Effetti del trattamento termico sulla durabilità del legno.

The use of woody material is becoming very common, but it is often limited by certain characteristics or the presence of certain defects. One of the features that can limit the use of wood is its low level of durability, namely the inherent ability to resist attacks by degradator organisms. To address this problem the low UNI EN 460 and 599 have been provided different treatments that could increase the degree of durability. Among the various treatments there is the thermal treatement, which provides that the material be subjected to temperatures between 160 ° C and 260 ° C in special rooms where the atmosphere can be controlled, to prevent combustion, for obtain a series of chemical-physical alterations. In reality there are several different types of heat treatment between them to temperature, time and intensity, but which lead to similar results. The most important alterations due to the heat treatment are that the chimica chage, the appearance of fractures in cell walls, the reduction in the mass, the reduction of the mechanical strength, the reduction in moisture contained, the increasing of dimensional stability, the increasing of durability, the reduction of wettability and the increase of darkness. In the particular the increase of durability, is due to the reduction of hygroscopicity that makes the environment most hostile for the organisms, the alteration of lignin and hemicellulose which make the material less attractive to xylophagous, and the production of substances that may have a biocidal effect. These substantial changes gives at the thermo-treated wood an higher resistance to atmospheric agents and to wood-eating organisms, making it more suitable for use outdoors, but its low mechanical resistance means that it can only be used for non-structural function. In reality the degree to which these changes occur depends on the type of heat treatment, and consequently before putting the material on the market, it should occur garanted that have the characteristics required by users. In the case of durability, the only direct measurement method consists in putting the material in contact with the biodegradator agents, so it can not therefore be applied in the industrial field. For this reason it was thought to use the analysis of other alterations to be able to predict the degree of durability conferred, since very often between them there is a close correlation. The analysis of chemical type on the carbon content and oxygen, and the analysis of the mass loss can be infact exploit, since a direct proportionality was observed between the latter and the durability conferred. A similar relationship was observed with the modulus of rupture, so for the mesurement of durability it can be used the method BING®, which analyzes the behavior of the wood to the longitudinal and transverse vibrations. Another method used to predict the durability is the CIELAB which uses the visible spectrum for identify the color of the wood, whoose the sheen presents a direct proportionality with the conferred durability. Finally, we can exploit the analysis with Electron Spin Resonance, which measures the absorption of the waves when the material is inserted in a magnetic field, since the relative intensity of the ESR and durabilità are in reverse proportionality. It was also observed a direct relationship with the kinetics of the temperatures, and in particular it whit the relative.