Dyestuffs diffusibility

The practical characteristic of a dyestuff is that when a textile is immersed in a solution containing a dye. the dye preferentially adsorbs onto and diffuses into the texiile. The thermodynamic equations defining this process have been reviewed in detail. The driving force for this adsorption process is the difference in chemical potential between the dye In the solution phase and the dye in the fiber phase. In practice it is only necessary to consider changes in chemical potential and to understand that the driving force is the reduction in free energy associated with the dye molecule moving from one phase to the other, as the molecule always moves to the siate of lowest chemical potential. 

They found sc C02 to be an excellent solvent for the dye. By controlling the temperature they were able to manipulate the diffusivity of the solution and to obtain dyeing uptakes comparable to those of conventional methods. In addition, their method allowed for easier recovery of residual dyestuff and eliminated the need for after-treatments such as reductive washing. 

Anthraquinoid compounds have also been employed as mediators for reduction of dispersed organic compounds, particularly for dyestuffs used in dyeing of cellulose fibers in 0.1 M NaOH [67], The reduction efficiency is characterized by comparing the maximum cathodic current of the anthraquinoid solution containing the dyestuff with the cathodic peak current without reducible vat dye. The limiting current density depends on the diffusion transport of the anthraquinoid compound, whereas the addition of dispersed dyestuff has a minor influence. 

B. Chem, C. W. Hui and G. Mckay, Film-pore diffusion modeling and contact time optimization for the adsorption of dyestuffs on pith, Chem. Eng. J., 84, 77-94,2001. 

Dye diffusion within the fiber is assumed to occur by migration of the dye molecule or ion in the plasticized polymer, rather than by diffusion of bulk water solution of the dyestuff . 

The exhaustion of disperse dyes, at equilibrium, is quite good, but the diffusion into the fibre is extremely slow. I he time required to reach equilibrium, therefore, is much longer than would be permissible in practice. By selecting disperse dyes with the most rapid rates of diffusion it is possible to dye pale to medium shades at the boil within a reasonable time. Dyestuffs which are suitable are shown below- and it will be observed that they all possess simple molecular structure. 

The very heavy shades such as blacks and navies cannot be dyed by conventional methods on polyester fibres, even with the help of carriers. Since the difficulty is the slow diffusion of the larger dyestuff molecules into the closely packed crystalline structure of the fibre, the obvious approach is to present simple molecules and cause them to combine to form a coloured pigment after they have entered. Brenthols and Naphthol AS products are not adsorbed by polyesters from aqueous solutions of their sodium salts. A limited measure of success was achieved by immersing the goods in a suspension of naphthoic acid anilide derivatives and then coupling with a diazotized base. The results were more successful, however, when coupling components with smaller molecules were used and a- and /3-naphthoIs, applied from suspension, were satisfactory but jS-hydroxynaphthoic acid proved to be the most suitable. 

For a discussion of reflectance spectroscopy, two types of reflectance must be defined, specular and diffuse. Specular reflectance is simply mirrorlike reflectance from a surface and is sometimes called regular reflectance it has a well-defined reflectance angle. Diffuse reflectance is defined as reflected radiant energy that has been partially absorbed and partially scattered by a surface with no defined angle of reflectance. The diffuse reflectance technique is widely used today for industrial applications involving textiles, plastics, paints, dyestuffs, inks, paper, food, and building materials. In the area of basic research, diffuse reflectance spectroscopy has been used in studies of solid-solid reactions, of species absorbed on metal surfaces, of radiation transfer, and of slightly soluble species. 

Dyeing with disprse colors depends on the ability of the dyestuff to diffuse into the fiber and upn the capacity of the fiber to absorb dye. These properties depend on the spinning, drawing, and heat-setting conditions, which, as is well known, are... 

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