As indicated in Chapter 2, little similarity exists between the two methods of dye classification (chemical functionality and method of application). However, there are some useful relationships that can be discussed and developed with regard to dye application class and the six representative fiber types: wool, cotton, cellulose esters, polyamides, polyester and acrylic (Table 3.
1). A further subdivision of dyes by method of application is given by Needles (ref. 1): (a) dyes containing anionic functional groups (acid, metal complex, mordant, direct and reactive); (b) dyescontaining cationic functional groups (basic); (c) dyes requiring chemical reaction before application (vat, sulfur and azoic); and (c) special dyeing techniques and classes of colorants (disperse, solvent, natural and pigment). Aftertreatments that improve dye fastness, the dyeing of fiber blends, and textile printing may also be included in the latter category. Since this subdivision and its interrelationship with fiber type and dye fastness properties are generally useful, each of these classes of dyes will be discussed in the order listed, followed by the types of dyeing and printing machines that are utilized.
APPLICATION OF DYES CONTAINING ANIONICFUNCTIONAL GROUPS 3.2.1 Acid dyes Acid dyes are water-soluble, contain one or more anionic groups (usually -S0 3 H), and are applied primarily to wool and polyamides. Most acid dyes contain azo groups, but there are a few that are in the anthraquinone or triphenylmethane chemical class. Adsorption of acid dyes by polyamides and by wool fibers is governed by species of polymeric functional groups that exist at different values of 113 pH and by the hydrophobicity of the dye. In neutral solution, the functional aminoand carboxyl groups exist as zwitterions: + NH 3 --F--COO", where F = wool or polyamide. Under acidic conditions, protonation of the carboxyl group occurs, and the fiber exists as + NH 3 --F--COOH. Conversely, deprotonation of the positively charged amino group occurs in basic solution and the fiber exists in the form NH 2 --F-COO". Sorption of acid dyes may be most appropriately characterized by the use of Langmuir-type isotherms, but Donnan equilibrium may also be utilized to explain the distribution of ions in the solid and solution phases. TABLE 3.1 Application class and suitability for natural and synthetic fibers (ref. 2). Class Wool Cotton Cellulose Polyamide Polyester Acrylics derivatives PA PE PAN Basic Direct Sulfur Azoic Ingrain Vat Acid levelling Acid milling Mordant Metal complex Disperse Reactive Pigment X X X X (X) X X X X X (X) X X X X X X X X X X X X X X X X X X = suitable; (X) = of secondary importance. Pigments may be applied to any substrate by the use of adhesives. Dye characteristics Affinity of anions Levelling dyes Poor Sulfuric acid 2-4 Good Low molecular wt. High solubility Molecular solns. Low Milling dyes Good Acetic acid 4-6 Moderate-poor High molecular wt. Low solubility Colloidal solutions High Super-milling dyes Very good Ammonium acetate 6-7 Very poor Hig...
