Dyebath dyeing rate

Isotherms. When a fiber is immersed in a dyebath, dye moves from the external phase into Lhe fiber. Initially the rate is quick but with time this slows and eventually an equilibrium is reached between the concentration of dye in the fiber and the concentration of dye in the dyebath. For a given initial dyebath concentration of a dye under given dyebath conditions, e.g.. temperature, pH, and conductivity, there is an equilibrium concentration of dye in fiber, D, and dye in the dyebath external solution, D,. Three models describe this relationship simple partition isotherm. Freundlich isotherm, and Langmuir isotherm. 

Balmforth et al. (jf.S.D.C., 1966, 82, 408) showed that there is a ma.xi-mum concentration of the carrier above which the take-up of the dye by the fibre decreases. This optimum carrier concentration corresponds approximately with the amount necessary to saturate both fibre and dyebath phases of the system. Excess will introduce a third phase, namely undissolved carrier, which will compete with the fibre for the dye. Increase in adsorption at equilibrium is brought about by bisphenyl, orthophcnyl-phenol and phenyl salicylate in the order in which they are set out. Benzoic acid, on the other hand, decreases the adsorption by the fibre at equilibrium and only increases the dyeing rate, probably by promoting the solubility of the dye. It is suggested, in fact, that water soluble carriers such as benzoic acid really act as extremely efficient dispersing agents. 

Various authors have studied the effect of the rate of liquor circulation in dyeing. Vickerstaff pointed ouT that, because of a lower concentration of the dye in the hquor near the surface of the fibre or trapped in the spaces of the yam compared with the external dyebath, the rate of dyeing is dependent upon the efficiency with which the dye is transported to the material being dyed, either by liquor circulation throngh the material or by the material s movement with respect to the dyebath. 

McGregor and Peters examined the mass transport process and defined two boundary layers. The hydrodynamic boundary layer is defined as a layer within which the velocity of liquor rises fiom zero to 99% of the main stream velocity. The layer within which the concentration of the dye rises fiom that at the fibre surface to 99% of the main stream is termed the diffusion boundary layer. The latter was regarded as a layer of Uqrrid which hindered the passage of dye from the bulk of the dyebath to the fibre smface. They showed that an increase in the main velocity stream or in the efficiency of hquor circulation decreased the thickness of the diffusion boimdary layer and hence increased the rate of dyeing. They found a main stream above which the dyeing rate no longer increased with an increase in velocity and where the thickness of the diffusion botmdary layer cotrld be considered zero. 

The dyeing rate is significantly affected by the temperature of the dyebath, increasing in all cases with an increase in temperatme. The effect of temperatrrre on dyeing rate may be expressed in a quantitative marmer by determining the... 

Rate of Diffusion. Diffusion is the process by which molecules are transported from one part of a system to another as a result of random molecular motion. This eventually leads to an equalization of chemical potential and concentration throughout the system, and in the case of dyeing an equihbrium between dye in the fiber and dye in the dyebath. In dyeing there are three stages to diffusion diffusion of dye through the bulk solution of the dyebath to the fiber surface, diffusion through this surface, and diffusion of dye from the surface into the body of the fiber to allow for more dye to diffuse through the surface layer. These processes have been summarized elsewhere (9). 

Although the rates of migration vary considerably from dye to dye and with different dyebath conditions, the generalized relationships between... 

Migration Exhaust Technique for Less than 0.5% Depth of Shade. Start at 50°C with sequestrant at pH 7.0. Add dye over 20 min and raise temperature to highest safe level depending on the dye (95°C with monochlorotriazinyl reactive system) at a rate of 1.5°C/min. Hold for 20 min and cool back to dyeing temperature. Alkali is portionwise added, or mechanically dosed, over 20 min and after a further 30 min the dyebath is dropped and the washing sequence begun. 

The compatibihty value is mainly related to the affinity of the dye for the particular fiber because for basic dyes on modified acryhc fibers there is htde possibihty for migration and therefore this does not play a significant part in determining compatibihty. The rate of dyeing of a specific mixture of dyes of the same compatibihty value is not determined by the value itself. The adsorption of cationic dyes is induenced by the presence of others in the dyebath the presence of cationic retarding agents and electrolytes also induences the rate of exhaustion. It is therefore possible to have a combination of dyes with a compatibihty value 5 that under specific dyebath conditions exhausts more rapidly than a combination based on dyes of compatibihty value 3. 

Transfer of Disperse Dye on Polyester. A specimen of dyed polyester is placed in a standard dyebath with an equal weight of undyed polyester and the dyeing cycle completed. The rate of transfer from dyed to undyed fabric is compared to that obtained with a range of five standard dyes and the dye under test is given the same number as the dye it most closely resembles. 

Dispersion Stability of Disperse Dyes at High Temperature. A disperse dye dyebath is treated under the desired test conditions at 130°C in a special apparatus (Gaston County Lab Dye and Chemical Tester) and filtered through cotton and polyester filters. The filter with the heaviest residue is then compared with a series of standard photographs of standard performance and rated equal to the one it most resembles (1 poor, 5 excellent). 

---