Wood heat treatment is an attractive alternative to improve decay resistance of low natural durability wood species. Nowadays, several types of thermal treatments of wood exist. These treatments differ mainly by the nature of the inert atmosphere used to avoid wood combustion which may correspond to: nitrogen, vacuum, steam or oil. Decay resistances as well as mechanical properties are strongly correlated to thermal degradation of wood cells wall components and consequently of treatment conditions from which depend chemical modifications. The aim of this study is to determine the effect of a new generation of treatment performed under vacuum on the chemical modifications occurring during treatment. For this purpose, beech wood has been treated under different inert conditions: vacuum and nitrogen. All treatments were performed at 220 °C for mass losses resulting from wood thermodegradation of approximately 12%. For each treatment condition concerning treated and untreated beech wood, extractives, Klason lignin, hemicelluloses and -celluloses content were determined as well as monosaccharide composition. Results show that extracts content were lower in the case of wood samples treated under vacuum, while lignin, hemicelluloses and a-celluloses contents were higher in the case of samples treated under steam and nitrogen, indicating lower wood degradation under vacuum treatment. Finally, effect of inert atmosphere on wood mechanical properties was evaluated. Results show that even if mechanical properties decreased after thermal treatment MOE, MOR and Brinell hardness were less affected under vacuum treatment comparatively to treatment performed under nitrogen.

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