Acid and Direct Dyes

The general wetfastness properties of acid dyes are normally enhanced in comparison to those of the disperse dyes due to the fact that the hair protein forms additional salt linkages with the anionic dyes. Moreover, the lightfastness is in most cases better than with disperse dyes. The application technique and killing pretreatments for acid dyes are similar to those for disperse dyes. 

In dyeing the hair and the suede portion a number of factors must be allowed for. The keratin of the hair contains basically the same amino acids as the collagen of the skin but in a different ratio. The keratin of the hair includes cysteine, which cross-links the polypeptide chain and imparts stability. The collagen of the skin does not have these substances, and the cross-links are made by the tanning agent. On the other hand, only L-hydroxyproline can be found in the collagen. As a result the thermal stability is different, and in addition the isoelectric points of the two polypeptides diverge. 

These facts play an important factor in the dyeing behavior. The isoelectric point of hair is at a pH of ca. 5, whereas untreated collagen has its isoeletric point at a pH of ca. 6, which varies with the kind of tannage. For pure chrome suede it is close to pH 7 or about pH 6 after slightly anionic retanning. Consequently, for good dye fixation the acidification at the end of a dyeing is around pH 3 for fur hair and ca. pH 4 for chrome-tanned suede. 

One method of classifying acid dyes is to divide them into groups according to their application behavior. The traditional wool/nylon/cotton classification is customarily employed by the Colour Index and is therefore used here, too. 

In practice, a less acidic pH and a lower temperature are used for fur than for wool. A pH of 4.5 and a temperature of 50-60 °C are sufficient, if used in combination with leveling agents. These are normally ethoxylated fatty alcohols or amines. The auxiliaries promote leveling and regulate the uptake of the dyes. Combination with an alkyl phosphate ester as carrier is recommended. 

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Table 2. Comparison of Basic, Acid, and Direct Dyes...

Hou (1992) used a simple screening test to determine whether acid and direct dyes precipitate at calcium concentrations typical of hard waters of the SE Piedmont region of the U.S. Of the 52 dyes tested, only three direct dyes (Direct Black 19, Direct Black 22, and Direct Blue 75) and seven acid dyes (Acid Red 88, Acid Red 114, Acid Red 151, Acid Brown 14, Acid Black 24, Acid Orange 8, and Acid Blue 113) precipitated. Although the Ca salts of acid and direct dyes were thought to be the most likely metal salts to precipitate after dye discharge to natural waters, the precipitation is not likely to occur unless dye concentrations exceed 0.02 to 0.6 mg/L, a level far greater than reported concentrations of dyes in surface waters. 

Baughman, G.L. 1995. Fate of azo dyes in aquatic systems. Part 3 The role of suspended sediments in adsorption and reaction of acid and direct dyes. Dyes and Pigments 27, 197-210. 

They resemble acid dyes in being sodium salts of sulphonic acids and are almost invariably azo compounds. I hey have a direct affinity for cellulosic fibres. Direct dyes are also frequently referred to as substantive dyes. Although they will dye protein fibres they are not used for this purpose except in special circumstances. The distinction between acid and direct dyes is often ill defined. C.I. direct red 37, for e.xample, may be applied as a direct dye to cellulose or as an acid dye to protein fibres. 

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. 

Clipson, J.A., Roberts, G.A.R, 1994. Differential dyeing cotton. Part 2—stoichiometry of interaction with acid and direct dyes. J. Soc. Dyers Color. 110 (2), 69-73. 

Mineral fillers are used to improve the physical and optical properties of paper. Usually, they comprise 5-15% of the weight and even more than 30% in some paper grades. The most common fillers are clay (kaolin), calcium carbonate, talc and titanium dioxide. Different dyes and pigments are added to the stock to colour the paper. Basic dyes are in most abundant use today, but acidic and direct dyes are also used. 

Uses Dispersant for improving dispersion of disperse dyes and sol. of most acid and direct dyes... 

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