Bis-phenol A polycarbonates

When dipoles are directly attached to the chain their movement will obviously depend on the ability of chain segments to move. Thus the dipole polarisation effect will be much less below the glass transition temperature, than above it Figure 6.4). For this reason unplasticised PVC, poly(ethylene terephthalate) and the bis-phenol A polycarbonates are better high-frequency insulators at room temperature, which is below the glass temperature of each of these polymers, than would be expected in polymers of similar polarity but with the polar groups in the side chains. 

Around Izod notch Low-density polyethylene Ethylene-propylene block copolymers Cellulose nitrate and propionate ABS and high-impact polystyrene Bis-phenol A polycarbonate... 

Independently at the General Electric Company in America, work was being carried out in search of thermally and hydrolytically stable thermosetting resins. As a by-product from this work the research team at General Electric also produced polycarbonates from bis-phenol A so that by 1958 production of bis-phenol A polycarbonates was being carried out in both Germany and the USA. 

Today about 75% of the market is held by General Electric and Bayer with their products Lexan and Makrolon respectively. Other manufacturers are ANIC (Italy), Taijin Chemical Co., Mitsubishi Edogawa and Idemitsu Kasei in Japan and, since 1985, Dow (USA) and Policarbonatos do Brasil (Brazil). Whilst this market is dominated by bis-phenol A polycarbonates, recent important developments include alloys with other thermoplastics, polyester carbonates and silicone-polycarbonate block copolymers. 

Of these four routes the first and third have been studied intensively, in particular in the preparation of the bis-phenol A polycarbonates. Since this polycarbonate is at present the only one of major commercial importance the following remarks will apply to this only unless otherwise stated. 

A study of the molecular structure of bis-phenol A polycarbonates enables one to... 

Table 20.1 Crystal structure date of bis-phenol A polycarbonates (after Prietschic) ...

It is found that observed values of Tg for the bis-phenol A polycarbonate are somewhat higher than estimated from data by the general rule-of-thumb relation ... 

To enhance the resistance to heat softening his-phenol A is substituted by a stiffer molecule. Conventional bis-phenol A polycarbonates have lower heat distortion temperatures (deflection temperatures under load) than some of the somewhat newer aromatic thermoplastics described in the next chapter, such as the polysulphones. In 1979 a polycarbonate in which the bis-phenol A was replaced by tetramethylbis-phenol A was test marketed. This material had a Vicat softening point of 196 C, excellent resistance to hydrolysis, excellent resistance to tracking and a low density of about l.lg/cm-. Such improvements were obtained at the expense of impact strength and resistance to stress cracking. 

The TMC polycarbonate homopolymer has a glass transition temperature of 238°C, nearly 100°C above that of the bis-phenol A polycarbonate. Therefore, copolymers will have intermediate glass transitions depending on the relative proportions of TMC and bis-phenol A. Commercial grades (marketed by Bayer as Apec HT) have Vicat softening points from 158 to... 

Polycarbonates with superior notched impact strength, made by reacting bisphenol A, bis-phenol S and phosgene, were introduced in 1980 (Merlon T). These copolymers have a better impact strength at low temperatures than conventional polycarbonate, with little or no sacrifice in transparency. These co-carbonate polymers are also less notch sensitive and, unlike for the standard bis-phenol A polymer, the notched impact strength is almost independent of specimen thickness. Impact resistance increases with increase in the bis-phenol S component in the polymer feed. Whilst tensile and flexural properties are similar to those of the bis-phenol A polycarbonate, the polyco-carbonates have a slightly lower deflection temperature under load of about 126°C at 1.81 MPa loading. 

Table 20.3 Comparison of mechanical properties of typical commercial bis-phenol A polycarbonates...

Typical mechanical properties for bis-phenol A polycarbonates are listed in Table 20.3. 

The electrical insulation characteristics of bis-phenol. A polycarbonates are in line with those to be expected of a lightly polar polymer (see Chapter 6). 

Figure 20.7. Effect of frequency on dielectric constant of bis-phenol A polycarbonate...

The very low water absorption of bis-phenol A polycarbonates contributes to a high order of dimensional stability. Table 20.6 shows how the water absorption of j in thick samples changes with time and environmental conditions and the consequent influence on dimensions. 

In spite of their rather complicated chemical structure, which consequently involves rather expensive production costs, the bis-phenol A polycarbonates have achieved an important place amongst the speeiality plastics materials. 

Other polymers are as rigid, others are as transparent, others are even both more rigid and as transparent, but the bis-phenol A polycarbonate is the only material that can provide such a combination of properties, at least at such a reasonable cost. The application of polycarbonates therefore largely arise where at least two and usually three or more of the advantageous properties are required and where there is no cheaper alternative. 

Introduction of aromatic or cycloaliphatic groups at R and/or Rj gives further restriction to chain flexibility and the resulting polymers have transition temperatures markedly higher than that of the bis-phenol A polycarbonate. 

Finally mention may be made of the phenoxy resins. These do not contain the carbonate group but are otherwise similar in structure, and to some extent in properties, to the bis-phenol A polycarbonate. They are dealt with in detail in Chapter 21. 

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