Aftertreatment with Formaldehyde

Because of a lack of storage stability in dyeings treated by this method, formaldehyde aftertreatment is mainly confined to dark shades (brown and black). Although the basic, simple process has remained virtually unchanged, its importance has declined sharply for ecological reasons. 

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The group of direct dyes with aftertreatment includes direct dyes that, after being applied to the fiber by the usual method, are subjected one of the following aftertreatments (1) Aftertreatment with cationic auxiliaries, (2) aftertreatment with formaldehyde, 3) diazotization of the dye on the fiber and coupling with suitable components (diazotization dyes), and (4) aftertreatment with metal salts. 

Direct Dyes. These water-soluble anionic dyes, when dyed from aqueous solution in the presence of electrolytes, are substantive to, i.e., have high affinity for, cellu-losic fibers. Their principal use is the dyeing of cotton and regenerated cellulose, paper, leather, and, to a lesser extent, nylon. Most of the dyes in this class are polyazo compounds, along with some stilbenes, phthalocyanines, and oxazines. Aftertreatments, frequently applied to the dyed material to improve washfastness properties, include chelation with salts of metals (usually copper or chromium), and treatment with formaldehyde or a cationic dye-complexing resin. 

Furthermore, direct dyes (see Section 3.3) are among the most important silk dyes. Their fastness properties can be improved by aftertreatment with metallic salts or formaldehyde. 

Aftertreatment The wetfastness of dyeings produced with acid dyes is often unsatisfactory. Formerly, tannic acid and tartar emetic were used, which form a 1 1 adduct on the material by hydrogen bonding. This process is expensive and said to be carcinogenic. Synthetic tanning agents (Synthanes) are preferred. They are condensation products of aromatic sulfonic acids with formaldehyde [106] or of phenols with formaldehyde, which are made water-soluble by reaction with bisulfite. These products deposit on the fiber surface and form an electrostatic barrier... 

The earliest polymeric cationic aftertreatments stemmed from the development of crease-resist finishes for cellulosic fibres. One such, promoted specifically for its colour fastness improvements when applied as an aftertreatment to direct dyeings, was a condensation product of formaldehyde with dicyandiamide (Scheme 10.82). Many similar compounds followed, such as condensation products of formaldehyde with melamine (10.212), polyethylene imine) with cyanuric chloride (10.213) and alkyl chlorides with polyethylene imine) (10.214 R = alkyl). 

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