Simulation of aqueous and supercritical fluid dyeing of a spool of yarn

Supercritical dyeing has risen good expectation in the last few years due to the attractive perspective of complete elimination of water from dyeing processes. A lot of research activities have been developed in the last decade both to deepen the knowledge of the interaction mechanisms between CO 2 , dye and polymers and to solve the technological problems of the process set-up. Among the last ones that of a non-uniform dye distribution when a yarn package is subjected to dye impregnation is one of the most undesired. The same problem is usually faced in the industrial practice of traditional aqueous dyeing and has always been solved empirically. Aim of the present work is to propose a simple model that simulates the dyeing process of a spool of yarn both with the aqueous and supercritical techniques pointing out the main characteristic times and allowing calculation of dye concentration profiles both in the dyeing ‘bath’ and along the yarn spool. The analysis of the influence of characteristic times ratios gives some indications on the best operative conditions that should be attained to guarantee an uniform dye distribution along the package radius both for aqueous and supercritical dyeing; the two processes, in fact, mainly differ for the order of magnitude of the characteristic times. To support the discussion arisen from the different roles of characteristic times on the model, a practical example of supercritical dyeing simulation is also performed for a specific PET package with a disperse dye already used in the literature at well defined working conditions.