Saminathan Ratnapandian

Relative colour strength (%)
125
100
75
50
0 0.025 0.05 0.075 0.1
Chitosan concentration (%)

Figure 5.7 Comparison of sum of the K/S values for Caspian dye

120

Relative colour strength (%)
110
100
90
80
0 0.025 0.05 0.075 0.1
Chitosan concentration (%)
Figure 5.8 Comparison of sum of the K/S values for Thar dye
It can be observed from Figure 5.7 and Figure 5.8 that incorporating 0.05% chitosan in the dye liquor produced the darkest shades for both mordants in combination with either dye when compared to samples dyed without chitosan. Further additions of chitosan reduced the colour strength. The observations contradict earlier results of exhaust dyeing, where an increase in the amount of chitosan used in pretreatment produced a proportional increase in the depth of shade [171, 174, 175]. As suggested by Chenite et al. [186] and Hirano et al. [187], it is likely that the chitosan film formed a gel in the presence of excess water and extended process time, which are typical characteristics of exhaust dyeing. Such a gel would have been able to attract additional quantities of dye leading to the darker shades reported by earlier researchers [171, 174, 175].

While analysing the padding results, in addition to the film-forming characteristic of chitosan the competing affinities between chitosan-dye, chitosan-cotton and dye-cotton also have to be taken into consideration. If it is assumed that at low chitosan concentrations cotton has an equal affinity for both chitosan and dye, then the thin film of chitosan absorbed an additional quantity of dye, leading to the observed increase in depth of shade. When higher amounts of chitosan were added to the pad liquor, the cotton absorbed a relatively lower amount of dye. The absorbed chitosan formed a thicker film which, in spite of having absorbed more dye, could not be readily penetrated by the dye especially in the low volume of liquor picked up. The combined effect was reflected as the lower colour strength at higher concentrations of chitosan.

The difference between the two dyes, Caspian and Thar, determined by FTIR in Chapter 3 becomes prominent when the effect of chitosan in increasing the depth of shade is considered. On examining Figure 5.7 and Figure 5.8, it is clear that the depth of shade increased less for Thar as compared to Caspian. It is possible that the subtle variations between the dyes and the resultant complexes alter the interaction with chitosan.

The increase in depth of shade was more pronounced when iron (II) sulfate was the mordant as compared to copper (II) sulfate. This may be due to the increased affinity of chitosan for Fe⁺⁺ as compared to Cu⁺⁺ ions [188, 189], translated as an increased affinity for the dye-mordant complex.

3. Fastness properties

The fastness properties of the cotton fabric pad-dyed in the presence of 0.05% chitosan are consistent with those obtained earlier. The results given in Table 5.2 indicate that chitosan does not significantly affect the fastness properties of the natural dyes investigated.

Table 5.2 Fastness ratings of cotton fabric dyed with Acacia dye and 0.05% chitosan