Abstract
UV-induced photodegradation has been studied of films of polyurethanes synthesised from (renewable) vegetable materials, namely rapeseed and sunflower oils. The techniques utilised were principally IR and UV-visible spectroscopy. UV(B)-photolysis of the cured films gave rise to IR bands in the carbonyl region at ca 1706 cm-1 and a UV band at 310 nm attributed to formation of a quinone methide, an assignment supported by the development of new IR bands at 1595, 1650, and 1695 cm-1 which are typical of this chromophore. In addition, intensification of a band at 1171 cm-1 is attributed to cross-linking of the soft segments in the polymer.
Plots of the growth of the 1706 cm-1 band on irradiation over time showed the usual profile for conversion of CH2 groups to carbonyl: these were normalised against the band at 2900 cm-1 of the polymer to give the carbonyl index (CI). The effect of adding the pigment TiO2 on the CI was monitored. The effect depended strongly on the nature of the TiO2; thus the Degussa P-25 pigment greatly accelerated photodegradation whereas TiO2 coated with a thin layer of silica or alumina, as in Kronos 2220 pigment, conferred photostability on the polymer.
Doping the TiO2 with metal ions markedly affected its behaviour; addition of Mo(VI) made the TiO2 more aggressive while doping with Cr(III) reduced its activity. Addition of the photostabiliser Tinuvin 770 had a dramatic effect in reducing the photo-oxidation of the polyurethanes.