Chemistry of dyeing process - Research Paper Example

The Chemistry of Dyeing Dyeing is said to have begun in about 20,000 BC with the earliest use of clothing to determine the status as well as the identity of the wearer (Tsatsarou-Michalaki 2). This practice of applying color on yarn and fabric went on for many years in many different cultures and even at present, the tradition of dyeing remains with us. Moreover, recent advances in modern technology and chemical techniques have brought this tradition to its maximum status so far. Nevertheless, with such an importance in our culture, a chemistry student needs to know how dyeing exactly takes place at the molecular level as well as other technical questions related to the practice of dyeing.

How Dyeing Occurs at the Molecular Level It is essential that a dye must bond strongly to the fibers of the fabric and must remain there even after several washings. The dye must also not fade when exposed to light. Moreover, it must color the fabric evenly. All these qualities of a good dye depend on the strength of bond it has on the fibers of the material. A dye molecule must contain two functional groups: the chromophore and the auxochrome (Price, “Basis of Colour”). The chromophore controls the color of the dye while the auxochrome is the part that forms the salt and helps improve the color of the dye (Price, “Basis of Colour”).

Moreover, the chromophore is electron-withdrawing while the auxochrome is electron-donating (Price, “Basis of Colour”). Either of these two parts may bond with the fibers of the fabric. The picric acid dye, C6H3N3O7, has a hydroxyl or OH- functional group that may either lose its H+ proton when it combines to a basic group on the fiber (Lane). The hydroxyl group of the picric acid dye then becomes an anion and strongly bonds to the end of the fabric that becomes a cation. (Lane) The fabric, on the other hand, has a variety of chemical structures.

Wool, for example, is a polypeptide with more than 170 different proteins with the repeating peptide unit -NHCHRCO- (“The Physical and Chemical”), with R as the acidic, basic or neutral functional group depending on the amino acid present in that particular point of the wool structure (Lane). The parts of the wool molecule that will bind to the hydroxyl or OH- group of the picric acid dye above will be any of the following: the amino or -NH2 group, the carboxyl or –COOH group at the ends of the polymer chain, or the specific amino acid on the R group (Lane).

For example, if the amino end of the wool binds to the picric acid, the hydroxyl or OH- group of the dye loses the H+ ion where it is received by the NH2 end of the wool, and the O- ion of the picric acid then binds to the ammonium or NH3+ ion of the fabric. (Lane) Wool may dye well with picric acid but a particular type of fabric may dye only with particular types of dyes. Although silk and nylon may behave in the same way with wool, cotton is different. Unlike wool, cotton has only hydroxyl groups and therefore does not dye well with dyes that produce anions and cations, such as picric acid (Lane).

Additionally, certain fibers like Dacron and Rayon do not have functional groups that allow chemical combination with dyes, and therefore have to use organic solvents to make the combination possible. (Lane) Why Dyeing with and without a Mordant Produces a Different Color Mordants are chemical substances “needed to set the color when using natural dyes” (Tsatsarou-Michalaki 6). Mordants are metal salts that act as fixing agents whose purpose is to “improve the color fastness of some acid dyes” and form “coordinate complexes” with metal ions (Price, “Protein Textile Dyes”).

Mordants are normally applied to fibers made from cellulose, wool or silk or those treated using metal salts (Ladha). Two commonly used mordants include sodium dichromate, Na2Cr2O7, and potassium dichromate, K2Cr2O7 (Price, “Protein Textile Dyes”). These mordants, or metallic salt mordants, practically help dyes, known as mordant dyes, bind to the fabric (Price, “Protein Textile Dyes”). The chrome mordant, or potassium dichromate, particularly makes dye colors brighter, and so does the copper and iron mordants (Tsatsarou-Michalaki 6).

In short, dying with or without a mordant will produce a different color. If cochineal dye, for example, has no mordant, it will produce a pink shade but if alum mordant is added to it, it will produce a crimson color. (8) The change in color depends on the presence of the coordinate complex that the mordant dye forms with the mordant. For example, the mordant dye called C. I. Mordant Black, C19H14Cl2N3NaO7S, binds with Chromium (III) ion from a chrome mordant (Price, “Protein Textile Dyes”).

The chromium ion of the chrome mordant binds simultaneously with the two hydroxyl or OH- groups and the nitrogen part of the C. I. Mordant Black Dye (Price, “Protein Textile Dyes”), thus producing a different shade of color in the fabric. Another example is that if the chromium ion of C. I. Mordant Black is displaced by an iron in a particular mordant, the resulting compound formed by the OH- and N parts of C. I. Mordant Black with iron is redder (Shore 103). The reason behind this change in color when a mordant is added to a mordant dye is the fact that a new compound possessed of a different color naturally forms from the combination of the mordant and the dye.

(Parnell 110) Why Varying the Acidity of the Dyebath Affects the Final Color A recent experimental study by Tsatsarou-Michalaki et al. has concluded that “the pH values of a dye bath have considerable effect on the dyeability of cationised cotton fabrics” (Tsatsarou-Michalaki 8). The explanation for this is that “as the pH increases, the dyeability decreases” (8). Since the dye is water-soluble and contains carboxyl or –COOH groups, these anionic groups would naturally interact with “protonated terminal amino groups of cationised…fibers at acidic pH via ion exchange reaction” (8).

This means that the –COOH group of the dye will bind with the –NH2 group of the fibers. The –COOH will lose the H+ ion and the ion will protonate the –NH2 resulting in a binding of –COO- and –NH3+ ions. However, these ionic interactions decrease because at pH > 2.5, the number of protonated terminal amino groups of the cationised fibers decreases (8). Thus, at a higher pH, there will be fewer ionic bonds that can form and thus less light energy can be absorbed by the dye molecule, and so the dye will have a weaker color.

(“Procion MX Dye”) Top of Form Bottom of Form Works Cited Ladha, DG. “Dyeing.” Dyes and Chemicals. Fibre2fashion.com, 2009. Web. 4 Mar 2011. Lane, Jim. “The Chemistry of Dyes and Dyeing.” ACS Lake Superior Local Section Home Page. American Chemical Society, 2000. Web. 5 Mar 2011. Parnell, Edward Andrew. Applied Chemistry: In Manufactures, Arts, and Domestic Economy. New York: D. Appleton & Co., 1844. Google Books. Price, Heulwen. “Basis of Colour.” Chemistry of Dyes. University of Bristol School of Chemistry, 2002. Web. 5 Mar 2011.

Price, Heulwen. “Protein Textile Dyes.” Chemistry of Dyes. University of Bristol School of Chemistry, 2002. Web. 5 Mar 2011. “Procion MX Dye.” The Dyebath Changes Color When Soda Ash is Added. The CSIRO Lectures, 2010. Web. 4 Mar 2011. Shore, John. Colorants and Auxiliaries: Organic Chemistry and Application Properties. 2nd ed. West Yorkshire, England: Society of Dyers and Colourists, 2002. Scribd Book. “The Chemical and Physical Structure of Merino Wool.” Files. Paula Burch’s Site, 2011. Web. 4 Mar 2011.

Tsatsarou-Michalaki, A., Hliopoulos, N., Priniotakis, G. and Mpoussias, Ch. “Natural Dyes-Unifying The Heritage Of The Past For An Eco-Friendly Future.” Papers. SynEnergy Forum, n.d. Web. 4 Mar 2011. Sources for Diagrams “File: Pikrinsaure.svg.” Wiki. Wikipedia, 2007. Web. 5 Mar 2011. Lane, Jim. “The Chemistry of Dyes and Dyeing.” ACS Lake Superior Local Section Home Page. American Chemical Society, 2000. Web. 5 Mar 2011. Price, Heulwen. “Basis of Colour.” Chemistry of Dyes.

University of Bristol School of Chemistry, 2002. Web. 5 Mar 2011. Price, Heulwen. “Protein Textile Dyes.” Chemistry of Dyes. University of Bristol School of Chemistry, 2002. Web. 5 Mar 2011.