Dyebath exhaustion 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. 

Holdstock C.R. (1988), The Measurement and Control of Dyebath Exhaustion PhD Thesis, University of Leeds, UK. 

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. 

As a general rule, the lower the dyebath pH the more rapid is the rate of initial adsorption by the amide fibre and the subsequent approach towards equilibrium exhaustion. Consequently, the attainment of a safely controllable rate of dyeing becomes easier the higher the pH at which dyeing begins. Eventually, however, a pH is reached at which the dyebath is no longer exhausted within a reasonable time. 

The electrolyte (i.e., sodium chloride or sodium sulfate), its concentration, and the speed of addition control the adsorptive behavior of the dyes and the degree of exhaustion. A high electrolyte content of the dyebath shifts the dyeing equilibrium toward the fiber. 

The dye liquor is made up in a manner similar to that described for Class A but the salt is omitted. It is added gradually during the period that the liquor is coming to the boil, and, if desired, the addition may be continued after the boil is reached. If the salt can be dissolved in a separate vessel and added through a drip feed, this method is greatly to be preferred. Surface-active compounds, of which Lyogen DK is an example, can be added to the dyebath with Class B dyes to reduce sensitivity to salt and exercise further control over the rate of exhaustion. 

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. 

The Abbott-Cox process relies upon exhausting the suspended pigment. In many cases exhaustion is difficult to control and irregularity in the shade can be the result. Vat dyes in their insoluble oxidized state are available in a super finely divided form which may be added to the dyebath and circulated through the goods on which they will be uniformly deposited without the use of an electrolyte to bring about coagulation. After a suitable period sodium hydroxide is added, followed by sodium hydrosulphite when the leuco compound becomes adsorbed on the fibre and is... 

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 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 addition of dye is used in controlled exhaustion as well as integration dyeing. Therefore, it is important to assess the performance of the metering pmnps in dosing calculated quantities of the solntion to the dyebath. 

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