Bisphenol A polycarbonate @PA-PC) is an important high performance engineering thermoplastic well known for its outstanding combination of toughness, transparency and heat resistance. These properties make it an ideal material for demanding applications where it is exposed to external stresses such as elevated temperatures, ultraviolet light and y-ray sterilization. However, on extended exposure to these conditions, BPA-PC slowly degrades, turning progressively more yellow, eventually leading to a decrease in its physical properties.Over the years, there has been numerous studies made to understand these degradative processes so as to better design more stable BPA-PC formulations. In this paper, this chemistry is briefly reviewed along with more recent work in this area with a special emphasis on the efforts made to identiijing the actual chemical species responsible for the observed yellow color and the chemistry responsible for their formation.
General ObservationsUnfortunately, investigations of yellowed BPA-PC generally show a minimum of new spectral features which might help identifjl the origin of the color. For example, samples which had turned yellow after exposure to heat or heat plus oxygen invariably showed only a general broad increase in their W-visible absorption with tailing beyond 400 nm into the visible spectrum which is the source of the observed yellow color. Fortunately, in the case of exposure to UV light and y-irradiation, this broad tailing is often accompanied by several new weak absorption peaks which, as will be discussed in the following sections, do give some clues to their chemical origins.
TbermolysisBecause of BPA-PC's highly aromatic structure, possessing only methyl and aromatic hydrogens, it is one of the most stable commercial polymers [ 11 showing TGA weight loss (N2, 10°C/min) only above SSOOC. However, it is generally melt processed well below this temperature, i.e. between its T, of 220-230°C and 340°C because heating BPA-PC above 34OOC leads to both MW loss and gelation reactions. This gelation reaction was observed by Schnell [2] to occur during the base-catalyzed melt synthesis of BPA-PC from diphenyl carbonate (DPC) and BPA and was postulated to be due to the base-catalyzed formation of o-phenoxybenzoic acids and esters previously reported by Fosse [3], eq. 1. Nonetheless, heating BPA-PC in the absence of air below its decomposition temperature generally does not lead to