Comparison of mechanical properties of heat treated beech wood cured under nitrogen or vacuum

摘要

Heat treated wood has been subjected to increasing interest during the last decade. This non biocidal treatment is an attractive alternative with a low environmental impact to improve decay resistance of low natural durability wood species. Nowadays, several types of heat treatment processes exist. These treatments differ mainly by the nature of the inert atmosphere used to avoid combustion of wood: nitrogen, steam pressure, oil or more recently vacuum. We have shown in a previous study that utilization of vacuum to perform thermal treatment instead of nitrogen allows to reduce considerably degradation of wood polysaccharrides. Indeed, it appears that for a similar 12％ mass loss generated by thermal degradation, thermodegradation performed under vacuum allowed to reduce degradation of sugar constitutive of hemicelluloses and formation of recondensation products within the wood structure. These results may be explain by the effect of vacuum allowing removal of volatile degradation products like organic acids, aldehydes and furans limiting therefore acidic degradation of polysaccharides and recondensation of volatile by-products. Decay durability tests, performed against different brown and white rots fungi, have shown no significant differences for vacuum and nitrogen heat treated samples, all presenting an improved decay resistance. According to these results, the aim of this short paper is to report the effect of vacuum versus nitrogen on the mechanical properties of heat treated samples making the assumption that lower degradation of wood polysaccharides during treatment performed under vacuum may reduce the weakening of mechanical properties. For this purpose, modulus of rupture (MOR) and modulus of elasticity (MOE) in bending and Brinell hardness (HB) were determined. Results show that the mechanical properties were lower degraded in the case of wood samples treated under vacuum.