Heat treatment of solid wood : effects on absorption, strength and colour

Wood is a useful construction material, but it has less desirable properties such as poor durability and poor dimensional stability. These problems can be reduced by using various chemical treatments. Increased environmental awareness has raised the demand for more environmentally friendly methods. Heat treatment is an alternative method for improving these properties with no use of chemical additives. When wood is heated, chemical changes start to take place inside the wood structure. These changes result in increased durability and dimensional stability. In this thesis, a method developed in Finland called ThermoWood has been used in which wood is heated in an atmosphere of superheated steam that serves as a shielding gas. Various property changes due to heat treatment have been studied in this thesis: Colour and strength (Paper I), internal checking (Papers II & III) and capillary absorption (Papers IV-VII). Some smaller unpublished studies are also presented. Paper I focuses on colour and strength response of birch to heat treatment. A method for measuring colour heterogeneity was devised. The results show that there is measurable colour heterogeneity present in birch after heat treatment. Paper I also examines the possibility of using colour for predicting mechanical strength in heat-treated birch, but it is concluded that it is not a suitable method. Impact testing did not give any clear results in strength loss, but it showed significant changes in mechanical failure mechanisms; it showed that heat treatment increased the mechanical failures between fibres in birch. When heat-treating boards thicker than 50 mm, there is a risk of internal checking, especially in spruce. The results show that internal checking in heat-treated spruce is caused by both thermal degradation and drying, but drying has the greatest influence on crack formation. One of the main reasons for heat-treating wood is to reduce the hydroscopic properties. Heat-treated wood has lower equilibrium moisture content and lower wettability as demonstrated by contact angle measurement. Increased water uptake has been discovered when Scots pine treated at around 170°C is subjected to free water in such way that capillary absorption is possible. The results in Paper VII indicate that extractives are a contributing factor to this increased absorption. The results also show that extraction by water soaking causes capillary water absorption in Scots pine to decrease in samples dried at 60°C and cause an increase in samples treated at 200°C. After extraction by water soaking, the reducing effect of heat treatment on capillary water absorption had disappeared.Godkänd; 2008; 20081106 (ysko)