Characteristics of Dyes

Write an essay on the chemistry of the dyeing process.

The chemistry of the dyeing process is based on the characteristics of the dyes. These include the ability of the dyes to carry out light absorption in the visible spectrum in the wavelength range of 400-700nm, presence of at least a single chromophore, presence of a conjugated system having alternating single and double bonds and exhibit resonance of electrons. The lack of any of these characteristics results in inability to of the dye to show colour (Kent, 2013). Dyes also consist of auxochromes that changes the colour of the dye and also influences the solubility of the dyes. Some of the examples of auxochromes are amino or hydroxyl groups, carboxylic acids, sulfonic acids, among others. Some examples of chromophoric groups are anthraquinone, azo, nitro, methane, phthalocyanin, among others (Shindy, 2016).

This essay will provide a description of the molecular mechanisms of the dyeing process, role of mordants and the role of acidity on the final colour of the dye.

The process of applying color to fabrics are called dyeing. Reactive dyes combine with the fiber directly, thereby giving rise to colorfastness. There are various methods of dyeing. These are bale, batik, beam, burl, cross, jig, piece and random dyeing. The rate of transfer of dyes to the fibers consists of four stages. These are convectional diffusion of the dye to the surface of the fiber within the dye bath, molecular diffusion of the dye through the hydrodynamic boundary layer, adsorption of the dye on the outer surface of the substrate and subsequent molecular diffusion of the dye into the fiber. This process is called sorption. The particle size distribution therefore affects the kinetics of the dyeing process (Lewis & Rippon, 2013 ). The diffusion of the dyes is described by the theory of diffusion as proposed by Fick. The diffusion of the mobile particles or the dyes is considered to be a random walk at the molecular level. The number of particles that diffuse through the cross-section of the fiber per unit time is called flux and it is proportional to the concentration gradient. The diffusion co-efficient also called diffusivity is a measure of the ease with which the mobile dye particles pass through a particular medium (Shamey & Zhao, 2013). The major classes of interactions that take place in dyeing are ionic, covalent and hydrophobic interactions. Ionic bonding involves the attraction of the negative and positive charges. In the solution, acidic groups like sulfonic or carboxylic groups lose a proton and in turn become anionic or negatively charged. Basic dyes like amines, on the other hand accept protons and become cationic or positively charged. Covalent bonding involves uncharged atoms and a stable configuration is achieved by either gain or loss of electrons. The atoms donate electrons to the shared orbitals and in doing so the atoms remain bonded due to the shared orbital. Co-ordinate bonds are a type of covalent bond, where the one atom donates all the electrons, which are shared by both atoms, thereby generating the bonds. Covalent bonding requires the presence of mordants. Hydrophobic stains or dyes bind to lipid rich regions (Chequer et al., 2013).

Methods of Dyeing

Fibers can be natural or synthetic and different dyes are required depending on the presence of different functional groups on the fibers. Acidic dyes contain –COOH or –SO₃H groups, which are attracted to the basic –NH groups present in the amide links of fabrics like silk, wool or nylon. The direct dyes bind to fabrics by the process of hydrogen bonding and bind to –OH groups present in cellulose fibers. The disperse or the vat dyes are insoluble in water and are oxidized while in solution and are physically bound to the fibers. The reactive dyes bind to hydroxyl or amine groups present on cotton fibers (Horrocks, 2018).

The mordants helps to act as dye fixatives that forms co-ordination complexes with dyes and helps in attaching to the fabric. Mordants are chemicals, which usually consists of a metal having a valency of 2 or above. However, other types of compounds can also act as mordants. Different mordants can give different colors to the same dye. The mordant thus allows a chemical reaction to occur between the fabric and the dye (Prabhu & Bhute, 2012). In the textile industries, mordants are used to fix the dyes on the fabrics. Metal mordants are polyvalent metal ions that form co-ordination complexes with dyes (Manian, Paul & Bechtold, 2016). The fundamental chemical reaction between a dye and a mordant involves the formation of 2 bonds. These are a covalent bond between the metal atom and the hydroxyl oxygen. The other bond is the coordinate bond formed between the metal and the double bonded oxygen referred to as chelation. Varying the amounts of the mordants can result in changes the color f the dye.  Mordants are applied in 3 ways. These are pre-mordanting, post-mordanting and simultaneous mordanting. In pre-mordanting process, the mordant is applied followed by the dye. In post-mordanting, the mordant is added after the dye, while in simultaneous mordanting, both dyes and mordants are added simultaneously. The addition of mordant to a dye solution results in sudden and dramatic changes in the color of the dye solution. This results due to the fact that a metal atom is incorporated into the delocalized electron system of the dye. The low energy levels of the metals results in its incorporation in the delocalized electron system, thereby lowering the overall energy. The absorbance of the dye and its color is therefore associated with the phenomenon (Morales-Oyervides et al., 2017; Nptel.ac.in, 2018).

Role of Mordants

There are various factors that play an important role in the dyeing process, but the most important is pH. pH plays a very important role on the adsorption properties of dyes. Controlling the pH of the dye bath is essential as it affects the dyeing cycle. pH also play an important role in the uptake of the dyes (Mondal & Islam, 2014). In acid dyeing, the low pH helps in the generation of hydrogen bonds that helps in the binding of the acidic dyes to the protein fibers of wool or silk. In the case of fiber reactive dyes, a high pH of the dye bath is essential as it helps to activate the cellulose fibers of the cotton fabrics, resulting in the formation of a cellulosate anion, which in turn reacts with the dye molecule leading to the production of a permanent and strong covalent bond, thereby intensifying the color (Pburch.net, 2018). For example, at acidic pH of 1-3, anthocyanins are present as red flavylium cation, while with increase in the pH, the color intensity of anthocyanin decreases due to the decrease in the concentration of the flavylium cation. Thus, differences in pH from acid to alkaline gives rise to different color tones (Wahyuningsih et al., 2017).

This essay therefore describes the chemistry of dyes and the chemical reactions associated with the dyeing process. It also describes the role of the mordants, which is to intensify the color of the dye. Moreover, pH also plays an important role as changes in pH results in changes in the color intensities of the dye.

Reference List

Chequer, F. M. D., de Oliveira, G. A. R., Ferraz, E. R. A., Cardoso, J. C., Zanoni, M. V. B., & de Oliveira, D. P. (2013). Textile dyes: dyeing process and environmental impact. In Eco-friendly textile dyeing and finishing. InTech.

Horrocks, M. (2018). How dyes attach themselves to fabrics. 4college.co.uk. Retrieved 19 January 2018, from https://www.4college.co.uk/a/Cd/fabric.php

Kent, J. A. (Ed.). (2013). Handbook of industrial chemistry and biotechnology. Springer Science & Business Media.

Lewis, D. M., & Rippon, J. A. (Eds.). (2013). The coloration of wool and other keratin fibres. John Wiley & Sons.

Manian, A. P., Paul, R., & Bechtold, T. (2016). Metal mordanting in dyeing with natural colourants. Coloration Technology, 132(2), 107-113.

Mondal, M. I. H., & Islam, M. K. (2014). Effect of pH on the dye absorption of jute fibre dyed with direct dyes. Oriental Journal of Chemistry, 30(4), 1571-1575.

Morales-Oyervides, L., Oliveira, J., Sousa-Gallagher, M., Méndez-Zavala, A., & Montañez, J. C. (2017). Assessment of the Dyeing Properties of the Pigments Produced by Talaromyces spp. Journal of Fungi, 3(3), 38.

Nptel.ac.in. (2018). Cite a Website – Cite This For Me. Nptel.ac.in. Retrieved 19 January 2018, from https://nptel.ac.in/courses/116104046/14.pdf

Pburch.net. (2018). What is the effect of pH in dyeing?. Pburch.net. Retrieved 19 January 2018, from https://www.pburch.net/dyeing/FAQ/ph.shtml

Prabhu, K. H., & Bhute, A. S. (2012). Plant based natural dyes and mordants: A Review. J. Nat. Prod. Plant Resour, 2(6), 649-664.

Shamey, R., & Zhao, X. (2014). Modelling, simulation and control of the dyeing process. Elsevier.

Shindy, H. (2016). Basics in colors, dyes and pigments chemistry: a review. Chemistry International, 2(1), 29-36.

Wahyuningsih, S., Wulandari, L., Wartono, M. W., Munawaroh, H., & Ramelan, A. H. (2017, April). The Effect of pH and Color Stability of Anthocyanin on Food Colorant. In IOP Conference Series: Materials Science and Engineering (Vol. 193, No. 1, p. 012047). IOP Publishing.