Direct Dyeing Techniques

Insolubilization. Insolubilization of compounds within textiles parallels the history of humanity the direct dyeing techniques for cotton were highly advanced in the Bronze Age. With the exception of fiber-reactive dyes discussed earlier, other cotton dyes, ie, vat and sulfur, are insolubilized within the fiber after an oxidization step. Insoluble metal oxides have been used to flameproof cotton, and zirconium compounds have been insolubilized on cotton to render the fabric microbial resistant (135) or mildew resistant (136) via a mineral dyeing process (see Textile Finishing). 

Radioactive tracer techniques have been used to study the absorption of various ions by cellulose from salt solutions and from dyebaths. Measurements have been made on the absorption of Na and SO4 ions from Na2S04 solutions at 25°C and at 90 C and from Na2S04 solutions containing a direct dye at 90°C. Measurements of the amount of dye absorbed were made using conventional techniques. The absorption of Br from NaBr solutions was also studied. 

Several recent scientific studies have been concerned with the application of high-performance liquid chromatography for the analysis of direct dyes in semipermanent hair coloring products primarily for quality control purposes [37, 42]. This technique has been used successfully for the complete separation of 18 dyes in a standard mixture and also for the separation and analysis of these dyes in eight commercial hair color products. 

Fibers may be identified by their response to different dyes. Animal fibers readily take up acid dyes, while plant fibers are more responsive to direct dyes. Combinations of dyes that will permit identification of different fiber types are commercially available. The most common commercial solution is Neocarmin. See technical literature or the Internet for more information. As in solvent testing, however, this technique is not suitable for making distinctions among similar fibers. 

Concentration and separation of dyes in nonionic surfactant anploying a cloud point technique has been explored by Tatara et al. (2(X)4). The researchers used oxyethylated nonionic surfactants and investigated their potential to separate two direct dyes and one basic dye for recovery. It was found that the method had some potential however, separation occurred slowly by accumulation of the organic solute in the surfactant-rich phase. Both surfactant and dye as well as other reaction parameters had to be selected appropriately for reasonable results. Cloud point extraction was also explored by Purkait et al. (2(X)4) for the direct dye Congo Red. Nonionic surfactant was used and recovered from the dilute phase by solvent extraction with heptane. 

A technique for the microscopic localization of antigens in or on various cells. In the direct staining technique, a fluorescent dye (e.g. fluoroscein) is linked to an antibody which is added to the tissue preparation. Sites where antibody has combined with antigen exhibit a brilliant fluorescence which can be seen microscopically. 

HPLC is often reported to be the technique of best choice for the quantification of food colorants. According to European Directive 94/36/EC, the quantities of synthetic colorants to be added to foods are restricted and thus reliable methods for their quantification must be established. Approved colorants, defined by E-coded numbers (Table 6.6.2), are permitted for non-alcoholic beverages, confectionery products, and even for caviar (dying fish roe). For example, a specific HPLC chromatographic method for the quantization of 14 synthetic food colorants belonging to azo dye, triphenyhnethane, or quinophthalone classes (E 102,104, 110, 122,123, 124, 127, 128, 129, 131, 132, 133, 142, 151) was reported to check their contents in caviar. ... 

A wide variety of enzymes have been used in conjunction with electrochemical techniques. The only requirement is that an electroactive product is formed during the reaction, either from the substrate or as a cofactor (i.e. NADH). In most cases, the electroactive products detected have been oxygen, hydrogen peroxide, NADH, or ferri/ferrocyanide. Some workers have used the dye intermediates used in classical colorimetric methods because these dyes are typically also electroactive. Although an electroactive product must be formed, it does not necessarily have to arise directly from the enzyme reaction of interest. Several cases of coupling enzyme reactions to produce an electroactive product have been described. The ability to use several coupled enzyme reactions extends the possible use of electrochemical techniques to essentially any enzyme system. 

Major applications of modern TLC comprise various sample types biomedical, pharmaceutical, forensic, clinical, biological, environmental and industrial (product uniformity, impurity determination, surfactants, synthetic dyes) the technique is also frequently used in food science (some 10% of published papers) [446], Although polymer/additive analysis takes up a small share, it is apparent from deformulation schemes presented in Chapter 2 that (HP)TLC plays an appreciable role in industrial problem solving even though this is not reflected in a flood of scientific papers. TLC is not only useful for polymer additive extracts but in particular for direct separations based on dissolutions. 

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