Activity #5A: Physical and Chemical Properties of Fibers: Unmercerized versus Mercerized Cotton Thread
In this activity, after reading a Student Essay, you will design a series of experiments to answer questions about treated natural fibers. You will also observe the effects of various chemicals on both natural and synthetic fibers.
Your clothing has both physical properties and a chemical structure that makes it rough or soft, easy to clean or easily stained, always wrinkled or neatly pressed. To understand why different fabrics have different properties, we need to begin with the fibers that form the textile product. Since ancient times, people of all civilizations have made fabric from natural fibers of plants (like flax and cotton), from the hair of animals (like sheep's wool) and from insects (such as worm's cocoons to make silk). As early as the 1600s, Europeans tried to reproduce these fibers in laboratories. Some scientists, in the early part of this century, succeeded in developing "artificial silk" or rayon by recombining plant fibers in new ways. These fibers, made in a laboratory using plant matter, were called cellulosics. In the 1930s, chemical companies hired scientists to develop wholly synthetic fibers from coal, tar, and water using chemical processes. Nylon was the first synthetic fiber to be sold commercially. Nylon differed from cellulosics such as rayon because it was made from materials not before used in fabrics and because the natural materials used in its production were combined chemically in unique molecular formulas. Since the development of nylon, chemical companies have marketed more than a dozen different synthetic fibers. So, fibers available today are either natural, cellulosic, or synthetic.
Chemists know that all fibers, whether natural or manufactured, are polymers, or long chains of matter. Polymerization is the process of joining molecules (small building blocks of matter) together. The length of the chain is called the degree of polymerization. In "simple polymerization," molecules of the same compound combine to form long chains. Cotton is a simple polymer composed of long chains of glucose (sugar) molecules (greater than 10,000 of them) combined to form cellulose. In contrast, the manufactured cellulosic fiber, rayon, has a lower degree of polymerization with about 350 glucose molecules joined together. In "copolymerization," two or more different kinds of molecules combine into a chain, each capable of polymerization itself. Wool and silk are co-polymers composed of many different amino acids (protein molecules), each of which is capable of binding to itself in long chains. In "heteropolymerization," two or more different kinds of units combine, but not all of them are capable of polymerization by themselves. There are no naturally produced heteropolymeric textile fibers. Synthetic fibers can be either simple polymers, copolymers, or heteropolymers.
Karl Ziegler once described the process of polymerization in a very clear and attractive way. A group of dancing couples symbolizes a number of molecules with double bonds. At the command of the dancing tutor the couples partially separate from each other, i.e., they only hold each other with one hand, while each partner takes with his or her free hand that of a partner from a second couple. Thus a long human chain is formed in a process much like polymerization.
|Polymer dance. Molecules with double bonds represented as dancing couples,|
who partially separate to form a chain.
The use of a particular fiber for a particular textile is determined by its physical and chemical properties. You can observe physical properties of fibers, such as strength and length, either by eye or under a microscope. You can't see the chemical properties of fibers -- they can only be determined by chemical tests because they concern the molecular structure of the fiber. When you test physical properties, no new substance is produced but a test of chemical properties produces a different material. For example, cutting a fiber (a physical test) produces shorter fibers while burning a fiber (a chemical test) leaves an ash.
The physical and chemical properties of any material are closely intertwined so that, in a chemistry laboratory, manipulation of the chemical properties of a fiber can modify its physical properties. For example, scientists can add loft, improve absorbency, and provide flame-retardant qualities by chemically changing the formulas of the fibers. Also, chemicals can be added as the fiber is produced, as it is spun, or as it is woven to achieve specific desired qualities. Some finishes may even be applied to the piece of clothing or other textile product before distribution, such as water-or stain-proofing to a coat or carpet. We usually think of fibers made from animal or plants as natural, while those produced in a laboratory are synthetic but, as you can see, the reality is more complex. Which is more "natural" -- nylon made from coal in a laboratory or cotton grown with chemical fertilizers and then treated in a laboratory to improve its feel and make it stain resistant?
Mercerization is a widely used chemical process for finishing cotton. Mercerization is accomplished by treating stretched cotton with a solution of caustic soda, a weak base (an alkali in chemical terms; soap is a familiar base) for a few minutes. The cotton is then washed and neutralized to remove any caustic soda residue. So mercerized and unmercerized threads are chemically different. In answering the following questions, you will do physical tests to check for chemical differences.
What To Do
Design experiments to compare white mercerized cotton thread with white unmercerized thread. Write a hypothesis for each question, then develop a procedure to perform each experiment.
Experiment 1: Which is shinier?
Which is stronger?
Which takes up more dye? (Use tea or soda as a quick dye.)
Which thread (mercerized or unmercerized) would you rather use for a shirt or a pair of pants? Why?
What tests did you devise to answer the previous questions?
How did the tests you used work?
Did you try any tests that didn't work?
Can you think of any other questions you might like to ask? How would you answer your questions?
In what ways are mercerized cotton and synthetic fibers such as nylon alike and in what ways are they different?
Think about testing the two kinds of thread to find out their physical properties as a kind of scientific experiment. You had a question and you figured out a way to answer it, much as a scientist would.
Is this the kind of work you thought scientists did?
Activity #5B: Physical and Chemical Properties of Fibers: Mystery Swatches
You will perform three sets of tests (experiments):
The results should tell you which fiber is natural (protein based) and which is synthetic and give some idea of each fabric's durability.
What You Will Need
*5-10 squares of nylon cloth
*5-10 squares of silk cloth
*red litmus paper
*hydrochloric acid (available in the chem lab)
*sodium hydroxide (available in the chem lab)
*dry cleaning solvent
*liquid detergent solution
*liquid soap solution
*test tubes, several with one-hole stoppers
*bunsen burner (or a candle)
*glass stirring rod
The Flame Test
The nylon is stronger and less reactive than the silk.
The Protein Test
When heated, animal fibers containing protein will give off nitrogen. The nitrogen will react with the water on the litmus paper producing ammonium hydroxide. Ammonium hydroxide is a base and will turn red litmus paper blue. The silk is composed of protein and should turn the red litmus paper blue; as a synthetic, nylon has no protein and so you should see no change in the litmus paper.
Other Chemical Tests
|#1 - water||#2 - water|
|#1 - acid||#2 - acid (hydrochloric acid)|
|#1 - base||#2 - base (sodium hydroxide)|
|#1 - bleach||#2 - bleach|
|#1 - solvent||#2 - solvent|
|#1 - detergent||#2 - detergent|
|#1 - soap||#2 - soap|
These tests help determine the delicacy of the fabric. Although the concentrations used are rather high, you do come in contact with acids and bases during the day. For example, lemon juice and most fruits are acidic, while soaps are generally basic. The reaction of the fabric to these types of chemicals determines the amount of wear and tear a garment constructed from it can withstand.
|Swatch #1 (nylon)||Swatch #2 (silk)|
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Last Revision: 6/5/98