Variations in commercial grades

The range of polycarbonates commercially available has increased greatly in recent years. The main differences between these grades are largely due to  

Structurally viscous grades are based on branched polymers (branching being effected by the use of tri- or higher functional phenols). These polymers exhibit a sharp decrease in viscosity with increasing shear rate which makes them particularly suitable for extrusion and blow moulding and also, it is claimed, in reducing drip in case of fire. 

Flame retardant grades usually employ additives (but see below), e.g. sodium 2,4,5-trichlorobenzene sulphonate, sometimes in conjunction with an antidripping agent which, it is claimed, cross-links the polymer as it bums thus reducing the tendency to drip. 

UV-stabilised grades exhibit reduced tendency to yellowing by the use of stabilisers such as benzophenone and benzotriazole compounds (see also Section 7.5.4). 

Whilst remaining speciality materials, several copolymers have been marketed ver the years in order to enhance certain specific properties whilst retaining the eneral characteristics of polycarbonates. There are also a number of homo-olymers in which the bis-phenol A has been replaced by another bis-phenol ompound. The most important of these are summarised below. 

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