CA2181813A1

CA2181813A1 - Method of bleaching jeans fabric

Info

Publication number: CA2181813A1
Application number: CA002181813A
Authority: CA
Inventors: Christoph Erwin Hartmann; Wolfgang Lex; Doris Preidt
Original assignee: Individual
Current assignee: United Initiators GmbH and Co KG
Priority date: 1994-01-28
Filing date: 1995-01-26
Publication date: 1995-08-03
Also published as: ATE159980T1; WO1995020643A1; EP0741777B1; GR3025703T3; AU681490B2; EP0741777A1; AU1536595A

Abstract

The invention concerns a one-step process for brightening dyed cotton fabrics which is characterized in that dyed cotton fabrics are treated, under conditions which are suitable for brightening the colours of cotton fabrics, with an aqueous bath which contains a peroxo compound as the active component but no added hypochlorites, permanganates and enzymes

Description

- 1 - 21~1813 "Bleaching jeans"
DESCRIPTION
The present invention concerns a process for brightening or blP~rh;n~ dyed cotton fabrics in the absence of added hypochlorites, pP~-n~n~tes and enzymes as well as a brightening bath suitable for carrying out this process.
For several years there has been an increased need for dyed cotton fabrics in particular for cotton jeans dyed with indigo or other dyes which are brightened or partially decoloured in further procP~; n~ steps either by mechanically supported washing processes or/and by chemical processes.
It is known that the overall colour tone of j eans fabrics can be toned down or brighten~d with hypochlorites such as NaOCl. This process has, however, the disadvantage that large amounts of waste water pol luted with AOX result, which have to be subsequently treated in order to comply with the limits for waste water which are bPc~m; n~ increasingly stringent. In addition public criticism of the use of products containing chlorine as well as of processes for manufacturing consumer products using ~hpm;~
containing chlorine is increasing due to the physiological risks of these substances. Manufacturers of dyed cotton fabrics in particular of jeans fabrics, therefore attempt to develop brightening processes which are less critical, in order to save disposal costs or additional production costs due to subse~uent expensive effluent processing methods.

- 2 - ~1~181~
It is known inter alia that dextrose as a reducing agent can be used as an alternative to hypochlorite bleaching which, however, has the disadvantage that the treatment temperature has to be at least 80C and the pH value of the brightening bath has to be adjusted to 13.
Furth. ~ the use of ozone as an oxidizing agent for brightening dyed cotton fabrics is known, but this process has, however, the disadvantage that the hRnrll in~
of ozone in technical production processes is quite difficult. In addition the cotton fibre is heavily damaged by treatment with ozone. Furth~ ~ ketone derivatives are known as reducing agents for brightening cotton fabrics. These substances allow normal conditions with regard to the 1 eat 1 temperature, pH value of the brightening bath and duration of treatment, but they have the disadvantage that the values f or the biological and chemical oxygen demand of the waste water (BOD5 and COD) are increased due to organic pollution by the washing liquor. In addition the white-blue contrast of cotton f abrics that are dyed with indigo is increased when uEing ketone derivatives as brightening agents which is undesired for the application.
In order to avoid the disadvantages of hypochlorite bleaching, cotton materials have recently been treated with potas6ium p~rrqn~RnRte to partially oxidize dyes in the cotton fabric in order to brighten cotton materials.
The disadvantage of this process is that the waste water from the brightening process is polluted with large amounts of heavy metals. Moreover the use of permanganates considerably in~:L ~ases operating costs . In addition the f ibres are coloured brown through MnO2 .
The use of hydrogen peroxide or peracetic acid to brighten dyed cotton fabrics i~ known. However, these _ 3 _ 2 1 ~:L 813 substances are not used themselves to carry out the brightening step but to neutralize cotton fabric which has already been brightened by hypochlorite or potassium pPr---n~;-nslte (cf. e.g. GB-A-2 213 842, US-A-5,205,835, JP-A-31027174; US-A-5,238,463; JP-A-3027173). However, as already described above these processes have the disadvantage of using hypochlorites or p~rr~n~n~tes.
Furth~ the brightening of dyed textiles in the presence of an enzyme such as peroxidase or oxidase and H2O2 as an oxidizing agent is known (WO-A-921867). This process is, however, expensive due to the use of enzymes and is theref ore not suitable f or large-scale processes .
The object of the present invention was therefore to at least partially eliminate the disadvantages of the state of the art and to provide a process for brightening dyed cotton fabrics which is comparable to the use of hypochlorite with respect to the achieved effect and costs but which has the advantage of lower BOD5 and COD
values in the waste water.
DE-OS-27 36 560 discloses a bl~hinq agent composition which is essentially composed of 50-99 . 9 % by weight of an inorganic peroxide which is capable of forming hydrogen peroxide in an aqueous solution and of a special complexing agent. There is no mention of peroxodisulf ates .
EP-A-0 531 849 discloses a process for bleaching materials made of cellulose dyed with direct or reactive dyes by treatment in a bleaching and retreatment bath which contains a bleaching agent, stabilizers, cationic _ '- and optionally non-ionic surfactants. Hydrogen peroxide is named as the bleaching agent.

` 21 1 13 _ 4 _ ~3 ~
Chemical Abstracts 92: 60299J and 60289A disclose a process for blea~hin~ dyed cotton fabrics using a bleaching lye which contains a peroxy ~ , ul.d and an oxime or peroxy ~ , .u.-d, a complexing agent and a sugar alcohol or an aldonic acid. Sodium perborate is disclosed as the peroxo ~ u--d.
The object according to the invention is achieved by a one-step process for brightening dyed cotton fabrics which is characterized in that dyed cotton fabrics are treated with an aqueous bath which contains a peroxo compound as the active ~, s~t but no added hypochlorites, p~ n~n~tes and enzymes under conditions which are suitable for brightening the colours of the cotton fabrics.
Surprisingly it turned out that the use of peroxo compounds as brightening agents has a very good effect on dyed cotton fabrics in particular jeans fabrics. The process according to the invention has the advantage compared to known processes that waste water pollution is avoided since there is no AOX pollution by hypochlorites and other environmentally harmful chemicals do not have to be used at all or only to a slight extent. The COD and BOD5 values as measures of waste water pollution are not increased by the peroxo and opposite effects can even occur since peroxo compounds introduce additional oxygen and thus decrease the oxygen demand of waste water.
For reasons of waste water and air pollution, the process according to the invention is preferably carried out using a brightening bath which contains minimal amounts of chlorine and in particular less than _ 5 _ 2~8 1813 5000 ppm, especially preferably less than 1000 ppm and most preferably less than 200 ppm chlorine (in the form of elemental chlorine, chlorite, hypochlorite etc. ) .
Suitable conditions f or brightening or bleaching the colours of the cotton fabrics are understood as those conditions of concentration of the brightening bath, temperature and duration of treatment under which the desired brightening of the colours is achieved without damaging the cotton f ibres .
A pern~o~ fate is used as the suitable peroxo _~.d for the process according to the invention.
Among the peroxo-disulfates an i~llk~l ino-earth te.g. Ca), alkali (e.g. Li, Na, K, Rb, Cs) or ammonium peroxodisulfate and in particular potassium, sodium or/and ammonium peroxodisulfate is in turn preferred.
The concentration of the peroxo compound can be varied tl~r~n~9;nq on the reaction conditions and the desired degree of brightening of the cotton fabrics. In general the peroxo ~_ ~ ' can be varied within a concentration of about 21 mmol/l (~;oLLe~ol~ding to ca. 5 g/l in the case of sodium peroxodisulfate) up to the solubility limit of the compound or up to a concentration above which the cotton f ibre is damaged . The concentration of the peroxo compound in the brightening bath is preferably in the range of 21 mmol/l to 2.1 mol/l, particularly preferably in the range of 42 mmol/l to 1.7 mol/l and most preferably 42 mmol/l to 1.3 mol/l.
The process according to the invention is preferably carried out at a temperature in the range of 15 to 90C, preferably of 40 to 80C and most preferably of 50 to ~ 81813 65C.
The time period f or carrying out the brightening process according to the invention depends on the desired degree of brightness, the temperature of the brightening bath and the concentration of the brightening reagent. This time period can be easily detF~rm;n-~cl by a person skilled in the art by routine experiments. When carrying out the process according to the invention on a terhn;c~l scale the reaction period is usually 2 minutes to 20 hours, preferably 5 minutes to 5 hours and particularly preferably 15 minutes to 1 hour.
The pH value of the brightening bath can be from 0 to 12, pH values in the range from 1 to 10 being preferred and in the range from 4 to 7 being especially preferred.
In order to adjust the pH range the brightening bath can contain a buffer system e.g. a phosphate buffer.
Further optional components of the brightening bath are surface-active agents e.g. surfactants, complexing agents and further agents which are known in the respective technical f ield.
The process according to the invention can be carried out in the usual devices and with the usual required amounts of the cotton fabrics such as those which are also used to carry out a bleaching process with hypochlorite. The dyed cotton fabrics are preferably treated with the brightening bath according to the invention in a washing drum.
In a particularly preferred on~horl;r L of the process according to the invention the brightening bath ~ X181813 additionally contains an activator which is selected from transition metal compounds or/and halogenide compounds. Examples of ~typical transition metals are titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, palladium, tin, molybdenum, silver, cerium, ruthenium, tungsten and rhenium.
Preferred transition metals are iron, copper, cobalt, zinc, silver, r-n~ln~e, chromium, nickel or mixtures thereof. The transition metals can be used in the form of metal salts or complex '~, metal salts being preferred. The transition metals are particularly preferably used in the form of their sulfate salts. Most preferred are iron(III) salts, iron(II) salts and coppertII) salts, in particular iron(III) sulfate, iron ( I I ) sul f ate and copper ( I I ) sul f ate .
The transition metal ollnr9 used as the activator is preferably added to the brightening bath according to the invention in an amount of up to 25 mmol/l in relation to the metal. The transition metal compound is preferably used in an amount of 0.05 to 15 mmol/l, particularly preferably of 0.05 to 8 mmol/l with respect to the metal.
The use of copper(II) sulfate, iron(II) sulfate or iron(IrI) sulfate as the activator is particularly preferred when ammonium peroxodisulfate is used as the peroxo compound. The use of iron(III) sulfate or iron(II) sulfate as the activator is also particularly preferred when sodium peroxodisulfate is used as the peroxo ~ '.
Examples of halogenide ~ ullds which are suitable a6 the activator are chloride, bromide and iodine _ '~

in particular bromide compounds. A illm, alkali metal and alkaline-earth metal halogenides are preferred and alkali metal halogenides are particularly preferred.
Most preferred are sodium or/and potassium bromide.
The halogenide - uul.d used as the activator is pref erably added to the brightening bath according to the invention in an amount of up to 500 mmol/l with respect to the halogenide ion. The halogenide compound is particularly preferably used in an amount of 0.5 mmol/l to 200 mmol/l, most preferably of 1 to 20 mmol/l with respect to the halogenide ion.
In comparison to activation with transition metal compounds, activation with halogenide ions and in particular with bromide ions has the advantage that the concentration of the peroxo compound can be considerably reduced without impairing the brightening effect. When bromide ions and persulfates are present simultaneously in the brightening bath, concentrations in the range of 21 mmol/l to 420 mmol/l persulfate (corresponding to 5 to 100 g/l in the case of sodium peroxodisulfate) and 0.5 to 200 mmol/l, in particular 1 to 20 mmol/l bromide are particularly preferred and optimal effects are already achieved at very low bromide concentrations.
When selecting optimal process conditions care should be taken that the concentrations of the halogenide cul~ ulld and the peroxo ~ ul-d are inversely proportional within the specif ied ranges .
A further r~mhorl;-^nt of the present invention comprises a treatment of the dyed cotton fabrics at an alkaline pH
value. The treatment of the cotton fabrics in AlkAl inf~
conditions has the advantage that an attack by H+ ions, 218:L813 g which can lead to a reduction in the tear resistance, is completely excluded. In this P~hQ~ the treatment is preferably carried out in a pH range of 9 to 13, particularly preferably of 10 to 12. Since peroxo c, _ ac, e.g. persulfates, only have a moderate brightening effect under ~lk 1l ;nP conditions without added hypochlorite, a suitable activator for Alk 1l ine pH
values is preferably added. Examples of such activators are transition metal complex, '- soluble at ~lk~l ;n~ pH values, e.g. complex c ul-ds of iron, copper, cobalt or nickel, and particularly preferably complex ~ c of iron.
Specif ic examples of suitable activators are complexes of transition metals with ammonia, polyamines such as EDTA, thiocyanate, cyanide or fluoride. Cyano complexes are preferably used, particularly preferably hexacyanoferrate (II) or hexacyanoferrate (III) complexes. The hexacyanoferrate complexes are used for example in the form of alkali metal salts, e.g. sodium or potassium salts.
The complexes are preferably used in an amount of up to 25 mmol/l with respect to the metal, particularly preferably in an amount of 0.05 to 15 mmol/l, most preferably of 0. 05 to 8 mmol/l with respect to the metal .
nhen transition metal complex c _ ~- are used as an activator a surprisingly strong brightening is achieved even at very low concentrations of peroxo compounds.
Therefore when brightening under ~lkll;nP conditions, the concentration of the peroxo ~ u~lds is in the range of e.g. 5 mmol/l to 2.1 mol/l, preferably 5 mmol/l to 1 mol/l, particularly preferably 5 mmol/l to 250 mmol/l. The treatment temperature is preferably 70 to 90C, particularly preferably ca. 80C. The duration of treatment depends on the concentration of the peroxo and activator compounds used, however good results were achieved with a treatment period in the range of 10 min to 1 h, particularly of 10 min to 30 min.
The dyed cotton fabrics used in the process according to the invention are pref erably dyed with dyes selected from the group comprising vat, direct and sulphur dyes.
Typical vat dyes are indigo, thioindigo and other indigoid dyes, indanthrene and other anthraquinoid dyes, phthalocyanine and naphthalene dyes.
All substances that are used to dye cotton fabrics and in particular jeans fabrics, ~sp~ciAl ly indigo, thioindigo and other indigoid dyes as well as sulfur dyes are particular preferred as dyes.
After brightening the dyed cotton or jeans fabrics by the process according to the invention, these are cleaned in the usual way e.g. by rinsing or/and neutralizing the brightening bath. Since the process according to the invention does not cause any discolouration e . g . yellowing of the cotton f ibres, an aftertreatment is generally not ~c~sAry. A further advantage of the process according to the invention is that the degree of brightness i . e . the brightening of the total colour tone of the dyed cotton fabrics can be very easily controlled by selection of the duration of treatment and temperature, concentration of the peroxo c~ ~ and if desired the activator compound.

In particular when using sodium peroxodisulfate and if desired iron(III) sulfate as the activator, the waste water does not contain any !h~m;~ which pollute the environment since sodium peroxodisulfate decomposes to 60dium sulfate and oxygen and iron sulfate also poses no problem since this substance is for example added in sewage plants as a f locculant . When peroxo compounds, in particular persulfates, are used in the brightening bath, then additional oxygen is even introduced into the bath and thus the oxygen demand of the waste water is reduced. The use of halogenide ions, in particular of bromide ions, at the low concentrations that are ne, ecsAry to activate the peroxo ~-c _ _I.d also does not lead to signif icant environmental pollution .
In addition the present invention concerns the use of a bath which contains 21 mmol/l to 2.1 mol/l of a peroxo compound as the active ~ ~ A' t and which is free of added hypochlorites, permanganates and enzymes to brighten dyed cotton fabrics. The peroxo compound is preferably selected from the group comprising H202, perphosphates, perborates, percarbonates, persulfates, percarbamide, peracetic acid and mixtures thereof. A
peroxodisulfate is particularly preferred as the peroxo compound, most preferred are potassium, sodium or/and ,n i llm peroxodisulf ate .
In addition the bath can contain an activator selected from the transition metal compounds at a concentration of up to 25 mmol/l in relation to the metal or halogenide compounds at a concentration of up to 500 mmol/ l with respect to the halogenide ion .
The bath particularly preferably contains 21 mmol/l to - 12 - 21~1813 420 mmol/l of a persulfate as the active component and 0.5 to 200 mmol/l halogenide ions as the activator.
A further subject matter of the present invention is the use of a bath for brightening dyed cotton fabrics which has an ;llk;~l inP pH value and contains 5 mmol/l to 2.1 mol/l of a peroxo compound as the active component and a transition metal complex ~ , ~UII~ soluble at ;~lk~l inP pH values and which is free of added hypochlorites, pPrr~n~ In~tes and enzymes.
It is intended to elucidate the invention by the following example EXA~IPLE
A 4 x 4 cm sample of a blue jeans fabric dyed with indigo was placed in 100 ml of a brightening bath. Then the pH value was detPrm;nPd. It was stirred continuously during the brightening process in order to ensure good penetration of the fabric with the brightening solution.
Subsequently the piece of textile was rinsed with distilled water in order to stop the brightening process. The degree of brightness was judged in the dry state (dried by hot-air at 50C in a drying chamber) using a textile sample which had not been brightened as a reference. The degree of the brightening effect (decrease in the blue colouration down to white) was scored in 5 steps as follows:
0 no decrease in blue colouration slight brightening 2 good brightening (desired effect)

3 stronger brightening 21~1813

4 6trongly bleached

5 very 6trongly bleached The brightening effect of ammonium peroxodisulfate (APS), potassium peroxodisulfate (PPS) and sodium peroxodisulfate (SPS) was ~Y;~m; ned.
1) Pr~l im;n Iry experiments: comparison of various per6ulf ate6 No. Bleaching Conc. Addit. Conc. Temp. Time Score Fabric agent APS 100 g/l - - RT 20 h 1 ok 2 PPS 45 g/l - - RT 20 h 0 . 5 ok 3 SPS 100 g/l - -- RT 20 h 1 ok APS (experim. 1) and SPS (experim. 3) show above all a usable bleaching effect. PPS (experim. 2) has a lower solubility but is comparable to APS and PPS with regard to the bleaching effect.
2 ) Inf luence of the concentration No. Bleaching Conc. Addit. Conc. Temp. Time Score Fabric agent 4 SPS 100 g/l -- - RT 21 h 2 ok 5 SPS 200 g/l -- -- RT 21 h 4 ok

6 SPS 300 g/l -- -- RT 21 h 4.5 ok Experiments 4 to 6 show that the bleaching ef f ect can be regulated by means o~ the concentration.

2~1 813 3 ) Inf luence of temperature No. Bleaching Conc. Addit. Conc. Temp. Time Score Fabric agent

7 SPS 100 g/l - - 60C 40 min 1 ok

8 SPS 100 g/l - - 80C 20 min 2 ok Experiments 7 and 8 show that the desired degree of brightness can be adjusted within a 6ignificantly reduced duration of treatment by selecting a higher temperature .
4) Effect of PPS with various activators It turned out that transition metal ~ ullds above all iron(III) in the form of iron(III) sulfate, iron(II) sulfate or copper(II) sulfate enhance the attack of persulfate on the indigo dye. It is remarkable that no significant damage to the fabric takes place during this process .

~181~13 -~v V ~ ~
R XO XO n XO ~ .Y O O
S~
V ~ ~ ~ I ~ N
C~
~ ~ O O O O O
.

V O O O O o O O
~ O ~
In _ ~1 V .~
o ~ O N ~D 00 5~ 0 ) If) V .G N ~ '1 In ~ N ~r _, ~) _ ~ ~
N ~
r ~4 3 V Z X V O
V 1-l ~ ~ ~ u~ ~ ~
U -- N N N N N ~ ~1 _I
O O ~1 ~ ~ ~r 10 'D

- 16 _ 8 8 Experiments 9 to 16 show that various activators are suitable for this application. Iron(III) (experiment 9), chromium (experiment 14) and iron(II) (experiments 15 and 16) showed the strongest effect.
5) Comparison of various activator dosages No. Bl-~a~h;n~ Conc. Addit. Conc. Temp. Time Score Fabric agent 17 SPS 250 g/l Fe2(SO4)3 1.25 60C 20 min 2 ok 18 SPS 250 g/l Fe2(S04)3 2.5 60C 20 min 3 ok 19 SPS 300 g/l Fe2(S04)3 2.5 60C 20 min 3.5 ok Experiments 17 to 19 show that the concentration of the activator must not be adjusted too high since otherwise an effect would result which may be too strong. Noreover the persulfate concentration and iron activator concentration have to be matched to one another. In this case values of 200 - 300 g/l SPS and 0.75 - 1.8 mmol/l iron(III) sulfate prove to be optimal.

u s, X X ~ ~ X
Ul o ~I N
U
t` O ~D O
r~ o O
.~ O o o o O O
U ~
o O O O
r~
- U
C) O O
-. : r r ~, I I o r Z Z ~ ~
O O O O
~ t~ N ~q ~ t`l q~
O
a O o .
~D Z N ~ ~I N

21gl81~

In contrast to experiment 20 the pH value was adjusted to 7 ln experiment 21 (preferred pH range 5 - 12) which shows that the desired degree of brightness of 2 is achieved even at this pH value. Experiments 22 and 23 show that the bleaching time could be reduced to 20 min using 300 g/l SPS even at pH 7. Experiment 25 shows that a degree of brightness of 2 can be achieved at a pH
value of 5 which was adjusted with a buffer mixture while adhering to the standard conditions.

8~
s ~Q
3 xo xo x xo xo -s~ ~ U7 O
., o R -1 0 o o o o - t E~ -- r p~ .C
O o O O O ~ --E~ ~ D 3 S
r1 U r1 ~ O - J
" ~0 ~ I ~ O ~D
S~
.~ ~ O
n ~Q N U~ U~ S-) C) 3 ~a ' S S
r1 0 0 0 0 0 0 O ~ O O O O O ~\ O
O
O
a .
o ~ ~ a o 21~1813 u , e ~o U
~1 h ,~ ,y, _ ~ t~
h U ~ ~q ~ ., u~
Q) ~ 3 ON ON N 1~ 'I
E-~ o e ~ U O O o o ~ .
O ~ ~ ~ o _I , ~ _I
~q C _I
~ o o~
- o e 1 1 ~ c u _ a) o ~ I
a) a r CP
.~ ; h U a) r ~ Z; Z (d tl t"
r' U ~ ~
.C O ~ N N N Ul ~ ~ h) 2 h r ~1 0 . H
tn tn ~
-- O ~I t`; I
CO Z ~ ~ ~

21~18~3 ,, X ~ X X Y X X X X
,~ o o o o o o o o o O ~ ~ ~ N N ~i ~ ~ 1 o -~1 0 0 0 0 0 0 0 0 0 N N N N N N N N N
~^
~; U O O O O O O O O O
~Q ~
U~ O ~ O N

h h x ~a ~ ~ ~ m ~ m ~2 r Z Z Z Z Z Z Z Z Z
o ~ . . . O O
O N N ~ O U~
~I
. . 1 a ~
o ~ 1~ ~ O ~ N

21 ~813 Experiments 34 and 35 show that the optimal effect can already be achieved using small amounts of bromide and a further increase of the bromide concentration does not result in further i~ V~ t. Experiments 36 to 39 show that in this range the degree of bleaching can also be regulated by reducing the bromide concentration.
Experiments 4 0 to 4 2 show that the ef f ect can be increased up to complete decolouration by increasing the SPS content in the bleaching vat.
In experiments 35, 40 and 41 the mole ratio was adjusted to 1: 50 (NaBr: SPS) .

`~ 2~81813 o ~, u~ In In U~ In ~, ~J
Hl .-1 .-1 ~ r-l r-l r1 .-1 rl N H
O O O O O O O O O O O O
O O O O O O O O O O O O
O
o F~ N '-i ~ ~ t`l ~ ~ .-i ~i 0 H H H H H H H H H H
H H H H H H H H H H
~ O ~ ~ N ~ N N
,0~ z ~r ~ ~r ~r ~ ~ ~ ~ ~ ~ I

21~813 Experiments 43 and 44 show that when an activator is not added there is only a slight ble~hin~ in the RlkRl ;nP
range. Experiments 45 and 46 show that the bleaching action of persulfate in the RlkRl ;nP range is c~nqidPrably amplif ied by the addition of an activator .
Experiment 47 shows that there is no detectable effect of the activator in the neutral range (pH 7.5).
Experiments 48 and 49 show that the ble~rh;nq action is considerably higher at a bath temperature of 80C than at a bath t, a~UL~ of 60C.
Experiments 50 and 51 show that the activation with potassium hexacyanoferrate (III) functions equally as well as activation with potassium hexacyanoferrate (II).
Experiment 52 shows that the activator concentration can be reduced further to ca. l mmol/l without wP~kPn; ng the brightening effect. Experiment 53 shows that a strong blPR~-h;n~ action is achieved even at a pH value of 12.
Experiment 55 shows that the brightening action can be regulated by a further reduction of the activator in the blP~- h; n~ solution .

Claims

- 25 -

1. One-step process for brightening dyed cotton fabrics, wherein dyed cotton fabrics are treated, under conditions which are suitable for brightening the colours of cotton fabrics, with an aqueous bath that contains a peroxodisulfate as the active component but no added hypochlorites, permanganates and enzymes.

2. Process as claimed in claim 1, wherein the peroxo compound is selected from alkaline-earth, alkali or ammonium peroxodisulfate or mixtures thereof.

3. Process as claimed in claim 2, wherein potassium peroxodisulfate, sodium peroxodisulfate or/and ammonium peroxodisulfate are used.

4. Process as claimed in one of the claims 1 to 3, wherein the peroxo compound is used at a concentration of 21 mmol/l to 2.1 mol/l, preferably of 42 mmol/l to 1.7 mol/l.

5. Process as claimed in one of the claims 1 to 4, wherein the cotton fabrics are treated at a temperature of 15 to 90°C, preferably of 40 to 80°C.

6. Process as claimed in one of the claims 1 to 5, wherein the brightening is carried out for a time period of 2 minutes to 20 hours, preferably of 5 minutes to 5 hours.

7. Process as claimed in one of the claims 1 to 6, wherein the brightening is carried out at a pH value of the bath which is from 1 to 10, preferably from 4 to 7.

8. Process as claimed in one of the claims 1 to 7, wherein a bath is used which additionally contains an activator selected from transition metal compounds or/and halogenide compounds.

9. Process as claimed in claim 8, wherein compounds of iron, copper, cobalt, zinc, silver, manganese, chromium, nickel or mixtures thereof are used as the activator.

10. Process as claimed in claim 9, wherein Fe(III) salts, Fe(II) salts or Cu(II) salts are used as the activator.

11. Process as claimed in claim 10, wherein Fe(III) sulfate, Fe(II) sulfate or Cu(II) sulfate is used as the activator.

12. Process as claimed in one of the claims 8 to 11, wherein the activator is used in an amount of up to 25 mmol/l with respect to the metal.

13. Process as claimed in claim 12, wherein the activator is used in an amount of 0.05 to 15 mmol/l, preferably of 0.05 to 8 mmol/l with respect to the metal.

14. Process as claimed in one of the claims 8 to 13, wherein ammonium peroxodisulfate is used as the peroxo compound and Cu(II) sulfate, Fe(II) sulfate or Fe(III) sulfate is used as the activator compound.

15. Process as claimed in one of the claims 8 to 13, wherein sodium peroxodisulfate is used as the peroxo compound and Fe(III) sulfate or Fe(II) sulfate is used as the activator.

16. Process as claimed in claim 8, wherein halogenide compounds are used as the activator.

17. Process as claimed in claim 16, wherein bromide compounds are used as the activator.

18. Process as claimed in claim 17, wherein sodium or/and potassium bromide is used as the activator.

19. Process as claimed in one of the claims 16 to 18, wherein the activator is used in an amount of up to 500 mmol/l with respect to the halogenide ion.

20. Process as claimed in claim 19, wherein the activator is used in an amount of 0.5 mmol/l to 200 mmol/l, preferably of 1 to 20 mmol/l with respect to the halogenide ion.

21. Process as claimed in one of the claims 16 to 20, wherein sodium peroxodisulfate is used and an alkali metal or alkaline-earth metal bromide, preferably sodium bromide or/and potassium bromide is used as the activator compound.

22. Process as claimed in one of the claims 1 to 6 or 8 to 9, wherein the treatment is carried out at an a alkaline pH
value and a transition metal complex compound soluble at alkaline pH values is used as the activator.

23. Process as claimed in claim 22, wherein the treatment is carried out at a pH value of 10 -12.

24. Process as claimed in claim 22 or 23, wherein complex compounds of iron are used.

25. Process as claimed in one of the claims 22 to 24, wherein cyano complexes are used.

26. Process as claimed in one of the claims 22 to 25, wherein hexacyanoferrate (II) or hexacyanoferrate (III) complexes are used.

27. Process as claimed in one of the claims 22 to 26, wherein the activator is used in an amount of up to 25 mmol/l with respect to the metal.

28. Process as claimed in claim 27, wherein the activator is used in an amount of 0.05 to 15 mmol/l, preferably of 0.05 to 8 mmol/l with respect to the metal.

29. Process as claimed in one of the claims 1 to 28, wherein cotton fabrics are used which have been dyed with dyes selected from the group comprising vat dyes, direct dyes and sulphur dyes.

30. Process as claimed in claim 29, wherein cotton fabrics are used which have been dyed with indigo, thioindigo or indigoid dyes.

31. Use of a bath which contains 21 mmol/l to 2.1 mol/l of a peroxo compound as the active component and which is free of added hypochlorites, permanganates and enzymes for brightening dyed cotton fabrics.

32. Use as claimed in claim 31, wherein the bath in addition contains an activator selected from transition metal compounds at a concentration of up to 25 mmol/l with respect to the metal.

33. Use as claimed in claim 31, wherein the bath in addition contains an activator selected from halogenide compounds at a concentration of up to 500 mmol/l with respect to the halogenide ion.

34. Use as claimed in claim 33, wherein the bath contains 21 mmol/l to 420 mmol/l of a persulfate as the active component and 0.5 to 200 mmol/l halogenide ions as the activator.

35. Use of a bath for brightening dyed cotton fabrics which has an alkaline pH value, contains 5 mmol/l to 2.1 mol/l of a peroxo compound as the active component and a transition metal complex compound soluble at alkaline pH values and which is free of added hypochlorites, permanganates and enzymes.