Surface pinking in titanium dioxide/lead stabiliser filled PVC profiles

The “pinking” phenomenon which is known to occur in white PVC profiles has been investigated in detail in terms of test evaluations and analysis methods for determining the mechanism and nature of the species responsible. An artificial weathering test method using a Microscal unit with submersion of samples in water was established where colour measurements and FTIR analysis proved the similarity of functional groups present to those in naturally pinked samples. Analysis of the artificially pinked samples using hydroperoxide concentration analysis, fluorescence spectroscopy, microscopic FTIR and scanning electron microscopy showed that artificially pinked samples presented a more extended degree of degradation than samples that remained white during the exposure in the Microscal Unit. Short chain conjugated polyenes were also identified as the most likely functional groups responsible for the development of the pink colour and interactions between the titania pigment and the lead stabilisers would seem to have an influence in the pinking development. In addition, it was found that there was an increase in production of hydroperoxy radicals at the onset of colour development. Addition of trace lead dioxide accelerated the production of pinking. Different titanium dioxide grades used in the PVC profiles showed differences in pinking in the artificial test, also an increase in the titanium dioxide concentration decreased the formation of the pinking colouration in artificial conditions The use of different analysis methods (Microwave Dielectric Spectroscopy and ESR) showed differences in carrier radical production and carrier radical trapping between the pigments used suggesting that a small difference in the coatings could play an important part in the pinking phenomenon. Finally, a mechanism was proposed involving a redox reaction between the titanium dioxide and the lead stabiliser that could facilitate the production of short polyene chains in the PVC matrix producing the pink colouration.