Aleš Straže

Influence of wood moisture content on the intensity of free radicals EPR signal

In this paper the influence of wood moisture content on the intensity of the free radicals electron paramagnetic resonance (EPR) signal in wood is described. Intensity of the free radicals EPR signal significantly correlates with wood moisture content.

Determination of the thermal conductivity of wood by the hot plate method: The influence of morphological properties of fir wood (Abies alba Mill.) to the contact thermal resistance

Abstract Thermal conductivity is frequently determined by the hot plate method. The influence of the anatomical and morphological properties of fir wood (Abies alba Mill.) was studied on the hot plate method and the mechanical treatment on the contact thermal resistance of the surface was addressed. It was confirmed that there is a contact thermal resistance between the surface of a specimen and the smoother metal plate of an experimental device. It amounted to 3.04×10-3 m2K W-1 in the direction of wood fibres and, similarly, 2.96×10-3 m2K W-1 in the transverse direction. A negligible contact thermal resistance – maximal 9% of the contact thermal resistance between the wood specimen and the metal plate – was determined at the contact of two wood surfaces. It is ascribed to the better fit of two wood surfaces compared to the fit of the wood surface with a smoother metal one. Irreversible morphological changes were determined in the real surface layers of mechanically treated test specimens. Abbot curves of real bearing profiles were additionally prepared, where the equality of longitudinal and transverse wood surfaces, as well as a sigmoid increase of bearing surface towards the inner wood tissue of test specimens was confirmed. The inverse proportional relation between the contact thermal resistance and the contact surface of wood was ascertained. The thermal conductivity of the combined and reintegrated composite of solid wood was invariable.

Sorption and Drying Characteristics of Xylite

The wood from which xylite is formed in a long-term process undergoes numous changes while still retaining the character of a hygroscopic porous material. To plan the optimal drying schedule of xylite, the permeability coefficient, diffusion coefficient, and surface emission coefficient were determined. The porosity of xylite is low due to the demolished capillary structure; concurrently with the blocked pits, the permeability of xylite is very low, with a permeability coefficient not higher than 2.12 × 10−10 m2/Pa s. The capillary flow of free water is insignificant, allowing the diffusion transport of water to prevail. The diffusion coefficient is considerably influenced by the xylites density. We confirm a significant increase in the bound water diffusion coefficient as the temperature increased (from 5.5 × 10−12 m2/s at 20°C to 46.3 × 10−12 m2/s at 40°C), and we predict that the activation energy will increase as the moisture content of xylite decreases (approx. 0.63 to 1.05%/% MC changes). As a consequence of the low surface resistance, the surface emission coefficient barely influences the moisture transport below the fiber saturation point.With an appropriate drying schedule, quality dried xylite can be used for various products as an equivalent substitute for dark-colored, high-density woods like ebony.

Evaluation of Material Characteristics of Xylite – Part 1. Influence of Moisture Content on Some Mechanical Properties

The structure of xylite as lithotype of lignite indicates that it predominately originates from tree stems, from the family Taxodiaceae or Cupressaceae. Despite drastic changes in the long term process of its transformation, it retains some relevant properties of wood, nowadays applicable for specific products. The research examined the influence of moisture content and temperature on modulus of elasticity (MOE) and modulus of rupture (MOR) as well as shear strength and hardness. With decreasing of moisture content, the bending strength and stiffness of xylite increased and was best fitted by power regression model. The increasing of testing temperature from 20 °C to 80 °C had negative effect on stiffness and bending strength of xylite. The hardness was positively correlated with density of xylite, whereas shear strength had the opposite tendency.

Influence of Initial Wood Quality and Drying Process on Utilization Grades of Sawn Spruce Timber

European spruce (Picea abies Karst.) belongs to the most easily dried wood species, but nevertheless cracking and warping often reduce the quality of dried sawnwood. Larger surface and end cracking was noticed at industrial drying practice in cold winter season, especially in using fast drying schedules. For the assessment of factors infl uencing the quality of dried wood some drying runs with varying drying conditions were carried out in an experimental kiln dryer. The quality of sawnwood was evaluated on green material using standard procedures and compared with the quality at the end of drying processes. Drying of spruce sawnwood at sharper drying conditions was more risky, resulting in a larger number of cracks as well as larger fi nal MC distribution, larger MC gradients and casehardening. Additionally, signifi cant correlation was confi rmed between the quality of dried wood and input quality of fresh material. More downgrading after the drying was observed in case of initially low graded material.

Lastnosti bukovine in njena raba / Properties of beechwood and its use

We present the structure as well as biological, physical and mechanical properties of European beech (Fagus sylvatica L.) wood in comparison with that of Norway spruce and selected hardwoods. Fine grain, high homogeneity, low anisotropy, medium to high density, favourable mechanical properties, machining, bonding, and finishing, as well as suitability for bending, are among the main advantages which enable wide use of beechwood. Large amounts of beechwood are currently used as fuel. However, for its optimal use, we should revitalize and increase the production of traditional and new semi-finished products like sawn timber, structural and decorative veneer, composite boards and products of chemical processing. In contrast, for energy purposes we should mainly use the residues from these wood-working procedures. According to the numerous favourable properties of beechwood and fairly good possibilities of increasing its durability, we should focus on the development of innovative products to increase its use for construction purposes in buildings.

Vpliv sušilnega postopka na kakovost in izkoristek bukovega žaganega lesa / Impact of the drying process on the quality and utilization rate of sawn beechwood

We estimated the quality of beech (Fagus sylvatica) sawn timber before and after the industrial normal-temperature convection kiln drying and after air drying. We analysed 42 mm thick and 2.6 m to 5 m long boards, where we measured the number and size of end- and surface cracks, and fissures close to the knots. The orientation and direction of the wood grain, the number and size of knots as well as occurrence of red-heart and twisting were also determined. We ranked the boards using these criteria before and after the end of the drying processes in accordance with the standard of the European Organization of the Sawmill industry (EOS). We found a significant effect of drying on the occurrence of drying defects, but differences between the two procedures were not confirmed. The increase in the number and size of the surface and end cracks, especially in the boards assessed to lower B- and C-classes was obvious. The latter mainly contributed to the lowering of the quality of the sawn timber after drying from the A- to B- or lower class. After the kiln and air drying 20 % and 33 % of the sawn timber quality was degraded, respectively. The presented research was part of a larger study where we followed the quality of the beech timber from the forest to the final product.

Fizikalne in mehanske lastnosti svežega in osušenega lesa v bukovih deblih, izruvanih med žledolomom

V raziskavi smo preucevali fizikalne in mehanske lastnosti lesa v deblih navadnih bukev (Fagus sylvatica L.), izruvanih med žledolomom. Lastnosti smo preucevali na vzporednih preizkusancih sveže in osusene bukovine, ki smo jih sistematicno vzorcili po radialni in vertikalni smeri dreves. Dolocili smo gostoto, togost, upogibno trdnost, strižno trdnost v smeri lesnih vlaken ter tlacno trdnost lesa v vseh anatomskih smereh. Togost smo primerjalno vrednotili med staticno 4-tockovnim upogibom in nedestruktivno z merjenjem hitrosti ultrazvoka ter z dolocanjem frekvencnega odziva. V drevesih, ki so izruvana obležala v gozdu od februarja do konca vegetacijske dobe 2014 in so imela delno ohranjen koreninski sistem, aktivno krosnjo in zadosten kapilarni tok, nismo zaznali sprememb gostote in mehanskih lastnosti, ki bi bile posledica razkroja lesa. Tako v svežem kot osusenem stanju smo ugotovili znacilni vzorec porazdelitve gostote in trdnosti. Gostota lesa je po visini dreves narascala, v radialni smeri od stržena proti periferiji debla pa smo zaznali rahel trend zmanjsevanja. Mehanske lastnosti z izjemo precne tlacne trdnosti se povecujejo tako po visini debla kot tudi od stržena proti kambiju in so bile vedno boljse pri osusenem kot pri svežem lesu. Zanesljivost nedestruktivnih metod za dolocanje togosti je vecja pri ocenjevanju osusenega lesa. Za oceno upogibne trdnosti z nedestruktivnimi metodami se vedno priporocamo vzporedno vizualno opredelitev znacilnosti lesa.

Structural and acoustic properties of African padouk (Pterocarpus soyauxii) wood for xylophones

The possibility of supplementing the subjective traditional assessment of wood quality for percussion instruments with reliable acoustic measurements was studied. Sawn boards of African padouk (Pterocarpus soyauxii Taub.) were selected in a timber yard and classified into five grades by using the dynamic combined specific modulus of elasticity. Then they were sawn into raw xylophone bars, which were afterwards traditionally graded by listening to the sound damping at flexural bending excitation and additionally by acoustic measurements. A high quality grade matching of xylophone bars and sawn boards was confirmed. Highly graded material was mostly quarter sawn with straight or moderately interlocked grain. The sound quality of raw xylophone bars was not influenced by the wood density. It negatively correlated with sound damping (tan δ) and positively with the specific modulus of elasticity and with absolute and relative acoustic conversion efficiency. The audial grading of xylophone bars could be improved or substituted by relative acoustic conversion efficiency.