Fiber Saturation Point Research Articles

Flexural properties of moso bamboo induced by hydrothermal treatment

The superior flexural properties of bamboo make it widely used in the field of bamboo furniture. However, thick bamboo strips are more susceptible to fracture in bending due to their significant fiber gradient variation, especially the nodes. In this study, the eco-friendly hydrothermal treatment was employed to augment the flexural properties of bamboo and mitigate the fracture. The results showed that the flexural strength, flexural modulus, flexural toughness, and flexural radius of curvature increased with the increase of strip thickness. The strip thickness was positively correlated with vascular bundle content. The structure of bamboo became more uniform after hydrothermal treatment and the nodes were no longer the susceptible fracture point. The moisture content was a pivotal factor influencing the flexural properties, which increased as hydrothermal treatment time and temperature increased. Furthermore, hydrothermal treatment increased bamboo's flexibility by softening it through moisture absorption and the facilitating molecular chain movement within the fibers. The optimal moisture content for bamboo strips when manufacturing curved components was around the fiber saturation point. These findings contributed to the refinement of the green manufacturing techniques for bamboo curved furniture components, aiming to foster sustainable environmental development.

Non-invasive method to measure bulk electrical resistivity of spruce wood – influence of moisture content and anatomical direction

ABSTRACT The correlation between electrical resistance and moisture content (MC) of wood has been studied for many years and is the basis for an important method for determining wood MC in practice. This study presents a non-invasive set-up using plate electrodes with a guard ring covered with silver foam and gold leaves to measure the direct-current (DC) electrical resistivity of spruce wood. The design ensures good electrical contact at a pressure of 0.68 MPa, without impairing the wood surface. Measurements were conducted across various moisture contents up to the fiber saturation point (FSP) for the three anatomical directions at a temperature of 21°C. The regression parameters between electrical resistivity and moisture content were determined with high coefficients of determination (R² = 0,97–0.98). The effect of MC on the electrical resistance was significantly more pronounced in the radial direction, and the electrical resistance in the axial direction was lower than in the radial and tangential direction. Those findings support the basic understanding of the electrical properties of spruce wood and provide a basis for non-invasive volume resistivity measurements of wood.

Acetylation of Aspen and Alder Wood - Preliminary Tests

An experimental study of the physical and mechanical properties of untreated and acetylated wood was conducted. The effect of acetylation on wood density was investigated. It was established that the density of the samples generally decreases after acetylation. As the level of acetylation increases, the fiber saturation point decreases in both conifers and hardwoods. Acetylation of wood helps to reduce the sorption properties of wood. The amount of swelling was analyzed. For untreated wood (aspen and alder), the volume swelling index is at the level of 7.5 %. Acetylation contributes to the stability of the geometric dimensions of structures, as the volume swelling index for aspen decreased by 4 times, for alder – by 2 times. An increase in the amount of swelling along the fibers is observed in all samples. Regarding the nature of the destruction of the samples during compression, the aspen (both untreated and acetylated) only crumples without visible signs of destruction. In some samples of alder, partial exfoliation is visible. It was found that acetylation has an ambiguous effect on the mechanical properties of wood of both species.

The Adhesion Performance in Green-Glued Finger Joints Using Different Wood Ring Orientations

Structural lumber is designed to meet the technical standards that ensure safety, cost-effectiveness, and sustainability. However, some tree species face limitations in their growth, which restricts their widespread use. An example of this is Nothofagus alpina, which has excellent mechanical properties but is not utilized much due to the challenges in extracting its timber and poor utilization, mainly because of the length of the wood. There is little information concerned with the uses and better management of small pieces using Nothofagus species, but it is still insufficient. This study investigates the adhesion performance of green-glued finger joints with varying wood ring orientations and moisture contents ranging from 21% to 25% using Nothofagus alpina. The primary aim is to assess how ring orientation and wet timber affect the green gluing process for creating larger wood pieces than sawn wood. The resulting products could meet the standards for wood serviceability number three for native Chilean wood. The findings indicate that finger joint performance improves with higher timber moisture levels. However, the orientation of the wood fibers did not significantly affect the performance under the tested conditions. It is important to note that this effect may become more significant near the fiber saturation point. These findings emphasize the need for a detailed protocol on the green gluing technique for Nothofagus alpina and the associated drying and surface processes in finger joint construction.

Moisture sorption and its induced deformation of juvenile wood at different radial positions in poplar wood at the tissue scale

Juvenile wood exhibits significant variations in its properties across the radial direction, which responds strongly to water and can impact its applications in various scenarios. Wetwood, a specific presence in juvenile wood, is more prevalent in certain species. In this study, we focused on poplar (Populus x euramericana cv. 'San Martino') and analyzed the moisture content (M) and hygroscopic deformation of juvenile wood near the pith (wetwood) and in juvenile wood near the bark (normal wood) in real-time at the tissue scale (earlywood and latewood). To achieve this, we employed a dynamic vapor sorption analyzer in combination with a Dino-Lite Edge digital microscope. By utilizing the Guggenheim-Andersen-de Boer (GAB) model and the Kelvin equation, we examined the relationship between the bound water and hygroscopic deformation. The results revealed that the M in green condition of wetwood was significantly higher than that of normal wood. However, the disparity in moisture sorption capacity between wetwood and normal wood below the fiber saturation point contributed only minimally to this phenomenon. Regarding for hygroscopic deformation, it was observed that not all wetwood samples exhibited a greater shrinkage/swelling compared to normal wood samples. This variation can be attributed to structural factors, such as differences in microfibril angle, crystallinity, and cell wall ratios. Furthermore, it was found that the swelling ratio caused by 1 % of bound water (SwR-B) increased with rising relative humidity (RH) within the range of 0–90 % RH. However, the SwR-B stabilized or slightly decreased in the range of 90 %-95 % RH. This behavior is believed to be linked to the formation of capillary condensed water at higher humidity levels. Additionally, we found that the swelling ratio resulting from 1 % of polylayer water exceeded that caused by 1 % of monolayer water.

Preparation and drying characteristics of poplar wood modified by hexacyclite

In recent decades, wood drying has emerged as an important method to ensure wood quality and improve wood utilization. However, the addition of modifiers to wood can hinder the transfer of moisture, making the drying process difficult. Therefore, the drying of modified wood is challenging. This study focuses on poplar wood and reports the preparation of modified wood capable of releasing health-beneficial negative oxygen ions. In particular, this study examined the effect of hexacyclite on the transfer of internal moisture and heat in wood. Hexacyclite was dispersed using sodium hexametaphosphate as a dispersing agent, and the modified poplar wood was prepared via vacuum pressure impregnation. The process involved six cycles of the following treatments: a vacuum treatment (−0.08 MPa, 20 min), followed by a pressure treatment (1 MPa, 120 min), and a final atmospheric treatment (5 min). The modified wood was dried via kiln drying, during which the overall wood temperature rapidly increased, with the total drying rate also increasing by 1.05%/h. The introduction of hexacyclite potentially altered the way modified wood bound with moisture, leading to permanent changes in the porosity of cell walls and the structure of polymers and thus altering the movement of water molecules. The modified wood continuously emitted far-infrared rays with a wavelength of 8–15 μm. In some large water clusters, after the far-infrared energy was absorbed, the movement of water molecules accelerated, thereby increasing the drying rate in the wood below the fiber saturation point. Ultimately, this resulted in the modified wood drying faster and thus exhibiting a better quality.

Elasto-plastic material model of green beech wood

Physically modelling the mechanical response of a tree by numerical simulation depends on having accurate data on the mechanical properties of green hardwood. Lacking such data, we developed and validated an orthotropic elasto-plastic (E–P) material model, based on the results of experiments performed on European beech (Fagus sylvatica L.) green wood, capable of including both the non-linearity and orthotropic properties of the material. We selected 655 clear samples with the special orthotropic structure of annual rings. All samples were prepared immediately after felling; their moisture content (MC) was 80% on average. The mechanical responses in normal directions and shear are represented by bi-linear stress–strain curves. The E–P model was validated by comparing the force–deflection response of three-point bending of green wood samples in a finite-element method (FEM) simulation (the average relative error was 4.6% for point-wise and 1.7% for integral-wise comparison). The output of this work was a consistent set of material constants for the E–P material model that is now available for the structural analysis of beech wood with MC above to fibre saturation point (FSP), especially green wood, subjected to relatively high loads (such that a plastic deformation appears) and that can very well predict a non-linear response above the proportional limits.

The effect of temperature and moisture content on drilling resistance measurements in wood (Pinus sylvestris L.)

ABSTRACT Drilling resistance (DR) measurements is widely used for nondestructive testing and evaluation of wood in growing trees and timber structures. A DR method can be applied in the regions with a cold climate and in wood with temperatures below zero degrees Celsius and varied moisture content (MC), which raises a question of correct interpretation of DR profiles and prediction of wood properties. Laboratory tests were conducted on Scots pine specimens with four levels of MC (7.4%, 11.0%, 23.5%, 147.9%) and temperature (+20°С, +2°С, −2°С, −10°С) variations. In-field tests were made on 33 growing trees at +18°С and −15°С. An IML-RESI PD 400 tool with a fixed rotational frequency of 2500 rpm and a feed rate of 1.5 m·min−1 was used. In most cases, wood temperature and MC had a significant influence on DR, but differences in an absolute value could be slight. A drastic increase in DR and feeding resistance was observed for water-saturated specimens tested at −10°C. For structural health assessment of timber structures at temperatures below 0°C, it is recommended to control wood MС. The effect of MC on DR was pronounced for groups of specimens with temperatures below and above the fibre saturation point. The in-field tests confirmed an increase in DR for frozen wood that can be used for studying water-conducting layers in growing trees.

Effects of the ice formation and its content on viscoelastic characteristics of wood exposure to low temperatures ranging from −125°C to 25°C

ABSTRACT The influence of ice formation and its content on the viscoelastic characteristics of wood were investigated in the temperature range between −125°C and 25°C. The stiffness and damping properties of the longitudinal (L), radial (R) and tangential (T) specimens were determined for different moisture content (MC) levels at or above the fiber saturation point (FSP) ranging from 25% to 300%. Results showed that the E′ value was correlated negatively with temperature for all specimens. Accompanied by the frozen free water inside the wood, the order of magnitude in E′ for specimens in the different anatomical directions trend toward an accord. The increment in E′ (ΔE′) of specimens was correlated positively with the ice content, and ΔE′ in the L direction was lower than those in the R and T directions at each MC level. The γ-relaxation ranged from −112.9°C to −94.8°C was observed for all orthotropic directions at 10 Hz, which was assigned to the reorientation of methylol groups and adsorbed water molecules in the wood cell walls. At each MC level, a lower peak temperature of γ-relaxation was observed in the L specimens compared to the transverse specimens. Notably, the γ-relaxation peak temperature in the E″ and tanδ spectrum showed obvious frequency-dependence, and the apparent activation energy (ΔH) of γ-relaxation over the measured MC range was about 38.08∼69.57 KJ/mol. The ΔH value was close for specimens in orthotropic grain orientations, indicating that this γ-relaxation phenomenon was not affected by its orthotropic structure and also related closely with the formation of ice in the wood cell lumens.

Moisture changes and surface checking development during drying of Masson pine wood

Deformation and surface-checking defects in wood are closely related to the drying stress caused by shrinkage and moisture content (MC) gradients throughout the drying process. In this study, 50-mm-thick Masson pine (Pinus massoniana Lamb.) lumber was used as the research object. The drying stress caused by shrinkage behavior and MC changes was monitored, and the development of surface checks was observed during the drying process. Results showed that when the average MC was 60%, the free water in the surface layer of lumber evaporated completely, causing the MC to decrease below the fiber saturation point (FSP). The surface layer was subjected to tension stresses, resulting in surface checks which primarily distributed in the latewood area and propagated longitudinally. When the average MC was between 30% and 40%, the combined effects of the high MC gradient and anisotropic shrinkage of the wood resulted in further expansion of the checks. As the average MC decreased to less than 20%, the MC of the core layer lumber decreased below the FSP, resulting in the closure or even disappearance of most surface checks. The critical point (average MC of 60%) of drying stress was obtained, which can benefit the fine-tuning of kiln-drying schedules and improve the drying quality of Masson pine wood.

Cell wall water induced dimensional changes of beech and pine wood

Hygroexpansion in timber construction is a phenomenon that occurs as a result of changes in the cell wall structure of wood caused by water. In this study, beech (Fagus sylvatica) and pine (Pinus taeda L.) wood were conditioned at different relative humidities with saturated salt solutions and saturated through vacuum pressure impregnation. The moisture content and dimensions of the samples were evaluated under various adsorption equilibrium states, as well as during drying from water saturation to an air-dried state. The results obtained using the two-dimensional time-domain nuclear magnetic resonance technique revealed two distinct types of cell wall water in both wood species, designated as B-water and C-water, as they displayed different mobilities. With increasing moisture content, the mobility of cell wall water increased; however, B-water maintained a constant level of mobility and formed water clusters when it accumulated in the high humidity region and at slightly above the fiber saturation point (FSP), which is evidence of the occurrence of free-bound water. The B-water, located in the amorphous region of microfibrils and their surrounding hemicellulose, exerted a predominant influence on the dimensional changes of the wood both under equilibrium and nonequilibrium conditions, in comparison to the C-water which located in the matrix of hemicellulose and lignin.

Adhesive strength and micromechanics of wood bonded at different moisture contents

The adhesive bonding of wood is preferably carried out at levels of moisture content close to those expected during service. In this study, the influence of non-ideal wood moisture content (MC) during adhesive bonding is investigated, using lap shear testing, nanoindentation and microscopy. Spruce lamellas were bonded at 5, 12, 25% MC and largely above fiber saturation point (∼55% MC) with melamine-urea-formaldehyde (MUF) and one-component polyurethane adhesive (1C PUR). Tensile shear strength decreased with increasing MC for both adhesive systems, even though the specimens were given ample time to post-cure, as testing was at standard MC 12%. Particularly above FSP, with the presence of free water, tensile shear strength was significantly reduced. For MUF, this is consistent with reduced mechanical properties measured with nanoindentation presuming that excessive moisture hindered adhesive cure. For 1C PUR, micromechanical properties of the bulk adhesive were not significantly affected by MC, the influencing factor was rather insufficient adhesion to the wood surface due to free water. The results propose the applicability of the adhesive beyond the recommended moisture ranges, but advise caution when bonding wood above FSP without process adjustments.

Dimensional Stability and Mechanical Properties of Gmelina arborea Roxb. Wood Thermally Modified through Open Reactor and Low-Pressure Closed Reactor Systems

This study focused on the thermal modification of Gmelina arborea Roxb. wood following processes using the open reactor and low-pressure closed reactor systems. The aim is to determine the optimum treatment conditions suitable for gmelina wood due to its poor drying characteristics using the low-pressure closed reactor thermal modification. Subsequent to thermal modification under both processes, the dimensional stability and mechanical properties of gmelina wood were investigated. Effects of the thermal modifications under the open and low-pressure closed reactor systems on mechanical properties were additionally reported. The outcome of this investigation revealed that mass loss increased with increasing treatment temperatures, but minimal mass losses were observed for samples modified in the low-pressure closed reactor system. Due to the low-pressure regime used in the closed reactor system, a lesser improvement was found in volumetric shrinkage, fibre saturation point and tangential-to-radial swelling compared to the improvement in these properties in the open reactor system. Results further revealed that the mechanical properties of gmelina wood deteriorated more rapidly after modification in the open reactor system. Since the properties of modified gmelina wood are comparable at 180 °C under both systems, the closed reactor system will be investigated further to arrive at a suitable treatment condition under higher pressure variations. The thermal modification of gmelina wood with the closed reactor system is more promising in delivering a better quality of modified gmelina wood.

The effect of moisture content over the fibre saturation points on the impact strength of wood.

The article's main aim is to assess the mechanical behaviour of linden under high-rate loadings (impact) and its change due to changes in moisture content (MC) over fibre saturation point. For assessing the mechanical properties of green wood, mainly the data of the dried wood is not applicable since the moisture content can drastically affect the mechanical properties of the wood. By testing both dried and high-moisture-content wood, we can understand a general viewpoint toward the effect of the moisture content on the impact behaviour of the wood. Several test samples were made of linden wood with different moisture content levels of 11%, 60% and 160%. A drop-weight impact machine tested the specimens to measure the reaction force of the hammer during a very short impact period. The results of the tests were parameters such as force-time chart, the maximum force required for crack initiation, the impact bending strength (IBS) and the work needed for crack initiation. The results indicated an increase in MC decreases the maximum force, work required for crack initiation and IBS drastically. However, when MC exceeded the fibre saturation point (FSP), there was no further influence on the force pattern and maximum required force.

Critical analysis of the mechanical properties of Dinizia excelsa wood in relation to moisture variations: implications for ABNT NBR 7190:2022

Brazil possesses an extensive forested territory conducive to the prevalence of numerous tree species, including Dinizia excelsa, commonly known as Angelim-Vermelho. The utilization of a wood species in a structural system necessitates a comprehensive analysis of its physical-mechanical properties. In this context, a crucial variable for study is the moisture content, the variations of which, a priori, can significantly impact wood properties. Although the Brazilian code prescribes specific models to adjust strength and stiffness concerning the reference moisture, allowing a linear increase as moisture content decreases, this phenomenon may not universally apply to all wood species. Some species may exhibit statistically constant or even decreased values, contradicting code predictions. Therefore, by employing analysis of variance (ANOVA) at a 5% significance level, this research aims to examine the impact of moisture content variation (ranging from 12% to the fiber saturation point – FSP) on 12 mechanical properties of Dinizia excelsa wood. Each type of test involved the creation of 12 specimens, totaling 360 determinations. Among the 12 mechanical properties examined, four showed no significant variations between 12% moisture and the FSP. This suggests that the code formulation inadequately addresses all properties for all wood species. Furthermore, the study revealed that correction models can result in strength values higher than experimentally obtained, thus overestimating structural capacities, and compromising safety. Therefore, a critical reassessment of the existing Brazilian code is imperative.

Effect of stacking position on drying rate during radio-frequency vacuum combined with mechanical press drying of Douglas-fir and Radiata pine

In this study, the effect of stacking position in the dryer on the drying rate during radio-frequency vacuum combined with mechanical press (RF/VP) drying was investigated for Douglas-fir and Radiata pine. Drying test was performed at a temperature of 40 °C using RF/VP dryer with approximately 3 m3. The results showed that the stacking position has a significant effect on the drying rate, with a more pronounced decrease in drying rate as the stacking position of specimens approaches the ground plate. Previous moisture content was an important factor in determining the drying rate in both species. The MC distribution of specimens in the dryer before drying was very irregular, but over time, specimens stacked on the RF charge plate side in the dryer tended to have lower MC distributions than those stacked on the ground plate side, which was serious at the specimens in contact with the upper ground plate. In addition, as the MC decreases below the fiber saturation point during RF/VP drying, the difference in MC distribution within the dryer according to the stacking position became clearer. These findings provide new insights into the effect of stacking position on drying rate during the RF/VP drying and are expected to improve RF/VP drying technology.

Environmental and ontogeny effects in wood fiber properties of Pterocarpus ericaneus (Poir.)

ABSTRACT The present study examined the wood properties of Pterocarpus erinaceus, a species found in West Africa's Guineo-Sudanian and Sudano-Sahelian zones. The primarily focus was on wood fiber properties, which serve as indicators of cambial growth and the transition between juvenile and mature wood. The aim was to analyze the factors contributing to the variability of the wood fiber properties. Specifically, it examined the effect of cambial age and ecological factors on fiber properties and analyzed how the fiber properties influence density and dimensional characteristics of the wood. Measurements of fiber properties were obtained from 451 specimens collected in the Guinean, Sudanian, and Sahelian zones in Burkina Faso, Niger, and Togo. The results indicate significant variations in average fiber parameters among the three climatic zones. This variability leads to an increase in the length, width, and thickness of the fibers along the north–south decreasing rainfall gradient. However, no noteworthy correlations were observed between cambial age and fiber properties, posing challenges in distinguishing between juvenile and adult wood. In terms of correlation, the analysis shows that fiber length has no effect on physical properties, but fiber width significantly influences basic density, wood shrinkage, and the fiber saturation point (FSP).

Phase transition behavior of water in original, heat-treated and acetylated poplar woods

Heat and acetylation modifications are often required to increase the added values of planted poplar woods. The phase transition behavior of water in modified woods is essential in predicting the physical, mechanical, and biodegradable properties of modified wood products but has not been fully explored. In the present study, the differential scanning calorimeter (DSC) was used to analyze the liquid-solid phase transition behavior of water in original, acetylated, and heat-treated poplar wood samples. The effects of moisture content (MC), different treatments (heat and acetylation treatments), and sample size (solid vs. powder) on the phase transition behavior were investigated in detail. The freezing and melting of water were clearly observed from peaks that occurred in the range of − 85–25 °C at a heating or cooling rate of 5 °C/min. Four different peak types with different shapes and positions were defined to characterize the free and bound water phase transition behavior in wood. More peak details were revealed from solid samples. Both freezing and melting temperatures of water in poplar wood samples were lowered with the decrease in MC, possibly resulting from the smaller pore size at lower MC. The modification methods, especially acetylation, reduced the hygroscopicity and associated fiber saturation point (FSP) of poplar wood samples remarkably, which further affected the bound water phase transition behavior of modified wood significantly. The freezing bound water in modified wood seemed to be more stabilized when subjected to cooling, and they tended to melt at higher temperatures.

Supercritical CO2 drying of New Zealand red beech to below the fibre saturation point reduces collapse distortion

Supercritical CO2 offers an alternative method of removing wood moisture and reducing cellular collapse compared to traditional drying techniques. The technique has been previously demonstrated for Pinus radiata and Eucalyptus nitens dewatering and was modified in this study for New Zealand red beech (Nothofagus fusca) heartwood, which is notoriously difficult to dry without causing excessive distortion. The technique was also successfully extended to drying below the fibre saturation point. A specific dewatering and drying schedule was developed for N. fusca because of negligible dewatering using a schedule previously designed for wood with an open hydrofluidic network of interconnected vessels. An anatomical assessment confirmed lumen pathways were occluded with tyloses and polyphenol resins. A fluid dynamics assessment concluded that permeability measurements are recommended together with tortuosity and porosity information for improved wood species dewatering characterisation. Using the dewatering and drying schedule, collapse was successfully reduced by 92% for both normalised internal wood area and void collapse when compared to oven-dried samples. The beech specimens took 18 days to reach 17.3% moisture content (MC) but displayed some checking from early dewatering depressurisation, compared to air-dried control specimens which showed no collapse or checking but took 6 months to reach 12% MC. Supercritical CO2 dewatering and drying could be combined with extractives separation, preservative treatment, and mechanical forming of wood in one plant to make a potentially economically viable process with improved energy, environmental and carbon footprints. A techno-economic analysis is suggested to fully compare supercritical drying of wood against conventional drying operations.

Modelisation of desorption isotherms and estimation of the thermophysic and thermodynamic properties of tropical woods in Cameroon: The case of Ayous and Ebony woods

A review at various models illustrates the necessary of choosing a model that can best describe each wood in relation to its isotherms of desorption. The theoretical models enable us to deduce several thermo physical parameters. The models of Dent and G.A.B are quite appropriate in the evaluation of the isotherms of desorption of Ayous and Ebony woods taking the humidity of the atmosphere air into consideration. The parameters of Dent’s model vary favourably with regard to the theoretical precisions. The level of humidity at fiber saturation points are at 0.2944 for Ayous wood and 0.19141 for Ebony wood. The specific surface area occupied by the woods under study decreases when there is a consequent increase in temperature. The specific surface area of Ayous wood ranges between 295 and 162 m2/g whereas that of Ebony ranges between 183 and 96 m2/g, while the wood temperatures vary between 20 and 60°C. It is realised that, there is a powerful relationship existing between the primary hydrophiles of Ebony and the first layer of mater molecules. The desorption heat of multi layers of our woods are superior to latent heat condensation of pure water. The heat condensation of water vapour constitutes a specific given which is superior to the heat isosteric adsorption of the woods in question. The soret effect is more important in the drying of Ayous wood than that of Ebony. However, temperature does not influence the soret effect for Ebony wood, relative air humidity is less than 0.85. This limits Ayous wood at 0.9. Above these limits soret effect increases when there is an increase in atmospheric air temperature.