Direct dyes temperature

Class B direct dyes have poor leveling power and exhaustion must be brought about by controlled salt addition. If these dyes are not taken up uniformly in the initial stages it is extremely difficult to correct the urdevelness. They are dyes that have medium—high affinity and poor diffusion. In their apphcation the cellulose is entered into a dyebath containing ordy dye. The salt is added gradually and portionwise as the temperature is increased and possibly the final additions made after the dyebath has come to the bod. 

Class C direct dyes are dyes of poor leveling power which exhaust well in the absence of salt and the only way of controlling the rate of exhaustion is by temperature control. These dyes have high neutral affinity where, resulting from the complexity of the molecules, the nonionic forces of attraction dominate. When dyeing with these dyes it is essential to start at a low temperature with no added electrolyte, and to bring the temperature up to the boil very slowly without any addition of electrolyte. Once at the bod the dyeing is continued for 45—60 min with portionwise addition of salt to complete exhaustion. 

Some direct dyes are sensitive to reduction or hydrolysis under alkaline conditions, particularly if temperatures above 100 °C are used (section 3.1.3) pH 6 is frequently favoured for stability and this can usually be achieved using ammonium sulphate. A few dyes give optimal results under alkaline conditions, using sodium carbonate or soap the tetra-amino dye Cl Direct Black 22 (12.22) is an example. Whether or not an addition is needed will depend on whether alkali is already present in the commercial brand. 

Direct dyes of the disazo diarylurea type, such as Cl Direct Orange 26 (3.5) and Cl Direct Red 79 (3.21), are particularly prone to decomposition in high-temperature dyeing because molecular breakdown can occur by hydrolysis as well as by the reductive mechanism under alkaline conditions. Thus the ureido linkage in this red dye may be broken by hydrolysis to give two monoazo dye fragments (3.22), or the azo groups can be reduced to yield a diaminodiphenylurea (3.23) and two molecules of H acid (Scheme 3.3). 

Because direct dyes become aggregated in aqueous solutions at normal temperatures, substantivity often cannot take effect until the temperature has risen. 

Walther obtained the first direct dyes by self-condensation of 4-nitrotoluene-2-sulfonic acid. So-called Sun Yellow is a mixture of different components, depending on the concentration of sodium hydroxide, the temperature, and the duration of the reaction. Oxidation of the intermediate dye and subsequent reduction with iron and hydrochloric acid gives 4,4 diaminostilbene-2,2 -disulfonic acid, which is used for fluorescent whitening agents and azo dyes. The shades are mostly yellow to red. The structure of Direct Yellow 11, 40000 [65150-80-3] (13 is one of the main components) probably contains a mixture of stilbene, azo, and/or azoxy groups. 

The high temperature stability of direct dyes is an important consideration if one wishes to use these dyes as the colorant for cotton when dyeing a polyester/cotton blend at 130°C.20 The key to success is to choose dyes that are resistant to hydrolysis. Suitable dyes include C.I. Direct Yellow 105, C.I. Direct Orange 39, and C.I. Direct Blue 80, whereas unsuitable dyes include C.I. Direct Yellow 44, C.I. Direct Red 80, and C.I. Direct Red 83. A quick examination of the structures... 

Tests for Identification of Some Synthetic Dyes. Two very simple reactions can confirm the presence of synthetic dyes. In the solvent stripping test, if the ammonia solution is heavily stained and it becomes irreversibly colorless upon the addition of zinc dust even at room temperature, the presence of an azo dye with sulfo group or groups is indicated (an acid or direct dye) (36, 37). The color of the solution in concentrated sulfuric acid can also be an important indication for identifying synthetic dyes. In this test, a few drops of concentrated sulfuric acid are dripped on a small sample of the dyeing, and the color of the sulfuric acid is observed after a few minutes. Intensive magenta red, red-violet, violet, blue, and green solutions indicate the presence of synthetic dyes (36, 37). 

Most dyes, when in solution, are either in molecular and partially ionized state, or exist in the form of ionic micelles similar to those of soap, as described in Chapter 9. Increase of temperature tends to break down micelles into less aggregated units. In the case of the acid and the direct dyes the chromophore-containing ions bear negative charges. These will be repelled by the zeta potential of the cellulosic fibres, but attracted by protein fibres when the aqueous phase is acidic. 

Fig. 16.3 Effect of temperature on exhaustion of some direct dyes...

There are a limited number of direct dyes (Table 16.4) which have a slow rate of adsorption and a high rate of migration, and these are most useful for shading when it is desirable to make additions at higher temperatures. 

It is known that there is a tendency for direct dyes to decompose when boiled for a long time owing to reducing action of the cellulose, especially when conditions are slightly alkaline. This effect is more pronounced at temperatures above 100°C and can interfere seriously with successful dyeing. Butterworth loc. cit.) examined a large number of substantive dyes and classified them into three groups. 

Classification of some direct dyes for high temperature dyeing... 

Many of the direct dyes pass the temperature of maximum exhaustion below 100 °C, and even at the normal boiling point adsorption has become diminished. This phenomenon is more marked at temperatures above 100°C and, after the dye has been levelled at the elevated temperature, it is often advisable to cool back to 85° to 90°C (185° to 194°F) to obtain the best value from the dyes. 

In their reduced state their dyeing properties resemble in many respects those of the direct dyes. They exhaust better in the presence of electrolytes and vary considerably with regard to the temperatures at which maximum e.xhaustion takes place. They are decomposed by acids, usually with the liberation of hydrogen sulphide and the precipitation of insoluble decomposition products. On exposure to air, or when acted upon by mild oxidizing agents, a portion of the sulphur is oxidized to sulphuric acid. 

There are a great number of union dyes available which are usually mixtures of neutral dyeing acid, and direct dyes, giving solid shades with careful manipulation of temperature and use of assistants. It is usually found that the cellulosic fibre dyes to a full shade at 60° to 70°C (140° to 158°F), and as the temperature approaches the boil an increasing amount of the dye is taken up by the protein fibre. At the boil some of the colour adsorbed by the cotton will be transferred to the wool so that, in time, the latter will become much heavier in shade. It is apparent, therefore, that a good solid result depends to a great measure upon control of temperature. It is usually necessary to add 5 to 20 per cent of common salt to promote exhaustfon of the direct dye on the cellulose, and with heavy shades as much as 40 per cent may be necessary. 

I-jther disperse dyes or, preferably, selected acid dyes will reserve the "ccllukisic fibre, and the polyamide will not be dyed by a limited number of direct dyes, provided the temperature does not exceed 90 C (194°F) and the dye liquor is maintained in a slightly alkaline condition. There are also a few direct dyes which will give a solid shade on the two fibres at a of 4 to 5, examples of which are as follows ... 

A single-bath method can be used if desired. The dyes are dissolved separately and added to the dyebath which already contains an ethylene oxide condensate which acts as an anti-precipitant. The dyebath is adjusted to pH 5 to 5-5 with acetic acid and sodium acetate and the goods are entered at 40 to 45°C (104 to 113°F). A period of 45 minutes is taken to raise the temperature to the boil, at which it is maintained for one hour. Alternatively, the acrylic fibre may be dyed first and then the same liquor is neutralized and the cellulosic fibre is dyed. Application of a cationic fixing agent improves wet fastness of direct dyes but copper after-treatment should be avoided because this can have an adverse effect on the light fastness of the cationic dye. Very good fastness is obtained if, after the acrylic component has been dyed, the cellulosic fibre is brought to shade with vat dyes. 

The experiments reported here were formulated with the intention of uncovering, in as much detail as possible, the environment existing within a cellulosic fiber in contact with an ionic aqueous phase. Measurements were made at room temperature ( 25°C) and at 90°C. The salt used most extensively was NaaS04 and measurements were made on the uptake of both cation and anion as a function of the concentration of the bath at 25°C and 90°C and as a function of pH at 25°C. In addition, the uptake of the direct dye, Chrysophenine G, and the effect of the dye on the salt uptake were studied at 90°C. It was also necessary to perform some measurements relating to the stability of the fibers. Finally, some measurements were made on the uptake of the Br ion from NaBr solutions. 

The recovery of direct dye by adsorption on cross-linked fiber was developed and appeared technically feasible. The concentration of amino group fixed in the adsorbent phase was 3.30 mol/kg dry fibers. Atypical direct dye, brilliant yellow was used. The breakthrough cmves for adsorption of the dye were measured for different flow rates, bed heights, influent concentration of the dye, and temperature (Hiroyuki et al., 1997). Chitosan fibers have been studied for the recovery of dyes and amino acids (Yoshido, 1993) but less attention has been paid to the use of this conditioning of the polymer for the recovery of metal ions. 

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