|Title of Invention||
|Abstract||TITLE: MEDIATOR SYSTEM. Mediator systems obtainable by mixing a salt of an electrochemically active compexing metal capable of forming a plurality of valence states with a hydroxyl-containing complexing agent, which may likewise be present as salt, and with a salt of an electrochemically inactive complexing metal in an alkaline aqueous medium, whrefor the molar ratio fo metal ion M2 to metal ion M1 is from 0.8:1 to 2:1 are useful for reducing dyes and dyeing cellulosic textile material.|
|Full Text||DyStar Textilfarben GmbH & Co. Deutschland KG DYS 2000/B 002 Dr. My
Dye reduction mediator system based on mixed metal complexes
The present invention relates to mediator systems obtainable by mixing a salt
of an electrochemically active complexing metal (M1) capable of forming a
plurality of valence states with a hydroxyl-containing complexing agent, which
may likewise be present as salt, and with a salt of an electrochemically inactive
complexing metal (M2) in an alkaline aqueous medium, wherefor the molar
ratio of metal ion M2 to metal ion M1 is from 0.8:1 to 2:1.
The invention also provides a process for reducing dyes, a process for dyeing
cellulosic textile material using these mediator systems and cellulosic textile
materials dyed by these processes.
Vat dyes and sulfur dyes are important classes of textile dyes.
Vat dyes are of major significance for dyeing cellulose fibers on account of the
high fastnesses of the dyeings in particular. To use these dyes, the insoluble
oxidized dye has to be converted into its alkali-soluble leuco form by a reducing
step. This reduced form has high affinity for cellulose fiber, goes onto the fiber
and once on the fiber is converted back into its insoluble form by an oxidizing
The class of sulfur dyes is particularly important for the production of
inexpensive dyeings having average fastness requirements. The use of sulfur
dyes likewise involves the need to carry out a reducing step and an oxidizing
step in order that the dye may be fixed on the material.
The literature describes a wide range of reducing agents for use on an
industrial scale, eg. sodium dithionite, organic sulfinic acids, organic hydroxy
compounds such as glucose or hydroxyacetone. In some countries sulfur dyes
are still being reduced using sulfides and polysulfides.
A feature common to these reducing agents is the absence of a suitable way
for regenerating their reducing effect, so that these chemicals are discharged
after use into the wastewater together with the dyebath. As well as the costs for
fresh chemicals to be used, this also creates the additional expense of having
to treat the wastewaters produced.
Further important disadvantages of these reducing agents are the very limited
means to influence their reducing effect or their redox potential under
application conditions in the dyebath and the absence of simple control
technology for regulating the dyebath potential.
A further group of reducing agents was discovered in the class of iron(ll)
complexes. Iron(ll) complexes are known with triethanolamine (WO-A-
90/15182, WO-A-94/23114), with bicine (N,N-bis(2-hydroxyethyl)glycine) (WO-
A-95/07374), with triisopropanolamine (WO-A-96/32445) and also with aliphatic
hydroxy compounds which may contain a plurality of hydroxyt groups and may
additionally be functionalized with aldehyde, keto or carboxyl, such as di- and
polyalcohols, di- and polyhydroxyaldehydes, di- and polyhydroxyketones, di-
and polysaccharides, di- and polyhydroxymono- and -dicarboxylic acids and
also hydroxytricarboxylic acids, preference being given to sugar-based
compounds, especially the acids and salts thereof, eg. gluconic and
heptagluconic acid, and citric acid (DE-A-42 06 929, DE-A-43 20 866, DE-A-
43 20 867, prior German patent application DE-A-199 19 746, unpublished at
the priority date of the present invention, and also WO-A-92/09740).
These iron(ll) complexes have a reducing effect which is sufficient for dye
reduction and which is described by the (negative) redox potential which is
measurable in alkaline solution at a certain molar ratio of iron(ll) : iron(lll).
Numerous of these iron(ll) complexes, eg. the complexes with triethanolamine,
bicine, gluconic acid and heptagluconic acid, also have the advantage of being
electrochemically regenerate and hence of usefulness as mediators in an
\ electrochemical reduction of dyes and also in electrochemical dyeing
It is further known to use mixtures of these iron complexes as reducing agents.
For instance, textil praxis intemational 40,pages 44-49 (1932) and Journal of
the Society of Dyers and Colourists, 113, pages 135=144 (1997) describe
mixtures of iron salts, triethanolamine and respectively citric acid or gluconic
acid. The latter paper also utilizes as mediators mixtures of iron salts, calcium
salts and gluconic acid and/or heptagluconic acid where the molar ratio of
calcium to iron is in the range from 0.5 to 0.75.
However, the known mediator systems have certain weaknesses. True, the
iron complexes based on triethanolamine or bicine have a sufficiently negative
redox potential for dye reduction, but they are not sufficiently stable in the more
weakly alkaline region at pH 11.5, which greatly limits their electrochemical
regenerability in indigo dyebaths for denim manufacture. True, the mediator
systems based on gluconate or heptagluconate have very good complex
stability in the pH range of 10-12, but the known systems have to have a
relatively large fraction of iron(ll) complex to achieve a redox potential of
-700 mV (Ag/AgCI, 3 M KCI reference electrode), as is required, for example,
to maintain the requisite bath stability for dyeing with indigo. But the large
fraction of iron(ll) complex required is disadvantageous especialy with regard to
dyeing with indigo in denim manufacture, since the textile material is here dyed
layer by layer by repeated immersion in the dyebath and subsequent air
oxidation of the dye, so that the mediator in the dyebath is completely oxidized
with every air passage and first has to be reduced again for the next dyeing
cycle, and this entails high electricity consumption, which in turn requires high
mediator concentrations or correspondingly large electrolytic cells by way of
It is an object of the present invention to remedy the disadvantages mentioned
and to make it possible to reduce dyes in an advantageous, economical
manner. More particularly, stable"mediator systems having a powerful reducing
action shall be provided.
We have found that this object is achieved by the mediator systems defined at
The invention also provides a process for electrochemical reduction of dyes in
an alkaline aqueous medium and also a process for dyeing cellulosic textile
material with vat dyes or sulfur dyes by electrochemical dye reduction in the
presence of metal complexes as mediators, which each comprise using the
mediator systems defined at the beginning.
The invention lastly provides cellulosic textile materials which have been dyed
by this process.
An essential feature of the mediator systems according to the invention is a
combination of the electrochemically active metal ion M1 with an
electrochemically inactive, but likewise complexation-capable metal ion M2 and
with a hydroxyl-containing but amino-devoid complexing agent in a molar ratio
of metal ion M1 to metal ion M2 of from 0.8:1 to 2:1, preferably from 0.9:1 to
1.1:1, particularly preferably about 1:1.
The mediator systems according to the invention are obtainable by mixing the
individual components, which may be used in the form of their water-soluble
salts, in an alkaline aqueous medium, which generally has a pH of about 10-
14. In the course of the mixing, the metal ions M1 and M2 are at least partially
complexed, preferably forming an approximately equimolar complex.
The amount of complexing agent is not critical and has only minor importance
given a predetermined ratio of reduced to oxidized form of the metal ion M1.
The minimum amount of complexing agent normally used will be the amount
theoretically required for completely complexing M1, ie. at least 0.5 mol,
preferably 1 mol per mole of M1. In principle there is no upper limit to this molar
ratio, but cost reasons will generally rule out the use of an amount of more than
5 mol, especially 3 mol, in particular 1.5 mol, of complexing agent per mole of
The metal ion M1 can be used not only in low-valent form but also in higher-
valent form. For example, in the case of the particularly preferred metal iron,
not only iron(ll) salts may be used but also iron(lll) salts, which are initially
readily reduced to iron(ll) electrochemically.
Useful hydroxyl-containing complexing agents for the purposes of the invention
include in particular aliphatic hydroxy compounds that have at least two
coordination-capable groups and that are likewise soluble in water or aqueous
organic media or miscible with water or aqueous organic media and that may
contain a plurality of hydroxyl groups and/or aldehyde, keto and/or carboxyl
groups. Specific examples of preferred complexing agents are:
di- and polyalcohols such as ethylene glycol, diethylene glycol,
pentaerythritol, 2,5-dihydroxy-1,4-dioxane, especially sugar alcohols
such as glycerol, tetritols such as erythritol, pentitols such as xylitol and
arabitol, hexitols such as mannitol, dulcitol, sorbitol and galactitol;
di- and polyhydroxyaldehydes such as glyceraldehyde, triose reductone,
especially sugars (aldoses) such as mannose, galactose and glucose;
di- and polyhydroxyketones such as, in particular, sugars (ketoses) such
dj- and polysaccharides such as sucrose, maltose, lactose, cellubiose
di- and polyhydroxymonocarboxylic acids such as glyceric acid,
particularly acids derived from sugars, such as gluconic acid,
heptagluconic acid, galactonic acid and ascorbic acid;
- di- and polyhydroxydicarboxylic acids such as malic acid, particularly
sugar acids such as glucaric acids, mannaric acids and galactaric acid;
hydroxytricarboxylic acids such as citric acid.
Particularly preferred complexing agents are the monocarboxylic acids derived
from sugars (especially gluconic acid and heptagluconic acid) and their salts,
esters and lactones.
It will be appreciated that it is also possible to use mixtures of complexing
agents. A particularly useful example thereof is a mixture of gluconic acid and
heptagluconic acid, preferably in a molar ratio of from 0.1:1 to 10:1, which
provides iron complexes that are particularly stable at high temperatures.
The metal ion M2 is preferably a metal ion which likewise will form stable
complexes with the complexing agent of the invention. Particular preference is
given to divalent metal ions, and calcium ions are very particularly preferred.
In particularly preferred mediator systems according to the invention the metal
ion M1 comprises iron(ll/lll) ions, the metal ion M2 comprises calcium ions and
i the complexing agent is gluconic acid and/or heptagluconic acid.
The particular advantages of the mediator systems according to the invention
are that they have a redox potential customary for dye reduction (about 12.5-13.5), but also at a lower
i concentration of low-valent metal ion M1 and hence at a lower concentration of
active complex, but will form a stable complex system even at lower pH values,
ie. at about 11-12, and so are altogether very useful as mediators for
electrochemical dyeing with indigo in particular.
That the redox potential of the electrochemically active complex would so
distinctly shift to what are more negative values in the presence of the
electrochemically inactive metal ion was unexpected. To illustrate this effect,
the redox potentials determined by means of electrochemical conversion trials
for a mediator system of iron, calcium and gluconate ions are reported in what
follows. The respective iron(ll)/iron(lll) ratio was determined photometrically
The mediator systems of the invention are very useful for the electrochemical
reduction of dyes.
The process of the invention is particularly important for reducing vat dyes and
sulfur dyes, particularly the class of indigoid dyes, the class of anthraquinonoid
dyes, the class of dyes based on highly fused aromatic ring systems and the
class of sulfur cooking and baking dyes. Examples of vat dyes are indigo and
its bromine derivatives, 5.5-dibromoindigo and 18.104.22.168-tetrabromoindigo, and
thioindigo, acylaminoanthraquinones, anthraquinoneazoles, anthrimides,
anthrimidecarbazoles, phthaloylacridones, benzanthrones and indanthrones
and also pyrenequinones, anthanthrones, pyranthrones, acedianthrones and
perytene derivatives. Examples of particularly important sulfur dyes are C.I.
Sulfur Black 1 and C.I. Leuco Sulfur Black 1 and sulfur vat dyes such as C.I.
Vat Blue 43.
The inventive process for reducing the dye customarily employs the mediator in
an amount not more than approximately that required by the dye reduction
stoichiometry. So one mote of an oxidized dye which takes up two electrons
per molecule to convert into the leuco form generally requires, 2 mol of a
mediator system according to the invention, based on the redox-active metal
ion supplying one electron. It will be appreciated that electrochemical
regeneration of the mediator can reduce this mediator quantity (in the case of
dyeing with vat dyes generally to about 0.1-1 mol of reduced mediator per
mole of dye, based on one liter of dyebath). The greater the deficiency of
mediator system, the higher the requirements the electrolytic cell has to meet.
The reduction process of the invention can advantageously be part of the
similarly inventive process for dyeing cellulosic textile material with vat and
sulfur dyes. Preferably, in this case, the dye is added to the dyebath in
prereduced form, for example in the form of an alkaline solution of catalytically
reduced indigo, and the dye fraction reoxidized by air contact during dyeing is
electrochemically reduced by means of the mediator systems according to the
The dyeing itself may be carried out as described in the references cited at the
beginning. Any known continuous and batch dyeing methods, for example the
exhaust method and the padding method, may be employed.
Because different dyeing processes and dyeing machines differ in the degree
of air access they allow, there will be some instances where appreciable
quantities of mediator system have to be used to cope with the oxygen from the
air. For instance, exhaust dyeing with vat dyes to medium depths of shade will
impose an additional requirement of about 1-10 mol of reduced mediator per
mole of dye, while continuous dyeing with indigo additionally requires about 2 -
10 mol of reduced mediator per mole of indigo.
The rest of the process conditions, such as type of textile assistants, use
levels, dyeing conditions, type of electrolytic cell and finishing of the dyeings,
can be chosen as customary and as described in the references cited at the
The dyeing process of the invention provides advantageous dyeing on all
cellulosic textile materials. Examples are fibers composed of cotton,
regenerated cellulose such as viscose and modal and bast fibers such as flax,
hemp and jute. Useful processing forms include for example staple, tow, yarn,
thread, wovens, loop-drawn knits, loop-formed knits and made-up pieces.
Machine forms can be pack systems, hank, package, warp beam, fabric beam
and piece goods in rope form or open width.
Dyeing with indigo in denim manufacture
250 ends of cotton yam (Nm 11.4, Ne 6.75/1) were dyed with indigo on a
laboratory dyeing range (from Looptex, Lugano, Switzerland) which was
coupled to an electrolytic ceil and is suitable for dyeing cotton yarn by the sheet
dyeing and the rope dyeing process.
The electrolytic cell was a multicathode cell (10 electrodes, 400 cm2 planar
surface area, total surface area 1.9 m2). The anolyte used was 5% by weight
sulfuric acid. Catholyte (dyebath) and anolyte were kept apart by a cation
exchange membrane. The cathode used was a stainless steel mesh, while the
anode used was a titanium electrode coated with platinum mixed oxide.
The dyeing was carried out as follows:
The cotton yam was initially prewetted in a cold wetting agent liquor (3 g/l of a
commercially available wetting agent) and, after squeezing off to 75% wet
pickup, dipped into the hereinbelow-described dyebath (11.251, room
temperature). After a dip time of about 25 sec and squeezing off to 75% wet
pickup, the yam was air oxidized at room temperature for 120 sec. This cycle of
dipping in the dyebath, squeezing off and air oxidization was repeated a
number of times. Thereafter, the dyed yam was rinsed with deionized water
The dyebath, which had been adjusted to pH 11.3, had the following
0.24 mol/l of iron(lll) chloride (40% by weight aqueous solution; 68.5 ml/I)
0.30 mol/l of sodium gluconate (99%; 65.4 g/l)
0.12 mol/l of sodium heptagluconate (22.5% by weight aqueous solution,
0.24 mol/l of calcium chloride (78.5% by weight aqueous solution; 29.6 g/l)
1.15 mol/l of aqueous sodium hydroxide solution (50% by weight; about
The dyebath was reduced prior to the start of dyeing. After 5 minutes of
electrolysis at 5 A a potential of -700 mV was reached, the cell voltage being
6.6 V. A 20% by weight alkaline aqueous leuco indigo solution (BASF) was
then introduced into the reduced dyebath, which was then used for dyeing.
The following 3 series were dyed with respectively 4, 6 and 8 cycles (3 dyeings
in each case):
45 ml of leuco indigo solution (corresponding to 1 g of indigo/I of dyebath), pH
in dyebath 11.35.
90 ml of leuco indigo solution (corresponding to 2 g of indigo/I of dyebath), pH
in dyebath 11.4.
180 ml of leuco indigo solution (corresponding to 4 g of indigo/I of dyebath), pH
in dyebath 12.5.
The dyeings obtained were of outstanding quality, being equivalent in depth of
shade and penetration to standard dyeings with hydrosulfite as reducing agent.
1. Modtator systems obtainable by mating a salt of an electrochemical
active complexhig metal (M1) capable of forming a pluratty of valance
states with a hydroxyl contaning comptexing agent, which may
Ikewise be present aa salt, and with a salt of an electrochemically
Inactive complex metal (M2) in an alkaline aqueous medium,
wherefor the molar ratio of metal lon M2 to metal lon M1 is from 0.8:1
2. Madiator systems as claimed in claimed 1, containing Iron (II) lons and/or
iron (III) loin as metal km M1.
3. Mediator systema aa claimes in claim 1 or 2, containing divalent metal
lona as metal Ion M2.
4. Medtator ayatema aa claimed In any of claims 1 to 3, containing
calclum lona aa metal ton M2.
5. Medtator systema as claimed in any of claims 1 to 4, wherein said
complexing agent la a hydroxyl-containing allphetic caroxylic acid.
6. Medtator systems aa claimed in any of claim 1 to 5, wherein said
metal ion M1 comprises ion (ll/lll) ions, said metal iom M2 comprises
caldum ions and said comptexing agent is gluconic add and/or or
7. A process for electrochemical reduction Of dyes such as dyess sulter
dyes, in an alloline aojusous medium using metal complekes as
meditor which comprisinf using a mediator system as claimed in any
of claims 1 to 6.
8. A process for dyeing cellulosic textile meterial with vat dyes or sulfer
dyes by electrochemical dye reduction In the presence of metal
complexes as mediators, whice comprises using a mediator system as
claimed in any of claimed 1 to 6.
9. A process as claimed in cliam 8, wherein the dye is addsd to the
dyebeth in prereduced from and the dye recoxidized by air contact
during dyeing is electrochemical reduced by means of the mediator
10. cellulosic textile material dyed by the process of claim 8 or 9.
Mediator systems obtainable by mixing a salt of an
electrochemically active complexing metal (M1) capable of forming
a plurality of valence states with a hydroxyl-containing
complexing agent, which may likewise be present as salt,and with
a salt of an electrochemically inactive complexing metal (M2) in
an alkaline aqueous medium, wherefor the molar ratio of metal ion
M2 to metal ion M1 is from 0.8:1 to 2:1 are useful for reducing
dyes and dyeing cellulosic textile material.
|Indian Patent Application Number||IN/PCT/2002/1009/KOL|
|PG Journal Number||12/2008|
|Date of Filing||06-Aug-2002|
|Name of Patentee||DYSTAR TEXTILFARBEN GMBH.& CO. DEUTSCHLAND KG.,|
|Applicant Address||GERMANY, ESCHENHEIMER TOR 2, 60318 FRANKFURT AM MAIN, A GERMAN COMPANY.|
|PCT International Classification Number||D06P1/22,D06P1/30|
|PCT International Application Number||PCT/EP01/02307|
|PCT International Filing date||2001-03-01|