Azoic dyes coupling

Azoic Dyes. These are used to produce cost-effective heavy yellow, orange, red, maroon, navy blue, brown, and black shades and are ptinted alongside other dye classes to extend the coloristic possibiUties for the designer. Two approaches are adopted. The common method ia the United States is to use both a naphthol derivative and a stabilized color base, usually in the form of a diazo imino compound in the same print paste. This mixture is soluble in dilute caustic soda and no coupling takes place at this stage. The dried prints are passed through steam at 100—105°C that contains acetic and/or formic acid vapor. As neutralization takes place on the print, the coupling occurs rapidly and the insoluble azoic dye is formed. 

Azoic dyeing of cellulosic fibres is a process that is used only to a small extent today. In this process, an azo pigment is formed by chemical reaction within the fibre. The cotton fibres are first impregnated with an appropriate coupling component such as the anilide of 3-hydroxy-2-naphthoic acid, 156, under aqueous alkaline conditions. The fibre is then treated with a solution of a stabilised diazonium salt, in which the... 

The stripping of fully developed azoic dyeings can often be carried out using a hot solution of sodium hydroxide (1.5-3 g/1), sodium dithionite (3-5 g/1) and a surfactant addition of anthraquinone (0.5-1 g/1) generally increases the effectiveness of the process. Yellow azoic dyeings are resistant, however, and can only be partially stripped [30]. On the other hand, stripping of naphtholated material before it has been coupled with the diazo component can be done quite effectively in boiling alkali. 

As recommended laundering temperatures have tended to fall in recent years, a bleach consisting of sodium perborate activated by addition of tetra-acetylethylenediamine (4.110 TAED) has become an important component of household detergent formulations. This system is effective at temperatures as low as 40-50 °C. A recent study of the effects of TAED-activated peroxy bleaching on the colour fastness of azoic dyeings has demonstrated that the sensitivity of these products can be related to their chemical structure. Electron-donating substituents in the diazo component enhance resistance to oxidative attack under these conditions, as do the size and complexity of substituents present in the coupling component [110]. 

Azoic dyes, the ice colors, are made on the fiber by coupling diazo-tized materials while in contact with the fibers. Low temperature keeps the diazonium compound from decomposing until ready to couple. 

Azoic dyes. Azo dyes contain at least one azo group (—N=N—) attached to one or often two aromatic rings. They are produced in textile fibers (usually cotton, rayon, and polyester), by diazotization of a primary aromatic amine followed by coupling of the resulting diazonium salt with an electron-rich nucleophile (azo coupling). A variety of hues can be obtained... 

The success which these azoic dyes ultimately achieved only came after a considerable effort in applied research, and a short historical account is, not without interest. Originally the Griesheim Elektron A.G. were making a red pigment by coupling metanitro-orthotoluidine with d-hydroxy-naphthoic acid which was used in printing inks, (3) ... 

It has been known for a long time that it is possible to dye acetate fibres with azoic dyes provided the right combinations are selected and suitable modifications in the method of application are made. Wallwork (J.S.D.C., 1935, 51,415) proposed a reversal of the norrnal procedure by impregnating with the base first, diazotizing, and then coupling with the /i-hydroxynaph-thoic acid derivative or, alternatively, impregnating with a mixture of the... 

Azoic dyes can be applied to unions but there are many complications. An alkali concentration of 4 g per litre is sufficient to keep most of the coupling components in solution, and the application should be carried out cold in as short a liquor as possible. The coupling is accomplished in a separate bath with the diazotized base and 25 g per litre of common salt to prevent the coupling component from migrating into the liquor. Development requires 20 to 30 minutes, after which the material is acidified and scoured. 

Azoic Dyes Azoic or naphthol dyes are formed in situ on the fabric through a coupling reaction of an aromatic alcohol or amine such as najh-thol (the coupling component) with a diazonium salt (the diazo component). The dye formed contains an azo group ... 

The fabric is usually impregnated first with the aromatic coupling component followed by immersion of the fabric in a solution containing the diazo component, with the azoic dye being formed instantaneously. Owing to the instability of the components, the dyeings are carried out near room temperature. The dyes have moderate fastness, and dyeings may be mordanted to increase fastness. 

Another synthetic dye used to color natural and polymeric textiles are azo (azoic) dyes. Azo dyes are made from the reaction of both a coupling and diazo compound in water into a diazonium salt on liber substrates, typically at temperatures below ambient. These dyes are becoming less prevalent as their chlorinated aromatic amine byproducts are toxic and the dyes themselves are not biodegradable. 

The alternative approach is to pad the fabric with the alkaline naphthol and dry, foUowed by printing directly onto this prepared fabric diazonium salts or stabilized diazonium salts. Coupling is instant and the only further treatment needed is to remove aU the uncoupled naphthol and surface azo pigment in a subsequent washing treatment. Because the choice of colors is limited from one naphthol component, other shades are obtained by using other classes of dye alongside the azoic colors, eg, reactives. This approach is widely used in the production of African prints. 

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