Influence of the pore structure of wood on moisture desorption at high relative humidities

Abstract

The influence of the porous structure on moisture desorption of two temperate and five tropical hardwoods was studied. Two experimental techniques were used to perform moisture desorption tests from full saturation at 25°C. The first one was the saturated salt solutions [between 33% and 90% relative humidity (RH)] and the second one was the pressure membrane method (above 96% RH). More emphasis was given to results obtained at high RH, given that sorption in that case is mainly governed by the capillary forces. The porous structure of these hardwoods was characterized by mercury intrusion porosimetry (MIP) and by quantitative anatomical analysis. The results showed that desorption of liquid water was very different among the hardwood species. The MIP technique appeared as an important tool to evaluate the fluid paths within wood, which permitted the prediction of water behavior in wood during drainage from full saturation at high RH. Quantitative anatomical results were very useful for explaining the first steps of drainage and mercury penetration in wood.     

Influence of the pore structure of wood on moisture desorption at high relative humidities

The influence of the porous structure on moisture desorption of two temperate and five tropical hardwoods was studied. Two experimental techniques were used to perform moisture desorption tests from full saturation at 25°C. The first one was the saturated salt solutions [between 33% and 90% relative humidity (RH)] and the second one was the pressure membrane method (above 96% RH). More emphasis was given to results obtained at high RH, given that sorption in that case is mainly governed by the capillary forces. The porous structure of these hardwoods was characterized by mercury intrusion porosimetry (MIP) and by quantitative anatomical analysis. The results showed that desorption of liquid water was very different among the hardwood species. The MIP technique appeared as an important tool to evaluate the fluid paths within wood, which permitted the prediction of water behavior in wood during drainage from full saturation at high RH. Quantitative anatomical results were very useful for explaining the first steps of drainage and mercury penetration in wood.     

压缩空气微爆破预处理对橡胶木单板 染色效果的影响

为了提高木材的渗透性,改善木材染色效果,采用压缩空气微爆破进行橡胶木单板预处理,探究微爆破压力、 处理次数对单板染色效果的影响;采用电镜和压汞分析法,表征处理前后单板表面形貌及微观孔隙。结果表明,处 理压力对单板上染率和表面色深影响显著,在优化工艺条件下（压力0.4 MPa、爆破4 次）,上染率和表面色深值 分别提高22.76%和19.69%;处理后木材的孔隙率和平均孔径均明显增大,比表面积略大,有利于染料的渗透。     

Effects of impregnation and heat treatment on the physical and mechanical properties of Scots pine (Pinus sylvestris) wood

Wood modification, of which thermal modification is one of the best-known methods, offers possible improvement in wood properties without imposing undue strain on the environment. This study investigates improvement of the properties of heat-treated solid wood. Scots pine (Pinus sylvestris) was modified in two stages: impregnation with modifiers followed by heat treatment at different temperatures. The impregnation was done with water glass, melamine, silicone, and tall oil. The heat treatment was performed at the temperatures of 180°C and 212°C for three hours. The modified samples were analyzed using performance indicators and scanning electron microscope micrographs. The mechanical and physical properties were determined with water absorption, swelling, bending strength, and impact strength tests. All the modifiers penetrated better into sapwood than hardwood; however, there were significant differences in the impregnation behavior of the modifiers. As regards the effect of heat treatment, generally the moisture properties were improved and mechanical strengths impaired with increasing treatment temperature. In contrast to previous studies, the bending strength increased after melamine impregnation and mild heat treatment. It is concluded that the properties of impregnated wood can be enhanced by moderate heat treatment.     

Effect of nanofillers on flame retardancy, chemical resistance, antibacterial properties and biodegradation of wood/styrene acrylonitrile co-polymer composites

Wood polymer nanocomposites (WPNCs) based on simul wood (Bombex ceiba, L.) were prepared by impregnation of styrene acrylonitrile copolymer, γ-methacryloyloxy trimethyl silane-modified TiO₂, SiO₂ nanoparticles and nanoclay intercalating mixture through vacuum impregnation. The impact of nanofillers on the physical properties, flame retardancy, water resistance, anti-swelling efficiency and biodegradability of the resultant WPNCs was investigated. Remarkable enhancement in wood properties such as flame retardancy, water resistance and anti-swelling efficiency was achieved with the treatment. The results showed that all the properties were maximum for wood samples treated with SAN/TiO₂ (0.5 %)/SiO₂ (0.5 %)/nanoclay (0.5 %). The presence of TiO₂ nanoparticles in WPNC exhibited antibacterial activity. The resistance to biodegradation was observed by incorporation of nanofillers into wood.     

Nonisothermal moisture movement in wood

In order to analyze the effect of temperature gradient on moisture movement during highly intensive drying, such as microwave-vacuum drying, the profile of the temperature and moisture content in sealed wood whose opposite faces were subjected to temperature gradient for a short time was measured. The ratio of the moisture content (MC) gradient to the temperature gradient (dM/dT) was calculated and the factors influencing moisture movement under nonisothermal conditions were discussed. The results indicate that moisture moved in wood from the warm surface to the cold one even if opposite faces of the sealed wood assembly were exposed continuously to different but constant temperatures for a short period. The moisture content on the cold surface was higher than that on the warm surface. The moisture content gradient opposite to the temperature gradient was established, and the dM/dT was below 0.9%/°C. The temperature in the sample and the distance from the hot surface of the sample was strongly linearly correlated. With an increase in temperature, initial moisture content and experimental time, the dM/dT was significantly increased. __________ Translated from Journal of Beijing Forestry University, 2005, 27(2): 96–100 [译自: 北京林业大学学报, 2005, 27(2): 96–100]     

Urea-formaldehyde microspheres as a potential additive to wood adhesive

Urea formaldehyde (UF) resins are important for wood industry due to their attractive properties at reasonable price. Particulate fillers added to UF are of interest with regard to improving the functionality of UF and also in terms of reduced UF consumption. To study their potential as filler, solid UF microspheres were synthesized and characterised respecting its morphology, chemical curing and thermal stability. Marigold flower structured spheres with diameters between 5 and 20 µm are presented and application trials demonstrated that high amounts of UF may be replaced by solid microspheres without impairing adhesive bond strength of solid wood bond lines. Fluorescence microscopy showed that microspheres greatly reduce adhesive penetration into the wood substrate, retaining the modified adhesive in the bondline. UF microspheres may thus be considered as viable filler for UF adhesives, particularly with regard to the possibility of endowing them with added functionality like self-healing properties.     

Methods for the determination of wood properties,Kraft pulp yield and wood fibre dimensions on small wood samples.

A trial set-up with methods for sampling, treatment and analysis of small wood chips are presented in this paper, to determine important wood and fibre properties, like basic density, dry density, volume swelling of wood, Kraft pulp yield, fibre length, fibre coarseness, fibre width, lumen width and fibre wall thickness. The required time for one sample is about 1.5 man-hour, but this requires relatively larger series and trained personnel. Acceptable measurement accuracy is achieved when the volume of the wood sample is at least 1.5 ml, except that of wood volume swelling. To gain acceptable measurement accuracy for volume swelling, the wood volume should be increased to at least 3 ml, and preferably more than 5–6 ml per sample. The level of pulp yield and wood density do not show a significant effect on the measurement accuracy for fibre cross-section dimensions. Fibre coarseness, on the other hand, has a significant influence on these accuracies. A double measurement of fibre coarseness will improve the accuracy to an acceptable level. The method presented here may, together with information about trees and growth locations, form the basis for greater insight into the mechanisms involved in development of wood and fibre properties in trees, which in turn may provide better control and utilisation of wood for pulp and paper production.Abbreviations CWD cell wall density in dry wood=1500 kg/m³ - Ww dry weight of wood (kg) - Vmax green (wet) volume of wood (m³) - Vmin dry volume of wood (m³) - BD basic wood density (kg/m³) - DD dry wood density (kg/m³) - VS maximum volume swelling of wood (%) - Wp dry weight of pulp (kg) - PY pulp yield (%) - C fibre coarseness, the average weight of a unit length of fibre (g/m) - CL average chip length (mm) - CWT average cell wall thickness (m) - FW average fibre width (m) - l average native fibre length in solid wood - L chip length - lc average fibre length in wood chip (mm) - Lc length-weighted fibre length in wood chip (mm) - lw native average fibre length in wood (mm) - Lw native length-weighted fibre length in wood (mm) - LW average lumen width (m) - llw average native fibre length, length weighted, in wood - X average fibre length in chip - Xlw average fibre length, length weighted, in chip     

Energy consumption of two-stage fine grinding of Douglas-fir wood

Fine wood powders have advantages over traditional coarse wood particles for various emerging applications. However, an efficient system to produce fine wood powders has not been well established. We investigated the comminution capability and efficiency of a two-stage grinding system consisting of a hammer mill circuit and an rotor impact mill circuit to convert wood feedstocks into fine powders. Air-dried forest harvest residuals were comminuted by the hammer mill circuit to three intermediate product sizes with geometric mean particle sizes of 1618, 669, and 316 µm. These intermediate products were then pulverized into fine wood powders with median particle sizes ranging from 35 to 250 µm. The specific energy consumption increased with the decrease of median particle sizes, with a transition at around 100 µm after which the energy consumption increased exponentially. This large-scale grinding trial provides the reliable energy consumption data for design and process economic analysis of mechanical biomass preprocessing.     

New cell wall and cell lumen wood polymer composites

Summary Furfuryl alcohol-based (FA) cell wall and methyl methacrylate-based (MMA) combination formulations were used to make wood polymer composites (WPC). Swelling gradients developed during preparation. Properties (density, hardness, water extractables, antiswell efficiency) gradients were observed along samples in 5 of the 7 treating formulations. Two cell wall treatments based on furfuryl alcohol did not show the gradients. The results suggest treating solution separations occurred during impregnation and the resulting nonhomogenious chemical produced the properties gradients. WPC made using the combination formulations based on MMA had properties which fell between cell wall and cell lumen formulations.It is gratefully acknowledged that Mr. Michel Andrew, Student Assistant, carried out the experiment part of this study     

Effects of borate impregnation on the response of wood strength to heat treatment

Based on the strong correlation between acidity and thermal degradation in wood reported in previous studies, the effect of borate impregnation as an alkali-buffering medium was investigated on the strength properties of thermally modified wood. Wood samples were impregnated with 0.1 M Sodium borate solution (pH=9) before they were subjected to heat treatment at temperatures of 180°C and 200°C for durations of 2 and 4 h. The borate impregnation results in some reductions in the severity of strength loss during heat treatment and this is invariably due to buffering effect of the alkali on the acidity of wood, which could have mitigated the degree of degradation. The positive effects of borate impregnation as a pretreatment on the strength properties of heat-treated wood depend on the degree of heat treatment. Hence, the use of borate impregnation as a pretreatment method for heat treatment is recommended only where a relatively mild heat treatment is involved.     

Moisture storage and transport properties of preservative treated and untreated southern pine wood

Moisture storage and transport properties of southern pine (Pinus spp.) wood were measured for implementation into hygrothermal models. Specimens were untreated or pressure-treated with alkaline copper quaternary (ACQ) preservative. Moisture storage was characterized with sorption isotherms in the hygroscopic region (high capillary pressures) and documented with mercury intrusion porosimetry in the overhygroscopic region (low capillary pressures). The data were then combined into a single moisture retention curve as a function of capillary pressure. Moisture transport was evaluated from steady-state water vapor transmission and dynamic capillary water absorption experiments. These data were used to calculate the moisture permeability over the entire range of capillary pressures using the diffusivity approach of Carmeliet et al. Moisture storage and transport properties were similar for the untreated and ACQ-treated southern pine, except for the permeability of the treated wood which was lower in the radial direction. The data presented here can be used to improve the accuracy of hygrothermal and combined hygrothermal–corrosion modeling simulations.     

Dimensional stabilization of wood treated with furfuryl alcohol catalysed by borates

Furfuryl alcohol has a sufficiently small molecular size, which allows impregnation into wood and can be polymerized in the cell walls. Polymerization can be accomplished using a catalyst, heat or penetrating radiation. This research illustrates a natural wood polymer composite (NWPC) that was obtained under the catalytic effect of borates. Furfuryl alcohol (FA) and borates were mixed at different ratios before treatment. When borates alone are used, they were totally leached from wood after cyclic leaching periods. However, this was not encountered with the mixture of furfuryl alcohol and borates. Furfuryl alcohol considerably improved antiswelling efficiency by about 85% for both Japanese cedar ( Cryptomeria japonica) and Scots pine ( Pinus sylvestris L.) wood specimens. Results of the leaching tests of borate indicated that boric acid readily lost its boron content in the early cycles of the leaching periods and the highest leaching rate of boron was observed at the first leaching cycle (512 ppm). When furfuryl alcohol mixed with borates, boron was released to the leaching water at slower amounts, suggesting the possibility of longer protection of treated wood in service.     

Studies on pre-treatment by compression for wood impregnation I: effects of compression ratio,compression direction,compression speed and compression-unloading place on the liquid impregnation of wood

To improve the impregnation of wood, the pre-treatment by compression was systematically studied in terms of effects of compression ratio, compression direction, compression speed and compression-unloading place on the liquid impregnation in poplar and Chinese fir. The results showed: the impregnation increased 0.0065 or 0.0074 g/cm³ for every 1% increase of compression ratio when the compression ratio was lower or equal to 50 and 40% for poplar and Chinese fir, respectively; it continued to increase afterwards while the variation was quite big. There existed a significant difference of the impregnation of wood compressed at different directions in Chinese fir, but not in poplar. There existed a significant difference of the impregnation of wood compressed at different speed in both species. The impregnation of wood is likely to be in favor of radial compression in terms of the amount of impregnation. 5 and 10 mm/min were recommended as a compromise of impregnation and pre-treatment efficiency. The impregnation of wood that the compression unloaded in water was about 18.2 (poplar) and 9.2% (Chinese fir) higher in amount and was much quicker in speed than that the compression unloaded in air, and the difference between them was significant, suggesting that compression unloaded in water is significant to improve the impregnation.     

Improved interaction between wood and synthetic polymers in wood/polymer composites

Summary This article describes the properties of wood polymer composites consisting of linear low density polyethylene (LLDPE) and wood flour (WF). In an attempt to improve the interfacial adhesion between the matrix and the filler, different compatibilizers were used. The interaction between polymer and wood were studied by comparing LLDPE/WF composites with composites when compatibilizer was added. The experimental measurements were conducted by impact and tensile strength testing and Scanning Electron Microscopy (SEM). The mechanical properties of the composites were improved with SEBS triblock copolymer modified with maleic anhydride and with the ionomer polymer, Surlyn, as compatibilizers. SEM fractography confirmed better adhesion between wood particles and LLDPE matrix when SEBS was present.This study was financed by the Swedish National Board for Industrial and Technical Development (NUTEK) which is gratefully acknowledged     

The calculation of EMC for the analysis of wood/water relations in Fagus sylvatica L. modified with 1,3-dimethylol-4,5-dihydroxyethyleneurea

Beech wood (Fagus sylvatica L.) was modified with 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU). The equilibrium moisture content (EMC) of wood modified with DMDHEU calculated on a dry modified basis is reduced. Previous results have shown that the modification with DMDHEU does not alter the capillary condensation; therefore, the reduction in EMC seems exaggerated. The equilibrium constants of the Hailwood–Horrobin model (K _d and K _h) and the molecular weight of a hypothetical polymer of modified wood capable of adsorbing one molecule of water (W _i) were calculated from the EMC on a dry modified wood basis (M) and on a dry wood basis (M _R). The hypothetical polymer was also calculated by stoichiometry (W _c) and compared to W _i to estimate the number of operative OH groups. The number of operative OH groups decreased when M was used, in contradiction with the previously obtained results of differential heat of adsorption (∆H _s). Therefore, the use of M _R is recommended for the analysis of moisture sorption in wood modified with DMDHEU.     

Real-time wood moisture-content determination using dual-energy X-ray computed tomography scanning

ABSTRACT

The estimation of the pixel-wise distribution of the moisture content (MC) in wood using X-ray computed tomography (CT) requires two scans of the same wood specimen at different MCs, one of which is known. Image-processing algorithms are needed to compensate for the anisotropic distortion that wood undergoes as it dries. An alternative technique based on dual-energy CT (DECT) to determine MC in wood has been suggested by several authors. The purpose of the present study was to evaluate the hypothesis that DECT can be used for the determination of MC in real time. A method based on the use of the quotient between the linear attenuation coefficients (μ) at different acceleration voltages (the so-called quotient method) was used. A statistical model was created to estimate the MC in solid sapwood of Scots pine, Norway spruce and brittle willow. The results show a regression model with R²?>?0.97 that can predict the MC in these species with a RMSE of prediction of 0.07, 0.04 and 0.11 (MC in decimal format) respectively and at MC levels ranging from the green to the totally dry condition. Individual measurements of MC show an uncertainty of up to ±0.4. It is concluded that under the conditions prevailing in this study, and in studies referred to in this paper, it is not possible to measure MC with DECT.     

Effect of nano-SiO2 on properties of wood/polymer/clay nanocomposites

Wood polymer nanocomposites (WPNC) based on nano-SiO₂ were prepared by impregnation of styrene acrylonitrile copolymer (SAN), SiO₂ nanoparticles modified with γ-trimethoxy silyl propyl methacrylate (MSMA), and nanoclay into wood. The structure of modified SiO₂ nanoparticles and WPNC was characterized by Fourier transform infrared spectroscopy (FTIR). XRD analysis showed the delaminated structure of SAN/SiO₂/clay-treated wood composites. The synergistic effect of nano-SiO₂ and nanoclay was investigated. Thermal stability of SiO₂ nanoparticles decreased after modification, while that of wood treated with SAN, SiO₂, and nanoclay improved. Morphological characteristics were examined by scanning electron microscopy (SEM). Mechanical properties, water uptake (%), dimensional stability, hardness, and flammability were found to improve due to incorporation of SiO₂ and nanoclay into wood polymer composites. Maximum improvement in properties was observed in the wood polymer composites containing SiO₂ and nanoclay at the ratio of 1:1.     

Impregnation of Scots pine and beech with tannin solutions: effect of viscosity and wood anatomy in wood infiltration

The impregnation process of Scots pine and beech samples with tannin solutions was investigated. The two materials involved in the process (impregnation solution and wood samples) are studied in depth. Viscosity of mimosa tannin solutions and the anatomical aspect of beech and Scots pine were analysed and correlated. The viscosity of tannin solutions presents a non-newtonian behaviour when its pH level increases, and in the case of addition of hexamine as a hardener, the crosslinking of the flavonoids turns out to be of great importance. During the impregnation of Scots pine (Pinus sylvestris L.) and beech (Fagus sylvatica L.), the liquid and solid uptakes were monitored while taking into consideration the different conditions of the impregnation process. This method allowed to identify the best conditions needed in order to get a successful preservative uptake for each wooden substrate. The penetration mechanism within the wood of both species was revealed with the aid of a microscopic analysis. Scots pine is impregnated through the tracheids in the longitudinal direction and through parenchyma rays in the radial direction, whereas in beech, the penetration occurs almost completely through longitudinal vessels.     

Raman spectroscopic study on the behavior of boric acid in wood

 Raman spectra of Japanese cedar (Cryptomeria japonica D. Don) treated by vacuum impregnation with aqueous boric acid solutions (8.1 × 10⁻² to 7.29 × 10⁻¹ mol dm⁻³) were recorded using a near-infrared laser as an excitation source. Raman spectroscopic measurements were carried out on treated wood blocks of two sizes: 20(T) × 20(R) × 5(L) mm (A-type) and 15(T) × 15(R) × 50(L) mm (B-type). Our attention was focused on a prominent band (ν ₁) assigned to a symmetrical stretching vibration of the BO₃ group because no Raman band due to boron species was observed except bands of B(OH)₃. We observed a change in ν ₁ band intensity with increasing boric acid concentration in the aqueous solution used to treat the A-type wood blocks and investigated the correlation between the intensity and the peak-top wavenumber. Raman line maps in the longitudinal direction of the treated B-type wood blocks revealed that B(OH)₃ is concentrated near the cut ends. These results suggested that two groups of B(OH)₃ exist in wood in terms of the chemical species in the nearest neighbor sphere. Received: March 11, 2002 / Accepted: June 26, 2002