James Richard Fromm
Giant molecules called polymers are made up by the linkage of simpler molecules (monomers) by a polymerization reaction into essentially endless chain structures. Polymers occur naturally, but the majority which are used commercially are manufactured from simple monomers. We now turn our attention to one of the most important industrial uses of polymers, man-made fibers and fabrics. The magnitude of this industry is indicated by the Table below.
|Area||Synthetic Fibers (Gg, 1985)||Man-Made Fibers|
Notes to Table: one Gg is also 1000 metric tons. Most of the production listed under "All Other" countries takes place in China or in Southeast Asia. Most man-made fibers are now synthetic; the remainder are the cellulosics, which includes rayon and cellulose acetate, and their relative importance is steadily decreasing. On a world basis in 1985, the production of synthetic fibers was about 50% polyesters, 25% polyamides, and 20% acrylics, with only about 5% of all other fibers.
While the synthetic plastics described in a different section can be drawn into fibers, these generally do not have properties desired in textiles. The first significant synthetic fibers were the cellulosic fibers, rayon and cellulose acetate. Cellulosic fibers are modifications of the natural polymer cellulose and are derived from it. Originally made from the cellulose of cotton fibers, these are now made from wood pulp. To make rayon, which is synthetic cellulose, wood pulp is treated with caustic (NaOH); after removal of impurities the aqueous mixture of cellulose is then treated with carbon disulfide. A simplified form of the reaction is:
[(C6H10O5)2.NaOH] + CS2 cellulose-O-CS-SNa.
Unlike cellulose, the product sodium xanthate or cellulose xanthate is soluble in dilute NaOH solution. The viscosity of this material increases on standing or aging, and when sufficiently aged it can be spun into fibers from small jets called spinnerets. The aging process can chemically be written as:
cellulose-O-C-SNa + H2O NaOH + cellulose-O-CS-SH.
On spinning the reaction is:
cellulose-O-CS-SH cellulose + CS2 + H2O.
Use of a slit rather than a jet to extrude cellulose xanthate produces the thin transparent packaging film called cellophane; cellophane is usually softened by addition of glycerol.
Cellulose acetate is used both as a plastic and as a synthetic fiber. It too is made from the cellulose of wood pulp, by treatment with acetic anhydride and acetic acid. The reaction is:
[C6H7O2(OH)3]n + nCH3COOH [C6H7O2(OOCCH3)3]n + 3nH2O.
Unlike cellulose, cellulose acetate is soluble in organic solvents such as acetone and solutions of this can be spun. Evaporation of the acetone solvent leaves fibers of cellulose acetate.
Perhaps the most well-known synthetic fiber is nylon, developed by Wallace H. Carothers at E.I. duPont de Nemours and Co. Nylon is a generic name for the polyamides rather than the name of a single polymer; the original nylon, known as nylon 66, is still the nylon produced in greatest quantity. The constituents of nylon 66 are 1,6-diaminohexane (trivial name: hexamethylenediamine)
H2N-(CH2)6-NH2 and adipic acid, HOOC-(CH2)4-COOH. Raw materials for these are variable; sources used commercially are benzene (from coke production or oil refining), furfural (from oat hulls or corn cobs), or 1,4-butadiene (from oil refining). Mixture of the two plus heating gives hexamethylenediamine adipate which then polymerizes to the polymer nylon 66. The polymerization reaction is:
nH2N(CH2)6NH2 + n(CH2)4(COOH)2 -[NH(CH2)6NHOC(CH2)4CO]n- + 2nH2O.
Since the polymer is insoluble, it is spun in the molten state.
The other industrially significant nylon, nylon 6, is made from caprolactam which polymerizes on heating in water. The polymerization reaction is:
Nylon is consumed primarily in home furnishings (U.S.A. 1981, 59%), mostly carpet; wearing apparel (U.S.A. 1981, 20%) and tire cord (U.S.A. 1981, 10%) are also major markets. Total production of all forms of nylon (U.S.A. 1981) was some 2.5 million pounds. Nylon can be machined and used for small, tough plastic parts in addition to its uses as a textile fabric.
Caprolactam is manufactured from benzene. The benzene is hydrogenated to cyclohexane, then air-oxidized in the presence of a catalyst to cyclohexanone, C6H10=O. Treatment of cyclohexanone with hydroxylamine, H2NOH, yields the oxime C6H10=NOH. In the presence of sulfuric acid, cyclohexanone oxime undergoes the Beckmann rearrangement to caprolactam. The use and properties of nylon 6 are similar to those of nylon 66.
Another major polymeric fiber group is the polyesters (Terylene, Dacron). Polyesters are produced by condensation of ethylene glycol, HOCH2CH2OH, with terephthalic acid, HOOCC6H4COOH. The polymer is usually produced from the methyl diester of terephthalic acid mentioned in the a previous section.
The polyester resin is then spun into fibers. Most of the polyester produced goes into wearing apparel (U.S.A. 1981, 62%). The rest goes into home furnishings (U.S.A. 1981, 17%) or tire cord (U.S.A. 1981, 10%). Total production (U.S.A. 1981) was some 4.2 billion pounds.
Other polymeric fibers include the polyacrylics or acrylics. Polyacrylics are produced from acrylonitrile, CH2=CHCN, which polymerizes to give polyacrylonitrile (Orlon). Acrylonitrile, in turn, is produced by a catalytic reaction of propylene and ammonia. Polyacrylics are used as a rug fiber and in other home furnishings to the extent of about 30% as well as in apparel (70%). Total production (U.S.A. 1981) was some 720 million pounds. The polymerization reaction is:
Some synthetic fibers such as glass and carbon are manufactured for special uses, as are many blends of the above types or special modifications, but the important commercial synthetic fibers are those described above.
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