6 Parts Superlose (18%) 55 Lustra clay (60%) 38 Colloidal silica (30%) 25 Example 6-Continued Parts Dimethylol urea 100%) 2 Zinc acetate 2 Acetic acid 1 Example 7 Parts Amioca starch (20%) 100 Lustra clay (60%) 75 Colloidal silica (30%) 50 Dimethylol urea (100%) 4 Zinc acetate 4 Acetic acid 1 Example 8 Parts Sodium carboxymethylcellulose (5%) 100 Lustra clay (60%) 33 Colloidal silica (30%) 16.7 Dimethylol urea (100%) 3.5 Zinc acetate 4 Zinc borate 2 Example 9 Parts Dextrin (30%) 67 Lustra clay (60%) 83 Colloidal silica (30%) 100 Dimethylol urea (100%) 6 Alum 2 Acetic acid 11 Example 10 Parts Dextran (7.35%) 109 Lustra clay (60%) 4 2 Colloidal silica (30%) 50 Dimethylol urea (100%) 4 Alum 2 Acetic acid 1 Butyl alc. 5

Example 11 Parts Polyacrylarnide (5%) 160 Lustra clay (60%) 42 Colloidal silica (30%) 50 Dimethylol urea 100%) 4.5 Alum (10%) 27 Acetic acid (50%) 2.5

Example 12 Parts Corn hull gum (8%) 100 Lustra clay (60%) 42 Colloidal silica (30%) 50 Dimethylol urea (100%) 4 Zinc acetate (25%) 24 Acetic acid (50%) 2 Butyl alc. 5

Example 13 Parts Gum ghatti (8%) 100 Lustra clay (60%) 42 Colloidal silica (30%) 50 Dimethylol urea (100%) 4 Zinc acetate (25 24 Acetic acid (50%) 2 Butyl alcohol 5 Example 14 Parts Tamarind kernel powder (5 100 Lustra clay (60%) 26.3 Colloidal silica (30%) 31.3 Dimethylol urea (100%) 2.5 Zinc acetate (25%) 15 Acetic acid (50%) 2 Butyl alcohol 5 Water 50 In the above examples it was found that the presence of the colloidal silica in the color permitted insolubilization of the color at substantially lower temperatures than would have been required in the absence of the silica. For example, the color formulation of Example 11 will readily cure at a temperature of about 200 F. In the absence of the colloidal silica, however, the formulation will not cure even at 350 F. for 45 seconds. The color of Example 2 will not cure even at 350 F. when colloidal silica is not present. When colloidal silica is present in a silica/ clay ratio of about 3:5, the color Will readily cure at 200 F. A polyvinyl alcohol-clay formulation will cure well at 350 F. in 20 seconds, but only to a fair degree at 260 F. When colloidal silica is present, however, in a silica/clay ratio of about 3:5, the color will readily cure at 140 F. in 20 seconds.

The following Examples 15-18 specifically relate to the use of polyvinyl alcohol as the adhesive. The specific proportions of pigment and adhesive necessary to achieve best all-around characteristics depend somewhat on the viscosity and degree of hydrolysis of the polyvinyl alcohol adhesive.

Example 15.A series of mixtures was prepared, using high viscosity (about 51), 98% hydrolyzed polyvinyl alcohol as marketed by du Pont under the trade name Elvanol 72-51. 100 grams of an 8% solution of the polyvinyl alcohol Was used in each instance, along with about 8 grams of (50%) Scriptite 45, 2 grams of alum and about 2 grams of 50% acetic acid. Lustra clay (60% Tests of sheets prepared according to the above indicated that the proportions listed in column C give the best results using Elvanol 72-51 adhesive. With too little Syton 200 in the composition, there was a tendency to tone at start up, and copies were not as clean as desired. With excessive amounts of Syton 200, the cured composition tended to be too hard, causing shorter correction life and increased tendency to halo.

Example 16.A series of mixtures, similar to those of Example No. 1, was prepared, using medium viscosity (about 24), 98% hydrolyzed polyvinyl alcohol was marketed by duPont under the identification Elvanol 71-24. 80 grams of a 10% solution of the polyvinyl alcohol was used, along with a cross-linking agent and catalyst. Six mixtures were prepared, using amounts of clay and Si0 identical to those specified in Example '15, as follows:

A B O D E F E1vano171-24 (10%) 80 80 80 80 80 8O Lustra Clay (60%)-- 53 53 53 40 40 40 Syton 200 0 28 53 80 108 Pigment/adhesive 4/1 5/1 6/1 6/1 7/1 8/1 Clay/SiOz 1/0 4/1 2/1 1/1 3/4 3/5 1 Also using Elvanol 72-60, which is 99% hydrolyzed and has a viscosity of about 60.

Elvanol 70-05 (20% Lustrawcolay (60%).-.

Test sheets made with the above mixtures were, in general, satisfactory but somewhat less so than sheets of Examples and 16. Stop-go characteristics tended to be poorer with mixtures A, B and C, and some halos were observed with mixtures D, E and F. Paper ribbon correction life was not good with mixture A.

Example 18.A series of mixtures, similar to the previous examples, was prepared using medium viscosity (about 24) 88% hydrolyzed polyvinyl alcohol, as marketed by du Pont under the identification Elvanol 5 222. 80 grams of a 10% solution of the polyvinyl alcohol was used, along with a cross-linking agent and catalyst. Six mixtures were prepared, as follows:

Tests of sheets made with the above mixtures indi cated that substantial toning occurred with mixture A, diminishing with increased amounts of Syton 200 until only a faint haze was observed with mixture F. However, at the high silica level of mixture F, both fabric ribbon and paper ribbon correction life are not satisfactory.

The toning tendency observed in Example 18 is probably caused by the presence in the adhesive of substantial acetate groups, which renders the adhesive excessively oleophilic. With greater hydrolysis, more acetate groups are replaced by hydroxyl groups, so that the adhesive is less oleophilic, and desired oleophilic-hydrophilic balance can be achieved by adding colloidal silica, without destroying correction life and other desired characteristics.

As a result of tests conducted using base stock coated with the mixtures of the above examples, certain conclusions are warranted, as follows: (a) The polyvinyl al cohol adhesive should be more than 88% hydrolyzed, and advantageously about 98% or 99% hydrolyzed. (b) The viscosity of the adhesive is not too critical, if within the range of about 66 to about 4, although different pigment/adhesive and clay/Si0 ratios may be desirable with adhesives of different viscosity. (c) Clay/SiO ratios should be less than 10/1 but not substantially less than l/1 and, apparently, the optimum ratio increases with decreasing viscosities. (d) Optimum pigment/ adhesive ratios apparently increase with increasing clay/SiO- ratios, and should preferably be less than 7/1 but greater than 3/1.

Substantial variation of the color formulations and procedures used according to this invention is possible depending upon the coating machine requirements, and requirements of the printing plate as to length of run, starting and stopping problems, etc. However, in all cases, the planographic printing plates made according to this invention produce clean, sharp copy, with good correction, toning, stop-go, and other desirable characteristics.

I claim:

1. A planographic printing plate comprising a base and a planographic printing surface thereon, said surface comprising a pigment mixture and the reaction product of a hydrophilic colloid adhesive and a cross-linking agent, said pigment mixture containing colloidal silica having a particle size of between about 7 to 30 millimicrons.

2. The planographic printing plate of claim 1 in which the colloidal silica comprises between about 10% to about 50% of the pigment mixture.

3. The planographic printing plate of claim 1 in which the adhesive is polyacrylamide.

4. The planographic printing plate of claim 1 in which the adhesive is polyvinyl alcohol which is more than 88% hydrolyzed.

5. The planographic printing plate of claim 1 in which the cross-linking agent is dimethylol urea.

6. The method of making a planographic printing plate which comprises applying a planographic printing surface to a base by coating said base with an aqueous coating composition comprising a hydrophilic colloid adhesive, a cross-linking agent, and a pigment mixture, said pigment mixture containing colloidal silica having a particle size of between about 7 to 30 millimicrons, and reacting the hydrophilic colloid with the cross-linking agent in the presence of the pigment mixture to form an insoluble planographic coating on said base.

7. The method of claim 6 in which the colloidal silica comprises between about 10% to about 50% of the pigment mixture.

8. The method of claim 6 in which the adhesive is polyacrylamide.

9. The method of claim 6 in which the adhesive is polyvinyl alcohol which is more than 88% hydrolyzed.

10. The method of claim 6 in which the cross-linking agent is dimethylol urea.

Worthen Dec. 19, 1950 Wolf et al. Sept. 6, 1960