FIELD OF THE INVENTION

This invention discloses a process for improving the hydrophiUcity in lithographic printing surfaces and thereby preparing improved lithographic printing plates.

BACKGROUND OF THE INVENTION

The art of lithography is well known and is practiced commercially in several fields including, for example, the electronic industry, newspaper industry, magazines, and the like. The field of lithography has been extensively written and commented upon in several books and articles. See, for example, Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 19, p. 140, John Wiley & Sons (1982) ; ibid, Vol. 20, p. 161 (1982) ; and Encyclopedia of Polymer Science and Engineering, Vol. 13, p. 373, John Wiley & Sons (1988) .

A typical lithographic printing plate comprises a substrate surface, typically made of aluminum, silicon and the like, with a photosensitive layer on top. When aluminum is the substrate material, generally the top of aluminum is anodically oxidized prior to deposition of the photosensitive layer. When printing images are photomechanically produced on such a printing plate, the applied photosensitive layers can penetrate too deeply into the porous oxide layer. This can result in inked areas of the non-imaged portion of the plate. This problem is referred to as "toning" in the industry.

It has been found in the past that improving the hdyrophilicity of the non-image areas generally tends to reduce this toning problem. Generally the hydrophiUcity is improved by reaction with a protective conditioning agent (also called passivating or sealing agent) . These

conditioning materials react with the surface oxide to seal the porous layer and render it more hydrophilic. Commonly used conditioners or sealers are the silicates, carboxylic acids or phosphonic acids which form the oxo-silicate, oxo- carboxylate or oxo-phosphonate, respectively. Polymer analogs are preferred since the surface hydrophilicity is enhanced by those unreacted groups on the polymer backbone which cover the surface of the treated substrate surface.

U.S. Patent No. 4,153,461 discloses the use of polyvinyl phosphonic acid ("PVPA") as a conditioning agent. Example 1 in that U.S. Patent describes the use of an aqueous solution of PVPA to treat the surface of oxidized aluminum.

While the use of PVPA improves the quality of the printing plates to some extent, the problem of sensitivity to background toning still remains. Thus, even plates that have undergone the PVPA treatment are known to have the problem, albeit to a less extent.

Thus, it would be desirable to have a process whereby good quality lithographic plates can be produced without the above toning problem.

It would also be desirable to have a process whereby the hydrophilicity of the non-image areas in the surface of printing plate substrates can be improved, thus improving the quality of images printed therefrom.

SUMMARY OF THE INVENTION

Applicants have discovered that treatment of lithographic printing plate substrate surface with a solution of polyvinyl phosphonic acid that has pH in the range about 2.5 to 6.6 surprisingly improves the hydrophilicity of the non-image areas in the surface and

also eliminates the background toning problem, thus resulting in printed images with excellent contrast, sharpness and brightness. At the same time, surfaces treated with PVPA solution alone (which is generally at pH=2.2), or with completely neutralized PVPA under the same conditions results in images with background toning still present.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one embodiment, the present invention discloses a process for improving the hydrophilicity of the non-image areas in the surface of a printing plate substrate, by treating the surface with a solution of PVPA which is maintained at a pH in the range 2.5 to 6.6. Such a process advantageously also improves the quality of the printed images prepared from the printing plate. Furthermore, the inventive process may be used on oxidized surfaces as well as those that are not oxidized.

The general process for the preparation of a printing plate is described below. The process starts with a suitable substrate. Suitable substrates include metal, silicon, plastics such as polyester and the like materials which are well known in the art. Suitable metals for lithographic use are steel, magnesium or aluminum and its alloys. Aluminum and its alloys are preferred because of their mechanical properties and relatively light weight. Of particular interest are alloys which maintain the advantages of aluminum but offer mechanical strength to prevent cracking or tearing during long runs on printing presses. Aluminum (or other metal) substrates are first cleaned to remove milling oils and surface impurities. Cleaning may be done by a variety of solvent or aqueous alkaline treatments.

Typical alkaline degreasing agents include: hot aqueous solutions containing alkalis such as potassium hydroxide, sodium hydroxide, trisodium phosphate, sodium silicate, and aqueous alkalis mixed with surfactants. Solvent type degreasers such as trichloroethylene, 1,1,1 trichloroethane, perchloroethylene can be used but are less popular because of increasing environmental and health considerations .

Solvent degreasing is done by immersion, spray or vapor washing with the listed agents. Preferably the substrate comprises aluminum, although the inventive process may conceivably be used with other substrate materials also. The substrate may be in the form of a plate, coil and the like. Generally, the aluminum substrate surface is cleaned and degreased with suitable reagents such as, for example, alkaline reagents like aqueous sodium hydroxide solution, aqueous potassium hydroxide solution and the like.

The metal surface may be smooth or roughened. Conventional surface roughening (graining) techniques include but are not restricted to chemical etching in alkaline or acid solutions, graining by dry abrasion with metal brushes and wet graining with aqueous slurries of abrasive particles, ball graining with metal shot and electrochemical graining whereby controlled pitting of the metal surface is achieved in aqueous electrolyte media under the influence of an electrical current . The electrolyte media may be acidic or basic, organic or inorganic; an inorganic acidic media, however, is preferred. The surface topography and roughness varies with each of these graining processes. Generally a mean roughness of about 0.2 to about 1 Ra is produced in a typical graining process. Ra is a measurement of average roughness of the surface and is defined as the arithmetic

potassium hydroxide, calcium hydroxide, ammonium hydroxide, tetramethylammonium hydroxide and tetrabutylammonium hydroxide. Preferred bases are sodium hydroxide and potassium hydroxide. Generally, for neutralization, a solution of the base in water is prepared which is then added to the PVPA solution while monitoring the pH at the same time. When the pH reaches the desired level, addition of base is stopped. The inventive conditioning solution thus contains partially neutralized PVPA, which refers to the fact that part of the PVPA exists as a salt with the base. When sodium hydroxide is used as the neutralizing base, for example, the conditioning solution exists as partial sodium salt of PVPA along with free acid groups.

The inventive conditioning process is generally done at temperatures ranging from ambient to about 212°F, preferably from about 100-200°F and typically from about 140-180°F. The PVPA solution at the desired pH is maintained at this temperature while the plate is treated with the conditioning reagent. The word "treated" refers to processes such as dipping, immersing, spraying and the like, which bring the surface into contact with the conditioning reagent. The conditioning is done for about 0.5-300 seconds generally, for about 1-60 seconds preferably, and for about 5-20 seconds typically. Then it is rinsed with water.

The rinsed plates are then dried with forced hot air, and then coated with a photosensitive layer. Photosensitive layers are made from photosensitive compounds that may be optionally mixed with suitable additives such as, for example, binder resins, photoinitiators, colorants, acid stabilizers, exposure indicators, surfactants and the like, known to those skilled in the art. Photosensitive compounds ("light sensitive compounds") useful in the practice of the invention include, for example, those described, in U.S. Patent Nos. 3,849,392; 3,867,147; 4,157,918, and 4, 183 , 788.

exposure, as is known to those skilled in the art. While the present invention may be practiced with both positive and negative type photosensitive materials, the present description describes the negative photosensitive layer materials to illustrate the invention. Furthermore, the illustration describes the use of diazonium photosensitive materials. No limitation in the nature or type of photosensitive materials is contemplated thereby. Some diazonium compounds useful for the practice of the invention are described, for example, in U.S. Patent Nos . 3,849,392; 3,867,147; 4,183,788 and 5,200,291.

As mentioned above, the light sensitive compound may optionally be mixed with additives as described above to form a photosensitive coating material, if so desired. For example, acid stabilizers may be used with diazonium compounds; suitable acid stabilizers are generally organic or inorganic acids. Examples include phosphoric, citric, benzoic, m-nitrobenzoic, p-toluenesulfonic, and the like as well as mixtures thereof. Preferably, the acid stabilizer is phosphoric acid. When used, the acid stabilizer is present generally in the amount of from about 0.02 % to about 2 %, and preferably from about 0.05 % to about 1.0 % based on the weight of the composition.

Exposure indicators (or photoimagers) which may be useful in conjunction with the present invention include 4- phenylazodiphenylamine, eosin, azobenzene, Calcozine Fuchsine dyes and Crystal Violet and Methylene Blue dyes . Preferably, the exposure indicator is 4- phenylazodiphenylamine. The exposure indicator, when one is used, is preferably present in the composition in an amount of from about 0.01% to about 0.35% by weight. A more preferred range is from about 0.02% to about 0.30% and, most preferably, the exposure indicator is present in an amount of from about 0.02% to about 0.20%, although the skilled artisan may use more or less as desired.

Colorants useful herein include dyes such as Rhodamine, Calcozine, Victoria Blue and methyl violet, and such pigments as the anthraquinone and phthalocyanine types. Generally, the colorant is present in the form of a pigment dispersion which may comprise a mixture of one or more pigments and/or one or more dyes dispersed in a suitable solvent or mixture of solvents . When a colorant is used, it is preferably present in the composition of this invention in an amount of from about 2.0% to about 35.0% by weight, more preferably from about 5.0% to about 30.0% and most preferably from about 5.0% to about 20% although the skilled artisan may use more or less as desired.

Resin binders may be used in the coating to increase the functional lifetime of the printing plates. If resin binders are desired, suitable binder resins are chosen based upon their compatibility with other coating components, the ability to be cleaned away during the development step in non-image areas of the plate and their resistance to wear on press. Resins with acid end groups are especially desirable since the acid helps stabilize the coatings based upon diazonium compounds and provides alkaline solubility for development.

In order to form a coating composition for the production of the photosensitive layer, the light sensitive compound is coated from a solution in a suitable solvent, wherein the light sensitive compound is present in amounts such that the concentration of the light sensitive compound is generally in the range 10-100% of the total solids in the dried coating upon drying (see below) , preferably in the range 30-75%, and typically in the range 40-60%. Suitable solvents for this purpose ^' include water, tetrahydrofuran, butyrolactone, glycol ethers such as propylene glycol monomethyl ether and methyl cellosolve, alcohols such as ethanol and n-propanol, and ketones such

as methyl ethyl ketone, or mixtures thereof. Preferably, the solvent comprises a mixture of propylene glycol monomethyl ether and butyrolactone. In general, after the solution is applied to an appropriate substrate by a suitable coating process, the solvent system is evaporated from the coating by a suitable drying process; some insignificant amount of solvent may, however, remain as residue.

The process to coat the light sensitive layer may be a batch or continuous process as is known to those skilled in the art. Some useful coating processes include dip coating, roller coating, slot type coating, spin coating, and the like, which deposit a thin uniform dry photosensitive coating on the surface once the solvents have been driven off. At the end of the coating operation and drying, generally a coating weight of from about 0.5g/m ² to about 2.5g/m ² , preferably from about 0.8 g/m ² to about 2.0 g/m ² and typically about 1.0 g/m ² is obtained.

The thus prepared photographic element is then exposed to suitable actinic radiation through a negative mask so as to yield a solid 5 on a 21 step Stouffer exposure wedge (supplied by Stouffer Graphic Arts Equipment Company, South Bend, Indiana) after development. Generally a radiation in the range of 300-400 nm is employed. The exposed plate is then developed with a suitable developer. Suitable developer may be aqueous developer, or aqueous developer mixed with a suitable organic solvent. An aqueous developer useful in the present invention comprises an aqueous solution containing one or more of the following groups: (a) a sodium, potassium or lithium salt of octyl, decyl or dodecyl monosulfate;

(b) a sodium, lithium, potassium or ammonium metasilicate salt;

(c) a lithium, potassium, sodium, or ammonium borate salt; (d) an aliphatic dicarboxylic acid, or

sodium, potassium or ammonium salt thereof having from 2 to 6 carbon atoms; and (e) mono-, di-, or tri-sodium, or -potassium phosphate.

Other suitable developers include water, benzoic acid or sodium, lithium and potassium benzoates and the hydroxy substituted analogs thereof as well as those developers described in U. S. Patent No. 4,436,807.

In conventional use, the developed plate is finished with a subtractive finisher such as a hydrophilic polymer. Examples include cold water soluble dextrin and/or polyvinyl pyrrolidone, a nonionic surfactant, a humectant, an inorganic salt and water, as taught by U. S. Patent No. 4,213,887.

For the purpose of improving the press performance of a plate prepared as described above, it is known that baking of the exposed and developed plate can result in an increase in the number of quality impressions over that otherwise obtainable. To properly bake the plate, it may be first treated with a suitable solution designed to prevent loss of hydrophilicity of the background during baking. An example of a suitable solution is disclosed in U. S. Patent No. 4,355,096. The plate prepared as above may then be heat treated by baking at temperature of from about 180 ^* C up to the annealing temperature of the substrate, preferably about 240 "C. The effective baking time is inversely proportional to the temperature and averages in the range of from about 2 to about 15 minutes.

In another embodiment, the present invention discloses improved lithographic plates prepared using the conditioning process described above. Such plates have utility in preparing colored sheets with excellent contrast .

The improvement in the hydrophilicity as well as in the quality of printing plates achievable through the process of the instant invention was demonstrated as follows. An aluminum coil (99.5 Al alloy) was cleaned, mechanically roughened, etched and then anodized as described above. The coil was divided into eight sections. Section 1 (control) was conditioned with an unneutralized PVPA solution (pH 2.0) as described in the U.S.patent No. 4,153,461 referred to above. Sections 2 through 6 were conditioned with PVPA solution partially neutralized with NaOH in accordance with the present invention at pH' s ranging from 2.9 through 6.6. Section 7 was conditioned with PVPA solution completely neutralized with NaOH solution (pH 7.1) . Section 8 was made with a different alloy of aluminum (3103) containing Cu and Mn (98.5% Al alloy) , and was conditioned with an inventive partially neutralized PVPA solution with pH 3.8.

Sections 1 to 8 were then coated with a photosensitive layer comprising a suitable negative acting diazonium compound. The diazonium compound used in this particular instance was the polycondensation product of 3-methoxy-4- diazo diphenyl a ine sulfate and 4, 4 ' -bismethoxy methyl- diphenyl ether, precipitated as mesitylene sulfonate, described in U.S. Patent 4,157,918. It was then exposed to UV light (365 nm) for 30 seconds, using a Teaneck exposure unit (sold by Teaneck Graphics Systems, Teaneck, New Jersey, using a L1250 UV light source from Olec Corporation, Irvine, California) . Development was then done with an aqueous developer sold under the trade name ND-143 by Hoechst Celanese Corporation, Printing Products

Division, Branchburg, New Jersey.

The sensitivity to background toning was measured by preparing printed images using the eight sections on a printing press, and then measuring the number of sheets ("copies") needed to print cleanly ("clean-out") after

application of the dampening rollers to each section. Table 1 lists the number of sheets needed to clean-out. The less the number of sheets needed, the better is the performance of the printing plate. As Table 1 indicates, Sections 1 and 7 would not clean-out even after rolling out >100 copies, while sections 2 through 6 and 8 needed much less (40-50 copies) to clean-out, demonstrating the superiority of the inventive process in improving the hydrophilicity of the non-image areas, the quality of the lithographic plates prepared and the quality of the images printed therefrom.

The following examples are illustrative of the invention but it is understood that the invention is not limited thereto.

EXAMPLES

In the following Example, coils made of aluminum alloys 1050 and 3103 (supplied by Alcoa Co., Pittsburgh, Pennsylvania) were used. Ambient temperature refers to about 20-30°C.

A coil of Aluminum (Alloy 1050, 99.5% Al alloy) was divided into eight sections (Sections 1 through 7) . Section 1 was A) degreased in a 1.3% NaOH solution at about 135°F for about 15 seconds, and then B) mechanically roughened using a 23% silica slurry in water by the action of rotating nylon brushes giving a mean roughness of about 0.45 Ra. The coil was then C) etched in a dilute caustic bath (1.5%) at 130'F to brighten the surface followed by D) anodization in 19% H ₂ S0 ₄ , to yield an oxide weight of 1.4 g/1. The oxide surface was E) conditioned/passivated by spraying the substrate with a 0.2% polyvinyl phosphonic acid solution in water at 160'F (pH= 2.2) . This section 1 of the web was the control section.

Another section (section 2) of the coil was treated with the same sequence of process steps A-D described above. At this point the polyvinyl phosphonic acid solution was partially neutralized by the addition of 50% NaOH to pH=2.9.

Another section (section 3) was similarly treated through process steps A-D followed by treatment with polyvinyl phosphonic acid solution neutralized as above to pH=3.6. Section 4 was neutralized to pH-4.5, section 5 to pH 5.4, and section 6 to pH 6.6.

Section 7 was treated through process steps A-D followed by conditioning with polyvinyl phosphonic acid solution which had been completely neutralized with NaOH to pH= 7.1.

Section 8 was made with a different alloy of Aluminum (3103) containing Cu and Mn (Al-98.5%) . Section 8 was treated through process steps A-D followed by treatment with polyvinyl phosphonic acid solution neutralized as above to pH=3.8.

The test sections 1 through 8 were then coated with a photosensitive coating (a diazo resin which was the polycondensation product of 3-methoxy-4-diazo diphenyl amine sulfate and 4,4 ' -bismethoxy methyl-diphenyl ether, precipitated as mesitylene sulfonate, described in U.S. Patent 4,157,918, and a modified polyvinylacetal resin described in U.S. Patent 4,940,646) and exposed to U.V. light through a negative mask. The plates were developed in the above-described ND-143 aqueous developer. The sensitivity to background toning was measured by running imaged plates on press in a dry scum cycle test using Kohl- Madden red ink on a Heidelberg printing press. The number of sheets needed to print cleanly was determined after application of the dampening rollers. The following Table shows the results of these experiments.

Table 1

Section Alloy PH # Sheets to Clean-out

1 1050 2.2 >100

2 1050 2.9 50

3 1050 3.6 40

4 1050 4.5 40

5 1050 5.4 40

6 1050 6.6 40

7 1050 7.1 >100

8 3103 3.8 40

As Table 1 indicates, Section 1 with no neutralization and Section 7 with complete neutralization, the print sheets would not clean-out even at >100 copies while the other partially neutralized sections cleaned out readily showing an increased hydrophilicity in the non-image areas of the lithographic printing plates .