Control of the adsorption of dyes on cotton

The present work examines and models physico-chemical aspects of dyeing processes of cotton fibre with a direct dye (C.l. Direct Yellow 162). The models and methods in this thesis were developed with the aim of providing analytical tools that could help to cut back dyeing process times compared to current standard industrial practice, to reduce the need for dye additions and re-works, and to diminish or eliminate the use of some types of auxiliary, such as levelling agents. Theoretical considerations revealed that three parameters appear to be central to the scaling of jet dyeing equipment: the liquor ratio, the number of fabric as well as of dyeliquor revolutions per minute and the dimensionless parameter L, which describes the influence of the tlow regime in the dyeing machine on the exhaustion kinetics. Experiments on the pilotscale jet dyeing machine showed that, under scaled-down industrial process conditions, the dye uptake rate was not affected by boundary layer effects. Several thermodynamic models were examined for their accuracy to predict the dye amount sorbed by the fibre under equilibrium conditions. Two versions of the Gouy-Chapman model and three models derived from the Donnan membrane equilibrium were evaluated. Best overall results were obtained with a GOlly-Chapman model using a variable fibre saturation molality for the dye. The average back-prediction error of this model for the dye on fibre amount was below two per cent. A newly developed model of the exhaustion kinetics of the dyeing process interprets the dye uptake rate as a combination of rapid dye adsorption at the fibre surface with slow diffusional dye transfer from the surface into the fibre interior. The model predicted the exhaustion values of isothermal experiments with an average accuracy of +1- 2.3%. It also accounted well for the effect of changes in the liquor ratio on the dye uptake rate and predicted the exhaustion speed satisfactorily in dyeings with changing salt dosing gradients. A series of experiments was carried out on the pilot-scale jet machine in order to identify the process parameters that significantly influence dyeing unlevelness. They showed that neither the dye amount nor selected non-ionic surfactants nor the holding time at maximum temperature had a notable effect. They also suggested that the critical dye uptake rate per contact was 1.0% when the exhaustion protile was linear. The signiticant intluence of the shape of the exhaustion curve on unlevelness was expressed in the newly introduced variable of the signiticant fibre surface molality change per contact. A statistical analysis indicated that the dye addition time, compensated for the dye-amount dependent effect of the First Strike, had a signiticant influence on un levelness, too.