The Chemistry Of Dyeing

The Chemistry of Dyeing: Reactive Dyes a lesson plan for beginning chemists from elementary through high school

written and prepared by Paula E. Burch, Ph.D. (copyright 2003)

(To prepare for the most effective use of this lesson, first construct models of the molecules pictured below, using a Zome building set.) Everything we do, from digesting our food to making art, involves chemistry. Everything is made of chemicals. Some dyes, such as the kind you can buy in the grocery store here in the US, really just stain clothes, so the dye washes out a little every time you wash it. A really good dye actually chemically attaches to the molecules of the fabric and can never be washed out. [bring out model of dye molecule blue MX-R] A molecule is much too tiny to see, but we can use models to show what the dye molecule is shaped like. Each of these balls represents a different sort of atom. These Cs are carbon atoms, like you see in charcoal. The Os are oxygen, like in the air you breathe, and these Cls are chlorine, like in bleach. This model shows you what the blue molecules in this bottle are shaped like. [note: in the drawing, the C or Carbon atoms are represented by any corner where two lines come together. All other atoms are spelled out with their own one- or two-letter abbreviation, Cl for Chlorine, O for Oxygen, etc.]

[show bottle of diluted blue MX-R] Different dye colors are made of different dye molecules. Here is a model of another dye molecule. [hold up model of dye molecule red MX8B] The water containing that kind of dye molecule is red, as you can see. [show bottle of diluted red MX-8B] You can see that the models are shaped a little differently. Each different shape of dye molecule absorbs light differently. That's what makes the different colors! The fabric your clothing is made out of is also made of molecules. Cotton, which grows on a cotton plant, is made of long strands of cellulose molecules, all twisted together. Cellulose is the same thing that wood is made of. Here is a model of a cellulose molecule: [model showing rope with -OH groups sticking out] If you put these two molecules, the

dye and the cotton, together, nothing will happen, unless you can get some of the atoms on the surfaces to come unstuck. If the H comes off of the cellulose, and the Cl comes off of one end of the dye molecule, the molecules will be able to react with each other and stick together. [wiggle -H of an -OH group on cellulose model, and wiggle a -Cl on dye model] How do we get the H and the Cl to get off of the cellulose and the dye? We just add another chemical, called sodium carbonate: [model of Na2CO3] What this does is increase the pH. That's how we say that it makes it less acid. You already know about some acids - vinegar and lemon juice are sour because they are acids. An acid has a low pH. The opposite of acid is called a base. You've probably seen a base in your kitchen, called baking soda. When you put baking soda in water, you get a high pH. You can taste the baking soda in your kitchen, but don't taste these chemicals! A high pH is all that is needed to get the dye and the cellulose ready to react. Sodium carbonate is stronger than baking soda, so it works better for dyeing. [pull H off of cellulose model and Cl off of dye model] [stick dye molecule model to cellulose model] All we have to do to make a permanent bond between the dye and the cotton is to put the dye on the cotton and add washing soda. We can put the sodium carbonate on the fabric before or after we put on the dye. After we put the dye and the sodium carbonate on the fabric, we just have to wait a while. While we wait, the reaction is happening - chlorines are coming off the of the dye molecules and hydrogens are coming off of the cellulose molecules. If they do this right next to each other, the dye then attaches to the cellulose, and a permanant bond is formed. If we leave it in a warm room for a few hours, we can then wash the excess dye out. We have to rinse it in cold water and wash it with detergent in hot water to get all the extra dye off. After all the excess dye is out, the dye left on the fabric is permanant and looks like this. [hold up dyed handkerchief] [demonstrate dyeing as follows] Materials: - one handkerchief for each child, already soaked in soda ash, wrung out, and placed in individual ziplock bags - one bottle each of fuchsia and sky blue dissolved in water Give each child a bag containing a slightly damp handerchief. Ask them which color they want, and whether they want one or two colors. Squirt small amount of dye into each child's bag as requested. Make sure bag is locked shut. Hand out another bag to double bag for increased spill resistance. Reactive dye is a class of highly coloured organic substances, primarily utilised for tinting textiles, that attach themselves to their substrates by a chemical reaction that forms a covalent bond between the molecule of dye and that of the fibre. The dyestuff thus becomes a part of the fibre and is much less likely to be removed by washing than are dyestuffs that adhere by adsorption. The very first fibre-reactive dyes were designed for cellulose fibres, and are still used mostly in this way. There are also commercially available fibre-reactive dyes for protein and polyamide fibres. In theory, fibre-reactive dyes have been developed for other fibres, but these are not yet practical commercially. The dyes contain a reactive group that, when

applied to a fibre in a weakly alkaline dyebath, form a chemical bond with the fibre. Reactive dyes can also be used to dye wool and nylon, in the latter case they are applied under weakly acidic conditions. The most important characteristic of reactive dyes is the formation of covalent bonds with the substrate to be colored, i.e. the dye forms a chemical bond with cellulose, which is the main component of cotton fibers. Fiber reactive dye is the most permanent of all dye types. Unlike other dyes, it actually forms a covalent bond with the cellulose or protein molecule. Once the bond is formed, what you have is one molecule, as the dye molecule has become an actual part of the cellulose fiber molecule. No wonder you can safely wash a garment that has been dyed in bright fiber reactive colors with white clothing, a hundred times, without endangering the whites in the least - even if it is all different bright colors, or even solid black! In contrast to all other dyes the reactive dyes bind chemically to the textile fibres, significantly improving the product's colour stability and washability. Thus reactive dying of cotton is currently the most widespread textile dying process in the world. http://www.jagson.com/reactive_dyes.htm

3.6.Textile and Dye Effluent Management Waste water from the textile industry contains a variety of polluting substances like excessive salts and dyes which make the soil saline. The presence of colour in the effluent pollutes the water bodies and prevents the penetration of light, which in turn reduces the photosynthetic activities of aquatic flora. Since the physical, chemical and physicochemical methods of colour removal are expensive, microbial methods were employed to reduce the colour and pollution load of the effluents. Among the bacterial cultures, Bacillus sp-2 recorded the highest percentage (91.91) of colour reduction in the dye effluent. In case of fungal strains and standard fungal culture, the standard culture recorded maximum colour reduction of 87.21% in seven days followed by wild strains of Phanerochaete sp. (82.01%) and Trametes sp. (76.07%). Chemical mutation was carried out and the mutants of Bacillus sp. 1 and 2 reduced the colour of dye effluent by 94.31% and 94.65% and Trametes sp. by 90.74%. The survey conducted along the course of Noyyal river from Tirupur to Orathupalayam dam revealed that the soil did not reach the category of problem soil but the quality of irrigation water was poor having a pH of > 8.12 and EC of > 6.4 dS m-1.

The sludge contained 16.6 % of total Ca and also other essential nutrients like S and K, thereby its ameliorative potential for the reclamation of acid and alkali soils was evaluated. The sludge application increased the plant height, DMP, seed yield and uptake of N, P, K, Ca and S by Sunflower crop. http://www.tnau.ac.in/scms/EnScience/Research.htm

The Indian textile industry is one of the largest in the world with a massive raw material and textiles manufacturing base. Our economy is largely dependent on the textile manufacturing and trade in addition to other major industries. About 27% of the foreign exchange earnings are on account of export of textiles and clothing alone. The textiles and clothing sector contributes about 14% to the industrial production and 3% to the gross domestic product of the country. Around 8% of the total excise revenue collection is contributed by the textile industry. So much so, the textile industry accounts for as large as 21% of the total employment generated in the economy. Around 35 million people are directly employed in the textile manufacturing activities. Indirect employment including the manpower engaged in agricultural based raw-material production like cotton and related trade and handling could be stated to be around another 60 million. http://texmin.nic.in/tnpl_chap1.pdf