Dyeing of denim with indigo

DYEING OF DENIM WITH INDIGO

R B Chavan ,
Indian Institute of Technology,
New Delhi – 110016
J N Chakraborty,
Regional Engineering College,
Jalandhar-144011, India

INTRODUCTION
Natural indigo , known in different names in different parts
of the world has been in use since around 7000 B.C. for
dyeing of cotton in attractive and bright blue shades. Plants
belonging to the genus Indigofera are the most valuable for
cultivation to produce natural indigo. The colouring matter in
the plant is present as a glucoside of indoxyl , known as
indican which is hydrolysed to the indoxyl by enzymatic
action and indigo is then obtained by subsequent oxidation
[ 1 ]. In ancient times, reduction of indigo was carried out in
fermentation technique which consisted of use of ripe fruit

and stale urine assisted by wood ash or lime as alkali . The
solution prepared in this way was left overnight for reduction
and solubilisation of indigo. The supernatent liquor was then
taken out for colouration through repeated dip - exposure to
air sequence at room temperature, till a deep shade was
produced ; a combersome as well as time consuming process
[ 1 , 2 , 3 ].
However, extensive research in search of a suitable
alternative in place of natural indigo to met growing demand
of it gave birth of modern indigotin by Adolf Von Bayer in
1880. BASF AG ( Germany ), after a lot of research work for
next 17 years placed commercial synthetic indigo in market
in 1897 [ 4, 5 ]. Indigo has the chemical structure as shown
below ( C.I. Vat Blue 1 ).
O O

N N
H H
Though natural indigo was known since ancient times and
the synthetic one was developed more than 100 years back,
research work started slowly in various areas of indigo

2

dyeing only in the 60's with an intensification since '80s to
cope up with increase in popularity of indigo dyed denim
worldwide. Enormous developments were proposed which
mainly comprised changes in conventional reduction and
application techniques, modification in design of
machineries, various ways to enhance indigo uptake,
quantitative estimation of indigo on cotton denim. Beside,
developments have also been made to improve wash down
property through efficient dyeing [ 6 - 36 ] .

APPLICATION OF INDIGO
Indigo is exclusively used to produce attractive blue shades
on denim alongwith desired wash-down property. However,
in its reduced and solubilised form, indigo possesses little
affinity for cotton - the extent of affinity is too less to
exercise exhaust dyeing ; rather several dip and nip technique
with intermediate air oxidation is followed for gradual build
up of shade. A suitable method of dyeing, viz. slasher dyeing
(sheet dyeing ) , rope dyeing ( ball warp dyeing or chain
dyeing ) or loop dyeing is followed for dyeing of warp yarn.
After dyeing, dyed warp yarn is woven using white cotton

3

weft to produce mostly a warp faced 3 × 1 twill with blue
face and white back and is then washed through specific
techniques to remove a part of dye in order to develop unique
fading property [ 37 ].
In slasher dyeing method, denim yarns in the form of a warp
sheet is pretreated in earlier compartments followed by multi
dip / nip indigo dyeing ; an after-washing, drying, sizing and
final drying completes the process. Handling of yarn is
minimum as once prepared warp sheet is directly processed
and is send to weaving section for its subsequent conversion
to fabric [ 38 - 41 ].
A slasher dyeing range may either consist of the indigo
dyeing range alone[ 42 – 45 ] or a continuous indigo dyeing
range with an integrated sizing range [ 46 ]. In the 1st type ,
yarn sheet moves forward through one or two pretreatment
boxes with the stages of dipping, squeezing and skying ;
washing and drying are the only after-treatments. On
requirement, two or more layers of warp sheet may be
processed simultaneously. Schematic diagram of a popular
slasher dyeing range is shown in fig. 1. The 2nd type

4

includes the steps followed in 1st type , extra attachments are
a sizing and a washing compartment as shown in fig. 2 .
Slasher dyeing is suited for better production schedules , with
a possible problem of centre to side variation . Superior
quality of yarn is needed to minimize breakage problems,
broken end roll wrap ups can cause catastrophic shut-down
problem as well as undesired waste. On the other hand ,
dipping , squeezing and oxidation - all require less time as
each yarn is independently subjected to treatments.
In rope dyeing method, yarns from creel are pulled by a ball
warping machine, pass through a lease stand consisting of a
special comb and lease rods to ensure proper registration of
ends. These drawn ends then pass through a condenser tube
assembly to merge these into a bundle followed by passing
the latter to form a rope comprising 350 - 400 ends. Ropes
are then wound on drums . Once the beam has been fully
wound , it is unloaded and mounted on dye range creel [ 7 ,
47 ]. Dyeing is done by passing ropes, upto 12 at a time,
through pretreatment bath followed by multiple dipping in
separate indigo baths with intermediate squeezing and

5

skying, as done in slasher dyeing . Schematic diagram of a
rope dyeing unit is shown in fig. 3 [ 48 ].
Rope dyeing method is important for higher production rate
with better quality dyeing and better fastness to wet / dry
rubbing as well as colour wash-down . Less breakage of
ends, shade consistency is better ; in contrast , more handling
of yarn to open ropes before sizing. However, efficiency of
slasher and rope dyeing methods depend on back feed to
dyebaths to maintain concentration of indigo and other
chemicals at a constant rate, which is a combersome work.
Moreover , consumption of chemicals and dye remain on
higher side with greater chance of uneven / tailing effect.
In loop dyeing , problems associated with slasher and rope
dyeing was solved through development of a new dyeing
technique, known as "loopdye 1 for 6", developed and
patented by Eckhardt Theodor Godau . In comparison with
other indigo dyeing methods with six colour troughs , here
there is only one brief indigo dyebath with one squeezing
unit. The process is offered by 'Looptex SA(CH)' and under
license by 'Inters(I) and Texima SA'(Brazil), is also of
modular construction. It comprises 8 - 16 warp beams ,

6

threading in feature , pre-wetting or pre-mercerising unit, the
twin - pad colour applicator, an integrated skying passage ,
two rinsing sections as well as an accumulator scray . Fig. 4
shows the sequence of operation used in this new technique
[ 49 - 53 ].

REDUCTION OF INDIGO
Sodium hydrosulphite reduces indigo at 30oC-50oC,
depending on type of indigo used, resulting in formation of
biphenols ( leuco-indigo) [ 54 ]. The reduced form is quite
stable in presence of NaOH to carry out dyeing at room
temperature.
However, hydrosulphite is very unstable. At the time of
reduction of dye , it gets decomposed thermally, oxidatively
and in various other ways [ 55 ], requiring 2 - 3 times higher
amount over the stiochiometric requirement [ 56 - 58 ].
Attempts were made to keep control over its use through
scientific analysis in course of dyeing [ 59 - 62 ].
Other reducing systems, viz. fermentation technique [ 1 ],
copperas method [ 1, 63, 64 ], zinc - lime method [ 1 ],
bisulphite - zinc - lime vat[ 1 , 65 ], thiourea dioxide [ 66 –

7

68 ], Sodium borohydride [ 69 -72 ], have fallen out of use
due to limitations associated with their application.

Eco-friendly reducing agents
In recent times, use of eco-friendly reducing systems for
indigo have attracted attention of researchers [ 73 ]. These
new reducing systems include hydroxyacetone, iron -
pentacarbonyl compounds, electrochemical reduction,
glucose – NaOH and iron (II) salt alongwith a suitable
ligand.
Hydroxyacetone, when used as a reducer ( reduction potential
~ - 810 mV) in RD (refined dyeing ) process [ 74 ], results
20% higher dye uptake alongwith less consumption of
auxiliary chemicals. Other advantages associated are higher
quality dyeing, better ring dyeing effect, better elasticity of
yarns, increased productivity, higher dye uptake, less
dyestuff in effluent. However, required reduction potential
for vatting is obtained at 100oC with higher concentration of
NaOH in this method and produced shade does not
correspond with that obtained with hydrosulphite and more
suitable for pad - steam method [ 75 ].

8

Use of iron-pentacarbonyl compounds, though has been
suggested as reducer, detailed report of this system has not
been disclosed so far [ 76 ] .
In electrochemical reduction technique, vat dye is directly
reduced by contact between dye and electrode , though a
reducing agent must be added to the reduced dyestuff
( though lower in quantity) in order to ensure corresponding
stability of the reduced dye - liquor . However, dyestuff
requirement for a specific shade is too higher [ 77 - 80 ].
Glucose in combination with NaOH can be used for
reduction of sulphur dyes ( reduction potential ~ - 550 to -
600 mV). Indigo requires nearly ~ -700mV for reduction ;
hence glucose - NaOH system can also be used , preferably
at boil, producing reduced indigo baths free from sediments
and highly stable for several hours. One problem related to
this reducing system is that padded textile requires more time
for oxidation in between two successive dips and indigo
uptake is less [ 81 ].
It has been reported in the literature that ferrous hydroxide is
a strong reducing agent in an alkaline medium [ 73 ]. The
reducing effect increases even more with increase in pH

9

value. Ferrous hydroxide is poorly soluble in alkaline
conditions and precipitates. It must be complexed [ 82 ] in
order to hold the ferrous hydroxide in solution . Fe++ has
hardly any reducing power as the central ion of complexes
which have the stable arrangement of the krypton shell.
However a stable complex with reducing power is obtained
with weaker ligands , e.g. gluconic acid. When dyeing with
vat dyes at 60oC, good dye uptake was achieved for optimum
concentration of iron(II) salt and molar ratio between this salt
and gluconic acid. However, dye uptake decreases with
decrease in concentration of iron salt , even too low an
iron(II) salt concentration gives lighter and unlevel
dyeings at iron(II) salt : gluconic acid - 1 : 1 and 1 : 2
(molar) due to insufficient vatting of dye. When molar
concentration of gluconic acid was reduced to 1 : 0.5 ,
unlevelled shades were produced with reduced brilliance [ 73
].
Iron (II) salts in combination with NaOH and a weak ligand
like gluconic acid, tartaric acid , citric acid or triethanolamine
, at suitable concentrations, can produce reduction baths with
potential ranging from –800mV to –1000mv. These reduction

10

baths are capable of reduction and dyeing with indigo at
room temperature. The reduction baths showed superior
stability in presence of indigo upto 24 hours. Deposition of
iron on dyed samples was also assessed and it was found that
gluconic acid as ligand causes least deposition , whereas
citric acid causes maximum deposition. Produced shades
were bright with no shifting of λmax . The baths were turbid
throughout dyeing [ 83 ].
It may be summarised here that, for quality dyeing, whatever
may be the reducing agent, reduction potential and alkalinity
of bath should be accurately adjusted for development of true
shade of indigo [ 84 - 90 ].

FACTORS INFLUENCING INDIGO UPTAKE
Reduced and solubilised indigo , though ionic in nature,
possesses negligible affinity for cotton ; a multiple dip / nip
technique is followed with intermediate airing for gradual
build-up of shade. Yarn to be dyed is dipped in indigo bath
for a specific time followed by a passage through padding unit
in order to achieve better penetration as well as distribution at
nearly 100% expression, succeeded by air oxidation. This

11

dip / nip followed by airing cycle is repeated till desired depth
of shade is produced, succeeded by a final oxidation for 3 - 5
minutes for complete conversion of soluble indigo to its
insoluble form.
As stated , desired depth of shade is primarily governed by
number of dips / nips , but dye uptake during each dip is
influenced by a number of other process parameters , viz. pH
of dyebath , immersion time , time of intermediate oxidation ,
concentration of indigo and temperature of dyebath. These
parameters affect build-up of indigo on cotton , degree of
penetration and various fastness properties.
The crucial factor is pH of indigo bath [ 54, 91 - 95 ] .
Depending on dyebath pH , indigo may exist in four
different forms as shown in fig. 5.
i) indigotin structure at lower alkaline pH .
ii) reduced but non-ionic form , at moderate alkaline pH.
iii) Mono-phenolate form , at relatively higher pH.
iv) Bi-phenolate form , at very higher pH.

Structures (i) and (ii) exist below pH 9 - 9.5 ; their relative
fraction depends on exact pH of bath. With slow increase in

12

pH , structure (i) collapses and gets converted to either (ii) or
(iii) or as a mixture of both. Further increase in pH slowly
converts all the (ii) to (iii) structures ( above pH 10 ). A pH
around 11.5 indicates that almost all indigo molecules are in
their mono-phenolate form. Any further addition of alkali
beyond pH 11.5 slowly attacks second C=O group of indigo
in presence of excess hydrosulphite with a result of
conversion of mono-phenolate to bi-phenolate form (iv). The
extent of conversion strictly depends on exact pH and excess
hydrosulphite in dyebath. As presence of excess
hydrosulphite is a must for successful indigo dyeing, only
addition of alkali , if made in excess above pH 12.5 , the
structure (iv) predominates. Indigo in the form of structure
(iv) shows reduced affinity for cotton and thus dye uptake
falls. Structure (i) shows no affinity , whereas structure (ii)
has negligible affinity , as (i) and (ii) are non-ionic forms of
indigo. Mono-phenolate form is the desired structure which
possesses highest affinity as well as strike rate to give higher
dye uptake. In this context , it is also to be noted that cotton
when dipped in alkaline bath acquires negative charge. Higher
the pH , higher the charge on fibre and more is the repulsion

13

between dye and fibre resulting in reduced dye uptake.
Another importance of pH is related to ease in washing. At
pH 10.5 - 11.5 , indigo shows maximum affinity as well as
higher strike rate on cotton due to lower solubility of mono-
phenolate form of indigo in this pH range, hindering
penetration inside cotton and results more imtensive surface
dyeing, facilitates achieving better wash-down effect [ 96 ].
It was mentioned earlier that relative fraction of mono-
phenolate and bi-phenolate forms of indigo can be positively
controlled by adjusting the dyebath pH [ 54 , 87 , 91 , 92 , 94 ,
95, 97 - 104 ] . pH is such a crucial factor in dyeing with
indigo that it not only controls dye uptake, but also
distribution of indigo on cotton yarn through control over
strike rate and hence wash-down properties with removal of
unfixed dyes [ 87 , 105 ].
Out of two soluble forms of indigo , viz. mono-phenolate and
bi-phenolate , the former has relatively lower water solubility
and higher strike rate - just opposite to that with bi-phenolate
species ( fig. 6 ). Hence, if pH can be kept within a range of
10.5 - 11.5 , most of indigo molecules will exist in bath in
their mono-phenolate form showing higher strike rate for

14

cotton , resulting poor penetration and higher surface
deposition of dyes , the so called ring dyeing effect [ 87 ,
91 , 105 ]. In ring dyeing , only a few surface layers are dyed
leaving interior of yarn undyed due to absence of adequate
time for penetration as well as thorough distribution ( fig. 7 )
[ 105 ]. If pH is raised above 11.5 , relative fraction of mono-
phenolate form gets reduced with instant increase in that of
bi-phenolate form, lowering strike rate and consequently
increases time for absorption - resulting dyeing of few more
layers than that obtained with mono-phenolate form . With
further increase in pH , mono-phenolate form go on
diminishing with a substantial rise in bi-phenoate form and
thorough dyeing of cotton occurs [ 105 ].
Efficient wash-down property of indigo dyed yarn depends on
amount of dye present on yarn surface. It is the 10.5 - 11.5 pH
range , which gives dyeings with desired washing
performance and a deeper shade with less amount of indigo.
Dyeing at higher pH ranges gives relatively lighter shades
as well as lowers wash-down property of dyeings [ 92 , 106
- 108 ].

15

Ionic nature of cellulose plays another key role to affect
indigo uptake. With increase in alkalinity of indigo bath ,
ionisation of cellulose ( expressed in gms - ion / l ) increases
with development of more anions, e.g. concentration of (Cell-
O-) at pH 10.0 is 3 ×10-3 gm-ions/ l, increases to 1.1 gm-ions/ l
at pH 13.0 . This in turn suppresses substantivity of indigo,
finally lowering dye uptake [ 87 , 105 ]. Calculation of
different forms of indigo in bath, though combersome, can be
made through study of equilibrium ionisation constant values
[ 107 ].
Effect of immersion time on final colour depth in indigo
dyeing shows that with increase in immersion time, dye
uptake increases initially , remain nearly same for little further
increase, but beyond certain limit falls considerably. An
immersion time of 30 seconds seems to be adequate,
prolonged immersion reduces colour depth mainly due to re-
reduction of oxidised indigo already on cotton through
previous dips and go back to dyebath . To get optimum
colour yield , oxidation time is fixed at 60 seconds or beyond
that. Due to very low affinity of solubilised indigo in bath for
cotton , increase in indigo concentration in bath though

16

initially can increase depth of colour , but beyond certain
concentration , does not show any remarkable change on dye
uptake. Increase in indigo concentration upto 3 g/l in bath
increases its uptake on cotton , beyond which it shows little
impact on dye uptake . Optimum depth of shade is obtained in
as high as ten dips and beyond that no such increase occurs.
But keeping in view limitation in machinery set up and a
lengthy process time , generally a 6-dip 6-nip technique is
followed [ 96 , 97 ]. However, it has been reported elsewhere
to work with lowest possible concentration of indigo and to
use maximum number of dips. By doing so , colour
uniformity , fastness properties and wash-down after dyeing
are improved. A rule of thumb is that more the number of dips
used for a given shade , the better [ 109 ].
Affinity of solubilised indigo for cotton decreases with
increase in temperature , initially at a faster rate followed by
very slow decrease. At lower temperature , affinity of indigo
is better and also re-reduction of oxidised indigo does not
occur typically. Hence , it is better to carry out dyeing
without any application of heat. It has been reported that
maximum dye uptake occurs if dyeing is carried out at room

17

temperature. With increase in dyeing temperature, dye uptake
falls relatively at a slower rate from 40oC - 60oC and beyond
that become almost parallel [ 96 , 97 ].
TECHNIQUES TO ENHANCE INDIGO UPTAKE
Alternate pH control techniques
Indigo uptake can be enhanced through proper control over
pH in the region 10.5 - 11.5. NaOH , which is used in indigo
dyeing is a very strong alkali , makes efficient dyeing
impracticable due to wide fluctuation in pH during dyeing .
Initially pH is highly sensitive to concentration of NaOH ,
a small change in NaOH concentration changes pH
remarkably [ 110 ]. The normal practice is to add NaOH at
higher concentration, such that final dyebath pH does not fall
below 10.5 , with a result of less dye uptake in initial dips.
This is because when very low levels of NaOH is used in an
attempt to achieve a dyebath pH of 11.0 , small changes in
NaOH concentration lead to very large changes in dyebath
pH. In fact , the dyebath pH may drop suddenly into a range
in which mainly non-ionised indigo is formed and
superficially deposited onto the denim yarn. Such a sorption
inhibits sorption of ionised dye , leading to streaking and

18

extremely poor crock fastness [ 110 ]. In contrast, use of
excess alkali makes lighter dyeings of higher alkalinity,
which creates trouble in finishing [ 111 ].
Attempts to substitute NaOH with other alkalis / bases
developed buffered alkali system of undisclosed composition ,
manufactured by 'Virkler company', USA, which when
applied in indigo bath at recommended concentration,
produces a stable pH in the range of 11.0 and little
fluctuation occurs in dye uptake throughout the period of
dyeing [ 112 ].
In another attempt, NaOH was substituted with different
organic bases. Before application, alkalinity of these bases
were evaluated and applied in various indigo baths keeping
equivalent alkalinity with that produced with NaOH. It has
been reported that dye uptake with these new systems were on
quite higher side than those obtained in NaOH method.
Stability of indigo baths were superior to NaOH baths even
after a lapse of 8 hours. Change in pH with time was also
studied and it was found most of the bases were capable of
maintaining pH in 10.5 - 11.5 region upto 4 hours [ 113 ].

19

Pretreatment of cotton
Indigo in solubilised state is anionic in nature and will be
attracted by cotton, if the latter is pretreated with cationic
compounds. Cotton was pretreated by immersing it in a
solution containing 200 g/l of a 12% aqueous 'Hercosett 57'
( cationic water soluble polyamine - epichlorohydrin resin )
solution and 10 g/l of 'Sandozin NI' for 5 minutes followed by
squeezing the material in padding mangle to achieve 80%
pick-up and dried at 100oC for 3 minutes. Treated cotton was
padded with indigo through 6-dip 6-nip technique. It has been
reported that dye uptake on pretreated cotton remained on
higher side than that on untreated cotton under identical
dyeing conditions [ 96 ].
Pretreatment of cotton with metal salts prior to padding with
indigo ( which may be done in several ways to synchronise
with batch or continuous dyeing methods) results substantial
increase in indigo uptake. It was also found that iron and
cobalt (II) salts were more effective even at very lower
concentration, viz. 1 g/l. Formation of insoluble metallic
hydroxides in situ cotton in presence of NaOH and air was

20

responsible to attract more anionic indigo molecules in bath
causing increase in dye uptake [ 114 ].
Introduction of a mercerising step prior to indigo padding
enhances indigo uptake considerably. In October , 1988 ,
ATME -1, Greenville, a 'DIMER' was shown, which is a unit
that incorporates real mercerising in the loop dye process for
indigo warp dyeing. The technique was originally developed
in Switzerland by loop-dye , but is now being produced and
marketed by 'Kuesters'. The unit simplifies warp production
with advantages like improved fabric appearance , handle ,
higher yarn strength , higher dye uptake and more intensive
ring spun yarn dyeing with shorter stone washing. Even it is
also possible to bottom with sulphur, indanthrene, reactive
and naphthol colours [ 115 ].

COST EFFECTIVE PRODUCTION IN INDIGO
DYEING
Multi-vat machine concept has regained its importance
through flexible adaptation of machine technology. Accurate
dosing, targeted dyestuff use , low dyestuff and chemical
losses are the main aspects of such production system.

21

According to literature, adequate dwell times in the steeping
bath ( approx. 6 m / bath ) and oxidation zones(>60 seconds ),
together with a high circulation rate with low dyestuff
concentration has been pursued in order to achieve optimum
dyeing [ 104 , 116 - 118 ] .
Due to lower dyestuff concentration in the effluent , ultra-
filtration equipment can then be equipped with smaller filter
surfaces to reduce investment and operating cost . Washing
water consumption can be kept low at 3 l / kg of warp.
Indigo vat , hydrosulphite and NaOH are continuously dosed
in proportional to speed in accordance with specifically
circulated quantities avoiding shade variation. Dosing is
controlled by a 'Semen Op 20 ' automation unit confirming a
high degree of processing reliability and reproducibility.
Dosing of dye is controlled via a special frequency control
pump. Measuring systems are available for bath
concentration, pH value, redox potential, bath temperature
etc.
Another unique way of cost reduction is ‘zero discharge
concept’, which works in ‘loopdye 1 for 6 process’ using
hydroxyacetone as reducing agent to produce waste-free

22

improved quality dyeing at reduced cost through recovery
and reuse of unused dye and waste- water [ 119 ].

QUANTITATIVE ESTIMATION OF INDIGO AND
OTHER VAT DYES

Indigo in dyed textile / dyebath / wash liquor is determined
spectrophotometrically at maximum wave length ( λmax ).
When indigo is in solid state , each of its molecules form H -
bonds with 4 adjacent dye molecules ( between C = O and
NH groups ) , makes it insoluble in water as well as in most
of the organic solvents [ 120 ]. Pyridine serves as a good
solvent . For spectroscopic estimation of indigo , a clear and
stable solution of indigo is an indispensable pre-condition for
consistent and reproducible results. Alternately, turbidity of
oxidised form of indigo may be a measure of quantitative
estimation. Several analytical methods have been devised in
this context, viz. colorimetry of leuco indigo , turbidity of
indigo in oxidised form , colorimetry of sulphonated indigo.
In solvent technique, a few methods based on colorimetry of

23

chloroformic , pyridine / water and dimethyl sulphoxide
extracts have also been reported [ 121 - 124 ].
The ‘colorimetry of leuco indigo method’ consists of
reduction of pure indigo with hydrosulphite in alkaline
medium. Absorbance is instantly read at 412 nm, as reduced
indigo has time-bound stability ; its spectrum changes
considerably in very short time intervals. This technique lacks
reproducibility [ 125 ]. However , the technique may be
dispensed with other methods , if stability of reduced indigo
can be increased . It has been mentioned in literature that
iron(II) complex with triethanolamine can reduce vat
dyestuffs and the reduced dyebath has high degree of stability
to temperature , time and atmospheric oxygen [ 126 ]. In this
technique, iron(II) salt concentration should be kept under
control , as iron ions below 400 nm tend to self absorption .
Turbidity of oxidized indigo has no linear relation between
turbidity and concentration, in some cases, higher values of
turbidity were also achieved on reducing the concentration.
This behaviour is possibly due to the excessive dimensions of
indigo particles as well as distribution of uneven sizes.
Addition of glycerine to stabilise the suspension did not

24

improve the results ( glycerine acts by increasing bath density
and retards particle fall ). Maximum absorbance was found to
be at 666 nm , but the slope of the calibration curve varied
according to the status of initial dyeing bath [ 126 ].
In ‘sulphonated indigo method’, incorporation of solubilising
groups of sulphuric acid in indigo structure makes indigo
water soluble, stable and deep blue coloured. The absorbance
is maximum at 605 nm [ 125 ]. Beer's law is fulfilled upto 15
ppm of concentration and the reproducibility of the method is
0.3 - 2%. However, before sulphonation, indigo should be in
oxidised and dried form - otherwise a greenish- brownish
solution is obtained . Indigo should also be free from
hydrosulphite. This analytical technique is hardly suited for
instantaneous , continuous control of indigo liquors [ 126 ].
Sulphonation of indigo with concentrated sulphuric acid at
75oC is also troublesome.

Solvent techniques
The colorimetry of chloroformic extract gives maximum
absorbance at 605 nm and good linearity is achieved in the
calibration curve upto indigo concentration of 10 ppm. At this

25

concentration , solubility limit of dye in chloroform is
reached and a blue interphase between the two layers is
formed during extraction [ 125 ].
Indigo structure has a similarity with that of pyridine.
Solubilisation occurs through H - bonding between these two .
Oxidised indigo can be dissolved upto a concentration of 40
ppm by boiling in pyridine - water ( 50 : 50 ) solution , which
is then photometrically measured at 625 nm . Measurement
must be done when the solution is hot to avoid precipitation
of indigo on cooling the solution. Indigo should be completely
free from hydrosulphite ; if available with dye , decomposes it
during heating with reagent solution . Presence of NaOH with
dye also changes absorbance as aggregation of dye alters with
pH. This method has a reproducibility of 2 - 4% [ 125 ] .
A completely new approach has been made by solubilising
oxidised indigo in Dimethyl Sulphoxide ( DMSO ). The
method does not have any of the problems as with earlier
methods. It is time saving , safe , not subjected to interference
and largely reproducible. Solubility of indigo is excellent
even at room temperature ; maximum absorbance is achieved
at 619 nm . The solution is highly stable for several days and

26

for indigo concentration upto 250 µg / ml , complete
dissolution takes place . The only problem lies in handling of
DMSO is that it is slightly detrimental to health, irritation in
contact with skin, itching etc.[ 127 ].
In some cases , denim is dyed with a combination of indigo
and sulphur or hydron blue dyes . To estimate contribution of
these two in the shade developed, indigo is 1st extracted
completely from sample at boil in pyridine - water solution.
The dyed sample under test is rinsed with methyl alcohol ,
dried at 40oC to remove residual pyridine and methyl alcohol.
Reflectance of the sample is measured. K/S calculated from
reflectance value at each wavelength are integrated over the
wavelength range , the result is an integrated value
( I.V.) [ 128 ].

CONCLUDING REMARKS

Though indigo is a class of vat dye, it can not be applied
according to conventional vat dyeing techniques due to its
negligible affinity for cotton, rather multiple dip / nip with
intermediate airing is to be followed. During dyeing, pH of

27

dyebath plays a crucial role to affect indigo uptake as well as
location of dye on cotton structure, though influence of other
factors can not be over-looked. Dyeing mostly takes place in
yarn stage, necessitating selection of specific machine
depending on form of yarn. Beside dyeing parameters, special
techniques through use of buffer / other alkalis / bases ,
pretreatment of cotton etc. may be introduced to enhance indigo
uptake. As far as quantitative assessment of indigo is concerned,
DMSO method has been found to be the most effective one.

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3. C S Hughey , Textile Chemist & Colorist , 15 , 6 ( 1983

) 103 / 13.

4. H Cassebaum , Melliand Textilberichte , 46 , 12

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LIST OF FIGURES

Fig. 1 Omez SpA. continuous slasher dyeing range.

Fig. 2 Slasher dyeing with integrated sizing range .

Fig. 3 Rope dyeing.

Fig. 4 Principle of the " Loopdye 1 for 6 " .

Fig. 5 Different ionic and non-ionic forms of indigo .

43

Fig. 6 Fractional form of each reduced species of indigo found
in dyebath as a function of pH.

Fig. 7 Simplified schematic of distribution of Indigo in the
cross-section of cotton denim yarn at different pH
conditions .

44

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