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GEORGE€3. KAUFFMAN California state University, Fresno Fresno. CA93740
The Ubiquity and Longevity of Fibers Raymond 6. ~ e ~ m o u r ' University of Southern Mississippi, Hattiesburg, MS 39406 George 6. Kauffman California State University, Fresno, Fresno, CA 93740
Because t h e r e i s evidence t h a t spiders (Araneae mesothelac) produced silk webs during the Devonian period of the Paleozoic era (350400 million years ago), fibers have been with us for many millennia. However, ancient humans used wool or other proteinaceous fibers instead of spider silk. Linen was produced from f l a x (Linum usitatissimum) as early as 12,000 years aeo, - makitlp flax one of man's first ~lti5;atedplantsil,. This strone fibcr is still uscd but was disolaced. to some extent, by k b e r s from t h e cotton plant (Gdssypium hirsutum). Silk, which, like t h e spider web, is spun through small orifices (spinnerets), was separated from the cocoon of the silkworm (Bombyx mori) by the Chinese Empress Hsi-Ling-Shi about 4600 years ago (2).However, as a result of the invention of the cotton gin in 1793 by Eli Whitney (1765-1825) and the mechanization of textile production in the early years of the 19th century, cotton became king. In 1940 it accounted for 80 % of textile fibers produced in the United States (3).However, since then, the production of natural fibers has declined, and polyester fibers are now king of fibers. The Molecular Structure of Fibers The worldwide production of fibers grew to more than 60 billion pounds before a unifying concept explaining the molecular structure of all fibers was proposed. I t is now known that these threadlike oroducts are actuallv eiant molecules (macromolecules)t&t differ from other na&ral molecules. such as elastomers and elastics. bv the streneth of the ~ntfrmoleculxbonds between the chGns and by thc tcndcncv of the libcr molecules to formcwstals Th'forces between molecules of elastomers such as natural rubber (Heuea bmziliensis) are weak London dispersion forces similar to the physical attractions between gas molecules, such as those in methane (CH4). These forces between nonpolar molecules, which are also called van der Waals forces, are based on transitory diooles caused by fluctuating changes in electron denskics'm the atams df these molecules. Thcse weak intermolecular forces arc readily ovcmome when mhbrr is stretched. In addition to these weak London disocrsion intermolecular forces, many plastics also have stronger dipole-dipole intermolecular bonds, like the attractions between hydrogen chloride molecules in which the positive and negative charges in one molecule are attracted to opposite charges in another molecule. Because of the presence of these dipoldipole intermolecular forces, i t is difficult to stretch plastic molecules, such as polyvinyl chloride.
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Most fiber molecules are also attracted to each other bv stronger dipole4pole forces in which a hydrogen atom in one molecule is attracted to an oxveen. " - , nitrogen. - . or fluorine atom in a chain of a n adjacent molecule. Because of the presence of these strong hydrogen bonds, it is very difficult to stretch these fibers. When unoriented fibers are drawn, hydrogen bonds are formed as the polymer chains are lined up (oriented) in the direction of the stretching force. These hydrogen bonds, which are common to cotton, wool. silk. and ravon. are also oresent in svnthetic fibers The unsuch'as nilon 6,6 ~polyhexamet~yleneadiP~mide). stretched fibers are noncrvstalline (amorohous). but the strctchcd fibers have a hig6 degree of ordc; and a;e shown to be highly crystalline hy X-r81y diffraction annlysis .4,. Producing Yarns from Fibers Natural fibers such as cotton and wool are relatively short and are called staple fibers. In contrast, silk, rayon, and synthetic fibers are continuous filaments, but they may be chopped up to form staple fibers. By definition, the ratio of length to diameter of a fiber (aspect ratio) must be a t least 100:l. Actually, even cotton fibers as short as 1in. in length have a n aspect ratio of about 1400, and the aspect ratio of silk, rayon, and synthetic fibers is more than 1million. Because relatively long fibers are required for the production of yarn, staple fibers must be aligned and opened mechanically by metal "fingers" on a picker. The batt from the picker, which resembles absorbent cotton, is then teased by wire pins in a process called carding in order to produce a ropelike strand of fibers, which is called a sliver. The slivers are then drawn by passing them through differential speed rollers on a revolving frame to form a yam, which is then wound on bobbins. Some filaments, like rayon, are produced by wet spinning, that is, a solution of a polymer is forced through small orifices (spinnerets) and coagulated in an acid bath. In the case of rayon, the solution is cellulose xanthate, which is produced by the reaction of carbon disulfide with cellulose in the presence of sodium hydroxide. Other filaments such as nylon or PET (polyester) are produced by melt spinning, that is, molten polymer is forced through small orifices (spinnerets), and the filaments are cooled. In another spinning process, called dry spinning, the solvent is removed by evaporation from a solution of the filaments after the solution has been forced through the spinnereta (5,6). In all cases the filaments are stretched (drawn) to increase the tensile strength by the alignment of attractive
Volume 70 Number 6 June 1993
449
Data for Major Fibers
Fibers Cotton Polyester Nylon 6,6 Acrylic Polypropylene
Tenacity (gid)
Elongation(%) 6 20 30 35 50
3 5.5 9 3 5
Rudyard Kipling (1865-1936) immortalized the term '8ank" in Stanza I of his poem, "The Vampire," with his expression "a rag and a bone and a hank of hair." Ahank of cotton yam contains 840 yards, but a hank of worsted yam contains 560 yards, and a hank ofwoolen yam contains only 300 yards. It is customary to report the tensile strength of most materials in units of pounds per square inch (psi) or megapascals (MPa) (1 MPa = 145 psi). However, textile technologists are abandoning the term hank and using the units of gramsldenier (gld) or tex. The denier is equal to the weight in grams of 9000 m of yarn, and the tex is the weight in grams of 1000 m of yam. The new trend is to use the unit newtonltex or decinewtodtex (dNItex), where
forces in adjacent chains. The hydrogen bonding that takes place when nylon 7 is stretched is illustrated in the above figure. An isotactic polymer is one in which the pendant groups are all on the same side of the polymer backbone. Because of the regularity of structure in isotactic polypropylene (it PP), there is a good fitting of the methyl pendant groups on adjacent polymer chains. This polymer can be melt spun and used as a low-melting strong fiber. The tensile strength of these fibers can be increased dramatically through chain extension by stretching filaments swollen solvents (7,. ~hesefibersare used for bdlet-resishy tant wearine aooarel. The advantaee of this excellent fitting of (it PP) polymer chains is readily demonstrated by the fact that atactic polypropylene (at PP), in which the methyl pendant groups are randomly arranged on the polymer backbone, is an amorphous nonfibrous polymer. Skeletal stmcturesof it P P and a t PP are shown below:
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C
I
.&
C
C-C-C-C-C-C
I
C
I
C
C
I C-C-C-C-C-C-c-C I I
C
it PP
450
Journal of Chemical Education
at PP
C
I
These units are somewhat cumbersome but certainly preferable to the weight in pounds of 840 yard hanks of yarn. The relative tenacity (gld) of major fibers and their elasticitv. exoressed as oercentaw eloneation. are shown in the table: The ubiouitv of fibers is demonstrated bv their universal use, which'is &reasing annually. The longevity offibers is attested to by the unearthing of textiles that were produced more than 12,000 years ago. Humans will continue to depend on fibers in the future, when we will be able to choose the best fibers for each s p e c f i use and not be entirely dependent on a specific source, such as cotton from plants, wool from sheep, silk from worms, or various fibers from synthetic polymers.
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Literature Cited ~~~
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1. Fisher,C.H. I o H i s t o ~ o f P o l y mSeioioand r %hn&dSeymour,R. B..Ed.; Mareel Dekker: New YmL, 198%Chap. 6. 2. Bittes A. S. In Amdrmie Am* E~yelopodla;Arae Publishing Co.: Princeton, NJ, 1981;Vol. 17. 3. ~ r ~ s i eJ. y .T 1n E n w l o p d h ofsdenm and %hno&; McGrsw Hill: New Yorh ,a",."", .".,,
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SeymoqR.B.; Carraher, C. E. Giant M o k u h s ; WWiley: New York. 1990: Chap. 9. 5. Mrmerieff,R. W. Man-ModeFibars; Newnss-Butteterths: London, 1975. 4.
6. Seymour, R B. Man-Mode F i b e r H o n d b o o k : E l s e ~New : Ymk. 1990. 7. &mn.Y.V.; Beseh, D.L. InEncyclapadvl orPo1ymrSclonnondEwiiri"~;WiIqr: New Yark, 1987;Vol.10.
