Spectroscopic studies of surface chemical composition and wettability of modified wood

Recent advances in spectroscopic methods used in the surface science field may provide new valuable information about the surface chemical composition of engineering materials. Such methods, combined with wettability analyses, have been applied in the development of well-designed adhesives and coating systems for newly developed and commercially available modified wood materials. The main objective of this paper is to demonstrate and present some aspects on the application of two different state-of-the-art spectroscopic methods for surface chemical composition studies of a complex material such as modified wood. The methods are X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), the former generating more quantitative data and the latter more qualitative data. The spectroscopic data are also combined with wettability data obtained from contact angle measurements using the Wilhelmy method. Modified wood samples were prepared from pilot plant or commercially produced acetylated, furfurylated and thermally modified wood. Effects of wood surface ageing, i.e. the time after machining, on the surface chemical composition and wettability were also studied. Results clearly indicate a hydrophobization process due to ageing of the unmodified and certain modified wood, probably mainly related to a migration and reformation of extractives in the surface. The surface composition and wettability of acetylated wood was not appreciably affected by the ageing process. Such findings could be quantified by the XPS measurements, which is further discussed and related to the different wood modification routes. ToF-SIMS is a powerful tool and complementary to XPS for identification of, for example, specific hydrophobic substances in the wood surfaces. In addition, this method provides ion images, mapping the lateral distribution of selected secondary ions signals within an analysed wood surface area.