How intraspecific radial variability of the European Oak’s may influence mild pyrolysis process and durability of the material

IRG/WP 16-40725

J Hamada, A Pétrissans, F Mothe, M Pétrissans, P Gérardin

Last decades wood is promoted as building material. Unprotected wood exposed to outdoor conditions undergoes a variety of degradation induced essentially by fungi attacks. Heat treatment of wood by mild pyrolysis (180°C T 240°C under inert atmosphere) is a preservation process with a weak environmental impact, and therefore, is viewed as an interesting alternative to the chemical impregnation methods. It is generally well recognized that final properties of thermally modified wood like decay resistance, dimensional stability, mechanical properties or color depend on wood species but also and heat treatment process and treatment conditions like time and temperature. However, in spite of different studies describing the effect of inter specific variability on wood thermal degradation, no study describes the effect of intraspecific variability of wood on thermo modification processes. As wood physical properties as well as chemical composition can vary between and within species, between stands and even within tree, we tested the effect of radial position of European oak wood (Quercus petraea Liebl.) on its thermal stability. Samples of heartwood, sapwood, juvenile wood, earlywood or latewood taken from the radii of 2 trees were ground to fine sawdust subjected to thermogravimetric analysis (TGA) at 220°C for 2 hours using the same heating program. At the same time, holocellulose, cellulose and extractives as well as extracted heartwood and sapwood were also subjected to TGA using the same procedure. Results indicated that heartwood was more sensitive to heat than sapwood, the inner side of heartwood being more sensitive than the outer side. Differences were also noticed between native and extracted wood, the latter ones being less sensitive to thermal degradation. These results were consistent with the stability of each wood cell wall component indicating that extractives were more susceptible to thermal degradation than holocellulose, holocellulose being more susceptible than cellulose. At the ring level, earlywood was shown to be slightly more sensitive to thermal degradation than latewood. It was therefore concluded that since wood radial position and the earlywood/latewood ratio determine the thermal stability of oak wood, the quality and the homogeneity of initial wood boards should be carefully controlled to avoid heterogeneity in the treatment leading to end products with different properties and quality.

Keywords: density, chemical composition, earlywood, heartwood, heat treatment, latewood, oak, sapwood, thermogravimetric analysis, variability, wood

Conference: 16-05-15/19 Lisbon, Portugal

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