Indian Journal of Fibre & Textile Research Vol . 25, June 2000, pp. 1 30- 1 37

Enhancement in indigo uptake on cotton through metal salt pretreatment

R B Chavan & J N Chakraborty

Department of Textile Technology, Indian Institute of Technology, New Delhi 1 1 0 0 1 6, India

Received 6 July 1 999; accepted 13 August 1 999

A simple metal salt pretreatment of cotton has been suggested before multiple dip and nip dyeing of cotton with indigo. Among the various metal salt pretreatments investigated, the pretreatment with FeS04 has been found to be most promisive. A tentative mechanism for improvement in colour yield due to metal salt pretreatment has also been suggested.

Keywords: Cotton, Denim, Ferric hydroxide, Ferrous sulphate, Indigo

1 Introduction Indigo, which belongs to the vat dye class is

exclusively used for the dyeing of cotton in the form of warp sheet or rope. These dyed yams are subsequently used for manufacture of universally popular denim fabric. The method of dyeing essentially consists of multiple dips and nips (6-dip 6-nip generally) with intermediate air oxidation. This special technique is essential because of the very low affinity of indigo towards cotton.

It has been reported in the literature that the affinity of indigo can be increased to some extent by adjusting the pH or by cationization of cotton. It has been found that the maximum colour yield of indigo is obtained when dyeing is carried out in the pH range of I O.S- 1 1 .S . This has been attributed to the presence of indigo in the monophenolate form in this pH range. This form of indigo has higher affinity for cotton compared to its biphenolate form, which exists at pH above I I .S . A mixture of Na2C03 and NaOH (SO:SO) has been found to be suitable to maintain the pH at optimum level and to increase colour yield as much as SO% compared to when NaOH alone is used as alkali . However, on commercial scale, the use of this alkali mixture was discontinued due to the formation of sludge' . The accurate control over dyebath pH is not possible by adjusting only NaOH concentration and this has led to develop alternative technique to control dyebath pH. One such development is the buffered alkali system of undisclosed composition recommended by Virkler Company, USA2. Chavan and Jahan3 have recommended the use of organic bases as a substitute for NaOH to maintain dyebath pH in optimum range where they have achieved an

increase in colour yield up to an extent of 2S-30% over when NaOH was used as an alkal i . Another novel attempt was made to improve indigo uptake by means of treating cotton fabric with a cationizing agent HERCOSETT S7 (Hercules) which is a water soluble polyamide-epichlorohydrin resin, an azaeti­dinium compound. This technique improves colour yield remarkably. It has also been reported that the sensitivity on colour yield is lost during dyeing with cationized cotton4. In this work, the principle of cationization of cotton to improve colour yield was exploited in an unique and simplified way. Cotton fabric was pretreated with different water soluble metal salts followed by padding with indigo. Colour yield for each metal salt treated fabric was measured in terms of K/S values.

2 Materials and Methods

Commercially scoured, bleached but unfinished plain woven cotton fabric was used.

BASF Indigo Pure (Microgranules) and c.1. Vat Blue 1 (ref. 5) were used. All metal salts and other chemicals applied were of laboratory grade reagent, except Na2S204 and laundry soap, which were of commercial grade.

2.1 Preparation of Padding Liquor

For padding, a dye solution of required concentration was prepared using a stock vat and dilution liquor. To prepare the stock vat, the required amount of NaOH and dye were added to 100 ml water, stirred well, heated up at SO°C followed by addition of Na2S204 and kept for I S-20 min to complete dye reduction. Dilution liquor was prepared

CHA V AN & CHAKRABORTY: ENHANCEMENT IN INDIGO UPTAKE ON COTTON 1 3 1

by dissolving the required amount of NaOH and Na2S204 in 1 litre water at room temperature and stirring well to get a clear solution. Dilution liquor (567 ml) was added to 1 00 ml reduced stock vat to get 667 ml final padding l iquor. Concentration of dye, NaOH and Na2S204 in stock vat and dilution liquor were as under (as per BASF guidelines)6:

For a padding liquor with dye concentration of 3 gllitre

Stock vat Dilution liquor Padding liquor Dye : 20 gil i tre

NaOH : 20 gliitre

Na2S204 : 20 g/litre

NaOH : 1 .35 gliitre Dye : 3 gliitre

Na2S204 : 2.0 gliitre Na OH : 4.4 1 6 gllitre

Na2S204 : 4.698 g/litre

2.2 Treatment of Cotton with Various Metal Salts followed

by Dyeing

Initial experiments consisted of padding cotton fabric with various metal salt� followed by drying and repadding with vatted indigo solution(6-dip 6-nip)with intermediate air oxidation after each nip and air oxidation for 3 min after 6th nip followed by 8- 1 0 rinses with hot and cold wash.

2.2.1 Treatment of Cotton with FeS04

Indigo dyeing of cotton pretreated with FeS04 was carried out using the following sequences:

FeS04 pad-7 Dry-7 Indigo pad

FeS04 pad-7 DrY-7 NaOH pad -7Wash-7 Indigo pad

FeS04 pad-7 Indigo pad

FeS04 pad-7NaOH pad-7 Indigo pad

FeS04 pad-7NaOH pad-7 Wash-7 Indigo pad

2.2.2 Indigo Padding of Samples

Indigo padding was done through a 6-dip 6-nip padding operation. Dipping of the fabric in dye liquor for 30 s followed by padding (80% expression) and airing for I min completes I -pip I -nip cycle. Samples for 6-dip 6 -nip padding were passed through 6 such consecutive cycles and a final airing for 3 min converted all the reduced dye on fabric to its oxidized state.

2.3 Evaluation of Dyed Samples

2.3.1 Measurement of KlS

The K/S values of dyed samples were measured with Computerized Colour Match System (Datacolor) at 620 nm Amax.

2.3.2 Determination of Fastness Properties7

Fastness to washing,light and rubbing was assessed in accordance with IS 764 : 1 979, IS 2454 : 1 985 and IS 766 : 1 988 respectively.

2.3.3 Estimation of Iron on Dyed Samplesx

Iron salts (Fe3+) when react with potassium thiocyanate in acidic medium develop blood red coloured complex. The intensity of red colour is in proportion to the iron concentration. This reaction was used for the estimation of iron in fabric samples.

3 Results and Discussion

Cotton fabric was pretreated by padding with various metal salt solutions ( l Ogilitre) followed by drying and 6-dip 6-nip indigo padding. Subsequently FeS04 was selected for further study and applied through different dry and wet methods at optimized concentrations. Colour yield and change in pH after each dip & nip as well as the fastness properties and iron content (before and after dyeing) of samples were compared.

3.1 Effect of Metal Salt Treatment on Colour yield

Cotton fabric was padded with I I different metal salts ( 1 0 gllitre), dried at 1 05°C and dyed with indigo by 6 -dip 6-nip padding technique. Colour yield was measured in terms of K/S values at 620 nm and is shown in Table I . The behaviour of these metal salts in terms of their effect on colour yield can be explained on the basis of the properties of metal hydroxides formed when the metal salt pretreated fabric comes in contact with alkali in dyebath during

Table I -Effect of metal salt pretreatment on indigo uptake

Metal salt K/S % yield

Untreated control 22.4 1 00.0 Ferrous sulphate (FeS04.7H20) 2S. 1 1 25.5 Cobaltous sulphate (CoS04. 7H2O) 2S.0 1 25.0 Nickel sulphate (NiS04.6H2O) 22.5 1 00.4 Copper sulphate (CuS04.5H20) 23.9 1 06.6 Aluminium sulphate [AI2(S04h I SH2O] 24.4 I OS.9 Magnesium sulphate (MgS04.7H2O) 22.4 1 00.0 Manganous chloride (MnCi2.4H2O) 26.6 I I S.7 Zinc sulphate (ZnS04.7H20) 22.9 \ 02.5 Chromium sulphate [Cr2(S04h6H2O] 24. 1 \ 07.7 Stannous chloride (SnCi2.2H2O) 23.4 1 04.2 Lead acetate [(CH)COOhPb] 24.7 1 1 0.2

1 32 INDIAN 1. FIBRE TEXT. RES. , JUNE 2000

indigo padding operation. From this, the above metal salts can be classified into following two groups:

Metal salts producing alkali insoluble hydroxides, e.g. FeS04, CoS04, NiS04, CUS04, MnCh and MgS04 Metal salts producing alkali soluble hydroxides, e.g. Ah(S04h ZnS04, Cr2(S04h SnCh and (CH)COO)2 Pb

From the effect of different metal salts on colour yield, it appears that only those metal salts whose metal hydroxides are insoluble in water and get converted to their higher oxidation state [from +2 (-ous) to +3 (-ic)] are effective in giving increased colour yield.

Based on the above assumption, iron, cobalt, manganese and copper hydroxides should have shown similar effects on colour yield. However, the order of increase in colour yield was as follows:

Fe(OHh :: Co(OHh > Mn(OH)) > CU(OH)2

This can be attributed to the force of attraction (+ ve charge) extended by these metals to the anionic dye molecules, which, in tum, varies from metal to metal .

Though the hydroxides of nickel and magnesium are insoluble in alkali, no increase in colour yield was observed. This may be due to the fact that the force of attraction rendered by these metal hydroxides was too feeble to attract dye molecules . In case of metal salts which get converted from their hydroxides to sodium salts, it may be possible that such a conversion process takes place slowly a�d while the metal salt is present in its hydroxide form before conversion to sodium salt form, attracted dye molecules, showing marginal increase in colour yi�ld, e.g. AI2(S04h, Cr2(S04h, SnCh �nd (CH)COO)2 P1:>:. Out FeS04 and CoS04, which showed highest colour yield, FeS04 was selected for further study on the basis of its easy availability at lower cost and eco-friendliness.

3.2 Methods of Application of FeS04

Five methods of application of FeS04 were used prior to indigo padding for studying the effect of FeS04 on colour yield. These methods were named as dry and wet methods. In dry method, the cotton sample was dried after FeS04 padding while in wet method, the intermediate drying was omitted. The sequence of operations were as under:

Dry Methods Method 1 : FeS04 pad-t Dry-t Indigo pad Method 2: FeS04 pad-t Dry-t NaOH pad -tWash-t

Indigo pad Wet Methods Method 3 : FeS04 pad-t Indigo pad Method 4 : FeS04 pad-tNaOH pad-t Indigo pad Method 5 : FeS04 pad-tNaOH pad-t Wash-t Indigo pad

3.3 Optimization of FeS04 Concentration in Dry and Wet Methods

3.3.1 Dry Method

Cotton fabric was padded with FeS04 solutions at different concentrations and the padded samples were dried at 1 05°C and rep added with 2g1litre indigo (6-dip 6-nip). The KlS values of dyed samples were measured and are shown in and Fig. I . It is observed that with the increase in FeS04 concentration, the KlS values of dyed samples increase steadily up to 5 g/litre and then become almost constant. However, for further studies, FeS04 concentration of 10 gllitre was selected.

3.3.2 Wet Method

Fabric samples were padded with FeS04 ( 1 0, 20, 30, 40 and 50 gllitre) followed by repadding with 2 gllitre indigo through 6 -dip 6-nip padding technique. The KlS values of dyed samples are shown in Fig. 2. It is clear from Fig. 2 that the KlS values increase very slightly in spite of considerable increase in FeS04 concentration. However, FeS04 concentration of 50 gllitre was selected for further studies.

15 �------��----�------'-----� o 5 10 15

Ferrous sulphate <X>nC.( gil ) 20.

Fig. I-FeS04 concentration vs colour yield in dry method

25 r-----------------------------� 20

ri � 15

10 +-____ .-____ -. ____ �------�--� 10 20 30 Ferrous sulphate <X>nC.(gJ1) 40 50

Fig. 2-FeS04 concentration vs colour yield in wet method

CHAVAN & CHAKRABORTY: ENHANCEMENT IN INDIGO UPTAKE ON COTION 1 33

3.4 Comparison of Colour Yield in Different Methods of Pretreatment

Cotton samples were pretreated with FeS04 in five different ways, dyed with reduced indigo dyebath by 6-dip 6-nip method and the colour yield after each dip/ nip was estimated in terms of KlS. The results are shown in Fig. 3 . It is found that the KlS values of FeS04 pretreated and indigo padded samples after 6-dip 6-nip padding are much higher in all the methods except method 3, where the final KlS value is little higher than that in conventional padding. Though the KlS after 1 st dip is higher in methods 1 , 2, 4 and 5, a lower KlS is found in method 3 . A sudden jump in KlS values on all FeS04 pretreated samples is observed during 2nd dip. Also in method 2, 3/4th of final colour yield is obtained during I st dip itself.

3.5 Fastness Properties of Dyed Samples

The fastness properties of FeS04 pretreated and dyed samples are shown in Table 2. The fastness results indicate that there is no effect of FeS04 treatment on wash and light fastness of dyed samples when compared to those obtained in the conventional method of dyeing whereas the rubbing fastness, particularly the wet staining, is severely affected by FeS04 pretreatment methods. This indicates that the additional colour yield compared to that in conventional indigo padding is mainly due to the surface deposition of dye on FeS04 pretreated samples. However, this surface deposition should be considered as desirable from the point of view of ease in getting stone-wash effects .

3.6 Change in pH during Various Dips

It has been well documented in the l iterature that

the pH has considerable effect on colour yield. The maximum colour yield is being obtained in the pH range of 1 0.5- 1 1 .5 . To understand whether the FeS04 pretreatment has any effect on pH of dyebath, which, in turn, is responsible for the increase in colour yield, the change in pH after each dip in al l methods of application was measured and the results are shown in Table 3. It may be seen from this table that there is a marginal decrease in pH. However, the pH value after 6-dip 6-nip remained much above th� optimum value ( 1 0.5- 1 1 .5).This indicates that increase in colour yield on FeS04 pretreated samples is not due to pH effect.

3.7 Mechanism of Dyeing

The contribution of FeS04 towards increase in colour yield is different in all the five methods of pretratment of cotton with FeS04. This may be

30 �--------------__ � __________ �

25

20

1/1 15 ...... ::.:: 10

5

__ Con_nol padding ..... F ....... IUIpI1'" pod · Dry • Indigo pod -.-Fenout .u�t. pad · Dry. NIOH pad- W .. MM'oo pld . -W-F ...... I� pod • Indigo pod -lIf-F ....... IUIpilate pod . NoaH pod . Indigo pod __ Forrouo .uIpI1l1. pod • NoaH pod • W.", . IndVo pod

o �---.----.---�--__ � __ � ____ � o 2 3 4 D i p l N i p

5

Fig. 3-Colour yield for different methods of ferrous sulphate pretreatment

Table 2-Fastness properties of FeS04 pretreated and dyed samples

Fastness property

Wash fastness (ISO-3)

Change in colour Staining

Light fastness Rub fastness

Change in colour Dry Wet

Staining Dry Wet

Conventional padding

5 4

7-8

4-5 4

4 3

Method I

5 4

7-8

4 .3

3 1 -2

Fastness rating Method 2 Method 3

5 5 4 4

7-8 7-8

4 4 3 2-3

3 3 1 -2

Method 4

5 4

7-8

4 2 3

3

Method 5

5 4

7-8

4

3 1 -2

l 34 INDIAN 1. FIBRE TEXT. RES. , JUNE 2000

Table 3-Change in pH during different dips

Dip/nip Conventional padding Method I

Nil 1 2.34 1 2.37 I 1 2.33 1 2.30 2 1 2.30 1 2.26 3 1 2.27 1 2.26 4 1 2.24 1 2.23 5 1 2.24 1 2.23 6 1 2.20 1 2.20

attributed to the basic chemistry of FeS04 and its subsequent conversIOn to the two states of hydroxides, i .e. Fe(OH)2 and Fe(OHh ; the contribution towards colour yield essentially depends on the state of the iron hydroxide. The difference between Fe(OH)2 and Fe(OHh is that Fe(OH)2 is unstable and releases electron, gets converted to Fe(OH)3 (in open air/ oxygen) making it electrically positive in nature. It is this electrically positive Fe(OH)3 which attracts anionic indigo molecule from the bath. Keeping this basic theory in view, the mechanism of colour yield in different dyeing methods can be explained as follows :

3.7.1 FeS04 pad� Dry� Indigo pad

During FeS04 padding and drying, a part of FeS04 gets converted into Fe2(S04)3 due to air oxidation. A part may also hydrolyse to Fe(OHh which immediately gets converted to Fe(OHh This means before dipping of the pretreated sample in dyebath, there were three different species, viz. FeS04, Fe2(S04)3 and Fe(OHh present on fabric. When this pretreated sample was dipped in indigo bath, FeS04 which was in major proportion comes in contact with NaOH in dyebath, got converted to Fe(OHh, whereas Fe2(S04h got converted to Fe(OHh. Fe(OH)3 produced earlier during FeS04 padding and drying remained unchanged. The presence of Fe(OHh which was in small proportion but being +ve in character could be responsible for increase in colour yield during the I SI dip. After I sl dip when air oxidation was carried out, the part of Fe(OH)2 also got converted to Fe(OHh This means that during the 2nd dip onwards only Fe(OH)3 was present on fabric, which being +ve in character attracted a substantial proportion of dye from dyebath. This mechanism is supported by the fact that during the 1 sl dip, there was a slight increase in colour yield, which was due to presence of small amount of Fe(OH)3 and large

pH Method 2 Method 3 Method 4 Method 5

1 2.35 1 2.35 1 2.36 1 2.35 1 2.27 1 2.22 1 2.26 1 2.24 1 2.24 1 2.20 1 2.23 1 2. 1 5 1 2.23 1 2.20 1 2.23 1 2. 1 1 1 2.23 1 2.20 1 2. 1 4 1 2. 1 0 1 2. 1 8 1 2.20 1 2. 1 6 1 2.02 1 2. 16 1 2.20 1 2. 1 0 1 2.00

proportion of Fe(OH)2 on fabric. After the 2nd dip onwards, the colour yield steadily increased, because of the fact that a constant amount of Fe(OH)3 was available on the fabric during each dip. This was due to the fact that during oxidation, anionic dye molecules got converted to non-ionic oxidized form and thus making free the positively charged Fe(OHh for further attraction of dye molecules in bath during each dip.

3.7.2 FeS04 pad� Dry� NaOH pad �Wash� Indigo pad

In this method, during FeS04 padding and drying, a part of FeS04 got converted to Fe2(S04)3 and Fe(OHh Treatment of fabric retaining these three components (FeS04 being the major part) in NaOH converted FeS04 to Fe(OHh, with its immediate oxidation to Fe(OHh Fe2(S04h in contact with NaOH got converted to Fe(OH)3 and the Fe(OH)3 formed earlier during padding and drying remained unchanged. So after padding with NaOH, all FeS04 molecules got converted to Fe(OHh Washing of this sample in water removed excess NaOH and superficial Fe(OHh Also, if any Fe(OH)2 molecule formed earlier had not come in contact with air, this washing supplied O2 to it to convert it to Fe(OHh Hence, before padding in indigo bath, the fabric retained only Fe(OH)3 molecules. It has already been stated that Fe(OH)3 is electrically positive. So, it may be expected that during I sl dip, the maximum colour yield should occur. When the indigo dye is in reduced form, it is water soluble and possesses anionic charge. However, when the indigo dye padded fabric is exposed to air, the dye gets oxidized and losses its anionic charge. Thus, a force of attraction exists between positively charged Fe(OH)3 and negatively charged dye molecule while it is in reduced soluble form. This force of attraction is lost when the dye is oxidized and loses its charge, thus making free positively charged Fe(OHh which further attracts

CHAVAN & CHAKRABORTY: ENHANCEMENT IN INDIGO UPTAKE ON COTTON 1 35

negatively charged reduced dye molecules during subsequent dip applications. This process is repeated after each dip and nip application, thus increasing the colour yield as compared to that in conventional method.

3.7.3 FeS04 pad � Indigo pad

The fabric was padded with FeS04 (50 gliitre) and without drying dipped into 'reduced indigo bath. In this process, since the wet fabric came directly in contact with dyebath, a considerable amount of FeS04 got stripped out into dyebath and got converted into Fe(OH)2 in presence of NaOH. This means that a part of FeS04 gets converted onto fabric as Fe(OHh and a substantial part in dyebath. The results (Fig. 3) indicate that in 1 st dip, indigo uptake onto fabric was less than that obtained in conventional method. This may be attributed to the fact that Fe(OHh stripped out into dyebath possessed feeble cationic charge, interacted with dye in the dyebath and reduced its transfer onto fabric, resulting in low colour yield. However, during subsequent dips/ nips, there was a steady rise in colour yield with a greater jump during 2nd dip. The reason of this is the same as explained earlier, i .e. Fe(OH)2 present in fabric got converted to Fe(OH)3 during air oxidation of indigo at the end of 1 st dip. The Fe(OH)3 being more cationic was responsible for attracting considerable amount of dye during 2nd dip, whereas this process of dye attraction in 2nd dip onwards remained constant. S ince the initial colour yield as compared to that in the conventional and other methods was low, the final colour yield was also low in this method of application. However, in spite of the low initial colour yield, the final colour yield after 6-dip 6-nip was slightly higher than that obtained in the conventional method. Among the five methods of pretrearment, this method has given least improvement in colour yield. This may be due to the opposing effect of Fe(OH)2 in dyebath and Fe(OH)3 on fabric.

3.7.4 Other Wet Methods

The results of colour yield obtained in other methods, viz. FeS04 pad�NaOH pad� Indigo pad and FeS04 pad�NaOH pad� Wash� Indigo pad, can also be explained on the basis of the above generalized mechanism and it may be concluded that the presence of Fe(OHh which is cationic in nature, is responsible for the increase in colour yield.

3.8 Influence of Iron Content on Colour Yield

To understand the effect of iron content In the fabric, estimation of iron was carried out before and after dyeing and the same was correlated with the final amount of colour yield after 6-dip 6-nip. The results are shown in Table 4. It appears from this table that a considerable amount of iron was lost into bath during dipping operation. The extent of loss of iron from the fabric into bath depends on the method used for pretreatment. In the first two methods, the initial iron content was low as compared to that in the subsequent methods 3, 4 and 5 . This is obviously because of the low initial concentration of FeS04 ( 10 gllitre) applied in methods 1 and 2 compared to 50 gllitre in methods 3,4 and 5 .

When the colour yield values obtained in different methods of pretreatment were compared against iron content, it was found that methods 1 ,2,4 and 5 showed more or less similar colour yield in spite of variation in iron content in fabric after dyeing. This indicates that there was not much effect of iron content on colour yield. This may be attributed to the fact that there is certain optimum l imit of iron content, beyond which no increase in dye uptake occurs and probably the level of iron content present in all fabric was above this l imit (Table 5) .

Table 4-Iron content of FeS04 pretreated samples before and after indigo padding

Method

Conventional padding Method I Method 2 Method 3 Method 4 Method 5

mg Ironl 1 00 g cotton Before indigo After indigo

padding padding

-0 -0 283.7 244.4 255.2 1 80.5 1 400.0 268.3 1 083.0 49 1 .4 653.8 467.0

Table 5-'-Concentration of FeSOi iron content vs colour yield [Indigo cone., 2 g/litre]

FeS04 (gllitre) KlS mg Ironll OO g cotton

Untreated control 1 6.5 -0 I 20.4 59.0 2 22.6 64.0 3 23.7 72.5 4 24.0 90.0 5 24.4 1 06.6 1 0 24.7 1 68 . 1 20 24.6 289.8

1 36 INDIAN J. FIBRE TEXT. RES., JUNE 2000

Dip/nip

Table 6-Indigo uptake on FeS04 pretreated samples without intermediate air oxidation

KlS value Untreated cotton FeS04 pretreated cotton

Padding with Padding without Padding with Padding without intermediate airing intermediate airing intermediate airing intermediate airing

I 1 0.5 9.2 2 1 2.6 1 3 .0 3 1 6.4 1 6.6 4 I S . I I S .2 5 20.7 1 9.9 6 22.4 2 1 .5

3.9 Stability of Fe(OHh

It is likely that the Fe(OH)3 which was formed onto the fabric during airing of dyed samples might get converted into Fe(OHh during subsequent dips due to the presence of Na2S204 in dyebath. Out of the two forms of iron hydroxides, i .e. Fe(OH)2 and Fe(OHh, Fe(OHh is mainly responsible for attraction of dye due to its electropositive nature compared to Fe(OHh. To confirm whether Fe(OH)3 gets converted to Fe(OHh during the dipping stage, the following experiment was conducted. Deposition of Fe(OH)3 on cotton is i ndicated by brown colour and that of . Fe(OHh by greenish blue. When the brown coloured ' . cotton fabric containing Fe(OHh was dipped in a blank bath containing NaOH and Na2S204 for 30 s, no change in colour of cotton fabric was observed. Had the Fe(OHh converted to Fe(OHh, a change in colour from brown to greenish blue was expected. However, the absence of change in colour indicates that the conversion of Fe(OH)3 to Fe(OH)2 in presence of Na2S204 had not occurred during dipping period of 30 s. Thus, electropositive Fe(OH)3 was available on the fabric during the multiple dip/nip operations of indigo padding and the presence of this electropositive Fe(OH)3 was responsible for increase in colour yield when the fabric was pretreatetl with FeS04.

3.10 Influence of Fe(OHh on Colour Yield

The interpretation of our observations on increased colour yield on FeS04 pretreated samples is based on the assumption that among the two hydroxides of iron,i .e. Fe(OH)3 and Fe(OHh. Fe(OHh is more electropositive as compared to Fe(OHh and it is mainly responsible for increase in colour yield. The presence of Fe(OH)2 has almost negligible contribution for increase in colour yield. The fol lowing experiment supplements the above statement.

1 1 .0 9.6 20.0 1 4.4 23. 1 1 7.6 26.6 20.4 26.7 22.0 2S.S 22.S

Cotton fabric was padded with FeS04 (50 gllitre) and treated with NaOH to have deposition of Fe(OH)2 on fabric and then dyed with indigo using 6-dip 6-nip padding technique without intermediate air oxidation . The conventional indigo dyeing was carried out in the same way but without intermediate air oxidation. The results of colour yield after each dip and nip are shown in Table 6, which indicates that there was a slight increase in colour yield compared to conventional dyeing. However, the colour yield was much less, particularly in case of FeS04-treated samples, without intermediate air oxidation. These observations clearly indicate that Fe(OHh is much

.less electropositive in nature compared to Fe(OHh and therefore

'the presence of Fe(OHh was mainly

responsible for increase in colour yield.

4 Conclusions

4.1 Selective metal salts, if applied on cotton prior to indigo padding, can increase colour yield considerably.

4.2 Amongst the various metal salts, FeS04 and CoS04 show higher increase in colour yield.

4.3 Amongst the five methods of application of . FeS04 on cotton prior to dyeing, all methods except the method 3 give substantial increase in colour yield.

4.4 There is an optimum l imit of iron concentration on fabric up to which increase in colour yield takes place. Beyond this optimum limit, there is no further increase in colour yield.

4.5 Dry methods are effective at lower concen­tration of FeS04 whereas wet methods are effective at higher concentration of FeS04,

4.6 Wash and light fastness properties of FeS04 pretreated and indigo padded cotton are equivalent to those obtained in conventional method, but rubbing

CHAVAN & CHAKRABORTY: ENHANCEMENT IN INDIGO UPTAKE ON COTION 1 37

fastness, especially wet staining, is severely affected for FeS04 pretreated and indigo padded samples. However, from the point of. view of ease of getting stone wash effect, this may not be a disadvantage.

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Technology option for Textile Industry ( lIT, New Delhi), 1 998, 63.

4 Chong C L, Poon L W & Poon W C, Text Asia, January ( 1 995) 55.

5 Colour Index, Vol. 4, 3,d edn (The Society of Dyers & Colourists), 1 97 1 , 4593.

6 Technical Information (BASF), September 1 995, 1 3 .

7 Handbook of Textile Testing, Part 4--ldentification and Testing of Dyestuffs and their Colour Fastness on Textile Materials (Bureau of Indian Standards, New Delhi), 1 988.

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