STYRENE POLYMER

Styrene Polymers. Styrene polymers of practical significance include polystyr- 

The most important hydrocarbon copolymers are styrene-butadiene rubbers (SBR) produced by free-radical emulsion or anionic polymerization. Anionic polymerization allows the manufacture of styrene-butadiene and styrene-isoprene three-block copolymers. 

At present all commercial polystyrene (with average molecular weights between 100,000 and 400,000) is manufactured by radical polymerization, which yields atactic polymers.476 Peroxides and azo compounds are commonly used initiators. The suspension process (usually as a batch process in water at 80-140°C) produces a product with relatively high residual monomer content.223 More important is the continuous solution process (usually in ethylbenzene solvent at 90-180°C), which yields high-purity product. Styrene can be copolymerized with numerous other monomers.477 One of these copolymers, the styrene-divinylbenzene copolymer produced by free-radical polymerization, has a crosslinked stucture and is used in 

The nonterminating nature of living anionic polymerization allows the synthesis of block copolymers,480,481 which are useful thermoplastic elastomers. They have many properties of rubber (softness, flexibility, resilience) but in contrast to rubber can be processed as thermoplastics 482,483 Block copolymers can be manufactured by polymerizing a mixture of two monomers or by using sequential polymerization. 

When a mixture of styrene and 1,3-butadiene (or isoprene) undergoes lithium-initiated anionic polymerization in hydrocarbon solution, the diene polymerizes first. It is unexpected, since styrene when polymerized alone, is more reactive than, for example, 1,3-butadiene. The explanation is based on the differences of the rates of the four possible propagation reactions the rate of the reaction of the styryl chain end with butadiene (crossover rate) is much faster than the those of the other three reactions484,485 (styryl with styrene, butadienyl with butadiene or styrene). This means that the styryl chain end reacts preferentially with butadiene. 

Sterically hindered phenols, such as 2,6 di-tert-butyl-p-cresol (BHT), long chain 3-(3,5 di-tert-hutyl-4-hydroxyphenyl)-propionate or sebacate are the preferred substances for the stabilization of polystyrenes. Preferred thio compounds are long-chain thiodi-propionates. The combination of thio compounds with sterically hindered phenols leads to thioether substituted phenols. Suitable phosphorous compounds include substituted phenylphosphites, such as tris-nonylphenyl phosphite (TNPP) or phos-phonite. 

While polystyrenes can also be used non-stabilized, copolymers with butadiene content, such as ABS, must be protected against thermal-oxidative damage by antioxidants alone or in combination with synergists. The most important antioxidants here are phenolic antioxidants and BHT (butylated hydroxy toluene). Phosphites are used as synergists and long-chain thiodipropionates as thiosynergists, respectively. The butadiene content determines the antioxidant or the mixture of antioxidant pius synergist needed, see Table 3.3 [86]. 

Impact toughness Butadiene content (%) Minimum quantity (%) Antioxidant w/o synergist SB 1 ABS  

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Resin and Polymer Solvent. Dimethylacetamide is an exceUent solvent for synthetic and natural resins. It readily dissolves vinyl polymers, acrylates, ceUulose derivatives, styrene polymers, and linear polyesters. Because of its high polarity, DMAC has been found particularly useful as a solvent for polyacrylonitrile, its copolymers, and interpolymers. Copolymers containing at least 85% acrylonitrile dissolve ia DMAC to form solutions suitable for the production of films and yams (9). DMAC is reportedly an exceUent solvent for the copolymers of acrylonitrile and vinyl formate (10), vinylpyridine (11), or aUyl glycidyl ether (12). 

In polymers such as polystyrene that do not readily undergo charring, phosphoms-based flame retardants tend to be less effective, and such polymers are often flame retarded by antimony—halogen combinations (see Styrene). However, even in such noncharring polymers, phosphoms additives exhibit some activity that suggests at least one other mode of action. Phosphoms compounds may produce a barrier layer of polyphosphoric acid on the burning polymer (4,5). Phosphoms-based flame retardants are more effective in styrenic polymers blended with a char-forming polymer such as polyphenylene oxide or polycarbonate. 

Foaming polystyrene resin prepared by blending with gas deHvers an opaque, low density sheet useful for beverage-bottle and plastic can labels as a water-resistant paper substitute (see Styrene polymers). 

Styrene is a colorless Hquid with an aromatic odor. Important physical properties of styrene are shown in Table 1 (1). Styrene is infinitely soluble in acetone, carbon tetrachloride, benzene, ether, / -heptane, and ethanol. Nearly all of the commercial styrene is consumed in polymerization and copolymerization processes. Common methods in plastics technology such as mass, suspension, solution, and emulsion polymerization can be used to manufacture polystyrene and styrene copolymers with different physical characteristics, but processes relating to the first two methods account for most of the styrene polymers currendy (ca 1996) being manufactured (2—8). Polymerization generally takes place by free-radical reactions initiated thermally or catalyticaHy. Polymerization occurs slowly even at ambient temperatures. It can be retarded by inhibitors. 

Figure 5 illustrates a typical distillation train in a styrene plant. Benzene and toluene by-products are recovered in the overhead of the benzene—toluene column. The bottoms from the benzene—toluene column are distilled in the ethylbenzene recycle column, where the separation of ethylbenzene and styrene is effected. The ethylbenzene, containing up to 3% styrene, is taken overhead and recycled to the dehydrogenation section. The bottoms, which contain styrene, by-products heavier than styrene, polymers, inhibitor, and up to 1000 ppm ethylbenzene, are pumped to the styrene finishing column. The overhead product from this column is purified styrene. The bottoms are further processed in a residue-finishing system to recover additional styrene from the residue, which consists of heavy by-products, polymers, and inhibitor. The residue is used as fuel. The residue-finishing system can be a flash evaporator or a small distillation column. This distillation sequence is used in the Fina-Badger process and the Dow process. 

Fig. 3. Typical creep behavior for rubber-modified styrene polymers.

Polymers of Styrene Derivatives. Many styrene derivatives have been synthesized and the corresponding polymers and copolymers prepared (61). Glass-transition temperatures for a series of substituted styrene polymers are shown in Table 3. The highest T is that of... 

Rubber-Modified Copolymers. Acrylonitrile—butadiene—styrene polymers have become important commercial products since the mid-1950s. The development and properties of ABS polymers have been discussed in detail (76) (see Acrylonitrile polymers). ABS polymers, like HIPS, are two-phase systems in which the elastomer component is dispersed in the rigid SAN copolymer matrix. The electron photomicrographs in Figure 6 show the difference in morphology of mass vs emulsion ABS polymers. The differences in stmcture of the dispersed phases are primarily a result of differences in production processes, types of mbber used, and variation in mbber concentrations. 

Polymerization Reactions. The polymerization of butadiene with itself and with other monomers represents its largest commercial use. The commercially most important polymers are styrene—butadiene mbber (SBR), polybutadiene (BR), styrene—butadiene latex (SBL), acrylonittile—butadiene—styrene polymer (ABS), and nittile mbber (NR). The reaction mechanisms are free-radical, anionic, cationic, or coordinate, depending on the nature of the initiators or catalysts (194—196). 

Other minor uses of ethyl chloride iaclude blowiag agents for thermoplastic foam (51) and styrene polymer foam (52), the manufacture of polymeric ketones used as lube oil detergents (53), the manufacture of acetaldehyde (qv) (54), as an aerosol propellent (55), as a refrigerant (R-160), ia the preparation of acid dyes (56), and as a local or general anesthetic (57,58). 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

C. A. Brighton, G. Pritchard, and G. A. Skinner, in Styrene Polymers Technology and EnvironmentalMspects, AppHed Science, New York, 1979. 

Cellulose acetate butyrate, f Acrylonitrile butadiene styrene polymer. 

KESKKULA, H., Chapter entitled Rubber-modified Styrene Polymers in Polyblends and Composites (Ed. bruins, p. f.). Interscience, New York (1970)... 

The tonnage of plasticisers consumed each year exceeds the annual tonnage consumption of most plastics materials. Only PVC, the polyolefins, the styrene polymers, the aminoplastics and, possibly, the phenolics are used in large quantity. 

BRIGHTON, c. A., PRITCHARD, G., and SKINNER, G. A., Styrene Polymers Technology and Environmental Aspects, Applied Science, London (1979)... 

Butadiene-Styrene Polymers A Review Report published by RAPRA Technology Ltd, Shawbury, England (1993)... 

Acrylonitrile-butadiene-styrene polymers are similar in stmeture, but the acrylonitrile hardens the polymer. Minute rubber particles act as stress-relief centers, making it good for large objects luggage or car body parts. It can be chrome plated, foamed, injection molded, blown, and alloyed wiih other pla.siic. . 

Occupational and environmental exposure to chemicals can take place both indoors and outdoors. Occupational exposure is caused by the chemicals that are used and produced indoors in industrial plants, whereas nonoccupa-tional (and occupational nonindustrial) indoor exposure is mainly caused by products. Toluene in printing plants and styrene in the reinforced plastic industry are typical examples of the two types of industrial occupational exposures. Products containing styrene polymers may release the styrene monomer into indoor air in the nonindustrial environment for a long time. Formaldehyde is another typical indoor pollutant. The source of formaldehyde is the resins used in the production process. During accidents, occupational and environmental exposures may occur simultaneously. Years ago, dioxin was formed as a byproduct of production of phenoxy acid herbicides. An explosion in a factory in... 

Methacrylate-butadiene-styrene polymers (MBS) and related materials chemically similar to ABS but often available in transparent form. 

Postelnek,W., Colemann, L. E., and Lovelace, A. M. Fluorine-Containing Polymers. I. Fluorinated Vinyl Polymers with Functional Groups, Condensation Polymers, and Styrene Polymers. Vol. 1, pp. 75— 113. 

I. Fiuorinated Vinyl Polymers with Functional Groups, Condensation Polymers, and Styrene Polymers. Vol. 1, pp. 75-113. 

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