Supercritical Fluid (SCF) | Supercritical Fluid Dyeing | Advantages of Supercritical Fluid Dyeing

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 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 consumer of water in all industries. Textile refining processes without using water have to be applied because costs of water and waste water 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.

Supercritical Fluid graph
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.

Advantages of
Supercritical Fluid Dyeing
This relatively new and innovative process offers unbeatable advantages compared to conventional dyeing.

  1. Qualitatively equivalent, partially better dyeing result
  2. No damage of the fibre
  3. Significantly shortened process and dyeing times
  4. Low dye and chemicals consumption
  5. No water consumption
  6. No reductive post laundry necessary
  7. CO2 is easily recyclable
  8. No drying process and therefore no drying devices necessary
  9. CO2 is innocuous and ecologically friendly
  10. 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.
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 CO2led in several steps through the plant.

1. Pretreatment
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 fibres. 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.

2. Dyeing
After the pre-treatment the actual dyeing process begins by switching of the dyestuff receiver D2 into the CO2circulation. 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 fibres. 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.

3. After Treatment
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.

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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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