Textile Notes related to fiber, yarn, fabric knowledge, spinning, weaving, processing, projects, knitting, Indian Traditional Textiles and denim manufacturing
Textile material can be dyed using batch, continuous or semi continuous process.
Batch processes are the most common method used to dye textile materials. There are three general types of batch dyeing machines:
In which fabric is circulated
In which dye bath is circulated
In which both the bath and material is circulated.
Jet dyeing is the best example of a machine that circulated both the fabric and the dyebath. Jet dyeing is used for knitted fabrics. For Terry-towels soft flow dyeing is use.
In jet dyeing machine the fabric is transported by a high speed jet of dye liquid.
As seen in the figure, this pressure is created by venturi. A powerful pump circulates the dyed bath through a heat exchanger and the cloth chamber. Cloth guide tube helps in circulation of fabric.
The vigorous agitation of fabric and dye formulation in the cloth increases the dyeing rate and uniformity. It minimizes creasing as the fabric is not held in any one configuration for very long. The lower liquor ration allows shorter dye cycles and saves chemicals and energy.
In soft flow dyeing machines the fabric is transported by a stream of dye liquor. However, the transport is
assisted by a driven lifter reel.
These machines use a jet having lower velocity that that used on conventional jet dyeing machines.
The soft flow machines are more gentle on the fabric than conventional jet machines.
The following are the features of a soft flow U-Type dyeing machine offered by Taxfab:
1. Machine pressure vessel and major wet parts made of stainless steel AISI 316/ 316 L, highly corrosion resistance material.
2. Heavy duty stainless steel centrifugal pump for optional dye liquor circulation. Highly efficient heat exchanger for fast heating and cooling.
3. A stainless steel filtering device placed in such a way for easy cleaning.
4. A unique design of jet nozzle can provide high discharge of liquore with subsequent pressure to ensure fast movement of fabric transport upto 300 Mtrs / Min., and the speed of fabric can be adjusted, required to desire quality.
5. A mirror polished fabric transport perforated basket for easy trouble free movement of fabric from back to the front of machine, perforated basket fabricated in such a way that welding part does not come in contact with fabric.
6. For preparing chemical, colour kitchen tank is provided made out of stainless steel 316, with required valves for auto dozing.
7. All valves is made of investment casting and is of stainless steel 3l6.
8.Electrical control panel with microprocessor to operate the machine is provided with pneumatic control circuits.
9. Magnetic level indicator duly calibrated for correct liquor measurement.
10.Take off reel with direct couple geared motor and stainless steel structure
11. All safety device required for a pressure vessel is incorporated with the machine.
A front view and side of the machine offered by them is as given below:
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Is Indigo Natural or Synthetic? How Was Indigo Produced?
The answer to the first part of the question is simple: both. Indigo began its textile journey as a natural dye obtained from plants. Later, with the development of modern chemistry, the same blue colouring substance began to be manufactured synthetically.
Today, most of the indigo used in the textile industry, especially in denim, is synthetic. But historically and culturally, indigo is deeply associated with natural dyeing traditions of India, China and many other parts of the world.
Indigo belongs to the category of water-insoluble dyes. More specifically, it is a vat dye. This means that indigo, in its blue form, does not dissolve in water and cannot be applied to fabric directly like many other dyes. It has to be chemically converted into a soluble form before dyeing, and then converted back into the blue insoluble form on the fibre.
Indigo is one of the oldest dyes known to human civilization. It is believed to have been used for dyeing in India and China from very early times, possibly as far back as 2000 BC. The name “indigo” itself is connected with India. The Greeks and Romans referred to it as a blue dye coming from India, often described as “Indian blue.”
Originally, indigo was obtained from plants, especially from species such as Indigofera tinctoria. The leaves of the plant contained the precursor of the dye, but not the ready-made blue dye in a simple usable form. The blue colour had to be developed through fermentation, reduction and oxidation.
This is what makes indigo so fascinating. It is not merely a colouring matter extracted like juice from a plant. It is a dye that requires chemical transformation before it becomes useful.
Chemical Structure of Indigo
The main colouring compound in indigo is called indigotin. Its molecular formula is:
\[
C_{16}H_{10}N_2O_2
\]
The structure of indigo consists of two indole-like units joined together through a central double bond. It also contains two carbonyl groups \((C=O)\) and two nitrogen atoms. A simplified way to understand the molecule is:
The indigo molecule is highly conjugated. This means that electrons are spread over a large part of the molecule. This extended conjugation is responsible for the absorption of visible light and the characteristic deep blue colour of indigo.
Visual 1: Chemical structure of indigo, \(C_{16}H_{10}N_2O_2\).
This structure also explains why indigo is stable and water-insoluble. For dyeing, the molecule has to be temporarily converted into another form.
How Natural Indigo Was Produced
Natural indigo was traditionally obtained from the leaves and stems of indigo plants. The harvested plant material was placed in a vat filled with water. In older traditional processes, urine or other alkaline fermenting materials could also be used. Fermentation was then allowed to take place.
Inside the plant, the main precursor of indigo is indican, a colourless glucoside. During fermentation, indican breaks down into indoxyl and glucose.
This step is important because the plant does not directly give a strong blue dye. It first gives indoxyl, which is the immediate precursor of indigo. When the fermented liquid is stirred or beaten with poles, oxygen from the air enters the liquid. This oxygen oxidises indoxyl into indigo.
As indigo is insoluble in water, the blue particles begin to separate out and settle at the bottom of the vat. The liquid above is drained off, and the remaining blue sludge or mash is collected. This mash is then dried in the open air and sold in the form of pressed cakes, lumps or powder.
This traditional method explains why indigo production was both an agricultural activity and a chemical process. The farmer grew the plant, but the dyer or processor had to understand fermentation, aeration, settling and drying.
Visual 2: Natural indigo production from leaves to indigo cake or powder.
Indigo as a Vat Dye
Indigo is insoluble in its blue form. Therefore, to dye fabric with indigo, it must first be reduced into a soluble form called leuco-indigo. This reduced form is pale yellowish or greenish and can dissolve in an alkaline dye bath.
In this reduced form, indigo can enter or deposit onto the fibre. When the yarn or fabric is removed from the dye bath and exposed to air, oxygen converts leuco-indigo back into blue indigo.
This is the magical moment in indigo dyeing. The material may come out of the vat looking yellowish-green, but slowly turns blue as it reacts with oxygen in the air.
Practical textile point: This conversion from insoluble blue indigo to soluble leuco-indigo, and then back to insoluble blue indigo, is the heart of vat dyeing.
Visual 3: Indigo vat dyeing cycle: reduction, dyeing and oxidation.
Why Stirring Was Important
In traditional indigo production, the fermented mass was stirred or beaten with poles. This was not merely a mechanical operation. It had a chemical purpose.
During fermentation, the precursor compounds were converted into reduced or reactive forms. When the liquid was stirred, air entered the vat. The oxygen in the air converted indoxyl into indigo. Since indigo is insoluble, it appeared as blue particles and settled at the bottom.
\[
\text{Fermentation develops the precursor}
\]
\[
\text{Stirring introduces oxygen}
\]
\[
\text{Oxygen converts the precursor into blue indigo}
\]
\[
\text{Insoluble indigo settles at the bottom}
\]
This is why the process required both patience and skill. Too little fermentation, too much fermentation, insufficient aeration or poor settling could all affect the quality of the final dye.
From Natural Indigo to Synthetic Indigo
Thus, indigo began as a natural dye. For centuries, India was one of the important sources of natural indigo. Indigo was exported as a valuable dye material, and the term “Indian blue” became associated with it.
However, in the nineteenth century, European chemists began studying the structure and synthesis of indigo. Adolf von Baeyer made major contributions to the chemistry of indigo and succeeded in synthesising it in the laboratory. Later, in 1897, BASF began industrial-scale production of synthetic indigo.
This changed the dye industry completely. Synthetic indigo gave manufacturers a more consistent, predictable and scalable source of blue dye. Natural indigo depended on crop quality, climate, fermentation conditions and extraction skill. Synthetic indigo could be produced in large quantities with more uniform strength and shade.
Over time, synthetic indigo almost completely replaced natural indigo in large-scale textile production.
Natural Indigo vs Synthetic Indigo
Point
Natural Indigo
Synthetic Indigo
Source
Plant-based, mainly from indigo-bearing plants such as Indigofera tinctoria.
Manufactured chemically.
Main colouring compound
Indigotin.
Indigotin.
Molecular formula
\(C_{16}H_{10}N_2O_2\)
\(C_{16}H_{10}N_2O_2\)
Consistency
May vary from batch to batch.
More uniform and predictable.
Scale
Suitable for craft, heritage and natural dyeing.
Suitable for industrial denim production.
Process control
Depends on fermentation and extraction.
Depends on chemical manufacturing controls.
Present use
Niche use in natural dyeing, handcraft and sustainable fashion.
Dominant in denim and industrial textile dyeing.
The important point is that the main blue molecule is the same: indigotin. The difference lies mainly in the source, impurities, production method, consistency and environmental profile.
Indigo and Denim
Today, indigo is most strongly associated with denim. Cotton warp yarns are dyed with indigo, while the weft often remains undyed or lightly coloured. In denim dyeing, yarns are repeatedly dipped into the reduced indigo bath and then exposed to air for oxidation.
This repeated dipping and oxidation builds up the blue shade gradually. One interesting property of indigo dyeing is that the dye often remains more concentrated near the surface of the yarn rather than penetrating fully into the core. This is one reason denim fades beautifully with wear.
As the outer surface of the yarn is abraded, some of the indigo is removed, revealing lighter areas. This gives denim its characteristic worn, faded and aged appearance. So, in denim, fading is not always a defect. It is often part of the desired aesthetic.
A Small Correction to Understand the Old Process Better
In older descriptions, it is sometimes said that hydrogen was created by microorganisms and acted as a reducing agent. This is a simplified way of explaining the process. More accurately, fermentation creates reducing conditions in the vat. These reducing conditions help convert dye precursors into forms that can later be oxidised into indigo.
Similarly, when the liquid is stirred, the aim is to introduce oxygen. Oxygen converts the soluble or reduced precursor into insoluble blue indigo. The blue particles then settle at the bottom.
\[
\text{Reduction during fermentation}
\]
\[
\text{Oxidation during beating or aeration}
\]
This reduction-oxidation cycle is also central to indigo dyeing on fabric.
Conclusion
Indigo is both natural and synthetic. Historically, it was obtained from plants and processed through fermentation, aeration, settling and drying. Later, chemists discovered how to produce the same blue colouring substance synthetically. With industrial production by BASF in the late nineteenth century, synthetic indigo gradually replaced natural indigo in most commercial textile applications.
The beauty of indigo lies in its chemistry. It is a water-insoluble vat dye. It must first be reduced to a soluble leuco form, then applied to the fibre, and finally oxidised back to the blue insoluble form.
This is why indigo is not just a dye. It is a story of plants, fermentation, chemistry, trade, denim and textile technology.
Related Reading on Indigo, Natural Dyes and Dyeing
This article is intended for educational and general textile knowledge purposes only. The chemical reactions shown are simplified to explain the main principles of natural indigo formation and vat dyeing. Actual indigo extraction, dye reduction, denim dyeing, effluent treatment and chemical handling require proper technical knowledge, safety precautions, process control and laboratory or mill-level validation.
Redder- Increase the conc. of Caustic , slightly decrease the conc. of Hydro Redder, Duller- Increase the con. of hydro Greener, Paler- decrease the con. of hydro Greener, duller- Increase the con. of caustic Bronzing- Increasing the con. of Hydro
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.
Practical 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.
