CA1242407A - Method of producing full colour images on aluminum - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A process for the production of full color, partial colored or two colored pictures, scenes, words, etc., on articles of aluminum or aluminum alloys. The process involves first forming a hard, dense, non-colored anodic coating of between about 10 microns and 26 microns on aluminum or aluminum base alloys by anodizing the aluminum in an acidic aqueous electrolyte comprising sulfuric acid, a polyhydric alcohol of from 3 to 6 carbon atoms and an organic carboxylic acid containing at least one reactive group in the alpha position; and coloring the coating by intimately contacting the unsealed coating with a paper containing a dye or dyes capable of subliming and then heating the surfaces causing the dye to sublime and transfer to the unsealed anodic film.

Description

BRIEF D~SCRIPTION OF THE INVENTION
Field of the Invention This invention relates to a process for the production of partial or full color images, pictures, scenes or the like on 5 articles o~ aluminum or aluminum alloys which have been previously specially anodized in order to obtain products which are useful for all types of industrial, commercial and consumer use. The products include decorative panels, nameplates, belt buckles, instrument panels, trophy articles, 10 and others.
Prior Art Many attempts have been made to economically produce multicolor images on aluminum. For example, U.S. Patent 3,264,158 describes a process wherein oil soluble dyes are 15 applied to a decal and an unsealed, anodized surface is then wetted with a solvent to cause dissolved dye to stain the surface. U.9. Patent 3,258,381 describes a similar process.
U.S. Patent 3,218,243 discloses a method wherein water-soluble dyes are used. U.S. Patent 3,242,037 describes yet another 20 process using a dye film on a solvent wetted anodic surface.
Another U.S. Patent 3,193,416, discloses a vat dye and compatible solvents. Another approach ~as taken in U.S.
Patent 3,515,598 whexein a silk screening type ink is utilized for screen printing dyes onto an anodized surface. A system 25 for coloring an anodized surEace was disclosed in U~S. Patent 3,718,548 wherein a meltable organic coloring matter is brought in contact with an anodized surface and is then melted into the oxide pores. Finally, U.S. Patent 3,079,309 discloses a system whexein a water base ink is manufactured using water soluble dyes and pigments. This is applied to an oxide surface wherein the surface is stained. None of these 5 prior art patents disclose a process capable of producing crisp, sharp images, or of producing pictures on a mass production basis, or of producing images having adequate clarity or durability. In addition, the processes of these prior art patents are cumbersome to practice and are not 10 capable of producing durable, full color photographs on aluminum which are comparable to those taken by a quality camera and printed on photographic paper.
U.S. Patent 3,363,557 discloses a heat transfer of indicia containing sublimable coloring agent. The process ; 15 of this patent is particularly directed to printing inks for textile coloring and those inks, in accordance with the invention, comprise an organic resinous binder which when deposited supplies a dry solid film which remains solid and dry upon exposure to elevated temperatures. This patent is 20 directed to and provides great detail about using ; sublimation type transfers for coloring cellulose and other organic materials used in the textile, carpet, plastic and garment industries. The patent gives several examples, each of which is concerned with placing images on textiles. In 25 addition, the patent mentions that images can be provided on ~Imetallic surfaces especially anodized aluminum". While the patent is clearly principally directed to the coloring of organic materials and the process disclosed will not provide any image on most metallic surfaces, it is disclosed that an image can be Eormed on an unsealed, anodized surface.
However, unless the anodic coating is of a very specific type, not disclosed in the patent, the image produced is fugitive, subject to fading and of limited or no commercial value. Attempts over the past few years to use the standard commercial anodizing processes, which are in general practice throughout the world, have resulted in colored 10 products which at first appeared satisfactory but later, thxough natural weathering and exposure to sunlight, proved valueless. The coatings produced by the heat transfers evaporated ~resublimed) or were destroyed by the natural or artificial ultra violet radiation.
We have now discovered that the use of a special anodizing techrlique produces a superior, extremely dense and hard anodic coating optimally suited to application of single or multiple colors by transfers produced as described in U.S.
Patent 3~363,557. It has been found that the anodizing system 20 disclosed in U.S. Patent 3,52~,799 can be modified to produce an ideal surface for subsequent coloring. This patented anodizing system was developed for produlcing- a white surface on space vehicles and was not intended to receive coloring.
In fact, the stark white surface was absolutely required to 25 reflect heat encountered by space vehicles and rockets.

Summary of the Invention The present invention provides a method of decorating articles of aluminum comprising the steps of:
anodizing said aluminum in an aqueous acid electrolyte consisting essentially of from about 165 to 250 grams per liter sulfuric acid, from about 10 to 30 milliliters per liter of an organic carboxylic acid containing at least one reactive group in the alpha-position wherein said reactive group is a hydroxy, amino, keto or carboxyl group, and from about 10 to 30 milliliters per liter of a polyhydric alcohol of from 3 to 6 carbon atoms, with the temperature of the electrolyte being main~ained at be-tween about 15C and 24C and the current density being maintained at bet,ween about 200 to 535 amps per square meter so as to form an anodized layer on the surface of said aluminum article, said layer having a thickness of between about 10 and 26 microns;
pla~ing a dry film in intimate contact with said anodized layer, said dry film containing a dye capable of subliming when heated;
heating said dry film while in intimate contact with said anodized layer for a time sufficient to cause at least a portion of said dye to sublime and condense within said anodized layer; and !
thereafter sealing said anodized layer on the surface of said aluminum article.
According to the present invention, a novel process is disclosed for the production of full color, partial color, or ~;
., any mixture of colors on aluminum or aluminum alloys. This includes but is not restricted to full color pictures, designs, images, and the like. The anodi~ing is carried out in a sulfuric acid electrolyte containing a polyhydric alcohol of 3 to 6 carbon atoms and an organic carboxylic acid containing at least one reactive group in the alpha-position. The preferred polyhydric alcohol is glycerine and the pre-ferred carboxylic acid is hydroxy acetic acid. Anodizing optimally is carried out with a current density of about 330 amps per square meter, with sufficient anodizing time to produce an anodic film thickness of between about 10 and 26 microns, The anodically formed aluminum oxide is then carefully washed to remove the electrolyte and is allowed to air dry. During the drying operation, care must be taken to keep the surface clean so that the pores in the anodic film will remain open. ~ heat transfer sheet, previously prepared by printing or hand painting using colorants which are capable of subliming when heated, is placed on the unsealed film and heated usually to about 160 to 220C during which time the image from the sheet is transferred into the pores of the anodic film in vivid detail. The completed rendering is then sealed.
DET~ILED DESCRIPTION
In order to obtain a durable anld desirable colorless hard anodic coating with its peculiar pore structure, it is absolutely critical that the anodic layer be maintained between 10 and 26 microns. It is preferable that the anodic layer is formed at a current density of between about 220 and 440 amps per - 5a ~ 9912-72 square meter, with the optimum current density being 330 amps per square meter and the optimum film thickness being 15 microns.
Further, the anodizing electrolyte must be maintained at a temperature of between 15 and 24C with the optimum being 21C
and its chemistry as disclosed in Table l.

Minimum Optimum Maximum Sulfuric ~cid 165 grams/liter 200 g/l 250 g/l Hydroxy Acetic Acid 10 milliliters/liter 20 ml/l 30 ml/l Glycerine 10 milliliters/liter 20 ml/l 30 ml/l It should be noted that the electrolyte of Table l is similar to that disclosed in U.S. Patent 3,524,799, but without any titanic acid salt being included. It has been found -that the titanic acid salt acts as a pigment resulting in some pore closure in the anodic film and a non-desirable surface for accepting the sublimation dyes from the heat transfer.
The transfers may be printed on any suitable substrate material, with paper being preferred and inks used for the pre-paration of transfers for the te~tile industry, such as are dis-closed in U.S. Patent 3,363,557 are accept~ble. The transfers may be printed by means of offset or gravure printing, for example.
Also, transfers can be hand painted ~' using these inks and the rendering so painted can then be transferred to aluminum in accordance with the present invention. Other printing or screening methods may also be used to produce the transfers.
To illustrate this unique and novel technology and to also compare it with val~eless technology, the following examples are provided:
Example 1 Heat transfers were produced by offset printing using 10 color separations made from a 35 millimeter slide of a lion taken in a jungle. The full color picture was printed on standard quality printing paper of size about 6.35 centimeters by 11.43 centimeters. The sublimation printing ink used was made by Colonial Printing Ink Company of New Jersey who 15 manufactures this type ink for making heat transfers for the garment and carpet industry. ~ transfer was placed tightly against a clean, steel surface and the surface was heated 190C for 2 minutes. No image was formed on the steel.
Example 2 The procedure was repeated as discussed in Example 1 with the transfer being placed tightly against clean metallic surfaces of tin, nickel chromium, zinc alnd anodized aluminum as used in the architectural and building industry. No image was formed on any of these metallic surfaces even when the 25 time of contact and the ~emperature of contact were varied.

It appears images were not formed on the metallic surfaces because there were no pores available to accept the dye as it sublimed from the printed paper. Conse~uently, the dye just evaporated into the air.
~xample 3 Anodized aluminum pieces of size 7.62 centimeters by 12.7 5 centimeters were produced in a standard sulfuric acid electrolyte conventionally used throughout the world. This electrolyte is normally 175 grams per liter sulfuric acid maintained at 22~. Anodizing is carried out at a current density of about 130 amps per square meter. More detail of 10 the process is described in the Metal Finishing Guidebook and Directory published by Metal and Plastics Publications, Inc., Hackensack, New Jersey. An anodic film of a thickness of about 15 microns was produced by anodizing in the conventional electrolyte at 130 amps per square meter for 35 minutes. The 15 aluminum pieces were rinsed free oE electrolyte with tap water and allowed to air dry. Care was taken not to touch or dirty the surface. Lion picture heat transfers produced as described in E~ample 1 were placed in intimate contact with the anodized surfaces and were heated to 190C. The 20 temperature was maintained for 2 minutes. The heat transfers were immediately removed and perfect im~es of the lion were faithfully reproduced on the unsealed anodized surface; the colors apparently having penetrated at least partially into the pores o~ the anodic film. These samples were then further 25 processed as follows:
Piece A was left, as decorated, in normal room light.
After 3 months, the image was apparently lighter. After months, the i~age was approximately half vivid and after one year the lion could hardly be discerned and the green jungle grass background was completely faded out.
Piece B was placed in 8~C water as used by some 5 anodizers to seal anodic surfaces. After 15 minutes, the piece was removed and the lion picture was observed. It was practically bleached out and the picture was unacceptable for any practical use.
Piece C was placed in a closed steam chamber, similar to 10 those commonly used for steam sealing of anodic coatings.
Saturated steam was formed in the chamber and the piece was sealed for 15 minutes. The picture of the lion, while not greatly faded, was streaked by dye which had run from the anodic film pores. The picture was worthless.
Piece D was sealed in a water solution of nickel acetate at a temperature of 93C and concentration of 5 grams per liter. Time of sealing was 15 minutes, in accordance with standard practices in the anodizing industry. The lion picture was apparently as bright as it was prior to sealing ZO but the surface of the picture had a slight velvet-like coating. This was easily removed by rubbing with a cloth.
The image appeared satisfactory. Several other samples were prepared in this manner and were tested as follows:
1. A sample was placed on a roof with a southern 25 exposure~ The image on the sample was noticeably faded in 5 days, and almost completely faded in 30 days. The picture was therefore unacceptable for commercial or decorative use.

~ . Samples were made into belt buckles that were worn by adults and youth. Periodically, during an 18 month time period, they were observed. The surface on all of the buckles was seriously scratched and light to heavy fading of the image was noted.
It is apparent from the above tests that the standard sulfuric acid anodizing process produces an unsatisfactory surface for coloring using sublimation dye heat transfer techniques. Further, it is also apparent that most metallic surfaces are also worthless as surfaces upon which a picture may be transferred. Based on the testing conducted, it became apparent that even conventionally anodized aluminum surfaces would not be satisfactory for use with aluminum. Hard anodizing processes including those used for engineering and industrial purposes were also considered. All known processes produced an integrally colored surface varying in color from light bronze to gray and black. These dark surfaces were obviously unsuited for decorating with bright colors, pastel shades, etc. Even non-standard, European anodizing processes 20 which utilize oxalic acid or mixtures of o~alic acid and sulfuric acid ~ere tried. These processes produced gray or dark surfaces unsuitable for decorating.! Chromic acid anodizing was also considered and tried but it also resulted in a gray surface. Further research was made with the standard sulfuric acid anodizing processes, attempting to obtain a more dense, durable surface. It was thought that by cooling the eleçtrolyte and increasing the current density ~ .

- 10 - g912-72 from the normal 130 amps per s~uare meter, a satisfactory surface might result. This was tried, but a gray surface resulted.
Anodizing temperatures of 20 up to 24C were tried with increased current density but all the anodic films produced became smutty and powdered off. The following examples are illustrative of use of the anodizing system of the invention as disclosed in Table l.
Example 4 Pieces of aluminum allo~ 5052 were anodized at 330 amps per square meter using the electrolyte disclosed in Table l, having a minimum concentration of chemicals. Electrolyte temperature was 20C and the anodic film was about lO microns thick. The lion was heat transferred to the clean air dried anodic film at a temper-ature of 190C by holding the transfer in contact with the anodic ~ilm for 1 minute using a hand flat iron. An almost perfect copy of the lion resulted. Its colors were just a shade light.
Example 5 An aluminum sample, decorated with the lion was prepared as discussed in Example 4 except the optimum anodizing electrolyte was used as disclosed in Table l. The electrolyte temperature was 21C. The anodic film thickness was about 15 microns The copy of the lion obtained matched the original 35 millimeter slide from which it was copied.
Example 6 An aluminum sampe was decorated as discussed in Example 5, except the anodic film thickness was about 26 microns. The ~;

: ~, lion picture was acceptable. However, it was not quite as bright as the one prepared per Example 5. It i5 believed that the pores in the thicker anodic film are smaller because of the thicker coating and thus cannot as readily accept the vaporized dye.
Example 7 An aluminumsample as decorated with the lion as discussed in Example 4 except an electrolyte temperature of 24C
was used and the maximum electrolyte strength was used as shown in Table 1. The anodizing current density was 440 amps per square meter, the anodization being carried out long enough to produce a film thickness of about 15 microns. A perfect picture of the lion resulted. The inventors observed that the anodic film seemed slightly softer than the previous samples when tested by drawing a metal working file across the surface.
Example 8 An aluminumsample was decorated with the lion as per Example 7, except a current density of 220 amps per s~uare meter was used. The picture of the lion was perfect except the anodic film seemed slightly softer as tested in Example 7 Example 9 An aluminumsample was decorated with a lion as per Example 7, except that a current density of 48 amps per square meter was used. The picture of the lion was perfect except the anodic film seemed soft when tested with a file per Example 7.
The anodic film appeared to be on the verge of burning (chalking).

~ ~1 Many decorated aluminum samples of the lion were prepared as discussed in Examples 4 through 9. They were accomplished on aluminum alloy 5052. Some were sealed in the nickel acetate solution per Example 3, piece D. Others were left 5 unsealed. Part of the 2 l/2 by 4 inch lion samples were converted to belt buckles and the others left as decorated aluminum plates. These articles were then kested for 18 months. The test results are summarized in Tables 2 and 3.

10 Decorated Samples Per Examples--Fading Observation*

Image Treatment/Exposure Examples Unsealed-inside exposure SF SF SF SF GF GF
Sealed-inside exposure NF NF NF NF SF SF
15 Outdoor Southern exposure-sealed GF SF GF SF GF GF
Outdoor Southern exposure-unsealed LI GF GF GF LI LI
Worn as belt buckles-sealed NF NF NF NF SF SF
Worn as belt buckles-sealed GF SF GF GF GF GF

*LI = Loss of Image SF = Slight Fading GF = Grea~ Fading NF = No Noticeable Fading Decorated Samples Per Examples--Abrasion/Scratch Resistance*

25 Image Treatment/Exposure Examples 4 l5 6 7 8 9 Worn as belt buckles-unsealed SS NS NS SS MS MS
Worn as belt buckles-sealed SS NS NS NS SS MS

*NS = No appreciable scratches SS = Slight scratches MS = Many deep scratches and abrasions From the tests conducted, it is apparent that the limits for the ancdizing electrolyte chemistry and film thickness are established for satisfactory practicing of the invention. It is evident that practicing the invention within the limits establishe~ produces highly acceptable products and is of great value while images produced by other anodizing techniques have no commercial or practical value.
The advanced technology disclosed in this specification was further evaluated by producing full-color samples of elk, aircraft, automobiles, mountain scenes, science fiction pictures, etc. Various aluminum alloys were used including 1100, 3003, 6061, 5005 and 2024. The rendering varied from a size of about 5 centimeters by 6.5 centimeters to about 38 by 38 centimeters. The anodic coating ~ilm was produced in the optimum electrolyte per Table 1. A curren-t density of 330 amps per s~uare meter was used and anodizing time was sufficient to produce a film thickness of about 15 microns.
15 The electrolyte temperature was held between 20 and 22C.
The heat transfers were produced on an offset, full color printing press using Colonial Heat transfer inks developed Eor the textile and carpet industries. Transfer temperature was 190C using a hand flat iron and also a standard heat 20 transfer press utilized in the "T" shixt heat transfer industry. Transfer time was from 1 to 2 minutes. The optimum time was dependent on the size gf the aluminum sheet and its thickness. The sheet thickness varied from about 0.08 centimeters to 0.3 centimeters. The completed work was 25 sealed by various means with results shown in Table 4, below.

Results of Sealing the Colored Image Perfect Slight Fading Image of Image 5 Saturated Steam--15 min. X
88C Water--15 min. X
5g/1 Nickel Acetate Solution--15 min.
(93C) X
It should be noted that nickel acetate sealing is the preferred method to seal images produced by this invention.
Example 10 An anodic film was produced on aluminum alloy 3003 in accordance with the optimum conditions described for producing the elk, aircraft, mountain scenes, etc. The clean dry film was then contacted with a hard painted landscape scene which was painted on heavy news print type paper using Colonial ~eat Transfer inks. The artist mixed the colors on a palette as if they were oil paints. The paper heat transfer and the prepared aluminum sheet were placed in a "T" shirt heat transfer press at a temperature of 190C for 2 minutes. A
perfect permanent metal scene of the previously painted rendering resulted; the color and clarity being preserved.
The rendering was then sealed in the previously described nickel acetate solution for 15 minutes.
Laboratory and metallurgical work ~as accomplished to determine why this invention produc~s highly satisfactorv, beautiful long lasting decorating work while a standard anodizing system is worthless. Taber abraser tests were made on film thicknesses of about 15 microns produced on aluminum alloy 5052 by this process~ The normal sulfuric acid anodizing process is used for comparison with the process disclosed in this invention because all other processes produce colored or otherwise unacceptable films. Results are depicted in Table 5.
_ABLE 5 Taber Abraser Tests*

Milligram Weight Loss for 3000 Cycles Sample anodized at 330 amps per s~uare me~er at 21C per this invention 11.6 Sample anodized at 130 amps per square meter at 21C standard sulfuric process 22.9 Sample anodized at 200 amps per square meter at 21C per this invention 15.7 Sample anodized at 530 amps per square meter at 21C per this invention 16.1 *Details on this test method are found in U.S. Military Specification In addition to the Taber abraser tests, the weight of the ; anodic films was determined by the method outlined in U.S.
Military Spacification MIL-~-8625. The results are shown in Table 6.

Average Anodic Film Weight in Milligrams Per Square Foot 330 amps per square meter anodizing per!this invention 2920 130 amps per square meter standard sulfuric acid anodizing 30 process 840 It appears that the process of the present invention produces outstanding decorated items and that those produced by other anodizing processes are unsatisfactory. The superior t~

anodic coating produced in accordance with the invention protects the colors deep inside the anodic coating pores from abrasion and it is believed that this dense film also shields the colors from degrada~ion by ultraviolet radiation coming from the sun or other sources.
Although a preEerred form of our invention has been herein disclosed, it is to be understood that the present disclosure is by way of example and that variations are possible without departing from the subject matter coming within the scope of the following claims, which subject matter we regard as our invention.

Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of decorating articles of aluminum comprising the steps of:
anodizing said aluminum in an aqueous acid electrolyte consisting essentially of from about 165 to 250 grams per liter sulfuric acid, from about 10 to 30 milliliters per liter of an organic carboxylic acid containing at least one reactive group in the alpha-position wherein said reactive group is a hydroxy, amino, keto or carboxyl group, and from about 10 to 30 milliliters per liter of a polyhydric alcohol of from 3 to 6 carbon atoms, with the temperature of the electrolyte being maintained at between about 15°C and 24°C and the current density being maintained at between about 200 to 535 amps per square meter so as to form an anodized layer on the surface of said aluminum article, said layer having a thickness of between about 10 and 26 microns;
placing a dry film in intimate contact with said anodized layer, said dry film containing a dye capable of subliming when heated;
heating said dry film while in intimate contact with said anodized layer for a time sufficient to cause at least a portion of said dye to sublime and condense within said anodized layer; and thereafter sealing said anodized layer on the surface of said aluminum article.

2. The process of claim 1, wherein the polyhydric alcohol is glycerine.

3. The process of claim 1, wherein the organic carboxylic acid is hydroxy acetic acid.

4. The process of claim 1, 2 or 3, wherein the dry film which is in intimate contact with said anodized layer is heated to a temperature of from about 160° to 220°C.

5. The process of claim 1, 2 or 3, wherein sealing of said anodized layer is done by immersing the anodized layer in a solution of nickel acetate.

6. The process of claim 1, 2 or 3, wherein sealing of said anodized layer is done by contacting said anodized layer with saturated steam.

7. The process of claim 1, 2 or 3, wherein sealing of said anodized layer is done by immersing said anodized layer in hot water at a temperature of about 88°C.

8. The process of claim 1, 2 or 3, wherein the anodized layer has a weight of at least about 2 milligrams per square centimeter and exhibits a maximum taber abraser weight loss of about 17 milligrams when tested at three thousand cycles.

9. The process of claim 1, 2 or 3, wherein the aqueous acid electrolyte does not contain any titanic acid salt.

10. A method for the production of color on aluminum or aluminum alloy article, which comprises:
anodizing said article in an aqueous acid electrolyte consisting essentially of from about 165 to 250 grams per liter of sulfuric acid, from about 10 to 30 millileters per liter of hydroxy acetic acid and from about 10 to 30 milliliters per liter of glycerine and not containing any titanic acid salt, with the temperature of the electrolyte being maintained at between about 15°C and 24°C and the current density being maintained at between about 220 and 440 amps per square meter, so as to form an anodized layer having a thickness of about 10 to 26 microns on the surface of said aluminum article;
washing the anodized aluminum article to remove the electrolyte and air-drying the washed article, while care is taken to keep the surface clean so that the pores in the anodic layer remain open;
placing a dry film having thereon a dye capable of subliming when heated, in intimate contact with said dried anodized layer of the article;
heating said dry film while in intimate contact with said anodized layer at a temperature of 160°C to 220°C for a time sufficient to cause at least a portion of said dye to sublime and condense within said anodized layer; and thereafter sealing said anodized layer on the surface of said aluminum article.

11. The process of claim 10, wherein sealing of said anodized layer is accomplished by:
(i) immersing the anodized layer in a solution of nickel acetate, (ii) contacting the anodized layer with saturated steam, or (iii) immersing the anodized layer in hot water at a temperature of about 88°C.