Disperse dyes solubilities

The traditional use of dyes is in the coloration of textiles, a topic covered in considerable depth in Chapters 7 and 8. Dyes are almost invariably applied to the textile materials from an aqueous medium, so that they are generally required to dissolve in water. Frequently, as is the case for example with acid dyes, direct dyes, cationic dyes and reactive dyes, they dissolve completely and very readily in water. This is not true, however, of every application class of textile dye. Disperse dyes for polyester fibres, for example, are only sparingly soluble in water and are applied as a fine aqueous dispersion. Vat dyes, an important application class of dyes for cellulosic fibres, are completely insoluble materials but they are converted by a chemical reduction process into a water-soluble form that may then be applied to the fibre. There is also a wide range of non-textile applications of dyes, many of which have emerged in recent years as a result of developments in the electronic and reprographic... 

A major drawback of synthetic thickeners when used with dyes is their sensitivity to electrolytes. Most soluble dyes behave as highly ionised electrolytes and disperse dyes contain anionic polyelectrolyte dispersing agents unless they have been formulated with nonionic systems specifically for use with acrylic thickeners. Consequently there is a loss of viscosity this can be quite pronounced although it depends on circumstances, particularly on the dye concentration. As already mentioned, this can be alleviated to some extent by copolymerisation with acrylamide during manufacture. Otherwise it is necessary to try to eliminate all electrolytes from the system or to increase the concentration of thickener. Such measures have their limitations in practice, however. Alternative synthetic thickening... 

These dyes have affinity for one or, usually, more types of hydrophobic fibre and they are normally applied by exhaustion from fine aqueous dispersion. Although pure disperse dyes have extremely low solubility in cold water, such dyes nevertheless do dissolve to a limited extent in aqueous surfactant solutions at typical dyeing temperatures. The fibre is believed to sorb dye from this dilute aqueous solution phase, which is continuously replenished by rapid dissolution of particles from suspension. Alternatively, hydrophobic fibres can absorb disperse dyes from the vapour phase. This mechanism is the basis of many continuous dyeing and printing methods of application of these dyes. The requirements and limitations of disperse dyes on cellulose acetate, triacetate, polyester, nylon and other synthetic fibres will be discussed more fully in Chapter 3. Similar products have been employed in the surface coloration of certain thermoplastics, including cellulose acetate, poly(methyl methacrylate) and polystyrene. 

The new Colour Index volume Pigments and Solvent Dyes lists some 350 solvent dyes and gives their chemical structures, unlike earlier editions which named 800 dyes but included few structures. This fall in numbers is not because of any decreased use but rather the general contraction in numbers of all dyes used in the textile industry. Solvent dyes have been introduced not by attempts to synthesise new colorants but by selection and in some cases modification of known disperse dyes to meet the technical requirements. The majority of solvent dyes are azo compounds but among the blue dyes there are anthraquinones. The aqueous solubility of some of the parent sulphonated dyes has been reduced to acceptable levels by formation of their salts with heavy metals or long-chain alkylamines. 

Dispersion properties and aqueous solubility of disperse dyes... 

A disperse dye suspension responds somewhat differently when a surface-active solubilising agent is added. At low concentrations of this type of additive the saturation solubility of the dye remains relatively little changed, but when the critical micelle concentration of the surfactant is reached a sudden marked increase in dye solubility is observed [57]. When micelles of the surfactant begin to form in the solution, these provide a more amenable environment into which the dye molecules or dimers can transfer. Above the critical micelle concentration the increase in solubility of the dye is directly proportional to the concentration of surfactant present [1,56]. 

Much of the available published data on the aqueous solubility of disperse dyes was accumulated during the 1960s in the course of studies of the mechanism of dyeing cellulose acetate with disperse dyes. Most of the dyes examined were low-energy types for dyeing acetate (Table 3.5). Particular attention was given to monoazo dyes derived from aniline or... 

Table 3.5 Aqueous solubility of disperse dyes of related structure [59-63 ...

Measurements of aqueous solubility and partition coefficient between cellulose acetate and water were compared for thirty disperse dyes and an approximate inverse relationship was postulated [60]. This can only be valid to a limited extent, however, because the partition ratio also depends on the saturation solubility of the dye in cellulose acetate. This property varies from dye to dye and is not directly related to aqueous solubility. The solubilities of four dyes in a range of solvents were compared with their saturation values on cellulose acetate. Solubilities in benzene showed no significant correlation. With the other solvents the degree of correlation increased in the order ethanol < ethyl acetate < 20% aqueous diethylene glycol diacetate (CH3COOCH2CH2OCH2CH2OCOCH3). The last-named compound was suggested as a model with polar groups similar to those in cellulose acetate [86]. 

Table 3.17 Times of half-dyeing and solubilities of disperse dyes in cellulose acetate at 85°C [114]...

Yen. C.-P.C., Perenich, T.A., and Baughman, G.L. Fate of dyes in aquatic systems II. Solubility and octanolAvater partition coefficients of disperse dyes, Environ. Toxicol. Chem., 8(ll) 9981-986, 1989. 

Bach, E. Cleve, E. Schiittken, J. Schollmeyer, E. Rucker, J.W. Correlation of solubility data of azo disperse dyes with the dye uptake of poly(ethylene terephthalate) fibres in supercritical carbon dioxide. Color. Technol. 2001, 117, 13-18. 

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