preferred according to the invention contain bisphenol a and 1 1 bis 4 hydroxyphenyl cyclohexane bisphenol z the polycarbonate mixtures according to the invention can be mixtures comprising homopolycarbonates and or copolycarbonates and the polycarbonate component a may differ from the polycarbonate component b not only in the higher molecular weight but it can also differ in that it has a different molecular composition aromatic polycarbonates which have molecular weights weight average mw for example measured by the light scattering method of about 28 000 to 40 000 preferably of about 29 000 to 39 000 and in particular of about 30 000 to 35 000 are to be understood as polycarbonate component b aromatic polycarbonates which have molecular weights weight average mw for example measured by the light scattering method of about 60 000 to 120 000 preferably of about 65 000 to 110 000 and in particular of about 70 000 to 95 000 are to be understood as polycarbonate component a they can be prepared by known methods for example from the abovementioned bisphenols it being possible to use for example phenol and p tert butylphenol as a chain stopper known polycarbonate preparation methods are described for example in u.