Strength and colour response of solid wood to heat treatment

Abstract: Heat treatment is a method for improving the durability and dimensional stability of wood. The treatment method used in this work is called Thermo Wood and is industrially applied in Finland. In 2004 production was 31146 m3. The method involves heating wood in a steam atmosphere where the heat causes chemical changes in the structure of the wood. The present work was done in order to improve the quality of the treated timber both by predicting strength through colour measurement and by understanding the mechanisms behind the internal checking in heat-treated wood. Besides the published papers there are some smaller unpublished studies presented in the thesis. In paper I the connection between colour and the strength reduction caused by the heat treatment is studied. The results show that colour measurements are not a good way of predicting strength loss on an industrial scale. However, they also show that on an experimental level it is an effective way to study the changes that occur in the wood during treatment. There is a problem with internal checking when heat treatment is performed on boards with dimensions greater than about 50 mm. In paper II the connection between internal checking and both mass loss and drying was investigated. The results showed that both mass loss and drying influence the development of internal checking. Paper III focuses on the effects of drying on internal checking. From studying the results from papers II and III together with the results of some of the unpublished studies, the conclusion is that drying stress is the main cause for internal checking in heat-treated wood.