Inorganic UV absorbers for the photostabilisation of wood-clearcoating systems: Comparison with organic UV absorbers

Inorganic UVabsorbers which are widely used today were originally designed neither as a UV blocker in coatings applications, nor for wood protection. In recent years however, there has been extensive interest in these compounds, especially with regard to their properties as a UV blocker in coating applications. In this work, we carried out a comparative study to look into some inorganic and organic UVabsorbers used in wood coating applications. The aim of this study is to determine the photostabilisation performances of each type of UVabsorbers, to seek possible synergies and the influences of different wood species. We have also searched to find eventual correlation between these performances and the influence of UVabsorbers on the film properties. Our study has compared the performances of the following UV absorbers: hombitec RM 300, hombitec RM 400 from the Sachtleben Company; transparent yellow and red iron oxides from Sayerlack as inorganic UVabsorbers; organic UVabsorbers Tinuvin 1130 and Tinuvin 5151 from Ciba Company. The study was carried out on three wood species: Abies grandis, tauari and European oak. The environmental constraints (in particular the limitation of the emission of volatile organic compounds VOCs) directed our choice towards aqueous formulations marketed by the Sayerlack Arch Coatings Company. The results obtained after 800 h of dry ageing showed that the Tinuvins and the hombitecs present better wood photostabilisations. On the other hand in wet ageing, with the hombitec, there are appearances of some cracks and an increase in the roughness of the surface. This phenomenon is absent when the Tinuvins are used. With regard to these results, the thermomechanical analyses relating to the follow-up of the change of the glass transition temperature (Tg) of the various coating systems, show a different behaviour between the two types of absorbers. However, contrary to organic UVabsorbers, inorganic ones tend to increase Tg during ageing, and, consequently decrease the flexibility of films. These phenomena can lead to the appearance of cracks found in the case of the wet weathering.