Dyebath temperature control

Experimental work on dyebath exhaustion control was also carried out using a pilot-scale radial flow package dyeing machine, and the results supported the flndings of the theoretical model. In particular, it was found that a quadratic profile was preferable to an exponential profile, which in turn was preferable to a linear profile. All controlled exhaustion dyeings gave better levelness than a standard (constant temperature ramp) dyeing method. This work was continued by Illett, who improved the mathematical solution of the Nobbs-Ren model and applied it to axial flow machines. 

The rate of exhaustion is usually controlled by adjustment of the dyebath temperature. In some cases it is also possible to control exhaustion by using a constant or nearly constant temperature, while affecting the composition of the dyebath by addition of salt, acid, alkali or other chemicals. pH influences the exhaustion at equilibrium , and the higher this value, the higher is the rate of dyeing at that moment. The value of the initial exhaustion (strike) inapH-controlled process must be selected in snch a way that the initial nnlevelness is no more than can be rapidly corrected by the migration of the dye. 

The effects of the addition of salt, concentration of dye and the dyebath temperature may also be considered in the design of the pH control algorithm. If the desired pH at any given cycle is not achieved, the next cycle s pH value should compensate for this shortcoming. A simple flowchart of this process is given in Fig. 7.26. 

Class B direct dyes have poor leveling power and exhaustion must be brought about by controlled salt addition. If these dyes are not taken up uniformly in the initial stages it is extremely difficult to correct the urdevelness. They are dyes that have medium—high affinity and poor diffusion. In their apphcation the cellulose is entered into a dyebath containing ordy dye. The salt is added gradually and portionwise as the temperature is increased and possibly the final additions made after the dyebath has come to the bod. 

In the application of tliesc it is essential that the dyeing should commence at a low temperature and no electrolyte is added to the dyebath at the commencement. The temperature is raised to boiling point very slowly and dyeing is then continued for a further to 1 hour. There is often a comparatively narrow temperature range over which exhaustion is most rapid, and it is during this period that control is most important. As an illustration a lew Class ( dyes are quoted in Cable 16.3 together with the range of maximum exhaustion. 

Han et al. [39] further found that the diffusion coefficient of the mononuclear HC red 3 increased with pH, with dye concentration (1.0 to S.Og/liter), and with increasing temperature (25 to 60°C). However, changing the dyebath solvent from water to 50vol % aqueous ethanol decreased dye uptake, but the diffusion coefficients remained similar in magnitude. The pH effect can be explained by increased swelhng of the hair, and the dye concentration and temperature effects are consistent with expectation for a diffusion-controlled interaction. The solvent effect occurs because the dye is more soluble in the ethanol-water system than in water alone, thereby increasing the affinity of dye for the solvent phase relative to the keratin and causing more of the dye to partition into the aqueous-ethanol phase and less into the hair. 

The change in the desired exhaustion rate in the model is governed by a change in temperature. In order to calculate the set temperature required for any given exhaustion, the dyeing period is divided into a series of control cycles . Within each cycle it is assumed that the rate of exhaustion has a first-order dependence on the current concentration of dye in the dyebath. 

The concentration of the dye was changed from 0 to 700 ppm while the temperature was maintained at 38°C (eqnilibrium temperature of the dyeing machine). Such comparisons can be nsed to confirm that the performance of the dye inventory at predicting the amonnt in the dyebath is satisfactory. Control profiles can also be tested in a similar marmer. 

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