Surface Treatment For Aluminum Plates

Abstract

A process for anodically oxidizing aluminum plates in an aqueous solution of trisodium phosphate at certain concentrations and conditions is disclosed. The process provides an aluminum plate which can be used as a printing plate support.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for the surface treatment of an aluminium plate, more particularly, it is concerned with a method for the surface treatment of an aluminum plate by an improved anodic oxidation treatment.

2. Description of the Prior Art

Aluminum (in the present specification this term includes aluminum alloys) has often been used as a support for printing plates because of its excellent flexibility and dimensional stability, light weight, price and ready availability. Utilizing these advantages, aluminum has further been used for building materials, improved weather resistance being obtained by forming an oxide layer on the surface of the aluminum. The formation of such an oxide film layer has hitherto been carried out by anodic oxidation in an electrolytic solution of sulfuric acid, oxalic acid or chromic acid, as is well known in the art.

While it is known that an aluminum plate can be used as a printing plate support, various problems with maintaining the hydrophilic property and printing resistance are encountered with such plates produced by prior art methods.

SUMMARY OF THE INVENTION

It is the principal object of the present invention to provide a method for the surface treatment of an aluminum plate by anodic oxidation to provide a printing plate which is chemically inert to an image forming layer to be coated thereon and which has an excellent hydrophilic property as well as excellent adhesiveness to an image forming layer.

In accordance with the invention, there is provided a method for the surface treatment of an aluminum plate, which comprises subjecting the aluminum plate to anodic oxidation in an aqueous solution containing 5-50 percent by weight of trisodium phosphate (based on the total aqueous solution by weight) at a liquid temperature of 20.degree.-40.degree.C and at current density of 5-20 volts, 0.8-3 amperes per 1 dm.sup.2 for 3-10 minutes. Practically speaking, the process is always practiced at atmospheric pressure, though if one wishes sub or super atmospheric pressure can be used. Little is to be gained by such operation, however.

DETAILED DESCRIPTION OF THE INVENTION

As the aluminum plate, pure aluminum plates and alloy plates such as aluminum alloys with copper, zinc, magnesium and iron are used. Such an aluminum plate is generally subjected to a pretreatment to remove stains and oily materials, for example, by immersing in a 5-15 percent by weight aqueous solution of sodium hydroxide for 30-90 seconds, washing with water and then treating with a 10 percent by weight solution of ammonium bifluoride. Any standard art technique for degreasing can be used in the present invention. Other known method include trisodium phosphate treatment and treatment with organic solvents such as trichlorostyrene, carbon tetrachloride and the like.

The anodic oxidation treatment characteristic of the invention is then carried out in an aqueous solution containing 5-50 percent by weight, preferably 10-30 percent by weight, of trisodium phosphate at a temperature of 20.degree.-40.degree.C and a current density of 5-20 volts, 0.8-3 amperes per 1 dm.sup.2, preferably 8-15 volts, 0.8-2 amperes, for 3-10 minutes, preferably 4-8 minutes. Excellent results are provided when the anodic film is deposited in an amount of about 10 mg to about 200 mg per square meter of support, more practically for commercial usage, at about 50 mg to about 100 mg per square meter of support. These ranges are not, of course, limitative, but provide a product of excellent properties. A relatively inert material such as lead is used as a cathode. Needless to say, the present invention is not limited to lead as a cathode, though this is commonly used because of its low cost and the fact it is not corroded by the electrolytic solution. Aluminum and other materials can also be used, though for economic reasons lead will usually be used.

The thus obtained aluminum support is an excellent support for plate making. In the case of using it as a support for plate making, a thin layer of a water-soluble high molecular weight compound can be provided on the anodic oxide film weight as occasion demands. Excellent results are obtained when such a water-soluble high molecular weight compound is provided in a thickness of 0.05 microns to 0.2 microns, better yet about 0.1 micron.

Illustrative of the water-soluble high molecular weight compound are polyvinyl alcohol, polyacrylamide, polyvinyl pyrrolidone, gelatin, hydroxyethyl cellulose, carboxymethyl cellulose, casein and sodium alginate.

The water-soluble high molecular weight compound is generally provided in an amount of about 10-30 per 1 m.sup.2 by coating or immersing. The hydrophilic property of the substrate is further increased by providing such a film of the water-soluble high molecular weight compound. Though not especially limitative, good results are provided when the high molecular weight compound has a molecular weight of from about 10,000 to about 500,000, better yet from about 50,000 to about 150,000. This includes most materials as above as they are commercially available. Moreover, a higher hydrophilic property is given by carrying out a graining treatment prior to the above described anodic oxidation.

An image forming layer is then provided thereon. Various conventional light-sensitive materials for photographic printing plates are used as such an image forming component. However, cinnamic acid ester type light-sensitive resins are particularly desirable in view of this printing resistance and light sensitivity. Typical of such light-sensitive resin are KPR (Kodak Photo Resist, manufactured by Eastman Kodak Co. KPR has, as its active chemical constituents the repeating group: ##SPC1##

polymeric materials as reported by Kato et al ("Gazo Gijutsu (Image Technique)" March (1971), page 18) and polymeric materials as disclosed in Japanese Patent Application Sho-46-22209. All of these materials are highly sensitive and useful for the production of printing plates.

These materials are high molecular weight compounds having the following recurring structural unit (generally, these materials are available at a molecular weight of from about 10,000 to about 500,000, and are most preferably used at a molecular weight from about 50,000 to 100,000): ##SPC2##

One of these light-sensitive high molecular weight compounds is dissolved in a suitable solvent, for example, cellosolve acetate, in which a pigment and/or a sensitizer is dispersed, and then coated onto the aluminum plate subjected to the above mentioned anodic treatment, thus obtaining the desired plate for printing plate manufacture. The thus resulting material is then brought into intimate contact with a transparent negative and exposed to a high voltage mercury lamp to yield insoluble exposed areas. The non-exposed areas, i.e., background, can be removed with a suitable organic solvent such as dimethylformamide or cellosolve acetate which does not affect the exposed areas. When a pigment is contained in the image area of the resulting printing plate, the quality of the image can readily be distinguished.

As the sensitizer, 5-nitroacenaphthene, Michler's ketone and 2,4,7-trinitrofluorenone are preferably used. A suitable pigment is copper phthalocyanine. When the plate is fitted to an offset press and printing is carried out on a sheet of paper with a printing ink, a print of good image quality can be obtained with a printing resistance of one hundred thousand or more.

The amount of sensitizer can vary greatly, and can easily be selected by one skilled in the art depending upon the high molecular weight compound used and the sensitizer selected. Generally speaking, however, from about 1 percent to about 15 percent of a sensitizer, more preferably 3 weight percent to 10 weight percent, is used in most commonly encountered systems, and such usually provides excellent results. This range is not, however, limitative.

As is apparent from the foregoing description, the printing plate using the aluminum plate processed according to the invention is excellent with respect to adhesiveness to an image forming film, water maintenance on non-image areas prevention of adherence of ink to incorrect portions of the printing plate and maintenance of water on correct portions of the printing plate and wear resistance, that is, printing resistance, which are favourably compared with those of the prior art.

The following examples are to illustrate the invention in more detail without limiting the same. Unless otherwise indicated, in all examples parts are part by weight.

EXAMPLE 1

An aluminum plate, the surface of which was grained by brushing, was immersed in a 10 percent by weight aqueous solution of caustic soda at room temperature for 30 seconds, washed with water and immersed in a 10 percent by weight aqueous solution of ammonium bifluoride for 90 seconds at room temperature to remove stains and oily matters on the surface. The aluminum plate was then washed well with distilled water, dried and then subjected to anodic oxidation by immersing it as an anode in a 20 percent by weight aqueous solution of trisodium phosphate at a liquid temperature of 30.degree.C for 5 minutes with a lead plate cathode. The voltage was 10 volts and the current density was 1 ampere per 1 dm.sup.2.

The plate was then taken out of the electrolytic bath and washed well with water. The thus treated aluminum plate was coated with a solution forming a light-sensitive layer having the following composition, followed by drying.

______________________________________ poly-.beta.-cinnamoyloxyvinyl ether 1 part (limit viscosity [.eta.]=0.216 dl/g determined in methyl ethyl ketone) 5-nitroacenaphthene 0.08 part dibutyl phthalate 0.1 part ethyl cellosolve acetate 19 parts ______________________________________

The thickness of the light-sensitive layer coated was about 1.5 microns.

The resulting printing plate was brought into intimate contact with a transparent negative, exposed for about 80 seconds with a 50 watt mercury lamp at 10 cm using a Plano PS Printer A 3 (made by Fuji Photo Film Co.) and developed, by rubbing the surface with an absorbent cotton pad impregnated with cellosolve acetate at room temperature for about half a minute. The high molecular weight compound at non-exposed areas was dissolved and the hydrophilic surface appeared.

When the resulting plate was fitted to an offset press and printing was carried out with wetting water, about sixty thousand prints of good image quality were obtained.

EXAMPLE 2

An aluminum plate as in Example 1 was subjected to anodic oxidation as in Example 1 and coated with a 0.3 percent by weight aqueous solution of polyacrylamide (molecular weight=12,000) in a coating amount of about 30 mg/m.sup.2 and dried. Then, the plate was coated with a light-sensitive solution having the following composition and dried.

______________________________________ p-phenylene diethoxy acrylate- 1 part 1,4-di-.beta.-hydroxyethoxycyclo- hexane polycondensate (1:1 molar) (2-benzoylmethylene)-1-methyl- 0.08 part .beta.-naphthothiazoline hydroquinone 0.02 part monochlorobenzol 19 parts pigment (Heliogen Blue K) 0.2 part ______________________________________

When the resulting plate was exposed for about 100 seconds using the light source of Example 1, developed with .gamma.-butyrolactone as in Example 1 washed with water, fitted to an offset press and printing was carried out, about 150,000 prints of good image quality were obtained.

EXAMPLE 3

An aluminum plate which had not been sand set was treated by immersion in a 10 percent by weight aqueous solution of caustic soda for 30 seconds and then in a 10 percent by weight aqueous solution of ammonium bifluoride for 90 seconds and washed well with distilled water. All processings were at room temperature.

Then the treated plate was then subjected to anodic oxidation by immersing it as an anode in a 20 percent by weight aqueous solution of trisodium phosphate at a liquid temperature of 30.degree.C for about 6 minutes with a lead plate cathode. The voltage was 12 volts and the current density was 1.2 amperes per 1 dm.sup.2.

The plate was then taken out of the electrolytic bath and washed well with water. The resulting aluminum plate was coated with the light-sensitive solution used in Example 1 to a thickness of about 1.5 microns, followed by drying.

When the resulting plate was exposed and developed, followed by printing, all as in Example 1, about thirty thousands good image quality prints were obtained, but the printing resistance was inferior to that of Example 1. This was possibly due to the fact that the water maintenance was lowered due to lack of sand setting.

EXAMPLE 4

An aluminum plate which had previously been grained by brushing was immersed at room temperature in a 10 percent by weight aqueous solution of caustic soda for 30 seconds and then in a 10% by weight aqueous solution of ammonium bifluoride for 90 seconds to remove oily substances thereon and washed with distilled water for 10 minutes. All these processings were at room temperature.

The thus treated aluminum plate was then subjected to anodic oxidation by immersing it as an anode in a 25 percent by weight aqueous solution of trisodium phosphate at a liquid temperature of 30.degree.C and applying a voltage of 12 volts at a current density of 1.5 ampere/dm for 4 minutes, using an apparatus as described in Example 1.

The plate was then taken out of the electrolytic bath, washed well with water and dried.

The plate was then coated with a 0.2 percent by weight aqueous solution of polyvinyl alcohol (hydrolysed to 90 percent, molecular weight = about 10,000) followed by drying, the dry thickness of the polyvinyl alcohol layer was 0.05 .mu.) and then coated with a light-sensitive solution having the following composition to give a coating thickness of about 2 microns and a proportion of about 2.3 g/m.sup.2.

______________________________________ poly-.gamma.-cinnamoyloxy-.beta.-oxypropyl 1 part methacrylate (molecular weight about 150,000-170,000) N-acetyl-4-nitronaphthylamine 0.08 part dioctyl phthalate 0.1 part pigment (Microlith Blue 4GK) 0.15 part methyl cellosolve acetate 6 parts methyl ethyl ketone 10 parts ______________________________________

The thus resulting printing plate, being dark blue, was brought into contact with a transparent negative, exposed for 100 seconds at a distance of 25 cm from a lamp (Toshiba SHL-100 UV) and then the organic substance at non-exposed areas dissolved off by rubbing with a cotton pad impregnated with dimethylformamide, followed by washing with water. There was thus obtained a sharp plate containing the pigment in exposed areas. When printing was carried out by means of an ordinary offset press, about twelve thousand sharp image prints were obtained.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims

What is claimed is:

1. A method for the surface treatment of an aluminum plate which comprises subjecting the aluminum plate to anodic oxidation in an aqueous solution containing 5-50 percent by weight of trisodium phosphate (based on the total aqueous solution weight) at a liquid temperature of 20.degree.-40.degree.C and at a current density of 5-20 volts, 0.8-3 amperes per 1 dm.sup.2 for 3-10 minutes, thereby forming an anodically oxidized layer on said aluminum plate.

2. The method of claim 1 wherein said anodically oxidized layer is formed in an amount of from about 10 mg to about 200 mg per square meter of said aluminum plate.

3. A method as claimed in claim 2 wherein said anodically oxidized layer is formed in an amount of from about 50 mg to about 100 mg per square meter of support.

4. A printing plate comprising image and nonimage areas, the image areas being carried on an aluminum plate which has been subjected to anodic oxidation as in claim 1.

5. The printing plate of claim 4 wherein said image areas comprise a photoinsulubilized polymer.