GB1582620A - Aluminium substrates useful for lithograpic printing plates - Google Patents

Description

(54) ALUMINIUM SUBSTRATES USEFUL FOR LITHOGRAPHIC PRINTING PLATES (71) We, POLYCHROME CORPORA TION, a corporation organized and existing under the laws of the State of New York, United States of America, of 137 Alexander Street, Yonkers, State of New York, United States of America, (Assignees of SIMON LONG CHU, EUGENE GOLDA, JEN-CHI HUANG and ALAN LEONARD WILKES), do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the treatment of aluminum surfaces, and more particularly to the treatment of aluminum surfaces to provide a surface thereon suitable for use in the production of lithographic printing plates.

There are many methods and processes which have been heretofore employed in the treatment of aluminum surfaces to render them suitable for use in the production of lithographic printing plates. One such method involves the electrolytic treatment of aluminum, for example, electrolytic treatment of aluminum, for example, electrolytic etching by use of a hydrochloric acid electrolyte, for such purpose. Various prior art publications, for example, U.S. Patents 3,072,546 and 3,073765 and British Patents 879,768 and 896, 563 describe the treatment of aluminum surfaces with hydrochloric acid while applying an alternating currect to the aluminum plates to render the plates suitable for lithographic use.While this treatment has been taught to be satisfactory, it apparently only functions properly with relatively pure aluminum, such as aluminum alloys having less than 0.5% impurity, such as Aluminum Association Alloy 1145. In addition, in the treatment of such Aluminum Association Alloys as 1100, a relatively large amount of electrical power has been required to obtain the degree of etching desired. Furthermore, heretofore, in the electrolytic etching of relatively impure aluminum sheetin8 for lithographic purposes, it has been experienced that an excessive amount of a black smut is created on the aluminum surface which requires additional subsequent treatment of the aluminum in order to render it suitable for lithographic use.It has also been found in the practice of the prior art processes that uniform etching of the surface is not obtained and the character of the grain imparted to the surface is not consistent, portions thereof being relatively coarser than others, thus yielding an undesirable irregular surface which is not ideally suitable for lithographic use. When the surface of the aluminum sheet is irregular and nonuniform, it can interfere with the subsequent printing process when the surface is subsequently coated with a photosensitive resin as is employed in normal lithographic processes as is well known to the skilled worker.

Heretofore, various suggestions have been made to overcome the disadvantages encountered in the practice of the prior art processes. One such suggestion in U.S.

Patent 3, 963,594 involves the use of a hydrochloric acid and gluconic acid electrolyte for etching. Other suggestions such as those contained in U.S. Patents 3,342,711; 3,365,380; and 3,366,558, refer to an electrolytic polishing effect obtained on aluminum and other metals using a mixture which may include various electrolytes such as sulfuric acid and gluconic acid. Japanese Patent 47-44046 also suggests a method of pretreating aluminum surfaces, also without solving the prior art problems. However, none of these proposed methods have been found to be completely successful in overcoming the disadvantages of the prior art methods.

It has now been discovered that by the practice of the present invention the disadvantages of the prior art may be successfully avoided while still obtaining the desirable results required. By practice of the present process, it has been found that aluminum surfaces may be roughened more uniformly, imparting a surface character thereto that is very satisfactorily conductive to their subsequent use for lithographic purposes than was previously known or anticipated.In addition, by the practice of the present invention, it is now possible to employ less pure alloys of aluminim, for example, Aluminum Association Alloys 1100 and 3003, to obtain aluminum sheets which possess relatively uniformly roughened surfaces which are emminentlysuitabe for use in the production of lithographic printing plates, while at the same time avoiding the detrimental characteristics and properties heretofore existant in such materials. More particularly, the practice of the present invention will permit the use of a wider variety of aluminium alloys for lithographic purposes, while at the same time imparting to the aluminium surfaces a consistent and uniform surface emminently suitable for use in the production of lithographic printing plates.In addition, the practice of the present invention results in economies in the use of electric power required to impart the desired roughened surface to the aluminium surfaces treated hereunder and also permits a less critical control of the elecerolytic bath solutions employed herein and an extension of the effective life of said baths.

According to the present invention a process is provided for the treatment of aluminium to provide thereon a substantially consistent and uniform roughened surface suitable for lithographic uses, which comprises: a) mechanically graining at least one surface of a lithographically suitable aluminium sheet, and b) then electrolytically etching said mechanically grained surface by treatment thereof with alternating current in an aqueous electrolytic bath containing an electrolyte of hydrochloric acid, nitric acid or combinations thereof.

The practice of the process of the present invention requires that the aluminium surface which is to be electrolytically etched for lithographic purposes must first be mechanically grained prior to the electrolytic treatment thereof. More particularly, in the production of lithographically useful aluminium sheets in according with the process of the present invention, at least one surface of aluminium sheet useful for lithographic purposes hereunder, is first mechanically grained to roughen the surface and the said mechanically grained surface is then electrolytically etched to yield a consistent and uniformly roughened surface emminently suitable for further treatment to produce very satisfactory lithographic printing plates.

The aluminum sheet which may be employed in the practice of this invention, include those which are made from aluminum alloys which contain substantial amounts of impurities, including such alloys as Aluminum Association Alloys 1100 and 3003. The thickness of the aluminum sheets which may be employed in the practice of this invention may be such as are usually and well known to be employable for such purposes, for example, those which are from 0.004 inches to 0.025 inches in thickness, however, the exact choice of aluminum sheet may be left to the discretion of the skilled worker.

The aluminum sheets employable in the practice of the present invention may be mechanically grained in accordance with the process of the present invention. By mechanically graining it is here meant processes by which the surface of the aluminum sheet may be mechanically roughened. The methods of mechanically roughening the surface of aluminum are generally known in the art and include such processes as wire brushing, sand blasting, wet mass slurry graining, dry brush graining and ball graining, these processes being generally taught in "Photography and Platemaking for Photo-Lithography by I. H. Sayre (Lithographic Textbook Publishing Co.) Pp 39-48.

While many of these processes provide satisfactory results in the practice of this invention, it has been found that satisfactory results are achieved when the mechanical graining of the aluminum surface is achieved by a wire brushing procedure, which is well known in the art.

After the surface of the aluminium has been mechanically grained in accordance herewith, the thus grained surface is then electrolytically etched. The electrolytic etching process of the present invention requires the electrolytic treatment of the mechanically grained aluminium with alternating current in an electrolyte bath containing an electrolyte of hydrochloric or nitric acid, or combinations thereof. It has been found that the most satisfactory results are obtained when a hydrochloric acid electrolyte is employed. The conditions under which the electrolytic etching is carried out are those gen rally employed for such purposes. For example, the current density employed for such purposes may vary from 2 to 100 amps per square decimeter of aluminium surface being treated. The temperature at which the electrolytic etching may be performed may vary from about 20 degrees to 80 degrees Centigrade. The time period during which the electrolytic etching may be carried out may vary from about 0.15 to 2.0 minutes per square foot of aluminium surface being treated. The exact parameters of the conditions under which the electrolytic etching may be carried out may.be varied and are within the purview of the skilled worker depending upon the results wished to be achieved in each specific case.

Subsequent to the electrolytic etching of the aluminum surface hereunder, the thus etched aluminum may be further treated to produce the desired lithographic printing plates. Thus, the electrolytically etched aluminum may be subsequently coated with a lithographically suitable photosensitive coating for such purposes, or alternatively, the electrolytical y etched surface may be anodized, for example, with direct current in a suitable electrolyte, such as sulfuric acid, prior to the application to the thus anodized surface of a lithographically suitable photosensitive coating.

The invention may be illustrated by the following Examples: EXAMPLE I A web of aluminum foil, AA 1100 alloy, was grained in an aqueous slurry containing 5.5% of pumice powder and 2.8% quartz powder with a constant pressure nylon brush, under the following conditions: Web Speed: 15 ft/minute Brush Speed: 400 RPM with oscillation Nylon Brush Thickness: 0.012 in.

Reflectance of grained surface: 18% + 2 Roughness of grained surface: Rz = 2 The thus pumice grained aluminum was then chemically cleaned and desmutted and subjected to electrolytic etching by treating the aluminum web having a speed of 3.0 ft/min. with alternating current of 175 amps for a period of 35 seconds in an aqueous electrolyte containing an HC1 electrolyte at a concentration of 2.5% by weight at room temperature. The resultant electrolytically etched aluminum is then rinsed with water and dried.

EXAMPLE 2 A sheet of aluminum foil AA 1100 alloy was grained and electrolytically etched in accordance with the procedures set forth in Example 1 (Plate A). A second sheet of aluminum foil, AA 1100 alloy, was electrolytically etched without prior graining in accordance with the electrolytic graining procedures set forth in Example 1 (Plate B).

Plate A and Plate B were then coated with the same suitable photosensitive coating to prepare presensitized lithographic printing plates which were then exposed to a light source through a negative transparency and then developed to prepare the finished printing plates. Each plate was mounted on a web offset press and run for printing on 48 pound abrasive newspaper stock and standard black offset ink at the rate of 10,000 impressions per hour. After 40,000 impressions, wear was observed on both the plate and the copies obtained with Plate B. After 88,000 impressions no wear was observed with either the plate or the copies obtained with Plate A.

This example demonstrates that the printing plate obtained with the present invention provides a longer, cleaner running printing plate than is obtained with out the practice of this invention.

EXAMPLE 3 In order to obtain a roughness characteristic as measured by standard Perthometer test (Rz) of the aluminum surface which is satisfactory for use in the production of printing plates, it is necessary to electrolytically etch the surface of the aluminum, which requires a certain amount of power consumption. By the practice of the present invention it has been found that in order to obtain a satisfactory roughness characteristic, i.e. Rz = 4, a substantial savings in the use of electric power can be realized.It has been found that the practice of the present invention will save substantial electric power when the aluminum is electrolytically etched as can be seen from the following Table A: TABLEA Required Power (KWHhl')for Power Aluminum Surface Electrolytic Etching Saved Smooth Al Alloy 1100 0.0165 0 Mechanically grained AlAlloyliOO 0.0125 24% The foregoing results demonstrate that the practice of the present invention provides substantial savings in the use of the electric power required for production of lithographic printing plates.

EXAMPLE 4 The procedure of Example 1 may be followed except that the aluminum surface is grained by wire brushes instead of a pumice-quartz slurry, providing equivalent results.

EXAMPLE 5 The procedure of Example 1 is followed except that an equivalent amount of nitric acid is substituted for hydrochloric acid yielding equivalent results.

WHAT WE CLAIM IS: 1. A process for the treatment of aluminum to provide thereon a substantially consistent and uniform roughened surface suitable for lithographic uses, which com- prises: a. mechanically graining at least one surface of a lithographically suitable aluminum sheet; and b. then electrolytically etching said mechanically grained surface by treatment thereof with alternating current in an aqueous electrolytic bath containing an electrolyte of hydrochloric acid, nitric acid or com

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. treated. The exact parameters of the conditions under which the electrolytic etching may be carried out may.be varied and are within the purview of the skilled worker depending upon the results wished to be achieved in each specific case. Subsequent to the electrolytic etching of the aluminum surface hereunder, the thus etched aluminum may be further treated to produce the desired lithographic printing plates. Thus, the electrolytically etched aluminum may be subsequently coated with a lithographically suitable photosensitive coating for such purposes, or alternatively, the electrolytical y etched surface may be anodized, for example, with direct current in a suitable electrolyte, such as sulfuric acid, prior to the application to the thus anodized surface of a lithographically suitable photosensitive coating. The invention may be illustrated by the following Examples: EXAMPLE I A web of aluminum foil, AA 1100 alloy, was grained in an aqueous slurry containing 5.5% of pumice powder and 2.8% quartz powder with a constant pressure nylon brush, under the following conditions: Web Speed: 15 ft/minute Brush Speed: 400 RPM with oscillation Nylon Brush Thickness: 0.012 in. Reflectance of grained surface: 18% + 2 Roughness of grained surface: Rz = 2 The thus pumice grained aluminum was then chemically cleaned and desmutted and subjected to electrolytic etching by treating the aluminum web having a speed of 3.0 ft/min. with alternating current of 175 amps for a period of 35 seconds in an aqueous electrolyte containing an HC1 electrolyte at a concentration of 2.5% by weight at room temperature. The resultant electrolytically etched aluminum is then rinsed with water and dried. EXAMPLE 2 A sheet of aluminum foil AA 1100 alloy was grained and electrolytically etched in accordance with the procedures set forth in Example 1 (Plate A). A second sheet of aluminum foil, AA 1100 alloy, was electrolytically etched without prior graining in accordance with the electrolytic graining procedures set forth in Example 1 (Plate B). Plate A and Plate B were then coated with the same suitable photosensitive coating to prepare presensitized lithographic printing plates which were then exposed to a light source through a negative transparency and then developed to prepare the finished printing plates. Each plate was mounted on a web offset press and run for printing on 48 pound abrasive newspaper stock and standard black offset ink at the rate of 10,000 impressions per hour. After 40,000 impressions, wear was observed on both the plate and the copies obtained with Plate B. After 88,000 impressions no wear was observed with either the plate or the copies obtained with Plate A. This example demonstrates that the printing plate obtained with the present invention provides a longer, cleaner running printing plate than is obtained with out the practice of this invention. EXAMPLE 3 In order to obtain a roughness characteristic as measured by standard Perthometer test (Rz) of the aluminum surface which is satisfactory for use in the production of printing plates, it is necessary to electrolytically etch the surface of the aluminum, which requires a certain amount of power consumption. By the practice of the present invention it has been found that in order to obtain a satisfactory roughness characteristic, i.e. Rz = 4, a substantial savings in the use of electric power can be realized.It has been found that the practice of the present invention will save substantial electric power when the aluminum is electrolytically etched as can be seen from the following Table A: TABLEA Required Power (KWHhl')for Power Aluminum Surface Electrolytic Etching Saved Smooth Al Alloy 1100 0.0165 0 Mechanically grained AlAlloyliOO 0.0125 24% The foregoing results demonstrate that the practice of the present invention provides substantial savings in the use of the electric power required for production of lithographic printing plates. EXAMPLE 4 The procedure of Example 1 may be followed except that the aluminum surface is grained by wire brushes instead of a pumice-quartz slurry, providing equivalent results. EXAMPLE 5 The procedure of Example 1 is followed except that an equivalent amount of nitric acid is substituted for hydrochloric acid yielding equivalent results. WHAT WE CLAIM IS:

1. A process for the treatment of aluminum to provide thereon a substantially consistent and uniform roughened surface suitable for lithographic uses, which com- prises: a. mechanically graining at least one surface of a lithographically suitable aluminum sheet; and b. then electrolytically etching said mechanically grained surface by treatment thereof with alternating current in an aqueous electrolytic bath containing an electrolyte of hydrochloric acid, nitric acid or com

binations thereof.

2. A process as claimed in claim 1 wherein the mechanical graining is obtained by treatment with a slurry of pumice and quartz.

3. A process as claimed in claim 1 wherein the mechanical graining is obtained by treatment with wire brushes.

4. A method of producing a lithographic printing plate which comprises applying a lithographically suitable photosensitive coating to the surface of an aluminum substrate which has been produced in accordance with a process as claimed in claim 1, 2 or3.

5. A process as claimed in claim 1 and substantially as hereinbefore described with reference to any one of the Examples.

6. A method as claimed in claim 4 and substantially as hereinbefore described with reference to Example 2.

7. A lithographic printing plate whenever produced by a process as claimed in claim 4 or claim 6.