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The wood of poplar (Populous sp.) species seem to be of low quality, mechanical resistance and is susceptible to microorganisms attacks, therefore, the application of thermal treatment is necessary, in order to improve some of its properties and increase its duration. Thermal modification of poplar (Populous sp.) wood was implemented at 180oC and 200oC, for 3, 5 and 7 hours in the presence of air and some of the most crucial mechanical properties were investigated. As the intensity of heat treatment increases, the mass loss of poplar boards is increased, attributed to moisture loss and the degradation and loss of chemical wood components. The bending strength decreased (2.05%-44.65%) compared to control, due to the treatments, while the elasticity decreased in all the treatment categories (5.66%-24.86%). The tangential hardness of poplar modified at 180oC for 3 and 5 hours, decreased by 10.24% and 14.59%, respectively and at 7 hours it grew by 2.35%, while at 200oC a decrease of 17.04-26.05% was recorded. The radial hardness decreased (2.81%-11.06%) at 180oC (3 and 5 hours) and when the duration reached 7h, it increased (9.28%), while at the treatments of 200oC radial hardness decreased (2.68%-18.86%). All the treatments reduced the impact bending strength of poplar specimens (24.27%-62.02%), even though they raised the resistance levels to compressive forces. Only the milder treatment achieved to decrease the roughness of poplar wood surface, while increasing further the treatment intensity, the degree of roughness showed a strong increase. [ABSTRACT FROM AUTHOR]

The effective specific heat of wood briquettes was determined by using a mathematical model developed for bulk pellets/briquettes and by measurements made in parallel and perpendicularly to the briquettes axis in the range of moisture contents below and above equilibrium moisture content. The mathematical model was based on the assumption that bulk pellets/briquettes are a porous system, consisting of solid particles and gas, with effective thermal properties. The model applied to the briquettes with the moisture content above the equilibrium moisture content is different to that applied to the briquettes with the moisture content below equilibrium moisture content, since alongside with wood fiber swelling, the magnitude and the number in the voids between chips increases. That is why, the briquettes density decreased in this moisture content range. New equations are proposed for this moisture content range considering a larger wood cell lumen that includes the chips interspaces. Experiments were performed at the temperature of briquettes ranging from 19 to 24°C and the average moisture content from 0 to 22.7%, dry basis. When increasing the moisture content above equilibrium moisture content, the briquettes expanded due to fiber swelling, but also due to the increase in the magnitude and number of chips interspaces. Both swelling and increase in briquettes voids influenced the effective specific heat. [ABSTRACT FROM AUTHOR]

Mechanical parameters of ash wood (Fraxinus excelsior) subjected to compression in radial direction, before and after its thermal modification and measured at moisture content close to the equilibrium moisture content of wood exposed in and outside (4 and 12%) were compared. Thermal modification of wood was performed at 190°C or 200°C for 2 h in industrial conditions. During the measurements, the moisture content of the modified and control samples was identical. The parameters compared included: modulus of elasticity, stress at proportionality limit, relative linear strain at proportionality limit and accumulated elastic energy. Changes in the mechanical parameters of wood induced by its thermal modification were found to depend on the modification temperature and wood moisture content. [ABSTRACT FROM AUTHOR]

An asymmetrical two-layer laminated densified wood product is prone to cupping when it is placed in a fluctuating moisture environment. This study was aimed at developing a 2D finite element (FE) model to simulate this situation. Two types of specimens with two thickness ratios of 0.18 (type I) and 0.58 (type II) were prepared. A 6-week moisture and temperature treatment was conducted. The cupping value of deformed specimens was measured for verification. The major results were: (1) The cupping value of type I specimens experimentally measured and predicted was 0.307 and 0.327 mm, respectively, while that of type II specimens was 0.440 and 0.396 mm, respectively; (2) FE and experimental results were in good agreement since the FE results lied in the 95 % confidence interval of the experimental results. The developed FE model can be used to predict the moisture-induced cupping of a two-layer laminated wood product. [ABSTRACT FROM AUTHOR]

Cupping is a critical issue that affects the performance of a two-layer laminated densified wood product in service. Many parameters including material properties and product geometry can influence the cupping. It is more efficient to study the influence of these parameters on cupping using a model than by testing. A model sensitivity analysis is helpful in identifying key parameters so that product design can be optimized. This paper expanded the discussion on the finite element model developed and its application. It was found that (1) swelling/shrinkage coefficient and modulus of elasticity in the tangential direction and equilibrium moisture content were three key material parameters; (2) increasing the modulus of elasticity could offset part of cupping; (3) thickness ratio of two wood layers, as a key geometry parameter, was independent of three key material parameters; and (4) the optimal thickness ratio of two wood layers was 0.12. [ABSTRACT FROM AUTHOR]

A large number of different heating technologies has been put into use for industrial scale thermal modification of wood. A useful classification of these processes is by the level of water vapour pressure, which ranges from vacuum to high saturated steam pressures. Only high water vapour pressure systems can maintain a finite moisture content during the heat treatment, but little is known about the water vapour pressure dependence of the thermal modification chemistry and the resulting modified wood properties. It is concluded from our analysis that the thermal wood reaction chemistry at the molecular functional group level is quite independent of the process and wood species. Wood properties that are strongly determined by wood chemical composition, such as the fungal durability and the equilibrium moisture content (EMC), can hence be equally achieved by all processes and for all wood species. This finding cannot be transferred to every other thermally modified wood property. [ABSTRACT FROM AUTHOR]

The thermal expansion behavior of dry solid wood was investigated by dynamic dilatometry and thermal mechanical analysis. Anomalous thermal expansion behavior was observed concerning the displacement change under a constant compression pressure, which was not previously reported. Wood submitted to temperatures below 0°C under dry conditions exhibited a large increment in the linear thermal expansion coefficient (CLTE) and a sudden drop in the CLTE around 50°C as well as above 130°C during heating. In subsequent cooling/heating processes, these anomalous behaviors remained at temperatures below 100°C, although less pronounced, and disappeared at temperatures above 100°C. These behaviors were clearly perceptible in the radial and tangential directions but not in the longitudinal direction. The CLTE depended strongly on the heat and moisture history of the samples and the effects are species-specific. [ABSTRACT FROM AUTHOR]

This study aimed to evaluate physical and mechanical properties and colour changes of fast-growing Gympie messmate wood subjected to two-step steam-heat treatments. To achieve this, Gympie messmate wood was thermally treated under different conditions. Combined steam (127°C and ∼0,1471 MPa) and heat treatments in an oven (180–240°C for 4 hours) were performed. Physical and mechanical properties were evaluated by weight loss, equilibrium moisture content, specific gravity, volumetric and linear swelling and static bending tests, while colour changes were studied using CIEL*a*b*technique. The main findings showed that the steam pre-treatment in autoclave influenced most of the technological properties evaluated, mainly for heat treatments performed in low temperatures (180–200°C). The most significant changes after thermal treatments were observed for dimensional stability, which increased as a function of temperature of treatment. On the other hand, mechanical strength of thermally modified wood was significantly affected, while stiffness did not change. Colour modifications due to the application of two-step steam-heat treatments confirm the possibility to using these samples for aesthetic purposes. [ABSTRACT FROM PUBLISHER]

Boards of poplar wood were subjected to thermal treatment at 180°C and 200°C, for durations of 2, 4, 6, 8 and 10 hours, in order to investigate the effect of heat treatment on the hygroscopic properties. Specifically, the change of equilibrium moisture content, density, swelling and absorption of wood were examined, in order to determine the significance of treatment temperature and duration in the enhancement of wood hygroscopicity. Generally, swelling in tangential direction proved to be quite higher than swelling in radial direction, while thermal treatment enhanced in great extent swelling in tangential and radial direction and the absorption of the specimens as well. Swelling in tangential direction decreased within the range of 8.17% and 33.3% for specimens treated at 180°C and the range of 43.85% and 65.09% for specimens treated at 200°C for 2-10 hours, referring to the final measurement after 72 hours of water immersion. In radial direction, the swelling decrease ranged between 2.47% and 28.45% for specimens treated at 180°C and between 19.62% and 60.87% for specimens treated at 200°C for 2-10 hours, respectively, after 72 hours immersion. Most of the heat treatments resulted also in absorption decrease, attesting the improvement of the poplar wood hygroscopic behavior. Finally, as it was proved, the higher the treatment temperature that is applied, the shorter is the duration, needed for the same swelling and absorption decrease level to be achieved. [ABSTRACT FROM AUTHOR]

OSB (oriented strandboards) made from Scots pine (Pinus sylvestris) were thermally post-treated at three different maximum treatment temperatures in the laboratory thermal treatment chamber of the Institute for Wood Technology Dresden (IHD). The OSB were supplied by a European manufacturer of OSB and bonded with methylene diphenyl diisocyanate (MDI). Post-treatment means that rather than thermally treating wood or strands before panel production, cut industry panels, i. e. 650mmx1200mm, were modified in IHD's lab kiln. The main goals of the thermal post-treatment of full-sized OSB were to improve the hygroscopic properties, i. e. to reduce the thickness swell (TS) and the equilibrium moisture content (EMC), as well as to improve the long-term properties creep behaviour and dimensional stability, without significantly reducing further mechanical properties like e. g. internal bond (IB) or bending properties, however. It was observed that the thermal post-treatment of the aforementioned OSB had a very positive effect on the moisture-related properties TS and EMC. The modified OSB showed far better values for said properties than the respective untreated ones. In can be concluded, that the thermal modification of OSB panels is possible and leads to very positive effects regarding their hygroscopic properties. [ABSTRACT FROM AUTHOR]

The thermal and dielectric properties of medium density fiberboard (MDF) with different moisture contents were measured by light flash and the parallel-plane capacitor method, respectively. The results show that increasing moisture content has a positive effect on both thermal properties and dielectric properties. The higher the moisture content, the higher the thermal conductivity and dielectric properties were. The thermal conductivity of MDF with different moisture contents varies in the temperature range of 25 to 150 ºC in a double-hump pattern rather than a proportional pattern. The dielectric constant decreases with increasing frequency up to 1000 MHz. The dielectric loss factor undulates within the frequency range of 1 to 100 MHz, and the peak value occurs at around 10 MHz. The results presented in this study can be used for radio frequency heating, wood building energy, material design, and radio frequency evaluation. [ABSTRACT FROM AUTHOR]

Birch wood ( Betula pendula) samples were treated in a thermal regime (140, 160, 180 °C) for 1 h and investigated by means of scanning electron microscopy (SEM). SEM microimages of the wood cross-section were taken from one and the same place before and after the thermal treatment (magnification 100-2,000×). The results of measurements of areas and linear sizes of the birch wood cells show significant changes, which depend on the thermal treatment conditions and the type of the cell: libriform, tracheid, vessel and ray. After the treatment at 180 °C, the integrity of wood morphological structure begins to break up. Voids and cracks are formed between fibres, thus leading to a decline in the mechanical properties of the wood. [ABSTRACT FROM AUTHOR]

Radio-frequency (RF) heating has been widely used in the wood industriesand it is essential to understand the factors affecting the heating rates. In this article, Chinese fir (Cunning hamialanceolata) was heated with RF energy intermittently at 2 min per cycle to study the effect of wood moisture content (MC), sample thickness, and RF power level on heating rates. The study revealed that wood MC significantly affected the RF heating rate (p < 0.01). The heating rate was the highest (5.0°C/cycle) at an MC around the wood fiber saturation point (FSP). The heating rates were lower with a lower MC of 15% (0.89°C/cycle) and a higher MC of 70% (1.2°C/cycle). Lumber thickness did not have a significant effect on the RF heating rate at the center of the lumber; however, the heating rates at various locations along the thickness direction were significantly different. The temperature at the lumber center was 20°C higher than the temperature at 10 mm below the surface during heating. The heating rate for lower RF power level was initially greater than the rate at higher power RF level and then rapidly decreased. However, the heating rate at the higher RF power level remained constant during the whole heating process. RF power levels had a significant effect on the internal heating rate of wood (p < 0.01). [ABSTRACT FROM AUTHOR]

The time-dependent mechanical behaviour (TDMB) of wood is important when using the material for structural purposes. Recently, a new method for predicting the TDMB by numerical modelling was established based on the assumption that TDMB is caused by the sliding of the microfibrils past each other. In this study, the TDMB is examined via creep experiments on small specimens of Norway spruce latewood. The results of these are compared with results from numerical modelling. The experiments include results at two levels of moisture content and three levels of temperature, enabling an investigation of these two climatic factors on TDMB of wood. It was found that the mechanical response of wood tissue is the sum of responses from both tracheids and middle lamella, with only the previous being reversible. The effect of moisture and temperature differed in that the latter affected the elastic and time-dependent responses equally. Moisture, on the other hand, reduced both the elastic properties and the activation energy barrier for sliding of the microfibrils, but furthermore changed the microfibril angle of the sample as a result of swelling. Hereby, moisture had a larger effect on the time-dependent response than the elastic. All of these effects were predicted by numerical modelling. [ABSTRACT FROM AUTHOR]

The moisture content of wood is known to have a significant influence on the wood's mechanical properties. Using wood as an energy absorber in impact limiters of packages for the transport of radioactive material, it is of particular importance to ensure the moisture content and thus relevant mechanical properties to be in specified limits. The paper surveys the influence of wood moisture content on the mechanical properties of wood. Different measuring methods are discussed with respect to in situ applicability, accuracy and effort. The results of an experimental analysis of the accuracy of hand held moisture metres using the electrical resistance method are discussed. Conclusions are drawn regarding the measurement of moisture content of wood upon delivery as well as of complete impact limiter assemblies. Requirements for quality surveillance during manufacturing of wood filled impact limiters are derived and it is exemplified how to meet them. Construction, manufacturing and inspection of impact limiter encapsulation with regard to leak tightness are addressed. [ABSTRACT FROM AUTHOR]

In this paper, anisotropic wood is thermally characterized using the transient hot-bridge method recently developed at PTB Germany. The main element of this method is the THB sensor which is composed of four tandem strips forming a Wheatstone bridge and which has the specificity of measuring in the same experiment both thermal conductivity and thermal diffusitnty. The adoption of special configurations allows us to characterize anisotropic materials. In this context, the axial, radial, and tangential thermal conductivities as well as the thermal diffusivities of three wood species are measured and analyzed for different moisture contents. Due to the difficulty of measuring thermal diffusivity using the conventional electrothermal methods, particular care will be given to this task. Thus, the paper will focus on the analysis of thermal diffusivity versus moisture content, as well as on its relation to thermal conductivity. [ABSTRACT FROM AUTHOR]

Wood-TiO2 (titania) composites were prepared by a sol-gel process, in which wood was impregnated with the precursor solutions prepared from tetrabutyl titanate (TBT), followed by a curing step. The surface morphology and moisture absorption behavior of the wood composites, as well as their thermal and mechanical performances, were examined. Environmental scanning electron microscopy (ESEM) analysis revealed that TiO2 gels were deposited principally in the cell lumens and partly in the cell walls, as confirmed by the energy dispersive X-ray (EDX) analysis. By this inorganic modification, the hygroscopicity of wood was significantly reduced and its dimensional stability was improved consequently. Greater amounts of TiO2 gel deposited in the cell lumens were not helpful in enhancing the hygroscopicity of wood. Thermal analysis (TG-DTA) showed that the incorporation of TiO2 gel retarded the thermal decomposition of wood matrix and improved the thermal stability of wood. The incorporated inorganic gel seemed to stiffen the wood cell walls, as indicated by the increased resistance of the wood composites to deformation and collapse in compression. [ABSTRACT FROM AUTHOR]

The ignition of various wood species by exposure to the radiation of a DKsR-10000 xenon lamp was investigated. Time dependences of ignition delay on the radiant energy intensity were obtained for various wood species: cedar, larch, pine, birch, and aspen. The effect of the initial temperature and moisture content of wood on the ignition time was studied. The ignition temperature of pine and birch wood for various radiant energy intensities was determined by an optical method. It is shown that the ignition time of pine wood decreased by a factor of 1.5-2 as the initial temperature of samples was increased by 100 degrees. It was found that the ignition temperature of wood increased with increasing radiant flux density and depended on the wood species. Increasing the wood moisture content by a factor of 10 increased its ignition time by more than 50%. [ABSTRACT FROM AUTHOR]

Abstract: This research aims at developing a new approach able to simulate 3-D heat and moisture transfer coupled with the mechanical behaviour of a wood during drying process. From the moisture content and temperature profiles, a 3-D formulation and a relevant constitutive model are used to calculate the stress/strain evolution within the board due to shrinkage and external mechanical loading. This allows a fast, comprehensive and realistic model to be implemented. The mechanical model takes into account the hydrous, thermal, mechano-sorptive and elastic deformations, as well as the changes of wood properties, caused by these processes, e.g. porosity, permeability, stress–strain relation, etc. The mathematical model describing simultaneous unsteady heat and moisture transfer between a gas phase and a solid phase during heat treatment has been developed. The conservation equations for the wood sample are obtained using diffusion equation and the 3-D incompressible Navier–Stokes equations have been solved for the flow field. The constitutive equations are discussed in some detail. ANSYS-CFX10 commercial code was used to solve the hygro-thermal problem and FESh++ for the mechanical behaviour. Experimental results obtained regarding temperature, moisture content and deformation profiles during industrial drying of black spruce wood are compared with the numerical results. Satisfactory agreement is obtained over a range of drying air temperatures. [ABSTRACT FROM AUTHOR]

In this study the thermoplastic flow behavior of steamed wood flour was investigated. First it was demonstrated that steamed Japanese beech flour flowed out of the nozzle under compression at high temperature in a thermal flow test with a capillary rheometer. The effects of the steaming temperature, steaming time, compressive pressure, and moisture content of wood flour on the thermal flow temperature were examined. It was shown that the higher the steaming temperature and compressive pressure, the lower the thermal flow temperature. Also, the thermal flow temperature of the sample steamed at 200°C for 20 min became lowest and increasingly higher over time. Furthermore, the thermal flow temperature became linearly low with increasing moisture content of the sample under 15%, whereas it became essentially constant over 15%. It is clarified that compressive pressure and moisture content as well as the steaming conditions profoundly affect the thermoplastic flow behavior of steamed wood flour. [ABSTRACT FROM AUTHOR]

Abstract: The present study is devoted to a numerical study with experimental validation of the high-temperature thermal treatment of three-dimensional wood pole. During the heat treatment process, the heat and mass transfer takes place between the solid and the drying medium, and the moisture evaporation occurs within the solid due to the capillarity action and diffusion. The development of the model equations, taking into account both bulk phases and interfaces of the multiphase system is described, starting from the microscopic scale. Fundamental to this model is the ability to quantify the effects of key material and geometric properties of the pole. The three-dimensional and unsteady-state mathematical model equations are solved numerically by the commercial package FEMLAB for the temperature and moisture content histories under different treatment conditions. A detailed discussion of the computational model and the solution algorithm is given. Heat treatment was applied on the test samples in an oven for three final temperatures (180, 200 and 220°C). A series of experimental tests aimed at determination of heat treatment schedules kinetics curves and the temperature and moisture profiles and there time evolution were carried out. A very good agreement between the experimental and predicted results was obtained, implying that the proposed numerical algorithm can be used as a useful tool in designing high-temperature wood pole treatment processes. [Copyright &y& Elsevier]

Wood-based composites, such as oriented strand board, are typically manufactured by consolidating mats of resinated wood elements under heat and pressure. During this process, the temperature and moisture content distributions within the mat greatly affect the properties of end products. To improve the fundamental understanding of mat consolidation during hot-pressing, a model is established to investigate the transverse compression behavior of aspen wood strands for a variety of combinations of temperatures (20–200°C) and moisture contents (0–15%). A regression approach is used to obtain the modulus-temperature-moisture relationship. In addition, elevated temperatures and moistures are found to influence the strain function of wood strands, which was previously assumed to be independent of these factors. [ABSTRACT FROM AUTHOR]

Abstract  Scots pine sapwood (Pinus sylvestris L.) and Douglas fir heartwood (Pseudotsuga menziesii Franco) specimens were exposed in double layer field trials at 23 different European test sites under different exposure conditions (in total 27 test sets). The material climate in terms of wood moisture content (MC) and wood temperature was automatically monitored over a period of up to 7 years and compared with the progress of decay. The overall aim of this study was to establish dose–response relationships between climate factors and decay as a basis for the service life prediction of wood. The “Scheffer Climate Index” based on weather data collected at official meteorological stations at the different test sites poorly correlated with the corresponding decay progress and was therefore not a suitable tool for estimating site-specific decay potential. In contrast, the use of the combined material climatic parameters MC and wood temperature led to a feasible dose–response function and turned out to be a useful basis for service life prediction of wood. [ABSTRACT FROM AUTHOR]