Impact of moisture content on dynamic mechanical properties and transition temperatures of wood

The dynamic shear modulus and the loss modulus of Betula alba, Ulmus parvifolia, Quercus robur, Acer platanoides, Tilia cordata, Fraxinus excelsior and Pinus sylvestris wood were measured using an inverted torsion pendulum within a wide temperature range. The glass transition temperature of the lignin–carbohydrate complex and the decomposition temperature of the wood cellulose were estimated. The temperature band from 170°C to 240°С shows the transition of the lignin–cellulose complex from the glassy to the rubbery state. Mechanical properties of different types of wood are affected by moisture and anatomical differences, but glass transition and decomposition temperatures are the same. More than 5% of moisture in the wood stored at normal conditions were found. After drying, the increase of dynamic shear modulus of wood over the entire region of the glassy state was observed. The intensity of maximum peak of dynamic loss modulus is also increased due to activation of the segmental motion of macromolecules of the ligno-carbon complex. The decomposition temperature of the cellulose crystallites was unchanged for specimens containing moisture and for dried specimens.     

Physical and mechanical properties of wood after moisture conditioning

Some properties of wood (hinoki:Chamaecyparis obtusa) moisture-conditioned by an adsorption process from a dry state and by two desorption processes (from a water-saturated state and from a state with a moisture content slightly below the fiber saturation point) were investigated. The moisture contents of wood conditioned by the adsorption process and by the desorption process continued to approach to one another for the moisture-conditioning period of over 50 weeks. Accordingly, sorption hysteresis should be regarded as a transitional phenomenon that occurs during the process of approaching the true equilibrium, which requires a long time. The wood conditioned by the desorption process beginning from a water-saturated state showed slightly smaller dimensions than those conditioned by the adsorption process with the same moisture content; however, the wood conditioned by the desorption process from a moisture content below the fiber saturation point showed slightly larger dimensions than those conditioned by the adsorption process. The wood conditioned by the adsorption process from a dry state showed a higher modulus of elasticity and modulus of rupture than did the wood conditioned from a water-saturated state with the same moisture content. The mechanical properties of the wood also varied based on the states at which the desorption process was started. This is a notable characteristic of the relation between the drying condition and the mechanical properties of wood.     

Influence of moisture content on the vibrational properties of hematoxylin-impregnated wood

The vibrational property of hematoxylinimpregnated wood was investigated from the aspect of moisture content dependence. The specific dynamic Young's modulus (E/) and loss tangent (tan) of hematoxylin-impregnated wood were determined in the relative humidity (RH) range of 0%–97%, and were compared with those of the untreated and some conventional chemically treated woods. The changes in theE/ and tan of wood with increasing RH were suppressed by acetylation and formaldehyde treatment because of a marked reduction in the hygroscopicity of the wood. Although the hematoxylin impregnation did not significantly affect the hygroscopicity of the wood, its influence onE/ and tan were similar to that of formaldehyde treatment at low RH and of acetylation at medium RH. It was supposed that at low to medium RH hematoxylin restrains the molecular motion of amorphous substances in the cell wall because of its bulkiness and rigidity. On the other hand, at high RH it seems to work as a plasticizer with adsorbed water molecules.     

Effect of moisture content on the ultrasonic acoustic properties of wood

We determined the ultrasonic velocity and en-ergy attenuation value of three tree species (basswood, elm, and fir) 1 per tree in different moisture content levels, using RSM-SY5 ultrasonic testing inst...     

Studies on pre-treatment by compression for wood drying III: the reduction of moisture content,the recovery rate,and mechanical properties of wood compressed at different moisture content conditions

As a follow-up report, the pre-treatment by compression for wood drying was systematically studied in terms of the reduction of moisture content (MC), the recovery rate (RR), and mechanical properties of wood compressed at different MC conditions. The results showed that MC after compression on water-saturated wood determined the critical value of MC before compression which were about 84 and 105% at a compression ratio of 60 and 40% for Poplar and Chinese fir, respectively. Beyond the critical value, MC after compression remained constantly at about 84% and decreased slightly from 105% for Poplar and Chinese fir, respectively. The MC reduction decreased with the decrease of MC before compression. The MC reduction was rather effective when the MC before compression was higher than the critical value and was recommended to pre-treat for the effectiveness of MC reduction. In addition, after the recovery, the wood volume and mechanical properties were well retained for the wood compressed at all MC conditions which were above fiber saturation point (FSP) before and after compression. Therefore, the pre-treatment by compression is viable in terms of the RR and mechanical properties at rather broad MC conditions above FSP. Moreover, the compression force needed for treatment was almost same at these MC conditions.     

Influence of wood moisture content and wood temperature on fungal decay in the field: observations in different micro-climates

In this study, Scots pine sapwood (Pinus sylvestris L.) and Douglas fir heartwood (Pseudotsuga menziesii Franco) specimens were exposed in double layer field trials at four different exposure sites and under different exposure conditions (in total ten test sets). The material climate of wood in terms of wood moisture content (MC) and wood temperature was automatically monitored over a period of 6 years and compared with the progress of decay. The aim of this study was to highlight the interrelationship between microclimate, material climate, and decay as a basis for the establishment of dose-response functions to be used for service life prediction of wood and wood-based products. Differences in resulting decay dynamics between the test sites as well as between the different types of exposure were quantified and discussed with respect to corresponding microclimatic and material climatic conditions. The time between the beginning of exposure and the first occurrence of visible decay varied between the sites and influenced the total decay development. The fundamental importance of direct decay factors, such as MC and wood temperature, were underlined and basic requirements for establishing dose-response-functions to be used in service life prediction models were derived.     

Effects of moisture content and specific gravity on static bending properties and hardness of six wood species

This study was designed to investigate the effects of moisture content (MC) and specific gravity (SG) on the bending strength and hardness of six wood species including Japanese cedar (Cryptomeria japonica D. Don), China fir (Cunninghamia lanceolata), western hemlock (Tsuga heterophylla), red meranti (Shorea spp.), Selangan batu (Shorea spp.), and red oak (Quercus spp.). The experimental results are summarized as follows: Effects of MC and SG on the strength (MOR), stiffness (MOE), and hardness (H _B) could be represented by a multiregression formulas. A negative correlation existed between these properties and MC, whereas a postive correlation showed between them and the SG. The changing rate of these properties induced by 1% MC changes varied with the wood species: 2.6% change in MOR was observed in Japanese cedar, China fir, western hemlock, red meranti, and Selangan batu; and 3.9% was found in red oak. For MOE, a 0.58% change was observed in Japanese cedar, China fir, and red meranti; western hemlock and Selangan batu exhibited 1.2% and red oak 2.5%. For hardness, a 1.1% change was observed in Japanese cedar, western hemlock, and red oak; red meranti and China fir exhibited 3.3%; and Selangan batu 1.8%.A part of this report was presented at the 48th annual meeting of the Japan Wood Research Society in Shizuoka, Japan, April 3-5, 1998     

Changes in the mechanical properties of wood during a period of moisture conditioning

Changes in the modulus of elasticity (MOE), modulus of rupture (MOR), and stress relaxation in the radial direction of wood (hinoki:Chamaecyparis obtusa) moisture-conditioned by the adsorption process from a dry state and by the desorption process from a moisture content slightly below the fiber saturation point were investigated. The MOE and MOR of wood conditioned by the adsorption process showed significant increases during the later stages of conditioning when the moisture content scarcely changed. However, with the desorption process they did not increase as much during later stages of conditioning, though they increased during early stages of conditioning when the moisture content greatly decreased. The stress relaxation of wood decreased with an increase in the conditioning period with both the adsorption and desorption processes. These results suggest that wood in an unstable state, caused by the existing state of moisture differed from that in a true equilibrium state shows lower elasticity and strength and higher fluidity than wood in a true equilibrium state. Furthermore, the present study demonstrates that the unstable states of wood induced during the course of drying, desorption, and possibly adsorption of moisture are slowly modified as wood approaches a true equilibrium state.     

Characterization of major,unused, and unvalued Indonesian wood species I. Dependencies of mechanical properties in transverse direction on the changes of moisture content and/or temperature

Mechanical property changes due to the moisture content (MC) and/or temperature changes were examined for 15 Indonesian wood species. A static bending test was carried out at 20°C, 65% relative humidity (air-dry), and water-saturated at 20°C (wet-20) and 80°C (wet-80). For individual test conditions, modulus of elasticity (MOE) and modulus of rupture (MOR) increased linearly with specific gravity regardless of wood species; however, maximum deflection did not correlate with specific gravity for any MC or temperature conditions. The relative values of MOE and MOR measured in wet-20 to air-dry conditions were variously affected from slightly to strongly depending on the wood species. However, the relative values always decreased markedly when saturated in water at 80°C, regardless of wood species. The relative MOE, MOR, and maximum deflection values due to the change in MC or MC and temperature combined were independent of specific gravity but may be dependent on wood type: softwood or hardwood.     

Effect of growth rate on wood specific gravity and selected mechanical properties in individual species from distinct wood categories

Summary This study examined the relationships of wood specific gravity and selected mechanical properties (MOR, MOE and Cmax) with growth rate in 16 timber species from four distinct wood categories: 1) first softwood category (FSC); 2) second softwood category (SSC); 3) diffuse-porous wood category (DPC); and 4) ring-porous wood category (RPC). And genetic, silvicultural and environmental influence on the relationships was briefly discussed. Statistical results show that the relationships of specific gravity and the mechanical properties with growth rate vary remarkably with both the wood property and the wood category. In general, the mechanical properties in the FSC species decrease remarkably with increasing growth rate, while they appear to be less influenced in the SSC species. Compared with the softwoods studied, the physico-mechanical properties in the hardwoods studied are remarkably less influenced. In the DPC species, growth rate generally has very a little influence on both specific gravity and the mechanical properties. In the RPC species, the physico-mechanical properties appear not to decrease with increasing growth rate, and in some species they even tend to increase. Among the three mechanical properties studied, MOE is remarkably less influenced by growth rate than MOR and Cmax. Compared with specific gravity, however, the mechanical properties are generally more influenced by growth rate. Therefore, the impact of growth rate on wood mechanical properties in a species can not be estimated exactly through the relationship of wood specific gravity with growth rate. Path analysis reveals that growth rate has a large effect on the mechanical properties which can be accounted for by the affected specific gravity. In addition to this indirect effect through specific gravity, growth rate still has an additional effect on the mechanical properties which can not be explained by specific gravity. In the SFC species, such effect is significant, and this, to a lesser extent, applies to the DPC species. However, this effect is not remarkable in the SSC species and may be negligible in the RPC species.     

Theoretical modeling of the effects of temperature and moisture content on the acoustic velocity of Pinus resinosa wood

To investigate the effects of temperature and moisture content(MC) on acoustic wave velocity(AWV)in wood,the relationships between wood temperature,MC,and AWV were theoretically analyzed.According to the theoretical propagation characteristics of the acoustic waves in the wood mixture and the differences in velocity among various media(including ice,water,pure wood or oven-dried wood),theoretical relationships of temperature,MC,and AWV were established,assuming that the samples in question were composed of a simple mixture of wood and water or of wood and ice.Using the theoretical model,the phase transition of AWV in green wood near the freezing point(as derived from previous experimental results) was plausibly described.By comparative analysis between theoretical and experimental models for American red pine(Pinus resinosa) samples,it was established that the theoretically predicted AWV values matched the experiment results when the temperature of the wood was below the freezing point of water,with an averageprediction error of 1.66%.The theoretically predicted AWV increased quickly in green wood as temperature decreased and changed suddenly near 0 °C,consistent with the experimental observations.The prediction error of the model was relatively large when the temperature of the wood was above the freezing point,probably due to an overestimation of the effect of the liquid water content on the acoustic velocity and the limited variables of the model.The high correlation between the predicted and measured acoustic velocity values in frozen wood samples revealed the mechanisms of temperature,MC,and water status and how these affected the wood(particularly its acoustic velocity below freezing point of water).This result also verified the reliability of a previous experimental model used to adjust for the effect of temperature during field testing of trees.     

Effect of moisture content of members on mechanical properties of timber joints

To investigate the effect of moisture content (MC) of members on the mechanical properties of timber joints, bending tests of precut joints and shear tests of dowel-type joints were carried out using timbers of Japanese cedar (Cryptomeria japonica D. Don) with three moisture conditions: green, kiln-dried with a MC target of 15%, and over-kiln-dried with a MC target of 5%. For the bending test, timbers were processed with a precut processing machine into “koshikake-ari” (a kind of dovetail joint) and “koshikake-kama” (a kind of mortise and tenon joint). A pair of members was jointed together without mechanical fasteners. Bolts (diameter = 12 mm) and nails (diameter = 2.45 mm) were used as dowels in the shear test. Bolted joints were constructed with one bolt and two metal side plates. Two nails and two metal side plates were used for the nailed joint. For precut joints, no clear effect of MC was recognized on maximum moment and initial stiffness. The maximum strength of mechanical joints assembled with kiln-dried wood was changed by the degree of drying. Stiffness of the joints assembled with kiln-dried specimens was larger than that of the joints assembled with green specimens.Part of this study was presented at the 7th International IUFRO Wood Drying Conference, Tsukuba, July 2001     

Poplar wood heat treatment: effect of air ventilation rate and initial moisture content on reaction kinetics,physical and mechanical properties

The kinetics of heat treatment as well as its effect on some physical and mechanical properties of poplar wood (Populus alba L.) were analysed in this research. Kinetic tests were performed at different treatment temperatures and two different air ventilation settings [low and high air exchange rate (AER) with the exterior]. The treatment kinetics was studied, starting from the oven-dry condition, according to the mass loss during time. The time–temperature equivalency was checked, the mass loss versus time formalised through a master curve. The analysis clearly showed how the heat treatment at low and high AER presents different degradation kinetics even if similar activation energy values were found. Some physical and mechanical properties of wood after treatments up to a mass loss of 7 and 10 % starting from oven-dry or standard environmental conditions state were also studied. All of the treated samples showed statistically significant differences compared to the untreated one. The treatments performed at 7 or 10 % of dry mass loss showed homogeneous behaviour. The same tendency was observed for the treatments starting at oven-dry or standard environmental conditions with the exception of Young’s modulus, which resulted in smaller reductions in wet starting condition when compared to dry samples.     

The viscoelasticity of wood at varying moisture content

Wood is regarded as a viscoelastic material. Creep deformations that arise from variations in the moisture content are described by a theory of hydroviscoelasticity developed by the author. Two different types of behaviour have been apparent: one, arising from a continuously increasing strain with periodic variation in the moisture content, and another with no cumulative effect. The theory has been applied to previously published experimental results concerned with beech, pine, hoop pine, klinki pine, along with birch and spruce plywood. Birch and spruce plywood have been used for experiments concerned with periodically-cycling bending moment and moisture content. The results obtained have been compared with the theory presented. Glue-laminated beams have been subjected to long-term outdoor loading extending for five years. A brief discussion is given of the results obtained.     

STUDY ON THE EFFECT OF WOOD MOISTURE CONTENT ON PERMEABILITY IN THE HYGROSCOPIC RANGE

Woodpermeabilityhascloserrelationshipwiththemanyfiledsofwoodworkingandusing,woodfluidpermeabi1-ityisrelatedtoinprocessingsuchaswooddrying;fireretartenttreatmen;pup1ingandpaper;woodpreservation.Theflowoffluidinwoodincludestwokindsoftransportprocess.Oneispenetrationfromwoodoutsidetoinside,suchas'woodpreservationandfireretartenttreatment,anotherisdiffusionfromwoodinsidetooutside'suchaswooddrying,Infact,woodpermeabilityinfulenceddirectlythequalityofwoodwaterheattreatment,Forthisreasontheresearchin…     

Evaluating the effect of heat treating temperature and duration on selected wood properties using comprehensive cluster analysis

In this study, twenty-five heat treatments were conducted at the various treating conditions of five temperatures and five durations. At each treatment, 15 poplar specimens were used. Twelve variables that represented wood color, physical and mechanical properties and durability were measured for both treated and untreated specimens. To evaluate the effect of heat treatment on the wood performance, a comprehensive cluster analysis was applied to the measured variables of treated and untreated specimens. Cluster analysis showed that treating and control specimens can be distinctly classified into 2, 3 or 4 clusters according to the intended purposes. Two clusters can represent the control group and twenty-five testing groups. Four clusters represented the control group and three groups of testing specimens having mild, moderate and severe treating conditions, respectively. At the mild treatment, modulus of rupture (MOR) was reduced <11 %. Wood durability increased to moderate resistant. At the moderate treatment, EMC decreased by 28 %, and MOR was decreased by more than 12 %. In the severe treatment, wood durability increased to resistant or highly resistant; however, its MOR was reduced half.     

Effect of moisture content on fatigue behavior in the non-linear region for wood

Summary For oven-dried wood applied non-linear deflection, the temperature rising AT was constant after the initially slight temperature rising, and then arose immediately before fatigue failure. Moreover, there was not the rapid reduction of dynamic stress as observed for moist wood. On the other hand, the process to fatigue failure for various moist wood depended upon the moisture content. For heating vs. moisture content, the heating increased with moisture content up to 13 to 20%, while it decreased gradually above 20%. The processes to fatigue failure at more than 13% moisture content were similar to one another. These results were explained on energy balance between the heating due to flexibility of moist molecules in wood substances and the energy consumption for moisture movement and vaporization. The present results supported the mechanism of fatigue behavior in the non-linear region which was proposed in the previous report.     

Effect of moisture content on the longitudinal tensile creep behavior of wood

In our previous report, we investigated the effect of the microfibril angle (MFA) in the middle layer of the secondary wall (S₂) on the longitudinal creep behavior of a thin homogeneous earlywood specimen sugi. In the present study, we investigated the role of moisture on the tensile creep behavior of wood. We discuss the creep behavior of the wood cell wall from the viewpoint of the composite structure of the cell wall and the properties of the constituent materials. A microtomed thin specimen of earlywood of sugi (Cryptomeria japonica D.Don) was used for the longitudinal tensile creep test. Creep tests were conducted at three moisture stages (oven-dry, air-dry, fiber saturation point) over a broad range of MFA. Results showed that the longitudinal tensile creep behavior was highly dependent on both the moisture content and the MFA. With a small MFA, the variation in the creep function among the three moisture states was very small. For a large MFA, the variation in the creep function was larger. At low moisture contents, the magnitude of the creep function was very small, while at high moisture content, it was very large except for the case of specimens with very small MFA. Those results show that the longitudinal tensile creep behavior was directly affected by the fine composite structure and the internal properties of the cell wall constituents.     

Micromorphology,moisture sorption and mechanical properties of a biocomposite based on acetylated wood particles and cellulose ester

Abstract

One of the major issues in a long-term perspective for the use of wood–plastic composites (WPCs) in outdoor applications is the moisture sensitivity of the wood component and the consequent dimensional instability and susceptibility to biological degradation of the composite. In this work, the effects of using an acetylated wood component and a cellulose ester as matrix on the micromorphology, mechanical performance and moisture uptake of injection-moulded WPCs have been studied. Composites based on unmodified and acetylated wood particles, specially designed with a length-to-width ratio of about 5–7, combined with both cellulose acetate propionate (CAP) and polypropylene (PP) matrices were studied. The size and shape of the wood particles were studied before and after the processing using light microscopy, and the micromorphology of the composites was studied using a newly developed surface preparation technique based on ultraviolet laser irradiation combined with low-vacuum scanning electron microscopy (LV-SEM). The water vapour sorption in the composites and the effect of accelerated weathering were measured using thin samples which were allowed to reach equilibrium moisture content (EMC). The length-to-diameter ratio was only slightly decreased for the acetylated particles after compounding and injection moulding, although both the unmodified and the acetylated particles were smaller in size after the processing steps. The tensile strength was about 40% higher for the composite based on acetylated wood than for the composite with unmodified wood using either CAP or PP as matrix, whereas the notched impact strength of the composite based on acetylated wood was about 20% lower than those of the corresponding unmodified composites. The sorption experiments showed that the EMC was 50% lower in the composites with an acetylated wood component than in the composites with an unmodified wood component. The choice of matrix material strongly affected the moisture absorptivity of the WPC. The composites with CAP as matrix gained moisture more rapidly than the composites with PP as matrix. It was also found that accelerated ageing in a Weather-Ometer® significantly increased the moisture sensitivity of the PP-based composites.     

In situ measurement of wood moisture content in high-temperature steam

Changes in moisture content of sugi (Cryptomeria japonica D. Don) wood during high-temperature saturated and superheated steam treatments were investigated. A system for in situ weighing of specimens was used, and the reduction of wood substance by heating was taken into consideration. At 160°C the loss of wood substance due to heating was significant and influenced the moisture content values, but it was almost negligible at 120°C. Treatment time and temperature affected the moisture content in saturated steam but not in superheated steam. Excess water in a saturated closed system appears to promote the decomposition of wood and condensation in or on specimens.Parts of this work was presented at the 49th, 50th, and 52nd Annual Meetings of the Japan Wood Research Society in Tokyo, April 1999; Kyoto, April 2000; and Gifu, April 2002