Dyebath concentration

Isotherms. When a fibei is immersed, in a dyebath, dye moves fiom the external phase into the fibei. Initially the late is quick but with time this slows and eventually an equiUbrium 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, eg, temperature, pH, and conductivity, there is an equiUbrium concentration of dye in fiber, Dj and dye in the dyebath external solution, D. Three models describe this relationship simple partition isotherm, Freundhch isotherm, and Langmuir isotherm. 

Stability Excellent to dilute acids and alkalis at dyebath concentrations. 

In theoretical treatments of diffusion, it is usital to characterise the rrrigration of dyes in terms of a diffusion coefficient D (mVs). Diffusion cocffiderrts are most usirally determined by the application of Pick s laws. Direct measuremerrt of diffusion coefficients can be carried out by measitring the rate of disappearance of dye from solution or by direct observation of dye migration within variorrs substrates. Diffusion coefficients that have been measmed by irrdirect methods (such as changes in dyebath concentration with time) are sometimes referred to as apparent diffusion coefficients. Such diffusion coefficients may not necessarily indicate the rates at which primary diffusion processes occrrr at the molecrrlar level inside fibres. 

The dye concentration at the surface of the fibre is related to the dye concentration in the liquor within the package (local dyebath concentration) according to the sorption isotherm of the dyeing equilibrium. 

Liquids. Some Hquid dyes are made directly from the thionation melt by additions of caustic soda and sodium hydrosulfide. Hydrotropic substances are sometimes added, either at the initial thionation stage or after the polysulfide melt is finished in order to keep the reduced dye in solution. Pardy reduced Hquids are also available. They are usually more concentrated than fully reduced Hquids, thus saving packaging and transportation costs. However, they require a further addition of reducing agent to the dyebath in order to obtain full color value. On the other hand, fully reduced Hquids are... 

Affinity values aie obtained by substituting concentiation foi activity in equation 4 foi the dye and, wheie appropriate, other ions in the system. A number of equations are used depending on the dye—fiber combination (6). An alternative term used is the substantivity ratio which is simply the partition between the concentration of dye in the fiber and dyebath phases. The values obtained are specific to a particular dye—fiber combination, are insensitive to hquor ratios, but sensitive to all other dyebath variables. If these limitations are understood, substantivity ratios are a useful measure of dyeing characteristics under specific appHcation conditions. 

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). 

Most commercial carriers are used in the dyebath at concentrations within the range 1-8 g/1 depending on active strength of the carrier concentrate, applied depth, liquor ratio and... 

A commercially branded product believed to be sodium trichloroacetate this is added at a concentration of 5% to the dyebath 20 minutes before the end of dyeing. Sodium carbonate is formed by hydrolysis of the trichloroacetate (Scheme 12.5), accompanied by a change of pH from 5.0-6.0 to 6.7-6.9 [2]. This reaction also releases the volatile AOX-generating chloroform, however. 

Dithionite and derivatives Minimising the concentration by optimising Oxidation of sulphite to sulphate (isothermal dyeing with oxidation in the dyebath for pale shades)... 

Measurements of the surface tension of aqueous solutions of various sulphonated and unsulphonated phenylazonaphthol dyes showed that the degree of surface activity (that is, the lowering of surface tension) tended to increase progressively with the degree of alkyl substitution in the series of dyes [7]. The surface-active behaviour of such alkylated dye ions ensures that they become more concentrated at the interface between the dyebath and the fibre surface, just as they do at the air-water interface of the dye solution. Foaming of dyebaths can be a serious practical problem with relatively hydrophobic dye structures solubilised by means of a single ionised group. 

As shown in Table 1 the wastewater limit for chromium is 0.5-1 mg/L and Cr is 0.1 mg/L. While conventional 1 2 and 1 1 dyes permit chromium concentrations in the dyebath at the end of the dyeing process of 3.0-13.0 mg/L Cr, the application of modem dyestuffs and optimized processes permits final concentrations to approximately 1 ppm. By general optimization of the process (e.g., dosage of acid), use of dyes with a high degree of exhaustion, and minimal concentration of free chromium [15], final bath concentrations below 4 ppm can be reached, even for black shades. By application of such procedures the exhaustion of the chromium should reach values of better than 95% of the initial value. 

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