Wood Thermal Properties

Wood thermal properties specify the answers to the questions related to heat transfer. The values of specific heat, thermal conductivity, and thermal diffusivity were simultaneously determined with quasistationary method. Wood is distinguished as a natural material for accumulating the energy by heat transfer, as isolating material, with the ability to slowly equilibrate its different temperatures. The measured thermal properties value of beech and fir wood samples support those conclusions. Known dependences of wood thermal properties on anatomical direction, density at given moisture content, temperature are modelled and incorporated into heat conduction equation to provide base for next evaluation of measured data. The heat conduction solutions, based on known wood thermal properties, are used in similar problems. It is shown that thermal properties influence the surface equilibrium temperature between skin and wooden sample and the solution of heat conduction equation describes the flux passing through the log as an element of log-cabin house. Also thermal diffusivity is a component of equation that determines the position of observed point of wood during conduction. The results served as a base point for planning the experiments, for designing the processes of heat transfer, for designing the furniture and wooden houses, for designing the machines and equipment in woodworking industry and others

Promjene konačnog sadržaja vezane vode u određenom volumenu drva bukve

The undesired wood instabilities are connected with the changes of bound water moisture content. The rates of finite changes of bound water moisture content in a given volume of wood were determined in the frame of the specimen dimensions. The derivation is based on the 1st Fick’s law and diffusion equation solution in three dimensions. The inverse solution of diffusion equation provided the diffusion coefficients in the principal anatomical directions. Beech wood was tested. The nonlinear regression is involved in the inverse solution. Therefore, the starting values of diffusion coefficients were computed according to the proposed three dimensional square root scheme. The evaluation of data revealed the random character of transport characteristics. The sufficient condition for the slowest finite change of ound water moisture content in a given volume of beech wood is the ratio 8.7:1.5:1.0 of dimensions in longitudinal, radial and tangential directions.Nepoželjna pojava nestabilnosti drva povezana je s promjenom sadržaja vode vezane u drvu. Promjene konačnog sadržaja vode vezane u određenom volumenu drva određene su u ovisno o dimenzijama uzorka. Derivacija se temelji na 1. Fickovu zakonu i rješenju jednadžbe difuzije u tri dimenzije. Inverzna rješenja jednadžbe difuzije daju koeficijente difuzije u glavnim anatomskim smjerovima. Test je proveden na uzorcima bukovine. Nelinearna je regresija uključena u inverzna rješenja, stoga su početne vrijednosti koeficijenata difuzije izračunane prema predloženoj shemi trodimenzionalnoga kvadratnog korijena. Ocjena rezultata otkrila je svojstvo slučajnosti kretanja vode. Dovoljan uvjet za najsporiju promjenu konačnog sadržaja vode vezane u određenom volumenu bukovine omjer je dimenzija 8,7 : 1,5 : 1,0 u uzdužnome, radijalnome i tangencijalnom smjeru

Some Physical and Mechanical Properties of Recent and Heat Treated Ash– Fraxinus Excelsior L.

Obrada jasena (Fraxinus excelsiorL.) na temperaturi od 200 oC uzrokovala je značajne promjene tehničkih svojstava odnosno fizikalnih i mehaničkih svojstava, tako obrađenog drva. Gustoća jasenovine u apsolutno suhom stanju signifikantno je manja od gustoće recentne. Ista je pojava uočena i kod utezanja u radijalnom i tangencijalnom smjeru i također volumnog utezanja. Istraživana mehanička svojstava: čvrstoće na tlak u longitudinalnom smjeru, statič ke čvrstoće na savijanje, tvrdoće u longitudinalnom, radijalnom i tangencijalnom smjeru toplinski obrađene jasenovine, također su signifikantno manja od recentne. Obrada jasenovine na visokoj temperaturi polučila je drvo veće dimenzionalne stabilnosti, odnosno manjih veličina linearnih i volumnog utezanja, ali manjih mehaničkih svojstava jasenovine.The aim of this paper is to study differences in some physical and mechanical properties of heat treated and recent wood. Wood used in this research was ash (Fraxinus excelsiorL.) from the area of Otok, region of Slavonia in Croatia. The felled tree was 85 years old with the diameter at breast height of 53 centimetres. A heart board was made from the tree trunk in the north-south direction, 4 metre long and 6 centimetres thick. The heart board was dried naturally with an average water content of 12 %. The heart board was sawn in four equal parts so that four heart boards were obtained – each 1 metre long. Looking from stump to top of the tree, the first and third heart boards were not heat treated and they were used for establishing the properties of recent ash. From the above four heart boards, samples were made for determining macroscopic, physical and mechanical properties of recent and heat treated ash. Treatment of ash (Fraxinus excelsiorL.) at the temperature of 200 oC caused significant changes of technical properties relatively physical and mechanical properties of the treated wood. Maximum possible number of samples was made for each physical and mechanical property in radial direction, i.e. from heart to bark. Maximum shrinkage was determined in radial and tangential direction, as well as maximum volume shrinkage, density in absolutely dry condition, nominal density and water content after a fortnight of soaking in water of physical properties. Determination was also made of compression strength parallel to the grain, static bending strength, hardness in longitudinal, radial and tangential direction. The investigation of some physical and mechanical properties of recent ash (Fraxinus excelsiorL.) and heat treated ash at 200 oC show diametrically different values of investigated properties. The investigation of mechanical properties of recent ash was carried out on samples with water content of 12 %, while the water content of the heat treated ash samples was 4 %. In order to be able to compare the results of mechanical properties, their values were converted to the water content of 4 % for recent ash and 12 % for heat treated ash. For increasing or decreasing mechanical properties of wood, correction factor was used in accordance with Forest Product Laboratory, Madison. The comparison of mean values of physical and mechanical properties of recent and heat treated ash was carried out in accordance with Mann-Whitney test with the help of statistical software Statistica. Density distribution in absolutely dry condition in radial direction of heat treated ash deviates from the distribution common for recent ash. The loss of mass with a relatively constant sample volume is evident in radial direction, and especially in the heart section. Mean value of density in absolutely dry condition of recent ash is higher by 8.4 % than density of heat treated ash. Shrinkage in radial and tangential direction and volume shrinkage of recent ash are higher that shrinkage in heat treated ash. Shrinkage in radial direction is higher by 70 %, in tangential direction by 62.2 % and volume shrinkage by 63.7 %. All above said physical properties of recent ash are significantly different from physical properties of heat treated ash. The investigated mechanical properties of recent ash are also significantly higher than mechanical properties of heat treated ash. Compression strength in longitudinal direction of recent ash is higher by 36.8 %, static bending strength is higher by 48 %, hardness in longitudinal direction (cross-cut) is higher by 31.3 %, hardness in radial direction (tangential cut) is higher by 22.8 % and hardness in tangential direction (radial cut) is higher by 25.9 % than the stated mechanical properties of heat treated ash at 200 oC. All investigated mechanical properties of recent ash differ significantly from these properties of heat treated ash. Dimensional stability of ash under such regime of heat treatment resulted in increased dimensional stability, and however the investigated mechanical properties are significantly lower. The above said percentages of higher values of mechanical properties of recent ash than of heat treated ash with water content of 12 % and 4 % indicate that they are similar at 12 % and 4 %. The similarity of these percentages indicates that the correction coefficients for calculating mechanical properties at different water contents is also satisfactory for heat treated ash

Relation of Chemical Composition and Colour of Spruce Wood

The visual inspection of fresh cut spruce wood (Picea abies, L. Karst.) showed the variability of its colour. Wood visual inspection is a part of wood quality assessment, for example, prior to or after its processing. The detail spruce wood colour analysis was performed using spectrophotometric data. The colour was measured by the bench-top spectrophotometer CM-5 Konica Minolta. The spectrophotometer was calibrated with a built-in white standard and on air. The whole analysis was performed in an xy chromaticity diagram supplemented with coordinate Y and CIE L*a*b* colour spaces. The ratio of the white chromophore amount to the amount of all achromatic chromophores is related to the Y coordinate. The ratio of the chromatic chromophore amount to all chromophores amount is saturation. The constructed model of the spruce wood colour is composed of four chromophores. The white chromophore belongs to holocellulose. The black chromophore belongs to lignin. The saturation is influenced by two chromophores. One of them belongs to extractives, another to lignin. The amounts of chromophores correlated with the spruce wood chemical composition. The chemical composition was measured using the procedures of Seifert, Wise, Sluiter, and ASTM. Moreover, the wood colour is affected by the moisture content

Colour and Chemical Changes of Black Locust Wood during Heat Treatment

Black locust is a fast-growing deciduous tree species with multiple industrial purposes due to its valuable traits. However, the heterogeneity of colour distribution between sapwood and heartwood limits its application. Thermal modification is an environment-friendly technology for improving various wood properties, especially dimensional stability, decay resistance, and colour homogeneity. In this work, black locust (Robinia pseudoacacia L.) wood samples were thermally modified at temperatures of 160, 180, and 210 °C. Extractives and main wood components were analysed by wet chemical methods, colour was measured by spectrometry, and structural changes by Fourier transform infrared spectroscopy. The obtained results show that the darkening of black locust wood, unlike other wood species of the temperate zone, is mainly caused by changes in extractives. Their content decreases during thermal treatment, but new chromophores are formed, especially in quinones. Degradation of hemicelluloses and the partial degradation of cellulose also contribute to colour changes. At higher temperatures, condensation reactions can occur in lignin, leading to the formation of some chromophores. Statistical analysis confirmed that temperature can be considered a very significant factor affecting the colour of the wood surface