Influence of drying temperature upon some mechanical properties of beech wood

The paper presents the results obtained in an experimental study concerning the influence of drying temperature upon the mechanical properties of beech wood (Fagus sylvatica L.). Sound wood samples without red heart were cut from white (unsteamed) beech timber parts, dried at different temperatures: 20 °C, 80 °C, 90 °C, 100 °C, 115 °C and same relative air humidity: 50%. After performing classical tests for evaluation of some selected mechanical properties, the following conclusions could be drawn: all bending properties (static bending strength, modulus of elasticity and impact bending strength) increased with increasing temperature, confirming thus the benefiting effect of heat upon wood plasticity. The tensile strengths, both parallel and perpendicular to grain, increased with increasing temperature, but only in the range below 100 °C; as soon as the temperature exceeded this value, the tensile strengths began decreasing. As far as compression strength parallel to grain, shearing strength and splitting resistance is concerned, no significant influence of temperature could be established. However, it seems that these properties are negatively affected by kiln-drying, as even with low kiln-drying temperatures these strengths are much lower than in case of air-drying.     

Effect of overdrying of yellow-poplar veneer on physical properties and bonding

The intent of this study was to follow the changes of various wood physical properties as the wood was overdried at high temperature and to determine if these changes correlated with the ability of the wood to bond with phenolic resins. Yellow-poplar (Liriodendron tulipifera L.) was dried from a water-soaked state at 170°C, 195°C, and 220°C for various lengths of time. Light reflectance, wettability, equilibrium moisture content (EMC), loss of wood substance, and compression shear bond strength were measured. Wettability was severely affected over time and was very quickly affected at high temperatures. The bulk properties, EMC and loss of wood substance, were slightly affected by drying for 224 min at 170°C but were more affected at the higher temperatures with time. The results for these bulk properties correlated well with light reflectance measurements regardless of temperature level. However, compression shear bond strength remained high and was unresponsive to either drying time or drying temperature. After these laboratory samples were analyzed, yellow-poplar plywood samples produced by an industrial manufacturer were tested for bond quality. One sample was composed of regularly dried veneer, and one sample was overdried.     

Density and some mechanical properties of densified and heat post-treated Uludağ fir,linden and black poplar woods

The effects of thermo-mechanical densification and heat post-treatment on air-dry density, modulus of elasticity (MOE), bending strength (MOR), and compression strength parallel to the grain (CS) of Uluda? fir (Abies bornmulleriana Mattf.), linden (Tiliagrandifolia Ehrh.), and black poplar (Populus nigra L.) wood samples were investigated. Samples were densified with compression ratios of 25 and 50%, and at 100 and 140?°C. Then, the heat treatment was applied to the samples at 185 and 212?°C for 2 h. According to the results of the study, density of all wood samples increased together with the increase of compression ratio. Regarding compression temperature, the highest density increase was obtained at 100?°C. Mechanical strength (MOE, MOR, and CS) in densified samples increased depending on compression ratio and increase of density. The highest strength increase was in black poplar samples and the lowest was in linden samples. After heat post-treatment, mechanical strength of samples decreased depending on increase in treatment temperature. However, strength values (MOR except) of samples which are 50% compressed and heat-treated were found higher than control samples.     

Microwave drying kinetics of jack pine wood: determination of phytosanitary efficacy,energy consumption,and mechanical properties

The aims of this study were to determine (1) the effects of microwave irradiation on the drying kinetics of jack pine wood, (2) the phytosanitary efficacy, and (3) the processing energy consumption and mechanical strength of the dried product. Microwave drying experiments were performed at 2.45 GHz frequency and at microwave powers ranging from 300 to 1000 W. Results indicate that higher microwave power and initial wood temperature and lower sample thickness increases the internal sample temperature, improves the drying rate, and reduces both drying time and energy consumption. The microwave irradiation efficacy to sanitize jack pine wood boards was determined in terms of temperature/time combinations based on actual drying kinetics according to standards for phytosanitary measures. The energy required to dry 12 mm thick wood board samples at microwave power ranging from 300 to 1000 W was in the range of 36.4–12.3 MJ/kg of water, respectively, for up to 65% energy consumption savings. The impact of microwave power on the mechanical properties was not statistically significant, although mechanical properties tended to decrease with increasing power.     

Strength of dried and re-moistened spruce wood compared to native wood

During the wood drying process, mechanical stresses and thermal conditions arise, which causes irreversible damage. Since drying of wood results in a general improvement of strength and elasticity, the negative drying effects are covered. In the present study, the mechanical properties of dried and subsequently re-moistened samples were compared to native wet wood. For oven dried/re-moistened macro-scale bending and compression strength a significant reduction of 16.5% and 15.0% was observed. Micro-scale specimens underwent a similar strength loss, i.e. 9.8% for tensile strength and 14% for compression strength. When the drying was performed at ambient temperature (20°C), no difference to native wood was observed in tensile testing, whereas a significant reduction of compression strength (10.0%) was found. SEM micrographs of tensile fracture surfaces of dried and re-moistened specimens showed a brittle and rather smooth transwall failure of tracheids, whereas native samples exhibited a more ductile character.

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Vergleich der Festigkeit von getrocknetem und wiederbefeuchtetem Fichtenholz mit frischen Proben

Zusammenfassung Während der Holztrocknung kommt es neben makroskopischen Spannungen auch zu mikroskopischen Spannungszuständen, sowie zu thermischen Veränderungen der Zellwandsubstanzen, die zu einer irreversiblen Schädigung der Holzsubstanz führen. Da ein Feuchtigkeitsentzug aus der Zellwand allgemein eine Verfestigung der Holzsubstanz verursacht, werden die Festigkeitsverluste während der Holztrocknung durch die zu vor geschilderte Verfestigung überdeckt. Durch den Vergleich von frischen und nach der Trocknung (103°C) wieder befeuchteten Proben konnte für makroskopische Biege- und Druckproben ein Festigkeitsverlust von 16.5% bzw. 15.0% nachgewiesen werden. Mikroskopische Zug- und Druckproben zeigten bei 103°C ähnliche hohe Verluste von 9.8 bzw. 14%. Trocknung bei 20°C führte nur bei den Druckproben zu einem signifikanten Festigkeitsverlust von ca. 10%. SEM Aufnahmen der Bruchflächen von Mikro-Zugproben zeigten ein glattes, sprödes Bruchbild bei den getrockneten/wiederbefeuchteten Proben. Hingegen waren die frischen Proben durch ein verformungsreicheres Bruchbild charakterisiert.

     

Influence of temperature of thermal treatment on surface densification of spruce

Spruce (Picea abies L. Karst) wood lamellae, thermally treated at 170, 190, 210 and 230 °C were surface densified by compression at a temperature of 150 °C to three degrees of compression. Immediate springback, set recovery, mechanical properties in 3-point flexure, Brinell hardness and density profiles measurements were used to determine the effect of thermal treatment on the properties of surface densified wood. The highest immediate springback occurred in wood specimens thermally treated at the highest temperature (230 °C) and decreased with decreasing thermal treatment temperature. The untreated samples had the highest set recovery, which decreased with the temperature of thermal treatment. The surface densification increased hardness and bending strength. The highest increase was in the case of untreated wood and decreased with the temperature of thermal treatment. The modulus of elasticity (MOE) and modulus of rupture (MOR) of surface densified wood decreased with increasing thermal treatment temperature. The trend was similar for specimens which were thermally treated but not surface densified. Surface densification increased the density of the specimens in the first few millimetres below the surface. The highest density was achieved in untreated specimens and the lowest in specimens thermally treated at the highest temperature.     

Festigkeitsuntersuchungen an Fichtenpressholz (FPH)

Sawn spruce wood was compressed perpendicular to grain to 50% of its original volume at a temperature of 140 °C in a multiple daylight press. The process had three steps: heating up—compression—and cooling. The mechanical properties of clear samples from densified wood were investigated thereafter. Test results show that the mechanical characteristics are influenced by wood anatomy, anisotropy and moisture content and that strength and stiffness are proportional to the increase of density. Strength, color, as well as swelling, depend on the parameters of the process especially temperature and duration of heating. Damages due to grain deviation or knots were rarely observed. Bending and tensile strength amount to 169 and 185 Mpa, respectively, compression strength perpendicular to grain benefits most from transverse compression reaching a factor of 4.4 times the non-densified reference samples.     

Fire resistance of wood treated with a cationic silica sol

Wood was treated with the cationic silica sol (CSS) Levasil 200S and dried at various temperatures (room temperature, 40, 60, 80 and 103 °C). A water leaching test revealed fixation of the silica in wood even after drying at room temperature. Maximum cross sectional swelling of the specimens decreased from 15.6 % (untreated control) to 13.0 %, when treated wood was dried at 103 °C; cell wall bulking values were also negative (?2.3 %), indicating a thermal degradation of the cell wall polymers catalyzed by the CSS. Penetration of the CSS into the cell wall did not occur. A simple flammability test revealed increased fire resistance of the treated wood. Mass loss and velocity of mass loss as well as burning time were reduced; glowing of the formed charcoal was completely prevented. The effectiveness increased with increasing weight percent gain of the CSS in the wood. Thermo gravimetric analysis under nitrogen atmosphere displayed only minor reduction in the initial temperature of thermal decomposition for wood treated with CSS as compared to the control. In the presence of oxygen the resulting charcoal showed comparable thermal behaviour to the control. The yield of charcoal after pyrolysis was increased to a minor extent (from 19.9 to 23.0 %), indicating that the release of combustible gases was hardly reduced. The mode of action of enhanced fire resistance due to CSS-treatment is discussed.     

Microwave treatment of <Emphasis Type="Italic">Eucalyptus macrorhyncha</Emphasis> timber for reducing drying defects and its impact on physical and mechanical wood properties

Microwave is a useful and proven tool for increasing permeability of refractory wood and, thereby, it can be used for reducing drying time and defects. However, strength loss is always a concern. In this study, green timber boards were treated with microwave in two intensities, low microwave (LMW), 89 kWh/m³, and high microwave (HMW), 95 kWh/m³, and compared with control boards. Samples of all three treatments were kiln dried together. Density and selected mechanical properties were also assessed after drying. LMW, the treatment which presented the best results, showed reduction in some drying defects, such as collapse (20%), surface check length (84%), internal check length (50%) and internal check width (70%). Density, MOE and MOR on static bending were unchanged, whereas reductions in shear strength (13%) and compression strength parallel to grain (10%) were observed. HMW, on the other hand, produced higher strength reductions and more drying defects than LMW. Fine adjustments of LMW microwave power might bring drying benefits without strength losses.     

Effect of high-temperature drying on properties of Norway spruce and larch

In timber drying, mechanical properties may be changed due to treatment temperature and treatment duration. In general, when increasing the kiln temperature, drying time is decreased and some timber properties are negatively affected. In this study, the effect of different drying temperatures (80, 120 and 170 °C) on equilibrium moisture content and sorption rate, on bending strength and stiffness was investigated for Norway spruce and larch from four proveniences and the results were compared to those obtained for heat-treated wood (Thermowood ^®). The experiments confirm earlier research that both treatment temperature and treatment duration affect the properties. High-temperature drying or treatment can be optimized for several applications, yielding strong but not so durable timber or vice versa.     

Influence of heat pressure steaming (HPS) on the mechanical and physical properties of common oak wood

Common oak (Quercus robur) was thermally treated applying a heat pressure steaming procedure. Physical and mechanical properties of treated and untreated samples were investigated extensively. Swelling, water absorption, water vapour resistance, porosity and thermal conductivity were tested and the mechanical properties of tensile, bending and compression strength and of Young’s modulus (static and dynamic) as well as Poisson’s ratio and shear modulus were determined. The tests were carried out in the standard climate 20 °C and 65 % relative humidity and also in all three anatomical main directions: longitudinal, radial and tangential. The equilibrium moisture content at 20 °C and 65 % relative humidity for HPS (heat pressure steamed) oak (determined in adsorption test) was 6.7 % and for untreated oak 9.1 %. Swelling in longitudinal direction was not affected: a reduction of 17 and 10 % could be observed in radial and tangential direction, respectively. The porosity of the treated samples was 53.9 % in comparison to the untreated samples with 51.0 %. The thermal conductivity depending on the modification procedure changed only slightly which was related to the different densities of the samples. The water vapour resistance of the modified samples increases compared to the untreated samples. The values are double (dry-cup) respectively three times (wet-cup) higher than those of the reference samples. The elastic properties were not influenced by heat pressure steaming. The MOE does not show a significant change depending on the treatment. Bending and tensile strength of HPS oak decrease. In longitudinal direction, the tensile strength drops by 26 % and the bending strength by 25 %.     

The potential of colour measurement for strength prediction of thermally treated wood

This paper investigates the effects of thermal treatment of birch with respect to colour and strength. Birch wood was treated at 175 °C and 200 °C for 0 h, 1 h, 3 h and 10 h. In bending-strength experiments, treatment was also performed at 185 °C for 2 h. Both static bending strength and impact bending strength were investigated using multivariate statistics (PLS) for correlation to process parameters, density, EMC, position in board, modulus of elasticity (only in static bending), colour and dimensions of samples. In static bending, two PLS models were designed, one based on process parameters and the other based on colour and EMC. From these models it was concluded that colour is not a useful parameter for prediction of strength. In impacted bending, the correlation was too small to give useful results. One test of static bending strength with matched samples was performed, and it showed a strength reduction of 43% when treatment was conducted at 200 °C for 3 h. Measurement of colour homogeneity of the treated boards showed that the colour is not homogeneous.     

Strength properties of Scots pine from harbour piles degraded by erosion bacteria

140-year-old Scots pine piles displaying various levels of degradation by erosion bacteria were excavated from the harbour of Hamburg and investigated. Bending and compression strength correlated to the basic density, moisture content and lignin content. The compression strength was determined both on samples with dimensions of 30 mm?×?20 mm?×?20 mm and using 6.6 mm-increment cores taken perpendicular to the grain direction. Both sizes of the samples showed almost identical results. The sapwood of all samples was completely water-saturated; the heartwood did not even achieve 50% water saturation. The sapwood samples showed a good correlation between moisture content and strength properties, whereas basic density correlated well with the strength properties of both sapwood and heartwood. With increasing bacterial attack and resulting lower basic density, the lignin content increased in relation to holocellulose. The measured lignin content of the sapwood showed a good conformity with all strength properties. Determination of the lignin content represents an alternative method for predicting strength values of waterlogged wood while leaving most of the test material intact.     

Inhibited wood degradation of cement-coated beech Laminated Veneer Lumber (LVL) for temporary in-ground applications

This paper investigates the long-term tensile properties of laminated veneer lumber (LVL) beech sections coated with cement and exposed to fungal decay. A set of LVL coupon (dog-bone) samples was stored in compost, tested in tension after 6 and 12 months and compared to reference samples stored at 20 °C and 65% relative humidity. Results showed that after 26 weeks of compost exposure, a fungus of the Ascomycota genus was identified in cement-coated samples using a molecular biology polymerase chain reaction (PCR) technique, which analyses the internal transcribed spacer (ITS) region of the ribosomal DNA. However, no visual deterioration was noticed. Still in cement-covered samples and after 12 months of exposure, a common white rot fungus was determined by DNA chip technology, but no fungal wood decay was visible in areas where the applied coating had a thickness of at least 5 mm. Decay in uncoated LVL samples was significant with the samples having an average residual strength equal to 7%. This compares to the tensile strength of coated samples, which only decreased by 65% relative to the reference samples. Strength and stiffness of coated samples did not differ significantly between 6 and 12 months of exposure. Preliminary investigations tend to show that the strength reduction in cement-coated samples is due to an alkaline degradation of the wood. The observed influence of the coating thickness on the visual fungal decay can probably be ascribed to the protection mechanism due to a physical fungal barrier with a high pH.     

Investigation on bonding quality of beech wood (Fagus orientalis L.) veneer during high temperature drying and aging

In the present study, the effects of high drying temperature and UV light induced aging on bonding quality of plywood manufactured from untreated and treated veneer layers were investigated. Rotary cut veneers with dimensions of 500 mm×500 mm×2 mm produced from beech (Fagus orientalis Lipsky) log were selected for topochemical, chemical and mechanical analyses. The veneer sheets were oven-dried at 100°C and 180°C after the peeling process. Afterwards, the surfaces were exposed to artificial UV irradiation in an UV chamber for 24 h, 48 h and 72 h representing natural sun irradiation of 2, 4 and 6 months, respectively. Topochemical distribution of lignin and phenolic extractives of the treated and untreated veneers was investigated on a cellular level using UV microspectrophotometry (UMSP). For the chemical characterization of accessory compounds high performance liquid chromatography (HPLC) was used. Furthermore, the shear and bending strengths of plywood manufactured from the treated samples are determined in order to study the bonding quality. The UV microscopic detection shows that after high drying temperature and aging treatment, lignin condensation occurs. With increasing drying temperature and aging duration, more phenolic extractives are situated in parenchyma cells and vessel lumens which can be proved by increased absorbance at 278 nm. The HPLC analysis of the treated tissue showed distinct signals of polymerized compounds such as catechin and 2,6-dimethoxybenzoquinone which are chromophoric compounds in discolored beech wood. The mechanical properties of plywood showed that with increasing drying temperature up to 180°C does not negatively affect shear and bending strengths of samples. After exposure of the veneers to UV irradiation (especially 6 months), decreasing shear and bending strengths of plywood samples can be observed.     

Study of Thermodynamic, Structural, and Quality Properties of Yacon (Smallanthus sonchifolius) During Drying

The thermodynamic properties and the changes in volume were studied during hot air drying of yacon. The results indicated high rates of moisture and water activity losses during the first 150 min of drying. Shrinkage of the product was evident during drying, with a reduction of about 89 % as observed by three-dimensional laser scanning and by the structural changes observed during scanning electron microscopy, indicating severe tissue shrinkage and collapse during air drying. The experimental sorption data of air-dried yacon were applied to various isotherm equations (GAB; Oswin; Chung–Pfost; Henderson, Smith, and Halsey). The Chung–Pfost model was best for characterizing the sorption behavior at 25, 30, 35, and 40 °C. The sorption isotherms of the dehydrated yacon were of type III, characteristic of high-sugar-content products. The values obtained for the differential enthalpy and entropy were higher at low moisture contents and decreased with increase in moisture content. The value obtained for the free energy of ?70.7 J?mol^?1 suggests the process was spontaneous. A color analysis was also carried out on the yacon samples dehydrated by hot air and by freeze drying, with and without prior blanching at 100 °C for 4 min. The parameter L* was significantly higher, and the values for a* and b* significantly lower for the freeze dried yacon as compared to the forced air dehydrated samples, resulting in lighter, greenish, and bluish samples.     

The influence of compression failure on the bending, impact bending and tensile strength of spruce wood and the evaluation of non-destructive methods for early detection

Bending strength (MOR) and bending Young’s modulus (MOE) according to DIN 52186 and MOE calculated on the basis of eigenfrequency and sound velocity were tested on small clear wood specimens of Norway spruce wood with and without compression failure. One group of specimens was climatised in a normal climate of 20°C and 65% relative humidity, while the other group was stored for one month under water before testing. The MOR of specimens with compression failure decreased about 20% on average (normal climate and wet) compared with the specimens without compression failure. The MOE of the specimens with compression failure was reduced only minimally compared with the specimens without compression failure stored in a normal climate, but very distinct differences (more then 30%) were found under wet conditions. The MOE of the specimens with compression failure calculated on the basis of eigenfrequency and sound velocity were not reduced or only minimally compared with the specimens without compression failure. It is therefore not possible to detect compression failure and to determine reduction in MOR using eigenfrequency or sound velocity. In addition, impact bending (DIN 52189), tensile strength and tensile MOE (DIN 52188) were tested on small clear wood specimens of Norway spruce wood with and without compression failure. The specimens with compression failure revealed an average reduction in impact strength of about 40% and an average reduction in tensile strength of about 20% compared with the specimens without compression failure, whereas tensile MOE of the specimens with compression failure was not reduced compared with the specimens without compression failure. The detection of compression failure by computer tomography (CT) was tested on Norway spruce wood boards 10 cm in thickness, and detection by optical scanner was tested on planed Norway spruce wood boards. CT recognised large compression failures easily, whereas the scanner was not able to detect them.     

Improvement of termite resistance,dimensional stability and mechanical properties of pine wood by paraffin impregnation

Paraffin has been used as surface protection of wood throughout the ages but its use for impregnation to improve wood resistance to biodegradation is recent. This study determined the main improvements on wood properties with paraffin impregnation. Healthy Pinus pinaster Ait. wood was impregnated with paraffin at different levels using a hot–cold process. Weight gain, equilibrium moisture content and dimensional stability (ASE) at 35 and 65 % relative humidity, termite durability against Reticulitermes grassei (Clément), bending strength, bending stiffness (MOE) and Janka hardness were determined. Density increased from 0.57 to 0.99, ASE ranged between 38–96 % and 16–71 % for 35 and 65 % relative humidity, respectively. Equilibrium moisture content decreased from 9.9 and 12.0 % to 0.8 and 3.6 % for 35 and 65 % relative humidity. Termite durability improved from level 4 to level 3 of attack, and higher termite mortality was found in treated wood (52 % against 17 %). Bending strength (MOR) increased with paraffin weight gain, reaching a 39 % increase. MOE also increased by about 13 % for wood with a weight gain around 80 %. Janka hardness increased significantly reaching about 40 % for wood with 80 % weight gain. Paraffin impregnated wood has improved properties with regard to equilibrium moisture content, dimensional stability and density, bending strength and Janka hardness, and resistance against termites.     

Air emission from timber drying: high temperature drying and re-drying of CCA treated timber

The nature and mass flows of atmospheric emissions from an industrial kiln, drying radiata pine (Pinus radiata D.Don) timber at 140°C dry bulb and 90°C wet bulb, which was subsequently copper-chrome-arsenic (CCA) treated and redried at 90°C dry bulb and 60°C wet bulb, were assessed by measuring the concentration of chemical components in the kiln atmosphere at regular time intervals and determining air-flows and temperature differentials across the stack. The volatile organic compounds (VOCs) emitted were trapped and analyzed by GC-MS. The two major VOCs found during high temperature pre-drying and CCA re-drying were α-pinene and β-pinene, which made up 87% and 60% of the total discharge (2433 and 145 g/m³ wood), respectively. Most of the VOC fraction was released during the early stages of drying. The polar compounds were trapped in water. Total combined amount of methanol, ethanol, acetic acid and formic acid released over the two runs were 283, 281, 117, and 260 g/m³ wood, respectively. The aldehydes were trapped in 2,4-dinitrophenyl-hydrazine solution and the aldehyde derivatives analysed by HPLC. The total release of formaldehyde and acetaldehyde during the high temperature pre-drying run were 13 and 15 g/m³ wood and the CCA re-drying run were 14 and 4 g/m³ wood, respectively.     

Effect of thermal treatment on some properties of lime wood

Thermal treatment of lime wood was performed in a drying oven at two temperature levels (180 and 200 °C) and for four durations (1, 2, 3 and 4 h). Mass loss, color change, swelling and hygroscopicity were investigated. The dimensional stabilization reached up to 66.4 % and the hygroscopicity reduction up to 33 %, both maximum values being attained at 200 °C/4 h, associated with a mass loss of 9.3 %. The results will be realized in the manufacturing of solid wood panels made of heat-treated lime wood lamellas for outdoor uses.