INDIGO DYEING

Indigo Dyeing – Various Methods Explained And Compared

Manufacturing Process.

The indigo dyeing and processing methods -currently used – for dyeing indigo warps were introduced from 1978- 1987 to obtain a higher productivity and savings in dyeing or to achieve the required darker shades (hard rock washing, super blue, soft denim), or softness of the yarn for final finishing. The following table gives you a comparison of the possible processing stages such as:

1- Indigo rope dyeing process

2- Indigo one sheet dye slashing

3- Indigo double sheet dyeing

4- Loop dye 1 for 6 (continuous dye slashing)

5- Loop dye 1 for 6 with dyemer (continuous mercerization dyeing and sizing).

For the five major Indigo dyeing methods for the basic denim, super blue denim, soft denim, stone wash denim, we also must take into consideration that a certain appearance of the garments is only achieved after a certain washing method. (Chemical washing, stone washing, hard rock washing), use of certain sizing agents (soft denim) or irregular appearance in warp or weft direction by using a yarn with slubs and neps (antic denim).

The final finishing methods have influence on fabric construction and dyeing methods.

CHEMICAL WASHED DENIM

The creator of snow wash denim or of f-side denim is Edwin Co. Limited Japan. These chemical washed types of ready made garments show less tensile strength and a harder hand.

STONE WASHING

With soft stones it takes 20 – 30 min. to achieve the desire surface appearance.

DRY BLEACH

Soft rock are socked with chloride and the garment treated in washing machines.

HARD STONE WASHING

Treatment takes approx. 3 – 4 hours. A very soft hand the garments is achieved. Due to the extended time of treatment in washing with hard rocks the strength of fabric is influenced. This treatment requires improved yarn quality and deeper dyed shades. Hard stone washing fades out partly at hems and stitched lines.

SUPER BLUE

Deepness shade is achieved by dyeing in 8 – 12 dye boxes with rope dyeing methods. With mercerized yarn where the colour stays at the surface of the yarn (ring dyeing) with continuous loop dye range with integrated mercerizing and dyeing unit (dyemer).

BLACK DENIM

Sulfur dyed, padazoic dyed etc. on continuous dyeing sizing machines or loop dye ranges.

INDIGO DYESTUFF AND ITS CHARATERISTICS

Indigo has despite many other blue dyestuffs kept its popularity. This by no doubt is achieved by the fact that Indigo has a number of properties which have in this combination not yet been achieved by other single dyestuffs.

The main properties are:

·         Pleasant colour shade.

·         Possibility to achieve by simple repeated dipping, deep marine blue shade.

·         Possibility to dye cotton in cold dye bath.

·         Competitive in price.

·         Possibility to achieve an acceptable colour fastness and the exceptional advantage by repeated washing of fading colour, to keep the colour shade that always a clear, pleasant blue shade result.

The following drawing shows the symbolic  built-up of dye stuff of the yarn according to the multi-deep process.

The sketch shows how Indigo layers one after the other are placed on the surface of the yarn then again scraped off, similar to knife through force or tension or by washing.

An Indigo has only a very low affinity and the depth of colour of the fibre is achieved by repeated dipping / oxidation. Each dipping cycle occur in a certain balanced manner as dye stuff is observed by the fibre at the same time and part of the already oxidized dye stuff is reduced and migrates from the fibres.

With increasing dipping operations , the  balance of absorption / migration moves towards migration.

A very important factor for the reduction of migration is the squeezing effect and the oxidation which follows. A high squeezing effect promotes a quick and thorough oxidation and reduces the migration and reduction in the following bye bath.

With the low squeezing effect ie  with high liquid absorption, the purely visual impression of a good oxidation can be deceptive as the outer colour skin looks blue. In the core of the thread however  less vat dye stuff may be deposited

Let us have a close look at the different dyeing systems.

CONVENTIONAL CLASSICAL CONTINUOUS INDIGO ROPE DYEING

The classical rope dyeing system is very labor intensive and consists of:

·         Ball warping

·         Indigo dyeing

·         Rebeaming on long- chain-beamer

·         Sizing

Yarn from the ring spinning machine is wound on automatic winding machines on to a suitable package either cylindrical or 5057 cone. The winders are directly linked to the ring spinning frames and the cops joint by splicing. OE yarns are directly creeled up on the Ball warper.

The required No. of ends (usually 380 – 420 ends) are assembled into a rope. These ends are wound onto a core. The rope is guided similar as a cross wound package and wound into a ball, length of ball approx. 12 – 15.000 meters.

A lease is inserted at the start and end of the rope. The facilitate Rebeaming every 1000 meters an additional lease is inserted. Usually 18 – 24 ropes are simultaneously process on the rope dyeing machine. Prior to dyeing, the ropes are boiled out and treated with caustic-soda and wetting agent to remove from the cotton oil, impurities which could influence the fastness for the dye.

To dye with indigo, the ropes are immersed into the dye-bath. To dye in rope 30 – 60 seconds immersion (20 meters yarn) and 60 – 180 seconds are required for the oxidation of the Indigo dyestuff to ensure that also ends in the centre of the rope are equally dyed. Please note that squeezing pressure is important- 5 tons- as fastness of colour and shade depends on even squeezing pressure. The comparatively long immersion and oxidation time requires a comparatively expensive equipment of machinery.

In order to obtain the required deep shade of blue colour the ropes are  immersed 5 – 6 times in a sequence of dye boxes with an oxidation range – so called skying – after each dye box. (Indigo belongs to the group of the vat dyes which is water-soluble in reduced solution and becomes an insoluble pigment when oxidized.

Having passed the dyeing and oxidation rage the ropes are guided through 2 or 3 washing boxes to wash off excessive dye .In the last box softener are added to ease the opening of the ropes. They are dried in a series of cans. The dried ropes which contain 380 – 420 ends are then deposited into large coilers .Rebeaming with 300 – 380 ends per rope is easier. These coilers are placed behind the long chain beamer where the Rebeaming and opening of the ropes takes place. In order to guarantee even yarn tension through Rebeaming on to a back beam ready for sizing the ropes are guided over a tension device which is placed approx.  10 -11 meters distance from the long chain beamer. Broken ends which very really happen during process of the rope dyeing are repaired at this process stage. Initially these machines were supplied without yarn stop motion but are available now a days on special request. This is of major importance as lost ends, fluff, 3 – tail ends and yarn remnants can cause inferior performance in weaving.

The so prepared beck beams are now sized in a sizing machine preferably with 2 size boxes. The size pick up varies between 8 – 10%. In Europe mainly modified starches with binders are used, whilst in USA certain low % of PVA is applied in combination with starches by some companies. Depending on the final finishing process (washed denim) with no filler also CMC gives excellent performance in weaving. Special size mixes for soft denim will be discussed separately. We recommend however not to use PVA for sizing of denim as a surface of denim may show a leather skinned appearance.

CONTINUOUS SLASHER DYEING SYSTEM

Contrary to the Indigo rope dyeing system, for the continuous slasher dyeing and sizing back beams are used. That means that the total No of ends required for a weavers beam are dyed, dried, sized and dried simultaneously. The back beam contains similar to rope 380 – 420 ends but distributed evenly over the width of 140 or 160 cm between the flanges so the end lay parallel to each other, warp length 12 – 15.000 meters, similar to the rope dyeing system the full No of ends are pretreated (washed) dyed in 4 dye boxes and oxidized, no softener are used in the last wash box.

CONTINUOUS INDIGO DYEING AND SIZING PROCESS

We must however consider that the squeezing effect is lower and therefore the danger of streakiness and shade variation from centre to out side is also higher. Consequently it would be better to reduce the warping width rather to 140 cm instead of using warper beams with 160 – 180 cm warping width.

The immersion time in the dye boxes is approx. 10 – 15 seconds and time for oxidation 30 – 60 second. The final result is a weaver’s beam. This system allows the installation of less expensive dye rage and less additional preparatory machinery.

One of the disadvantages previously when warp preparation (knots, weak, thick places) was not kept at a very high level was that ends sown in the dyeing range could cause major color variation through machine stops.

DOUBLE SHEET CONTINUOUS DYEING

Patents applied for double shade dyeing by E. Godau date back as 1976. With the system dyeing sizing is done in 2 operations. The main reason for dyeing of 2 sheet simultaneously is achieved a more even dyed sheet, that means 8000 – 8200 end are dyed, oxidized, dried and the full length of the warper beam 12.000 – 15.000 meters flange diameter. These beams are transported with the aid of air cushions to the sizing machine and the yarn sheet sized in double size boxes. Immersion time and oxidation time is the same as with continuous slasher dyeing.

Indigo full-width warping process

With double sheet dyeing the linear warp thread density is doubled. Therefore:

·         Squeezing effect is increased,

·         An even squeezing over the whole width is achieved,

·         Condensation and concentration of ends at one spot show compared to single sheet dyeing no colour strips formation, streakiness or shading in the finished fabric.

·         The production out put of the dyeing is increased by 75%.

·         Dyeing of 3 layers of yarn simultaneously is possible but very difficult to control the beaming on 3 big warp batches.

Unfortunately the double sheet dyeing machine as well as the rope dyeing range cannot be linked with a sizing machine which must be regarded as disadvantage as the processes of dyeing and sizing must be carried out separately.

Loop dye system 1 for 6

Similar to the sheet dyeing systems 10 – 16 warper beams with the total number of ends required for the weaver beams are used. The warper beams are placed in a moveable warp creel which can be loaded whilst one set is in potation.

The yarn sheet is guided to the soaking bath through a feed-in system with tension compensation rollers. The soaking bath has the task to prepare the yarn for the following dyeing operation.

The yarn sheet after having been immersed into a single indigo dye bath runs into a long loop where oxidation takes place. As you can see from the slide the back beams are inside the yarn sheet passes through the dye box as often as necessary to obtain the required deepness of shade. One of the advantages is:

Ideal, utilization of Hydrosulphite through squeezing 4 – 6 layers simultaneously and oxidation of yarn in a comparatively long oxidation loop. After the oxidation the yarn sheet is guided through 2 washing boxes into a yarn accumulator and finally on to a series of drying cans, dried up to 25 – 30% final moisture content prior being immersed into size boxes, dried and wound onto a weavers beam.

The molecules are controlled DC drive to maintain warp tensions. Temperatures are automatically controlled as well as the PH value in the dye box. The automatic control unit of the PH value supplies automatically hydrosulfite and caustic soda to stabilize the present value from the start to end of a dye set.

All rollers arein  contact with the dyed sheet are fluted, they keep the sheet in position and reduce deposit of dye and build-up of other deposit (fluff). In addition to width is controlled by guides to ensure even distribution of the yarn layer over the whole width of the dyed sheet.

All accumulators placed between washing boxes and drying cans guarantees a continuous production of the dye range when a weaver beam has to be exchanged at the head stock.

The creels can be loaded with back beam with 1200 mm diameter which allows to warp approx. 36.800 meters of yarn, count No 7, 5 (tex 78) or 50.000 m count No 10 (tex 60). This means that depending on the count normally one cyl-spool is used in warping to fill a warper beam.

WARPING SPEED

PRODUCTION

Speed varies between 1000 m/min, 35 m/min. No 5, 5 (tex 107) and 42 m/min. No 10 (tex 60)

MACHINE STOPS DURING SIZING

For 36800 m, 4 recorded on expansion comb.

WASTE OF MATERIAL

Approx. 15-20 kg per set.

TIME REQUIRED FOR CHANGE OF SET

2 hour

Linear warp densities in the squeeze.

The linear density in the nip is calculated in the same manner as for sizing.

Q= linear thread density

F= ends in cm -1

D= diameter of yarn

D= 0,921 mm = 0,921 = 0, 29125

 Nm 10 = 3.1622

Q= F x D

As already mentioned previously the warp density has an influence on colour fastness. A higher squeezing effect is achieved due to the over laying of the watp ends this also gives more side to side squeezing, therefore reduce strips formation. The high squeezing effect results also in better, quicker and proper oxidation and better colour fastness.

It must be mentioned that recommended dip and oxidation times on warp dyeing ranges are of little use if not the squeezing effect is taken into consideration. Under a given squeeze pressure, for instance 500 kg the squeezing effect of the mentioned 4- dyeing systems can be compared. We can see that similar squeezing effect can be achieved with the loop dyeing system 1 for 6 (75%) as with rope dyeing 70 – 110%. Double dyeing with 2 layers width with 150 cm gives approx. 80% squeezing effect, whilst single sheet slasher dyeing varies depending on count between 100 – 130%.

Loop dye 1 for 6 Rope-dyeing Slasher-dyeing

As only one short indigo dye bath is deeded only one feeding tanks, no separate feeding of chemicals is necessary.

Dye liquor is use in 1500 1 instead of 6-4500 1, therefore less chemicals are in  use at same time.

·         Lowe power consumption.

·         Fine counts can be dyed as well (No 30)

·         For dark shades, black shades or other shades needed by fashion, other indanthrene dye stuff can be directly added into the indigo dye bath (indanthrene yellow or orange).

LOOP DYE 1 FOR 6 COMBINED WITH DYEMER

The demand for dark shades specially dark marine blue for super blue denims also led to new ideas in indigo dyeing ranges have been increased between 8-15 dye boxes with corresponding oxidation ranges.

In some cases Hydroxyaceton has been specially treated with high frequency. Besides achieving a darker shaded with the desire greenish touch it is very suitable for biological treatment.

In order to achieve ring dyeing, mercerized yarn has also been used. Mercerizing prevents penetration of dye stuff into the inner code it is suitable for this purpose to obtain an optical blue effect and superior colour fastness and behavior in washing. Mercerizing is very costly, therefore new ways in continuous mercerizing and indigo dyeing was found.

DYEMER

The dyemer range is integrated.

For impregnation padder for hot caustic solution is placed after the heating system. The yarn is guided over cold cylinders and with an adjustable roller the tension of the yarn sheet can be adjusted according to the required tension prior to the scouring in 2 more boxes.

After having passed this, the yarn sheet is immersed into the dye box and the same process as with the loop dye method is repeated.

CONTINUOUS DYEING AND SIZING ON THE DYEMER RANGE

If fashion needs other colours than indigo blue it is comparatively simply to dye and size on this range.

Slasher Dyeing Vs Rope Dyeing – A comparison

Comparison On	Slasher Dyeing	Rope Dyeing
Required Space	Less	More
Mercerizing	Easy	Difficult
Manpower	Less	More
Dye bath	Less capacity	More
Yarn	Can use Ne 1-30 without major change	Can use Ne1-16 without major change
Flexibility	Flexibility to produce denim in different colors and small quantities	Less flexible. Difficult to change colors.
Cost	Cost of production is lesser compared to Rope Dyeing	Cost of production is more compared to Rope Dyeing

About the author: Adnan is a textile engineer and working with a reputed denim mill in Pakistan.He is looking after new denim developments.

Indigo Rope Dyeing : Some Important Technical Considerations

Denim - Developments.

Rope Dyeing is considered a superior dyeing technology where the dyeing uniformity achieved is better than other Indigo Dyeing technologies like Slasher Dyeing. 
However, Rope Dyeing is a also a more difficult dyeing technology. One needs to master its nitty gritties to get the best out of the system.

I , recently came across a very well written article on Rope Dyeing and thought it appropriate to share here.  The article mentions in great details the technical points to be kept in consideration while using Rope Dyeing..

Read on only if you are very technically oriented !

Notes on Yarn for Rope Dyeing

* Yarn faces stress and stretch at ball warping, rope dyeing, rebeaming, sizing and loom shed so elongation of yarn should be more than stretch at (ball warping + Robe Dyeing+ Rebeaming + Sizing )= (2-3%) + Loom shed (about 5%)

* Tension at Ball warping should be less by 7-8% of single yarn strength.

* Strength CV should be within limits as it may give rise to weak points

* The tendency of yarn to migrate at rope dyeing can be countered by less micronaire of yarn ( should be around 3.8-4.2)

* More dropping of short fibers at long chain beaming is good

*The sensitivity settings for neps is set at +280 for rotor spun yarn and not +200 as in case of ring spun yarn. The reason for this is that the structure of rotor spun yarn is intrinsically different from that of conventional ring spun yarn. Neps in rotor yarn tend to be spun into the solid yarn body rather than remaining on the yarn surface, which is typical of ring spun yarns. Although embedded in the yarn core, these neps still represent a short mass defect and will therefore trigger the imperfection counter upon exceeding the preset value. However, compared to neps that are attached to the yarn surface, fully embedded neps are barely perceptible for the human eye. Thus, in order to balance the typical visual appearance of rotor spun yarn with the imperfection counts, +280 sensitivity setting is a common convention for rotor spun yarns.

Comparison of Ring Yarn with OE yarn at Ne 7

Process of dyeing of sulphur color in Indigo Dyeing Range:

1st Wash tank: mercerisation by taking 22% NaOH ie. 250 gpl 
2nd Wash Tank: Hot Wash 
3rd Wash Tank: Cold Wash 
2. In 1st and 2nd dye bath take sulphur color 6-8% on the weight of the yarn sheet. Temperature 90 deg. cel. The solution contains the following: 
1. solubalised sulphur color: 150 gpl 
2. Na2S–> reducing agent: It is added to increase its reducing power 
3. Caustinc Soda –> 10 gpl–> reducing agent 
4. Wetting agent–> 2gpl 
5. Antioxident Sulphide ( Glucose paste–> 5gpl). This is added to prevent the oxidation of of Sulphide solution. It will always remain in reduced form 
( Alos if the shade is slightly greyish, one can add tiny tinge of sulpher blue–> 20gpl) 
in III, IV and V dye bath–> cold wash 
in 6th dye bath. We take H2O2(30%)+Acetic Acid(2:1 by weight). H2O2 acts as an oxidising agent. But as it acts on neutral pH (=7) and after cold bath the solution is slightly alkaline, to make it neutral wil add acetic acid. Acs in alkaline pH, oxidising action of H2O2 will be similar to the bleaching action, which may cause tendering in the fabric. 
7th and 8th Dye Bath: Cold Wash 
Wash Box Number 4: Here washing is done with detergent and soda ash at 60-70 deg.c 
5th and 6th Wash Box: Hot Wash 
7th wash Box: Here softner is added at 25 gpl. It is cationic softener with pH 4.5 to 6.5. As during oxidation of sulphur, strength is reduced by 10%. On a yarn sulphur is of two types : 
1. Free Sulphur 
2. Reacted Sulphur. 
The free sulphur will react with moisture in the atmosphere to form: 
H2O + S –> H2SO4 
Which tenders the yarn. Now at acidic pH reaction is much faster. So we add only a small amount of softener (25 gpl) as against that in indigo which is 100gpl. 
3rd Point 
Over all during sulphur dyeing and storing, the yarn strength is reduced by 15% as compared to Indigo. 
4th point 
If ball formation takes place of sulphur dyed warp at loom shed, then we can taken in 4th dye bath little Na2S+Caustic to reduce the free sulphur.

Technical Considerations in Rope Dyeing for Indigo dyed Denim.

The passage of yarn in rope dyeing is as follows: 
Pre-scouring –>hot wash–>cold wash –> Dye baths–> hot wash–>cold wash–> application of softener 
lets discuss these processes one by one: 
Pre-scouring 
1. The objectives of pre-scouring are the removal of wax content from cotton, removal of trapped air from cotton yarn and Making yarn wet 
2. This is done at 90 o C 
3. We use the following ingredients at pre-scouring stage: 
Caustic Soda: Its quantity depends upon the quality of cotton fibres used in the mixing. Generally we take 2-4% of caustic soda. It removes the wax by the action of soapanification. 
Wetting agent: It is anionic in nature 
Sequestering Agent: Even with the use of water softening, it is very difficult to find the desired softness in water ( about 2-3 ppm) . So we use the agent to make the water soft. 
4. Why Trapped Air should be removed. The reason for this can be understood as follows: 
In 1 kg of yarn, there is approximately 2 litres of air. 1 litre of air decomposes 1.8 litres of Sodium Hydrosulphide. It will cause uneven dyeing and more consumption of Sodium Hydrosulphide ( hydro). 
5. Absorbency of yarn may be checked after scouring. 
Hot wash 
As some caustic is carried by the yarn after pre-scouring, so hot water is given at 70-800C. If this is not done, this yarn will go into the dye-bath which will change the pH of the dye-bath. 
Cold Wash 
After hot wash, yarn temperature is more. To bring it back to its room temperature, cold wash is given to it. 
INDIGO DYEING 

1. Indigo is not a perfect vat color. It may be called a trash vat color. The constant of substantivity for other colors is 30, for indigo it is only 2.7. So there is a need of 5 to 6 dye baths and make the use of multi-dip and multi-nip facility to increase the penetration. 
2. The dyeing is done at room temperature as indigo belongs to Ik class of vat dyes, where dyeing is done at room temperature and oxidation is done by air only and not by chemicals. If oxidizing agents are used, they will cause stripping of colors. 
3. Indigo is not soluble in water. So it is reduced with Sodium Hydrosulphide. Then caustic soda is added to make sodium salt of vat colors to make it soluble. To reduce 1 kg of Indigo, 700 gms of sodium hydrosulphide is required. However some extra SHS needs to be taken to avoid some decomposition of SHS. 
Practically it is prepared in the following sequence 
-Take indigo 
-Add caustic 
-Then reducing agent 
4. When caustic is added to indigo, it is an exothermic reaction. It is allowed to cool down, then before sending it to feeder, sodium hydro-sulphide is added. Reducing agent is not added first as it will be decomposed first, so consumption of it will increase. It is also not advisable to take solubalised vat, as offered by some companies due to the following reasons: 
a. If it is used after 6 months, there will be a decomposition of sod. Hydrosulphide. It will become partially soluble. Then to make it soluble again, more SHS has to be added. 
b. Transportation is difficult 
c. Cost is more 
5. Feeding System 
Rat of flow of yarn is given by 
((No of ropes x no of ends x speed of machine)/ count x 1.693 x 1000) 
in kg of yarn / minute 
So we can determine the rate of feed of indigo. It is very important that replenishment of indigo is there as any variation will result in the change of shade and also if level is more, there is a problem of over-flow. 
6. If total capacity of dye bath for example is 15000 litres, then circulation must be 3 times the volume. If it is less then there are 100% chances of getting a lighter shade. 
7. Core and ring dyeing effect 
This effect is obtained by multidip-multinip facility 
8. pH of the Dye bath should be kept in between 10.5-11.5. At this pH , sodium salt of Indigo is mono phenolic form. At this form, the strike rate of dye is very high. So after washing, there will be a better dye effect. At pH 11.5 to 11.7, at this affinity is less, so dye effect will be less prominent. 
pH is controlled by the addition of caustic soda. 
9. Testing of Hydro 
TOTAL HYDRO 
We take 10 ml of indio with SHS in 30-35 ml of water. It is set for one minute and shaken. As air will decompose SHS. So vacuum created will fetch the water from above. If 3 ml of water is required, then concentration of hydro is 3 gpl. As a thumb rule, concentration of total hydro should be min. 1.5 gpl. 
REDUCED HYDRO 
It is the hydro that is used for the reduction of Indigo. It should be around 0.7 ( 1000 kg of Indigo needs 700 kg of hydro to reduce it). For testing we take 10 ml of dye solution and 30 ml of water and 5-6 drops of 40% formaldehyde and shake it for one minute. The water that goes gives the readings of the reduced hydro. 
Total Hydro- Reduced Hydro = free hydro 
If Total hydro is min. 1.5 gm/lit. then free hydro must be min. 0.5 gms/ litre which acts as buffer 
10. Also hydro reduction capacity is measured by mV meter which measures the Redox Potential. 
It should be around 760-800 
Through the day, the redox potential should be +- 20 mV of the norm. If it is more then the process control is a failure. 
Caustic–> It is around 0.4 to 0.5 times the hydro used. 
Washing 
Rubbing fastness of indigo is very important. On a scale of (1-4), it is 2. Washing is done to improve the rubbing fastness. 
Wash at 60 deg.–> Wash at 60 deg.–> Wash at room temperature–> wash with softener 
Why Softener: 
1. The rope is going to be opened at Long Chain Beamer. It the softener is not used, opening will be hampered. 
2. It is generally 1.2% of the weight of the yarn. It is a cationic softener. It is always having pH in the range of 4 to 55. Softening is done at room temperature. If high temperature is used there is always some chance of tendering of yarn. 
3. Concept of Buffer pH is given by Virkler USA, they say by addition of this, there is 40% less consumption of Indigo for same shade depth. 
4. Metering Consumption 
If solution is of 900 litres 
10% Indigo–>90 litres 
Hydro–> 90*.7 = 63 kg 
Caustic–> 63*0.445= 28 kg.

It belongs to a VAT class of dyes. It has a dark blue color wit a bronze lustre. It belongs to KI class of dyes. In this class, dyeing is done at cold and air oxidation is done to reoxidise the dye. It can be applied on both cellulosic and protein fibres. For protein fibres, a weaker alkaline solution is used. It can be reduced by NaOH and Na2SO4 in water to give monophenolate and biphenolate ions as complete solution. Reduced form of Indigo is called leuco indigo. Leuco has got low affinity for cellulosic fibres. Dye take up can be improved by: 
1. Either mercerisation of cellulosic fibres before by dyeing 
2. or by adopting multidip, squeeze and airing process, so that dye is coated on the fibre layer by layer 
Indigo can be further developed into Halogenated derivatives and sulphonated derivatives. Halogenated derivatives give better fastness properties and brighter shades, whereas sulphonated derivatives gives a soluble blue dye, good dye and is applicable on protein fibres. 
reaction : 
Image 1 
Although indigo is a vat dye, it can be regarded as a ‘trash’ dye, the dyeing and fastness properties are in no way comparable to other class of vat dyes. It is due to these properties, that make it an excellent dye for denim. The on tone fading and the bleach down properties of indigo blue has generated a lot of denim jeans fashions like stone, ice faded looks, etc. These special effects cannot be simulated by the other classes of dyes. 
During Preparatiuon of stock vat, the following points must be remembered: 
1. Vatting temperture should be as close to room temperature as possible. 
2. Stirring should be minumum, unnecessary stirring affects the stability of reduced vat. 
3. Volume of the reduced vat should be kept constant for every stock vat batch, as the constant volume will ensure a constant replenishing amount. 
For rope dyeing system, with chemical replenishment 
with Stock replenishment 
Indigo: NaOH: Na2SO4 :: 1:0.8:0.8 
Without stock replenishment 
indigo: NaOH: Na2SO4:: 1:1:1.2 
Chemical Feeding 
NaOH: Na2SO4: : 1: 1.2-1.6 
ie. for approximately 60 gpl of NaOH–> 120 gpl of hydro is required

Indigo Dyeing process control

1.        Concentration of Hydrosulphite 
It is measured by vatometer. It should be from 1.5 gpl to 2.5gpl , or by redox potential of dye bath which should be from -730 mV to -860 mV. 
2. Caustic Soda or pH value 
Should be from 11.5-12.5 
3. Dye concentration in Dye bath 
it is measured by spectrophotometer. It should be in g/l 
Guidelines 
High Indigo Concentration –> Shade is greener and lighter 
Low Indigo Concentration –> Shade is dull and Red. 
High pH or Caustic Concentration –> Redder and lighter 
Low pH or caustic concentration –> greener and darker 
Dipping Time 
Longer the dipping time, better will be the penetration and lesser will be the ring dyeing effect. It varies from 15-22 seconds. 
Squeeze Pressure 
High pressure will lead to lower wet pick up and result in lesser color and better penetration. At rope dyeing, squeeze pressure is 5-10 tonnes, ie. wet pick up is as low as 60%. Hardness of squeeze roller is about 70-75 deg. shores. It sqeeze rolls are too hard then there are chances of slippage and uneven yarn tension.. If squeeze rollers are too soft then shading will occur. Surface of the squeeze rolls should be ground twice a year. 
Airing Time 
It should be 60-75 seconds. Longer airing time results in high tension on the yarn and subsequent processes will become difficult. 
Drying 
Insufficient or unevenly dried yarns will result in poor rebeaming 
Calculation of Replenishing Dye feed/min 
Conc. of stock vat is g/l= 90 
range speed in yards/min=25 
count = 7s 
totoal ends = 4100 
Wt of yarn dyed /min= (4100*25*1000)/(7*840*202)= 7924 gms 
shade desired = 2% 
Amount of dye to be replenished/min= 158.5 gms 
Effect of pH 
At pH of 10.5 to 11.5, there will be formation of more monophenolate ions, which lead to higher color yield, as strike rate of the dye to the yarn bundle is very high, and wash down activities will be very good. 
At pH higher than this, dye penetration will be less and wash down characteristics are also poor. 
Testing 
1. Alkalanity in Dye Bath Liquor 
Pipet 10.0 ml of vat liquor into 100ml of distilled water in a 150 ml beaker. place under continuous agitation and insert the electrodes of a pH meter caliberated at pH 7.0 with standard buffer solution. 
Titrate with tenth normal HCl ( 0.1 HCl) to pH 7.0 (ml = A) 
calculate 
g/l of NaOH = A *0.40 
2. Hydro in Dye bath Liquor 
Add 2 ml of 37% HCHO to 150 ml beaker. Add 2 ml of dye range liquor . Add 6 ml of 25% glacial acetic acid solution prepared by diluting 1 part acid with 3 parts water. Add 2 ml of starch/KI indicator. Add ml of water. Titrate with 0.046 N ( prepared by diluting 460 ml of 0.1 N Iodine to one liter ) solution until the color changes from emarald green to bluish purple. 
G/l of hydro= mo fo 0.046N of Iodine 
Importance of High Concentration of Free Hydrosulphite 
The clearest shades with minimum reddish streaks are observed at by relatively high conc. of hydrosulphite. On the other side, with lack of hydrosulphite, the leuco indigo is less dissolved and thereby adheres to a greater extent to the fibres. With lack of hydrosulphite furthermore, the amount of unreduced dyestuff by oxidation at the upper level of the liquor and through activiation of unfixed dyestuff, gets separated from the fibrous material would constantly rise as the reducing agent for creating leucoform would be missing. Under these circumstances a reddish bronze like shade results due to dispersion of not reduced dyestuff in the yarn. The min. proportion of hydrosulphite should be around 1.3 to 1.5 gpl in case of rope dyeing and 3-4 gpl in case of sheet dyeing. Also to avoid the lack of hydrosulphite or Indigo at certain places in the immersion, vat, the whole quantity of the liquor should be circulated 2-3 times every hour. 
Reaction Time 
At very short reaction time, an adequate liquor exchange ( i.e. the amount of chemicals consumed and replaced by fresh addition of reduced indigo) is not assured. This has a negative influence on dyeing and depth of dye penetration. In addition to this the time available for diffusion of dyestuff until oxidation commences is too short. To ensure an even and good depth of dye penetration by dyeing in several passages, the reaction time should be 20-30 sec. for each vat (eg. at a speed of 20m/min for a reaciton time of 10 seconds, the immersion path should be maximum 3.3 meters). 
A reaction time exceeding 60 seconds should be avoided as the amount of dyestuff again get reduced and released may again supersede that of additionally take up dye stuff, resulting in higher shades. 
Softening Agent: 8 g/lit 
Drying: Rest humidity should be 30% and then sized. 
Addition of chemicals 
1. Red Tinge: reduce addition of NaOH, increase slightly Na2S2O3 
2. Darkish Red: increase Hydro 
3. Light Greenish: decrease Hydro 
4. Dark Green: Increase Caustic

Indigo dyeing calculations

For 12 ropes, at 24 m/min, of 344 ends of 14000 m length of 7s count. 
wt of yarn = (12*344*14000*100*453.6)/(7*840*36*2.54*1000) kg= 5000 kg 
at 24 m/min, a lot of 14000 m will be completed in 14000/24 = 583.3 min 
at 1.8% shade 
100 kg of yarn needs–> 1.8 kg of Indigo 
5000 kg of yarn needs –> 90 kg of dye 
at 100 gpl 
100 gms of dye = 1 lit of solution 
90 kg of dye = 900 litres 
900 litres should be completed in 583.3 min 
1 litre would be completed in = 583.3/900= 38.8 seconds 
so flow rate will be 38.8 seconds / litre 
Similarly flow rate of caustic and hydro can be determined 
Hydro is taken around 100 gpl 
caustic is taken around 90 to 100 gpl

Indigo preparation sequence

In a tank of 1000 liters: 
a. take 400 litres of water (soft) 
b. add setamol ws–> 4 g/l (stirring) ( dispersing solution) 
c. Add 100 kg of Indigo ( at 1.8 % shade -see the indigo calculations- stirring) 
d. add caustic soda –> stirring ( for solubilising and pH) 
e. allow to cool it for 2/3 hours 
f. Add hydrosulphide ( As reducing agent) 
g. Make the solution to 1000 l by adding water. 
If pH is fluctuating, if it is > 11.7 then hydro is added (2-3 kg), if (<11.2) then caustic is added. 
For 100 kg of Indigo, 
Caustic Required= 90 kg 
Hydro Required= 80 kg

Difference Between Rope Dyeing and Sheet Dyeing

About the author : M Bilal Tariq has worked as a Deputy General Manager in a reputed Denim Mill in Pakistan . He is a textile Engineering Graduate and also maintains a Denim Blog

Important considerations in dyeing with vat dyes

Vat dyes are the primary choice where the highest degree of fastness to industrial laundering, weathering and light are required. Based on temperature, amount of caustic soda, hydrosulphite and salt, used in dyeing, vat dyes can be classified into four main groups:

• IN dyes require high temperature, and a large amount of caustic soda and sodium hydrosulphite;
• IW dyes require medium temperature and a medium amount of caustic soda and sodium hydrosulphite with salt added;
• IK dyes require low temperature and a small amount of caustic soda and sodium hydrosulphite with salt added; and
• IN Special dyes require more caustic soda and higher temperature than IN dyes.

Generally speaking, vat dyes have a very rapid strike, a good degree of exhaustion and a very low rate of diffusion within the fibre. Vat dyes of different chemical structure may differ in the solubility of their sodium leuco-vat, stability towards over-reduction, stability towards over-oxidation, substantivity and rate of diffusion. Commercial competitive dyes have fairly equal particle sizes. Large particle sizes give dispersions of poor stability. For some vat dyes, colour yield decreases with increasing particle size. The effect is generally dye-specific.

The main stages in the dyeing of cotton with vat dyes are as follows:

• Conversion of insoluble vat pigment into soluble sodium leuco-vat anions [reduction]
• Diffusion of sodium leuco-vat anions into cellulosic fibres
• Removal of excess alkali and reducing agents by washing off
• Oxidation of the soluble dye into insoluble pigmentary form within the cellulosic fibres, and
• Soaping, during which the isolated molecules of vat pigments are re-orientated and associate into a different, more crystalline form

Important requirements of vat dye reducing agent are as follows:

• A level of reducing power (reduction potential) sufficient to reduce all commercial vat dyes to their water soluble form, quickly and economically
• Conversion of the vat dyes into products from which the original pigment can be restored (no over-reduction)

Various reducing systems for vat dyes have been proposed and used. The most common type of reducing agent used for dyeing with vat dyes is sodium hydrosulphite, commonly known as hydros but more correctly known as sodium dithionite, which has the chemical formula Na2S2O4. Although a part of the hydros is used up in the reduction of vat dyes, a large part of it may be destroyed by its reaction with oxygen in the air (oxidation), particularly at higher temperatures.

The rate of reduction of vat dyes depends upon various factors, such as the following:

• Particle size of the dye,
• The temperature, time and pH during reduction, and
• Concentration of the reducing agent

The stability of alkaline solutions of reducing agents may decrease with increased temperature, greater exposure to air, greater agitation and lower concentration of the reducing agent. Vat dyes of the Indanthrene type may produce duller or greener shades at dyeing temperatures higher than 60 C due to over-reduction. Over-reduction can be prevented by the use of sodium nitrite if the reducing agent is hydrosulphite. In the case of thiourea oxide, over-reduction cannot be prevented by nitrite.

The factors influencing the rate of dyeing with vat dyes include the following:

• Type of the substrate,
• Temperature,
• Liquor ratio,
• Concentration of dye, and
• Concentration of electrolyte

Mercerized cotton gives a higher rate of dyeing as compared to un-mercerized cotton which in turn gives higher rate than the grey material. At low temperature, the rate of exhaustion is low which might promote levelness but the rate of diffusion is also low. At high temperature, the rate of exhaustion is high which might decrease levelness but the rate of diffusion is high. Maximum exhaustion, penetration and levelness can be obtained by starting the dyeing at low temperatures in the leuco stage and slowly raising the temperature. Some dyes may not be stable to very high temperatures, so the stability of dyes to temperature must be taken into account. The reducing efficiency of sodium hydrosulphite in caustic soda solutions at high temperatures decreases rapidly in the presence of air. The higher the liquor ratio, the slower is the rate of dyeing. Most of the dyes exhaust more rapidly at low concentrations, increasing the risk of unlevel dyeing in light shades. Some have the same rate of dyeing irrespective of the concentration. The higher the concentration of electrolyte, the higher is the rate of dyeing.

The purpose of rinsing before oxidation is to remove any loose dye, excess of reducing agent and alkali to lower the pH and establish conditions favourable for oxidation. The higher the temperature and/or pH of the rinsing bath, the lower is the colour strength. Very high pH and temperature during rinsing may also result in the dulling of the shade. The ideal is to do rinsing thoroughly at low temperature at a rinsing bath pH value of 7.

The purpose of oxidation is to convert the water-soluble leuco from of the vat dye, back into the insoluble pigment form. The important variables for the oxidizing step are:

• The type and concentration of oxidising agent,
• The type of pH regulator and pH during oxidation, and
• Temperature during oxidation

The oxidizing agent must provide a level of oxidation potential sufficient to oxidize the reduced vat dye into insoluble pigment, with no over-oxidation i.e., beyond the oxidation state of the original pigmentary form of the dye. Poor control of pH during oxidation may result in uneven oxidation and a lower temperature may result in slower oxidation. A pH below 7.5 should be avoided to prevent the possible formation of acid leuco form of vat dyes. The optimum pH for oxidation is 7.5-8.5. The acid leuco form of vat dye is difficult to oxidize, has little affinity for fibre and is easily rinsed out. The higher the temperature, the faster is the oxidation, the optimum temperature being 120-140 F.

The purpose of soaping after oxidation is to remove any dye that is not diffused into the fibre and to stabilise the final shade. This results in improved fastness properties and resistance to any shade change to a resin or other finish or to consumer use. Important soaping parameters are:

• Time,
• Temperature,
• Type and concentration of soaping auxiliaries

Even when no detergent is used, the dyeings exhibit good colour strength and good fastness properties. Washing with water alone tends to give a slightly higher colour yield. It is best to carry out soaping without any detergent at boiling temperature. After soaping the fabric is rinsed and dried Both exhaust and continuous dyeing methods are used to apply vat dyes. Exhaust dyeing processes are mainly used for dyeing of loose stock, yarn and knitted fabrics. Woven fabrics can also be dyed by exhaust method but for large batch sizes, the continuous method is mostly used.

Pad dyeing methods are usually a preference in case of woven fabrics particularly, if these are in large batches. The commonly used pad dyeing methods are pad-jig, pad steam and pad thermosol. The most popular method for dyeing woven fabrics in a continuous manner is pad-dry-pad-steam method, consisting of the following key steps:

• Impregnation of the fabric in a bath containing vat dye, dispersing agent, anti-migrant and a non-foaming wetting agent
• Squeezing the impregnated fabric to a given pick up level
• Drying the fabric to achieve a uniform distribution of the vat pigment throughout the fabric
• Impregnating the fabric with a solution of caustic soda and sodium hydrosulphite, with the optional use of salt
• Expressing the impregnated fabric to a given pick up level
• Steaming the fabric to bring about reduction of the dye to the soluble leuco form and to promote diffusion of the dye into the cellulosic fibres, and
• Rinsing, oxidation, soaping, rinsing and drying the fabric

Intermediate drying is one of the most important steps in the pad-dry-pad-steam process where the most common problem migration can take place. Important factors on which migration depends are as follows:

• Dye constitution,
• Dye formulation,
• Pick-up,
• Additives in the dye padder,
• Residues of wetting agents and lubricants on the fabric,
• Fabric structure, and
• Drying conditions

After drying, the fabric is padded with an alkaline solution of sodium hydrosulphite, after which the fabric undergoes steaming. Almost 40 % of vat dyeing problems are related to improper steaming conditions. Ideal steaming conditions are controlled temperature and moisture, freedom from air, and sufficient dwell time. After steaming, the fabric undergoes rinsing, oxidation and soaping.

The most important control steps in vat dyeing are reduction, absorption and oxidation. The reduction and oxidation can best be controlled by metered addition of chemicals. The advantages of metered addition of hydrosulphite are as follows:

• Better levelling by slower vatting
• No need of levelling agent
• Protection from over-reduction
• Control of initial rate of dyeing (strike)
• Possibility of warm pre-pigmentation to give optimum fabric/liquor movement
• Good reproducibility
• Reduction of sulphite/sulphate effluent pollution, and
• Automatic monitoring of vat state and the redox potential by means of measuring and regulating technology

The dosage of hydrogen peroxide in the oxidation tank, by measuring and controlling the pH has the advantages of constant pH during the production run, the presence of enough peroxide for oxidation and controlled speed of oxidation.