Saminathan Ratnapandian

Figure 2.4 Polymers of condensed tannins

3. Chemicals

The sources of the various chemicals used are listed in Table 2.2. All the chemicals were of laboratory reagent grade except the industrial grade chemicals indicated.


Table 2.2 Chemicals used and their suppliers

Chemical	Supplier
Acetic acid (glacial)	Merck
Chitosan*	Sigma-Aldrich
Copper (II) sulfate (CuSO4.5H2O)	Merck
Delinol VB/LU	Impala Chemicals
Dextrose	Impala Chemicals
Iron (II) sulfate (FeSO4.7H2O)	Ajax Finechem
Nitric acid (70% w/w)	Sigma-Aldrich
Non-ionic detergent	SDC
Potassium sulfate (K2SO4)	Chem Supply
Prisulon 1550S	CHT
Sodium alginate NVS*	CHT

Sodium carbonate	Ajax Finechem
Sodium dithionite	Impala Chemicals
Sodium hydroxide	Chem Supply
Thiourea dioxide	Impala Chemicals
Tryptone soya agar	Oxoids, Australia
Tryptone soya broth	Oxoids, Australia

*Chemicals were of industry grade

2. Dyeing methods

Exhaust dyeing recipes provided by the dye supplier were followed to produce benchmark samples for the selected dyes. A two-step process is generally required for the application of both mordant and vat dyes. Hence from among the different padding techniques described in Chapter 1, the pad (dye) – dry –pad (chemicals) – steam method (Section 1.2.2.2) was adapted for use with the chosen natural dyes. This method was modified and used throughout the research. The modifications are discussed in relevant chapters. Steaming was carried out at 100^○C using saturated steam for the required durations. All colouration trials were conducted in triplicate for each variable investigated. Consistency of shade obtained was the measure for process repeatability. Evaluation of other properties of the dyed fabric was limited to samples obtained from a single padding trial.

1. Mordant dyes

As mentioned in Section 1.2.1.2, copper (II) sulfate and iron (II) sulfate are widely used as mordants in natural dyeing. Hence, padding with the selected dyes was carried out in individual combinations with the above traditional mordants, either copper (II) sulfate or iron (II) sulfate. Initial dye-mordant ratio (1:6) and process sequence (simultaneous mordanting) were based on the information available from limited literature [94, 95]. In optimising the padding process, the first step was to identify the process sequence that would yield the darkest shade for a given concentration of dye. This ideal process sequence was used while determining the optimum dye-mordant ratio for a given dye concentration.
Two aspects of improvement to the padding process, from Section 1.2.2.5, were attempted using the optimised recipe and sequence. They were either a) application of the emerging technology of Atmospheric-Pressure Plasma (APP) pretreatment of fabric; or b) incorporating chitosan, derived from chitin, a byproduct of the seafood industry, in the pad liquor. The effects of varying the APP parameters or the amount of chitosan used were evaluated. Increased dye uptake leading to darker shades for a given amount of dye was the objective of these experiments. In addition, one step patterning was attempted using APP. Similarly, the consequential antimicrobial functionality brought about by the use of chitosan was evaluated.

2. Vat dyes

As a vat dye, indigo has to be reduced to its water-soluble colourless leuco form to be soluble for colouration. Oxidation of the leuco compound, after being absorbed, regenerates the dye inside the textile, imparting a fast blue colour. This reversible reaction is shown in Figure 2.5 [13].

Figure 2.5 Solubilising reaction of indigo

Many authors [42, 110, 116, 117] list widely recorded traditional methods of indigo- reduction as:

1. 
   the fermentation method
   
2. 
   the copperas vat method
   
3. 
   the zinc-lime vat method; and
   
4. 
   the bisulfite-zinc-lime vat method.
   

Each of the above methods has its own benefits and drawbacks. Inconsistent reduction of indigo and the time involved are the main drawbacks that have been identified.

Similarly, traditional indigo printing is equally varied and is summarised in Figure 2.6. Premature paste oxidation is the main concern in indigo printing [118-121].

Figure 2.6 Various types of indigo printing

Commercial dyeing using indigo commonly employs sodium dithionite (sodium hydrosulfite or hydros) as reducing agent and sodium hydroxide (caustic soda) as alkali in the solubilising step [13, 109]. The ready decomposition of sodium dithionite in alkaline solutions is usually compensated by adding it to the dye bath in excess and at frequent intervals [122-124]. Further, the decomposition products, such as sulfur dioxide and thiosulfate, also pose effluent disposal problems [125]. Caustic soda, on the other hand is a strong alkali and needs careful handling and disposal [126]. In the case of indigo printing, sodium dithionite is too unstable for application by the all in method where dye, alkali, thickener and reducing agent are applied together. It is usual therefore to utilize a more stable reducing agent such as sodium formaldehyde sulfoxylate or the equivalent zinc salt of the same compound. These reducing agents pose their own usage and effluent issues [127]. Thus, the use of the above chemicals poses technical and environmental problems.

The search for alternate reduction methods for indigo has given rise to research literature on use of electro-chemical, biodegradable organic chemicals and enzymatic

systems [25, 125, 128]. The use of pre-reduced indigo has also been encouraged [116]. The requirement of additional equipment and an increase in process time are some of the drawbacks of the above developments. Hence, in this research, substitution with chemicals that are stable and possess lower environmental impact for solubilising indigo was investigated. The chemicals used were thiourea dioxide (TUD) as reducing agent and sodium carbonate (soda ash) as alkali. TUD was selected because it has a reduction potential slightly higher than that of sodium dithionite [50, 129].

The padding parameters that produced the darkest shade for a given amount of indigo reduced by the action of the selected alternate chemicals were determined. Comparison dyeing was made between natural and synthetic indigo using the alternate chemicals. In addition, the effectiveness of the above process was evaluated in the area of indigo printing.

3. Evaluation

   1. Colour measurement

Instrumental colour evaluation of the conditioned dyed samples was carried out using a Datacolour 600 spectrophotometer with 10^○ LAV (Large Area View) observer using D65 illuminant. An average of three measurements of colour strength (K/S) or reflectance was recorded.

Colour measurement is based on the ratio between total light absorbed K and scattered S by the substrate as defined by the Kubelka-Munk equation given in Equation 2.1 below.

K _ (1 R)²