Strenght and Colour Response of Solid Wood to Heat Treatment

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 m. 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 Besides the published papers there are some smaller unpublished studies presented in the thesis.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 From studying the results from papers II and III together with the results of some of the unpublished studies, the conclusion is that drying stress is the main cause for internal checking in heat-treated wood. Preface As a child I wanted to become a scientist or a rich businessman. Then, when I was fourteen, I realised that I really enjoyed going to school and learning new things all the time. So, one day I came home and told my mother that I was going to remain a student my whole life. Now sixteen years later I am a doctoral student, which means I can both do research and be a student. I haven’t become rich, but I am getting paid while I am fulfilling the other two dreams, which must be considered a success. Therefore, I would like to thank Vinnova and the industrial consortium (Valutec, Stora Enso, Södra, Nordiskt Rostat Trä, Norra Skogsägarna, Martinsons Trä and IUC Trä) that have made this possible by financing my project. The work was carried out at Luleå University of Technology, Division of Wood Science and Technology. I would like to thank my supervisor Professor Tom Morén for supporting me and trusting me enough to let me take the initiative and make my own decisions. I would like to give special thanks to everybody who has contributed to my work: Margot Sehlstedt, Joakim Wänstedt, Stefan Huszics, Aleksandra Igumnova and Yu Chen. Also thanks to Brian Reedy for reviewing the language in this thesis. I would also like to take the opportunity to give a big thanks to Margit Sandström, Monica Tjerngren and Eva-Stina Nordlund for all their help with the administrative part of this work. Finally, huge thanks to my beloved wife Anna-Lena and my daughters Tyra and Line for making it all worthwhile. Skellefteå, November 2005