Fiber Saturation Research Articles

Specific heat capacity of wood between −140 and 50 °C in dry and wet state

Abstract Specific heat capacity of wood is an important material property for fundamental research and timber technology. However, the existing database for heat capacity values is limited, especially for data below 0 °C. Therefore, the aim of this study was to investigate the specific heat capacity of wood with different moisture contents (dry, below and above fibre saturation) in lower temperature. Beech (Fagus sylvatica) and scots pine (Pinus sylvestris) were investigated via differential scanning calorimetry (DSC), equipped with a liquid nitrogen-cooling device, using the sapphire standard. Specific heat capacity for beech and pine were linear within the investigated temperature region. Values above 0 °C are in agreement with existing literature. Both species showed similar results. Based on the obtained results the specific heat capacity for bound water was calculated. The presented results are an important step for increasing the database of wood heat capacity. These findings enable future research for calculating heat-flux, diffusion and moisture transfer in wood.

Flexural characteristics of Eucalyptus nitens timber with high moisture content

The demand for timber resources in the building industry has been increasing. Plantation Eucalyptus nitens is of interest because of its sustainable supply and potential for structural applications. However, few design standards cover strength values of plantation eucalypt timber, especially flexural failure below and above the fibre saturation point, which is an important mechanism of failure in bending members used in the building industry. Static bending tests were undertaken using a universal testing machine to examine nonlinear bending behaviour of 130 fibre managed E. nitens small clear wood samples at low and high moisture contents (MC). The mean bending modulus of rupture (MOR) was 80.7 MPa for low MC and 59.0 MPa for high MC. The high MC samples exhibited larger displacements at low ultimate loads, while the low MC samples showed abrupt failures at relatively small displacements with high ultimate loads. The design characteristic values for low and high MC E. nitens were 68.5 MPa and 39.8 MPa, respectively. This research demonstrates that fibre managed E. nitens timber is a promising timber for structural applications, especially when exposed to water, as the MOR reduction of E. nitens timber above FSP is relatively lower than those of P. radiata, which is a traditional construction material.

Differences and similarities between kraft and oxygen delignification of softwood fibers: effects on chemical and physical properties

The fiber properties after oxygen delignification and kraft pulping were studied by looking into the chemical characteristics and morphology. The effect of the two processes on the fibers was evaluated and compared over a wider kappa number range (from 62 down to15). Wide-angle X-ray scattering, nuclear magnetic resonance and fiber saturation point were used to characterize the fiber network structure. Fiber morphology and fiber dislocations were evaluated by an optical image analysis. The total and surface fiber charges were studied by conductometric and polyelectrolyte titrations. The fiber wall supramolecular structure, such as crystallinity, size of fibril aggregates, pore size and pore volume, were similar for the two processes. The selectivity, in terms of carbohydrate yield, was equal for kraft cooking and oxygen delignification, but the selectivity in terms of viscosity loss per amount of delignification is poorer for oxygen delignification. Clearly more fiber deformations (2–6% units in curl index) in the fibers after oxygen delignification were seen. Introduction of curl depended on the physical state of the fibers, i.e. liberated or in wood matrix. In the pulping stage, the fiber continue to be supported by neighboring fibers, as the delignified chips maintain their form. However, in the subsequent oxygen stage the fibers enter in the form of pulp (liberated fibers), which makes them more susceptible to changes in fiber form.Graphic abstract

Peningkatan Kerapatan Kayu Samama Melalui Pre-kompresi Asam Sitrat (Density Improvement of Samama Wood by Pre-compression of Citric Acid)

Samama wood (Anthocephalus macrophyllus (Roxb.) Havil.) is a potential fast-growing species of Sulawesi and Maluku. This study aimed to increase the density of the wood through citric acid pre-compression. The temperature/time pressing formula and the optimal concentration of citric acid for fixation were also determined. Water saturated samples of (5x5x4) cm3 (L = longitudinal x T = tangential x R = radial) were pre-compressed at 100 °C for one hour to reach drying set. Subsquently, the samples were soaked for 4 hours in a citric acid solution of 5% and 10% concentration, drained and wrapped in aluminum foil before re-pressed at 180 °C for 10, 20, 30, 40, 50, and 60 minutes. Fixation was measured by soaking the samples into water for 24 hours at room temperature. The results showed that the moisture contents at fiber saturation point ranged from 33.15-33.94%, with density of 0.46 g cm-3 and oven dry density of 0.37 g cm-3. The L, T, and R shrinkages were 0.18-0.20%, 4.13-4.14%, and 2.53-3.10%, respectively; while the T/R ratio was 1.33-1.63%. Pre-compression can only be done at a compression target of 25% with compression level of 19.57-20.01%. Pre-compression increased the oven dry density of 17.11-20.13% to 0.44-0.45 g cm-3. After thickness recovery, the weight of the oven dried samples increased by 1.79-2.72% at the 5% citric acid concentration and by 12.04-15.25% at the 10% citric acid concentration. Permanent fixation achieved at 180 °C for 50 minutes pressing time with 10% citric acid concentration.

Rheological properties of cement pastes with cellulose microfibers

The rheological properties of cement pastes prepared using kenaf-plant cellulose microfibers (CMFs), which was incorporated for the purpose of internal curing, were investigated. The main test variables are the length and mass fraction of the CMFs. CMFs of lengths of 400 μm and 5 mm were included in the mixtures at 0.3, 0.6, 1, and 2 wt.% of the cement. Dry CMF particles were wetted with water to the fiber saturation point using vacuum filtration immediately before mixing. An optimum shearing protocol was designed to minimize the shear-induced structural breakdown of cement pastes with the CMFs under hysteresis loops of the shear strain rate. It consisted of an initial pre-shearing step at a high shear strain rate of 80 1/s, three acceleration and deceleration cycles with a maximum shear strain rate of 40 1/s, and a rest step before each acceleration. The mixtures’ flow curves were well fitted to the Herschel–Bulkley fluid model with a minimum coefficient of determination of 0.9993. The yield stress of cement pastes was at least 34.3% higher for longer CMFs at the same dose. However, the mixtures exhibited similar plastic viscosities with a coefficient of variation of only approximately 5.8%.

Bound Water Content and Pore Size Distribution of Thermally Modified Wood Studied by NMR

The physical and mechanical properties of thermally modified wood (TMW) have been comprehensively studied; however, the quantitative analysis of water states and cell wall pores of TMW is limited. In this work, Douglas fir and Norway spruce were thermally modified at 180, 200 and 220 °C, and then studied by NMR cryoporometry method. The results show that thermally modified samples had lower fiber saturation point and the bound water content than the reference samples at all the experimental temperatures, indicating the reduced hygroscopicity due to thermal modification (TM). In addition, TM decreased number of hygroscopic groups, which can be implied by the decreased proportion of bound water sites, and TM also increased the proportion of small voids for bound water clusters. An increase in TM intensity resulted in lower bound water content and a smaller number of hygroscopic groups. In summary, the NMR method detected the water states and pore size distribution and confirmed that TM decreased the fiber saturation point and hygroscopicity of wood by reducing the bound water content and proportion of bound water sites in wood cell walls.

How Bound Water Regulates Wood Drying

For most wood-based product uses it is essential to remove a large part of the water content from wet or green (fresh-cut) wood, to reduce further dimensional variations under varying humidity conditions, improve its mechanical characteristics, and protect it from biological attacks. However, the internal mechanisms of drying are not fully described. Here we observe drying at different scales using macroscopic measurements (weighing), nuclear magnetic resonance measurements allowing to distinguish bound- and free-water contents, and x-ray computed tomography images of air-liquid interfaces at the smallest pore scale (wood lumens). We show that during wood drying, even well above the fiber saturation point, bound-water diffusion in cell walls (instead of capillary effects) ensures the extraction of liquid water from pores and its transport towards the surface of evaporation, and thus controls the drying rate. The distribution of bound-water content (uniform or heterogeneous) along the main sample axis and the drying-rate evolution depend on the competition between the external conditions and a characteristic rate of transport due to bound-water diffusion. For sufficiently slow drying this distribution remains homogenous until free water is fully extracted. An original physical phenomenon is thus at work, which plays a major role in regulating water extraction, in that it maintains a constant drying rate and a homogeneous distribution of the (mean) water content throughout the material. These results provide sound concepts for modeling and controlling drying properties of wood materials. They open the way to the understanding or control of the properties of many other materials containing two water types in food or civil-engineering applications. Our results complete recent observations that bound-water diffusion also controls imbibition in hardwood and finally show that transfers between bound and free water play a major role in the interaction of plantlike systems with water.

Dewatering Green Sapwood Using Carbon Dioxide Undergoing Cyclical Phase Change between Supercritical Fluid and Gas.

Conventional kiln drying of wood operates by the evaporation of water at elevated temperature. In the initial stage of drying, mobile water in the wood cell lumen evaporates. More slowly, water bound in the wood cell walls evaporates, requiring the breaking of hydrogen bonds between water molecules and cellulose and hemicellulose polymers in the cell wall. An alternative for wood kiln drying is a patented process for green wood dewatering through the molecular interaction of supercritical carbon dioxide with water of wood cell sap. When the system pressure is reduced to below the critical point, phase change from supercritical fluid to gas occurs with a consequent large change in CO2 volume. This results in the efficient, rapid, mechanical expulsion of liquid sap from wood. The end-point of this cyclical phase-change process is wood dewatered to the cell wall fibre saturation point. This paper describes dewatering over a range of green wood specimen sizes, from laboratory physical chemistry studies to pilot-plant trials. Magnetic resonance imaging and nuclear magnetic resonance spectroscopy were applied to study the fundamental mechanisms of the process, which were contrasted with similar studies of conventional thermal wood drying. In conclusion, opportunities and impediments towards the commercialisation of the green wood dewatering process are discussed.

Study of the collapse and recovery of Eucalyptus urophydis during conventional kiln drying

The collapse of Eucalyptus urophydis wood during conventional kiln drying prevents the widespread use of its wood products. This research aims to explore its collapse and recovery characteristics, as well as the effect of continuous and cyclic drying. The collapse was measured by total shrinkage, and cell deformation was observed at three critical points using scanning electron microscopy. The collapse and recovery were investigated by integrating the shrinkage curves and observing the cell micro-deformation. The results showed that the continuous drying rate was faster than that of cyclic drying, especially when the moisture content was below the fiber saturation point. Compared with relative humidity, the temperature had a greater effect on the tangential absolute dry shrinkage. Although cyclic drying decreased collapse, its effect was apparent only at low temperatures and was weak at high temperatures. The collapse recovery time was affected by temperature, and recovery occurred quickly at high temperatures, and the moisture content range was longer. In contrast to continuous drying, the high relative humidity period during cyclic drying led to earlier collapse recovery. The degree and trends of microscopic collapse were consistent with the macroscopic scanning measurements.

SOME PROPERTIES OF OLIVE WOOD GROWN IN AYDIN AND KAHRAMANMARAŞ REGIONS

Son yıllarda hızla artan yapılaşma nedeniyle kesilen meyve ağaçlarının ve budama atıklarının yakacak yerine orman ürünleri sektöründe değerlendirilmesine yönelik çalışmalar artmıştır. Türkiye, zeytin hasat alanı bakımından Dünya’da 6. sırada yer almaktadır. Geniş zeytin alanlarına sahip Aydın ve Kahramanmaraş’ta yeni başlayan bir inşaatın, şantiye sahasından kesilen zeytin (OIea europaea L.) ağaç gövde odunlarının kimyasal, morfolojik ve fiziksel özellikleri belirlenerek, karşılaştırma yapılmıştır. Holoselüloz, α-selüloz, lignin, alkol-benzen çözünürlüğü, sıcak su çözünürlüğü, soğuk su çözünürlüğü, %1 NaOH çözünürlüğü ve kül miktarı sırasıyla Aydın örneklerinde %70.4, %39.8, %23.0, %4.88, %12.5, %11.0, %20.4, %1.15 ve K.Maraş odunlarında ise %58.6, %36.0, %25.3, %17.0, %20.7, %15.9, %28.3 ve %0.79 tespit edilmiştir. Morfolojik özellikleri olarak zeytin ağaç liflerinin uzunluğu, genişliği, çeper kalınlığı ve lümen çapı belirlenmiştir. Bu değerler sırasıyla Aydın’dan alınan örneklerde 0.83 mm, 15.8 µm, 4.32 µm, 11.5 µm ve K.Maraş odunlarında ise 0.78 mm, 12.3 µm, 4.12 µm, 8.17 µm’dir. Zeytin ağaç odunlarının hava kurusu yoğunluğu (D12) 0.88 g/cm3, tam kuru yoğunluğu (Do) 0.78 g/cm3, hacim yoğunluk değeri (R) 0.70 g/cm3, hacimsel daralma (βv) %9.16-10.68, hacimsel genişleme (αv) 10.1-10.9, lif doygunluk noktası (LDN) %13.1-15.6 ve maksimum rutubet içeriği (MMC) %75.3-77.1 olarak tespit edilmiştir.

Preparation and Characterization of a Type of Green Vacuum Insulation Panel Prepared with Straw Core Material.

The Vacuum Insulation Panel (VIP), regarded as the most promising high-performance thermal insulation material, still has application limitations because of its high cost. In this paper, VIPs using natural straw as the core material are prepared. The fiber saturation point (FSP) is important in order to determine the optimum for the use of renewable straw materials as a potential VIP core. The microstructure of straw core material, together with the relationship between the moisture content, the diametral compression strength, and the thermal conductivity of as-prepared straw VIPs are investigated. Compression characteristics of straw core material and heat insulation mechanism within the straw VIP envelope enclosure are analyzed. Total thermal conductivity of a straw VIP is sensitive to both the inner pressure and the moisture content of straw core material. The optimum drying process for straw VIPs is heating the straw core material at a temperature of 120 ℃ for 60 min, with its center-of-panel value being about 3.8 mW/(m·K).

Effect of air velocity, temperature, and relative humidity on drying kinetics of rubberwood

Kiln drying of rubberwood lumbers is a complex transport phenomenon for realistic modeling and simulation. To decouple this complexity, researchers usually divide their research into two parts. The first one is single-lumber drying kinetics to describe how wood lumber responds to its surface conditions. Then they combine this drying kinetics with a lumped transport model or dispersion model or computational fluid dynamics. The mathematical models are then solved numerically to predict the industrial kiln drying behaviors. This work focuses on the drying kinetics of stacked rubberwood lumbers using hot air at different air velocity (0.5, 1.5, 2.5, 3.5, 4.0 m/s), relative humidity (6–67% relative humidity (RH)) and temperature (60–100 °C). The drying kinetics followed the conventional drying theory. However, the two drying periods, namely constant and falling rate (CRP and FRP), were not distinct. As the air velocity increased, the transition from CRP to FRP is faster. The middle of the transition period (at critical moisture content, CMC) moves closer to the fiber saturation point (FSP). The overall mass transfer coefficients in the falling rate period for stacked rubberwood drying were lower than those predicted by the Ananias correlation. Hence, a modified formula was proposed, representing the overall moisture transfer coefficients as a function of air velocity, temperature, relative humidity, and lumbers thickness for the range of variables under investigation satisfactorily. In general, the drying rate and the overall moisture transfer coefficient increased with increasing air velocity, drying temperature, and decreasing RH. Relative humidity directly affects the driving force of moisture transfer rate because higher RH is associated with higher equilibrium moisture content. A lumped parameter model for kiln drying was also developed. After being integrated with the estimated mass transfer coefficient, the model can predict the moisture profiles in lab-scale kiln drying satisfactory, although the model needs more validation data. These kinetic parameters and correlation for stacked rubberwood drying can be used in more complex models and process optimization in future research.

Heats of wetting and sorption for wood pellets

Wood pellets have become a flagship biofuel in transitioning from coal-based heat and power generation to renewable energy. However, the hygroscopic nature of wood pellets is a long-standing challenge in storing and transporting this commodity. The heat of moisture sorption contributes to the self-heating and spontaneous combustion of wood pellets. The objective of this paper is to investigate the evolution of heat in moistened pellets and to compare them to that of wood chips. Wood pellets at several moisture content levels were immersed in liquid water and the rise in the pellets’ temperature was recorded. The calorimetric method was used to calculate the heat of wetting for the immersed pellets. The heat of wetting was at its maximum for completely dry pellets. The heat of wetting decreased linearly as the moisture content increased towards a fiber saturation point of about 0.17 (decimal dry basis). Statistically, the maximum heat of wetting of pellets is the same as that of wood chips. However, the rate of heat release of pellets is twice of that of chips.

Full-field tracking and analysis of shrinkage strain during moisture content loss in wood

Abstract Anisotropic shrinkage is a typically feature in wood, which is of critical importance in wood drying. In this study, the shrinkage strains over each growth ring were determined by a full-field strain measurement system during moisture content (MC) loss. Color maps were used to visualize the full-field distribution of displacement and shrinkage strain under different MC conditions. The variation of tangential and radial shrinkage strain from pith to bark, as well as the anisotropic shrinkage in heartwood and sapwood were studied. Both of the displacement and strain values increased as the MC decreased. From pith to bark, the tangential strains were higher at two poles as compared to the center, showing a parabolic distribution below fiber saturation point. While for radial shrinkage strain, a minor difference was observed except for the MC of 10%. An intersection between tangential and radial shrinkage ratio curve was observed at the MC of 28%. Both expansion and shrinkage in tangential direction were larger than radial counterparts, and the transformation from expansion to shrinkage occurred at the MC region of 32–28%. In addition, the shrinkage in heartwood was larger than sapwood, whereas anisotropic shrinkage in sapwood was more pronounced as compared to heartwood.

Finite-element-based moisture transport model for wood including free water above the fiber saturation point

Timber constructions and engineered wood products exhibit a strong moisture-dependent behavior, which needs to be sufficiently describable by models to avoid critical designs. For wood, several models exist, able to describe the specific heat and moisture transfer mechanisms under normal service conditions as well as for particular situations during the production process, like the kiln drying of sawn timber. However, a unified model combining free water transport with the multi-Fickian approach below the fiber saturation point and the corresponding stable numerical implementation able to handle several possible states of water (bound water, water vapor and free water) and applicable to both drying and infiltration problems, is still missing.In this work, we link models for all those transport processes by reasonable coupling mechanisms. Numerical difficulties occurring at transition between different states of water are handled appropriately, leading to a stable and reliable simulation tool, which is implemented through a three-dimensional Abaqus user element. Thus, this contribution extends the applicability of heat and moisture simulation tools for wood to more critical service conditions, where wooden components might get into direct contact with free water. This may happen due to insufficient supervision during construction or in the production process of new types of wood composites. We anticipate that with the developed tool potentially critical designs can be identified by simulation instead of by time-consuming experiments or experience. This is of importance because the presence of free water and the resulting reduction in stiffness and strength is associated with a higher risk of local failure. This would allow more targeted designs of engineered wood products and efficient optimization strategies, especially with respect to the moisture behavior.

Shrinkage and Swelling of Greek Chestnut Wood (CastaneaSativaMill.) in Relation to Extractives Presence

The study of various woods shrinkage is of particular practical importance because the wood’s property to shrink and swell with the adsorption or absorption of moisture from the atmosphere is a main cause of many defects that occur in wood and furniture during weather conditions changing. In the present study, measurements of the shrinkage of chestnut coppice wood samples were performed before and after extraction. The extraction was carried out with hot water for 6 and 12 days. Afterwards, the dry density, radial and tangential shrinkage were determined both in air-dry conditions and in absolute dry conditions. Moreover, the fiber saturation point was determined for the selected samples. The most significant result arrived from this study is that, now, in conjunction with the other researches have been made, there is enough evidence for chestnut wood from Mediterranean coppice forests, to claim that maximum radial shrinkage is about 3.83% (ranging from 3.40% - 4.12%) and maximum tangential shrinkage is about 6.58% (ranging from 6.21% - 8.20%).

Propagation velocity model of stress waves in larch wood (Larix gmelinii) three-dimensional space with different moisture contents

Based on the effects of stress wave propagation in larch (Larix gmelinii) wood, the propagation mechanism of stress wave was explored, and a theoretical model of the propagation velocity of stress waves in the three-dimensional space of wood was developed. The cross and longitudinal propagation velocities of stress wave were measured in larch wood under different moisture contents (46% to 87%, 56% to 96%, 20% to 62%, and 11% to 30%) in a laboratory setting. The relationships between the propagation velocity of stress waves and the direction angle or chord angle with different moisture contents were analyzed, and the three-dimensional regression models among four parameters were established. The analysis results indicated that under the same moisture content, stress wave velocity increased as the direction angle increased and decreased as chord angle increased, and the radial velocity was the largest. Under different moisture contents, stress wave velocity gradually decreased as moisture content increased, and the stress wave velocity was more noticeably affected by moisture content when moisture content was below the fiber saturation point (FSP, 30%). The nonlinear regression models of the direction angle, chord angle, moisture content, and the propagation velocity of stress wave fit the experiment data well (R2 ≥ 0.97).

Influence of temperature and moisture content on bark/wood shear strength of black spruce and balsam fir logs

The effects of temperature (T) and moisture content (MC) on bark/wood shear strength (BWSS) were studied. Fifteen stems of black spruce (Picea mariana (Mill.) B.S.P.) and balsam fir (Abies balsamea (L.) Mill.) logs were selected and cross-cut into three 1.25 m log sections, corresponding to bottom (1.3 m), middle (3.8 m), and top (6.3 m) positions. BWSS was measured at five temperatures, ranging from 10 to − 30 °C, and at five levels of sapwood moisture contents (SMC), obtained by air-drying, from green state to near to the fiber saturation point (FSP). Bark and sapwood properties, MC and basic density (BD) were also determined. Temperature had a significant effect on BWSS for both species. This property was similar between 10 and 0 °C, and significantly increased as temperature decreased below 0 °C. However, the influence of temperature on BWSS depended on SMC and it varied between the two species. For black spruce, for each temperature between 0 and − 20 °C, BWSS showed similar values for SMC between 157 and 62%. At − 30 °C, BWSS showed a tendency to increase with SMC. For balsam fir, the BWSS increase due to the decrease in temperature was more important as the SMC increased. BWSS increased as SMC approached the FSP of wood for both species. Among the studied covariates, inner bark MC and inner and outer bark BDs significantly affected BWSS. Inner bark MC and SMC affected BWSS similarly. Multiple regressions were developed for prediction purposes, which explained 68 and 69% of BWSS variability for black spruce and balsam fir, respectively.

Evaluating the Potential to Modify Pulp and Paper Properties through Oxygen Delignification.

The potential to modify pulp and paper properties by oxygen delignification was assessed by looking beyond the ordinary purpose of oxygen delignification. Pulps with the same kappa number were obtained by both pulping and the combination of pulping and oxygen delignification, and the mechanical and chemical properties were compared. The oxidation of pulp components leads to an increase in carboxylic acid groups in the fibers, resulting in a large influence on fiber swelling, seen as an increase in the water retention value and fiber saturation point. The introduction of charged groups appears to replace some of the morphological changes caused by refining and enhance the strength of fiber–fiber joints, generating pulps with better refinability and higher tensile strength. Oxygen delignification was able to improve the tensile index with 6% at the same sheet density and less refining energy, when the amount of total fiber charges was higher than 140 μekv/g.

Anatomical and physico-mechanical properties of Acacia auriculiformis wood in relation to age and soil in Benin, West Africa

Acacia auriculiformis is increasingly used as timber in Benin, while little is known about its wood characteristics and the factors affecting such characteristics in the country. The determination of these characteristics is necessary for understanding the functioning of this species and its uses. The aim of this study was to evaluate anatomical and physico-mechanical properties of Acacia auriculiformis in relation to age and soil type in Benin, West Africa. Nine different age plantations (young 6–7 years old, middle-aged 9–11 years old, aged 15–29 years old) were sampled on three soil types (ferruginous, sandy and vertisol). In total, 30 trees were felled for determination of vessel diameter and vessel frequency from pith to bark, basic density, density at 12% moisture content, tangential and radial shrinkages, fiber saturation point, anisotropy of shrinkage, modulus of elasticity, compression strength, and bending strength. Age is the main factor influencing the physico-mechanical characteristics of the species and Acacia auriculiformis could be exploited at 15 years old for use as timber. Still, a high deformation ratio is noted on ferruginous soil. Besides, wood density is negatively correlated with anisotropy of shrinkage, which is an asset for the use of high density wood for structures. In contrast, the parameters of the vessels are weakly correlated with wood density. Further studies are necessary to understand the densification process of Acacia auriculiformis wood in order to optimize the production of high-density wood of the species, which is important for the use of wood as timber.