Saminathan Ratnapandian

Dye	Mordant (60 g/l)	CuSO4.5H2O
FeSO4.7H2O
Dye sequence	PDS			PSD			PDS			PSD
Mordant sequence	PDS	PSD	PDS		PSD	PDS		PSD	PDS		PSD
Thar	Pre-mordanting	1.85	1.65	2.97		1.68	1.89		2.56	2.58		2.31
	Post-mordanting	1.80	1.52	1.69		1.56	2.36		2.63	2.36		2.77
	Meta-mordanting	1.62			1.54			0.46			1.89
Caspian	Pre-mordanting	1.43	0.64	1.38		0.71	1.18		1.47	1.23		2.65
	Post-mordanting	2.48	1.59	1.56		1.42	1.76		2.11	1.81		2.77
	Meta-mordanting	0.81			0.73			0.64			1.17

In most cases, post-mordanting gave higher K/S values compared to the other methods. During pre-mordanting and meta-mordanting trials it was observed that dye-mordant complexes were formed in the padding liquor or on the fabric surface. As suggested by Ali [136], pre-mordanted samples were likely to have leached out the mordant during padding of dye, thereby facilitated complex formation in the dye bath. These complexes were unable to sufficiently penetrate and adhere to the substrate and hence were washed off during soaping and rinsing. Therefore, post-mordanting was selected as the optimum technique for padding with dyes derived from the Acacia family.

The current optimum mordanting technique contradicts an earlier study [94] that claim that meta-mordanting yields better results than pre-mordanting when padding cotton fabric with A. arabica willd. It is possible that the source and purity of dyes used, surface dyeing effect and final shade obtained, and reaction rate between mordant and dye could have contributed to the discrepancy. It should be noted that comparative literature that combines the padding of natural dyes and post-mordanting is not available.

4. Optimum mordant concentration

A single dye concentration of 10 g/l approximating a nominal 1% shade and the optimised technique and sequence determined above were used to identify the ideal mordant concentration. In accordance with the results discussed in Section 3.5.3, only post-mordanting trials were conducted. On the same basis, process sequence combinations of PSD (dye) → PSD (mordant) in case of iron (II) sulfate mordant and PDS (dye) → PSD (mordant) copper (II) sulfate mordant were followed.

No definitive assumptions can be made based on the K/S values at a single wavelength of 440 nm as listed in Table 3.6. As discussed earlier, additional factors had to be considered while evaluating neutral shades. Although the K/S values at 440 nm were not the highest at these mordant concentrations, 15 g/l for copper (II) sulfate and 5 g/l for iron (II) sulfate gave the most level visually observed shades for both dyes. Samples mordanted at these concentrations exhibited minimal or no colour bleeding during the soaping off process. Samples mordanted at higher than the above levels leached excess mordant into the soaping bath. The leached mordants tinged the bath a characteristic blue or grey. Further, the fabrics dyed using these mordant concentrations retained a supple handle. The other samples acquired a rough handle, probably due to incomplete removal (or redeposition) of excess mordant. In general, an unbalanced dye-mordant combination resulted in splotchy dyeing. These results lead to the inference that at the dye-mordant ratio of 10 g/l dye to either 15 g/l copper (II) sulfate or 5 g/l iron (II) sulfate, the dye-mordant-textile complex was completely balanced. Hence, these dye- mordant ratios are termed as the ideal dye-mordant concentrations in this thesis.

Table 3.6 K/S values at 440 nm for cotton fabric padded with different mordant concentrations