Sustainable Dyeing Process in Textile Industry

Sustainable Dyeing Process in Textile Industry

Devanshi Goyal
3rd Year, Textile Design
School of Fashion Technology, Pune, India
Email: devanshi.g19@gmail.com

Introduction:
Natural dyes find use in the colouring of textiles, drugs, cosmetics, etc. Owing to their non-toxic effects, they are also used for colouring various food products. In India, there are more than 450 plants that can yield dyes. In addition to their dye-yielding characteristics, some of these plants also possess medicinal value. Though there is a large plant resource base, little has been exploited so far. Due to the lack of availability of precise technical knowledge on the extracting and dyeing technique, it has not commercially succeeded like the synthetic dyes.

Although plants exhibit a wide range of colours, not all of these pigments can be used as dyes. Some do not dissolve in water, some cannot be adsorbed onto fibres, whereas others fade when washed or exposed to air or sunlight. It remains a mystery, why plants reward us with vibrant dyes.

Primitive dyeing techniques included sticking plants to fabric or rubbing crushed pigments into cloth. The methods became more sophisticated with time and techniques using natural dyes from crushed fruits, berries and other plants, which were boiled into the fabric and which gave light and water fastness (resistance), were developed.

Sources of Natural Dyes and Natural Mordants:
Natural dyes can be sorted into three categories: natural dyes obtained from plants, animals and minerals.

Although some fabrics such as silk and wool can be coloured simply by being dipped in the dye, others such as cotton require a mordant.

When dyes need mordants for fixation or adherence to the fabric they are called as Adjective dyes. Common mordants are

• Alum (usually used with cream of tartar, which helps evenness and brightens slightly)
  • Potassium Aluminium Sulphate (PAS or alum) can be used on animal fibres. It requires a hot mordanting bath. When combined with tannins, it works on plant fibres, too.
  • Aluminium Acetate is a cold mordant that works best on cellulosic fibers and silk.
• Iron (or copper) (which saddens or darken colours, bringing out green shades) Iron sulfate comes in form of iron crystals. It is best used on plant fibres. It should be used sparsely on animal fibres, as it might deteriorate them. Iron acetate is a much gentler choice for dyeing wool and silk, but it can’t be purchased, only homemade. Iron acetate has a short shelf life and takes ca. 2 weeks to develop at home, using steel wool and white vinegar.
• Tin (usually used with cream of tartar, which blooms or brightens colours, especially reds, oranges and yellows)

Copper, tin, and chrome are some of the other metals that can be used in natural dyeing. They might be potentially unsafe to humans or ecosystems.

Blue vitriol (which saddens colours and brings out greens shades)

There are other substances that allow for better uptake of plant colour, like, for example, soy milk. But in contrast to mordants, soy milk does not form a permanent bond with the fibres and dyes. Rather than a chemical connection, it works as a sort of mechanical “protein film” that layers on top of the fibre and glues the dye and the fibre. This “glue” is soluble and might degrade over time. Therefore, soy milk is not a mordant, it is a binder.

Soy Milk: Soy milk is a great alternative for dyers, who don’t sell their work and don’t care about the extreme longevity of the colour. The connection it forms with fibres is not fully stable and might degrade when exposed to sunlight or washed. It is a good way of improving colour uptake, though, and it’s safe to use around kids.

Tannins: Another alternative to metal mordants is tannins. They can be found in many leaves, seeds, roots, and stems. Just like soy milk, they are not able to form any insoluble bonds with fibres, but they might improve light fastness. They will usually stay true or might even darken when exposed to sunlight but bleach if vigorously washed.

There are three types of mordant:

1. Metallic mordants: Metal salts of aluminium, chromium, iron, copper and tin are used.
2. Tannins: Myrobalan and sumach are commonly used in the textile industry.
3. Oil mordants: These are mainly used in dyeing turkey red colour from madder. The main function of the oil mordant is to form a complex with alum used as the main mordent. If they need no mordants, such as lichens and walnut hulls, they are called substantive dyes.

Singeing:
Singeing is a textile finishing process commonly used to remove protruding fibres from the surface of fabrics, resulting in a smoother appearance. While the traditional singeing process involves the use of an open flame or heated metal plates, sustainable alternatives to achieve similar results with reduced environmental impact, sustainable Indian alternatives to traditional singeing:

1. High-Pressure Water Jet Singeing: This method uses high-pressure water jets to remove protruding fibres and create a smoother fabric surface. It is considered environmentally friendly as it eliminates the need for an open flame and reduces energy consumption.
2. Air Jet Singeing: Air jet singeing involves using compressed air to remove loose fibres from the fabric surface. It is energy- efficient and can be a sustainable alternative to flame singeing.
3. Laser Technology: Laser technology can be used for precise fibre removal without the need for heat or flames. It offers control and reduces the risk of fabric damage. Laser singeing is an eco-friendly alternative.
4. Mechanical Singeing: Mechanical methods, such as mechanical brushing or shearing, can be employed to remove protruding fibres. While these methods may still require energy, they eliminate the need for open flames.
5. Plasma Treatment: Plasma treatment can modify the fabric’s surface and remove loose fibres without the use of heat or flames. It is an eco-friendly and precise method.

Desizing:
Desizing is a process in textile manufacturing that removes sizing agents applied to warp yarns during the weaving process. These sizing agents improve the weaving efficiency and prevent yarn breakage. In traditional textile processing, desizing often involves the use of chemicals and water.

Some alternatives to traditional desizing methods:

1. Fermentation: Fermentation was a common method used for desizing textiles. Fabrics were soaked in water and allowed to ferment, often using natural fermenting agents like sour buttermilk or rice water. The natural enzymes produced during fermentation helped break down the sizing agents.
2. Fermented Rice Water: Rice water, obtained by soaking rice in water and allowing it to ferment for a few days, was used as a desizing agent. Fabrics were soaked in the fermented rice water, which contained natural enzymes that assisted in breaking down the sizing materials.
3. Boiling with Alkali: Fabrics were boiled in water with the addition of alkaline substances such as wood ash or lime. The alkaline environment helped in breaking down the starch or sizing agents on the fabric.
4. Boiling with Soapnut (Reetha): Soapnut, a natural plant-derived soap, was used to prepare a solution that fabrics were boiled in. The saponins in soapnut helped in breaking down the sizing agents on the fabric.
5. Boiling with Enzyme-Rich Plants: Some plants with natural enzymes, like papaya, were used to prepare solutions in which fabrics were boiled. The enzymes from these plants assisted in desizing the fabric.
6. Clay Desizing: A mixture of clay and water was used to soak fabrics. The clay particles helped in breaking down the sizing materials, and the fabrics were rinsed thoroughly to remove the desized sizing agents.
7. Fermented Jaggery Solution: Jaggery, a traditional non-centrifugal cane sugar, was used to prepare a fermented solution in which fabrics were soaked. The fermentation process helped in breaking down the sizing agents.

Enzyme desizing:

Amylase Enzymes: These enzymes can effectively break down starch-based sizing agents commonly used in textiles. Enzyme desizing is a more eco-friendly option as it reduces the need for harsh chemicals and conserves water. It is suitable for various fabrics, including cotton and blends.

Mechanical methods:

1. Ultrasonic Desizing: Ultrasonic technology can help disintegrate and remove sizing agents from textiles using sound waves. It reduces the need for chemicals and water in the desizing process.
2. Steam Desizing: Steam Treatment: Steam can be used to soften and remove sizing agents from textiles. This method requires less water than traditional washing processes.
3. Supercritical Carbon Dioxide (SCCO2) Desizing: SCCO2 Processing: In SCCO2 desizing, supercritical carbon dioxide is used to remove sizing agents. It’s a clean and efficient method that minimizes water usage and chemical waste.

Scouring:
Scouring is a crucial step in textile processing that involves the removal of impurities, oils, and natural waxes from fibres or fabric. This process often uses significant amounts of water and chemicals, making it essential to explore sustainable alternatives, some sustainable Indian alternatives to traditional scouring processes:

1. Boiling with Alkaline Substances: Fabrics were boiled in water along with alkaline substances such as wood ash, soda ash, or slaked lime. The alkaline environment helped in breaking down and removing natural impurities and oils from the fabric.
2. Boiling with Soapnut (Reetha): Soapnut, a natural plant-derived soap, was used to prepare a solution in which fabrics were boiled. The saponins in soapnut helped in breaking down impurities and oils from the fabric.
3. Fermented Jaggery Solution: Jaggery, a traditional non-centrifugal cane sugar, was used to prepare a fermented solution in which fabrics were soaked and boiled. The fermentation process helped in breaking down impurities and oils.
4. Boiling with Plant Extracts: Extracts from certain plants with natural cleansing properties, such as neem leaves, were used to prepare solutions for boiling fabrics. These plant extracts assisted in the scouring process.
5. Clay Scouring: A mixture of clay and water was used to soak fabrics. The clay particles helped in breaking down and removing impurities from the fabric, and the fabrics were rinsed thoroughly.
6. Fermentation with Cow Urine: In some traditional practices, cow urine was used in combination with water to ferment and prepare a solution in which fabrics were soaked and boiled. The fermentation process was believed to aid in scouring.
7. Rinse with Running Water: Fabrics were rinsed thoroughly with running water in natural water sources like rivers or streams. The mechanical action of flowing water helped in removing impurities from the fabric.
8. Supercritical Fluid Scouring: Supercritical carbon dioxide (SCCO2) can be used as an alternative to water in scouring. It is non-toxic, recyclable, and energy-efficient, making it a sustainable option for certain textiles.
9. Ultrasonic Cleaning: Ultrasonic technology generates high-frequency sound waves to create tiny bubbles in water, which agitate and clean the fabric. This process reduces water consumption and the need for chemicals.
10. Sustainable Detergents: Using biodegradable and environmentally friendly detergents can reduce the environmental impact of scouring. These detergents break down more easily in water and are less harmful to ecosystems.
11. Combined Scouring and Dyeing: Implementing integrated processes that combine scouring and dyeing can reduce water and energy usage by eliminating intermediate washing steps.
12. Water Recycling and Closed-Loop Systems: Textile manufacturers can implement closed-loop water systems to recycle and reuse water in the scouring process, reducing overall water consumption.
13. Green Chemistry: Using sustainable chemicals that are less toxic and have lower environmental impacts can make traditional scouring processes more eco-friendly.
14. Low-Temperature Scouring: Lowering the scouring temperature can reduce energy consumption while still effectively removing impurities.

Bleaching:
Bleaching is a textile processing step that is used to remove natural colour or stains from textiles and to achieve a uniform white or light-coloured appearance. However, bleaching processes often involve the use of harsh chemicals like chlorine-based bleaches, which can have negative environmental and health impacts there is a growing emphasis on sustainable and eco-friendly alternatives to bleaching, some methods alternatives to bleaching processes:

• Sunlight Bleaching: Sunlight has been traditionally used to bleach textiles. Fabrics are spread out in open areas to be naturally bleached by the sun’s rays, which have a bleaching effect on the fibres.
• Rice Water: Rice water, the starchy water left after washing rice, was traditionally used as a natural bleach for fabrics. Fabrics were soaked in rice water and then exposed to sunlight for bleaching.
• Bleaching with Ash: Wood ash or ash derived from burnt plant materials was used to bleach textiles. Fabrics were soaked in a solution made by boiling ash in water, followed by thorough rinsing and drying in the sun.
• Plant-Based Bleaching Agents: Certain plant materials, like soapnut (reetha) and Indian madder root (manjishtha), were used to prepare natural bleaching solutions for fabrics. These solutions were used to treat textiles before dyeing.
• Fermented Rice Water: Fermented rice water, obtained by allowing rice water to ferment for a few days, was used to bleach and prepare fabrics for dyeing. The acidic properties of the fermented rice water helped in the bleaching process.
• Drying in Dew: Fabrics were sometimes bleached by spreading them out in the open overnight to absorb dew, which was believed to have a natural bleaching effect.
• Boiling in Water with Turmeric: Fabrics were boiled in water with turmeric, a traditional practice believe to have bleaching properties. Turmeric is also known for its antiseptic and skin-brightening qualities.
• Oxygen-Based Bleaching Agents: Hydrogen Peroxide: Hydrogen peroxide is an eco-friendly alternative to chlorine-based bleach. It breaks down into water and oxygen during bleaching, leaving no harmful residues behind.
• Enzyme-Based Bleaching: Enzymes like peroxidases can be used to break down fabric colour and stains. Enzymatic bleaching is more environmentally friendly and gentler on fibres compared to chemical bleaching.
• Biological Bleaching: Certain microorganisms can be employed to bleach textiles naturally. This method is eco-friendly and reduces the need for chemicals.
• Electrochemical Bleaching: Electrochemical methods can be used to achieve bleaching effects without the need for harsh chemicals. These methods use electricity and specialized equipment to modify fabric surfaces.
• Sustainable Dyeing and Fabric Selection: Selecting fabrics that do not require extensive bleaching to achieve the desired colour can reduce the need for bleaching processes during production.
• Effluent Treatment and Recycling: Implementing effective effluent treatment and water recycling systems can help manage the environmental impact of bleaching processes.

Mercerization:
Mercerization is a textile finishing process that strengthens and enhances the lustre of cotton and other cellulosic fibres by treating them with a caustic soda solution. While mercerization can improve the appearance and performance of cotton fabrics, it can involve the use of chemicals and significant energy consumption, for a true mercerization process, caustic soda (sodium hydroxide) is a critical component, and it’s not a traditional or natural substance. Some alternatives to mercerizing:

• Bio-Enzyme Treatment: Enzymatic treatments using cellulase enzymes can provide similar surface enhancements to mercerization. These enzymes soften the fabric, improve luster, and reduce the need for harsh chemicals.
• Tencel (Lyocell) Fibre Blending: Blending cotton with Tencel (Lyocell) fibres can improve the fabric’s luster and strength while maintaining eco-friendly properties. Tencel is made from sustainably sourced wood pulp and is known for its smooth, shiny appearance.
• Mechanical Finishing: Mechanical treatments, such as brushing or shearing, can be used to enhance the fabric’s surface appearance without the need for chemical processes.
• Optical Brightening Agents (OBAs): OBAs are chemicals that can be used to enhance the brightness and whiteness of textiles. While they are not an alternative to mercerization, they can help achieve similar visual effects without caustic soda treatment.
• Eco-Friendly Mercerization: If mercerization is deemed necessary, adopting eco-friendly mercerization processes that use less caustic soda and optimize energy consumption can reduce the environmental impact.
• Effluent Treatment and Recycling: Implementing effective effluent treatment and water recycling systems can help manage the environmental impact of mercerization processes.
• Leveraging Natural Properties: Leveraging the natural properties of different cotton varieties and blends can reduce the reliance on mercerization for desired fabric characteristics.

Acid dyes:
Acid dyes are synthetic water-soluble dyes that are typically used for dyeing protein-based fibres like wool, silk, and nylon.

• Fermented Plant Extracts: Fermented extracts from certain plants were used to create an acidic environment for dyeing. These plant extracts were often rich in tannins, which have acidic properties.
• Fermented Fruit Juices: Fermented juices from fruits or vegetables with natural acidic properties, such as pomegranate, were used as mordants or additives to create an acidic pH for dyeing.
• Boiling with Vinegar: Fabrics were sometimes boiled with vinegar, a natural acidic substance, to create the required acidic conditions for dyeing with natural dyes.
• Sour Buttermilk: Sour buttermilk, which has natural acidity due to fermentation, was used to treat fabrics before dyeing with natural dyes. The acid in buttermilk helped in setting the natural colours.
• Indigo (Indigofera tinctoria): Indigo is one of the oldest and most widely used natural dyes in textiles worldwide. It produces various shades of blue and is often used in Banarasi sarees for its richness and depth.
• Madder Root (Rubia tinctorum): Madder root is a plant-based dye that produces shades of red, ranging from pale pinks to deep reds.
• Turmeric (Curcuma longa): Turmeric is a common spice that also serves as a natural dye. It produces shades of yellow.
• Pomegranate Rind: Pomegranate rind is used to produce various shades of yellow and brown. It is known for its natural and subtle hues.
• Onion Skin: Onion skins can be used to achieve warm shades of yellow and orange. They are readily available and eco-friendly.
• Henna (Lawsonia inermis): Henna, primarily known for its use in temporary tattoos and hair dye, can also produce orange to brown tones when used as a fabric dye.
• Manjishtha (Rubia cordifolia): Manjishtha, a medicinal plant, is used to produce shades of red. It is valued for its natural and gentle colour.
• Catechu (Acacia catechu): Catechu, obtained from the heartwood of Acacia catechu, is used to produce shades of brown and is historically used in Indian textiles.
• Alizarin (obtained from the roots of the madder plant): Alizarin is a red dye obtained from the roots of the madder plant and has been historically used to achieve red hues in textiles.

Direct dyes:
Direct dyes are water-soluble dyes that are typically used for dyeing cellulose-based fibres such as cotton, rayon, and other plant-based fibres.

• Turmeric: Turmeric has been traditionally used to obtain yellow and orange hues on fabrics.
• Henna: Henna leaves can produce shades of orange and brown, often used for dyeing fabrics and as a natural dye for skin and hair.
• Onion Skin: Onion skins were used to achieve warm shades of yellow and orange, and they are readily available.
• Beetroot: Beetroot extract can provide pink to red hues when used as a dye. Pomegranate Rind: Pomegranate rind extract was used to achieve yellow and brown tones. Sandalwood: Sandalwood extract was used for light yellows and oranges.
• Myrobalan: Myrobalan fruit and seeds were used to achieve shades of yellow and brown.
• Marigold Flowers: Marigold petals were used to achieve bright yellow and orange hues.
• Hibiscus Flowers: Hibiscus petals were used to obtain red shades.
• Iron (ferrous sulfate): Iron was used as a mordant to shift colours towards darker shades and create unique effects in dyeing.
• Fermentation: Natural fermentation processes were utilized to create an acidic environment suitable for dyeing and achieve direct dyeing effects.

Vat dyes:
Vat dyes are a type of dye that is typically used for dyeing natural fibres such as cotton.

Natural vat dyes are dyes derived from plant-based or natural sources that are used in a similar way to traditional vat dyes. These dyes have the ability to be reduced to a water-soluble form and then oxidized to an insoluble form on the fabric, providing excellent colour fastness. Some commonly used natural vat dyes:

• Indigo: Indigo is one of the most well-known and widely used natural vat dyes. It produces a range of blue shades and has been used for centuries in various cultures. It’s derived from the leaves of the indigo plant.
• Tyrian Purple: Tyrian purple is a historic natural dye obtained from certain species of sea snails. It has a rich purple colour and was highly valued in ancient civilizations.
• Logwood: Logwood is a natural dye obtained from the heartwood of the logwood tree. It produces various shades of purple, blue, and gray.
• Madder: Madder is a plant-based dye derived from the roots of the madder plant. It produces shades of red, orange, and brown.
• Turmeric: Turmeric is a spice derived from the turmeric plant’s rhizomes. It produces a vibrant yellow colour and is often used as a natural dye.
• Weld: Weld is a plant that yields a bright yellow dye. It has been used historically for its yellow dyeing properties.
• Pomegranate: Pomegranate peels can be used to create yellow and brown colours. It’s a natural dye obtained from the outer covering of pomegranate fruit.
• Onion Skins: The outer skins of onions can produce various shades of yellow and orange when used as a dye.
• Quebracho: Quebracho is a natural dye derived from the quebracho tree’s bark. It yields shades of red and brown.
• Walnut: Walnut hulls or shells can be used to create brown to black natural dye.

Basic dyes:
Natural basic dyes are dyes derived from plant-based or natural sources that possess basic properties, meaning they can be used as bases for dyeing various materials, particularly textiles. These dyes are typically alkaline in nature and are often used in conjunction with mordants to enhance colourfastness and achieve different shades. Some examples of commonly used natural basic dyes:

• Turmeric: Turmeric is a bright yellow-orange dye derived from the rhizomes of the turmeric plant. It is one of the most common natural dyes used as a base and is known for its vibrant colour.
• Weld: Weld is a yellow dye obtained from the leaves and stems of the weld plant. It produces a bright yellow colour and has been used historically.
• Pomegranate: Pomegranate peels can yield yellow and brown dyes. The dye is obtained from the outer covering of the pomegranate fruit.
• Onion Skins: The outer skins of onions can produce various shades of yellow and orange when used as a natural dye.
• Henna: Henna is a plant known for its use in temporary body art. It can also be used to create orange to reddish-brown dyes on fabrics.
• Annatto: Annatto is a reddish-orange dye derived from the seeds of the achiote tree. It’s used to produce orange and yellow hues.
• Lac: Lac is a dye derived from the resin of the lac insect. It can produce shades ranging from red to violet, depending on the mordant used.
• Cutch: Cutch is a brown dye obtained from the heartwood of the acacia catechu tree. It can produce various shades of brown.
• Hibiscus: Hibiscus flowers can yield a range of colours, including reds and pinks.
• Beetroot: Beetroot can be used to create pink to red dyes.

Reactive dyes:
Reactive dyes are a class of dyes that form a covalent bond with the fabric or fibre they are applied to. They are known for their excellent colourfastness and brightness. While traditional reactive dyes are typically synthetic, efforts are being made to create natural alternatives or modify existing natural dyes to behave as reactive dyes.

Natural reactive dyes aim to achieve the benefits of reactive dyes while using natural and sustainable sources. However, it’s important to note that the exact chemistry and properties of traditional reactive dyes may be challenging to replicate entirely using only natural sources. Nevertheless, advancements are being made in this field to develop natural alternatives.

Here are some approaches and sources that are being explored for natural reactive dyes:

• Plant-Based Reactive Dyes: Researchers are investigating ways to modify natural dye molecules to make them reactive and capable of forming covalent bonds with fibres. This involves chemical modifications to impart the reactive properties.
• Enzyme-Assisted Dyeing: Enzymes can be used to facilitate the bonding of natural dyes to the fabric fibres, creating a more permanent and colourfast bond. Enzymes can improve the wash and light fastness of natural dyes.
• Bioengineered Natural Dyes: Advances in biotechnology enable the production of modified natural dyes through genetic engineering. This involves modifying the genes of natural dye-producing plants to enhance their properties, including reactivity.
• Chitosan-Based Reactive Dyes: Chitosan, a natural biopolymer derived from chitin, can be used as a base for creating reactive dyes. Chitosan-based dyes can form covalent bonds with fibres, providing similar benefits to traditional reactive dyes.
• Tannin-Based Reactive Dyes: Tannins, naturally occurring polyphenols found in various plant tissues, can be chemically modified to create reactive dye molecules that bond with fibres in a reactive manner.

Indian Traditional Printed and Dyed Textiles:

Innovative Dyeing Techniques:

Waterless technologies for sustainable dyeing:
The dyeing process of textiles varies depending on the type of fabric. Cotton dyeing is a longer and more water and heat-intensive process, due to the negative surface of cotton fibres.

This means that usually cotton only takes up about 75% of the dye that is used. In order to make sure colour holds, dyed fabric or yarn is washed and heated over and over again, producing huge amounts of wastewater.

COLOURZEN
ColourZen uses a patented technology that pre-treats cotton before it is spun. This pretreatment makes the dyeing process faster, reduces 90% of water usage, 75% less energy and 90% less chemicals that would otherwise be needed for effective dyeing of cotton.

AIRDYE
Dyeing synthetic fibres, such as polyester, is a shorter process and 99% or more dye fixation (99% of the dye that is applied is taken up by the fabric). However, this does not mean that current dyeing practices are more sustainable. Air Dye uses dispersed dyes that are applied to a paper carrier. With heat alone, Air Dye transfers dye from the paper to the textile’s surface. This high heat process colours the dye at a molecular level. The paper that is used can be recycled, and 90% less water is used. Also, 85% less energy is used because the textiles do not need to be soaked in water and heat dried over and over.

DYECOO
DyeCoo uses CO₂ to dye textiles in a closed-loop process. “When pressurized, CO₂ becomes supercritical (SC-CO₂). In this state CO₂ has a very high solvent power, allowing the dye to dissolve easily. Thanks to the high permeability, the dyes are transported easily and deeply into fibres, creating vibrant colours.”

DyeCoo does not require any water, and they use pure dyes with 98% uptake. Their process avoids excess dyes with harsh chemicals and no wastewater is created during the process. They have been able to scale up this technology and have commercial endorsements from both textile mills and end-users.

Pigments from microbes
Most of the clothing we wear today is coloured using synthetic dyes. The problem with these is that valuable raw materials, such as crude oil are needed during production and the chemicals added are toxic to the environment and our bodies. Even though natural dyes are less toxic than synthetic dyes, they still require agricultural land and pesticides for the plants that made up the dyes.

Labs across the world are discovering a new way to create colour for our clothing: bacteria. Streptomyces coelicolour is a microbe that naturally changes colour based on the pH of the medium it grows inside. By changing its environment, it is possible to control what type of colour it becomes. The process of dyeing with bacteria begins by autoclaving a textile to prevent contamination, then pouring a liquid medium filled with bacterial nutrients over the textile in a container. Then, the soaked textile is exposed to bacteria and is left in a climate-controlled chamber for a couple of days. The bacteria is “live dyeing” the material, meaning that as the bacteria grows, it is dyeing the textile. The textile is rinsed and gently laundered to wash out the smell of the bacterial medium, then let to dry. Bacterial dyes use less water than conventional dyes, and can be used to dye many different patterns with a vast range of colours.

FABER FUTURE
Faber Future, a UK-based lab, is using synthetic biology to program the bacteria to create a large range of colours that can be used to colour both synthetic and natural fibres (including cotton).

LIVING COLOUR
Living Colour is a biodesign project based in the Netherlands that is also exploring the possibilities of using pigment-producing bacteria to colour our clothes. In 2020, Living Colour and PUMA teamed up to create the first-ever bacterial dyed sports collection.

WEREWOOL
Werewool is taking inspiration from nature to produce colourful textiles that come from proteins. One of these proteins is from Discosoma Coral which produces a bright pink colour. The DNA of this protein can be copied and placed into bacteria. This bacterium can then be woven into a fibre to make coloured fabric.

WE ARE SPIN DYE
We aRe SpinDye dyes recycled materials from post-consumer water bottles or wasted clothing before they are spun into yarn. Their technology melts colour pigments and recycled polyester together without the use of water, which reduces overall water usage by 75%. In recent news, H&M has used We aRe SpinDye®’s dyeing process in their Conscious Exclusive collection.

HUUE
Huue makes sustainable, biosynthetic indigo blue meant for the denim industry. Their technology does not use petroleum, cyanide, formaldehyde or reducing agents. This eliminates massive amounts of water pollution. Instead of using toxic chemicals, huue. uses sugar to make dye. They use proprietary bioengineering technology to create microbes that mirror nature’s process and consume sugar to enzymatically produce dye.

Sustainable Dyeing Innovations:
Sustainable dyeing innovations can help reduce water usage, replace wasteful practices with efficient and cost-effective ones, and minimize the impact on our ecosystems. New sustainable advancements that enhance the dye ability are ultrasound, ozone, plasma, ultraviolet, gamma illumination, laser, microwave, particle implantation, air-dye and other waterless technologies.

Exhaust dyeing:
Exhaust dyeing process is also termed as batch, discontinuous, direct or coordinate dyeing. Direct dyeing involves the direct application of dye to fabric without the help of any fixing agents. This process is so called exhaust because the dye molecules slowly get transferred from a comparatively large volume dye bath to the substrate or material that is to be dyed. The exhaust dyeing process is used for staple fibre dyeing. Yarn and fabric could be dyed by exhaust dyeing method. Dye solution or dye bath is produced by dissolving the dyestuff according to required liquor ratio. Then textile material is immersed in to the dye first solution. Initially the surface of the fibre is dyed when dyes contact with the fibre, then the dyes are entered in the core of fibre. Proper temperature and time are maintain.

Continuous dyeing:
Continuous dyeing process typically consists the following. Dye application, dye fixation with heat or chemicals and finally washing. Various sequential operations are used for the continuous dyeing of fabric. An initial padding stage is common to all sequences. It involves immersion of the fabric in the dye liquor contained in a trough of minimal volume, which is kept constantly replenished from a stock tank. A liquor ratio is as low as 1:1 may be used; in general, low-substantivity dyes are used in continuous dyeing process. Next, the fabric passes in open width through a ‘nip’. The nips are the padding mangle, in which heavy rollers (called bowls), pressed closely together along their length, and are rotated in opposite directions to carry the fabric through the system at a constant speed, squeezing out the superfluous dye liquor. Heavier fabrics are passed through two consecutive troughs and a second nip, using a three-bowl mangle.

Continuous dyeing has been found to be most suitable for woven fabrics. Mostly continuous dye ranges are designed for dyeing blends of polyester and cotton. Sometimes Nylon carpets are also dyed in continuous processes, but the design ranges for them is unlike that for flat fabrics. Warps are also dyed in continuous process. Very good examples of such warp dyeing are long chain warp dyeing and slasher dyeing using indigo.

Few continuous dyeing processes includes pad-stream process, pad-dry process and thermosol process.

Pad-Steam:
Pad Steam dyeing is a procedure of steady colouring in which the texture in open width is padded with dyestuff and is then steamed. It’s an ideal dyeing machine for reactive dyeing. Light, pale and medium shades can be dyed in this machine. Continuous roller steamer is used for diffusion of reactive, vat, sulphur dyes and direct dyes into cellulosic fibres in an atmosphere of heat and moisture that is created by saturated steam injected into the steamer.

Pad-Dry:
In this process fabric is padded in a padder with reactive dye in presence of an alkali then padded fabric is passed through a squeezing roller into a dryer. As a dryer cylinder, stenter etc. may be used. During drying due to higher temperature fixation of dye in fibre increases and at the same time water is removed by evaporation. After drying fabric is washed in a washing machine to remove unfixed dye.

Thermosol:
This method is continuous method of dyeing with disperse dye. Here dyeing is performed at high temperature like 180-220◦C in close vessel. In this process time of dyeing should be maintained very carefully to get required shade and to retain required fabric strength.

Semi-continuous dyeing:
In the process of semi-continuous dyeing that consists of pad-batch, pad-jig, pad-roll the fabric is first impregnated with the dye-liquor in, what is called a padding machine. Then it is subjected to batch wise treatment in a jigger. It could also be stored with a slow rotation for many hours. In the pad-batch this treatment is done at room temperature while in pad-roll it is done at increased temperature by employing a heating chamber. This helps in fixation of the dyes on to the fibre. After this fixation process, the material in full width is thoroughly cleansed and rinsed in continuous washing machines. There is only one point of difference between Continuous and semi-continuous dyeing process is that in semi-continuous dyeing, the dye is applied continuously by a padding. The fixation and washing remaining discontinuous. Liquor ratio in semi-continuous dyeing is not of much importance and is not taken as a parameter. Some of semi-continuous dyeing processes include pad-roll process, pad-jig process and pad-batch process. One of the widely used techniques for semi-continuous dyeing process is the Pad Batch Dyeing a schematic diagram is given here for the semi-continuous dyeing process.

Pad batch process:
It is mainly used in the dyeing of cellulosic fibre like cotton or viscose (knit and woven fabric) with reactive dyes. Pad batch dyeing is a textile dyeing process that offers some unique advantages in the form of versatility, simplicity, and flexibility and a substantial reduction in capital investment for equipment. It is primarily a cold method that is the reason why it is sometimes referred to as the cold pad batch dyeing.

Electrochemical dyeing:
The main ecological impact of vat dyeing today is the use of dyeing auxiliaries and reducing agents, which end up in the waste water after their use in the dyeing process. Electrochemical dyeing is one alternative to overcome these problems and increase the attractiveness of vat dyeing. In principle for this process the reducing agent is replaced by electrons from the mains helping to achieve a wastewater-free process. Additionally, the vat dyeing process can be monitored and steered by measuring the redox potential in the liquor. This positively influences the quality and cost of the process. There are two techniques by which electrochemical dyeing can be done: direct or coordinate electrochemical colouring and indirect electrochemical colouring.

Foam dyeing:
Foam dyeing is an attractive alternative to traditional dyeing methods due to the potential environmental benefits and supply chain savings. The main dyeing element in this process is foam, using air instead of water to carry the chemistry or dye onto the fabric. Foam is the key factor in foam dyeing process. Foams are formed using foaming agents and usually foam is mainly obtained from aqueous solution which is then spread on the textile material. These agents must produce foam instantly, should not get affected by temperature, quick wetting process and ability to stabilize itself. Foam may be of dispersion foam or condensation foam. Dispersion foam is mixing of gas with the liquid while condensation foam is producing gas within the liquid physically or chemically.

The continuous methods of foam dyeing have the following steps:

• Foam generation.
• Foam application to the substrate.
• Foam distribution with simultaneous drainage and diffusion of the liquid into the substrate Foam collapse and release of active substance.
• Fixation of the active substance.

Advantages

1. Fixation of dye into fibre can be improved.
2. Diffusion of dye into fibre can be enhanced.
3. Stability of the fibre dyed obtained is high.
4. Outcome is more in short time duration.
5. Waste generation is less and energy saving process

Microwave technology:

Mechanism:
The fabric material is washed prior to dyeing. Hot water is added to the microwave container containing fabric material and dye powder is added to it. The container is closed and covered properly. Then the container is placed inside the microwave and treated at high temperature for few minutes. After that dye solution is added again and the process is repeated. Then the container is removed and cooled. The dye gets absorbed to the fibre leaving the cloudy water. Then water is filtered and the fabric is dried in shade. The main drawback in microwave dyeing technology is that uniform dyeing cannot be obtained and the depth of dyeing is also not even. Colouring of fabric material occurs but most of the dye stays in water only and is washed out during rinsing process.

Advantages:

1. Dye-uptake and fixed percentage of the reactive dye can be improved.
2. Salt and the alkali dosage in the dyeing bath get greatly reduced.
3. The tensile strength of the fabric is retained.

Ultrasonic wave dyeing technology:

Mechanism:
Cavitation occurs when ultrasonic waves are absorbed into the liquid system. This results in release of entrapped gases in the liquid medium such as the textile material or dye solution. The effect of ultrasound technology on dyeing process can be explained in three methods:

Dispersion: Breaking of micelles and high molecular weight compounds to form uniform dispersion in the dye solution.

Degassing: Release of entrapped gases from the fibre capillaries.

Diffusion: Penetration of dye into the fibre material. Interaction occurs between the dye and fibre resulting in bond formation.

Advantages:

1. Energy saving process and temperature required is also low.
2. Operating time and chemical usage are also less.
3. Product quality can be improved.
4. This method is suitable for water insoluble to hydrophobic dyes.
5. It requires less processing cost.

Disadvantages:
The main drawback of using ultrasonic wave technology in dyeing process is difficulty in producing uniform ultrasound waves and high intensity in a large vessel.

Plasma Technology:
Plasma technology is mainly used for inducing surface modifications and also for enhancing the property of textile materials for increasing dyeing rates, for colour improvement, diffusion and adhesion of coated dyes. The textile material to bicoloured is placed inside the chamber and plasma is incited. The particles get generated and then interacts with the surface of the textile material. A thin nanometre sized film is formed on the surface of the material and the surface is structured with functional groups.

Advantages:

1. Chemical and Water discharge is less.
2. Colour obtained is bright and durable.
3. This method alters the surface of fibre than modifying inside the material.
4. Effect on environment is very less.

Disadvantages:

1. This treatment produces harmful gases such as ozone and nitrogen oxides during operation.
2. High cost of plasma device.
3. Less availability.
4. Requires skilled operator.

Air-Dye technology:
Textile wet processing industry is one of the highest water-consuming industries. 17-20% of today’s industrial pollution is the result of the textile colouring treatment, contributing to 72 toxic chemicals in water supplies, 30 of which are permanent processes. To reduce these water contaminations, a new technology called “Air-Dyeing” has been introduced.

Mechanism:
This method does not require water for dyeing instead this employs air to enter into fibres. In this method, the fabric is first heated and then the dye is injected directly into the fibres in the form of gas. The outcome of this technology is more beneficial than any other conventional dyeing methods such as vat dyeing, cationic dyeing, etc. The colour after dyeing process results in rich look and lasts for a longer period of time.

Advantages:

1. The Air-Dyeing uses 95% less water and 86% less energy than conventional fabric dyeing processes.
2. Only 1% of Air-Dyed fabrics are damaged during this process.
3. Highly flexible and maximum colour durability is obtained.
4. No post-treatment or finishing is required.
5. Reduces the industry’s share of global warming by 84%.

The Air-Dye process radically reduces the environmental profile of the colour application process while improving the use phase performance of the finished fabric. By removing the requirement of water at the point of colour application, Air-Dye technology creates a significant opportunity to localize production for regions of the globe that lack the water resources traditional methods require. Because traditional processes require considerable energy to heat the water and dry the fabric, Air-Dye technology also significantly reduces the energy required at the point of colour application. As Air-Dye technology matures, expect to see additional benefits from increased efficiency in power usage, power source, and the direct application of dye without a donor media. Air-Dye is a clear response to an increasing awareness of the environmental impacts associated with traditional dye application processes and a new technology for improving the process of colouration and decoration of textiles.

Supercritical fluid dyeing:

Mechanism:
The dye and fibre are added to the reaction vessel. The components present in a CO2 dyeing system are CO2 gas cylinder, pressure pump, temperature controller, vessel, heating and cooling system. The whole system is pressurized with CO2 up to 800 Psi. Continuous stirring is done with agitation speed of 1000 rpm. Temperature of about 180°C is maintained. Then pressure is raised to 3500 Psi and the system is maintained at these conditions for 2 hr. Finally, the pressure is released and dyed fibre is removed.

Advantages:

1. No discharge of wastewater/contaminated water into environment.
2. CO₂ causes swelling of fibre thus enabling dyes to diffuse at faster rate.
3. Energy required for dyeing process is low compared to other conventional methods.
4. Drying process is not required after dyeing.
5. Supercritical fluids cause no pollution, non-flammable and are nontoxic.
6. Diffusion rate is comparatively higher.

Pigment dyeing:
Pigment dyeing is not colouring in the genuine sense as the shade sticks on the texture as a result of the binding agents. During the process of pigment dyeing, no real synthetic response happens between the dye and the texture. Rather, what happens is that the shades get situated on the texture with the assistance of binders. Pigments are not dissolvable in water and demonstrate no liking or affinity for fibre. Along these lines, regular dyestuff-based colouring conditions are not achievable for pigment dyeing. To overcome these drawbacks, another sort of colours has been detailed for use in fabric strands. These are kept up in a steady scattering in the medium of water by anionic surfactants. This sort of shade is known as pigment resin colour (PRC), essentially utilized as a part of printing.

Ozone technology for dyeing:
Ozone is a naturally occurring gas that has both beneficial and hazardous effect on the environment. It is mostly present in the stratosphere and protects the earth from harmful ultraviolet radiation entering it. It is a pungent smelling gas. Ozone gas can also be produced artificially by various methods such as Electrolysis, Corona discharge and UV radiation. Ozone is a strong oxidising pungent smelling gas. Ozone gas is helpful in surface modification and improving fibre durability through a process termed ozonation. The dyeing ability through ozonation process depends on factors such as pH, temperature, water level and ozone dosage level.

Bio-based dyeing technology:
Conventional dyeing techniques have negative impact on environment though they result in rich colourful products. The presence of toxic chemicals, heavy metals and other hazardous substances affect humans who wear it. To overcome these issues, new technique namely “bio-based dyeing” has been developed with more benefits such as safe, eco-friendly, durable and also cost-effective. These dyes are also known as natural dyes. Plants, animals and microbes are used for this type of dyeing process. Compared to plants and animals, microbe-based dyeing is more effective with high efficiency. Downstream processing can be eliminated using bio-based dyeing technique. The dye is in liquid state and dyeing can be done in batch or continuous mode. The dyeing process depends on several parameters such as type of textile material, production conditions, requirement of product quality, etc.

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