Lecture 015.014

DISCOLOURATION OF PLASTIC OBJECTS. THE IDENTIFICATION OF YELLOW AND ORANGE SYNTHETIC ORGANIC PIGMENTS IN PLASTIC OBJECTS
BY SUZAN DE GROOT, HENK VAN KEULEN

Colourants (pigments and dyes) in plastics play an important role in the appearance and perception of an object. However more and more plastic objects show colour change due to problems with colourants; colourants can fade and also the transfer of colourant to adjacent differently coloured plastics may take place.
All synthetic organic pigments fade under the influence of light; previous research by the authors (de Groot 2015) focused on the red organic pigments in plastics, this continuing research is now focused on the orange and yellow synthetic organic pigments in the very broad range of pigments available for colouring plastics. In combination with blue colourants yellow pigments also play a role in the colouring of green coloured plastics, here the fading of the yellow pigment may cause the colour of the plastic shift to the blue hues.
The red pigment research proved that Raman spectroscopy and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) are the most suitable combination for the identification of synthetic organic pigments in plastics. Therefore these techniques have been used for building databases of reference spectra of yellow and orange pigments. Using these databases it was possible to identify several yellow and orange pigments in plastic objects.
The identification of the organic pigments in plastics can contribute to formulating exhibition lighting guidelines for a particular object of art. When it is known which colourants are present in an object, the light-fastness can be derived from literature and the sensitivity class can be determined. Objects containing pigments with poor light fastness may only be exposed to a low illumination dose, whereas others may be exposed to a higher dose.

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