Textile dyeing is a major cause of the generation of large amounts of coloured effluent. The ecosystem is harmed when these effluents are directly dumped into natural water sources. In contrast, supercritical waterless dyeing technology requires only a supercritical fluid for circulation and dyeing. Once this is achieved, the process can be completed without generating wastewater by lowering the temperature and pressure. A supercritical fluid possesses both the liquid quality of dissolving materials into their constituent parts and the gaseous property of penetrating any substance. The most widely utilised supercritical fluid is carbon dioxide, as it is inexpensive, readily available, non-toxic, and non-flammable. Above 31°C and 73 bar pressure, its physical characteristics lie between those of a liquid and a gas. The dyeing cycle is significantly shortened as operational procedures are simplified. This method offers significant environmental benefits, including reduced water usage and the effective recycling of dyes. Ongoing research and technological advancements make SC-CO2 (supercritical carbon dioxide) dyeing a viable long-term solution for more environmentally friendly textile manufacturing.

SC-CO2 is a state of carbon dioxide achieved when it is subjected to high pressure and temperature, resulting in a fluid that exhibits properties of both gases and liquids. This unique state allows SC-CO2 to act as an effective solvent, especially in environmentally friendly applications. Supercritical carbon dioxide is held at or above its critical temperature (31.1°C) and critical pressure (7.38 MPa), resulting in properties that differ significantly from regular CO2. This state allows SC-CO2 to exhibit characteristics of both gases and liquids, making it an effective solvent and extraction medium.1 In textile applications, SC-CO2 is increasingly utilised for dyeing and functionalisation due to its environmental benefits and efficiency. SC-CO2 dyeing significantly reduces water usage and eliminates harmful effluents, addressing the pollution issues associated with traditional dyeing methods.2
This method enhances dyeing efficiency, as techniques such as flash explosion treatment improve dye uptake in cotton fibres, optimising colouration performance.3 The process allows for the recovery and reuse of dyes, reducing waste and energy consumption. SC-CO2 is non-toxic, non-flammable, and does not contribute to ozone depletion, positioning it as a greener alternative to traditional organic solvents. The supercritical state of carbon dioxide presents unique advantages for textile dyeing, making it an increasingly popular choice in the industry. Its properties facilitate a more efficient, eco-friendly dyeing process compared to traditional methods.4

Process of Supercritical CO2 Dyeing
The dyeing process begins with preparing a dye solution, where one or more dye materials are dissolved in a suitable solvent along with any necessary additives. This solution is then used to pre-treat the textile material, resulting in a dye-coated fabric that can achieve a uniform intensity of colour due to a controlled concentration of dye materials on the surface. The pre-treatment facilitates the penetration of dye molecules into the textile’s pores and capillaries, enhancing the overall efficiency of the dyeing process. Once the textile material is pre-treated, it is placed inside a supercritical fluid dyeing vessel.

The process begins with converting CO2 into a supercritical state by applying high pressure and temperature, typically around 11 to 23 MPa and 353 to 393 K. The dye dissolves in the SC-CO2 environment and interacts with the textile fibres, such as polyester and nylon. SC-CO2 is influenced by pressure and temperature; higher pressures generally increase solubility. Adding water in controlled amounts enhances dye uptake by swelling the fibres and improving dye reactivity, achieving optimal colour saturation. The dyeing apparatus is designed to recycle CO2 and residual dye, minimising waste and environmental impact.5

  • Preparation: The process begins with the cooling of CO2 from a storage tank into a liquid state, which is then heated to reach supercritical conditions in a dye kettle.
  • Dyeing: Textiles are immersed in SC-CO2 where the dye, often a ‘disperse’ dye, is introduced. Moisture can enhance dye uptake, with optimal saturation leading to deep colours and high fixation rates (70-92 per cent).
  • Colour Fixation: After dyeing, the dyed textiles undergo a static developing phase in SC-CO2, allowing for controlled colour fixation without dye migration.
  • Recycling: The system is designed to recover and recycle both the dye and CO2, significantly reducing waste and resource consumption.

The temperature, pressure, and concentration of water are the main critical variables that impact the supercritical carbon dioxide dyeing process. Raising the temperature has little effect on the dyeing results for polyester, suggesting a complicated link between temperature and dyeing efficiency. Pressure is a key factor in improving dye absorption and solubility. Higher pressures (up to 278 bar), according to studies, improve colour strength, especially for polyester. Pressure enhances the solubility of dyes in SC-CO2, which is advantageous for dye uptake. When water is present, dyeing performance is greatly improved. Deeper colours and higher fixation percentages are produced at the ideal saturation levels.6

Dyeing Process with SC-CO2 Machinery
The machinery involved in the dyeing process, particularly for SC-CO2 dyeing, encompasses various innovative designs and technologies to enhance efficiency and quality. These machines typically utilise mechanical and hydraulic systems to ensure optimal dye application and fabric handling. The primary components of SC-CO2 dyeing plant would include dyeing vessels, separators, Modern PLCs and HMI terminals with safety interlock logic, as well as CO2 hold-up tanks, heat exchangers, and other components as well as software for reliable outcomes and safe functioning. The dyeing process involves loading the fabric, pressurising the system to reach a supercritical CO2 state, colouring the cloth, and then releasing the dyed fabric by depressurising the system.7

  • Loading: A high-pressure dyeing tank is filled with the fabric, necessary dyes, and supporting ingredients.
  • Pressurisation: To attain the supercritical state, the vessel is filled with carbon dioxide, which increases both the temperature and pressure.
  • Dyeing: To get the required colouring, the dyes dissolve and penetrate the textile fibres in the supercritical CO2 state.
  • Depressurisation: The colourful fabric remains inside the fibres as the CO2 transforms into a gas when the pressure is lowered.

Principal Benefits
With considerable financial and environmental advantages over conventional water-based dyeing techniques, supercritical carbon dioxide dyeing has come to light as a viable substitute. Due to its special state, SC-CO2 can break down dye molecules and permeate fibres effectively without water—a significant benefit over traditional dyeing techniques.

Waterless Process: The primary advantage of SC-CO2 dyeing for the environment is its waterless operation, which tackles the textile industry’s excessive dependence on water. Compared to standard dyeing methods, which frequently involve hazardous chemicals, this technique uses less water and eliminates wastewater formation.

Less Chemicals: SC-CO2 dyeing minimises or completely does away with the requirement for auxiliary chemicals, such as fixatives and salts, which are frequently employed in water-based procedures. As a result, there is less environmental pollution and less chemical waste.

Energy Efficiency: Compared to water dyeing, the procedure uses less heat and less energy overall because there is no need for drying after dyeing.

Dye Recycling: The SC-CO2 process makes it possible to recover and repurpose CO2 and dyes, which promotes environmental and economic sustainability.

Environmental Benefit
Compared to conventional dyeing techniques, supercritical carbon dioxide dyeing offers a notable environmental benefit. This creative method improves environmental sustainability and dyeing efficiency.8

1. Water Saving: In conventional dyeing methods, large amounts of water are used as a medium to dissolve dyes and transport them into textile fibres. In SC-CO2 dyeing, carbon dioxide in its supercritical state acts as the solvent, eliminating the need for water. Since water is an essential part of conventional dyeing processes, SC-CO2 dyeing eliminates this use of water, resulting in a significant reduction in total water consumption.

Traditional dyeing processes generate large quantities of polluted wastewater containing residual dyes, chemicals, and salts. In contrast, SC-CO2 dyeing produces no wastewater, as there is no liquid phase involved. The CO2 and dyes can be recovered and reused.

2. Utility Saving: There are several environmental advantages to using SC-CO2, especially when it comes to lower energy and chemical waste. Green solvent SC-CO2 efficiently replaces organic hazardous solvents, reducing the production of toxic waste and improving process efficiency. Using SC-CO2 results in a much lower E-factor, a measure of waste per unit product mass, suggesting a more environmentally friendly method. When compared to conventional procedures, SC-CO2 can recover up to 50 per cent of solvents, drastically reducing waste compared to traditional methods.

3. Energy Efficiency: Lower operating temperatures and pressures compared to conventional technologies make SC-CO2 processes more energy-efficient. Its unique properties, such as high diffusivity and low viscosity, enhance extraction and purification processes, reducing energy consumption. With lower energy consumption due to the elimination of water heating and drying, the overall carbon footprint of the dyeing process is reduced. SC-CO2 dyeing eliminates the need for water and auxiliary chemicals, leading to significant cost reductions (up to 20 per cent) and improved colour fastness in dyed products. The innovative use of SC-CO2 allows for the recycling of dyes from waste textiles, promoting a circular economy in textile production.

Economic and Technological Limitations
Implementing supercritical carbon dioxide dyeing technology commercially presents several challenges that need to be addressed for successful adoption.9

1. Dyeing Efficiency: Achieving optimal dye fixation remains a challenge, as variations in pressure and temperature can affect colour outcomes. For instance, while polyester showed increased colour saturation with pressure, other fibres like silk and wool did not respond similarly. The presence of water significantly enhances dyeing efficiency, but managing moisture levels in SC-CO2 systems is complex.

2. Economic Factors: The transition to SC-CO2 technology requires substantial capital for equipment and training, which can deter smaller manufacturers. The specialised dyes compatible with SC-CO2 can be more expensive than traditional dyes, impacting overall production costs.

3. Regulatory and Environmental: The apparel industry faces stringent environmental regulations, complicating the integration of new technologies like SC-CO2 dyeing. The cost comparison between SC-CO2 dyeing and traditional dyeing methods reveals significant differences in both initial setup and operational expenses. The initial investment for SC-CO2 dyeing systems is higher due to the need for specialised equipment, including high-pressure vessels and cooling systems. However, this setup can lead to long-term savings by eliminating the need for water treatment facilities associated with traditional dyeing. SC-CO2 dyeing reduces operational costs by minimising energy consumption and eliminating drying processes, which are significant in traditional methods. The recovery and reuse of SC-CO2 also contribute to lower material costs over time.

Other Major Limitations
The use of SC-CO2 for dyeing fabrics presents several limitations regarding the types of fabrics and dyes that can be effectively utilised.10

1. Fabric Compatibility: Not all fabrics respond equally to SC-CO2 dyeing; hydrophobic materials like polyester and nylon yield high colour value and fixation. On the other hand, natural fibres like cotton need to be altered in order to improve the absorption of dye. As the number of cloth layers increases, the levelling qualities may decrease, which could impact the dyeing uniformity. This biodegradable fabric, made of polylactic acid, shrinks at higher temperatures and has difficulty producing vivid colours, especially in darker tones.

2. Limited Dyes: Nylon and dispersion dyes are principally used in the SC-CO2 dyeing procedure. Different dye types might not produce colours or fixation at the same intensity. Dyeing with SC-CO2 often results in insufficient colour depth, particularly for darker shades, as seen in poly (lactic acid) fibres. The dye uptake is significantly improved at higher temperatures, which can lead to fibre damage, such as shrinkage and hardening. Water considerably improves the dyeing process’ effectiveness, suggesting that SC-CO2 dyeing may not work as well without it, which could restrict its use.

3. Process Complexity: The dyeing process requires precise control of temperature and pressure, which can complicate operations. The need for moisture to enhance dyeing efficiency adds another layer of complexity, as optimal water saturation is crucial for achieving deep colours.

4. CO2 Recovery and Recycling: A comprehensive CO2 recovery system includes components like raw gas separators, compressors, and purification towers, effectively removing impurities and ensuring high-quality recovered CO2. The design of recovery and separation kettles, which utilise molecular sieves, enhances the purity and drying of recovered CO2, facilitating its reuse in dyeing processes.

5. Safety and Security: Handling SC-CO2 in dyeing facilities requires strict safety precautions due to its unique features and operational requirements. Because SC-CO2 works at pressures that are usually between 225 and 278 bar, it needs strong equipment to survive these circumstances. Leaks and malfunctions must be avoided by performing routine maintenance and inspections on pipe systems and pressure vessels. Localised accumulation of CO2 can cause asphyxiation, therefore adequate ventilation needs to be maintained.

Innovation in SC-CO2
Innovations in SC-CO2 dyeing focus on enhancing dye solubility, improving dye uptake, and optimising process efficiency. Recent research highlights several promising approaches. Using organic small molecules as assist-dissolving agents has been shown to significantly increase the solubility of disperse dyes in SC-CO2, achieving dye uptake improvements from 20-30 per cent to 60-70 per cent. Advances in SC-CO2 technology have facilitated the functionalisation of textiles and improved extraction techniques for natural dyes, enhancing the efficiency of dyeing processes. Computational fluid dynamics simulations have been employed to model industrial-scale SC-CO2 dyeing processes, addressing flow uniformity and pressure dynamics to enhance dyeing consistency. Acid, direct, and reactive dyes colours that are appropriate for dyeing these organic fabrics, hold no solubility in SC-CO2 as SC-CO2 cannot break the hydrogen bonds found in many natural fibres. Further research and innovation for the procedure is necessary because working with natural fibres requires that the fibres are either altered, or a brand-new fixing mechanism must be created.11