4.2.1. Dyeing process:

The dyeing takes place in following steps

1. Dissolution of dye in CO₂
2. Transport to the fibers
3. Adsorption of dye on fiber surface and finally
4. Diffusion of dye into the fiber takes place

The sample to be dyed is wrapped around a perforated stainless steel tube and mounted inside the autoclave around the stirrer. Dyestuff powder is placed at the bottom of the vessel and the apparatus is sealed, purged with gaseous CO₂ and preheated. When it reaches the working temperature, CO₂ is isothermally compressed to the chosen working pressure under constant stirring. Pressure is maintained for a dyeing period of 60 mins and afterwards released. The CO₂ and excess dyes are separated and recycled. After this dyeing procedure, the dry sample is removed and rinsed with acetone if necessary to remove the adhering residual dye.

4.2.2 Effect of temperature and pressure:

The influence of temperature on the dyeing is mainly due to the increase in the diffusion rate of dyes in the polymer and thus affects the dyeing time. Pressure regulates the solubility of the dye stuff. The diffusion coefficients of the dye dissolved in the supercritical medium are higher than in water, leading to generally very short dyeing time. At low temperature, the solubility of the dye stuff in CO₂ is high and with low pressure and high temperature the dye content is small but its penetration into the fiber is facilitated.

Since dyeing virtually takes place from gaseous phase, whereby the dyestuff is homogenously distributed, a high degree of levelness is achieved. For some fabrics extensive extraction of spinning oils should be avoided due to undesirable hardening of the handle of the fabric. The aim of extraction II with cold CO₂ at the end of dyeing process is to remove the unfixed dye and simultaneously decrease the temperature as fast as possible below the glass transition temperature to avoid the extraction of fixed dye from the fiber.

4.3. Uses of Supercritical CO₂:

Supercritical CO₂ is used for:

• Dry cleaning process: Earlier supercritical CO₂ was tried for this process but due to the damage to the buttons liquid CO₂ was preferred.
• It is used as a medium for extracting materials like natural wax, paraffin wax, knitting oil from fibers, yarns and fabrics.
• Another application is the sterilization and disinfection of textiles and related material in the medical field.

4.4. Comparison with conventional dyeing process:

In conventional method of dyeing, water, dyes, and other auxiliaries are used to enhance the efficiency of dyeing process. The cost of waste water treatment and of arranging water of acceptable quality is becoming serious concerns. Either the water available is too hard or not available in sufficient amount or therefore dyeing plants cannot be set up at some places. Compared to this, use of supercritical CO₂ completely avoids the use of water and other auxiliaries, thus creating no effluent. Drying is also not required as CO₂ is released in gaseous state. CO₂ can also be recycled upto 90% and energy required is about 80% less compared to conventional dyeing. Dyeing is only carried on for 2 hrs compared to 3 or 4 hrs of conventional dyeing.

4.5. Advantages:

1. elimination of water treatment and water pollution
2. no need of drying textiles
3. gives good rubbing fastness
4. dyeing occurs with high degree of levelness
5. CO₂ is non toxic obtained from natural resources and can be easily recycled in dyeing process
6. Dyeing houses may be started on sites where there is water scarcity