Distribution and penetration of tung oil in wood studied by magnetic resonance microscopy

Water repellents, as environmentally-friendly treatments, are gaining popularity as non-biocidal solutions for wood protection. Drying oils, in addition to waxes and organosilicon compounds, are one of the most important water repellents for wood. Tung oil has so far been proven to be one of the best performing oils for wood protection. However, tung oil, similarly as other oils, does not penetrate deeply into wood, due to its high viscosity. In order to improve the penetration of oil into wood, a vacuum-pressure procedure has to be applied. The species used in this study are important in Central Europe: sweet chestnut heartwood (Castanea sativa), European larch heartwood (Larix decidua), Scots pine heartwood and sapwood (Pinus sylvestris) and Norway spruce (Picea abies). Oil uptake depends on the applied impregnation method and on the wood species used. Retention of tung oil was higher after an impregnation process than with the immersion procedure. Magnetic resonance imaging (MRI) was applied to elucidate the influence of wood species on oil penetration and distribution in wood after treatment. High spatial resolution MR imaging is highly sensitive to changes of liquids in wood and is therefore also very appropriate for monitoring oil penetration. Furthermore, with a good signal to noise ratio of MR images, the method can also discern among specimens with different patterns of oil distribution, as well as between areas of early-wood and late-wood.