Waterless Dyeing Technology in Textile Processing

Waterless Dyeing Technology in Textile Industry

Pallavi Sunil Gudulkar
Department of Textiles (Textile Chemistry)
DKTE’S Textile & Engineering Institute, Ichalkaranji, India
Intern at Textile Learner
Email: pallavigudulkar@gmail.com


1. Introduction
The textile sector is considered to be one of the largest water consumers. Water is utilized extensively in traditional textile dyeing, both in terms of fresh water input and effluent discharge. Many pretreatments and finishing operations, such as washing, scouring, bleaching, and dyeing, require water as a solvent. On average, 100-150 liters of water are required to process 1 kg of textile material, with the dyeing process consuming 16% of this total. Water is the world’s most valuable natural resource, with more than 97 percent of it being salty and only 2.5 percent being fresh water. More than two-thirds (68.7%) of 2.5 percent freshwater is frozen as snow and ice, with the remaining one-third retained as groundwater below ground. Only 0.3 percent of the world’s freshwater appears to be available as surface water in lakes, ponds, rivers, and streams. As a result, in order to save this finite resource of freshwater, manufacturers should develop alternate textile dyeing techniques.

2. Water Consumption in the Textile Industry
The textile industry, with the exception of agriculture, is the most water-intensive industrial sector. The world’s need for fresh water is expected to rise by 40% by 2030 as the world’s population grows, and by 2050, an estimated billion or more people would be without the water they require on a daily basis. To color just two pounds (one kg) of cotton fabric, up to 26 gallons (100 liters) of water is required! An average-sized textile mill uses roughly 1.6 million gallons of water each day. Depending on the type of dye used, dyeing water usage ranges from 30 to 50 liters per kg of cloth. Yarn dyeing uses around 60 liters of water every kilograms of yarn. The dyeing sector accounts for 15–20 percent of the overall waste water flow. Governments, NGOs, and consumers have put pressure on the textile sector to do a better job protecting the environment and to adopt a more proactive approach to decreasing the industry’s environmental footprint in recent years. The intense examination isn’t going away any time soon. Indeed, as governments speed up efforts through new laws to strengthen monitoring and minimize water pollution by the textile sector around the world, the industry is likely to see greater regulation.

Water consumption in textile industry
Figure 1: Water consumption in textile industry

To truly reduce environmental effect, the textile industry must take responsibility and implement sustainability initiatives. The textile industry has spent years attempting to find innovative ways to reduce water use, such as lowering the liquor ratio or employing textile dyes that require less water during the dyeing process.

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3. Waterless Dyeing
Waterless dyeing is a method of dyeing that does not require the use of water. It’s a process that doesn’t need any water and uses less energy than standard dyeing methods while still producing vibrant colors in solids and designs. The technique, time, chemicals, and auxiliaries used in the waterless dyeing process were all reduced. There are two waterless dyeing technologies as follows:

  1. Air dyeing Technology
  2. Supercritical fluid dyeing technology

3.1 Air dyeing:
The dyeing fluid is first atomized, then blended with high-pressure airflow, and lastly sprayed over the fabric to be dyed in an airflow dyeing machine. Usually a small amount of water is used because water only serves as a solvent for the dyeing liquor and the dye chemicals come into direct contact with the fabric. Airflow dyeing machines provide the advantages of high efficiency, energy savings, and environmental preservation over overflow dyeing machines.

The fabric passes through an air-steam mixture generated by a blower, which is the basic principle behind the aerodynamic system. This means that, unlike a hydraulic dyeing machine, cloth transport does not require a dye bath or an aqueous medium. To put it another way, the fabric transfer is possible without the use of liquor. From feeding to the completion of the operation, as well as during drainage and filling procedures, the material is constantly moving.

Machines used in air dyeing technology
Figure 2: Machines used in air dyeing technology

3.1.1 Advantages of air dyeing:

  • During the color application procedure, it does not pollute the water. No hazardous waste is emitted and no water is lost when dye is transported by air rather than water.
  • It significantly reduces energy requirements, decreasing expenses and satisfying the most stringent global responsibility requirements.
  • Air-Dye technology avoids toxic wastewater generated by textile dyeing. Water scarcity affects one in every three people on every continent, and the problem is getting worse as the world’s population grows, cities expand, and homes and enterprises use more water.
  • According to an independent assessment, Air-Dye uses up to 95 percent less water and up to 86 percent less energy, contributing 84 percent less to global warming, depending on the fabric and type of dyeing. Furthermore, some companies claim that Air-Dye technology allows them to print to order, reducing waste and extra output.
  • Because the dyes are injected into the fabric rather than applied to it, bleaching and cleaning chemicals have no effect. Using Air-Dyeing technology, a manufacturer may save 1,132,500 megajoules of energy, 157,500 gallons of water, and 57,500 gallons of water for every 25,000 T-shirts sold (KgCO2 equiv. emissions).

The air-dye technology is still relatively new, and the cost of machine installation is high. As a result, the researcher’s future focus in textile dyeing could be the development of low-cost simple air-dye coloring procedures for natural and synthetic materials that use less water and emit almost no pollutants.

3.2 Supercritical fluid dyeing:
In this type of waterless dyeing process supercritical carbon dioxide used. When carbon dioxide is heated to over 31°C and compressed to above 74 bar, it becomes supercritical, a state of matter that resembles an expanded liquid or a heavily compressed gas. In other words, above the critical point, carbon dioxide has both liquid and gas qualities. In this manner, supercritical CO2 has liquid-like densities, which is beneficial for dissolving hydrophobic dyes, as well as gas-like low viscosities and diffusion qualities, which can result in faster dyeing times than water. The extraction of spinning oils, dyeing, and removal of residual dye can all be done in one facility in the carbon dioxide dyeing process, which only requires changing the temperature and pressure conditions; drying is not necessary because CO2 is expelled in a gaseous state at the end of the process. CO2 can be easily recycled, up to 90% when the extracted materials is precipitated in a separator.

Schematic diagram of Supercritical Fluid Dyeing
Figure 3: Schematic diagram of supercritical fluid dyeing

3.2.1 Advantages:

  • This CO2-based dyeing process offers a sustainable alternative to conventional water-based dyeing process.
  • The result is that no processing water or chemicals are required, with a 50% reduction in energy consumption for the process.
  • Water contamination and water treatment are eliminated.
  • Textiles do not need to be dried.
  • It has a high rubbing fastness.
  • Dyeing takes place with a great degree of uniformity.
  • Carbon dioxide is a non-toxic substance that may be easily regenerated in the dyeing process.
  • On areas where there is a lack of water, dyeing houses can be established.

3.2.2 Disadvantages:

  • The dyeing process is carried out at a pressure of 260-280 bar and a temperature of 130°C. Such high pressures require a unique design of the textile machinery, and upgrading needs a large investment. For the textile business, pressures of 260-280 bar are exceptional, and they may create mental limitations.
  • For industrial labor safety concerns, a control device for monitoring CO2 concentrations in air must be installed due to the usage of suffocating gas CO2.
  • Due to the non-additive behavior of the dye components, traditional color measuring will meet some additional challenges in this application.
  • The machine is still not suitable for coloring natural (hydrophilic) fabrics since it can’t break hydrogen bonds, which makes supercritical carbon dioxide diffusion difficult.
  • At such high temperatures and pressures, reactive dyes, direct dyes, and acid dyes may be destroyed.
Machine used in Supercritical Fluid Dyeing
Figure 4: Machine used in supercritical fluid dyeing

4. Conclusion
The above techniques of dyeing can be depending on to save fresh water rather than spending large amounts of it on conventional water dyeing, as the need for fresh water is increasing day by day for daily activities. In waterless dyeing technology, the washing step is also removed, preventing water contamination from waste water discharge into fresh water resources. The more of these techniques are implemented, the more water can be conserved.

5. References

  1. Recent Progress in Waterless Textile Dyeing by Iqbal Mahmud and Shantanu Kaiser
  2. Waterless Dyeing by Swati Jain
  3. https://www.slideshare.net/alaminmasum1/waterless-dyeing-presentation
  4. https://www.researchgate.net/publication/345669188_Recent_Progress_in_Waterless_Textile_Dyeing
  5. https://www.textilesphere.com/2020/08/waterless-dyeing-techniques-technologies_19.html

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