Supercritical carbon dioxide assisted dyeing: A novel approach - II

Supercritical fluids offer advantages in textile processing as they combine the valuable properties of both gas and liquid, say M Subramanian Senthil Kannan and R Nithyanandan

Description of a supercritical dyeing system

The dyestuff/supercritical carbon dioxide/fiber system will in this respect, represent a three-component/ three-phase system. The three components are the gas, the dyestuff and the fiber polymer. In their solid state, dyestuff and polymer are present in the form of three separate phases besides the supercritical mixture. The dyestuff is dissolved in the supercritical fluid, transferred to, absorbed by and diffused into the fiber. In the first approximation the system is described as the distribution equilibrium of the dyestuff between fluid and fibers. A more exact definition of the thrmodynamic processes involved in this system will have to consider the solubility of carbon dioxide in the polymer and in the solid dyestuff as well as the solubility of the polymer in the fluid. For the sake of simplification, the dyestuff will be considered as pure component, whereas the solubility of carbon dioxide and polymer in the solid dyestuff can be neglected. The solubility if the polymer in the fluid is so low that it can be neglected as well. All other mixtures can, however, significantly affect the dyeing process.

Model of the dyeing process

In the following we shall discuss the principle underlying the dyeing by supercritical media drawing on the example of a theoretical dyeing of polyester (PES). In the first instance, the supercritical fluid performs the two essential functions of aqueous liquor, namely the transfer of the dyestuff and of the heat to the fibres. The model is subdivided into four steps:

1. Dissolution of the dyestuff

2. Transfer to the fibre

3. Absorption of the fibre surface

4. Diffusion into the fibre.

The solubilising power of carbon dioxide in the supercritical state corresponds to that of the weekly polar solvents. In contact to the conditions governing dyeing in an aqueous liquor, the disperse dyestuff is transferred to the fibre out of a molecularly disperse solution and not by micelles which will then allow it’s molecularly disperse liberation. In this respect, there exists a certain similarity to solvent dyeing techniques which, some times ago, were frequently discussed as an option to dye polyester fibres, which, however, did not gain acceptance for environmental and toxicological reasons. Other reasons for discarding this option included high prices and unfavorable energy balances in the case of recovery of the solvents. All these shortcomings are avoided in the case of carbon dioxide. On the other hand, potential merits of solvent dyeing techniques are surpassed in many respects. The density and thus the dissolvability of the supercritical fluid is more or less similar to relative properties of liquids, whereas the viscosity is similar to that of a gas. This has an impact on the dyestuff transfer. Due to its low viscosity, the fluids will readily enter pores and capillaries of fibres and/or fibre bundles. The penetration, for instance of yarn packages, by the supercritical fluid will cause a substantially lower pressure drop. In a practical case, this means high degrees of solved molecules such as for dyestuff are higher by more than three powers of ten compared to those of liquids. This will allow a faster mass transport and, therefore, significantly higher dyeing rates. Due to the favorable diffusion properties of the supercritical fluid, even the times needed for the dissolution of the solid dyestuff will be cut to a negligible minimum.

The state of the dyestuff in a super critical solution can virtually be described as gaseous. This means that it will be absorbed by the fibre at a rate comparable to the high diffusion rates corresponding to that of a gas. In addition, the dissolved dyestuff will be quickly available for diffusion into the boundary layers. This results in high degrees of levelness and low convection in spite of high absorption rates. In addition, the absorption equilibrium will be achieved very quickly, which in turn will favorably influence the degree of levelness. In this connection, we would like to mention that in the case of using dyestuff free carbon dioxide and changing of processing parameters it is possible to extract dyestuff from the fibre.

A crucial difference to dyeing process using a liquid phase is to be seen in the fact that the solubility of the dyestuff in a supercritical fluid can be continuously changed across a wide range. The distribution balance dyestuff-fluid/dyestuff-polymer can in fact be continuously shifted in favor of the polymer until after expansion of the gas to standard pressure the solubility in the fluid will be equal to zero, where a theoretical exhaustion level of 100 percent is reached. In the case of using liquid media this would only be possible by evaporation of the solvent. Spectrophotometric measurements in a supercritical medium during stepwise reduction of the density have shown that short-time over saturated solutions will be formed which accelerate the absorption of the dyestuff molecules lack other condensation nuclei during the gaseous phase.

Supercritical carbon dioxide will be partly dissolved in the polymer. It has a softener-like effect which accelerates the diffusion processes by increasing the chain mobility of the polymeric molecules. This indicates possibilities of cutting dyeing times and/or an option to use lower dyeing temperatures. As soon as the fluid expanded to the atmospheric pressure again it will completely lose its capacity to dissolve the dyestuff. Any unfixed dyestuff will drop out during the expansion phase in the form of a dry powder and can be disposed of. The textile goods leave the dyeing equipment in a dry state and do not contain any solvent because the carbon dioxide is completely eliminated.

(To be continued)

The authors, Mr Kannan is with Technology Innovation Group, The Arvind Mills, Naroda Road, Ahmedabad and Mr Nityanandan is with SGS India, Tirupur, Tamilnadu.