Direct dyes after treatment

Direct dyes have only modest fastness to washing, which may be improved by after-treatments such as metal-complex formation (section 5.5.3) or by diazotisation of the dye on the fibre and further coupling of the diazonium salt with an insoluble coupling component (section 1.6.14). In addition to their use on cotton and viscose, direct dyes are important in the dyeing of leather. The cheapest members of this class are also used in the coloration of paper, since for this purpose fastness properties are largely irrelevant and price is all-important. 

Printing with Direct Dyes is common only for inexpensive goods, because of the extended steaming time required and the limited wetfastness of the prints, even after treatment with cationic quaternary products [5, p. 514],... 

The product has practically no Influence on the light fastness and the shade of goods dyed with reactive and direct dyes. The handle of synthetic textiles may become somewhat fuller after the treatment with FIXAPRET COC. 

These dyes are directly applied on fibers, most often cotton, wool, flax, or leather in a neutral or alkaline bath. They have low wet-fastness, and frequently need after-treatments. Direct Black 38 (Figure 18), a triazoic compound dye used for cotton, wool, and silk, has been implicated in patients wearing black clothes, with concomitant immediate-type reactions in some cases. 

Basic dyes, being cationic, can, under certain conditions, be precipitated by direct or acid dyes which are anionic. The two, therefore, cannot be used together except at very low concentrations. Advantage can he taken of this mutual precipitation because the direct dye acts as a mordant when on dyed cellulosic materials and, therefore, basic dyes can be used for after-treatment to brighten shades. 

The lack of wet-fastness of direct dyes is undesirable for many purposes, particularly for materials which will be expected to withstand washing. Many methods have been used to increase the molecular weight of the dye, after it has been adsorbed by the fibre, to render it less soluble in water and therefore more fast to wet treatments. If the molecule is too great it will not enter the fibre or, if it could, tlu dye would be extremely unlevel-dyeing. It is therefore necessary that the preliminary deposition should be achieved with a soluble and comparatively small molecule, and the reaction rendering it less soluble should be carried ut in situ. 

There are many direct dyes of which the wet-fastness is improved by after-treatment with a copper salt such as copper sulphate or, alternatively, spedally prepared mixtures of copper salts and cationic resin-fixing scents. Some of the dyes which have been specifically developed for this treatment are the Cuprofix, the Cuprophenyl, and the Coprantine colours, as well as many others not necessarily classified in distinctive groups. Examples of mixtures of copper salts and cationic products for after-treatment are Resofix C and Ct rantex B. 

The present practice of copper after-treatment owes its origin to the discovery in 1915 that some of the oo dihydroxy direct dyes would form complexes with copper. The hue of the new derivative was usually somewhat duller, but boA the light- and the wet-fastnesses w ere improved. An example is Sirius Super Green BTL in which the ethoxy and hydroxyl groups are in the oo positions as shown below ... 

There are direct dyes the wash-fastnesses of which are improved by after-treatment with potassium bichromate in the following maimer. Immerse for 20 to 30 minutes at 60° to 80°C (140° to 180°F) in a solution containing 1 to 3 per cent of potassium or sodium bichromate and 1 to 2 per cent of acetic acid (30 per cent). Fastness to light as well as washing is sometimes improved by substituting 1 to 2 per cent of bichromate, 1-2 per cent of copper sulphate, and 2-4 per cent of 30 per cent acetic acid. 

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. 

Chitosan treatment has shown improved dye uptake on cotton fiber both in presence and absence of electrolyte as discussed earlier. However, the dye was mostly confined to surface of fabric due to lack of penetration of parent chitosan. Fastness properties were also found to be deprecated. A study on the effect of nano chitosan treatment on the direct dyeing of cotton is reported in this paper [94], Further, the dye bath was made slightly acidic with acetic acid [0.5 g/L] and treated for 15 min after the conventional dyeing was over [i.e., after the 60 min] [Figs. 19.43 and 19.44]. 

From the data in Table 3.1, it can be seen that the changes in dye absorption are dependent on the type of dye used. When reactive dye was used, color strength increased after the plasma treatment, whereas for the direct dye, color strength decreased. In the case of natural dyes, strength increased after plasma treatment. Thus it is seen that the type of dye is important (Bhat et al., 2011). 

Direct dyes have an inherent substantivity for cellulose fibers. The dyebath is gradually heated and electrolyte added to promote exhaustion and diffusion. The most attractive feature is the simplicity of the dyeing process. However, a cationic after-treatment is necessary for most direct dyeings to enhance washing fastness (Cook, 1982 Shore, 1995b). The wash fastness of the apparel item, even with an aftertreate-ment, is only satisfactory for pale and some selected medium shades. Consequently direct dyes have been largely replaced by reactive dyes. 

Direct dyes n. A class of dyestuffs that are applied directly to the substrate in a neutral or alkaline bath. They produce full shades on cotton and linen without mordanting and can also be applied to rayon, silk, and wool. Direct dyes give bright shades but exhibit poor washfastness. Various after treatments are used to improve the washfastness of direct dyes, and such dyes are referred to as after treated direct colors . 

The In Vitro 3T3 neutral red uptake phototoxicity test (3T3 NRU PT) was included in Annex V Method No. 41 to Directive 86/906/EEC (2000) and accepted by OECD (Test Guideline 432, 2002) and the US FDA (2003). The 3T3 NRU PT test is used with a basic screen to identify acute phototoxic potential. It is based on a comparison of the cytotoxicity of a chemical when tested in the presence and in the absence of exposure to a non-cytotoxic dose of UVA/Vis light. Cytotoxicity in this test is expressed as a concentration-dependent reduction of the uptake of the vital dye, NR, 24 h after treatment with the test chemical and irradiation. 

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