Effect of coloured pigmentation on titanium dioxide photo catalysed PVC degradation

Abstract

Most organic coatings, regardless of colour, are pigmented with titanium dioxide (TiO₂) to increase their covering power. TiO₂ in its anatase crystal form is highly photoactive under ultraviolet (UV) irradiation. Photo activation of TiO₂ causes the formation of free radicals, which can interact with the organic matrix, causing accelerated degradation of the polymeric coating. This releases carbon dioxide (CO₂) via a series of reactions initiated by surface bound hydroxyl radicals. To identify which type and concentration of coloured pigment provides the most photostable formulation of TiO₂ containing organic coating, model coatings containing different coloured pigments and loadings have been assessed using a novel flat panel irradiation reactor. The rate of CO₂ evolved by the irradiated sample, measured using a Fourier transform infrared (FTIR) spectrometer, has been used to quantify the degradation induced by individual pigment combinations. Yellow chromium titanate pigments are ineffective in stabilisation of the TiO₂ driven oxidation largely owing to poor light absorption properties and carbon black pigments are only effective at higher loadings where light absorption outweighs charge percolation. Red organic pigments can potentially sensitise degradation while the blue and green pigments tested offer the best stabilisation of TiO₂ driven degradation over the range 0·1–10% per hundred resin (PHR) owing to their superior absorption of near ultraviolet A (UVA) light (at 380 nm) which activates the TiO₂.