Attempts to enhance wood durability by modification with heat and vegetable oils

Abstract: The research is presented as a composite thesis containing a theoretical explanatory review based on three scientific articles. The thesis covers the investigation of wood modification methods, which in the future could possibly be developed further to enhance the performance of wood products without harming the environment. The target applications for the studied methods were non-structural wooden materials for outdoor exposure, such as cladding, facades and decking. Therefore, proper durability and dimensional stability under outdoor conditions and resistance to decay were the most important qualities to be improved by the application of the methods. The methods included heat treatment in steam, heat treatment in oil and oil impregnation of heat treated wood. All the treatments were performed mainly on Scots pine, but also on Norwegian spruce and European varieties of ash, aspen, birch and beech. The first article covers the investigation of methods for oil heat treatment and oil impregnation of wood. The oil treatments were designed and performed at laboratory scale. The second and third articles cover studies concerning the behavior of steam heat treated, oil heat treated and oil impregnated wood boards in terms of their resistance to cyclical climate conditions and brown rot fungi. The climate conditions were recreated in laboratory by letting the samples to soak in water, freeze in a freezer and absorb heat from an infrared lamp. The results showed that degradation processes within wood during oil heat treatment were similar to the degradation processes during heat treatment in steam, but with the difference of a small oil uptake when heat treating in oil. When heated wood is directly placed to cool down in oil at room temperature, then it absorbs significantly more oil than during the process of heat treatment in oil. However, oil seems to be rather unstable inside wood. After the samples were impregnated with oil in the experiments, the oil migrated throughout the wood and leached out when exposedto water and heat. By visual observation it was found that colour degraded to a certain extent in all untreated and treated species exposed to the cyclical weather conditions used in the study. It can also be noticed, that the method used to create the cyclical weather conditions (mentioned above), in which the wood was exposed to periods of soaking, freezing and warming, showed to be an effective method to test the wood for outdoor applications. It was observed that both heat treatment and oil impregnation improved durability of wood against brown rot fungi. However, the oil impregnation method needs, probably, to be further developed to protect wood in the long term due to oil leakage from the wood.