Dyeing process dispersion factor

An important assumption, however, is assumption 6, since it separates convection and dispersion. Assumptions 7 and 8 aim to further incorporate the adsorption factor into the model. According to the theory and practice, these assrrmptions are reasonable. These basic assiunptions, and special definitions for some individttal cases, provide a basis for mathematical modelling of the dyeing process. 

The rate of reduction of a vat dye depends partly on the intrinsic chemical properties of the dye and partly on the size and physical form of the dispersed particles undergoing this reaction. The physical factors are much less important than the chemical aspects [26]. The vatting process entails conversion of the insoluble keto form into the soluble sodium enolate (section 1.6.1). The reaction takes place in two stages at ambient temperature. Extremely rapid reduction to the hydroquinone is followed by slower dissolution in the alkaline solution. At higher temperatures, however, the dissolution rate approximates more closely to the rate of reduction. Temperature and dithionite concentration are the important variables and the rate of reduction is much less dependent on dye or alkali concentration. 

Colorants are typically defined as pigments and dyes that, when added to plastics, define a specific color. The subject of colorants has been discussed in numerous books and papers. However, in the context of this chapter, a dye is defined as a substance that is soluble in the resin system and produces color only by the absorption of light and no scattering of light. Pigments are not soluble in the resin system and therefore must be mixed into the resin by one of many dispersion processes. Factors such as pigment particle size and ease of mixing or dispersion directly influence their... 

Furthermore, sample dispersion during the FASS-CE process is reduced by coating microchan-nel walls with methylcellulose to suppress EOF. The preconcentration factors attainable using this approach are excellent, with a concentration increase of 1100-fold reported for two fluorescent dyes (Figure 50.8). 

In the release process, the dye-filled particles were dispersed in 5mL EtOH. One sample was stirred under UV light irradiation and the other was stirred in the dark. After filtration, the concentration of the OBN molecules released into EtOH was determined by the absorbance (A) of the solution at Amax (644 nm) using UV/visible spectroscopy. The release data of the two samples are shown in Fig. 13.20. There are two factors influencing the release rate the effective pore size... 

The optimum amount of UV absorber varies with each chemical type and application, but in most cases is in the 0.25-1.0% concentration range. Other factors to consider while selecting the proper type and amount of UV absorber are color contribution, FDA sanction for food packaging applications, and, of course, add-on cost. The UV absorbers are incorporated into plastic substrates by dry blending, extrusion compounding, and then conversion into the final product by various processes such as injection molding, blown film extrusion, multifilament spinning, and sheet extrusion. Also, UV absorber dispersions can be prepared and dyed onto textile fabrics, which will not only UV-stabilize the polsrmer substrate but also improve the color stability of the dyes used to color the fabric. 

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