Claims:WE CLAIM:
1. A process for colouring an aluminium sheet with an indigo dye, said process comprising the steps of:
forming a film of aluminium oxide on a surface of the aluminium sheet to obtain an aluminium oxide coated aluminium sheet;
immersing said aluminium oxide coated aluminium sheet at predetermined temperature for predetermined time period in an indigo dye solution having an alkaline pH to obtain an indigo dyed aluminium sheet; and
exposing said indigo dyed aluminium sheet to hot water to form a layer of transparent material on said aluminium oxide film.
2. The process as claimed in claim 1, wherein said film of aluminium oxide is formed by anodizing said aluminium sheet in an electrolyte bath for a time period ranging from 45 minutes to 60 minutes.
3. The process as claimed in claim 1, wherein said indigo dye solution is at temperature ranging from 20º Celsius to 40º Celsius.
4. The process as claimed in claim 1 or claim 3, wherein said indigo dye solution is at room temperature.
5. The process as claimed in claims 1 to 4, wherein said indigo dye solution is a leuco indigo dye solution having pH ranging from 8 to 14.
6. The process as claimed in claim 5, wherein said leuco indigo dye solution has pH more than 9.
7. The process as claimed in claim 1, wherein said aluminium oxide coated aluminium sheet is dyed with said indigo dye solution for a time period ranging from 20 seconds to 60 seconds.
8. The process as claimed in claim 1, wherein said indigo dyed aluminium sheet is exposed to hot water for a time period ranging from 2 minutes to 5 minutes.
9. The process as claimed in claim 8, wherein temperature of hot water ranges from 80º Celsius to 100º Celsius.
10. The process as claimed in claim 1 or 8, wherein hot water is boiling water.

Dated this 10th day of June, 2019

For Arvind Limited,
By their Agent

(GIRISH VIJAYANAND SHETH) (IN/PA 1022)
KRISHNA & SAURASTRI ASSOCIATES LLP
, Description:FORM 2
THE PATENTS ACT, 1970
(39 OF 1970)
&
THE PATENTS RULE, 2003
COMPLETE SPECIFICATION
[See Section 10, Rule 13]

A PROCESS FOR COLOURING AN ALUMINIUM SHEET WITH AN INDIGO DYE;

ARVIND LIMITED, A COMPANY INCORPORATED UNDER THE COMPANIES ACT, 1956, WHOSE ADDRESS IS NARODA ROAD, AHMEDABAD - 380025, GUJARAT, INDIA

THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED

TECHNICAL FIELD OF THE INVENTION
The present disclosure relates to aluminium coloration processes.
BACKGROUND OF THE INVENTION
Conventionally, an aluminium sheet is coloured using a powder coating process or an electrolytic process. In the powder coating process, free-flowing dry powder of a desired colour is applied on the aluminium sheet. The coating of the powder is typically applied electrostatically. In the electrolytic process, the colour is typically applied on the aluminium sheet after cleaning and anodizing the aluminium sheet. However, it is not possible to colour an aluminium sheet with an indigo dye using any of the aforementioned processes. Further, if an aluminium sheet is coloured with a blue colour using any of the aforementioned processes, the blue tone obtained is not as visually pleasing as the blue tone obtained in case of a fabric dyed with an indigo dye. Another conventional approach to colour aluminium sheet is to use a pigmentation process. However, the aluminium sheet coloured using the pigmentation process does not show properties such as uneven and gradual wash-down of the colour as shown by the indigo dyed fabric.
Therefore, there is felt a need of a process for colouring an aluminium sheet with an indigo dye.
SUMMARY OF THE INVENTION
The present disclosure envisages a process for colouring an aluminium sheet with an indigo dye. The process comprises the following steps. A film of aluminium oxide is formed on a surface of the aluminium sheet to obtain an aluminium oxide coated aluminium sheet. The aluminium oxide coated aluminium sheet is dyed by immersing it in an indigo dye solution having an alkaline pH for a predetermined time period and at a predetermine temperature to obtain an indigo dyed aluminium sheet. The indigo dyed aluminium sheet is exposed to hot water to form a layer of transparent material on said aluminium oxide film.
BRIEF DESCRIPTION OF THE ACCOMPNYING DRAWING
Figure 1 illustrates a flow chart depicting a process of the present disclosure.
DETAILED DESCRIPTION OF THE INVENTION
Although specific terms are used in the following description for sake of clarity, these terms are intended to refer only to particular structure of the invention selected for illustration in the drawings, and are not intended to define or limit the scope of the invention.
References in the specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, characteristic, or function described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.
The present disclosure envisages a process for colouring an aluminium sheet with an indigo dye. The steps of the process (100) are now elaborated in subsequent paragraphs with the help of Figure 1.
Initially, an aluminium sheet to be dyed is cleaned using a suitable cleaning solution (step 105). In an embodiment, the aluminium sheet is cleaned using cleaning solutions such as, but not limited to, Sodium Hydroxide (NaOH) and Nitric acid (HNO3). Cleaning the aluminium sheet using Nitric acid is known as acidic cleaning, whereas cleaning the aluminium sheet using Sodium Hydroxide is known as alkaline cleaning. In an embodiment, Sodium Hydroxide and Nitric acid used are have concentration of approximate 15%. In first step, the aluminium sheet undergoes cleaning. Cleaning the aluminium sheet helps in removing any carbonaceous deposits, traces of oil lubricants and polishing compounds or sanding grits, oxide films, and the like from the surface of the aluminium sheet. Further, the aforementioned cleaning method is useful in case etching the surface of the aluminium sheet to remove aforementioned deposits is not permitted due to application requirements.
In the next step, the aluminium sheet is subjected to anodizing (step 110). In anodizing, a film of aluminium oxide is formed on the aluminium sheet. The anodizing of the aluminium sheet is carried out in the presence of an electrolyte bath. In an embodiment, the electrolyte bath contains sulphuric acid. More specifically, the aluminium sheet is immersed in the electrolyte bath containing sulphuric acid stored in an anodizing tank. An electric current is passed through the electrolyte bath. A cathode is mounted in the anodizing tank. The aluminium sheet acts as anode. When the current passes through the electrolyte bath, sulphuric acid begins to decompose, and the hydrogen ions separated in the sulphuric acid begins to move towards the cathode, where the hydrogen ions are reduced to hydrogen gas. The chemical reaction of the same is represented below.
2H+ + 2e- H2(g)
Simultaneously, negatively charged anions, i.e. hydroxide, sulphate and maybe oxide ions, move to the anode. The electrical charge in the circuit causes positively charged aluminium ions (Al3+) to be generated in the anode and in turn move towards the cathode. At the Anode surface, i.e., at the surface of the aluminium sheet, the aluminium ions react with the oxide/hydroxide ions to form a film of aluminium oxide (in the case of the hydroxide ion, hydrogen ions are released into the solution). The chemical reaction of the same is represented below.
Al ? Al3+ + 3e-
2Al3+ + 3O2- ? Al2O3
2Al3+ + 3OH- ? Al2O3 + 3H+
Net chemical equation is:
2Al + 3H2O ? Al2O3 + 6H+ + 6e-
It is to be noted that the film of aluminium oxide is integral with the aluminium sheet, and thus, cannot be chipped or peeled off from the surface. Further, the film of aluminium oxide is porous in nature. The film of aluminium oxide thus formed is useful for secondary process such as colouring.
For better coloration of an indigo dye on the aluminium sheet, the anodizing on the aluminium sheet is carried out for more than 45 minutes. In an embodiment, anodizing is carried out for a time period ranging from 45 minutes to 60 minutes.
The aluminium oxide coating contains 12% – 15% sulphate ions which facilitate movement of the hydrogen ions in the bath, thereby reducing the voltage required for anodizing.
In accordance with the present invention, the thickness, porosity, and other characteristics of the oxide film may vary according to the concentration and nature of the electrolyte bath, the alloy being anodised, the temperature of the bath, the voltage, and current density applied. In an embodiment, the sulphuric acid is at temperature ranging from 20º Celsius to 23º Celsius. In another embodiment, molar concentration of the electrolyte ranges from 2.5 mol/litre to 3.5 mol/litre. In yet another embodiment, voltage maintained during anodizing ranges from 15 Volts to 20 Volts, whereas the current density maintained ranges from 1.4 A/m2 to 1.8 A/m2. It is to be noted that, at high electrolyte molar concentration and temperature, and at lower voltage or current densities, a thin oxide film is formed on the surface of the aluminium sheet. On the other hand, hard and dense oxide film is formed on the surface of the aluminium sheet at low electrolyte temperature and high current densities.
After anodizing, an aluminium oxide coated aluminium sheet is obtained. In an embodiment, the thickness of porous oxide film formed on the aluminium sheet ranges from 10 µm to 25 µm.
In the next step, the aluminium oxide coated aluminium sheet is dyed with an indigo dye solution for a predetermined time period and at predetermined temperature to obtain an indigo dyed aluminium sheet.
The indigo dye solution is a water based solution. Water, alkali, and reducing agent are used to make the indigo dye water soluble.
In an embodiment, the aluminium oxide coated aluminium sheet is dyed for a time period ranging from about 20 seconds to about 60 seconds.
In an embodiment, the step of dyeing the aluminium oxide coated aluminium sheet includes a sub-step of immersing (step 115) the aluminium oxide coated aluminium sheet in the indigo dye solution for the time period ranging from about 20 seconds to about 60 seconds. When the aluminium oxide coated aluminium sheet is immersed in the indigo dye solution, the indigo dye is absorbed in pores of the aluminium oxide film. After dyeing, an indigo dyed aluminium sheet is obtained.
Typically, organic dyes are heated to temperature ranging from about 50º Celsius to about 60º Celsius in conventional dyeing processes. In an embodiment of the present disclosure, the indigo dye solution is at temperature ranging from about 20º Celsius to about 40º Celsius. In another embodiment, the indigo dye solution is at room temperature.
Further, the indigo dye solution is a leuco indigo dye solution having alkaline pH. In an embodiment, the leuco indigo dye solution has pH ranging from 8 to 14. In another embodiment, the leuco indigo dye solution has pH more than 9.
In the next step, the indigo dyed aluminium sheet is subjected to sealing. In sealing, pores of the aluminium oxide film are closed. To seal the pores, the indigo dyed aluminium sheet is exposed to hot water (step 120) for a time period ranging from 2 minutes to 5 minutes to form a layer of transparent material on the aluminium oxide film. More specifically, the indigo dyed aluminium sheet is immersed in hot water for the aforementioned time period. Water reacts with aluminium oxide to form the mineral known as Boehmite (AlO.OH). The chemical reaction of the same is represented below.
Al2O3 + H2O ? 2AlO.OH
Boehmite is a hard, transparent material with a greater volume than the aluminium oxide. Thus, the pores of the aluminium oxide get closed. Closing the pores of the film facilitates retention of the indigo dye in the film.
In an embodiment, the hot water is boiling water. In another embodiment, temperature of water ranges from 80º Celsius to 100º Celsius.
The process, of the present disclosure, can be used for colouring an aluminium sheet of any dimension and thickness. In an embodiment, the thickness of the aluminium sheet, coloured with the indigo dye using the process of the present disclosure, ranges from 20 Gauze to 24 Gauze.
The colour depth obtained on an aluminium sheet, coloured with an indigo dye using the process of the present disclosure, is uniform. Further, blue tone obtained on the sheet is visually pleasing. The surface of the aluminium sheet remains smooth after applying the indigo dye using the process of the present disclosure. An aluminium sheet dyed with an indigo dye can be used for artistic application, interior decoration, travelling bags, mobile back cover, and many other similar applications as it is aesthetic in nature.
The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, to thereby enable others, skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omission and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the application or implementation without departing from the spirit or scope of the present invention.