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Chap._800008 - Properties and Applications The...

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Unformatted text preview: * Properties and Applications The characteristic properties of the styrene-butadiene thermoplastic elastomers stem from the inherent incompatibility of the polystyrene and polybutadiene blocks. In the bulk material the twa different blocks aggregate into separate domains leading to a dispersion of polystyrene domains (as the lesser component) in a continuous matrix of polybutadiene. At ordinary temperatures the polystyrene domains are rigid and immobilize the ends of the polybutadiene segments, ' in effect serving both as cross-links and filler particles. Thus the system has the strength of a filler—reinforced vuicauized amorphous e1 astomer (see Table 20.1). At temperatures above the glass transition temperature of polystyrene (lOO‘C), the polystyrene domains are readily disrupted and the material may be melt-processed. The maximum service temperature of components fabricated from the copolymers is about 60'C. TIME 20.] Typical values for various properties or vulcanizatcs or common die)“: clastnmers [h] NR and IR BR SBR Sits NM‘R (‘R “W Gum ivulc'anirrire [rutfilhui] Specific gravity 0.93 0.93 0.94 0.9-1 4.0] LO LI Tensile strength (M Pit) lT-Zl ”-6.9 lat-2.8 ”—26 JAE—b!) Zl-ZH (lhffini) 1500-1000 300-1000 200-400 I 700-3700 500w um 3000—4000 Reinfim'ml I'lrfi':trli:rlll‘ [fill-'11] Tensile strength [M Pal Zl—lll [4-2-1 [4-24 6.9n2I ZIAZK Zl—le llbf/in‘J Jilin—$000 20004500 31300—3500 lDOOr-JlKll'l 3000-40“) 3011,4000 Elongation til break PM 3004700 300-700 3004700 500—1000 300-700 100-700 Hardness (Shun.- AJ Ell-[00 30400 40400 4045 BED-[00 Ill—IOU Still'ening temperature [°C) —30 to —45 ~15 to —50 ~20 to ~45 ~50 lu —-60 —15 to ~55 — It) In —30 W. Nil Natural rubber SIIR Slymic-hulndiune rubber NI!!! Aurylunilrile-huludienc rut-her [R High cis-l.J-pulyimprene [synthetic] SIDS SIylmn-hulndieue-styrcne CR l-‘ofychloruprene BR Polyhutudiene tribloek cupolymer Styrene-butadiene thermoplastic elastomers find use in such applications as footwear soles and heels, elastic bands audadhesives. 8.4 Acrylonitrile-Butadiene Copolymer (Nitrile rubber) In 1930 it was found that acrylonitrile was a promising comonomer in polybutadiene it gave rise to oil-resistant rubbers. The first small-scale manufacturing unit for acrylonitrile-butadiene rubber (nitrile rubber) was started in 1934. Substantial quantities of acrylonitrile—butadiene rubber were produced under the government sponsored synthetic rubber programme (as Government Rubber-Acrylon; rile or GR-A). Acrylonitrile—butadieue copolymers have since maintained their position as significant speciality rubbers. * Preparation The commercial preparation of acryloni trile~butadiene copolymers is generally by emulsion polymerization and the techniques used are entirely similar to those described for styrene-butadiene rubbers. Both 'hot' and 'cold' processes may be employed but now only 'cold' processes (using temperatures in the range 5-30 0C) are operated commercially. 'Low', 'medium‘ and ‘high' solvent-resistant grades of copolymer, respectively containing approximately 25, 30 and 40% by weight of acrylonitrile, are commonly produced. The butadiene content of a copolymer is made up of trans-1,4, cis-1,4- and 1,2-units; in, for example, a copolymer containing 72% butadiene _ and prepared at 28 DC, the respective contributions are 77.5, 12.5 and 10%. Copolymers prepared by 'cold' and 'hot' ' processes differ in that the former are less branched and have a narrower molecular weight distribution. ' * Properties and Applications Nitirle rubbers are compounded and processed in much the same manner as natural rubber; they may be vulcanized with either conventional sulphur systems or peroxides. As with styrene-butadiene rubbers, gum vulcanizates have I low strength but reinforcement with carbon black leads to vulcanizates with tensile strength approaching that obtained with natural rubber. Acrylonitrile—butadiene rubbers are used almost invariably on account of their resistance toward non-polar solvents such as petrol and oils. ...
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