Textile Fiber Identification An Organic-Polymer Laboratory Robert L. Flachskam Jr. and Nancy W. Flachskam Kentucky Wesleyan College, Owensboro, KY 42301 Most graduates in chemistry who enter industry will be involved in some way with macromolecules ( I ) . Therefore the new ACS guidelines for undergraduate chemistry curricula emphasize the need to increase student exposure to polymer chemistry (2). However, few first-year organic laboratory texts contain experiments beyond the preparation of simple addition and condensation polymers ( 3 6 ) . We have developed a laboratory introducing students to an important application of polymers-textile fibers. The introductory lecture describes the major textile fibers, including their history, physical structure, and chemical composition (6-9).The laboratory consists of identifying seven readily available fibers--silk, wool, cotton, acetate, nylon, polyester, and acrylic-by burning and by chemical tests.' This experiment is appropriate at the end of a one-year sequence in organic chemistry. Classification of Fibers Fibers are classified by source. Natural fibers, from animals or plants, have been used for thousands of years. Human-made fibers, regenerated from natural fibers or synthesized from petrochemicals, have only been available for about 80 years. Natural Animal Fibers Natural fibers, such as silk and wool, contain fibrous proteins that are composed of condensation polymers of amino acids. The amino acids are linked by amide (peptide) bonds and have varying R groups. (See structure 1.)
NH-CH-C L
R
n 1
L
OR
CHiOR
J n
Za, R = H
Zb,R = COCH,
Silk consists of fibroin, which contains the simple R groups-H, 4 H 3 , and-CH20H.It is extrudedby silkworms as filaments that can grow up to a mile long. The filaments have triangular cross-sections that give silk fabrics a pearly sheen and a luxurious feel. The exposed hydroxyl groups readily form hydrogen bonds with water molecules, thus making silk garments comfortable because they wick moisture away from the body Wool, the hair covering of animals such as sheep and goats, consists of keratin. It contains a significant amount of cysteine, the sulfur-containing amino acid that forms bonds. These cross-links give wool its disulfide (3-%) unusual crimped structure and wrinkle-releasing charac-
' These tests could be combined with microscopic analysis (11). 200 Blackford Avenue, P.O. Box 429, Middlesex, NJ 08846. 1044
Journal of Chemical Education
teristics but also attract the larvae of clothes moths and carpet beetles, which feed on wool textiles. Natural Plant Fibers Vegetable fibers are composed of cellulosic polymers. The most common is cotton, which grows from the seed of the cotton plant as a P-linked condensation polymer of P-D-glucose (2a).Each glucose unit contains three hydroxyl groups that readily interact with each other and with water molecules to form hydrogen bonds. Cotton fabrics also wick water away from the body, but the hydrogen bonds are easily formed and broken, resulting in wrinkled fabrics. Regenerated Fibers Regenerated fibers were the first humau-made fibers. A natural product, such as wood pulp, cotton linters, or peanut protein, is dissolved in a solvent and extruded through a sdnneret into a coarmlatine - bath or air to form a filament. Acetate, for example, is produced by chemical modification (partial acetylation) ofthe hydroxyl groups of cellulose (2b). Acetate's physical and chemical properties are different fromthose ofcelldose. Acetate is solublein acetone and cellulosics are not. SyntheticFibers Synthetic fibers, the second type of human-made fibers, are made entirely from chemicals. The polymers are generally produced from materials in pellets that are melted or dissolved in a solvent and forced through spinnerets. The characteristics of the fiber may be significantly changed by modifying the cross-section or texture and by graftpolymerization or copolymerization. Nylon-6,6 (31, a polyamide, is a condensation polymer of adipic acid and hexamethylenediamine. Dacron polyester (4) is a condensation polymer made from dimethyl terephthalate and ethylene glycol. Acrylic (5)is an addition polymer of acrylonitrile.
In Flame
Burns & Chars
Out of Flame
Stops Burning
Residue
~rukhable Bead
I
Odor
I
Burning Hair I
Fiber identity
Protein silk wool
Keeps on Burning
I ~ s h I
I
Burning Paper
I
Cellulosic cotton
I
Melts & Burns
I
Stops Burning
Keeps on Burning
I
~laitic Bead
I
AcAd Chemical
I
I
plastic Bead
I
~
Chemical
I
Synthetic Regenerated nylon i Synthetic polyester acetate acrylic
Procedure. The sample is added to 10 mL of 3 M NaOH in a test tube and warmed to boiling. The solution is cooled, two drops of 0.25 M P ~ ( O A Care ) ~ added, and the color is observed. Cellulose Test
Cellulose-based fibers are detected by treating a sample with sulfuric acid and then iodine. A blue-black color is produced immediately with cotton and in 1-2 h with acetate. Procedure. The sample is placed on a watch glass, and 2 mL of 14 M H2S04 are poured on it. The sample is transferred with forceps to a beaker containing 2 drops of iodine solution3in 10 mL of water. The color and time are noted. Protein Test
In the Biuret protein test, the peptide bonds in protein fibers complex with alkaline cupric ions to give a characteristic violet color. Silk and wool give positive tests.
Fiber Burning Tests. Fiber Tests The seven fibers are identified using four different types of tests: burning tests, chemical tests for the elements nitrogen and sulfur, structure-specific tests for cellulose and protein, and solubility tests (10).
Procedure. The sample is placed on a watch glass, and 5 drops of 0.05 M CuS04 are placed on it. After 5 min, the sample is dippedusingforcepsinto 3 M NaOH for 10 s. The color is observed.
Fabric Samples
Solubility Tests
Cntreated, pure fibcr samples that are ap mpriate for these tests are available from TestfBbrics, Inc. One yard of each of the seven fabrics (itxms #611, ,527,400, 100,306A. 738,974) is enough for 400 students' and can be obtained for a total wst of about $40. Students are given a 5-cm square of each fiber. Chemical tests are carried out on 0.5-cm squares.
'k
Burning Tests
The way a fiber burns allows classification on the basis of four characteristics: behavior in the flame, behavior out of the flame, residue, and odor. Results for the fibers are summarized in the figure. Procedure. A 0.5-x-5-cm strip of the fiber is held by forceps, and the end is placed in a Bunsen burner flame for 2 s. Then the sample is removed from the flame. If necessary, the flame is extinguished. The odor is observed by gently wafting smoke toward the nose. The cooled residue is examined.
The solubility of a fiber depends on the solvent. Formic acid dissolves silk, acetate, and nylon. Acetone dissolves only acetate. Procedure. The sample is added to1 mL of each solvent in a test tube, stirred, and its behavior noted. Conclusion Over the years our students have demonstrated great interest in this exoeriment because it involves a .~.d i c a t i o n s of qualitative organic analysis to everyday materials, and the). like thechallenge of identifyingunknowns. We recom. mend this experiment because it familiarizes the students with the chemical structure of the fibers and teaches them basic chemistry in the process. Overall analvses are accurate. While some students find the burning t&ts difficult to interpret, fiber identities are readily confirmed with the chemical tests. The burning tests are included because, with a little practice, it is a quick, simple, and commonly used method to obtain information about a fabric.
Nitrogen Test
Nitrogen is detected by heating the sample in the presence of solid calcium hydroxide; ammonia is liberated. Silk, wool, nylon, and acrylic give positive tests. Procedure. The sample is placed in a test tube, and a few granules of C ~ ( O H )are Z dropped directly onto it. The sample is strongly heated with a Bunsen burner. Moist red litmus paper is held over the mouth of the test tube without touching the inside of the tube. If the litmus paper turns blue, nitrogen is present. Sulfur Test
Sulfur is detected by treating the sample with sodium hydroxide and lead acetate; the characteristic brown-black color of lead sulfide appears. Only wool gives a positive result.
Literature Cited 1. Bregar. B. PlostiesNems 1880, Aug 13, 10.
2. Worthy, W. Cham. En#. Nema 1989 67(18), 4 M 0 . 3. Lehman, J. W.Opemfionnl OrgonicChemisfr)i-AL&mtorCourse,kdd.;All~" 81 Bacon: Boston, 1988: Chapter 21. 4. Rabe*,R.M.:Gilbe*,J. C.;Radewsld,L.B.;Windgraue,R. SModemE~dmentol OrganicChPmisfry, 4th ed.: Saunders: NewYork, 1985; Chapter 24. 5. Williamson, K L.Mocmsmh ondMiemsmlPogonic Ezpdmnfa: Heath: Lerington, MA, 1989: Chapter 67. 6. Jaaeph,M. L.E~snfialsofTatiIes.4thd.; Holt,Rininhhrt,rtd W m m t o t o : N ~ ~ Y ~ ~ k ,
.""". .on-
7. Butler, S.;Malott, S. J. Chpm.Edu. IsB1,58,2953W. 8. Hyde, N. "BIk-The &en of Teini1es":Noliond Ckomphic 1984.165.2-49. Ckwmphie IS=, 273,552691. 9. Hyde, N. "Wool-FabrieofHlsto$;Noti-I 10. Rid1ey.A.;Williams, D. SimplaEzp~manfsin Tertik Science; Heineman: London, 1974: pp 79-81. 11. Ref 10, pp 77-79.
Prepared by dissolving 12.7g of iodine in a solution of 20 g of KI in 30 mL of water and diluting to 100 mL with water.
Volume 68 Number 12 December 1991
1045
