CA1044511A - Lithographic printing plate and method of making the same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A lithographic printing plate is disclosed which comprises a base and a lithographic printing surface thereon. The printing surface is formed of a positively charged colloidal silica and an insolubilized hydrophilic polymer which is a nonionic or cationic polymer, or a mixture thereof. The surface layer exhibits longer life and avoids the need for additional water-resistant barrier coats.

Description

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The present invention relates to li~hographia printing plates and to a method for making the same, More particularly, the present invention relates to a lithographic printing plate in which tt~e printing surEace or face coat is comprised of an insolubilized hydrophilic polymer and a positively charged or cationic colloidal silica.
The present invention is particularly applicable to the preparation of paper based li~hographic printing plates and will be described with reference to the same, although it will be apparent to those skilled in the art that the invention has other applications, for instance the preparation of a litho-graphic printing surface or face coat on a metal or other rLgid base.

BACKGROUND OF THE INVENTION

Paper based lithographic printing plates and methods for making the same have been well known for a considerable period of time. Lithography depends upon the immiscibility of a greasy lithographic printing ink and an aqueous etch or lithographic solution. In use, a paper lithographic printing plate is first imaged in a known manner with typed, written, or drawn copy material to be reproduced. The image may also be obtained in other ways, for instance by xerography; e.g.
Electrofax (Trademark, Radio Corp. of America) and Xerox (Trademark, Xerox Corp.). The grease-receptive imaging materiaL
employed makes the imaged areas ink receptive and water-repellent (i.e. hydrophobic). The remaining non-imaged surface is water-receptive and ink-repellent (i.e. hydrophilic).
The imaged plate is placed on a plate cylinder of an off-set Auplicating press~ The over-all surface of the plate is then treated with an aqueous wet-out liq~id which wets all portions oi the plate except those areas that have been imaged i~

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~0~5~1 and are water-repellent. The press inking roll then passes over the surface of the plate and deposits a film of ink only upon the ink-receptive imaged areas. In the printing operation, the ink from the imaged areas is transferred in reverse to a rubber off-set blanket which in turn prints directly onto a paper sheet so as to form a copy.
A number of different coating formulations and tech-niques have been utilized in the past to provide paper based sheets with suitable lithogra~hic printing surfaces. In this regard, the use of colloidal silicas is known. Prior United States patent No. 3,055,295 describes the preparation of litho-graphic printing plates 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, the pigment containing a colloidal silica having a particle size between about 7 and about 30 millimicrons. Exemplary silicas listed are those marketed by E. I. duPont de Nemours and Co. under the trade-marks Ludox S.M., Ludox L.S. and Ludox H.S.; and also by the Mon-santo Chemical under the trademark Syton 200. Exemplary hydrophi-lic colloids are polyvinyl alcohol, unmethylolated polyacrylamide . and others. The patent also lists a number of cross-linking agents which can be employed. Although the purpose of the cross-linking agent is to insolubilize the coating, it is stated in the patent that the colloidal silica greatly assists the insolubili-zation of the hydrophilic colloid so that a substantially lower temperature can be used to insolubilize the colloid. Other representative patents employing colloidal silicas are United States Patents numbers 2,681,617; 2,741,981; 3,220,346; 3,254,597;

3,270,667; and 3,455,241.
In the above patents, speci~ic colloidal silicas, where ~ .

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4~,~ed ~t~ks ,^ gi~en, are the same as those set forth in~patent No. 3,055,295.
All of the silicas are characterizable as being negatively charged and to applicant's knowledge a positively charp~ed silica has not heretofore been employed in the preparation of a litho-graphic printing plate. One reason for this i9 believc-d to be that conventional pigments and insolubilizers employed in the preparation of lithographic face coatings are strongly alkaline, and thus incompatible with a positively charged silica.
The coatings of the aforementioned prior patents suffer from a number of disadvantages. For one, it has traditLonally been necessary to apply to the printing plate base, particularly where the base is paper or of a cellulosic material, one or more barrier coats offering water resistance and compensating for surface unevenness. Obviously, the application of such coats adds to the cost of the printing plate.
Another disadvantage associated with conventional colloidal silica containing face coats is the short life of such coats in the absence of special barrier coats. Also the use of conventional negatively charged silicas requires employing in the face coat formulation additional additives such as pigments, insolubilizers, cross-linking agents, and other materials to achieve the desired properties of clean copy, good toning, good imaging, stop-go properties and others commonly associated with high quality planographic printing plates. Also, it has conven-tionally been necessary to take special steps to prevent curlingor wrinkling of the printing plates while on a press.

SUMMARY OF THE INVENTION

It has now been discovered that the foregoing disadvan-tages may be overcome by employing as the lithographic printing ` 30 surface or face coating an aqueous based formulation comprising 4.

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an insolubilized hydrophilic binder and a colloidal silica wherein the silica is a positively charged or cationic colloidal silica, the hydTophilic binder being a nonionic or cationic polymer, or a mixture of the sameJ and thus compatible with the silica.
Accordingly, one aspect of the invention provides a lithographic printing plate comprising a base and a lithographic printing surface thereon, said printing surface comprising colloidal silica and insolubilized hydro-philic polymer, the colloidal silica being positively charged and the silica and hydrophilic polymer being present in a weight ratio of at least about four parts silica to one part polymer.
Another aspect of the invention provides a process for preparing a lithographic printing plate comprising a base and a lithographic printing surface thereon, said printing surface comprising colloidal silica and an insolubilized hydrophilic polymer, said process comprising the steps of applying to said base said colloidal silica and an insolubilizable hydrophilic polymer wherein said colloidal silica is a positively charged colloidal silica, the silica and hydrophilic polymer being present in a weight ratio of at least about four parts silica to one part polymer, and then drying said surface.
Preferably, the colloidal silica is an aqueous dispersion of positively charged colloidal silica particles having a dense silica core coated with a polyvalent metal-oxygen compound, e.g. a metal oxide, the sol also containing a monovalent anionic counter ion such as chlorine.
An example of a suitable positively charged or cationic silica ~ which can be employed in accordance with the concepts of the present invention -~ is an aqueous silica sol marketed by E.I. duPont de Nemours and Co. under the registered trademark Ludox 130 M. This silica is stated to have the ^~ approximate chemical composition by weight;
% SiO2 26.0 3Q % A1203 4.0 % Cl 1.4 % MgO0.2 , . . ..
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A12o3 being the metal oxide, chlorine being the monovalent anion. The mole ratio of alumina to surface silica is 1:1 (the mole ratio should be in the range 1:2 - 2:1), and the alumina is present as a polymeric coating of the positively-charged Al-0-Al species on the silica core.
The preparation of the positively-charged colloidal silica (Ludox 130M~ is disclosed in United States Patent No. 3,007,878. Essentially, an aquasol of dense silica particles having a surface area of 20 to 600 square meters per gram is mixed with an aqueous solution of a basic salt of a metal :
having a mono~alcnt mnion, e.g. chlorine, wherein the le ratio of anion to -:~.'~' :.;
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-104~5~1 metal atoms i9 less than tx-2)~ 1 where X is the valence of the metal. The metal can be any Inetal having a valanc0 of 3 to 4, and the anion can be any stable ~onovalent anion other than hydroxyl. Prefe~rred salts are stated to be the chlorides and nitrates of aluminum, zirconium and thorium.
In the method under 3,007,878, the proportion of salt to silica is such that the mole ratio of m~tal atoms in the salt to silica in the aquasol is from 2 x 10 5A:1 to 2 x lO A~l where A i9 the surface area in square meters per 8ram of the particles in the aquas~
Although applicant is not to be held to any particular theory as to the reason for the surprising effectiveness of a positively-charged colloidal silica in the preparation of litho-graphic printing plates, it is believed that the aluminum or other metal being trivalent or tetravalent is weakly bonded with the anion such as chlorine providing a plurality of reactive sites both acidic and cationic which permit reaction both with the hydrophilic binder, e.g. polyvinyl alcohol and with the basic or hydroxyl groups of the cellulosic fibers of the paper ~0 base. In this regard, the present invention is useful where the base is other than paper; specifically where the base has hydrophilic or basic chemical groups, e.g. NH2, OH, capable of reacting with the cationic aluminum. One example of such a base is an alu~inum plate having a surface made alkaline by washing with caustic. This results in a filling or coating of the pores of the base providing a continuous water resistant hydrophilic film. In other words, it is believed to create an indirect bond between the polyvinyl alcohol and cellulosic fibers or coated substrate in addition to the bond that exists directly between the polymer and base or substrate.

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~0~5~, Preferably the positively-charged colloidal silica and hydrophilic polymer are employed in the ratio of about 10:1, although other ratios in the range of about 4:1 to about 15:1 can be employed.
A preferred hydrophilic polymer is polyvinyl alcohol, although other hydrophilic polymers may be employed. One suitable polyvinyl alcohol is that manufactured by E. I.
duPont de Nemours and Co. and marketed under the trademark Elvanol. The polyvinyl alcohol should be at least about 88%
hydrolized, preferably fully hydrolized and preferably also should be of a mèdium viscosity grade. Another suitable poly-vinyl alcohol is one manufactured by the Bordon Co. under the trademark Lemol.
Other suitable hydrophilic colloids include water soluble modified starch (manufactured by Hercules Powder Co.
under the trademark Ceron N 4S); corn hull gum, guar gum, dextran, dextrin, carboxymethyl or hydroxyethyl cellulose, polyacrylamide, polyacrylic acid (manufactured by Rohm ~ Haas `' Co. under the trademark Acrysol A-3~; styrene-maleic anhydride copolymer (manufactured by Texas Butadiene and Chemical Corp.
under the trademark, SMA 6000-N); oxidized cornstarch (marketed by National Starch and Chemical Corp. under the trademark Flokote 64); casein, and albumin. These materials may be applied as a pre-coate with or without pigment followed by application of the cationic colloidal silica. Where compatible, , these materials may be applied in a mixture with the cationic silica.
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10~5~1 If desired, a barrier coat can be applied to the base cellulosic sheet, prior to the application of the face coat, to obtain a longer run master. When so employed, the barrier coat should be capable of providing resistance to water and also improving uniformity of the surface. Coatings of this type are well known in the lithographic printing art.
For the purposes of the present invention, the barrier coat should be anionic or nonionic. Suitable such coa~s are 4~ L 5t~cS
described in prior~patent No. 2,721,815, containing about 10 ~ 60% elay or other filler and smaller amounts of adhesive (10-30%), solvent (0~4%) and hardening agent (1-5%). Suitable formulations contain, in addition to the clay, amounts of silica, satin white, casein, zinc caseinate, carboxymethyl eellulose, alum, and others. One such formulation contains 16%
easein, 1-1/2% ammonia, 60% clay, 19-1/2% satin white, 3%
formalin and sufficient water to reduee the solids content to about 30%. The disclosure of the prior patent No. 2,721,815 is incorporated herein by reference.
Optionally, and in the preparation of a master suitable for a medium run master, the lithographic printing plate can eomprise a cellulosie or other base having thereon a single hydrophilic binder-silica coat of the present invention.
The binder~silica eoat may be employed in the amount of 0.5-6 lbs. per ream (defined as 3,300 sq. ft.) of base paper, If the silica face coat is applied over a barrier coat, a . lesser amount, less than 1 lb. per ream may be used, although for - longer run masters, up to 2 lbs. per ream should be employed.
An advantage of the present invention is that the `; face coat can be applied directly to a high wet strength paper without the need for a barrier coat. Ihus~ an improved long~
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life lithographic printing plate can be obtained in a sin~le off-machine coating operation,only. This represents an economic as well as a technical advance in the art.
As a further advantage, the present invention avoids the need for insolubilizers, the positively charged silica acting as an insolubilizer for the polyvinyl alcohol or other hydrophilic polymer. Thus, the positively charged silica in the presence of heat reacts and cross-links polyvinyl alcohol by eliminating HCl rendering the polymer insoluble in a fountain

5~1u~ion, A still further advantage of the present invention is that longer life is obtained. By the invention, high quality lithographic printing plates free of swelling or image distortion, and stable to curl or wrinkling are provided capable of giving 1000 to 10,000 copies.
A still further advantage of the invention is that ~
less hydrophilic polymer is required, on the order of 0.4-0.5 ;
pounds per ream, or l/lOth of that required with conventional coatings. In this regard, the coating may be applied by a single bladeapplication.
A still further advantage is that the master prepared using the face coat of this invention has outstanding de-inking properties; in that, if one unskilled in the art inadvertantly ~ "inks up" the master it is only necessary to wipe excess ink i 25 from it with a cotton pad. apply a small amount of conventional ;etch, run 10 or 20 copies and original qual1ty is restored.
A still further advantage is that the master may be removed from the press and later be reused after applying a small amount of etch solution.
s 30 A still further advantage is that ~he master is ., 9 ' . ' ' . : :
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compatible wi~h conventional inks, etch solutions and fountain solutions.
A still further advantage is that errors and smudges may be eradicated by erasure and then brushing on a dilute solution of the face coat.
Other advantages are tha~ the face coating holds an image better, and can be employed where the image is applied by xerography, by typing, wax pencil, or by other means.
Other advantages will become apparent to those skilled in the art.
The invention will become clear from the following examples. It is understood that all percentages are percentages by weight and temperatures are in degrees Centigrade, unless otherwise stated.

A solution consisting of 20.9% Ludox 130 M and 2%
polyvinyl alcohol (99% hydrolized Elvanol 7130) was coated with a number 10 Meyer Rod (3.1 pounds per ream) on an 8.5 lb. clay-protein coated base sheet (having 20 lbs./ream barrier coat applied in equal amounts to both sides) and dried at 110C.
The barrier coating is a typical clay protein barrier coat prepared to about 46% solids and containing the following ingredients:
H T Clay 66,4%

Dispex N40 Dispersant 0.2%
(trademark, Allied Colloids Inc.) Casein 6.8%
Borax 0.6%
Styrene Butadiene Latex 24.4%
Formaldehyde 1.6%
Sufficient ammonia to bring the pH to 9O0 :

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Approximately 9,000 good quality copies with no background were obtained from a typewritten master. Approxi-mately 6,000 good copies were obtained from a master that was imaged using a "Xerox"* copy machine.

A solution consisting of 20.9% Ludox* 130 M and 2%
polyvinyl alcolhol ~Elvanol*7130) was applied with a No. 3 Meyer Rod (0.63 lb. per ream) on the coated paper in Example 1. Approximately 3,000 good quality copies were obtained from a typewritten master.

This example illustrates by comparison results ;
obtained employing a negatively charged silica in accordance with the concepts of the prior art. A solution consisting of 20.9% Ludox* AM (a negatively charged colloidal silica) and 2% polyvinyl alcohol (Elvanol*7130) was applied with a No. 10 Meyer Rod to the same coated paper of Example 1, as in Example 1. After printing 200 copies from a typewritten master significant background was apparent. After 600 copies the print had a broken appearance. In the same manner, a ' coating consisting of 23% Syton* 200 and 2% polyvinyl alcohol i gave at most 700 copies.

This example illustrates the quality obtained with a single off-machine coating operation to produce a bond-like sheet which functions as a master. A 63 lb. per ream wet-strength ~;

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sheet was wringer pre-coated with a 20.9% Ludox 130 M and 2%
polyvinyl alcohol 7130 formulation in place of a conventional starch formulation. This operation would normally be done on a paper machine. The sheet was then 10 Rod coated (3.1 lbs.
per ream) with the same formulation (off-machine) and tested on an offset press. Approximately 7,000 perfect copies were obtained using typewritten, ballpoint, and crayon imaging techniques.

The effects of molecular weight, concentration of polyvinyl alcohol and the effect of concentration of Ludox*
130 M on the durability of the face coat are summarized in the following Table 1. Elvanol*7260, a high viscosity grade, and 7130, a medium viscosity grade, were examined. The coated sheet of Example 1 was used and the solutions described were 10 rod coated in successive runs to supply 3.1 lb. per ream.

-Grade of Concentration Concentration Number of Perfect 2~ Run 'Elvanol* of'Elvanol*% 'of Ludox*130M% '' Copies'Obtained 1 7130 1 20.9 600 2 7130 2 20.9 9,000 3 7130 ' 4 20.9 1,600 4 7130 7.5 22.5 300 7130 4 15.0 1,800

6 7130 2.7 27.3 1,300

7 7130 2 23.0 11,000

8 7130 0 30.0 300

9 7260 2 20.9 2,500 7260 4 20.9 1,200 Runs 2, 3, 5, 6, 7, 9 and 10 are within the scope of the present invention. Optimum results were achieved with a ratio of " * trademarks ,~

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~044511 colloidal qilica to hydrophilic polymer of about 10tl or higher (runs 2, 7 and 9), Run 6 employed this ratio, but the results were not as good (although still within the ~cope of the invention), possibly due to higher concentrations of both ~ Ludox 130M and Elvanol~ Ratios of about 4:1 (run 5) and 5:1 (runs 3 and 10) produced satisfactory results. Although the best results were achieved in run 7 with a silica/polymer ratio of about 12tl, excessive viscosity at significantly hi~her ratio9, and dilution of binder or polymer, set a practical upper limit of about 15~1. The above results also show that better results are obtained with th.e medium viscosity grade polyvinyl alcohol.
It was found that partially hydrolyzed polyvinyl alcohol (Gelvatol 40/10 or 40/20 from Monsanto Co., registered . lS trademark9 of Monsanto Co.) was significantly inferior to . the fully or more than 88% hydrolyzed grades.

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Other filler materials, such as zirconium acetate, zinc chloride, zinc acetate, titanium dioxide, talc, zinc ~0 silicate and aluminum silicate have been observed to contribute ~ ~
'. in some cases to improvement in resistance to toning. For --. example, using 0.5% zirconium acetate in the face coat ~ described in Example 4, the number of copies before random ..
~ tonlng was observed was raised from 9,000 to 10,600.

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Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A lithographic printing plate comprising a base and a lithographic printing surface thereon, said printing surface comprising colloidal silica and insolubilized hydrophilic polymer, the colloidal silica being positively charged and the silica and hydrophilic polymer being present in a weight ratio of at least about four parts silica to one part polymer.

2. The printing plate of claim 1 wherein said hydrophilic polymer is selected from cationic and nonionic hydrophilic polymers and mixtures of the same.

3. The printing plate of claim 2 wherein said hydrophilic polymer is polyvinyl alcohol and wherein the silica and polyvinyl alcohol are present in a silica: polymer ratio of about 4:1-15:1, said polyvinyl alcohol being at least 88% hydrolyzed and of medium or high viscosity grade.

4. The printing plate of claim 3 wherein the silica and the hydrophilic polymer are present in a ratio of about 10:1, said polyvinyl alcohol being essentially fully hydrolyzed.

5. The printing plate of claim 1 wherein said base is cellulosic.

6. The printing plate of claim 1 wherein said base has chemical groups reactive with the cationic silica, said groups being selected from hydrophilic and basic groups.

7. A process for preparing a lithographic printing plate comprising a base and a lithographic printing surface thereon, said printing surface com-prising colloidal silica and an insolubilized hydrophilic polymer, said process comprising the steps of applying to said base said colloidal silica and an in-solubilizable hydrophilic polymer wherein said colloidal silica is a positively charged colloidal silica, the silica and hydrophilic polymer being present in a weight ratio of at least about four parts silica to one part polymer, and then drying said surface.

8. The process of claim 7 wherein said hydrophilic polymer is selected from cationic and nonionic polymers and mixtures of the same.

9. The process of claim 8 wherein said hydrophilic polymer is polyvinyl alcohol and wherein the silica and polyvinyl alcohol are present in a silica:
polymer ratio of about 4:1-15:1, said polyvinyl alcohol being at least 88%
hydrolyzed and of medium or high viscosity grade.

10. The process of claim 9 wherein the silica and the hydrophilic poly-mer are present in a ratio of about 10:1, said polyvinyl alcohol being essentially fully hydrolyzed.

11. The process of claim 7 wherein said base is cellulosic.

12. The process of claim 7 wherein said base has chemical groups reactive with the cationic silica, said groups being selected from hydrophilic and basic groups.