Saminathan Ratnapandian

Limitations of natural dyes
Some typical limitations of natural dyes are inadequate availability and low colour yield and efficiency during both cultivation and dyeing [32, 39, 48]. Their unsuitability for colouring synthetic and synthetic-blended fabrics is another drawback. A widely accepted disadvantage is their poor performance as regards to desirable properties when compared to most synthetic dyes [30, 49]. The most significant shortcoming is their inherent variation, introduced by changes in growing and or harvesting conditions.
Unlike synthetic dyes, most natural dyes are usually made up of more than one compound. The type of compounds present and their interaction (among themselves and the substrate) result in unique shades. A skilled dyer can utilise these same drawbacks to create specialty products for niche markets.

2. Fabric colouration

Colour can be introduced into textile materials at all stages such as fibre, yarn, fabric and garment. However, fabric colouration is the most popular and involves a wide variety of machines and methods. Dyeing and printing are the broad classifications of fabric colouration, as depicted in Figure 1.3. A simplistic description of dyeing is that the fabric acquires a single colour uniformly throughout. Similarly, printing may be defined as a process where one or more colours are applied in a desired pattern on the fabric [11, 12]. Dyeing and printing are described further in the following paragraphs.

Figure 1.3 Fabric colouration methods [11, 12, 50]

1. Batch dyeing

This is an intermittent method used for dyeing short lengths of fabrics. It involves bringing together the fabric and dyes, which are partially or wholly dissolved in a transfer medium, usually water, so that the colour can be absorbed by the fabric. The dye is adsorbed on the fabric (fibre) surface and ideally diffuses (or is absorbed) into the gross structure. Depending on the dye and constituent material of the fabric, the interaction may be either chemical or physical. The process is generally called exhaust dyeing because it is usually conducted using dye liquor which is a dilute solution of dyes in water. The ratio of material to liquor can range from 1:4 to 1:30, normally termed low-liquor to long-liquor dyeing [11, 12]. In order to facilitate the initial dye