Changing the color or dyed textile substrates

Abstract

The present invention relates to a process for obtaining color changes on dyed textile substrates by treating the dyed textile substrates with an electrochemically generated aqueous solution of reducing or oxidizing agents, which comprises controlling the cell current in such a way that the solution, when in contact with the dyed textile substrate, has a suitable redox potential to obtain the color change.

Description

The present invention relates to a process for changing the color of dyed textile substrates by the action of reducing or oxidizing agents which are electrochemically generated in aqueous solution.

Bleaching is of fundamental importance in the manufacture of textile products, its original function being to destroy the natural colored constituents of the fibrous materials prior to the dyeing operation.

Various bleaching processes are employed. The until recently widely used bleaching processes employing oxidized chlorine compounds (hypochlorite, chlorine dioxide) have very largely disappeared from the textile industry because of the wastewater burden due to AOX. Today, it is customary to use inorganic or organic peroxides (hydrogen peroxide, peracids).

In the textile washing field, perborate compounds and percarbonates are widely used bleaching chemicals.

These processes all have in common that the substances mentioned are directly added to the bleach bath in a defined amount as solid or liquid chemicals and, after the operation has ended, pass into the wastewater together with the treatment bath. Process control options are limited to the definition and maintenance of appropriate process conditions, especially rate of addition of the chemicals, pH, temperature, treatment time and liquor ratio. Bleaching operations employing chlorine compounds, especially hypochlorite, are today only used for particular applications. A particularly important application is the bleaching of fully made-up indigo-dyed garments. For example, denim garments are bleached in large drum washers by treating them with 10 ml/l of sodium hypochlorite solution and 1.0 g/l of sodium carbonate at 40° C. and a liquor ratio of 10:1 for 20 min. This production technology is practiced in a wide variety of process versions, since the controlled destruction of dye is used to create a wide range of effects. What is essential is not a complete bleaching of the dye, but a partial destruction of the dye on each article.

Reductive bleaching processes are employed on azo dyes in a wide variety of process stages. In textile printing, for example, the discharging of a dye is normally based on the reductive destruction of azo dyes by the local application of reducing agents (formaldehydesulfoxylates). Similarly, off-shade dyeings are stripped off again, ie decolorized, by means of suitable reducing agents (sodium dithionite), so that the textile material may be redyed. These processes are directed to the complete destruction of the dye and accordingly the application conditions are chosen so as to produce a substantial bleach.

The literature already describes a wide range of electrochemical processes for generating reducing and oxidizing agents, but these processes are mostly used to produce the chemicals in bulk and not in situ. However, it is already known that the use of electrochemical processes to generate reducing agents or oxidizing agents in situ can lead to various advantages.

For instance, according to WO90/15182, iron-triethanolamine complexes can be used as regenerable reducing agents for dyeing vat dyes, sulfur dyes and indigo and also for decolorizing azo dyes. Similarly, according to Bechtold et al. (J. Chem. Soc. Faraday Trans. 1993 89(14) 2451-2456, J. Appl. Elchem. 2001 31 363-368), other complexing agents such as gluconic acid, hydroxyethylenediaminetriacetic acid, etc. are suitable as well. DE 195 13 839 A1 describes a process for the electrochemical reduction of vat dyes using alkaline iron-triethanolamine complexes in an electrochemical apparatus having a large cathode surface area (graphite felt cathode). These processes all have in common that the electrochemical reduction is part of a dyeing process, ie the dye is completely reduced in each case. This also applies to the reductive destruction of the azo dye in use example 3 of WO90/15182. There the cathodically generated Fe(II)-triethanolamine complex is used to prepare a faultily dyed textile for a subsequent redyeing.

The electrochemical in situ generation of hypochlorite has already been investigated at length, the publications in this field focusing on the generation of hypochlorite-containing solutions for subsequent use in bleaching operations (Rengaranjan et al., India. bull. Electrochem. 1993 9(11-12) 642-643 and Tong C., CN88101765). Similarly, the anodic generation of hypochlorite has also been described for textile washing and bleaching (see EP 0 002423 A1).

The decolorization of reactive dyes on textiles and in wastewaters by electrochemically generated hypochlorite has been described for example by Nishibe K. (Dekni Kagaku oyobi Kogyo Butsuri Kagaku 1980 48(4) 267-270) and Vijayaraghavan K. et al. (Color. Technol., 2001 117 49-53), although such processes have been studied for pulp bleaching in particular (Varennes S., et al. Can. Appita J. 1994 47(1) 45-49, Fadali O., Egypt. Bull. Electrochem. 1993 9(1) 21-24 and Cellul. Chem. Technol. 1991 25(3-4) 181-187, Schwab G., U.S. Pat. No. 4,617,099, Nassar M. M. et al., J. Appl. Electrochem. 1983 13(5) 663-667).

The electrochemical generation of oxidizing organic compounds for bleaching indigo jeans materials is described in DE 198 43 571 A1 and DE 197 23 889 A1. The former reference describes a potentiostatic or galvanostatic operation with regard to the working electrode (anode), while the latter merely indicates the cell voltage to be employed. The use of metal complexes of manganese and of iron to support the electrochemical oxygen bleach in pulp bleaching was extensively investigated by Oloman (Can. Tappi. J. 1993, 76(10) 139-147, Tappi 1994 77(7) 115-126). A cathodic generation of hydrogen peroxide by gas consumption electrodes is likewise described in the literature (Cooper J. F., U.S. Pat. No. 5,456,809).

A wide range of redox systems are thus known from the prior art which can be electrochemically converted into the “active” form, which can be the reduced form or the oxidized form. But none of these systems has so far been used in industry, above all because of their unsatisfactory technological performance and hence poor cost-benefit ratio.

Prior art processes concentrate on the use of electrochemical techniques in place of conventional chemicals. Accordingly, these electrochemical operations are run galvanostatically or potentiostatically with regard to the working electrode. In the potentiostatic mode, the potential of a working electrode (cathode or anode) is defined in relation to a reference electrode introduced into the solution, whereas in the galvanostatic mode it is the current introduced into an electrolytic cell arrangement that is defined. Such techniques are admittedly useful for preparing chemicals, but completely unsuitable for the reproducible generation of defined color changes of the kinds required these days to manufacture, especially fashionable, items of clothing (reference may be made for example to the “stone-wash” process to produce the used look, of utmost importance in the manufacture of jeans). This applies especially to the teachings of DE 197 23 889 A1 and DE 198 43 571 A1.

It is an object of the present invention to provide a process whereby defined and reproducible color changes can be obtained on dyed or colored textiles. It has now been found that this object is achieved when the electrochemical procedure is adapted to the textile material to be treated. In other words, the setting of a suitable redox potential is of utmost importance.

The present invention provides a process for obtaining color changes on dyed textile substrates by treating the dyed textile substrates with an electrochemically generated aqueous solution of reducing or oxidizing agents, which comprises controlling the cell current in such a way that the solution, when in contact with the dyed textile substrate, has a suitable redox potential to obtain the color change.

For the purposes of the present invention, color changes are especially lightenings, hue shifts and deliberate unlevelnesses (microscopic or macroscopic).

The reducing or oxidizing agents required to obtain the desired color changes are electrochemically generated according to the process of the invention, customarily in an electrolytic cell or an apparatus which can perform the functions of an electrolytic cell. In other words, a suitable redox system or redox couple is introduced into the cell and the reducing or oxidizing agent is generated therefrom by electrochemical means. Suitable electrolytic cells will be known to one skilled in the art. They are generally made up of a working electrode, a counterelectrode and also power feed and power supply. Where appropriate, the cell contains a membrane to divide the anode and cathode spaces; but, depending on the redox system used, it is also possible to use a simpler and less costly undivided form of the cell. Particularly suitable electrolytic cells have a large active electrode area and accordingly preference is given not only to planar electrodes, for example in the form of plates, but also to three-dimensional electrodes (foraminous sheets, wire cloths, webs, felts, flowing electrodes, shaking electrodes, porous sintered plates), particular preference being given to multicathode systems. When large electrode areas are used, the requisite concentrations of active redox system can be kept low, which is particularly important for the economic viability of the process according to the invention. Suitable electrode materials are known to one skilled in the art and are to be selected according to the requirements, particular preference being given to noble metals, stainless steel, graphite and carbon and also titanium sheet coated with noble metal oxide.

Advantageously, the electrolytic cell is constructed as a flowthrough cell which communicates directly with a treatment assembly into which the aqueous solution containing the reducing or oxidizing agent (treatment liquor) is pumped and in which the color change on the textile material is generated. The solution can subsequently be returned back into the electrolytic cell for regeneration. There is no continuous loading and unloading of the machine for preparatory rinses or a wash after the electrochemical treatment step.

It is advantageous for the electrolytic cell to be used successively on plural treatment assemblies by connecting it up during the electrochemical treatment step and disconnecting the circulation after the treatment has ended. Useful treatment assemblies include the apparatuses customary for wet processing or washing textiles. Suitable are in particular textile machines, such as a yarn dyeing machine, jigger, jet dyeing machine or continuous machine. The treatment of textiles in tube form is carried out for example in overflow machines or jet dyeing machines, the treatment of piece goods takes place on beam dyeing machines and the treatment of fully made-up garments on fully-fashioned machines or drum dyeing machines.

In a particular embodiment of the process according to the invention, the electrolytic cell is directly configured as a treatment assembly, or parts of the treatment assembly act as electrodes, so that electrolyte circulation between electrolytic cell and treatment assembly is obviated. The electrodes can in this way be disposed in the direct vicinity of the textile material, so that locally confined dye change can be achieved.

The geometry of the electrodes depends on the desired result. If the electrode generating the reducing or oxidizing agent is for example pressed in the form of a stamp onto the textile which contains electrolyte or is in the treatment bath, a patternwise color change can be obtained after the current has been switched on. In another variant according to the invention, fully made-up parts, for example trousers, are pulled over electroconductive supports of appropriate shape, which act as electrodes, and subjected to the process according to the invention.

In a preferred embodiment of the process according to the invention, the dyed textile material is subjected to mechanical and/or hydrodynamic treatment at the same time as being subjected to the action of the reducing or oxidizing agent. This is accomplished for example by stirring or recirculating in the treatment apparatus. When drum washers are used, the mechanical treatment is accomplished for example by agitating the textile material in the rotating drum, whereas in the case of the treatment of dyed textile material in rope form or open width a mechanical treatment can be effected for example by squeeze systems, rolls or rollers, for example immersion squeeze systems etc.

However, any mechanical treatment required can also be effected by adding abrasive materials to the treatment liquor. Useful abrasive materials include for example pumice, suitable plastics or metals. A mechanical treatment at a microscopic level can be effected by adding suitable abrasive powders, for example abradant or metal powder. A mechanical treatment can also have coupled to it a hydrodynamic treatment, for example in a squeeze system, in a drum washer, during the treatment in a jet dyeing machine or overflow dyeing machine. A specific hydrodynamic stress can also be caused by an intensive liquor flow (spray tubes, aspirating, sieve drum washer) or for example by vibration or ultrasound.

What is decisive for the obtention of a reproducible color change is the setting of a suitable redox potential in the solution in immediate neighborhood to the dyed textile substrate. The redox potential need not be mandatorily kept constant at one value, it is also possible for defined different redox potential levels to follow each other. A person skilled in the art is easily able to determine the necessary redox potential for the desired color change by simple experimentation for a system which otherwise remains the same. For this purpose, the redox potential of the treatment solution can be measured with a redox electrode, for example a Pt combination electrode with Ag/AgCl 3 M KCl reference electrode.

In the above-described case where the electrolytic cell and the treatment apparatus form a single unit, the redox potential in the treatment liquor may preferably be measured and controlled by a measurement of the rest potential, in which case the working electrode also serves as the measuring electrode and the electrolytic current is interrupted for the period of the potential measurement. The reference electrode required can be installed alongside the working electrodes, since the potential measurement is only carried out during the currentless period of the working electrode. The time required to measure the potential must be determined in preliminary tests, since, once the current has been switched off, a minimum time period is necessary to allow the working electrode to adapt to the solution potential.

To obtain reproducible results, the redox potential is preferably measured in the treatment apparatus, more preferably in the immediate vicinity of the dyed textile substrate. In general, the redox potentials needed vary with the dye and the general conditions, but preferably range from +100 to +2000 mV and more preferably from +400 to +1600 mV in the case of oxidations and preferably from −300 to −1800 mV and more preferably from −400 to −1200 mV in the case of reductions.

The necessary potentials are set by controlling the cell current, current densities generally being in the range from 0.1 mA/cm² to 1 A/cm² and preferably in the range from 1 mA/cm² to 0.1 A/cm².

The process of the invention can be carried out with reducing or oxidizing agents. Reducing agents are generated cathodically and oxidizing agents anodically from reversible redox systems. For example, halides, for example sodium chloride or sodium bromide, can be subjected to anodic oxidation to form hypohalites which are oxidizing agents. In general, any inorganic or organic redox systems can be used whereby the necessary potential can be obtained. A person skilled in the art is therefore easily able to find suitable redox systems.

Preferably, the process of the invention is carried out with inorganic reducing or oxidizing agents.

Useful Reducing Agents Include for Example:

• • cathodically generated metal complexes with inorganic or organic ligands and in which the metal is present in a low, ie reduced, valency state, preferably iron(II) or tin(II) complexes with inorganic or organic ligands, more preferably iron(II) complexes containing a 2-hydroxyethyl group or a polyhydroxycarboxylic acid in the ligand,
  • substituted. Examples are triethanolamine and iron (II) complexes of polyhydroxycarboxylic acids such as for example gluconic acid and heptagluconic acid, anthraquinone compounds, such as 1,2-dihydroxyanthraquinone or anthraquinonesulfonic acids,
  • tin(II) compounds, such as hexahydroxystannite in alkaline solutions,
  • dithionite generated by cathodic reduction, which can be used in neutral and weakly acidic solutions.

Useful Oxidizing Agents Include for Example:

• • halogen-oxygen compounds, preferably hypochlorite and hypobromite. Particular preference is given to the electrochemical production of mixtures of hypobromite and hypochlorite from mixtures of NaCl (0.01 mol/l to 5 mol/l) and NaBr (0.001 mol/l to 1 mol/l), which leads to an unexpected intensification of the bleaching effect. The chloride:bromide molar ratio is preferably between 5 and 100,
  • metal complexes with inorganic or organic ligands and in which the metal is present in a high, ie oxidized, valency state, preferably metal complexes of iron(III) and of manganese(III) with inorganic or organic ligands, more preferably iron(III) 2,2′-dipyridyl, Fe(lII) hexacyanoferrate and Mn(III) transcyclohexane-1,2-diamine-N,N,N′,N′-tetraacetate),
  • cycloaliphatic, heterocyclic or aromatic compounds which contain an NO, NOH or HNR—OH group, more preferably 2,2,6,6-tetramethylpiperidin-1-yloxyl (TEMPO) and violuric acid or
  • hydrogen peroxide generated cathodically by oxygen reduction or other electrochemically regenerable inorganic or organic peroxo compounds.

The reducing and oxidizing agents mentioned can each also be used in mixtures with other reducing and oxidizing agents respectively, in which case the mixing ratios within each mixture are not critical.

The reducing and oxidizing agents are preferably used in the concentration range from 0.1 mmol/l to 5 mol/l and more preferably between 1 mmol/l and 0.1 mol/l.

The process according to the invention is customarily carried out at temperatures which are adapted to the dye, to the redox system and especially to the color change to be obtained. The process according to the invention is preferably carried out between 15° C. and 150° C., more preferably between 20° C. and 95° C. and most preferably between 40° C. and 60° C.

As regards the dyed textile substrates to be treated, there are no technical restrictions, since the process according to the invention can be adapted to the material. They can be present in the form of fibers, yarns, wovens, knits or already as wholly or partly made-up products. The dyed textile substrates are preferably composed of fiber materials composed of cellulose fibers, synthetic fibers or blends thereof.

Particular preference is given to dyed textile substrates composed of cellulose fibers and blends thereof with manufactured fibers, such as polyester or polyamide fibers. But textile substrates composed of polyester or polyamide fibers can be used as well.

Nor are there any restrictions with regard to the dyes to be changed. For instance, reactive dyes, vat dyes, sulfur dyes, direct dyes, naphthol dyes, disperse dyes, acid dyes, cationic dyes or metallized dyes can all be color changed according to the invention. Cellulose fibers can be dyed with any class of dye customary for this substrate, preference being given to reactive dyes, vat dyes, sulfur dyes or direct dyes. Substrates composed of polyester fibers can be dyed with disperse dyes for example.

The process according to the invention is particularly preferable for treating indigo-dyed cotton substrates.

It will be appreciated that the dye and the redox system have to be adapted to each other, ie the reducing or oxidizing agent has to be able to react with the dye to be able to change the dyeing. For example, dyes containing azo groups can be reductively cleaved or indigo can be subjected to a reversible dye reduction.

The treatment liquor is preferably circulated between the treatment assembly and the electrolytic cell in such a way that the electrochemically generated amount of active chemicals can be transported into the treatment assembly as well and the given concentration of chemicals to be reacted does not limit the current density of the electrolytic cell. This adaptation can be effected by customary calculations based on electrolytic current and concentration of redox system used. If, for example, the cell current is 20 A and the concentration of redox system is 0.05 mol/l and the concentration of starting chemical is not to drop below 0.04 mol/l, a circulation rate of 1.24 l/min is necessary between the treatment assembly and the cell.

Further parameters which may have to be taken into account in the process according to the invention are the liquor ratio, the pH and also assistants such as dispersants, detergents, lubricants or enzymes.

The liquor ratio must be taken into account because the redox potential generates a certain concentration of active chemicals and the amount used will vary with the liquor ratio. Since many redox chemicals are strongly pH-dependent, the pH influences not only the measured redox potential but also the chemical reactivity.

The addition of dispersants and/or wetting agents to the treatment liquor can modify the penetration of the treatment liquor into the dyed textile material to be treated and also the degree of detachment and dispersion of the dye pigments from the textile material. Lubricants can be used to reduce any shearing effects which arise.

Enzymes such as cellulases can detach dyed fiber fragments from the dyed textile substrate to be treated and so intensify any abrasive treatment.

Redox-active enzymes such as laccase can under suitable conditions augment dye destruction and hence augment or modify the desired color change.

All the aforementioned assistants are inexpensive. Which type is used in which amount in the process of the invention, if at all, is easily ascertainable by one skilled in the art.

FIG. 1 is a schematic view of an arrangement which is suitable for carrying out the process of the invention. An electrolytic cell made up of a working electrode b, a membrane d, a counterelectrode e, current feeds c and current supply a is used to generate the reducing or oxidizing agent (desired form of the redox couple used) in the liquor k, which is pumped through a circulating system g into the treatment assembly m. The treatment assembly holds the dyed textile substrate f to be treated, which is subjected therein to additional mechanical and/or hydrodynamic stresses. The redox potential in the liquor h in the treatment assembly is measured by the redox measuring means i. The value measured here forms the basis for the adaptation of the supply current strength of the cell current supply a via the control loop l. This provides control over the redox potential and hence over the treatment effect to be achieved on the dyed textile substrate. After use, the treatment liquor is dropped via the outlet j from the treatment assembly, which subsequently effects for example rinsing processes with the textile substrate. The dropped treatment liquor can be sent to a regenerating stage.

FIG. 2 shows an arrangement in which the electrolytic cell and the treatment assembly form a unit and the two electrodes are disposed in the immediate vicinity of the dyed textile substrate to be treated, and also the two working phases. During electrolysis, the power supply a feeds electricity through lines c to the working electrode b and to the counterelectrode e. The dyed textile substrate f to be treated contains the amount of electrolyte required for electrolysis and hence the cell content h and the treatment liquor k are identical. Since potential measurement is for geometric reasons relatively costly and inconvenient, the measurement is carried out, after the power supply has been interrupted, as a rest potential measurement with the aid of line i, which is connected to the working electrode b, and of the reference electrode n. Potential measurement and current control to set the desired redox potential therefore take place intermittently.

USE EXAMPLE 1

Lightening of Indigo with Hypochlorite at pH 10.0-10.2

Construction of Electrolytic Cell:

Anode: expanded titanium metal with Pt mixed oxide coating, two electrodes, each 10 cm in active length and 3.0-3.1 cm in width.

Diaphragm: Nafion cation exchanger membrane

Cathode: stainless steel about 100 cm²

The treatment apparatus/electrolytic cell circulation is provided by a peristaltic pump rated 150 ml/min. Inside the treatment apparatus, the liquor is agitated by a magnetic stirrer (500 rpm) with heating. The vessel contains the Pt electrode and reference electrode and also the temperature measurement and a pH measurement system. Constant pH during electrolysis is ensured by metering alkali into the anolyte.

Anolyte: volume 850 ml, 1 g/l of sodium carbonate p.A. and 10 g/l of NaCl p.A.

Catholyte volume: 350 ml. Catholyte composition same as anolyte.

Fabric weight: 11.18 g of indigo-dyed woven cotton (denim).

Initial temperature: 23.5° C., treatment temperature 50° C.

A redox potential of +440 to +470 mV is set at a pH of 10.0 to 10.2 for a period of 35 min by controlling the cell current in the range from 100 mA to 500 mA (cell voltage 2.40 to 5.10 V).

Sample 1 is treated for 15 min and sample 2 for 35 min under these conditions.

The removed samples are rinsed in cold water, whizzed and dried at 110° C.

The continuous lightening is discernible from the L values (CIELab system).

Representation in Lab coordinates:

	L	a	b
	Original sample:	24.30	+1.02	−13.10
	Treated sample 1:	26.00	+0.94	−13.98
	Treated sample 2:	27.13	+0.83	−13.66

USE EXAMPLE 2

Lightening of Indigo with Hypochlorite pH 7.8-8.6

The electrolytic cell is constructed and the process is carried out as in use example 1.

Anolyte: volume 850 ml, 0.5 g/l of sodium carbonate p.A, 0.5 g/l of sodium bicarbonate and 10 g/l of NaCl p.A, catholyte composition same as anolyte.

Fabric weight: 10.86 g of indigo-dyed woven cotton (denim).

Initial temperature: 24.8° C., treatment temperature 50° C.

A redox potential of +700 to +720 mV is set at a pH of 8.1 for a period of 32 min by controlling the cell current in the range from 200 mA to 500 mA (cell voltage 3.10 to 4.80 V).

Sample 1 is treated for 12 min and sample 2 for 32 min under these conditions.

The removed samples are rinsed in cold water, whizzed and dried at 110° C.

The continuous lightening is discernible from the L values (CIELab system).

Representation in Lab coordinates:

	L	a	b
	Original sample:	24.30	+1.02	−13.10
	Treated sample 1:	26.44	+1.02	−13.89
	Treated sample 2:	27.48	+0.99	−14.33

USE EXAMPLE 3

Lightening of Indigo with Hypochlorite pH 10.2-10.6

The electrolytic cell is constructed and the process is carried out as in use example 1.

Anolyte: volume 800 ml, 1.0 g/l of sodium carbonate p.A, 0.1 g/l of sodium bicarbonate and 10 g/l of NaCl p.A, catholyte composition same as anolyte.

Fabric weight: 10.93 g of indigo-dyed woven cotton (denim).

Initial temperature: 21.1° C., treatment temperature 50° C.

A redox potential of +455 to +555 mV is set at a pH of 10.5 for a period of 45 min by controlling the cell current in the range from 0 mA to 500 mA (cell voltage 5.15 V).

Sample 1 is treated for 25 min and sample 2 for 45 min under these conditions.

The removed samples are rinsed in cold water, whizzed and dried at 110° C.

Representation in Lab coordinates:

	L	a	b
	Original sample:	24.30	+1.02	−13.10
	Treated sample 1:	26.75	+0.72	−14.31
	Treated sample 2:	26.14	+0.71	−14.39

USE EXAMPLE 4

Lightening of Indigo with Hypochlorite/Potassium Bromide at pH 8.1-8.6

The electrolytic cell is constructed and the process is carried out as in use example 1.

Anolyte: volume 800 ml, 0.5 g/l of sodium carbonate p.A, 0.5 g/l of sodium bicarbonate and 10 g/l of NaCl p.A, 0.1 g/l of potassium bromide, catholyte composition same as anolyte.

Fabric weight: 10.90 g of indigo-dyed woven cotton (denim).

Initial temperature: 22.9° C., treatment temperature 50° C.

A redox potential of +720 to +750 mV is set at a pH of 8.1 to 8.6 for a period of 40 min by controlling the cell current in the range from 50 mA to 500 mA (cell voltage 2.4 to 5.0 V).

Sample 1 is treated for 20 min and sample 2 for 40 min under these conditions.

The removed samples are rinsed in cold water, whizzed and dried at 110° C.

Representation in Lab coordinates:

	L	a	b
	Original sample:	24.30	+1.02	−13.10
	Treated sample 1:	48.89	−5.78	−2.56
	Treated sample 2:	54.27	−65.0	+0.89

USE EXAMPLE 5

Lightening of Indigo with Hypochlorite/Potassium Bromide at pH 8.9-9.5

The electrolytic cell is constructed and the process is carried out as in use example 1.

Anolyte: volume 800 ml, 0.5 g/l of sodium carbonate p.A, 0.5 g/l of sodium bicarbonate, 10 g/l of NaCl p.A and 0.1 g/l of potassium bromide, catholyte composition same as anolyte.

Fabric weight: 10.78 g of indigo-dyed woven cotton (denim).

Initial temperature: 21.3° C., treatment temperature 21.3 to 22.2° C.

A redox potential of +710 to +745 mV is set at a pH of 8.9 to 9.5 for a period of 30 min by controlling the cell current in the range from 50 mA to 500 mA (cell voltage 2.2 to 5.1 V).

Sample 1 is treated for 10 min and sample 2 for 30 min under these conditions.

The removed samples are rinsed in cold water, whizzed and dried at 110° C.

Representation in Lab coordinates:

	L	a	b
	Original sample:	24.30	+1.02	−13.10
	Treated sample 1:	27.33	+0.49	−14.95
	Treated sample 2:	30.53	−0.92	−15.48

USE EXAMPLE 6

Lightening of Indigo with Hypochlorite/Potassium Bromide at pH 7.5-7.7

The electrolytic cell is constructed and the process is carried out as in use example 1.

Anolyte: volume 800 ml, 1.0 g/l of sodium bicarbonate, 10 g/l of NaCl p.A and 0.1 g/l of potassium bromide, catholyte composition same as anolyte.

Fabric weight: 10.78 g of indigo-dyed woven cotton (denim).

Initial temperature: 21.9° C., treatment temperature 21.9 to 22.9° C.

A redox potential of +710 to +815 mV is set at a pH of 7.5 to 7.7 for a period of 30 min by controlling the cell current in the range from 20 mA to 500 mA (cell voltage 1.65 to 5.1 V).

Sample 1 is treated for 33 min and sample 2 for 63 min under these conditions.

The removed samples are rinsed in cold water, whizzed and dried at 110° C.

Representation in Lab coordinates:

	L	a	b
	Original sample:	24.30	+1.02	−13.10
	Treated sample 1:	28.32	+0.27	−15.18
	Treated sample 2:	28.41	+0.11	−15.31

USE EXAMPLE 7

Lightening of Indigo with Hypochlorite/Potassium Bromide at pH 7.1-7.7

The electrolytic cell is constructed and the process is carried out as in use example 1.

Anolyte: volume 800 ml, 1.0 g/l of sodium bicarbonate, 10 g/l of NaCl p.A and 0.1 g/l of potassium bromide, catholyte composition same as anolyte.

Fabric weight: 10.34 g of indigo-dyed woven cotton (denim).

Initial temperature: 21.1° C., treatment temperature 21.1 to 22.0° C.

A redox potential of +759 to +825 mV is set at a pH of 7.7 to 8.3 for a period of 30 min by controlling the cell current in the range from 500 mA (cell voltage 4.8 to 5.05 V).

Sample 1 is treated for 10 min and sample 2 for 30 min under these conditions.

The removed samples are rinsed in cold water, whizzed and dried at 110° C.

Representation in Lab coordinates:

	L	a	b
	Original sample:	24.30	+1.02	−13.10
	Treated sample 1:	35.17	−3.05	−11.55
	Treated sample 2:	43.05	−5.70	−4.88

USE EXAMPLE 8

Lightening of Reactive-Dyed Fabric with Violuric Acid at pH 4.6

The electrolytic cell is constructed and the process is carried out as in use example 1.

Anolyte: volume 770 ml, 1.0 g/l of violuric acid, 12 g/l of acetic acid and 4 g/l of NaOH,

Catholyte: 300 ml of aqueous sodium hydroxide solution 40 g/l.

Fabric weight: 4.1 g of reactive-dyed cotton (red).

Initial temperature: 26.7° C., treatment temperature 37.1 to 53.3° C.

A redox potential of +604.7 to +633 mV is set at a pH of 4.6 for a period of 36 min by controlling the cell current in the range from 0 mA to 500 mA (cell voltage 5.3 V).

Sample 1 is treated for 16 min and sample 2 for 36 min under these conditions.

The removed samples are rinsed in cold water, whizzed and dried at 110° C.

Representation in Lab coordinates:

	L	a	b
	Original sample:	43.66	+64.15	+6.26
	Treated sample 1:	43.57	+58.80	+3.11
	Treated sample 2:	44.54	+57.51	+2.91

USE EXAMPLE 9

Lightening of Reactive-Dyed Fabric with Violuric Acid at pH 4.6

The electrolytic cell is constructed and the process is carried out as in use example 1.

Anolyte: volume 800 ml, 1.0 g/l of violuric acid, 12 g/l of acetic acid and 4 g/l of NaOH,

Catholyte: 300 ml of aqueous sodium hydroxide solution 40 g/l.

Fabric weight: 4.55 g of reactive-dyed cotton (red).

Initial temperature: 35.5° C., treatment temperature 35.5 to 55.8° C.

A redox potential of +608.7 to +661 mV is set at a pH of 4.6 for a period of 55 min by controlling the cell current in the range from 200 mA to 500 mA (cell voltage 3.2 to 4.55 V).

Sample 1 is treated for 31 min and sample 2 for 55 min under these conditions.

The removed samples are rinsed in cold water, whizzed and dried at 110° C.

Representation in Lab coordinates:

	L	a	b
	Original sample:	43.66	+64.15	+6.26
	Treated sample 1:	44.86	+56.39	+2.39
	Treated sample 2:	47.68	+53.40	+1.67

USE EXAMPLE 10

Lightening of Sulfur Black 1 Dyed Fabric with Violuric Acid at pH 4.6

The electrolytic cell is constructed and the process is carried out as in use example 1.

Anolyte: volume 800 ml, 1.0 g/l of violuric acid, 12 g/l of acetic acid and 4 g/l of NaOH,

Catholyte: 300 ml of aqueous sodium hydroxide solution 40 g/l.

Fabric weight: 4.62 g of cotton dyed by the pad-steam process with 180 g/l of Sulfur Black 1.

Initial temperature: 24° C., treatment temperature 24 to 53° C.

A redox potential of +564 to +615 mV is set at a pH of 4.6 for a period of 31 min by controlling the cell current in the range from 200 mA to 500 mA (cell voltage 3.25 to 5.2 V).

Sample 1 is treated for 11 min and sample 2 for 31 min under these conditions.

The removed samples are rinsed in cold water, whizzed and dried at 110° C.

Representation in Lab coordinates:

	L	a	b
	Original sample:	20.00	+0.34	−0.80
	Treated sample 1:	19.72	+0.10	−0.41
	Treated sample 2:	20.67	+0.30	+0.35

USE EXAMPLE 11

Color Change on Sulfur Black 1 Dyed Fabric with Violuric Acid at pH 4.6

The electrolytic cell is constructed and the process is carried out as in use example 1.

Anolyte: volume 800 ml, 1.0 g/l of violuric acid, 12 g/l of acetic acid and 4 g/l of NaOH,

Catholyte: 300 ml of aqueous sodium hydroxide solution 40 g/l.

Fabric weight: 4.62 g of cotton dyed by the pad-steam process with 180 g/l of Sulfur Black 1.

Initial temperature: 21.6° C., treatment temperature 21.6 to 52.6° C.

A redox potential of +477 to +582 mV is set at a pH of 4.6 for a period of 31 min by controlling the cell current in the range from 0 mA to 500 mA (cell voltage 5.2 V).

Sample 1 is treated for 20 min and sample 2 for 40 min under these conditions.

The removed samples are rinsed in cold water, whizzed and dried at 110° C.

Representation in Lab coordinates:

	L	a	b
	Original sample:	20.00	+0.34	−0.80
	Treated sample 1:	18.61	−0.10	−0.59
	Treated sample 2:	18.94	−0.13	−0.22

USE EXAMPLE 12

Local Dye Destruction with Hypochlorite/Potassium Bromide

A woven fabric which has been dyed with indigo or red reactive dye and has a mass of 12.4 g for example is wetted with four times the amount (about 50 ml) of a solution of 1.0 g/l of sodium bicarbonate, 10 g/l of NaCl p.A. and 0.1 g/l of potassium bromide. A Pt electrode having a surface area of 2.25 cm² serves as a working electrode and a stainless steel electrode serves as a counterelectrode. The potential of the working electrode is measured against Ag/AgCl 3M KCl after interruption of the current and after a delay time of 2 min for adjustment of the potential.

The color changes achieved as a function of the redox potential achieved (measured after 2 min current interruption) are reported in table 1. Original indigo sample see use example 1, original red reactor dyeing sample see use example 8.

TABLE 1
	Potential
	(2 min)	Time	Current	Voltage
Dye	(mV)	(s)	(mA)	(V)	L	a	b
Indigo	+800	15	50	4.1	30.71	−0.36	−14.54
Indigo	+1070	30	50	4.0	34.15	−1.26	−13.68
Indigo	+1130	60	50	4.1	41.39	−3.25	−11.78
Indigo	+1230	180	50	4.1	54.44	−6.45	−2.80
Red	+1110	15	50	4.1	61.34	+27.33	+10.10
Red	+1040	30	50	4.1	58.59	+31.57	+9.09
Red	+1130	60	50	4.1	62.28	+26.07	+10.93
Red	+1100	180	50	4.1	67.99	+20.99	+15.67

USE EXAMPLE 13

Local Dye Destruction by Iron(II/III) Complexes

A woven fabric which has been dyed with indigo or red reactive dye and has a mass of 12.4 g for example is wetted with four times the amount (about 50 ml) of a 0.024 mol/l iron(II) complex solution (triethanolamine and polyhydroxycarboxylic acid as ligands). A Pt electrode having a surface area of 2.25 cm² serves as a working electrode and a stainless steel electrode serves as a counterelectrode. The potential of the working electrode is measured against Ag/AgCl 3M KCl after interruption of the current and after a delay time of 2 min for adjustment of the potential.

The color changes achieved as a function of the redox potential achieved (measured after 2 min current interruption) are reported in table 2. Original indigo sample see use example 1, original red reactor dyeing sample see use example 8.

TABLE 2
	Potential	Time	Current	Voltage
Dye	(mV)	(s)	(mA)	(V)	L	a	b
Indigo	−950	30	15	2.0	40.79	−2.04	−11.98
Indigo	−938	60	15	2.0	39.99	−2.16	−13.96
Indigo	−945	180	15	2.2	38.38	−2.44	−11.73
Red	−825	30	15	1.9	46.91	+57.25	+2.01
Red	−916	60	15	1.9	50.20	+53.11	+0.17
Red	−824	180	15	2.0	52.69	+48.00	−0.09

Key to Drawings:

(Right to left, top to bottom)

FIG. 1

Current strength control

Potential measurement

FIG. 2

Electrolytic process

Current strength control

Potential measurement

Claims

1. A process for obtaining color changes on dyed textile substrates by treating the dyed textile substrates with an electrochemically generated aqueous solution of reducing or oxidizing agents, which comprises controlling the cell current in such a way that the solution, when in contact with the dyed textile substrate, has a suitable redox potential to obtain the color change.

2. The process of claim 1, wherein the aqueous solution of reducing or oxidizing agent is generated in an electrolytic cell which is constructed as a flowthrough cell and which communicates directly with a treatment assembly into which the aqueous solution containing the reducing or oxidizing agent is pumped and in which the color change on the dyed textile substrate is generated.

3. The process of claim 1, wherein the aqueous solution of reducing or oxidizing agent is generated in an electrolytic cell which is constructed as a treatment assembly in which the color change on the dyed textile substrate is generated.

4. The process of claim 1, wherein the redox potential is +100 to +2000 mV in the case of oxidations and −300 to −1800 mV in the case of reductions.

5. The process of claim 1, wherein inorganic reducing or oxidizing agents are used.

6. The process of claim 1, wherein the reducing agent used is cathodically generated from a reversible redox system.

7. The process of claim 6, wherein the reductive redox system used comprises

cathodically generated metal complexes with inorganic or organic ligands and in which the metal is present in a low, reduced, valency state,

substituted anthraquinone compounds,

tin(II) compounds, in alkaline solutions, or

dithionite generated by cathodic reduction, in weakly acidic solution.

8. The process of claim 1, wherein the oxidizing agent used is anodically generated from a reversible redox system.

9. The process of claim 8, wherein the oxidative redox system used comprises

halogen-oxygen compounds,

metal complexes with inorganic or organic ligands and in which the metal is present in a high oxidized, valency state,

cycloaliphatic, heterocyclic or aromatic compounds which contain an NO, NOH or HNR—OH group, or

hydrogen peroxide generated cathodically by oxygen reduction or other electrochemically regenerable inorganic or organic peroxo compounds.

10. The process of claim 6, wherein the redox systems are used in the concentration range from 0.1 mmol/l to 5 mol/l.

11. The process of claim 3, wherein the redox potential is +400 to +1600 mV in the case of oxidations and −400 to −1200 mV in the case of reductions.

12. The process of claim 6, wherein the reductive redox system used comprises

cathodically generated metal complexes with inorganic or organic ligands and in which the metal is present in iron(II) or tin(II) complexes with inorganic or organic ligands,

substituted anthraquinone compounds,

hexahydroxystannite in alkaline solutions, or

dithionite generated by cathodic reduction, in weakly acidic solution.

13. The process of claim 6, wherein the reductive redox system used comprises

cathodically generated metal complexes with inorganic or organic ligands and in which the metal is present in iron(II) complexes containing a 2-hydroxyethyl group or a polyhydroxycarboxylic acid in the ligand,

1,2-dihydroxyanthraquinone or anthraquinonesulfonic acids,

hexahydroxystannite in alkaline solutions, or

dithionite generated by cathodic reduction, in weakly acidic solution.

14. The process of claim 8, wherein the oxidative redox system used comprises

hypochlorite and hypobromite,

metal complexes with inorganic or organic ligands and in which the metal is present in a metal complexes of iron(II) and of manganese(III) with inorganic or organic ligands,

2,2,6,6-tetramethylpiperidin-1-yloxyl (TEMPO) and violuric acid or

hydrogen peroxide generated cathodically by oxygen reduction or other electrochemically regenerable inorganic or organic peroxo compounds.

15. The process of claim 8, wherein the oxidative redox system used comprises

hypochlorite and hypobromite,

metal complexes with inorganic or organic ligands and in which the metal is present in a metal complexes of iron(III) 2,2′-dipyridyl, Fe(III) hexacyanoferrate and Mn(III) transcyclohexane-1,2-diamine-N,N,N′,N′-tetraacetate,

2,2,6,6-tetramethylpiperidin-1-yloxyl (TEMPO) and violuric acid or

hydrogen peroxide generated cathodically by oxygen reduction or other electrochemically regenerable inorganic or organic peroxo compounds.

16. The process of claim 9, wherein the redox systems are used in the concentration range between 1 mmol/l and 0.1 mol/l.