Dyeing Of PET Fibre Using Supercritical Carbon Dioxide (SCCO2)


What is supercritical fluid?


Any gas that is above its critical temperature is able to retain the free mobility of gaseous state but if pressure is increased its density will tend to increase towards liquid. Such highly compressed gases are supercritical fluids and that is the reason they are able to combine properties of both liquid and gas. As shown in phase diagram below.


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Introduction:


The textile industry is believed to be one of the biggest consumers of water. On average an estimated 100 kg of water is needed to process 1 kg of textile material. Water is used as a solvent in many pretreatment and finishing processes, such as washing, scouring, bleaching and dyeing. Although there have been efforts to reduce the water input such as altering conventional equipment, recycling water and reusing wastewaterwater usage is still high in the textile industry. Non-aqueous systems of dyeing can reduce or completely eliminate the amount of water used. Reducing water use provides environmental benefits as well as cost savings. Among the most promising of the non-aqueous systems is the use of supercritical carbon dioxide (CO2).


Concern over volatile organic solvent emissions and the generation of aqueous waste streams has prompted a number of chemists and chemical engineers to seek new, cleaner methods for polymer synthesis1 and polymer processing. The use of supercriticalcarbon dioxide (scCO2) has attracted particular attention in both of these areas for the following reasons:


  1. CO2 is non-toxic, non-flammable,chemically inert, and inexpensive.
  2. Supercritical conditions are easily obtained: Tc (CO2) = 31.1C; Pc,(CO2) = 73.8 bar.
  3. The solvent may be removed by simple depressurisation.
  4. The density of the solvent can be tuned by varying the pressure
  5. Many polymers become highly swollen and plasticised in the presence of CO2.


Comparison:


Other advantages of the supercritical CO2 dyeing process are:


  • Contaminated waste water streams are not produced.
  • Dispersants are not required to solubilize a disperse dye in water.
  • Solubilities are controllable by pressure, allowing control of the dyeing intensity and color.
  • Take-up of carbon dioxide by the polymer fiber causes it to swell slightly giving faster diffusion within the polymer.
  • Viscosities are lower making the circulation of the dye solutions easier.
  • No preparation of processing water (by desalting) is required,results in cost saving.
  • No effluents are necessary.
  • There is low energy consumption for heating up the liquor.
  • Energy is preserved because drying processes are no longer required (conventional dyeing processes consume about 3,800 kJ per Kg of water evaporated).
  • No air pollution due to recycling of the carbon dioxide is generated.
  • There are substantially shorter dyeing times .
  • Environmentally acceptable formulations of dyestuff - no dispersants or adulterants are necessary.
  • No chemicals such as levelling agents, pH regulations etc. have to be added.
  • Non-exhausted dyestuff is recuperated in the form of a powder-therefore no waste.
  • Dual eco-friendly.



Note: At present, supercritical dyeing with CO2 is confined to synthetic fibers. For natural fibers the diffusion of supercritical CO2 is hampered by its inability to break the hydrogen bonds present in many natural fibers, including cotton, wool and silk. A further problem is that reactive dyes, direct dyes and acid dyes which are suitable for the dyeing of natural fibers are insoluble in supercritical CO2 and also dye may pptd out at such high pressure and temperature.


Dyeing of PET by Supercritical CO2:


Materials needed: Polyethylene Terephthalate (PET), Azo Dye, industrial grade Carbon dioxide with diptube

Apparatus: An apparatus for dyeing in supercritical carbon dioxide is consists of a temperature controller, a vessel heater which surrounds the vessel, a stainless steel dyeing vessel of 50ml capacity (with a quick release cap), a manometer, a Varex HPLC carbon dioxide pump and a cooler for cooling the head of the carbon dioxide pump. The apparatus was pressure-tested for use up to 350 bars(5,076.321 PSI) and 100 degree Celsius. A side arm connects the top and the bottom of the cell outside the heater to allow the supercritical carbon dioxide to circulate by thermal convection.


Procedure: The process of dyeing by supercritical fluid begins with placing of PET packages inside the vessel in a dry state. CO2 is allowed to enter the dye vessel and the operational pressure and temperature(conventional dyeing temp of fibre)achieved. The dye is dissolved in circulating CO2 in the chamber. The Dyeing takes around half an hour under optimal conditions. The concentration of the dye compound in the supercritical CO2 determines the shade. This shade can be manipulated by density of supercritical CO2.Small quantities of modifier can increase the solubility of the dye. The dyeing cycle ends with the depressurization of the system and collection of excess dyestuff in the Recovery Vessel. A Typical Plant And Process Flow Sheet Is Shown Below,


Disadvantage:

  1. Equipment permitting operation at the require temperature and pressures with holding capacities up to one cu.m. are considered to be difficult and the employed for high pressure extraction processes, many step towards an industrial-scale application in textile plant already being accomplished.
  2. High cost investment, however it can covered up by Quality of fabric and also from carbon credit provided by GOVERNMENT OF INDIA.
  3. It is a batch process, so processing of long length fabric (continuous) is not possible.

Conclusion:


Dyeing in supercritical carbon dioxide has been identified as one of the best alternatives to water-based dyeing and the same has been dealt in detail in this paper. But, this favourable concept is waiting for its commercial implementation. The successful commercialization of the above said concept will definitely improve the economics of dyeing by the way of elimination of wastewater discharges.


Reference:

  1. Dyeing in Supercritical Carbon Dioxide,http://www.geocities.com/nitiz_sood/dye.html
  2. Farias, Len, Senior Textile Chemist for Cotton Incorporated.  Personal Interview. January, 2008.
  3. Milmo, Sean. Textile Outlook International. Developments in Textile Colorants. January-February 2007
  4. ETTERS, J.N., Textile Res. J., Vol. 64 (7), 1994.
  5. CHANG, K.-H., BAE, H.-K., SHIM, J.-J., Korean J. Chem. Eng., Vol. 3 (3), 1996
  6. DRAPER, S.L., Montero, G.A., SMITH, B., BECK, K., Dyes and Pigments, Vol. 45, 2000
  7. VAN DER KRAAN, M., FERNANDEZ, M.V., WOERLEE, G.F., VEUGELERS, W.J.T.,WITKAMP, G.J., 4th Int. Symp. on High Pressure Process Technology and Chem. Eng., sept. 22-25, Venice, Italy, vol.1, 2002,
  8. SCHMIDT, A, BACH, E., SCHOLLMEYER, E., Dyes and Pigments, Vol. 56, 2003

 

The Author is an Engineering student 3rd year textile processing SCET College, Surat