engleski

Analysis of moisture diffusivity at thermo-modified and innate beechwood

Heat treatment (HT) of wood is a promising technique for improvement of its dimensional stability, resistivity against biological and environmental factors as well as its durability. A heat treatment, besides positive impacts, also changes cellular structure of wood, its chemical characteristics, significantly reduces the wood density and most of mechanical properties [1, 2]. HT wood has a high potential for using it in outdoor applications, where can be exposed to continuously changing climatic conditions, causing heat transfer and moisture diffusion. The HT beechwood (Fagus sylvatica L.), used in the experiments, passed standard 1-day heating procedure at saturated steam conditions, consisted of pre-drying-, heating-, conditioning- and cooling phase. Clear, oriented wood specimens, (n = 7), 20 mm (width) by 100 mm (length), having various thicknesses (5, 10, 15 and 20 mm), were made afterwards from untreated- (C) and heat treated (HT) wood. Five side surfaces sealing of specimens using epoxy resin followed thereupon and conditioning at 20 °C and 40 °C having 33% relative humidity (RH). Diffusion experiments were made in adsorption, by exposing of specimens to 75% RH. The specimens were periodically weighted in short time interval, where the thickest material was also layered into 3 mm thick slices, for gravimetrical determination of moisture content (MC) profile. Diffusion coefficients were determined by traditional unsteady state method, using fractional change in average MC and a Newman equation for separating internal and external mass resistance [3]. Diffusivity was additionally analysed numerically by finite difference method, to asses the influence of moisture content. The heat treatment, according to the traditional unsteady state method, reduced moisture diffusion for 20% at low tested temperature (DC20 = 5.8×10- 11 m2 s-1 ; DHT20 = 4.7×10-11 m2 s-1). Mean moisture diffusion was for 2.6 times higher at 40 °C (DC40 = 1.56×10-10 m2 s-1 ; DHT40 = 1.25×10-10 m2 s-1). The diffusion ratio (DHT/DC ≈ 0.8) remained similar at both tested temperatures (Fig. 1). The finite difference method also confirmed the influence of heat treatment on moisture diffusion. It gave similar values of diffusion coefficients, for 25% higher at 20 °C (DC20 = 1.0×10-1o m2 s-1 ; DHT20 = 5.9×10-11 m2 s-1), and 10% lower at 40 °C (DC40 = 1.4×10-10 m2 s-1 ; DHT40 = 6.6×10-10 m2 s- 1), whereas variability of results was higher. The greater variation is ascribed to variability of wood properties, to uneven conditions during HT process, as well as to the achieved moisture content. The innate wood maintained higher MC at established climate conditions, and reached higher equilibrium moisture content (EMC) at initial and final state (Tab. 1). Additionally, increase of diffusion coefficient with the increase of moisture content was confirmed during adsorption process, more pronounced at innate beechwood specimens. Lower diffusivity of HT beechwood can be generally ascribed to lower hygroscopicity, most probable due to removal of hemicelluloses. In coincidence, the decrease of wood density presumably has positive, but less significant influence on moisture diffusivity of HT wood. We believe therefore, that physical changes of HT wood, like possible micro deformations of the cell wall structure, can not be neglected at mass transfer experimentations on this material.

Thermo-modified wood; moisture diffusion; surface emissivity; finite mass model; equilibrium moisture content

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