Globally, dyeing and finishing processes consume millions of gallons of water, mostly due to multistep processes that are largely inefficient. After the processes are completed, much of this water becomes polluted with unfixed dyestuff and chemicals such as auxiliaries and detergents. In response, dye manufacturers have developed dyestuffs and application processes that aim to increase the exhaustion of dye on to the fiber and also reduce substantially the need for auxiliary chemicals. In this case, supercritical fluid dyeing is remarkable. The ideal situation is to be able to dye fibers without the need for a water‐based process, and this can be achieved by dyeing from supercritical fluids. Carbon dioxide (CO2) is widely produced commercially and is mainly used as a fluid for supercritical owing to its nontoxicity, non-hazardousness, and non- corrosiveness..
What is Supercritical Fluid (SCF):
Supercritical fluid is one kind of fluid which is highly compressed gases which combine properties of gases and liquids in an intriguing manner. It is a substance which can be either liquid or gas, used in a state above the critical temperature and critical pressure where gases and liquids can coexist. It shows unique properties that are different from those of either gases or liquids under standard conditions. A supercritical fluid has both the gaseous property of being able to penetrate anything, and the liquid property of being able to dissolve materials into their components.
Supercritical fluid refers to the phase of a substance with both temperature and pressure higher than the critical point (the point where liquid and gaseous phases of a substance become indistinguishable). Supercritical carbon dioxide (scCO2) is formed at high temperature and pressure, typically around 40–120°C and over 100 bar (Figure 1). In the supercritical state it behaves like a gas as well as a liquid. The gaseous properties enable it to diffuse into polymers, giving a swelling and plasticising action, whilst its liquid properties enable it to dissolve disperse dyes. This phase of a substance enjoys many advantages and can replace water in the dyeing process. The supercritical fluid normally used is carbon dioxide (CO2), as the critical temperature and pressure are easier to achieve than that of other substances. Moreover, carbon dioxide is cheap, non toxic and also non-flammable without residue, so it is suitable for industrial use.
Supercritical Fluid Dyeing:
In textile industry, water is the usual medium for dyeing and cleaning. This is the reason why textile industry is one of the biggest consumers of water in all industries. Textile refining processes without using water have to be applied because costs of water and wastewater are increasing more and more, the legislator fixes more rigorous limit values for the sewage load and also the water resources become more and more limited in several areas.
In cooperation with the DTNW (German textile research centre Northwest) Uhde has developed a textiles-refining process which is uncoupled completely from the water cycle. Therefore the new, ecologically sensible ways which are enough, moreover, also for the highest quality requirements are opened to the dyeing by textiles. Supercritical fluid dyeing is an eco-friendly and sustainable dyeing process.
The cost of supercritical dyeing machines is more expensive than conventional machines for aqueous dyeing, but the operating costs are lower and the quality of the dyed polyester is comparable in terms of color yields and fastness properties.
The Dyeing Process:
The textiles to be dyed are wrapped on a dyeing beam to achieve an equal dyeing result. The dyeing beam is placed in the dyeing autoclave C and the dyestuff is filled into the receiver D2. The pressure vessels are closed and is CO2 led in several steps through the plant.
In the first step the textiles are cleaned from pollutions and sticking auxiliary materials from the production because the rests of wax, oils and other hydrophobic substances can disturb the dyeing process. With the pressurization pump P1 liquid CO2 from the buffer tank D1 is compressed to supercritical pressure and heated up in the heat exchanger E1 to supercritical temperature. The supercritical CO2 flows through the textiles in the dyeing autoclave C and, besides, solves carefully all sticking pollutions from the fibers. The loaded CO2 flows via a expansion valve and becomes by the pressure decrease gaseous. Thereby the solution power is reduced and the extracted pollutions precipitate and are collected in the separator S. Afterwards the cleaned CO2 is liquefied in the condenser E3 and is led via the buffer vessel D1 back into the circulation.
After the pre-treatment the actual dyeing process begins by switching of the dyestuff receiver D2 into the CO2 circulation. The supercritical CO2 solves the dyestuff in the dyestuff receiver and flows through the dyeing autoclave C. The CO2 loaded with dyestuff is delivered through the textiles and the dyestuff is adsorbed in the fibers. After the dyeing autoclave the CO2 flows through a filter to the circulating pump P2 and afterwards is fortified in the dyestuff receiver with fresh dyestuff and is led as long as in the circulation, until the desired dyeing intensity of the textiles is achieved.
After finishing the dyeing step the CO2 circuit and dyed Material are cleaned from excess dyestuff. Therefore the dyestuff receiver is taken out of the CO2 circuit and the loaded CO2 is expanded via the expansion valve into the separator. The excess dyestuff precipitates fall out and is collected in the separator. The CO2 is circulated as long as the plant and the textiles is cleaned from the excess dyestuff leftovers.
After finishing the complete dyeing process the CO2 circulation is stopped and the dyeing autoclave is depressurized to atmospheric conditions. The dyed textiles are taken out of the autoclave.
Advantages of Supercritical Fluid Dyeing:
In traditional water-dyeing technology, textiles should undergo multiple processes with the help of aid agents, chemical salts, surfactants and reduction clearing agents. In contrast, for the supercritical waterless dyeing technology, only supercritical fluid is needed for dyeing and circulation, after which the pressure and temperature can be lowered and the whole process is finished, without producing any wastewater. Also, as carbon dioxide automatically detaches from textiles and remaining dyes, the latter can be reused. More importantly, as operation procedures are reduced, the dyeing cycle is also shortened from several hours to 15 to 60 minutes; energy is also saved due to the lower operational temperature.
This relatively new and innovative process offers unbeatable advantages compared to conventional dyeing:
- Qualitatively equivalent, partially better dyeing result
- No damage of the fiber
- Significantly shortened process and dyeing times
- Low dye and chemicals consumption
- No water consumption
- No reductive post laundry necessary
- CO2 is easily recyclable
- No drying process and therefore no drying devices necessary
- CO2 is innocuous and ecologically friendly
- Energy savings – by the short process times, the lower heat capacity of the CO2 compared with water and no need for drying of the material after the dyeing.
Supercritical fluid dyeing of other synthetic fibers, such as nylon and polypropylene, has not been quite as successful as with polyester and is not commercially available, mainly because of lower dye sorption and poorer fastness properties. However the use of special reactive dyes enables nylon fibers to be dyed, though this has not been done commercially. Also, despite much research, the dyeing of natural fibers is not successful because of the insolubility of ionic dyes in scCO2. Although disperse dyes have been tried with these fibers, they inherently lack substantivity and have proved unsuitable. It is possible that reactive disperse dyes may be a possibility for natural fibers, but as yet there is no commercially viable process using them.
- Emerging Technologies for Textile Coloration Edited byMohd Yusuf and Mohammad Shahid
- An Introduction to Textile Coloration: Principles and Practice By Roger H. Wardman
- Textiles and Environment Dr. N N Mahapatra
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.