23 April 2010

Article: the effects of white pigments and extenders (1990)

Title: Studies on the photochemical stability of synthetic resin-based retouching paints: the effects of white pigments and extenders
Authors: Paul M. Whitmore and Catherine Bailie
Reference: Cleaning, retouching and coatings: Contributions to the 1990 IIC Congress, Brussels, 1990, pp. 144-149
Links: http://www.cmu.edu/... (pdf, 568 Kb)

The durability of a paint vehicle can be strongly influenced by the pigments which it contains. This report describes an initial investigation of the photochemical degradation of paints formulated with a commercial poly(n-butyl methacrylate) and a variety of white pigments and extenders. Upon exposure to near-ultraviolet (UV) light, this resin predominantly cross-links, eventually becoming almost completely insoluble. White pigments such as rutile TiO2 and Green Seal zinc oxide do not alter this tendency but they do slow the cross-linking rate, probably by their ability to absorb UV light. By contrast, a nonabsorbing light scatterer like barium sulfate increases the rate of cross-linking, perhaps by increasing the path length of light as it becomes diffused within the paint film. Finally, pigments such as anatase TiO2 and Kadox 515 zinc oxide not only decrease the overall degradation by screening UV light, but also increase the relative importance of chain-breaking chemistries through their photocatalytic properties. Even so, loss of volatile degradation products renders these films as insoluble as the purely cross-linked systems.

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