Abstract
Cationic dyes containing primary to quaternary amino groups undergo complex arid dissocia-tion reaction. There are several molecular species of cationic dyes in each pH region. By the use of a model cationic dye, bis(m-nitrobenzyl) amine (BNB) which showed a simple spectral change at various pH, the dyeing mechanism of acrylic fiber was investigated in detail at various pH by the cylindrical film roll method at 95°C. The acid dissociation constant of BNB was determined spectrophotometrically to be 7.4 in aqueous HC1-CH3COONa (ionic strength I=-0.1) solution at 25°C. Both cationic and non-ionic species, [2] and [1] of BNB are present in a neutral or a weakly acidic aquous solution (Equation ( 1 )). On the other hand, acrylic fibers have two kinds of dyeing sites, strong acid groups from the fragment of polymerization catalyst or from a monomer, and weak acid groups. The amount of effective dyeing stites in acrylic fibers changed at various pH, as shown in Table 1, which was confirmed by the pH dependence of the adsorption of [2] (Fig. 2). In the dyeing of acrylic fibers, there are three kinds of dyeing mechanism, which work simultaneously at various pH (Table 2). The adsorp-tion isotherms of [1] and [2] were obtained at pH 1.5, 4.2 and 6.8 (Fig. 1). The adsorptions of [1] and [2] were also measured over pH range from 0.2 to 6.8 (Fig. 2). The concentration distributions of [1] and [2] for the diffusion in polyacrylonitrile were obtained at pH 5.0, 6.0 and 6.8 (Fig. 3-6). At pH lower than 4.2, the distribution of [2] was exclusively obtained. The contribution of various dyeing mechanisms was discussed by these results. It may be concluded that the acid dissociation reaction of BNB does not occur in acrylic fibers.
