Lithographic Plate Finishers And Cleaners

Abstract

This invention relates to the treatment and maintenance of lithographic printing plates. More specifically, it is concerned with finishing, cleaning, and dampening lithographic plates with aqueous solutions containing an alkali metal-polyphosphate and an inorganic oxidant to render the same more efficient.

Description

FIELD OF THE INVENTION

The efficiency of lithographic printing plates depends upon the oleophilicity of the printing areas and hydrophilicity of nonprinting areas. The "gumming" or finishing of lithographic plates after image development is the standard way to enhance and maintain the hydrophilicity of the nonprinting areas. Indeed gumming of plates prior to long press stops and before plate storage is the general practice to ensure the nonprinting areas remain hydrophilic. The primary purpose of a fountain solution is to dampen the plates during printing to maintain nonprinting areas free of ink. A finishing solution is applied immediately following development and press stops and plate storage. Accordingly the efficiency of said solutions depends on the reactivity of the support material and image with the same.

Various materials e.g., glass, wood, synthetic polyester films and other suitable rigid or flexible materials have been used as supports for lithographic compositions. Certain metals, particularly those of zinc, stainless steel, aluminum, and aluminum alloys, have been commercially accepted in the planographic printing industry to support the lithographic element. However, it is also well known that these metals are somewhat active and readily oxidized. Therefore it is necessary to treat the surfaces of such plates to avoid corrosion or oxidation which will cause the plates to scum on press.

The light-sensitive, image-forming coating may be similar to the diazo compounds disclosed in Schmidt et al., U.S Pat. No. 3,188,210, the combined diazides and interpolymeric resin type taught in Neugebauer et al., U.S. Pat. No. 3,180,723 or the photopolymerizable compositions of assignee's Alles, U.S. Ser. No. 560,889 filed June 27, 1966 abandoned, but photopolymerizable subject matter first refiled as continuation-in-part application Ser. No. 690,732, Dec. 15, 1967, U.S.P. 3,458,311, July 29, 1968 and assignee's Fan, U.S. Ser. No. 772,054 filed Oct. 30, 1968 U.S. Pat. No. 3,558,322, Jan. 26, 1971.

BACKGROUND OF THE INVENTION

Tremendous efforts have been directed toward remedying the inherent disadvantages characteristic of metal support plates. The various techniques involved chemical etching, mechanical graining or merely degreasing the plate and predominantly desensitizing or finishing with a suitable solution. Due to the difference between zinc and aluminum in chemical activity, best desensitizing of each metal is attained by using specific solutions for each metal. In this instance we are particularly concerned with aluminum and alloys thereof.

Earlier finishers or desensitizers generally comprised water-soluble gums such as cellulose gum or gum arabic salts and various other salts in water. Upon acidification and subsequent release of acidic groups, the gum substance formed a strongly adsorbing film with the metal surface. Each component having a definite purpose when applied separately or combined to cofunction with the others to provide a water-wettable surface.

The well-known practice of "gumming" does, in fact, modify the surface of the metal to prevent formation of metallic oxide films and imparts considerable corrosion resistance thereto, however, not all of the inherent disadvantages or other difficulties characteristic of lithography were overcome.

Conventional gums may be deficient and present at least one of a variety of problems. Said gums may fail to adequately desensitize the nonimage areas under certain circumstances. They may "blind" or render the image area hydrophilic thus preventing ink acceptance. Often such gums are incapable of desensitizing those areas which have been inadvertently degraded prior to finishing e.g., contaminated or oxidized. They may leave too thin or weak a film to protect the plate from being sensitized by picking up dirt, grease, ink etc., during normal handling. In a similar manner certain gums may not protect the plate during storage, and blinding or deterioration may occur under slightly abnormal conditions. The tendency for natural gums to coagulate and clog equipment upon partial evaporation and phase separation limits the use of certain gums in mechanical systems. The finishing solutions of the present invention are capable of remedying all of the problems discussed above.

It is the primary objective of the present invention to provide a finishing solution which will render the nonprinting areas of a lithographic plate more water acceptive and grease repellent while the printing areas remain ink acceptive.

An objective of this invention is to provide a finishing solution which will render the nonimage areas resistant to atmospheric corrosion and any deleterious effects normally caused by prolonged exposure in humid conditions. An objective of this invention is to provide a gum-free fountain solution by which the hydrophobic printing area and hydrophilic background areas in lithographic printing plates are readily maintained.

An objective of this invention is to provide a finishing solution which, when properly applied, will protect lithographic plates during long storage.

An objective of this invention is to provide finishing solutions which, when properly applied, can restore and desensitize lithographic plates which have been inadvertently oxidized and show scumming on the press.

An objective of this invention is to provide a finishing solution which can also profitably be employed as a fountain solution.

SUMMARY OF THE INVENTION

The aqueous finisher-fountain solution for lithographic printing plates contains an alkali metal polyphosphate, phosphoric or citric acid, and sodium or potassium nitrate, perchlorate, permanganate, or persulfate. Said solution should have a pH ranging from 1.0 to 5.0 and is applicable to systems involving manual or mechanical processing of conventional lithographic elements. The fountain solutions of this invention may contain additional ingredients to further facilitate desensitizing of the nonprinting areas.

PREFERRED EMBODIMENTS

As briefly discussed above, any metal support may be used, for example, aluminum, zinc, stainless steel, etc. Aluminum and suitable alloys thereof have proven most efficient as conventional photolithographic plate supports, and accordingly are the preferred support materials used in practicing this invention. Since the desired thickness is dependent upon the type of printing press and the conditions incurred thereon, such support materials may be obtained commercially, as foils, sheets, or laminates of various thicknesses ranging from about 0.006-inch to 0.020-inch or more. Also, said commercial support material may be available with a treated surface to which an undercoating or the presensitized lithographic composition may be applied.

A typical composition may be of the type previously discussed or a photoactivable system such as that disclosed in assignees Chang et al., U.S. Ser. No. 731,733 now U.S. Pat. No. 3,549,367 or Fan, U.S. Ser. No. 722,054 now U.S. Pat. No. 3,558,322. Said compositions, essentially comprising a monomer, photoinitiator, and sensitizer(s) dispersed in a soluble polymeric binder, are coated on the aluminum support. When thoroughly dried, the lithographic plates may be exposed and processed in the conventional manner.

Upon sufficient exposure to high-energy actinic radiation for 20 to 120 seconds ore more through a suitable image-bearing negative, the exposed areas become hardened and insoluble.

Adequate development may be achieved by gently applying a conventional aqueous-alkali solution to the just exposed lithographic composition for 10 to 120 seconds at 65 to 95.degree. F. and thoroughly washing the same immediately thereafter. Such a developer may be the type disclosed in assignee's application, Lam U.S. Ser. No. 801,740 comprising an aqueous solution of an alkali metal silicate and a water-miscible organic solvent.

The aqueous finishing solutions may contain alkali metal polyphosphates in amounts from about 1 percent to the limit of solubility, phosphoric or citric acid in amounts of 1 to 20 percent, and the oxidizing agents in amounts of 1 to 5 percent or more, although quantities exceeding 5 percent are usually no more effective, the balance is mainly water, and the percentages are by weight of the total weight of the solution.

Preferred concentration ranges are 1 to 15 percent of alkali metal polyphosphates, 2 to 12 percent of the phosphoric or citric acid, and 1 to 15 percent of sodium or potassium nitrate, perchlorate, permangate, or persulfate.

Accordingly, the suitable alkali metal polyphosphates are those having the general formula XMPO.sub.3 .sup.. YM.sub.2 O.sup.. ZO, wherein M is an alkali metal, preferably those of sodium or potassium, X is an integer of 2 to 20, Y is an integer of 1 to 5, and Z is 1 or 0.

Phosphoric and citric acids have been found to be the only two acids useable in this invention. Preferably, the acids are added in an amount between 2 to 12 percent or more of the total weight of the intended volume. Acid concentrations are adjusted within the prescribed range to avoid reaction beyond that normally required to adequately form the ink-repellent surface in the nonimage areas.

Many adjuvants e.g., accelerators, buffers, preservatives wetting agents etc., may be incorporated in the instant solutions to slightly alter process activity. However, inorganic oxidizers such as the sodium or potassium nitrates, perchlorates, permanganates, persulfates and other suitable salts thereof are preferred. A particular salt or suitable combination thereof is added in amount of approximately 2 percent by weight. Since these oxidizers act as catalysts, their precise concentrations are not critical and can be varied within the limits given above.

Proper application of the finisher composition of the instant invention results in desensitization of the background or nonprinting areas only of an aluminum, aluminum alloy or stainless steel supported printing plate bearing a conventional lithographic element. Accordingly the desensitized areas become water-receptive and resistant to the greasy lithographic ink, as the image areas remain oleophilic, upon adequate treatment with a solution of the preferred acid, alkali metal polyphosphate, and inorganic oxidant. Also the conditioned plates are stable during storage. In a manner similar to that of any commercially accepted multipurpose process solution, the preferred solutions herein are readily adaptable to both manual and mechanical systems.

Sufficient conditioning may be achieved by any conventional manual method such as gentle application of the finishing-cleaning solution over the entire plate surface with a cotton pad, sponge, or squeegee and immediate buffing down to dryness. The solution may be applied at room temperature.

Conventional mechanical systems may involve subjecting the processed plate to a gentle to moderate spraying of the solution followed by squeegee roller treatment and blower drying.

This invention will be further illustrated by examples of processed photopolymerizable elements finished with the essential acidic-alkali metal polyphosphate-oxidant solutions and various modifications disclosed herein. Also said solutions may be applied to the plates as a fountain solution. However, it is not intended that the scope of the present invention be limited to the examples.

EXAMPLE I

Several photopolymer lithographic printing plates were processed in preparation for comparative-finishing tests with a preferred solution of the present invention and several conventional gums. A free-radical photopolymerizable composition similar to that described in Chang and Fan, Ser. No. 731,733 filed May 24, 1968, wherein lophine dimers are combined with Michler's ketone (4,4'bisdimethylaminobenzophenone) was coated on an aluminum plate. Said composition was thoroughly mixed, and a uniform layer of the same of about 35 to 50 mg. per dm.sup.2 was coated on a brush-grained aluminum sheet approximately 12 mil thick.

The photopolymerizable plates were then dried and subjected to appropriate light exposures of 20 to 50 seconds through a suitable negative process transparency. Exposures were made on standard equipment, such as the carbon arc or nuArc "Flip Top" Plate Maker, Model FT26L to the xenon light source. Several of the exposed plates were then developed in an aqueous alkali metal silicate solution such as that taught in Assignee's copending application, Lam U.S. Ser. No. 801,740, filed Feb. 24, 1969. Upon completion of development, the image-bearing elements were treated with a preferred solution or one of several conventional gum finishers prior to printability tests. The finishers were applied immediately after development and thorough rinsing by gently swabbing over the entire plate surface with a sponge. The commercially available finishers, identified as "A" and "B", essentially comprised gum and phosphoric acid in combination with various conventional ingredients. All finishers were applied at room temperature, and the preferred solution "C" had the following formula:

Components Amount Distilled water 100 ml. Sodium hexametaphosphate 15 g. Sodium nitrate 2 g. Phosphoric acid 2 g. *Octyl phenoxy polyethoxy ethanol containing 9-10 ethoxy groups (10 percent solution) and a pH of 2.00. 4 drops * This ingredient is optional and may be replaced by other similar surfactants.

Shortly thereafter the plates were buffed down to dryness and stored under various conditions for two weeks. Said conditions were room temperature, tropical oven and high humidity. Following the storage period, the plates were mounted on a Heidelberg KOR single-color offset press for printing with black offset ink. Upon close examination, of several samples, representative of many prints obtained therefrom results were as recorded below: --------------------------------------------------------------------------- TABLE 1

Conditions Finisher A B C __________________________________________________________________________ Room Clean Clean Clean Temperature Background Background Background Tropical Oven Clean Heavy Clean (120.degree. F. & 80% Background Scumming Background Relative Humidity) High Humidity Slight Severe Clean (85.degree. & 90 percent Scumming Scumming Background or more Relative Humidity)

As indicated by the table, the plate finished with the preferred solution (C) yielded prints comparable and, at times, superior to those obtained from similar plates finished with commercially accepted gums, A and B. Unlike both gums following the two week exposure to high humidity and the tropical oven in the instance of gum B, the preferred solution did not scum. Significant oxidation was prevented. Accordingly said solution rendered the image-bearing plate more storable and continuous reproduction of a sharp image in excess of 2,000 prints was evident of lasting, good printability.

EXAMPLES II-XIV

A quantity of lithographic printing plates bearing a free-radical photopolymerizable composition similar to that of Example I was prepared for printability tests in the manner described in Example I using various preferred solutions and operable modifications thereof. The particular formulas and the results obtained therefrom are recorded as follows:

As disclosed in table 2 below, the above-identified solutions except that of Example XII, proved readily applicable as finishers to be employed when practicing this invention. There was no scumming, and after several thousand prints, the same tones characteristic of the original were reproduced.

EXAMPLES XV-XVI

Additional solutions comprising essentially the same ingredients, but of lesser concentrations, as the preferred formulation in Example I were prepared for use as fountain solutions on a Harris Intertype Sheet-Fed press. Respective working strength of said solution, XV and XVI, were 10 and 25 percent of the above-mentioned preferred solution of Example I. Said press is equipped with a reservoir having a partially immersed driver-roll so positioned to transfer fountain solution to the printing roll by means of a series of contiguously situated rolls. In this instance, processed photopolymerized litho plates such as that described in Example I were used with Litho Spark-Dri Black ink. None of the problems associated with dampening systems were incurred.

The diluted acidic-alkali metal polyphosphate and sodium nitrate solutions having a small amount of ##SPC1## octyl phenoxy polyethoxy ethanol therein proved efficient as fountain solutions. Solutions XV and XVI sufficiently dampened the press roll thereby maintaining the desired printable plate surface throughout the run. Printing was suspended for one-half hour and resumed without treating the plates with additional preferred finisher. Nevertheless, it was apparent as several thousand scum-free prints were made, the fountain solution dried down on the plate during the press stop protected the plate from contamination and oxidation which usually occur with conventional dampening solution.

EXAMPLE XVII

A number of lithographic printing plates, similar to those of Example I, were exposed and developed as in Example I with appropriate developer. Oxidation was intentionally promoted on the plates by placing on the plates damp paper towels which were allowed to dry in contact with the plates. Several plates were then finished with a preferred solution, such as that disclosed in Example I, or a conventional commercial gum, and prepared for printability tests in the prescribed manner. Prints obtained from those plates finished with the finisher of Example I did not show oxidation scumming, whereas, the other plates showed various degrees of scumming. This illustrates the ability of solution of this invention to desensitize oxidized background, thereby preventing scumming on press.

EXAMPLE XVIII

A stainless steel lithographic plate was cleaned with a conventional trisodium phosphate cleaning solution, prepared for coating, and coated as in Example I. After exposure and by processing, as in Example I, a portion of the plate was treated with the finisher composition of Example I. The untreated portion scummed badly, while the treated portion was free of scum when tested on the Heidelburg press.

In addition to the foregoing examples, the compositions were used to finish a diazo composition on an aluminum wipe-on plate, as well as other commercial presensitized lithographic plates on aluminum supports, including grained and silicated supports. On all such plates, the advantages of the invention were realized.

When applied in the prescribed manner, the preferred finisher offers several advantages over those compositions designed for the same purpose in addition to providing an economic means of desensitizing the nonprinting areas and rendering the same hydrophilic.

The preferred finisher-cleaning solution offers an advantage in that it penetrates or solubilizes any treatment layer or residual development film to attack the metal surface of the background areas only. Hence, the efficiency of these solutions is not appreciably altered by any conventional presensitized lithographic element or subcoatings thereof. Accordingly finisher activity is not hindered by any treated stainless steel or treated aluminum surface, e.g., brushed-grained, silicated, anodized etc., or any layer applied thereon.

Another advantage is that said solutions are nontoxic and do not present the potential hazards characteristic of certain commercially accepted compositions, such as the fluorides or chromate/dichromate combination.

Another advantage is that these acidic-alkali metal polyphosphates desensitize areas very well even after oxidation has occurred.

Another advantage provided upon use of said solutions is the greater resistance to abnormally high humidity and atmospheric oxidation which permits increased plate storage life.

An additional advantage is the lasting homogeneity of these solutions. The ingredients of said solutions do not decompose and remain soluble. Accordingly the preferred solutions do not "scale" or precipitate and are readily adaptable to use in automatic processors. These solutions are also immune to bacteriological deterioration which is a distinct drawback associated with conventional compositions containing various types of natural gums.

One part by volume of the finisher solution may be diluted from 1 to 20 parts by volume of water to form a solution.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A lithographic plate finishing solution having a pH of 1- 5 and consisting essentially of:

a 1-15 percent of an alkali metal polyphosphate of the formula XMPO.sub.3 .sup.. YM.sub.2 O.sup.. ZO, where M is Na or K, X is an integer of 2 to 20, Y is an integer of 1 to 5, and Z is 1 or 0,

b 2- 12 percent of phosphoric acid or citric acid,

c 1- 5 percent by weight of sodium or potassium nitrate, perchlorate or persulfate, and

d about 95 to about 68 percent of water, the percentages being by weight and based on the total weight of the composition.

2. A fountain solution consisting of 1 part by volume of the finisher solution defined in claim 1, diluted with from 1 to 20 parts by volume of water.