Bonding with Color: The Chemistry and Outreach Opportunities of Tie Dye



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Bonding with Color: The Chemistry and Outreach Opportunities of Tie Dye

Anne E. Moody -- Truman State University



amoody@truman.edu

Alpha Chi Sigma 49th Biennial Conclave

July 2008, Indianapolis IN
The Gamma Theta chapter began to tie-dye in the fall of 1992, after I learned about it at a workshop at a Biennial Conference on Chemical Education. We started by offering it as an activity for our freshman chemistry majors in a one-credit-hour seminar course. For the freshmen, it works well both as a new chemistry application and as an ”ice-breaker” for the students to get to know each other. We have also tie-dyed with non-science majors as part of a course laboratory exercise. The structures of the dye molecules and reaction mechanisms of the process make interesting additions to Organic Chemistry courses. Truman chemistry students at all levels enjoy learning chemistry from the “real-world” and creating their own unique T-shirt.

Before long, we began to use the activity as a fund-raiser, marketing to the general public in our community. We also began offering it as a fun science activity to school kids of all ages. We have tie-dyed with pre-schoolers, kindergarteners, 2nd and 4th graders, middle school students, and high school chemistry students. We have even taken it all the way to Kansas City to serve the high school chemistry classes of one of our alums who teaches there! Although any day is a good day to do chemistry, National Chemistry Week is a great time to promote science to people of all ages in this way.

The ages and interests of the participants determine how much science and chemistry is presented to the groups. The general public sees the activity as a fun craft, but they do ask how it relates to chemistry while they are doing it. The elementary school kids can learn some new science words (e.g., molecules and bonds), and that science is both useful and a whole lot of fun. High school students can absorb a lot more of the scientific details of the process. They know about acids and bases and bonding, and they can clearly understand that they are making new chemical bonds between the fabric and the dyes when they tie-dye.

This paper is an attempt to explain some of the chemical nature of dyes and fibers, and to give you some ideas about using this activity both to promote chemistry in your community and to make some money for your chapter. It is my attempt to help you learn more of this chemistry than your participants might want to know! The practical considerations for setting up an actual tie-dye event and some helpful handouts to give to your customers are given separately.



History of Fabric Dyes. Before advances in science allowed fabrics to be dyed with synthetic dyes, textile chemists relied on naturally occurring pigments to dye their clothing (Epp, 1995 and 1999). These earliest dyes usually interacted with the fabric through ionic interactions. They were mostly obtained from plant materials, which contain a variety of pigments such as chlorophyll (green), indigo (blue), and alizarin (red), to name a few. Metal ions were added to these pigments as mordants to add colorfastness and wetfastness (the ability of the dye to remain with the fiber during conventional washing). Later, small insects and animals (such as ants, frogs, and shellfish) were used to obtain pigments; for example, carminic and kermesic acids are red dyes derived from insects.

It was not until 1856 that the first synthetic dye was prepared by William Henry Perkin. Attempting to prepare quinine as a treatment for malaria, Perkin mixed aniline with allyl chloride. He initially obtained a yucky mess, but when he added an alcohol solvent to clean it up, a dark purple crystal formed. This fortunate accident was the beginning of the synthetic dye industry.


Structure Makes the Color. Textile and synthetic chemists eventually determined that the extended conjugated  systems in these organic molecules were responsible for a pigment's visible color. Light in the visible portion of the electromagnetic spectrum is just the right amount of energy to promote an electron from a  molecular orbital (MO) to the antibonding (*) MO in such molecules. The number of double bonds in the conjugated system, as well as the number and types of additional substituents attached to the  system, affect the exact energy difference between these MOs. Thus, these same factors affect the colors (i.e., wavelengths) of the light that is absorbed, and that which is reflected back to our eyes. Several example dye structures are shown below, all of which have extended conjugated  systems that allow them to exhibit color that our eyes can detect.
Dye – Fiber Interactions. Dyes adhere to fabrics using three basic mechanisms as defined by Aspland (1992): physical sorption, mechanical retention, and fiber reaction.

  • In physical sorption, or direct dyeing, the dyes are attracted to fibers by ionic or polar interactions and do not usually have a high degree of wetfastness. Orange II and Martius Yellow are examples of direct dyes. (Epp, 1995)


Examples of Direct Dyes:




  • In mechanical retention, or vat dyeing, the dye molecule is introduced into the fiber in a soluble form. While the freshly dyed fabric is drying, the dye undergoes a chemical change that makes it insoluble in water. Indigo, a dye used in blue jeans, works in this manner. This class of dyes is generally limited in range of color but has an increased degree of wetfastness. (Epp, 1999)


Examples of Vat Dyes:

Indigo


(from plants: indigo and woad)

Tyrian Purple

(from shellfish)



The Redox Chemistry of Indigo:



  • Finally, dyes undergoing fiber reaction form covalent bonds between the fiber and the dye. Because the dye is covalently bonded to the fiber, this class of dyes demonstrates excellent wetfastness. Also because reactive dyes are synthetic, different dyes can be engineered with a wide variety of colors (Aspland, 1992). The structures and the mechanisms of the reactions of these dyes with fabrics will be discussed more extensively below.

Obviously, the structure of the fabric also influences the dye-fabric interactions. Cellulose is a non-branched polysaccharide of glucose monomers attached by  14 linkages and is found in many plant cell walls, including cotton. What gives this structure rigidity and non-water solubility is its hydrogen-bonding pattern. Each hydroxyl group extending from a glucose monomer is involved in hydrogen bonding to other hydroxyl groups of other glucose monomers within the polymer. This hydrogen bonding reduces the molecule's polarization, and thus decreases its solvation. It also brings individual polymer fibers close together, increasing its rigidity.

During early development of the reactive dyes little work was done with cotton. The cellulose fibers of the fabric were thought to be unreactive because of its structural properties mentioned above. After all, both the dye and the fiber must be reactive to cause the bond to form! Instead, wool was the primary fiber candidate for fiber-dye reaction exploration. Wool is a protein, and as such, it contains many highly reactive mercapto and amino substituents on its amino acid monomer units, as well as hydroxyl groups. Even so, once a method of gentle base modification of cellulose proved to be safe and effective, reactive dyes became widely used for cotton since it is such an inexpensive fabric.
Structures of Some Fibers:

Cotton:

R = H


(Acetate) Rayon:





Wool: a protein and thus a polyamide:




Fiber Reactive Dye Properties. The primary dyes used in tie-dyeing are Procion dyes, which are dicholorotriazinyl dyes. These dyes all contain variations of the same units: water solubilizing group(s) (W) connected to the chromophore (D), the bridging link (Q), the reactive group (RG), and the leaving group (X).

General Procion Dye Structure: W-D-Q-RG-X

Where W = water solubilizing group(s)

D = Chromophore of the dye

Q = bridging Group

RG = reactive group



X = Leaving group

  • Some common water solubilizing groups are sulfonate and amino groups. These groups can also fine-tune the color of the chromophore as described below.

  • The chromophore is generally an aromatic species forming a portion of a larger extended conjugated species. The primary determinant of the color perceived is the number of multiple bonds in this system. Substituents further fine-tune the wavelengths of light absorbed or reflected by the chromophore. Such substituents could include alkyl, carbonyl, amino, hydroxyl, or sulfonate groups.

  • In the Procion dicholorotriazinyl dyes, the bridge is generally a nitrogen atom. Under certain conditions, this nitrogen can be deprotonated, which affects the facility of the dye-fiber bond formation.

  • The reactive group in dichlorotriazinyl dyes is an unusual aromatic ring component. It contains a six membered ring, with alternating carbon and nitrogen atoms. One of those carbons connects to the bridging group and the other two carbons are attached to the leaving groups.

  • The leaving group is generally chlorine.

S
pecific Examples of Procion Dye Structures:










Reactions of Procion Dyes. Under basic conditions, the hydroxyl groups on cellulose can lose their hydrogens, making them good nucleophiles. Then, these cellulose oxygen anions can react with the reactive group of the Procion dyes in an aromatic nucleophilic substitution reaction (SNAr). Note that SNAr reactions are understood to proceed through an addition-elimination mechanism, forming a tetrahedral addition intermediate before the leaving group is eliminated to form the product (see reactions below). Another interesting feature is that the different oxygen anions on the cellulose have different reactivities, with the anion at the six-position being the most reactive.

The Reaction of a Generic Procion Dye with a Cellulose Oxygen Anion:


Remember that our basic solution still contains free hydroxide ions that can also react with these Procion dyes. Two reaction processes are in competition, the rates of which are pH dependent. At relatively low pH (but still basic conditions), the reaction with hydroxide leads to hydrolysis of the carbon chlorine bond, forming a non-reactive alcohol.

The Hydrolysis of the Carbon-Chlorine Bond a Generic Procion Dye:


At higher pH (even more strongly basic conditions), the bridging group (a secondary amino group) can be deprotonated by the hydroxide; the resulting anion can pick up a proton at a different nitrogen, again forming an unreactive species. This last process reduces the reactivity for the dye to the fiber by several powers of ten.

Acid-Base Chemistry that Inactivates the Procion Dyes:


Notice that, since the dye molecule has two leaving groups, it can react twice with cellulose. Given cellulose’s uniquely hydrogen bonded structure and the closeness of the molecules within the cellulose fiber, the dye molecule can attach to two different cellulose molecules. When this happens, one dye molecule forms a bridging group between two molecules of cellulose.
Conclusion. Tie-dye uses chemistry in a fun and creative way. The Gamma Theta chapter has used this activity for many different purposes. You can let the nature of your audience determine how much of this chemistry you present. You can bring as much or as little background chemistry to the group as you see fit. Use the accompanying handouts on practical considerations of running these events as a guide for your first tie-dye, but your own environment and the type of participants you are working with will affect the details of your future tie-dye events. Be prepared, stay organized, enlist the help of others; but most of all – have fun!
References

Aspland, J. R. (1992). “Chapter 5: Reactive Dyes and Their Application.” Textile Chemist and Colorist. 24:5, 31-36.

Epp, Dianne, N. (1995) The Chemistry of Natural Dyes. Terrific Science Press. Middletown Ohio.

Epp, Dianne, N. (1999) The Chemistry of Vat Dyes. Terrific Science Press. Middletown, Ohio.

Koehler, Christopher, S. (1999) “Queen Victoria’s wedding attire started a color fashion wave that swept Europe.” Today’s Chemist at Work. February 1999: 85-91.

Zollinger, H. (1998) “Chapter 6: Reactive Dye-Fibre Systems.” A. Johnson ed. The Theory of Coloration of Textiles. Society of Dyers and Colourist.





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