WO2013191645A1

WO2013191645A1 - Electrolytic reduction of sulfur dyes

Info

Publication number: WO2013191645A1
Application number: PCT/SG2012/000219
Authority: WO
Inventors: Shao-Lin Lim; Sundar Pethaiah SETHU
Original assignee: TECHNODYE DYING COLORS Pte Ltd
Current assignee: TECHNODYE DYING COLORS Pte Ltd
Priority date: 2012-06-21
Filing date: 2012-06-21
Publication date: 2013-12-27
Anticipated expiration: 2014-12-21

Description

ELECTROLYTIC REDUCTION OF SULFUR DYES

Technical Field

The present disclosure relates to dyes, specifically sulfur dyes, which require reduction prior to application to a substrate.

Background

Sulfur dyes are widely used to color many textiles. The stereochemistry of various sulfur dyes is unknown. Accordingly, sulfur dyes are classified according to the chemical structure of their starting materials. Starting materials then undergo a sulfurization process resulting in a specific color dependent upon the starting material. For example, yellow, orange, and brown sulfur dyes may be formed using a starting material such as aromatic amines, diamines, and their acyl and nuclear alkyl derivatives with or without a nitroaniline and nitro- or amino-phenol. The type of sulfurization used may vary. For example, one type of sulfurization used is a "sulfur bake". Another type of sulfurization is a polysulfide bake used, for example, on the starting material polynitrodecacyclene to form shades of brown. A polysulfide melt sulfurization is also used to sulfurize other indophenol-type intermediates, quinoneimine, and phenazone structures. In accordance with the present disclosure, a sulfur dye is any dye requiring chemical reduction prior to application of the dye to a substrate.

Application and adherence of sulfur dyes to textiles requires chemical conversion of the sulfur dye via a redox reaction. In a reduced state, sulfur dyes have an affinity for the textiles and absorb thereon. Following absorption, the sulfur dye is oxidized, for example, by air drying. Oxidation of the dye results in a dyed textile with color-fastness.

Traditionally, sulfur dyes have been reduced using sodium sulfide alone or sodium sulfhydrate with an alkali such as sodium carbonate or sodium hydroxide at a high temperature. Depending upon the color of the dye, alkaline sodium dithionate or alkaline sodium formaldehyde sulfoxylate may also be used as a reducing agent. Unfortunately, these reducing agents have a high biotoxicity and carcinogenicity, and the effluent waste generated by their use is environmentally hazardous.

Several strategies for altering the chemical state of sulfur dyes without the use of toxic solvents have been proposed. These include the use of glucose with sodium hydroxide, enzymatic reactions, or use of cationic membranes. Cationic membranes however, are costly and tend to clog with ions from the solvent in which the sulfur dye is dispersed. Unfortunately, an effective, cost efficient, environmentally friendly method for reducing sulfur dyes is still lacking.

Summary

In accordance with a first aspect, there is disclosed a method for reduction of a sulfur dye including exposing a sulfur dye in an oxidized state to an electrical current in the presence of a porous membrane, wherein upon reduction the sulfur dye is drawn across the porous membrane.

In accordance with a second aspect, there is disclosed an apparatus for electrolytic reduction of sulfur dye including a tank for containing a solution, the tank having a cathode and an anode; and a porous membrane dividing the tank into an oxidation portion comprising the anode and a reduction portion comprising the cathode; wherein dye contained in the reduction portion is reduced when electrically driven by the anode and the cathode.

In accordance with a third aspect, there is disclosed a method of dying a substrate including applying an electrolytically reduced sulfur dye to a substrate, and oxidizing the electrolytically reduced sulfur dye in the presence of the substrate to form a dyed substrate; wherein the electrolytically reduced sulfur dye has been reduced by exposing a sulfur dye in an oxidized state to an electrical current in the presence of a porous membrane.

In accordance with a fourth aspect, there is disclosed an apparatus for electrolytic reduction of a sulfur dye including: a tank; a power source; and a porous membrane, wherein oxidized sulfur dye present in the tank is exposed to an electrical current from the power source, undergoes reduction, and is drawn across the porous membrane.

The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.

Brief Description of the Drawings

Embodiments of the disclosure are described herein with reference to the drawings in which:

FIG. 1 is a front view of the apparatus of the present disclosure;

FIG. 2 is a front view of the apparatus of FIG. 1 with reaction components; and

FIG. 3 is a front view showing the flow of electrons through the apparatus of FIG. 1.

Detailed Description

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein.

The present disclosure provides an effective, cost efficient, environmentally friendly method and apparatus for reducing sulfur dyes. The method and apparatus utilize electrolysis through a generic, inexpensive, porous membrane to reduce sulfur dye. This method does not require the use of hazardous chemical methods of reduction but rather uses an ionic solution in water to electrolytically reduce the sulfur dye.

As stated above, formation of sulfur dye requires sulfurization of a sulfur dye starting material. The resulting sulfur dye is in an oxidized state and is not capable of adhering to a substrate. In order for the sulfur dye to adhere to a substrate, the sulfur dye must undergo reduction from an oxidized state to a reduced state. Reduction includes altering the oxidation state of the sulfur dye through the addition of an electron. In accordance with the present disclosure, reduction of a sulfur dye may be achieved through electrolysis, i.e., electrolytic reduction.

The electrolytic reduction process employs electrons to reduce the sulfur dye compound. The electrolytic reduction of the sulfur dye may be carried out in a tank or other container forming an electrolytic cell. An electrolytic cell is an apparatus that is capable of facilitating reduction through the introduction of electrical energy into an electrolyte solution. The sulfur dye may be added to the solution in order to undergo reduction, i.e., transformed to a state capable of adherence to a substrate.

The apparatus of the present disclosure is described herein with reference to FIG. 1. The apparatus 10 includes a tank 12 and a power supply 14. Anode 16 and cathode 18 are attached to power supply 14. As the reaction is one that introduces electrical energy into the electrolyte solution, the anode 16 is positively charged (+) and the cathode 18 is negatively charged (-). Between the cathode 18 and the anode 16, there is a placed a filter 120. In embodiments, a circulation pump 20 may also be included in the apparatus 10.

When in use, as depicted in FIG. 2, the tank 12 is filled with an electrolyte solution 100. The sulfur dye 1 10 is added to the electrolyte solution 100. The anode 16 is separated from the cathode 18 by a filter 120. In embodiments, a catalyst 130 is present between the filter 120 and the cathode 18. In embodiments, a catalyst 130 may also be on the cathode 18. The circulation pump 20 may be used to circulate the electrolyte solution 100 containing the sulfur dye 110 from the tank 12 to the space between the anode 16 and cathode 18.

As depicted in FIG. 2, between the cathode 18 and the anode 16, there is a placed a filter 120, which may function like a salt bridge. Generally, a salt bridge in an electrolytic cell or tank separates the cell into two portions, one portion having the cathode and the other having the anode. In other words, the salt bridge separates the electrolytic cell into the reduction portion and the oxidation portion and allows only the flow of ions to maintain a balance in charge between the oxidation and the reduction portions of the apparatus 10. The salt bridge does not allow the mixing of fluids in the oxidation portion and the reduction portion of the electrolytic cell. The voltage level of the power supply 14 may be, for example, about IV to about 6V. Any other suitable voltage levels can also be used.

FIG. 3 depicts the flow of cations (+) and anions (-) of the electrolytic solution 100 through the apparatus 10. Sulfur dye 110 is added to the electrolytic solution 100. The sulfur dye 110 is reduced, i.e., picks up a free electron and is drawn through the filter 120 toward the cathode 18 thereby separating the reduced sulfur dye 110' from the oxidized sulfur dye 110. The sulfur dye 110 is circulated by the circulation pump 20 from the tank 12 into the area between the anode 16 and the cathode 18.

A first portion of tank 12, which includes the anode 16, is called an oxidation portion 12a and a second portion of tank 12, which includes the cathode 18, is called a reduction portion 12b. In other words, the filter 120 divides tank 12 into the oxidation portion 12a and the reduction portion 12b. The oxidation portion 12a and the reduction portion 12b include the electrolyte solution 110 in which the anode 16 and the cathode 18 are suspended or placed.

In electrochemistry, an electrolyte is a conducting medium in which the flow of current (in the form of electron transportation) is accompanied by the movement of charged particles in the form of ions. Electrolytes are mostly liquids, but they can also be solids soluble in a liquid solvent. Liquid electrolytes can be solutions of acids, bases and salts. The electrolyte used in accordance with the present disclosure may be ionic solution of an alkaline electrolyte. In embodiments, the ionic solution is a metallic base solution. Examples of a metallic base that may be used to form the ionic solution include, for example, sodium. In embodiments, the ionic metallic base may be sodium hydroxide.

The concentration of the electrolyte in the solution may be, for example, from about 0.1 g/L to about 2.5 g/L. In some embodiments, the concentration of the electrolyte solution is about 2.5 g/L. In embodiments, the electrolyte used is sodium hydroxide (NaOH). The electrolyte is mixed with water (H₂0) and added to tank 12. The sulfur dye 110 to be reduced is circulated by the circulation pump 20 to the reduction portion 12b of tank 12. The dye in the reduction portion 12b is reduced when electrically driven by the anode 16 and the cathode 18. This reaction is described in detail below.

As stated above, the exact chemical structure of sulfur dyes has yet to be identified.

However, they are primarily high molecular weight heterocyclic compounds containing a sulfur group. The most commercially valuable dyes contain as chromophores, ring structures of thiazole (1) thiazone (3H-isophenothiazin-3-one) (2), and or thianthrene (3) d

(1) (2) (3).

Upon reduction in the presence of sodium, the sulfide rings and bridges above are converted to -SNa groups with sodium ions forming loose bonds with the sulfur in a solution containing sodium. Described below are the reactions taking place at both the anode 16 and the cathode 18. The reactions at the anode 16 are as follows:

NaOH Na⁺ + OH^"

The sodium hydroxide molecules break down into sodium ions and hydroxide ions. The sodium ions are positively charged and the hydroxide ions are negatively charged. Further, the hydroxide ions breakdown into oxygen, water, and electrons as illustrated in the reaction below:

40H^" > 0₂ + 2H₂0 + 4e^" The resulting oxygen gas bubbles up from the oxidation portion 12a. The electrons generated are attracted towards the anode 16. The positive charge of the sodium ions reduces the sulfur present in the sulfur dye molecules.

The sodium ions and dye, being positively charged, are repelled by the anode 16 and pass through the filter 120, toward the cathode.

The reactions at the cathode are as follows:

2H₂0 + 2e^~ » H₂ (g) + 20H^"

The water molecules in the electrolyte react with the electrons from the cathode to produce hydrogen gas and hydroxide molecules. The hydrogen gas bubbles up from the reduction portion 12b. The hydroxide ions in the reduction portion 12b react with the positive sodium ions to form sodium hydroxide.

The free electrons in the reduction portion 12b react with the sulfur dye particles to reduce the particles. The reduced sulfur dye particles may now be soluble in an aqueous solution. This reduced sulfur dye may now be capable of attaching to a substrate such as fabric and imparting color to the substrate.

The tank of the present disclosure may be any tank that does not degrade upon exposure to any of the components of the electrolytic reduction process. The tank may be, for example, stainless steel, plastic, metal, or other non-reactive materials.

The sulfur dye is added to the electrolyte solution. In embodiments, the sulfur dye is added to the electrolyte solution at a concentration of about 1 g/L to about 100 g/L. In some embodiments, the sulfur dye is added to the electrolyte solution at a concentration of about 25 g/L. The cathode and the anode used may be any electrodes. In embodiments, the cathode and/or anode may be a porous electrode. Porous electrodes are also referred to as foam electrodes, mesh electrodes or 3-D electrodes because of their structure. Porous electrodes, because of their porosity have a surface area higher than non-porous electrodes, available for electron exchange and current collection during the process of oxidation and reduction in an electrolytic cell. The higher surface area accelerates the process of reduction of the sulfur dye in the reduction portion and the oxidation of sodium in the oxidation portion. The electrolyte used in the electrolytic cell may actually pass through the recesses and cavities in a porous electrode, thus utilizing the higher surface area of the electrodes.

Alternatively, the anode may be in the form of, for example, a plate or a sheet electrode. A plate electrode has a substantially planar, non-permeable surface. As the area available for electrolytic reaction at the electrode is a function of the surface area, a planar plate electrode may have a greater surface area for reaction. In embodiments, the surface area available for the electrolytic reaction is approximately twice the surface area of the electrode plate, including both the sides of the plate. The thickness of the porous electrodes may range from about 0.1 mm to about 20 mm.

In embodiments, the anode may be a plate electrode and the cathode may be a foam electrode. As described above, the surface area available for electrolytic reaction in a foam electrode includes the surface area exposed to the pores, substantially increasing the surface area available for the electrolytic reaction. For example, a plate electrode and a porous electrode of the same dimension, the surface area of the porous electrode available for electrolytic reaction is far higher than the surface area available in a plate electrode. As the pores or holes in the porous electrode increase, the surface area available for electrolytic reaction also increases. Thus the cathode may have a substantially higher surface area available for reduction when compared to the anode. In embodiments, the surface area of the cathode used for reduction is, equal to the surface area of the anode. Hence, the cathode is not fully utilized for reduction. Whereas, if both the anode and the cathode are porous or foam electrodes, the surface area of both the anode and the cathode are approximately equal and therefore the anode and the cathode can be fully utilized for the electrolytic reduction, which increases the speed of the reaction. In embodiments, the anode can be a plate electrode.

Moreover, the electrodes can also have a coating of a catalyst on their surfaces. To enable the catalyst to remain coated on the electrode, the catalyst can be coated on the surface of the electrode using a polymer or ionomer solution. To prepare the solution, the catalyst is mixed with the solution and then applied on the electrode. Alternately, the catalyst can also be coated on the surface of the electrode without the help of the polymer or ionomer.

The power source used for the reaction may be any power source capable of providing the needed voltage. Any voltage suitable for use in an electrolytic reaction may be used. In embodiments, the voltage level of the power source may be about IV to about 6V.

The filter used in accordance with the present disclosure may be a porous membrane. The porous membrane may be a porous synthetic or natural polymer membrane. In embodiments the membrane is a synthetic polymer membrane. The pore size of the porous membrane may be, for example, from about lOOnm to about 700nm. Examples of synthetic polymer porous membranes that may be used include, for example, those constructed of polyethylene, polypropylene, polyvinyl acetate, ultra high molecular weight polyethylene (UHMWPE), polytetrafluoroethylene (PTFE) or blends of these available from manufactures such as DeWAL^®, Toray^®, and Porex^®. The porous membrane may be made of a single type of polymer or multiple polymer, copolymer, or polymer blends. In embodiments, the porous membrane is an UHMWPE membrane from DeWAL . Previous attempts at electrolytic reduction of sulfur dyes have employed cationic membranes. However, cationic membranes are costly. Furthermore, cationic membranes filter ions present in the electrolyte solution other than the reduced sulfur dye, resulting in quick clogging and reduced filtration by the membrane. Thus, cationic membranes are not considered a viable option for use in the present disclosure. As stated above, in embodiments, in order to prevent solution neutralization prior to complete sulfur dye reduction, a catalyst may be used. Catalysts that may be used in accordance with the present disclosure include platinum group metals. Platinum group metals include, for example, platinum, palladium, iridium, rhodium, ruthenium and osmium. Additional catalysts that may be used include platinum black, ruthenium black, nickel, gold, silver, aluminum, antimony, cadmium, copper, indium, iron, kovar, lead, tin, cobalt, stainless steel or manganese. In some embodiments, platinum is used as a catalyst. In other embodiments, palladium is used as a catalyst. The catalyst may also include compounds and combinations of any of the aforementioned catalysts. The particle size of the catalyst is in the range of about 1 nm to about 50 nm.

The catalyst may be located in or on one or more of the electrodes or between the membrane and the cathode. In embodiments, the catalyst is located in the cathode. In embodiments, the catalyst is located in between the membrane and the cathode. In embodiments, the amount of catalyst used in the reaction (i.e., added to the area between the membrane and the cathode) is from about 0.1 mg/cm² to 1 mg/cm². In some embodiments, the amount of catalyst used is about 0.1 mg/cm². In embodiments, the catalyst is located in both the cathode and in between the membrane and the cathode.

The catalysts described above accelerate the reduction process to different rates. Among the catalysts mentioned above, all of them speed up the electrolytic reduction process. The catalyst to be used is selected based on the rate of the electrolytic reduction that is required.

Reduced sulfur dyes may be used to dye substrates after which the sulfur dye is oxidized and the dye retained by the substrate. In accordance with the present disclosure, substrates suitable for dying with sulfur dyes include, for example, cellulose and cellulose derivatives or materials containing cellulose fibers. Cellulose may be, for example, microcellulose, cellulose powder, carboxymethyl cellulose, hemp, switchgrass, willow, poplar, adhesives, binders, wood pulp, and paper. Substrates containing cellulose fibers include, for example, cotton, cellophane, rayon, and linen. Substrates including synthetic fibers such as polyamides, polyesters, and acrylic fibers, blended with substrates containing cellulose fibers, may also be dyed using sulfur dyes. Silk, although not cellulosic, is also subject to dying by sulfur dyes and considered a substrate in accordance with the present disclosure. The substrates may be used to form denim, cotton and/or cellulose-synthetic blend garments, cellulose-synthetic blend or cotton knit fabrics, and the like. Any combination of the aforementioned substrates is also contemplated by the present disclosure for dying with sulfur dyes.

In order to dye the substrate, the reduced sulfur dye is applied to the substrate. The reduced sulfur dye may be applied to the substrate while in an aqueous or electrolyte solution. Following application of the reduced sulfur dye to the substrate, the sulfur dye is oxidized. The dye may be oxidized by air drying or exposure to a peroxide or acidic solution. The resulting oxidized dye-substrate complex is water insoluble and "color fast" meaning the dye is retained after washing.

Claims

1. A method for reduction of a sulfur dye comprising:

exposing a sulfur dye in an oxidized state to an electrical current in the presence of a porous membrane, wherein upon reduction said sulfur dye is drawn across said porous membrane.

2. The method of claim 1, wherein said exposing occurs in the presence of an electrolyte solution.

3. The method of claim 1, wherein said exposing occurs in the presence of a catalyst.

4. The method of claim 2, wherein said electrical current is generated in said electrolyte solution by a power supply having a cathode and an anode.

5. The method of claim 4 wherein said porous membrane is placed between said cathode and said anode.

6. The method of claim 4 wherein said catalyst is placed between said membrane and said cathode.

7. The method of claim 1, wherein said sulfur dye in an oxidized state is not drawn across said porous membrane.

8. The method according to claim 2, wherein the solution has a concentration of oxidized said sulfur dye in said electrolyte solution is from about 1 g L to about 100 g/L.

9. An apparatus for electrolytic reduction of sulfur dye comprising:

a tank for containing a solution, the tank having a cathode and an anode; and

a porous membrane dividing the tank into an oxidation portion comprising the anode and a reduction portion comprising the cathode; wherein dye contained in the reduction portion is reduced when electrically driven by the anode and the cathode.

10. A method of dying a substrate comprising:

applying an electrolytically reduced sulfur dye to a substrate, and

oxidizing said electrolytically reduced sulfur dye in the presence of said substrate to form a dyed substrate;

wherein said electrolytically reduced sulfur dye has been reduced by exposing a sulfur dye in an oxidized state to an electrical current in the presence of a porous membrane.

11. The method of claim 10 wherein said oxidizing is air drying.

12. The method of claim 10, wherein said dyed substrate is color fast.

13. A substrate prepared by the method of claim 10, wherein the substrate is selected from the group consisting of cotton and cellulose- synthetic blends.

14. An apparatus for electrolytic reduction of a sulfur dye comprising: a tank;

a tank;

a power source; and

a porous membrane, wherein oxidized sulfur dye present in said tank is exposed to an electrical current from said power source, undergoes reduction, and is drawn across said porous membrane.

15. The apparatus of claim 13, wherein the power source further comprises an anode and a cathode.

16. The apparatus of claim 14, wherein said cathode is separated from said anode by said porous membrane.

17. The apparatus of claim 13, wherein said tank further comprises an electrolyte solution.

18. The apparatus of claim 13 further comprising a catalyst.

19. The apparatus of claim 17 wherein said catalyst is selected from the group consisting of platinum, palladium, and combinations thereof.

20. The apparatus of claim 13, wherein the porous membrane is selected from the group consisting of: polyethylene, polypropylene, polyvinyl acetate, ultra high molecular weight polyethylene (UHMWPE), polytetrafluoroethylene (PTFE) and blends thereof.

21. The apparatus of claim 13, wherein the porous membrane has a pore size of about 100 nm to about 700 nm.