3.1.3. Syntan + cation after treatment
As discussed, the two-stage, syntan + cation after treatment process was developed to enhance the effectiveness of commercial syntans in improving the wash fastness of various non-metallised and pre-metallised acid dyes on nylon. Table 2 and Fig. 6 show that the application of this after treatment increased the lightness and
reduced the colour strength, respectively, of the dyeings prior to wash testing. The finding that the extents of the reduction in fk values and increase in L* values were greater for the syntan + cation after treatment than the syntan after treatment can
be attributed to more dye having been desorbed from the dyed substrate during the two hot, aqueous stages of the syntan + cation process than from the single, hot, aqueous stage of the syntan process. The colorimetric data in Table 2 show that, for each of the three dyes, after treatment with the syntan + cation process reduced the chroma and imparted a slight reddish hue to the dyeings, these findings being very similar to those obtained for the application of the syntan alone. In terms of the fastness of the dyeings, Fig. 6 shows that the reduction in colour strength of the syntan + cation treated dyeings that occurred during repeated washing was greater than that achieved for the dyeings which had been after treated with the syntan alone. Indeed, the fk values of the syntan + cation after treated dyeings which had been subjected to five, consecutive wash tests was more or less the same as that of their non-after treated counterparts, for each of the three dyes used. Hence, the colour strength data in Fig. 6 showed that after treatment with the syntan + cation process did not improve the fastness of the three dyeings to repeated washing.
However, this observation was not supported by the staining results obtained (Table 3) which showed that the syntan + cation after treated dyeings displayed slightly superior fastness after five washes than their non-after treated counterparts. This finding might be explained in terms of both the amounts of dye that were
removed from the dyeings during the after treatment process and the amount of vagrant dye available for redeposition on adjacent multifibre strip materials during wash testing. After treatment with the two-stage, syntan/cation process removed dye from the dyeings, as evidenced by the respective fk and L* values (Table 2 and Fig. 6). This means that there was less dye present in the case of the syntan + cation after treated dyeings and, therefore, less dye was available for removal during wash testing, resulting in less vagrant being available for redeposition onto the adjacent multifibre strip materials (Table 3).