A Multi-Modal Investigation into Titanium Dioxide Degradation of Oil Paints: Shedding Light on the Effect of Environmental Factors

Schmitt, Thomas

doi:https://doi.org/10.21985/n2-84w9-ar66

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A Multi-Modal Investigation into Titanium Dioxide Degradation of Oil Paints: Shedding Light on the Effect of Environmental Factors

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Since its introduction as an artist’s pigment in the earth 20th century, titanium white has become one of the most common white pigments. The early formulations of titanium white contained anatase, which has been studied to facilitate degradation of oil paint under UV illumination. Around the 1940s production shifted to a less photocatalytically active form, rutile. Although these pigments are not traditionally seen as active under visible light illumination, interactions within the paint mixture may result in increased photocatalytic activity. In this thesis, we present findings from a series of works painted by Jackson Pollock in the 1940s investigating the pigments used in white paints. Raman spectra from different locations on each of four paintings identify the phase of titanium dioxide present as well as some other pigments present. We further probed one painting, Eyes in the Heat, using reflection FT-IR (r-FT-IR) to interrogate the binder and other pigments in 4 distinct white paints. We identified signs of degradation in anatase and rutile containing white paint areas, suggesting TiO2 facilitated degradation has taken place. Further, we expand on the existing understanding of TiO2 facilitated degradation of linseed oil, by examining the effect of visible light and crystallographic phase (either anatase or rutile) on the reactivity of TiO2. We combine experimental chemical characterization via FT-IR with computational calculation through Density Functional Theory (DFT) modeling of the TiO2-oil system. Under visible and UV lightillumination, samples containing rutile or anatase (but not BaSO4 or pure linseed oil) based paint showed signs of degradation products when probed with FT-IR. Density Functional Theory (DFT) modeling of interactions between anatase TiO2 and oleic acid, a fatty acid component of linseed oil, showed the formation of a charge transfer complex, providing one possible mechanism for the visible light activity observed in artificial aging. Finally, we observed mass changes under UV aging at moderate (50 % RH) and high (80 % RH) humidity conditions through the use of the Quart Crystal Microbalance (QCM) as a high frequency rheometer. We observed a significant increase in mass loss in high humidity UV irradiated samples, and were able to measure a corresponding change in mechanical properties. These results suggest that both anatase and rutile based paints are more active under UV and visible light than originally thought due to complex interactions within the paint film.

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