Polymers

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Polymers

帖子 由 Giraffe 于 周五 七月 23, 2010 12:56 pm

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.
The most well known natural polymers are proteins (polymers of amino acids), nucleic acids (polymers of ribose or deoxyribose sugars with attached purine or pyrimidine bases), and the polymers of glucose (starch, glycogen, cellulose). Synthetic polymers were originally derived from these natural polymers. The first commercially successful synthetic polymer was cellulose nitrate (Celluloid, 1869) which was first practically molded as a substitute for ivory in billiard balls. Nitration of cellulose, [C6H7O2(OH)3].xH2O, produces mixtures of cellulose trinitrate, called guncotton, and cellulose dinitrate, called pyroxylin. John Wesley Hyatt discovered that pyroxylin, when mixed with camphor, becomes a thermoplastic, a substance which can be molded when heated. Unfortunately, cellulose nitrate is also an explosive and its use in motion picture film and in billiard balls occasionally produced spectacularly inflammable incidents. Cellulose acetate, discussed in a following section, soon replaced it.

The second development was that of casein-formaldehyde plastics (A. Spitteler, 1899) made using formaldehyde (H2C=O) and casein obtained from milk. These polymers are no longer of industrial significance. Phenol (C6H5OH)-formaldehyde resins (Bakelite, 1909) were developed in the United States by the Belgian-born chemist Leo Baeckeland while searching for a substitute for varnish shellac .Heating these resins under pressure gave soft solids which could be molded and then hardened; they were both safe and economical. These early polymers have now been replaced by others based on simpler monomers.

The polymer industry is normally divided into three areas on the basis of the type of product manufactured: synthetic plastics, man-made textile fibers, and synthetic rubber. Some polymers have properties which permit their use in more than one of these areas.



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回复: Polymers

帖子 由 Giraffe 于 周五 七月 23, 2010 12:56 pm

Manmade Fibers and Fabrics

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, manmade fibers and fabrics. The magnitude of this industry is indicated by the Table below.
Table: Production of Man-made Fibers and Textile Fabrics

Area Synthetic Fibers Manmade Fibers
(Gg, 1985) (Gg, 1985)
U.S.A. 30.9 35.2
Western Europe 26.5 32.8
Japan 14.2 18.1
All Other 58.8 76.5
World 130.5 162.6
Notes to Table: one Gg is also 1000 metric tonnes. Most of the production listed under "All Other"countries takes place in China or in Southeast Asia. Most manmade 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 softenedby 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: 2nCH2NH(CH2)4C=O --> -[NHCH2(CH2)4CONHCH2(CH2)4CO]n-.

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: nCH2=CHCN --> -[CH=CH-CH=N-CH=CH-CH=N]n-.

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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回复: Polymers

帖子 由 Giraffe 于 周五 七月 23, 2010 12:57 pm

Synthetic Rubber

Natural rubber is a polymer found in the sap of the rubber tree, Hevea brasilensis, originally native to Brazil. The rubber tree grows throughout the tropics and is cultivated in plantations, primarily in Southeast Asia; Malaysia and Indonesia are the most significant sources. The sap is collected, and on exposure to air and mild heat gives natural rubber. Chemically, this is a polymer of 2-methyl-1,3-butadiene (isoprene), CH2=C(CH3)-CH=CH2. The polymerization reaction is: nCH2=C(CH3)-CH=CH2 --> -[CH2-C(CH3)=CH-CH2]n-.
Natural rubber has long been known, but became valuable only with the development of vulcanizing (heating in the presence of sulfur) developed by Charles Goodyear, which gives a much more rubbery and coherent substance.
The development of synthetic rubber was a German program for many years. Germany produced some 2500 tonnes of methyl rubber (polymer of 2,3-dimethyl-1,3-butadiene) during World War I, but returned to natural rubber at the end of the War. Specialty rubbers were produced and research was carried out in Germany and elsewhere prior to World War II, but the synthetic rubber industry of the world arose during World War II. At the present time over 75% of the rubber used in the United States is synthetic, while on a world basis about 65% of the rubber used is synthetic.
The most significant forms of synthetic rubber used today are the Buna S, Buna N, and synthetic natural rubbers developed in Germany. Other types (Thiokol, Neoprene) have somewhat different properties and specialty uses. Buna-S, by far the most significant, has been made since 1933 in Germany. It is a copolymer of 1,3-butadiene and styrene. The polymerization reaction is: nCH2=CHCH=CH2 + nC6H5CH=CH2 --> -[CH2CH=CHCH2CH(C6H5)CH2]n-.
The Buna-N rubber, which is soil-resistant, is a copolymer of 1,3-butadiene and acrylonitrile. The polymerization reaction is: nCH2=CHCH=CH2 + nCH2=CHCN --> -[CH2CH=CHCH2CHC=N]n-. Like the Buna-S type, Buna-N rubber can be vulcanized.
True synthetic rubber, a polymer of isoprene, was achieved almost simultaneously in 1955 by several major rubber manufacturers using different catalysts. The process is a polymerization of isoprene itself and the product is virtually identical to the natural Hevea product. The feedstocks for the synthetic rubber industry are products of the petrochemical industry.
Rubber, either natural or synthetic, is normally vulcanized, most often with sulfur, although for some specialty uses other agents are used. The majority of rubber used is used for automobile tires. For this and similar applications a filler or reinforcing agent must be added, of which carbon black is by far the most common.

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