ABSTRACT
As previously mentioned, almost all the techniques used for wood densification are based on two distinct characteristics of wood, viz: the influence of the moisture content on the glass transition temperature (Tg) of the wood and the poor thermal conductivity of the wood. experiments have shown that, during wood-surface processing, the large deformation (compression) produced is not permanent, since the densified cells open widely at high humidity and high temperature. As described with respect to uniform densification in Chapter 6, the fixation of the compression-set can be achieved mainly by eliminating the stored elastic strain energy through thermo-Hydro-Mechanical post-treatments, where the wood is heated in saturated steam to a temperature of 200 °C (in a closed system) for about 4 minutes, or to a temperature of 160 °C or 180 °C for about 60 or 20 minutes, respectively. the consequences of these treatments are a rather limited degradation of the wood and its mechanical properties. the investigation by rautkari and Hughes (2009) has shown that, to totally fix the compression-set in the densified surface, it was necessary to heat wood in steam to 200 °C for more than 2 hours. under such conditions, densified wood will obviously degrade significantly and lose a great amount of its mechanical strength. this phenomenon has been clearly shown by Gong et al. (2010), who heat-treated undensified wood as well as densified wood at three temperatures: 190 °C, 200 °C and 210 °C. they report that the hardness, Moe, nail withdrawal resistance and wearing resistance of undensified aspen after heat treatment were higher than the values for heat-treated densified aspen. the heat treatment processing time was not reported, but it is evident from the negative impact value on Moe (reducing the values for densified and undensified wood by 24% and 32%, respectively, as compared to that for non-heat-treated densified wood) that this time should be several hours. it is important to note that the level of heating the processing time and moisture content of the element during post-treatment have an important influence on the wood degradation during post-heat-treatment. if the aim of surface densification is to increase the wood surface wear and hardness by densification, then the conditions of post-treatment should be such that it does not greatly reduce the compressed wood properties.
