Saminathan Ratnapandian

Copper (II) sulfate (g/l)
Metal (mg)
Applied by padding on 10 g fabric, 80% wet pick up	Retained by
	Cotton	Cotton and dye	Wool	Wool and dye
5	10.2	0.86		2.49
10	20.4	1.29		8.87
15	30.6	3.52	6.49	10.40	28.52
20	40.8	5.88		18.69
Iron (II) sulfate (g/l)	Metal (mg)
	Applied by padding on 10 g fabric, 80% wet pick up	Retained by
		Cotton	Cotton and dye	Wool	Wool and dye
3	4.8	2.52		1.02
5	8.1	5.22	6.12	2.88	6.04
10	16.1	5.78		4.07
15	24.2	7.57		5.11

The difference in optimum mordant concentration (5 g/l for iron (II) sulfate and 15 g/l for copper (II) sulfate) indicates that the dye reactivity varies according to the mordant. The observed optimum concentrations are much lower than the recommended

concentration of 60 g/l mordant for meta-mordanting [94]. The low amount of mordant required for dyeing and the high metal retention percentage reduce effluent load.

When padding and exhaust dyeing are considered, there is a significant difference in the liquor ratio. Padding requires approximately 1:2 (calculated from 80% pick-up during padding of dye and mordant) while a ratio of 1:30 is recommended for exhaust dyeing. The mordant levels and time for dyeing are lower than the 5% OWM mordant levels and dyeing time of 45 minutes suggested for exhaust dyeing by the dye supplier. As mentioned earlier, the present study identified a significantly lower quantity of mordant as compared to that reported by other researchers. These observations are summarised in Table 3.8. Overall it is evident that padding is an attractive option as compared to exhaust dyeing using the natural dyes investigated.

Table 3.8 Comparison of dyeing process consumables (10 g fabric sample)

Dyeing process	Exhaust	Padding [94]	Padding*
Water (ml)	300	20	20
Time (min)	45	20	20
Mordant (g)	0.50	0.48	0.12 or 0.04

* Padding parameters determined in this investigation

5. Fastness properties

Good fastness to rubbing, washing and light are desirable properties in dyed textiles. As evident from the test results shown in Table 3.9 all fastness ratings are 3-4 or better.

Table 3.9 Fastness testing results for pad-dyed cotton

Dye		Thar (A. catechu)			Caspian (A. nilotica)
Mordant		CuSO4.5H2O (15 g/l)	FeSO4.7H2O (5 g/l)	CuSO4.5H2O 15 g/l)		FeSO4.7H2O) (5 g/l)
Washing test		3–4	3–4	3–4		3–4
Light fading test		4–5	3–4	4–5		3–4
Rubbing test	Dry	5	5	5		5
	Wet	5	5	5		5

Both dyes, when mordanted using either copper (II) sulfate or iron (II) sulfate, exhibited excellent rubbing fastness. There was no staining of the white fabric under both dry and wet conditions. During washing, the colour did not bleed either into the liquor or onto the adjacent fabric. However, a colour tone change was observed after washing, leading to a grey scale rating of 3–4. This change may be attributed to long exposure (30 min) to a high temperature (60˚C) during standard washing. A similar change but of a temporary character was observed at the end of the light fading test. Samples subjected to testing for fastness to light regained their original tone of colour after conditioning at 20 ± 2^○C and 65% RH for 24 hours. It is possible that at elevated temperatures one or more of the bond angles in the dye-mordant-cotton complex undergo changes, altering the shade. The change is reversible in the absence of excess moisture during light fading but becomes permanent when this restraint is removed during washing. Fading to light at 48 hours of exposure was rated 4–5 against blue wool standards.

The fastness results agree well with those obtained by Patel et al. [94]. Tannins, the main components of dyes derived from the Acacia family, are inherent mordants that are used to improve the fastness properties of other natural dyes. This factor combined with the metallic salts used in this study resulted in strong covalent bonds between cotton and the dye-mordant complex, leading to the robust fastness ratings observed. The results support the findings of Gupta [37], who postulates that the characteristics of mordants play a more important role for the fastness properties of natural dyes than the dyes themselves. Samples treated with copper (II) sulfate showed less significant colour change as opposed to those treated with iron (II) sulfate. This may be due to the more stable structure of the copper-dye-cotton complex.

6. Conclusions

Cotton fabric can be padded with natural dyes and a low mordant concentration to yield shades that are equivalent to those obtained by exhaust dyeing. The padded samples possessed acceptable rubbing, washing and light fastness properties. Post-mordanting is a better procedure than pre-mordanting or meta-mordanting when the levelness and depth of shade obtained are considered. Copper (II) sulfate and iron (II) sulfate as mordants produced beige and grey shades respectively with the same natural dye from
the Acacia family. Each mordant-dye combination is unique in terms of the optimum amount of mordant required for the same amount of dye. The optimal mordant concentration was determined to be 15 g/l copper (II) sulfate and 5 g/l iron (II) sulfate respectively for 10 g/l dye. AAS results confirmed a definite increase in the quantity of metal retained by the dyed fabric as compared to mordanted fabric. FTIR analyses identified significant differences between the two dyes investigated; however, these differences diminished on forming complexes with iron or copper and cotton (dyed fabric). The difference seen in the reflectance curves may be attributed to variations in bond angle and orientation in the dyed fabric. The optimum process parameters, namely mordant concentration, process sequence and the post-mordanting technique, were adopted in the next stage where improvements to the padding of natural dyes were attempted.

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Chapter 4