CO₂ DYEING

When carbon dioxide is heated to above 31ºC and pressurized to above 74 bar, it becomes, supercritical , a state of matter that can be seen as an expanded liquid, or a heavily compressed gas. Characteristic of a supercritical fluid is a high (liquid-like) density that enables dissolution of compounds. For dyeing in supercritical carbon dioxide, the CO₂ is heated to 120ºC and pressurized to 250 bar. The CO₂ penetrates synthetic fibres, thereby acting as a swelling agent during dyeing, i.e. enhancing the diffusion of dyes into the fibres. In other words, the glass transition temperature of the fibres is lowered by the penetration of the CO₂ molecules into the polymer. This accelerates the process for polyester by a factor 2. Finally, the CO₂ is able to transport the necessary heat from a heat exchanger to the fibres.

During the dyeing of polymer fibers, CO₂ loaded with dyestuff penetrates deep into the pore and capillary structure of fibers. This deep penetration provides effective coloration of these materials which are intrinsically hydrophobic. The process of scouring, dyeing, rinsing, drying and removing the excess dye can be carried out in the same batch.