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 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.     

Effects of high temperature drying in nitrogen atmosphere on mechanical and colour properties of Norway spruce

Heat treatment and drying processes of wood are always accompanied by material changes. These changes are partly caused by oxidative reactions. This study aims to compare high temperature drying in inert gas atmosphere with conventional kiln drying of spruce wood. The mechanical properties as well as colour changes were measured after drying. In general, higher compression strength was observed for samples dried at high temperatures in nitrogen atmosphere. However, tensile and bending strength proved no significant difference. Corresponding to the strength values increased stiffness values were also observed for the compression samples, whereas for tension and bending samples no significant effect was visible. Up to a temperature of approx. 125 °C discolouration is prevented by drying samples in nitrogen atmosphere.     

Influence of steaming on selected wood properties of four hardwood species

Influence of steaming on various mechanical and physical properties of two European (Robinia pseudoacacia L., Quercus robur L.) and two tropical (Intsia bijuga, Hymenolobium petraeum) hardwood species were investigated. Each of these wood species requires adequate adhesive systems for the use as dimension stock because of their highly reactive surface chemistry. In order to optimize the gluing behaviour of the timbers involved, steaming processes with five different sets of parameters (steaming time and temperature) were carried out. In addition to the adhesive test, bending strength, hardness, and colour of the modified timbers were examined. The result of steaming highly depends on the wood species. For black locust, steaming is a suitable method of colour homogenization and colourization. Short term, low temperature treatment improves the adhesion performance also, whereas the colour change value reaches its maximum in the case of long term, high temperature steaming. Hue shift of oak and sapupira was inconsiderable for any applied set of parameters, only a small L* decrease was observed at higher temperatures. The colour of merbau samples shifted slightly during the treatment. Bending strength and hardness of wood samples in all of the four wood species decreased during the treatment. However, steaming time is more important than temperature while aspect of colour change both time and temperature has the same significance. Adhesive properties of sapupira can be greatly improved by hydrothermal treatment.     

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.     

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.     

Effect of drying temperature and beeswax content on physical properties of whey protein emulsion films

The objective of this work was to study the effect of drying temperature and the beeswax (BW) content on the physical properties of whey protein emulsion films. For this purpose, films were obtained by the casting method and dried at two selected temperatures (5 and 25 °C). Film thickness, water vapor permeability (WVP), solubility and mechanical properties were measured. The results showed that the decrease in drying temperature from 25 to 5 °C reduced the WVP and increased the solubility of the films containing BW. The effect of drying temperature on the mechanical properties was significant in the tensile test but not in the puncture test. The addition of BW decreased the WVP and the solubility and also had a significant effect on the evaluated parameters in both mechanical tests. In general, this effect was observed at both drying temperatures studied. Therefore, taking into account the several applications of the coatings the optimization of coating formulations and drying conditions is of vital importance.     

The mechanical properties of densified VTC wood relevant for structural composites

The mechanical properties of densified wood relevant for structural composites were studied. Low density hybrid poplar (Populus deltoides × Populus trichocarpa) was densified using the viscoelastic thermal compression (VTC) process to three different degrees of densification (63, 98, and 132%). The modulus of rupture (MOR) and the modulus of elasticity (MOE) of the control (undensified) wood and of the VTC wood were determined. The bonding performance of the control and VTC wood, using two phenol-formaldehyde (PF) adhesives, was studied. Four different 3-layer composites were also prepared from undensified and VTC wood, and tested in four-point bending. The results showed that the bending properties of the VTC wood (MOR and MOE) were significantly improved due to the increased density. The bonding performance of VTC wood with PF adhesives was comparable with or better than in the case of the control wood. Increased density of the face layers in the 3-layer VTC composites was advantageous for their mechanical performance.     

Changes in dimensional stability and mechanical properties of Eucalyptus pellita by melamine–urea–formaldehyde resin impregnation and heat treatment

This study investigated the influence of heat treatment on the dimensional stability and mechanical properties of untreated and low molecular weight melamine–urea–formaldehyde (MUF) resin treated Eucalyptus pellita wood. Wood samples were heat treated under vacuum atmosphere in laboratory conditions at temperatures between 160 and 240 °C for 2–10 h. The results showed that anti-shrink efficiency and anti-swelling efficiency of MUF resin-impregnated heat-treated eucalypt wood were improved by up to 47 and 49 % at 240 °C for 10 h separately, which were greater than those of heat-treated wood. In relation to mechanical properties, the modulus of elasticity and modulus of rupture decreased with increasing temperature and time, but the reduction of properties appeared to be smaller for MUF resin-impregnated heat-treated wood. Therefore, heat treatment combined with resin treatment of eucalypt wood shows potential to improve the wood quality of solid wood products.     

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.     

Physical and mechanical properties of thermally modified wood from E. grandis

This study was aimed at evaluating the effect of thermal treatment on the physical and mechanical properties of Eucalyptus grandis wood. Boards were taken from 5.9-year-old E.?grandis trees. The boards were thermally modified at temperatures up to 220°C in the Laboratory of Wood Drying and Preservation from UNESP, Botucatu, SP, in Brazil. The results indicated that the thermal modification caused significant decreases of up to: (1)?49.3% and 10.5% in the equilibrium moisture content and in the density of wood, respectively; (2)?53.3% in the volumetric swelling; (3)?52.3% in the modulus of rupture at static bending, without changes in the compressive strength parallel to grain and the respective modulus of elasticity; and (4)?20.7% in the Janka hardness. It was found that the specific gravity and compression parallel to the grain test were not the most suitable ones for evaluating the quality of thermally modified wood.     

Investigation of drying characteristics in superheated steam drying of UF-impregnated Chinese fir

Urea formaldehyde (UF) resin-impregnated Chinese fir (Cunninghamia lanceolata (Lamb.)Hook.) was dried at different temperatures in an atmospheric pressure superheated steam dryer. Drying characteristics, moisture content, drying rate, temperature profile, drying defects, and color change were investigated. The moisture content was reduced from 66.21 to 11.79% within 30 h without causing severe drying defects; in contrast, the conventional hot air process required 7–8 days. After 25 h of drying, the temperatures at both the center and the surface of wood remained stable. After 34.5 h, the surface temperature gradually approached the steam temperature. The color of the superheated steam dried Chinese fir appeared slightly more intense yellow and red than the control. Investigation of the UF-impregnated Chinese fir wood by scanning electron microscopy (SEM) revealed that the majority of the lumens and voids, including the microvoids in wood structure, was filled with urea formaldehyde resin.     

Untersuchungen zu ausgewählten Eigenschaften von Fichtenklangholz aus Graubünden

This survey focuses on the mechanical properties of spruce tonal wood from three different locations in the canton of Grisons, Switzerland. In addition, the material from one location was stored under water for three months in order to test the influence of extractives. The tests included the analysis of the annual ring structure, measurements of the sound velocity with longitudinal and transverse waves in the three main directions, bending strength, bending modulus of elasticity (MOE), eigenfrequency, logarithmic decrement of the damping, sorption and swelling. Single specimens were taken to measure the extractives content. The tested wood properties suggest tonal wood of high quality for all the locations. No significant differences of the wood properties, except density-related ones, were found between the three locations. The water storage resulted in the removal of extractives, but no significant change in properties could be found.     

Deformation properties of finnish spruce and pine wood in tangential and radial directions in association to high temperature drying

The set of papers sums up the results of an extensive project to quantify primarily the creep characteristics but also other deformation properties of Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) wood under conditions relevant in the high temperature drying process. The programme included tension experiments on tan-gentially and radially oriented specimens both under wet (saturated, green) conditions and drying conditions at temperatures 95–125 °C. Also, a limited strength test series was carried out with spruce. This first paper contains the documentation of the experimental techniques used as well as reports the results obtained about shrinkage, hygrothermal deformation and modulus of elasticity. The results of the strength tests are also included.     

Physical properties of burnt timber, with special focus on the drying performance

Burnt wood has been found to perform different from sound (green) wood when dried together, but also with regards to other physical and mechanical properties. In this study the drying performance of wood burnt to different degrees in recent plantation fires was investigated, and the physical, chemical and wood anatomical properties of these different wood types were determined. Different cell structure and chemical composition due to thermal degradation could be observed as well as different drying performance and variation in mechanical properties. An explanation for the deviant drying performance was attempted with the observed structural changes.     

Some physical and mechanical properties of thermally modified juvenile and mature black pine wood

In this study, test samples of juvenile and mature black pine wood were treated for 3 h at temperatures of 140, 170, and 200 °C. Mass loss, density, equilibrium moisture content (EMC), modulus of rupture (MOR), modulus of elasticity, and impact bending strength (IB) of the separate samples were determined. The purpose of the tests was to determine how heat treatment at the three temperatures influenced the properties of the juvenile and mature wood. The results showed that heat treatment had greater effects on mass loss, EMC, and density of juvenile wood than of mature wood. The results also showed that heat treatment had lesser effects on the MOR, modulus of elasticity, and IB of juvenile wood than of mature wood. The results clearly indicated that heat treatment had different effects on the properties of juvenile and mature black pine wood.     

Strength reduction of spruce wood through slow freezing

Some selected mechanical properties of spruce wood (Picea abies L.) were determined after freezing green timber boards under different conditions. The influence of the freezing rate and the time of exposure to negative temperatures were evaluated by applying three different freezing conditions which may occur in winter when green timber is stored in an open yard. It was found that a high freezing rate (?10 °C/h) does not affect wood strengths at all, while slow freezing (by ?1 °C/h) significantly reduces all mechanical wood properties, especially MOR, MOE and the compressive strength (by 20…30 %), the Janka hardness (by 18 %), and also the tensile strength (by 10 %). A longer time of exposure, involving repeated freezing and thawing due to natural temperature variations led to further reduction of MOR, MOE and the compressive strength (up to 37 %), but without further affecting the tensile strength, shear strength and hardness. The obtained results may be useful to industrials with respect to a more careful planning of green timber purchase and storage in wintertime.     

The influence of certain factors on thickness loss after the hot gluing of cut lamellas into solid wood layered boards

An experimental laboratory investigation has been carried out into the hot gluing of solid wood layered boards made out of 5.3 mm thick solid-wood lamellas, the latter being produced by the lengthwise veneer cutting technique, the object of the research being to determine some of the reasons (humidity of wood before heating, temperature of wood before cutting into lamellas, and temperature and pressure during the gluing of boards) for the permanent thickness loss which occurs during gluing. Spruce wood (Picea abies (L.) Karst.) was used in the experimental tests. Before cutting into lamellas, this wood was classified into two humidity groups, and heated to three different temperatures. The dried lamellas were used, after suitable preparation, for the two outer layers, which were glued, together with a middle layer of sawn lamellas, by the hot gluing process. A total of 162 laboratory boards, of length 500 mm, width 475 mm and thickness from 24 to 25 mm, were hot glued, using melamin-urea-formaldehyde glue, at three different gluing temperatures and three different gluing pressures. Regression analysis of the measured results of thickness loss showed that the influence of the studied factors on thickness loss was linear, and that thickness loss depends the most on gluing pressure, followed by gluing temperature. It was also found that a higher wood humidity results in a slightly greater thickness loss than the wood’s temperature before cutting into lamellas.     

Creep behaviour of commercial wood based boards under long-term loading at room conditions in Taiwan

The purpose of this study was to investigate the creep behaviour of commercial wood based boards (sanded and laminated) under long-term loading at room conditions in Taiwan. Ten types of commercial wood based boards were used to test their physical and mechanical properties in this study. Four types of these boards were used for the same test after laminating with red oak (Quercus spp.) veneer or balanced paper. The bending creep was measured at room conditions with central concentrated loading. The values for creep deflection of commercial wood based boards 18 mm thick decreased from 1.06–1.19 mm ti 1.00–1.12 mm after laminating veneer or balanced paper. The relative creep (120 days) of those specimens decreased from 1.71–3.17 to 1.43–2.04 after laminating with veneer or balanced paper in four months’ loading. The creep deflections of specimens increased with increasing loading time. The linear regression function ist/y=a+bt, wheret is time,y is deflection. When theb value is larger, the boards displayed good creep resistance. The residual bending strength of wood based boards after creep test would be retained at about 80–95%, and the residual bending strength between particleboard proved more significant than between fiberboard.     

Improvement of the heat treatment of Jack pine (Pinus banksiana) using ThermoWood technology

Thermal treatment in inert atmosphere is used to preserve wood without utilisation of toxic chemical agents. In addition, this process increases the dimensional stability of the wood matrix and results in attractive dark colour. On the other hand, it can deteriorate the mechanical strength and the flexibility of wood. For this reason, heat treatment parameters (such as maximum temperature, heating rate, the duration of the first plateau at constant temperature (100–120°C) and the second plateau at the maximum treatment temperature, humidity, final cool down rate) must be optimised in order to benefit from advantages of thermal treatment without deteriorating significantly the mechanical properties of wood. Correlation between wood properties and thermal treatment parameters depends not only on the wood species but also the environment (climate, soil) where the given species grow. This paper presents a study on thermal treatment of Canadian Jack pine (Pinus banksiana) using a medium size prototype furnace. The aim of this study was to optimize the set of parameters used during industrial treatments. The possibility of shortening the process time without causing any deterioration in wood quality was also investigated. The results show that increasing the maximum heat-treatment temperature increased the dimensional stability of Jack pine and darkened its colour. This parameter did not affect the modulus of elasticity but it decreased the modulus of rupture of Jack pine. A slight reduction in gas humidity during the initial warming up period permitted to shorten the drying period and at the same time increased the mechanical strength. This improvement helps save energy and increase productivity.