GB2062885A - A method of producing a white lithographic substrate - Google Patents

Abstract

There is disclosed herein a method of making aluminum supporting base sheets for lithographic plates by graining and subsequently etching a surface of an aluminum sheet in a trisodium phosphate bath. The sheet may subsequently be anodized and/or treated with an interlayer composition prior to being coated with a photosensitive composition.

Description

SPECIFICATION A method of producing a white lithographic substrate This invention relates to the preparation of supporting bases for lithographic printing plates, and more particularly to novel aluminum lithographic plate supporting bases having a substantially whiter surface than heretofore produced.

It is well recognized that the substantial majority of photosensitive lithographic printing plates now produced are made from an aluminum based sheeting. This aluminum sheeting has, over the years, been found to suffer from certain disadvantages which prevent the direct application of the light sensitive coating to the aluminum base sheet. It has been found that upon direct application of the light sensitive composition to the aluminum base sheet, and subsequent exposure thereof to light, and the removal of the non-imaged areas for the preparation of a lithographic printing plate, the resultant printing plate has many undesirable characteristics which render it unacceptable for commercial use in the printing industry.Among the disadvantages suffered by such plates can be included the fact that the removed non-image areas do not possess sufficient hydrophilic and oleophobic characteristics, and therefore on printing will pick up background imperfections which will be imprinted on the final copy. In addition, since the aluminum is rather soft metal, it does not have the resistance properties to sustain long press runs where more than about 100,000 copies are required. Further, the characteristic of the aluminum surface is such that a problem is encountered in achieving a strong bond between the light sensitive composition and the aluminum base sheet thus causing the image area of the plate to be dislodged from the surface, and renders the printed copy imperfect.

Heretofore, in the production of metal presensitized lithographic printing plates, it had been found beneficial to provide a variety of surface treatments to the aluminum surface to impart thereto any of a number of beneficial characteristics. Such include surface graining, etching, anodizing and coating with interlayer bonding materials prior to the application of a photosensitive coating. U.S. Patent 4,090,880 teaches chemical etching in trisodium phosphate with subsequent anodizing in sulfuric acid and interlayer application. U.S. Patent 3,891,516 shows graining with an aqueous pumice slurry with subsequent anodizing to form a dark surfaced plate.

The prior art teaches that is its desirable to treat the metal sheet substrate surface receiving the light sensitive coating material, which when exposed to light and developed becomes the printing surface of the printing plate, with an undercoating substance that forms a strong bond with the metal sheet substrate and with the light sensitive coating material.

Many such undercoating treatments are known in the art for manufacturing longer running lithographic plates, and can be used on the sheets of this invention. U.S. Patent Nos.

3,160,506 3,136,636, 2,946,683, 2,922,715, disclose a variety of suitable materials for undercoating bonding substances onto plates and methods for applying them. Alkali metal silicate, silicic acid, alkali zirconium fluoride and hydrofluozirconic acid solutions presently are the most important commercial bonding substances.

The art teaches several two step etching and graining treatments which are distinct from the instant invention and possess variant surface characteristics. As previously mentioned, U.S.

Patent 4,090,880, particularly in examples 1-4 teaches an aluminum degreasing and etching in trisodium phosphate followed by a mechanical graining step, however this produces a dark surfaced plate. U.S. Patent 3,891,516 teaches degreasing and cleaning in sodium hydroxide, graining with a slurry of pumice and anodizing to purposefully produce a hard dark plate surface.

The present invention provides a method of preparing an extraordinarily white surfaced aluminum sheet which enhances the visibility of an exposed and developed photosensitive coating placed on its surface. The invention comprises the steps of first graining the aluminum surface, for example, by wire brushing or abrading with a wet pumice slurry followed by a subsequent etching in trisodium phosphate. Significantly, the reverse order provides a dark surface plate whereas the sequence of the present invention provides a substantially white surfaced plate. Importantly, the surface topography of this aluminum is such that an excellent lithographic plate may be produced when it is coated with a photographic composition. Also, the shelf life and press life of such a plate are termed excellent.

It is an object of the present invention to provide a novel lithographic plate having improved image clarity, visibility and plate whiteness.

It is another object of the present invention to provide an improved lithographic printing plate which exhibits excellent shelf life and press life.

As the first step in the process of this invention, a sheet metal substrate, preferably aluminum and the alloys thereof especially those aluminum compositions suitable for the manufacture of lithographic printing plates such as Alcoa 3003 and Alcoa 1100, which may or may not have been pre-treated by standard graining, cleaning, degreasing, and/or etching techniques as are well known in the art, is grained, preferably by abrading with a brush, for example, a wire brush, or rubbing with a wet graining mass such as an aqueous pumice slurry. This grained sheet is then immersed in an aqueous trisodium phosphate solution. Preferred solution parameters include a trisodium phosphate concentration of up to about 30% although this is not critical. The solution is preferably maintained from room temperature to the boiling point with immersion times of up to about 5 minutes.It has been found that by increasing immersion time and/or solution temperature plate whiteness increases. For a given solution temperature and immersion time, a change in solution concentration does not appreciably change plate whiteness. A preferred embodiment would employ a 5% trisodium phosphate solution at 85"C for 45 seconds.

The following table reports the optical density of plates produced by first degreasing an aluminum sheet, graining it with an aqueous pumice slurry and immersing it in a trisodium phosphate solution under the stated conditions. All measurements were made with a Macbeth Surface Reflection Densitometer, Model RD514.

TABLE Concentration is in percent by weight of trisodium phosphate in water. Temperature is in degrees centigrade. Time is in seconds. Optical density is in arbitrary units with whiteness increasing as the optical density figure decreases.

Concentration Temp. Time Optical Density 5% 55 C 15 Sec. 0.60 5% 55'C 30 Sec. 0.65 5% 55 C 45 Sec. 0.66 5% 70'C 15 Sec. 0.52 5% 70'C 30 Sec. 0.48 5% 70'C 45 Sec. 0.34 5% 85 C 15 Sec. 0.38 5% 85'C 30 Sec. 0.25 5% 85'C 45 Sec. 0.19 10% 55'C 15 Sec. 0.60 10% 55'C 30 Sec. 0.65 10% 55'C 45 Sec. 0.56 10% 70 C 15 Sec. 0.48 10% 70 C 30 Sec. 0.45 10% 70'C 45 Sec. 0.37 10% 85'C 15 Sec. 0.36 10% 85'C 30 Sec. 0.26 10% 85 C 45 Sec. 0.23 20% 55'C 15 Sec. 0.61 20% 55'C 30 Sec. 0.67 20% 55'C 45 Sec. 0.49 20% 70'C 15 Sec. 0.45 20% 70 C 30 Sec. 0.45 20% 70 C45 Sec. 0.35 20% 85 C 15 Sec. 0.33 20% 85 C 30 Sec. 0.25 20% 85'C 45 Sec. 0.21 30% 55 C 15 Sec. 0.56 30% 55'C 30 Sec. 0.47 30% 55'C 45 Sec. 0.42 30% 70'C 15 Sec. 0.48 30% 70'C 30 Sec. 0.34 30% 70'C 45 Sec. 0.28 30% 85'C 15 Sec. 0.35 30% 85 C 30 Sec. 0.25 30% 85'C 45 Sec. 0.22 The etched sheet may then optionally be anodized by a method well known in the art such as, in an aqueous solution of sulfuric, chromic, or phosphoric acid having a concentration of from about 0.5 to 25% by weight of acid in water.Anodizing preferably takes place in a bath maintained at a temperature of from about 1 5C to 35C for about from 1 to about 20 minutes at about from 5 to 20 volts and at a current density of about from 10 to 70 amperes per square foot.

The etched sheet may optionally be coated by spraying, brushing, dipping or other means with a composition suitable for use as an interlayer for lithographic plates. Coating compositions employable in the practice of this invention include aqueous solutions of alkali metal silicate, silicic acid, the Group IV-B metal fluorides, polyacrylic acid, the alkali metal zirconium fluorides, such as potassium zirconium hexafluoride, or hydrofluozirconic acid in concentrations of 0.5 to 20% by volume. A preferred concentration range is from 3 to 8% and the most preferred range is from 4 to 5%.

The etched sheet may optionally be both anodized and treated with an interlayer composition or treated with an interlayer composition and then anodized with an optional second interlayer treatment.

It is to be understood that the foregoing parameters are necessarily interdependent and various combinations and modifications of said parameters are operable in the context of the present invention. The hereinbefore mentioned parameters are specifically not intended to limit the scope of the instant invention.

The resulting sheet substrate may than be coated with a photosensitive composition suitable for lithographic purposes. The photosensitive compositions which may be satisfactorily employed in the practice of this invention are those which are lithographically suitable and are actinic and ultraviolet light reactive. The photosensitive compositions which may be employed in the practice of this invention are those which are negative or positive acting and include such photosensitive agents as aromatic diazo compositions, such as those containing diazonium salts or diazides and photosensitive compositions containing materials capable of being polymerized, cross-linked or hardened by exposure to light.

The following examples are provided to illustrate the operation of the present invention and in no way limit its scope.

Example 1 Two sheets of Alcoa lithographic grade 3003 aluminum were degreased in a 5% trisodium phosphate solution. Sheet A, according to the prior art was subsequently grained with a wet pumice slurry and anodized in an 18% sulfuric acid electrolyte for 60 seconds at 14 amps. The optical density was measured at 0.75 by the heretofore described instrument. Sheet B, according to the present invention, was immersed again in a 5% trisodium phosphate bath after the pumice graining step for 45 seconds at 85"C. An optical density of 0.19 Example 2 Example 1 was repeated except the graining was conducted by abrading with a wire brush.

Similar results were obtained.

Example 3 Three sheets of Alcoa lithographic grade 3003 aluminum were degreased in a 5% trisodium phosphate solution and grained with a wet pumice slurry. Plate A, according to the present invention was again immersed in a 5% trisodium phosphate bath for 45 seconds at 85"C. Plate B was etched in a 25% sodium hydroxide etching bath for 5 seconds at 50"C. Plate C was immersed in a 30% aqueous phosphoric acid etching bath for 40 seconds at 80"C. Each plate was then anodized in an 18% aqueous sulfuric acid electrolyte for 60 seconds at 14 amps; provided with a sodium silicate interlayer treatment and coated with a photosensitive composition comprising paradizo diphenylamine condensed with paraformaldehyde and reacted with 2-hydroxy-4-methoxy benzophenone sulfonic acid. One sample of each sheet was subjected to accelerated shelf life testing. Plate A maintained its ability as a commercially acceptable lithographic printing plate for 18 months while plates B and C were useful for only 12 months.

Another sample of each plate was exposed to ultraviolet radiation through a mask, developed, and run on a printing press. Plate A produced 70,000 commercially acceptable reproductions whereas plates B and C each produced only 42,000 commercially acceptable reproductions.

Claims (11)

1. An aluminum sheet suitable for use as a lithographic substrate produced by a method which comprises the steps of first graining said sheet and them immersing said sheet in an aqueous trisodium phosphate bath at a concentration of up to 30% by weight for up to five minutes at a temperature of from room temperature to the boiling point of the aqueous trisodium phosphate bath.

2. An aluminium sheet as claimed in claim 1, wherein the first graining step is conducted by abrading with a brush or an aqueous pumice slurry.

3. An aluminium sheet as claimed in claim 1 or 2, wherein the solution concentration is up to 10% by weight, the temperature is from 70"C to the boiling point of the aqueous trisodium phosphate bath and the immersion time is from 30 seconds to 90 seconds.

4. An aluminium sheet as claimed in claim 1, 2 or 3, which has been subsequently anodized.

5. An aluminium sheet as claimed in any of claims 1 to 4 which has subsequently been treated with an interlayer composition selected from aqueous solutions of alkali metal silicate, silicic acid, the Groups IV-B metal fluorides, polyacrylic acid, the alkali metal zirconium fluorides or hydrofluozirconic acid.

6. The sheet of claim 5 which has subsequently been anodized.

7. An aluminium sheet as claimed in any of claims 1 to 6, which hEz subsequently been coated with a photosensitive composition.

8. An element produced by the method of a) degreasing an aluminum sheet; and b) graining the surface of said sheet by abraiding with a brush or an aqueous pumice slurry; and c) immersing the sheet in an aqueous trisodium phosphate bath wherein the concentration of trisodium phosphate is up to 30% by weight, the bath temperature is from room temperature to the boiling point of the aqueous trisodium phosphate bath and the duration of immersion is up to five minutes; and d) anodizing said sheet; and e) treating said sheet with an interlayer composition selected from aqueous solutions of alkali metal silicate, silicic acid, the Group IV-B metal fluorides, polyacrylic acid, the alkali metal zirconium fluorides or hydrofluozirconic acid; and f) coating at least one surface of said sheet with a photosensitive composition.

9. An element as claimed in claim 8 wherein said graining step (b) is conducted with an aqueous pumice slurry; said step (c) is conducted with a bath concentration of up to 10% with a bath temperature of from 70"C to the boiling point of the aqueous trisodium phosphate bath for from 30-90 seconds; and said anodizing step (d) is conducted with sulfuric or phosphoric acid.

10. An element as claimed in claim 9 wherein said photosensitive composition comprises the condensation product of paradiazo diphenyl amine with paraformaldehyde reacted with 2-hydroxy-4-methoxy benzophenone-5-sulfonic acid.

11. An aluminium sheet as claimed in claim 1, substantially as hereinbefore described in any one of the Examples.