Dye classes and dyeing process fundamentals

Prior to the development of synthetic dyes, natural colorants were the only means of dyeing textile fabrics. Today, natural dyes are only used in handcrafted fabrics. Although some niche boutique fashion ranges also use natural dyes as part of their theme, the volume of such fabrics is extremely low. Natural colorants have four key weaknesses they can provide only a limited range of colors, shades are difficult to reproduce, they have poor fasmess to laundering, and they have very poor fastness to light. 

For other major apparel fibers such as wool, silk, and nylon a dye class referred to as acid dyes is routinely used for coloration. Reactive dyes have also been developed for wool and are widely used for fashion apparel items because of their bright, broad color range. A range of mordant dyes is also available for wool and other animal fibers. The mordant dyes provide very high levels of fastness, but the shade range is limited, the shades are typically dull, and the application process is complicated. For acrylic fibers the dominant dye class is the basic dye. For polyester apparel, the insoluble disperse dye range is almost exclusively used. 

Dye substantivity The term substantivity refers to the ability of a dye to transfer from a solution onto a textile fiber and to set (Ronette, 2(XX)). The quantitative measurement of the force with which the dye is captured by the fiber is described as affinity. In practice, substantivity is often used as a qualitative description of the affinity of a dye for a particular fiber. The substantivity of a dye generally depends on its solubility, its ionicity, and its molecnlar size and stmctnre. Substantivity is favored 

Dye exhaustion In exhaust dyeing, the fabric starts absorbing dye as soon as dye is added to the bath. The term dyebath exhaustion describes Ihe rate and extent of the dyeing process (Broadbent, 2005). Exhaustion is an important sustainability issue the greater the absorption, the less dye that is discharged to waste. 

Dye diffusion The movement of a dye from the fiber surface into the fiber is diffusion (Broadbent, 2005). Optimal dyeing and hence maximum color yield requires uniform distribution of dye throughout the fiber (Khatri et al., 2014). In shades requiring two or more dyes, it is important for the dyes to have similar diffusion coefficients. Minor changes in the process conditions can lead to different distributions or take up each dye. This causes off-shade or batch-to-batch reproducibility problems. 

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