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: