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Development of a water-free dyeing process for cotton is essential for the textile industry due to ecological and economical reasons. In this study, a dyeing method is described where cotton has been effectively dyed in supercritical carbon dioxide (scCO 2 ). Excellent dye fixation of 100%, and colour strength (K/S) values up to 30, were achieved in a small batch reactor and in a scale-up vessel. A series of non-polar reactive dyes with fluorotriazine as reactive group were synthesised at our laboratory. Fluorotriazines were found to be the best dyes for dyeing cotton and their reaction with cotton was improved by adding small quantities of acids to the reaction medium. H 3 PO 4 and HAc were tested at different concentrations on cotton dyed with fluorotriazines. Evenly dyed pieces of cotton without any damage to the cotton fibres were observed in all experiments. An important step forward has been made for the future commercialization of a green process for industrially dying cotton in scCO 2 . Elimination of water and its costly treatment can be now achieved in the cotton dyeing process.

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A new simple method to improve the dyeability of cotton in supercritical carbon dioxide, which does not require any chemical modification of the cotton, is described here. The affinity of the cotton is enhanced through physical interactions between the pretreatment solvent and the cotton structure. Addition of cosolvents and dissolution of the dye greatly improved the coloration and fixation of the cotton. Experiments with different solvents as pretreatment and as cosolvents were performed. Methanol was found to be the best solvent for the pretreatment. Dimethylsulfoxide (DMSO) as cosolvent and dye carrier provided the highest cotton coloration ( K/ S 15.3), but a fixation of only 2%. An increase in fixation to 62% was achieved when methanol was the cosolvent and the dye carrier.

The dyeing of polyester textile in supercritical carbon dioxide (scCO 2) was investigated experimentally. The influence of temperature and density of the scCO2 on the process was studied in the ranges 85—125°C and 400—550 kg/m3. The dye saturation concentration in the polyester increased and the distribution coefficient decreased with temperature, the latter showing a logarithmic dependence on the reciprocal of temperature. Increasing the fluid density led to an increasing saturation concentration and a decreasing distribution coefficient. When the right temperature and solvent density were chosen for the supercritical process, the same dye concentration could be attained as in aqueous dyeing. It was found that the adsorption of the dye on the polyester followed Nernst adsorption, as is the case in aqueous dyeing. The experiments showed that the dyeing was exothermic, with a negative change of entropy. The thermodynamic characteristics of supercritical and aqueous dyeing were concluded to be roughly the same, with similar saturation concentrations, thermodynamic affinities and heats and entropies of dyeing.

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Reactive dyeing of cotton in supercritical carbon dioxide is a green alternative to reduce water consumption and wastewater. Cotton could be successfully dyed and the reaction kinetics was determined; however, several issues need to be clarified for further optimization of the dyeing process. For instance, it was observed that the difluorotriazine dye reacted very fast with cotton but its kinetics showed a negative activation energy. Furthermore, it seemed that fixation was strongly related to the nature of cosolvents methanol and dimethyl sulfoxide. Finally, the dihalogenotriazine dye reacted selectively with cotton in the presence of methanol, commonly used as a model for cotton. A molecular modeling approach was applied to provide explanations for the above-mentioned experimental phenomena. In combination with experimental data, molecular modeling was shown to be a powerful tool for the understanding and optimization of the process of cotton dyeing in supercritical carbon dioxide.