US 3756826 A
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United States Patent Oflice Patented Sept. 4, 1973 1 3,756,826 TREATMENT OF ALUMINUM PREPARATORY TO APPLICATION OF PHOTOSENSITIVE COATING THERETO Walter G. Zelley, Lower Burrell, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa. No Drawing. Nov. 15, 1971, Ser. No. 199,031
Int. Cl. G03c 1/94 U.S. Cl. 96-86 12 Claims ABSTRACT OF THE DISCLOSURE Use of ammoniacal or amine tartrate, citrate, malate or the like at pH 8.1-9.9 in anodizing aluminum preparatory to application of photosensitive coating thereto.
BACKGROUND OF THE INVENTION Field of the invention Description of the prior art In U.S. Pat. No. 2,714,066, Jewett et a1. claimed to have produced for the first time a commercially acceptable pre-sensitized metal-backed planographic plate. They attributed their success to a silicate treatment for rendering the surface of aluminum permanently hydrophilic in addition to having a scum-preventing and tone-reducing film. The patentees claimed that the smooth silicatetreated surface of their plate had the property of tenaciously holding an image formed by exposure to light of light-sensitive organic nitrogen-containing material, e.g., light-sensitive diazo compounds, on the surface theretof. Subsequently, Fromson, in U.S. Pat. No. 3,181,461, found that, although the sensitized image-forming diazo resin will adhere to a layer formed by the reaction of aluminum with the silicate, a light-sensitive material such as polyvinyl alcohol containing a bichromate will, after exposure to light, wash or rub off easily. Fromson found, however, that such a light-sensitive material, when applied to a layer consisting of the reaction product of the silicate and porous aluminum oxide, after exposure to light, will adhere firmly and indefinitely to the plate. The same patentee, in U.S. Pat. No. 3,280,734, disclosed that watersoluble materials such as ammonium bichromate, sodium bichromate, potassium bichromate, ferric ammonium oxalate and any of the water-soluble dyes that will dye aluminum oxide surfaces, when deposited on the oxide surface before the application of the light-sensitive material thereto, would, when the solvent is substantially driven off, be helpful as pore filler materials and as substrates for application of the photosensitive material thereto.
SUMMARY OF THE INVENTION After extended investigation, I have found that the aluminum surface can be prepared for application of a photosensitive or light-sensitive material such as a diazo coating thereto in a single step by employing in the anodizing bath which imparts the aluminum oxide coating to the aluminum an ammoniacal or amine ester of a water-soluble dibasic or tribasic organic acid having at least one hydroxyl group, e.g., tartrate, citrate or malate. To obtain the desired tartrate, citrate and malate or like anodizing bath according to my invention, I adjust the pH to from about 8.1 to about 9.9 with ammonia (preferably ammonium hydroxide) or an amine, preferably to a pH of from about 8.8. to about 9.2, the optimum being about 9'. Thus, tartaric, citric, malic or like acid, according to my invention, may be converted in the anodizing bath to the aforementioned amine or ammoniacal tartrate, citrate, malate, or like ester by this adjustment of pH. The preferred compounds used for adjusting the pH according to the invention are ammonium hydroxide and triethanolamine.
In order to insure that a satisfactory image is produced when the photosensitive coating is subsequently applied to the aluminum anodized according to my treatment method, when exposed to light to develop the image, it is preferred to perform the anodizing at a temperature of at least about F., preferably at a temperature of from about to F. (60 to 76 C.), 150 F. (65.6 C.) being optimum. At below 130 F. (54 C.), only an undesirably weak image is produced. Use of temperatures higher than F. also tend to result in undesirable images. The preferred anodizing time is 2-5 minutes. At less than 1 minute, the image produced may be somewhat weak. In the greater than 5-minute range, there is some danger of background yellowing. An anodizing period of about 3 minutes gives an optimum coating. A current density of about 3 amp/ft. (a.s.f.) is adequate for consistent satisfactory results according to the invention at a reasonable voltage level, e.g., at from about 20 to about 26 volts. The preferred amount of acid or ester employed in the bath according to the invention is at least about 0.5% by weight.
Experimental work has shown that, for consistent results, sodium or potassium hydroxide cannot be satisfactorily substituted for the ammonia or amine in adjusting the pH for the anodizing bath. Experimental work has also shown that oxalic acid cannot be satisfactory used in this process to produce a material on which a reasonably good image can be developed.
Conventional aluminum lithographic plate or sheet, such as Aluminum Association Nos. 1100 and 3003, are useful according to the invention, that is, may be treated by the anodizing procedure described hereinabove. The process is also generally usable on other aluminum base non-heat treatable sheet alloys, particularly on alloys containing at east 85% aluminum. Conventional caustic or acid etching treatments may be employed to prepare the aluminum for treatment according to the invention, particularly when it is in the form of a brushed grain or ball grained plate, due to the residual silica abrasive on the grained surface. For example, a light caustic etch (10 sec. and 5% NAOH at 135 F.) followed by a nitric acid desmutting treatment or a treatment in nitric acid plus ammonium bifluoride (10-15 sec. in 50% vol. HNO'g, 25 g./l. NH HF at room temperature) may be used to remove any impacted abrasive. In any event, it is helpful if the surface is well cleaned prior to the anodizing treatment used according to my invention.
The oxide film formed by the citrate, tartrate, malate, or like anodizing treatment of this invention will be in the order of 0.00001-inch when 3 minutes processing time is employed. This is sufficient for bonding of the photosensitive coating which is applied to the coated aluminum prior to development of the image thereon.
The concentration of the tartaric or other acid used according to the invention is not critical, the minimum requirement, as stated hereinabove, being generally about 0.5% by weight (5 g./l.). Obviously, for economic reasons it is not particularly profitable to employ concentrations higher than about 30 g./l. or 3% by weight, although slightly higher concentrations may be employed if desired.
If desired, according to the invention the required 8.1 to 9.9 pH may be obtained by adding the ammonium or DESCRIPTION OF THE PREFERRED EMBODIMENT The following examples are illustrative of the invention:
Example 1 Press-size plates of grained aluminum (Aluminum Association No. 1100) lithographic sheet were pretreated to remove any residue as follows:
(1) 10 sec. NAOH at 135 F. (2) 30 sec. 50% HNO at 80 F.
The pretreated plates were then anodized according to the procedure of the invention as follows:
2 min. 3% tartaric acid at 150 F., 3 a.s.f., anodic, pH 9.0 adjusted with NH OH Subsequent to the foregoing steps according to the process of the invention, the plates were treated as follows:
( 1) Rinse (2) Deionized water rinse One of the plates was tested by applying a diazo resin thereto according to conventional procedure for adapting it for production of an image thereon, exposing it to light through a negative, and then developing the image. A good image was produced.
Example 2 A lithographic plate was prepared similarly to those of Example 1. It was treated with a conventional wipe-on chemical to make it susceptible to production of an image thereon. Upon exposure, an image was produced which looked good. Approximately 1700 impressions were made 7 therefrom, and the resulting copy was also good.
Example 3 A 3003 (Aluminum Association designation) aluminum base alloy was cleaned and treated according to the anodizing process described in Example 1 hereinabove. A good image was produced upon subsequent treatment with a diazo resin and exposure to a light source.
The steps of Examples 4 and 5 are given principally in imperative form, in the order performed, for the sake of simplicity.
Example 4 Brush grained 1100 alloy litho sheet.
Etch sec in 5% NAOH at 135 F. to remove residual abrasive. Desmut in 50% HNO 80 F., 30 sec.
Anodize for 2 min. in ammonium tartrate electrolyte, 150 F., 3 a.s.f. Electrolyte was prepared by dissolving 30 g./l. tartaric acid and neutralizing to pH 9 with ammonium hydroxide.
Apply wipe-on diazo photosensitive coating. Expose with negative in platemaker. Develop image.
Mount on press.
Example 5 As-rolled 1100 alloy litho sheet.
Etch 30 sec. in 5% NAOH at 150 F. Desmut 30 sec. in 50% HNO at 80 F.
' 4 Anodize for 3 min. in ammonium tartrate electrolyte, 150 F., 3 a.s.f. Electrolyte was prepared by dissolving 37 g./l. ammonium tartrate and adjusting to pH 9.
Apply wipe-on diazo photosensitive coating. Expose with negative in vacuum frame and arc light. Develop image.
In Examples 4 and 5 appropriate water rinses are employed after the chemical and electrochemical treatments.
While the invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass all embodiments which fall within the spirit of the invention.
Having thus described my invention and certain embodiments thereof, I claim:
1. A process for production of a surface suitable for coating with a photosensitive coating and then develop ing an image thereon by exposure to a light source, said process comprising forming an anodic oxide coating on aluminum by anodizing it in an aqueous dispersion of an amine or ammoniacal salt of a water-soluble dibasic or tribasic organic acid having at least one hydroxyl group and maintaining the pH of said dispersion during said anodizing at from about 8.1 to about 9.9.
2. The process of claim 1 wherein the temperature of the dispersion is maintained during the anodizing at a temperature of from about F. to about 190 F.
3. The process of claim 1 wherein the pH of the dispersion is maintained during the anodizing at from about 8.8 to about 9.2 and the temperature of the dispersion at from about F. to about F., and the anodizing is continued for a period of from about 1 minute to about 5 minutes.
4. The process of claim 1 wherein the amine or ammoniacal salt is produced in the dispersion by adding thereto a sufiicient amount of a compound selected from the group consisting of amines and ammonium compounds to obtain the pH of from about 8.1 to about 9.9.
5. The process of claim 4 wherein the compound is ammonium hydroxide or triethanolamine.
6. The process of claim 1 wherein, subseqeunt to the anodizing, the aluminum is coated with a photosensitive material.
7. The process of claim 1 wherein, subsequent to the anodizing, the aluminum is first treated with a diazo compound and thereafter an image is developed on the surface by exposure to a light source.
8. The process of claim 1 wherein the aluminum subjected to the anodizing is in the form of plate or sheet.
9. The process of claim 1 wherein the dispersion contains at least about 5 g./l. of the salt.
10. The product formed by the process of claim 1.
11. The product formed by the process of claim 6.
12. A process for production of a surface suitable for preparation for developing an image thereon by exposure to a light source, said process comprising forming an anodic oxide coating on aluminum by anodizing it in an aqueous dispersion of an amine or ammoniacal salt of a water-soluble dibasic or tribasic organic acid having at least one hydroxyl group and during the anodizing maintaining the temperature of the dispersion at from about 130 F. to about F. and the pH at from about 8.1 to about 9.9.
References Cited UNITED STATES PATENTS 3,321,385 5/1967 Fazzari 9686 R NORMAN G. TORCHIN, Primary Examiner R. L. SCHILLING, Assistant Examiner US. Cl. X.R. 96-49; ll749