Abstract
Even when formulated with heat stabilizers, clear ultramarine tinted bisphenol-A polycarbonate (BPA-PC) can become discolored when processed at too high a temperature or when scrap material is reprocessed too many times. In the past, it has not been clear if this discoloration was due to yellowing of the base resin or to degradation of the pigment or to some combination of the two processes. By taking advantage of the fact that the species responsible for the blue color in ultramarine pigments are paramagnetic sulfur compounds, electron paramagnetic resonance and visible spectroscopic studies were undertaken to assess the relative importance of the two processes under a range of processing conditions and to study the roles played by several common BPA-PC stabilizers in retarding and/or promoting them.
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