Dependence of the predicted outdoor lifetime of bisphenol-A polycarbonates on the terrestrial UV irradiance spectrum

The outdoor lifetimes for sheets of bisphenol-A polycarbonate (BPA-PC) having different concentrations of phenolic terminal groups were predicted as a function of the average terrestrial UV irradiance spectrum measured at several latitudes. The lifetimes were based upon chain scission rates predicted to occur in a thin surface layer of unstabilized BPA-PC sheet, and were less than the outdoor lifetime reported for sheets of stabilized material. These rates were derived from chain scission quantum yields measured at 287 and 308 nm in thin solid films of BPA-PC. In general, these rates will be extremely dependent on latitudinal, seasonal, and possibly diurnal variations in the wavelength distribution of terrestrial UV irradiance, and on the concentration of phenolic end groups. Reducing the concentration of phenolic terminal groups by a factor of two in commercial BPA-PC by acetylation (capping) lowers the chain scission quantum yield, reduces terrestrial UV absorbance, and nearly doubles the predicted outdoor lifetime of the material at North latitudes of 9°, 39°, and 71°. High concentrations of terminal phenolic groups in BPA-PC induce a cross-linking reaction which competes with chain scission at 308 nm and predominates at 287 nm.