INTRODUCTION
Dyes are colored organic compounds used to impart color onto cloth. The current process for dyeing textiles is operative, but inefficient and harmful. The primary function of water in the dyeing process is to rinse excess dye off of the fabrics that have been colored. All of the current commercial dyeing methods use a significant amount of water, and pollute most of that water during the process. For each product manufactured, the typical water to dye ratio is 15:1.
Textiles leave one of the largest water footprints on the planet and dyeing poses an especially big problem. Textile dyeing is the No. 2 polluter of clean water on earth, following agriculture. Dye houses in India and China are notorious for not only exhausting local water supplies, but for dumping untreated wastewater into local streams and rivers.
The industry’s challenge is to adopt more water-friendly technologies to dye cotton and polyester, the two most mass marketed textiles. So what can companies do to mitigate the effects of this timeless, yet toxic, dyeing process?
Many factories could start by tackling the wasteful dye-to-water ratio. A 1-to-30 ratio is common. Reaching a 1-to-10 dye-to-water ratio is an accomplishment. The manufacturer does not need more dye, diluting a dye simply means wasting more water: much of the answer in solving the waste involved in dyeing textiles lies in a factory’s mechanization.
WATER CONSUMPTION IN THE TEXTILE INDUSTRY
With the exception of agriculture, the textile industry is the heaviest industrial consumer of water. By 2030, it is estimated that the world demand for fresh water will increase by 40 percent with the increased population, and by 2050, an estimated billion-plus people will also lack the water they need for daily living. Up to 26 gallons (100 liters) of water are needed to dye just two pounds (one kilogram) of cotton fabric! The daily water consumption of an average sized textile mill having is about 1.6 million liters. Specific water consumption for dyeing varies from 30 – 50 liters per kg of cloth depending on the type of dye used. The overall water consumption of yarn dyeing is about 60 liters per kg of yarn. Dyeing section contributes to 15% – 20% of the total waste water flow.
In recent years, there has been increased pressure placed on the textile industry by governments, NGOs and consumers to do a better job protecting the environment and to take a more proactive approach in reducing the industry’s environmental footprint. The intense scrutiny is not likely to abate anytime soon. In fact, the industry will likely see tighter regulation as governments step up efforts through new legislation to improve oversight and reduce water pollution by the textile industry around the world.
There is a critical need for the textile industry to take responsibility and adopt sustainable business practices to truly reduce environmental impact. For many years, the textile industry has tried to identify new ways to reduce the water consumption in the industry, such as lowering the liquor ratio or using textile dyes that require less water during the dyeing process.
COMPANIES WORKING ON WATERLESS TEXTILES
- ColorZen: Its process modifies cotton’s molecular structure and allows dye to settle within the fibres without requiring the massive discharge of water, eliminating the need to rise off fixing agents that keep a fabric’s coloring consistent. Compared to conventional processes, ColorZen claims its technology can finish cotton fabric using 90% less water and 75% less energy.
- AirDye: Instead of water, the company’s technology uses air to disperse dye. AirDye’s process embeds dye within textile fibres instead of merely on them, so color lasts longer and is more resilient to chemicals and washings.
- Adidas (“Dry Dye” Technology): Instead of water, Adidas’ supplier usescompressed and pressurized carbon dioxide as the agent to disperse dye within polyester fabric. The CO2, which takes on liquid-like properties, is contained in stainless steel chambers. After the dyeing cycle the CO2 becomes gasified, and dye within the cotton fibres condenses as it separates from the gas. The CO2 is then recycled and pumped back into the dyeing vessel. Adidas claims using CO2 is a safe and environmentally-friendly option because the gas is contained and can be used repeatedly without the risk of any emissions.
- DyeCoo: Uses supercritical CO2 gas rather than water to infuse fabric with color. Special temperature- controlled pressure chambers force the carbon dioxide to act as a fluid similar to water (the supercritical fluid CO2) which causes the polymer fiber to swell allowing the dispersed dye to easily diffuse within the polymer, penetrating the fibers, and carrying the dyes into the fabric and dyeing it.
DYECOO: WATERLESS DYEING
Dyecoo, a Dutch-based company founded in March 2008, is the world’s first supplier of industrial CO2 dyeing equipment and a leading innovator in CO2 dyeing technology and processes. It enabled the textile industry to have environmentally sustainable operations by reducing the amount of water and energy consumed during the dyeing process; and lowered the amount of CO2 emitted and waste generated.
DyeCoo uses supercritical carbon dioxide gas rather than water to infuse fabric with color. Special temperature controlled pressure chambers force the carbon dioxide to act as a fluid similar to water (the supercritical fluid CO2) which causes the polymer fiber to swell allowing the dispersed dye to easily diffuse within the polymer, penetrating the fibers, and carrying the dyes into the fabric and dyeing it.
DyeCoo is believed to be the first company to successfully apply the scCO2 process to the commercial dyeing of polyester fabric (the most commonly used fabric in the world), and research is already under way to apply the technology to other natural and synthetic fabrics.
DYEING WITH CARBON DIOXIDE
When carbon dioxide is heated to above 31 degrees Celsius and pressurized to above 74 bar, it becomes supercritical, a state of matter that can be seen as an expanded liquid, or a heavily compressed gas. Above the critical point, carbon dioxide has the properties of both a liquid and a gas. In this way supercritical carbon dioxide, has liquid-like densities, which is advantageous for dissolving hydrophobic dyes, and gas-like low viscosities and diffusion properties, which can lead to shorter dyeing times compared to water. With such properties, they have minimal surface tension which allows for better penetration into materials.
During the dyeing process, CO2 is heated to 120 degrees Celsius and pressurized to 250 bar. In its expanded liquid state, the CO2 penetrates the textile fibres, and acts as a swelling agent during the process, which enhances the diffusion of dyes into the fibres. The glass-transition temperature of the fibres is lowered, which accelerates the rate the dye penetrates the textile.
To fix the dye onto the fibres, CO2 is loaded with dyestuff that penetrates deep into the pore and capillary structure of fibres, providing effective coloration of these hydrophobic materials. After the dyeing process, the remaining CO2 is gasified in order to precipitate the dye and the clean CO2 can be recycled by pumping it back to the dyeing vessel.
Compared to water dyeing, the extraction of spinning oils, the dyeing and the removal of excess dye can all be carried out in one plant in the carbon dioxide dyeing process which involves only changing the temperature and pressure conditions; drying is not required because at the end of the process carbon dioxide is released in the gaseous state. The carbon dioxide can be recycled easily, up to 90% after precipitation of the extracted matter in a separator. Furthermore, dyeing and removing excess dye can be carried out in the same vessel; and residue dye is minimal and may be extracted and recycled.
NIKE AND DYECOO
In 2013, NIKE, Inc. celebrated the opening of a waterfree dyeing facility featuring high-tech equipment to eliminate the use of water and process chemicals from fabric dyeing by entering into a strategic partnership with DyeCoo. NIKE, Inc. has named this sustainable innovation “ColorDry” to highlight the environmental benefits and unprecedented coloring achieved with the technology.
You begin to understand the potential of ColorDry technology when you consider it takes an estimated 30 liters of water to dye a single t-shirt using traditional methods. Take that to a global scale and you’re looking at an estimated 5.8 trillion liters of water used by the apparel industry each year to dye fabric.
ColorDry technology eliminates water by using heat and pressure to convert liquid CO2 to supercritical fluid carbon dioxide, or “SCF” CO2, which then permeates and carries the dye into the fabric.
Not only does ColorDry eliminate water from fabric dyeing, it also reduces energy consumption by around 60% compared to traditional dyeing, eliminates the use of process chemicals, and uses nearly 100% of dye in the process, practically removing the potential for wastewater pollution.
NIKE PLANT USING DYECOO MACHINES IN TAIWAN
DISADVANTAGES
The technology has some disadvantages regarding the dyeing process and machine structure.
- The dyeing process is carried out at 260-280 bar and 130 °C. Such high pressures require a special design of the textile machinery and up-scaling requires a very significant investment. Pressures of 260-280 bar are unusual conditions for the textile industry and may cause mental restrictions
- Due to the use of suffocating gas CO2, a control device for monitoring CO2 concentrations in air has to be installed for industrial labor safety reasons
- Classical color measurement will face some new challenges in this application, due to non additive behavior of the dye components
- The machine is not suitable for dyeing natural (hydrophilic) fibers as the diffusion of supercritical carbon dioxide is hampered by its inability to break the hydrogen bonds
- Reactive dyes, direct dyes and acid dyes may be damaged at such high temperatures and pressure
CONCLUSION
“Color the earth beautiful and kill it with sweet poison”
We talk about saving water every day, but have we ever considered that this vital natural resource is vastly being wasted and polluted in the process of textile dyeing. It is obviously not possible to cut down the process of dyeing altogether. What we can do is find a suitable alternative to this problem. This is where the need of innovations like DyeCoo play a crucial role. This technological advancement has modified the textile dyeing process in such a way that water consumption is minimal and the energy used is controlled to a large extent.
An Article by :- Swati Jain
National Institute of Fashion Technology