Volume 1 Issue 2
Supercritical Carbon-dioxide Assisted Dyeing of Textiles: An Environmental Benign Waterless Dyeing Process
The textile industry is among the industries with the highest water consumption; water is being utilized at all stages during chemical wet processing of textiles. The water effluent contains dyes as well as dispersing agent and various chemicals. The treatment of these effluents leads to the increase in the ultimate cost. Since costs for water and waste water are continually rising, and the legislator lays down increasingly stringent limits for the pollution of waste water, textile dyeing and finishing methods have to be used in future which will use no or very little water. Various new techniques, such as plasma dyeing, e-control method, solvent induced dyeing method, have been developed to overcome the present deficiency in the conventional dyeing system. One such method is the use of supercritical carbon dioxide (SC-CO2) instead of water for the dyeing of textiles, particularly made from synthetic fibres (polyester, acetate, nylon). Carbon dioxide is available in abundance, ecologically harmless, non -toxic and non-explosive. When placed above the critical point (31.1°C and 73 atmospheres), CO2 becomes a remarkable solvent for many natural and synthetic dyes, which are utilized for the colouration of cotton and various synthetic fibres. The dye solution in the supercritical CO2 is carried to the fibre to be dyed. Under certain conditions, the gas-like diffusion of supercritical CO2 disperses the dye evenly into the small pores and crevices of the fibre. In the dyeing process, the dyestuff powder is fed into the autoclave. The dyeing equipment is flushed with liquid CO2 and preheated liquidified CO2 picks up the dye and performs dyeing of the package as in solvent dyeing. This dye gets absorbed onto the fiber. When the pressure is released CO2 becomes gaseous and loses its dissolving ability and the dye residues are separated after liquidification. Carbon dioxide, free from dye, goes back into the collecting tank after completion of the dyeing process. The circulation of CO2 is stopped, dyeing autoclave is depressurized and the unused dye powder gets deposited at the bottom of the machine. Thus, super-critical dyeing does not produce any drainage and may be considered as an eco-friendly dyeing approach.
supercritical, carbon dioxide, dyeing, solvent, eco-friendly