The effect of molding conditions on resin, colorant, stabilizer interactions in bisphenol-A polycarbonate containing ultramarine pigments

Even when formulated with heat stabilizers, clear ultramarine tinted bisphe nol-A polycarbonate (BPA-PC) can become discolored when processed at too hi gh a temperature or when scrap material is reprocessed too many times. In t he past, it has not been clear if this discoloration was due to yellowing o f 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 respon sible for the blue color in ultramarine pigments are paramagnetic sulfur co mpounds, electron paramagnetic resonance and visible spectroscopic studies were undertaken to assess the relative importance of the two processes unde r a range of processing conditions and to study the roles played by several common BPA-PC stabilizers in retarding and/of promoting them.