Surface modification of solid wood using different techniques

The rising demand for consistent, high quality, as well as high density wood species has raised interest in different wood modification techniques. One modification method is wood densification, where solid bulk wood can be compressed, following softening, until its density reaches that of the cell wall (1.50 g/cm3). It is well known that many of the properties of solid wood correlate with its density and as such can be enhanced by increasing the density. By targeting the compression to the surface of solid wood only, the surface properties can be enhanced. The main aim of this thesis was to evaluate the effect of different wood surface densification methods on surface properties. Three methods were evaluated; (i) a lamination technique, where low density wood was laminated to a high density compressed wood surface, (ii) surface modification where densification was accomplished using a single sided heated press and (iii) with a single sided heated press assisted by frictional heating. With the lamination technique, it proved easy to generate the desired hard surface, but an adhesive is needed. The difference between compression with and without friction is that when using friction the process temperature is rather difficult to control, but an even layer of extractives are formed on the surface. Without friction, however, extractive spots are seen on the surface. The results show that the process parameters used in surface densification have a significant influence on the vertical density profiles, which in turn have an influence on surface hardness. On the other hand hardness correlates with the degree of densification and the thickness of the densified surface. Moreover, wettability decreased significantly, caused by the closure of lumens, a smooth surface and an extractive layer on the surface. The extractives on the surface were analysed by FTIR-ATR spectroscopy and X-ray photoelectron spectroscopy. Using FTIR-ATR spectroscopy there was no trace of extractives, but with X-ray photoelectron spectroscopy an extractive layer was identified, this was most probably due to the penetration depth of the IR-beam, which is rather high, compared to X-ray photoelectron spectroscopy, which is highly surface sensitive. The extractive layer could provide a natural coating and reduce the need for further coating. Furthermore, a potential measure error was found when measuring the vertical density profile of composites with large differences in densities, caused by the calibration process.