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Chap._1100007 - (b Ester Interchange In this method the...

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Unformatted text preview: (b) Ester Interchange . In this method, the polymer is obtained by ester interchange between bisphenol A and diphenyl carbonate, ' In order to obtain high yields of polymer and high molecular weights, almost complete removal of phenol from the reaction mixture is required. In a typical process, a mixture of bisphenol A — TH’ m and diphenyl carbonate together with a basic ; "“OQ‘EQO“ t "GO—COrOQ :‘ catalyst (eg. lithium hydride, zinc oxide) is - CH’ melted and agitated at about 150 °C under CH3 nitrogen. éI'he temperature is then raised to —o©—¢~©~0—Co— + 2" 00H about 210 C over 1 hour and the pressure is . CH reduced to about 20 mm Hg. By the end of this _ J , n time most of the phenol has been distilled off. The reaction mixture is then heated for a further period of 5-6 hours during which time the temperature is raised to about 300 °C and the pressures is lowered to about 1 mm Hg. With increasing viscousity, the _ reaction is eventually stopped while the material can still be forced from the reactor under nitrogen. The 3 extruded material is then pelletized. Compared to direct phosgenation, the ester interchange method of preparing the polycarbonate has the following advantages: solvents (and attendant recovery operations) are not required; purification of the polymer is unnecessary; and the product does not require densification. On the other hand, the ester interchange method does require more complex equipment (since high temperatures and low pressures are involved) and the high viscosity of the polymer melt makes it impracticable to handle products with molecular weights greater than about 50 000. Nevertheless, the ester interchange method is usually preferred to direct phosgenation in commercial operations. Properties Bisphenol A polycarbonate is a transparent rigid substance. The bisphenol A polycarbonate chain is very stiff because of the presence of the aromatic rings. This stiffness leads to the high glass-transition temperature (Tg=150 ”C) and also restricts crystallization (Tm= 220 "(3) in normal mouldings. Highly crystalline material can be obtained only by special techniques such as heating the polymer it 180'C for several days or slow evaporation of solutions. The material has outstanding rigidity and toughness, which properties are retained at both low and elevated temperatures; the maximum permissible service temperature is about 135 °C. The resistance of the polycarbonate to deformation under load and the dimensional stability in humid atmospheres are also outstanding. However, polycarbonate exhibits environmental stress cracking in various media such as hydrocarbon vapOurs, moisture at elevated temperatures and soap solutions. The stability of bisphenol A polycarbonate towards aqueous solutions of inorganic and organic acids, salts and oxidizing agents is very good. The ester group is, however, readily hydrolysed by bases. Amines and aqueous ammonia lead to rapid and complete breakdown to bisphenol A; aqueous solutions of strong alkalis attack the surfaces of specimens causing progressive saponification. Bisphenol A polycarbonate has good oxidative stability, largely because of the absence of secondary and tertiary carbon atoms. The polymer is stable in air up to 150 0C over long periods; at higher temperatures some oxidation and cross-linking occur. Ultraviolet light is strongly absorbed by the polycarbonate and causes crazing and degradation. However, such effects are restricted to the surface and in-depth deterioration does not occur Bisphenol A polycarbonate may be melt-processed by all of the standard techniques although its melt— viscosity is rather high. Despite its fairly high cost, the polymer has found in wide variety of uses. The largest application is in the electronics/business machine field for such parts as connectors, terminals and covers. An important new application is for compact discs. Other uses include glazing, safety glasses, medical devices and domestic appliance housing. ...
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