Combination of heating and preservative impregnation of wood for outdoor exposure

Sammanfattning: The research is presented as a composite thesis containing a theoretical explanatory review based on four 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 in an efficient and economical way. The target applications for the studied methods were non-structural wooden materials for outdoor exposure, such as cladding, facades and decking. Therefore, weather resistance, appearance and durability were the qualities to be studied.The methods were based on the combination of heat and impregnation with liquid preservatives. The following methods were included in the study: heat treatment in steam, heat treatment in oil with subsequent oil impregnation of heat treated wood and impregnation with artificial chemicals (copper, furfuryl alcohol and phenol formaldehyde) enhanced by preheating the 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 behaviour 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 fourth article covers the study of impregnation of preheated wood with copper-based, furfuryl alcohol and phenol formaldehyde solutions; phenol formaldehyde treated wood was tested in the accelerated weathering test in order to find out if the treatment blocks wood resin coming out from the knots.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; similarly, impregnation with copper-based preservative after preheating was deeper compared to impregnation without preheating. 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 exposed to 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 could 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. It was found that the phenol formaldehyde treatment using hot-and-cold bath process significantly reduced the migration of resin and extractives through knot in painted wood panels.