Chap._700003 - coho—co Cato—$0 ~CH-—¢~ ~CH,—$~ —...

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Unformatted text preview: coho—co Cato—$0 ~CH,-—¢~ ~CH,—$~ . —. . ~CH,—(|I~ —cu,—o~ c,H,o-—co canon-co cum 0 Clflao— 0 19.5%....7,‘ ~ct~i,—t;~ + C,H,0H :_ H g -"_“.. O i czupitfo ~CH.—CH~ -.cu,-cu- As with other rubbers, acrylate rubbers must be vulcanized before they exhibit useful technological properties. It is possible to crosslink straight polyacrylates by various means. For example, treatment with a peroxide leads to abstraction of tertiary hydrogen and subsequent crosslinlcing. Various basic compounds such as sodium hydroxide also bring about vulcanization by the liberation of alcohol. In general, however, vulcanization of straight polyacrylates are relative weak and do not have good ageing properties. Consequently, most of commercial acrylate rubbers are invariably vucalized through reactive sites introduced into the polymer by copolymerization, discussed later. The important charateristics of acrylate rubbers are good oil resistance and good heat resistance; carbon black reinforced materials are very well in air uo tp 200 °C. It is these properties which account for the major use of acrylate rubbers, i.e. oil seals for automobiles. CH3 CH3 I R—O* | CH2=CII ~——-—-+ ~(CH2-Cl3)x- coocn3 (3:0 9 CH3 Table 6.2 Typical values for various properties ofsome polyialkyl methaerylatels [3' Methyl Ethyl n-Propyl n-Butyl' Specific gravity [.19 Lil Tensile strength L06 L05 (MPa) 62 34 23 ( I bf/In’) 9000 5000 4000 I 503 Elongation at break (Va) 4 7 5 230 M Isoburyl' 3400 Poly(methyl methacrylate) (PMMA) is by far the predominant polymethylacrlate in the use in rigid applications, mainly due to its relatively high softing temperature and strength. PMMA was first produced by ICI and Rohm and Haas in 1930s. The polymer is usually prepared by bulk polymerization. The subsequent casting is used in the manufacture of PMMA sheet. Typically, monomer is stirred with peroxide at 90 0C for about 10 minutes to give a syrup which then cooled to RT. Colourant, plasticizer and UV absorber, if needed, are added at this point. At this point the degree of monomer conversion is about 20%. The syrup is then poured into a casting cell, then heated to convert most of monomers to polymer sheet. PMMA is a hard and rigid transparent substrace and is somewhat tougher than‘polystyrene but is less tough tham the ABS polymers. The transmission of normal ineldent light 1s about 92%. PMMA is amorphous polymer, soluble in some organic solvent, but 13 very good resrstance to water. The polymer is also very good outdoor weathering, marketly snpenor to most other thermoplasatics. The Tg of PMMA is 105 °C. The application of PMMA includes aircraft glazing, display signs, lighting fittings, bathroom fitting, pressure theating wood, etc. ...
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