Investigating the effect of artists’ paint formulation on degradation rates of TiO2-based oil paints

Abstract

This study reports on the effect of artists' paint formulation on degradation rates of TiO 2 -based oil paints. Titanium white oil paint exists in a multitude of different recipes, and the effect of the formulation on photocatalytic binder degradation kinetics is unknown. These formulations contain, among others, one or both titanium dioxide polymorphs, zinc oxide, the extenders barium sulfate or calcium carbonate and various additives. Most research performed on the photocatalytic degradation process focusses on pure titanium white-binder mixtures and thus does not take into account the complete paint system. Since photocatalytic oil degradation is a process initiated by the absorption of UV light, any ingredient or combination of ingredients influencing the light scattering and absorption properties of the paint films may affect the degradation rate. In this study three sets of experiments are conducted, designed using the design of experiments (DoE) approach, to screen for the most important formulation factors influencing the degradation rate. The benefits of using DoE, compared to a more traditional 'one factor at a time approach' are robustness, sample efficiency, the ability of evaluate mixtures of multiple components as well as the ability to evaluate factor interactions. The three sets of experiments investigate (1) the influence of the TiO 2 type, (2) the impact of different mixtures of two types of TiO 2 , ZnO and the additive aluminum stearate and (3) the influence of common extenders in combination with photocatalytic TiO 2 , on the photocatalytic degradation of the oil binder. The impact of the formulation on the degradation rate became apparent, indicating the shortcoming of oversimplified studies. The protective effect of photostable TiO 2 pigments, even in a mixture with photocatalytic TiO 2 pigments, as well as the negative effect of extenders was demonstrated. Furthermore, the ambiguous role of ZnO (photocatalytic or not) and aluminum stearate is highlighted. Neither can be ignored in a study of degradation behavior of modern oil paints and require further investigation.

Determining The Presence of Photocatalytic Titanium White Pigments via Embedded Paint Sample Staining: A Proof of Principle

“…All samples reach a correct 'yes' or 'no' classification based on prior testing of the pigment's photocatalytic activities and aging behavior, under the criterion color change = photocatalytic material present. However, two samples do not exactly follow the expected behavior (van Driel et al 2018b). The one sample which was based solely on ZnO (R8) shows much less photocatalytic discoloration than expected, based on previous artificial aging studies under UV, where it showed similar oil degradation rates to uncoated anatase.…”

“…d Sample from the JPI-CH CMOP project, from a case study painting supplied by the Stedelijk Museum (Steyn forthcoming). e Sample acquired (from painting at the Bouwfonds Kunstcollectie) during the Ph.D. project of the author in the scope of 'The white of the 20th century -An explorative survey into Dutch modern art collections' (van Driel et al 2018b). f Sample was part of the investigation performed by the Cultural heritage agency of the Netherlands and the Gemeentemuseum in The Hague (van den Berg et al 2012).…”

“…The reference samples were selected, based on prior research (van Driel et al 2018a(van Driel et al , 2018b, to represent a range of extreme properties (photocatalytic versus photostable and inert) and common compositions found in modern oil paints (primarily those based on TiO 2 , in a range of pigment volume concentrations, with some also containing BaSO 4 , ZnO, and aluminium stearate). A tube paint was added to the selection, chosen because of its similarity in composition to one of the tested reference mixtures, and because it is a commercially available artist's paint.…”

“…The reference pigments had been tested for photocatalytic activity by a dye degradation test and by artificial aging under UV in previous studies (van Driel et al 2016(van Driel et al , 2018b. The Talens tube paint (R5) and the canvas board were analysed by X-ray diffraction (XRD).…”

“…The aim to develop a test usable on samples from objects was initially envisioned to be an adaptation of such a dye system. However, its development was considered redundant in light of the work of Mills and co-workers on an indicator ink available as a commercial product (Mills et al 2005(Mills et al , 2013(Mills et al , 2014a(Mills et al , 2014bMills and McGrady 2008;Mills and Wells 2015;Baudys, Krýsa, and Mills 2017;van Driel et al 2018b). This product, developed over the past decade, is called a photocatalytic activity indicator ink (PAII) and was developed and validated to determine photocatalytic activity of TiO 2 -containing self-cleaning tiles and windows.…”