Saminathan Ratnapandian

Mordant dyes
Vat dye

Padding with natural dyes
Process optimization

Figure 2.1 Design outline for padding natural dyes

Commercially available natural dyes were used to avoid the inconsistencies of collecting plant source, extracting and purifying the dye. It was also decided to use at least industrial grade chemicals for all recipes instead of any available traditional natural counterparts.

The final shade obtained was the deciding factor while optimising the dyeing process. All dyed materials were evaluated in accordance with relevant Australian or international testing standards.

1. Selection of materials

   1. Fabrics

The use of fabrics produced from cotton or wool, natural fibres, enhances the sustainable aspect of the study. Further, they are the most compatible with natural dyes [28, 32, 48] and were chosen for this investigation. Mercerised and bleached cotton fabric was received from Bruck Textiles, Australia. Ready-for-dyeing 100% merino wool fabric was sourced from Macquarie Textiles, Australia. Both fabrics were used as

received without any further chemical treatments. The fabric specifications are given in Table 2.1.

Table 2.1 Fabric specifications

Characteristic		Cotton	Wool
Weight (g / m^2)		297	230
Yarn count (Tex*)	Warp	42	36
	Weft	81	41
Ends/cm		32	36
Picks/cm		20	24
Weave		3/1 twill	2/1 twill

*Tex – weight in grams per 1000 m of yarn

2. Dyes

Two classes of natural dyes, namely mordant and vat, were selected. Representative dyes from each class were chosen based on widespread use and established large scale availability. Mordant dyes were obtained from Alps Industries Limited, Ghaziabad, India. Vat dyes were obtained from Kraftkolour, Melbourne, Australia.


Vat dye
Indigo (CI Natural blue 1), the blue dye used since ancient times, represents the vat dyes. The title ‘king of colours and colour of kings’ may be rightfully conferred on indigo because of its traditional use and cultural significance spread across the globe [13, 41, 42, 107-110]. Today, it retains its allure as the principal dye employed to colour the popular blue jeans [13, 43, 108].

Many plant species (sp.) belonging to Leguminosae, Polygonaceae, Apocynaceae and Acanthaceae families produce this dye. Some of the well known indigo sources are knotweed (Polygonum sp.), woad (Isatis sp.) and common indigo (Indigofera sp.) [13, 28]. Each species of indigo-producing plant may possess its own mix of precursors such as Indican (Indigofera sp.) or Isatan (Isatis sp.). The precursors are converted into pale-yellow (colourless) indoxyl by enzymatic reduction. This colourless secondary metabolite is extracted in water by crushing of the plants. During air-oxidation, two indoxyl groups combine to produce indigotin, the main colouring matter of indigo.

Indigo formation is shown in Figure 2.2 [111]. The combination of precursors and degree of interaction vary according to plant species and growing conditions ultimately determining the final yield and purity. This variation also results in isomers among which indirubin, the red shade of indigo, is well identified. The separation of these isomers (impurities) is not cost-effective.