Zur dynamischen Torisonselastizität von Holz

The changes in torsional elasticity of untreated and fire retardant impregnated beech and spruce specimens under cyclic temperature load were determined in torsional vibration tests. A temperature of 60°C already caused a permanent 25% reduction in torsional modulus of elasticity which remained unaffected by further temperature rises and by repeated heating and cooling cycles up to 200°C. Above 200°C oxidation presumably occurred with wood strength deteriorating visibly. Also, previously water saturated specimens showed greater losses in torsional elasticity than air-dried or dried ones. Fire retardant treatment with acid salt mixtures reduced torsional modulus of elasticity by 35% already at room temperature and subsequently led to a rupture of specimens at 200°C. Apart from fluctuations in absolute values both wood species reacted similarly to temperature load and protective treatment.     

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.     

Effect of grain orientation and surface wetting on vertical density profiles of thermally compressed fir and spruce

The effects of grain orientation and surface wetting on wood densification by compression in a hot press were evaluated for two commercial Canadian wood species, balsam fir (Abies balsamea) and black spruce Picea mariana. The vertical density profiles (VDP) of wood densified at 180 °C could be engineered to achieve different properties depending on press closing rate, wood permeability and annual ring orientation. The lower permeability of spruce caused it to split frequently during hot pressing. For balsam fir, at a press closing time of 2 min, the compressed wood with an original grain angle of 0° (radial compression) shows widened high density bands due to collapse of low density earlywood adjacent to the dense latewood. All grain orientations show higher density areas close to the wood surfaces similar to those of wood-based composites. However, when wood was preheated without pressure for 5 min followed by a press closing time of 2 min, water migrated to and plasticized the board centre causing it to be densified while the surface density remained low. Wood surface plasticizing with water or urea solution causes some localized surface densification, but the effect was not great.     

Bending properties of compressed wood impregnated with?phenolic resin through drilled holes

The effect of drilling on the permeability of Japanese cedar and the bending properties of resin treated compressed wood were examined. Compressed wood was manufactured by impregnating aqueous phenolic resin solutions into the heartwood through drilled holes and polymerizing the impregnated resins using a hot press to maintain the deformation. The compressive deformation was carried out at 150°C for 1 h to one-half of the original specimen thickness. A dye solution permeated the entire specimen when the density of drilled holes exceeded approximately 5000?holes/m². When the compressed wood was manufactured under the drilling condition, the resin type was found to greatly affect the bending strength and the failure mode of the specimens in bending test.     

Bending performance of 3-layer beech (Fagus sylvativa L.) and Norway spruce (Picea abies (L.) Karst.) VTC composites bonded with phenol–formaldehyde adhesive and liquefied wood

Low quality beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) were densified with viscoelastic thermal compression (VTC) process to two different degrees of densification, and lamellas were used to manufacture different types of 3-layer VTC composites. Bending properties of 3-layer VTC composites bonded with phenol formaldehyde (PF) adhesive and liquefied wood (LW) were determined and compared to 3-layer composites produced with undensified beech or spruce wood lamellas. Morphology of VTC spruce wood of higher density was analysed with fluorescent microscopic technique. All composites produced with densified beech lamellas and bonded with PF adhesive had significantly higher values of modulus of rupture (MOR) and modulus of elasticity (MOE) than composites produced with undensified lamellas. Densified spruce lamellas contributed to better bending performance of 3-layer VTC composites bonded with PF adhesive to some extent. Furthermore, composites bonded with LW had significantly lower MOR and MOE values compared to composites bonded with PF adhesive. Study of VTC spruce wood microstructure showed that densification caused non-uniform deformation of cell wall structure, in which cell wall fractures were observed.     

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.     

Reducing the set-recovery of surface densified solid Scots pine wood by hydrothermal post-treatment

The mechanical properties of wood surface are of particular interest in applications where mainly the surface is exposed to use, such as flooring boards. Wood surface densification aims to improve these properties by compressing only the first few millimeters beneath the surface. Scots pine (Pinus sylvestris L.) sapwood was surface densified to three degrees of compression at 150 °C using a specially designed heated press. Half of the specimens were hydrothermally post-treated at 200 °C. To study the influence of the degree of compression and hydrothermal post-treatment on the set-recovery, specimens were subjected to water soaking-drying-cycles. The surface densification process resulted in a minor mass loss regardless of the degree of compression, whilst the hydrothermal post-treatment led to an average mass loss of 3.8 % in surface densified specimens and of 4.1 % in control un-densified specimens. Furthermore, considerable fixation of compressive deformation was obtained by hydrothermal post-treatment at 200 °C.     

Zur Beurteilung der mechanischen Eigenschaften von Preßvollholz

Pine sapwood specimens of standardized dimensions were densified in a press, in radial direction at 10% humidity down to 90, 70, and 50% of their original dimensions, while control specimens were diminished in size by planing. Compression strength in fiber direction and static bending strength of densified and planed specimens were determined. Bending tests were carried out at constant span length as well as at span lengths adjusted to varying specimen heights. Strength values and quality indices obtained for densified specimens were compared with those of undensified and diminished specimens. Results show that compression and bending failure load transferred by densified wood elements is remarkably lower than that of non-densified elements. The sole advantage of densified wood elements is that their dimensions perpendicular to fiber can be reduced, making their use in constructions more economic spacewise. This, however, will rarely be of great practical significance.     

The wettability and bonding performance of densified VTC beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) bonded with phenol–formaldehyde adhesive and liquefied wood

The influence of viscoelastic thermal compression (VTC) on surface wettability and bonding performance of wood was evaluated. Low quality beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) were densified with the VTC process to different degrees of densification. Control and densified strips were bonded with phenol–formaldehyde (PF) adhesive and liquefied wood (LW). Shear strength of bonded assemblies was determined after 1 week of conditioning at 20 °C and relative humidity of 65 %. Wettability was determined on the basis of the contact angle of water, PF adhesive, and LW using the Wilhelmy method. Results showed that densification of beech and spruce wood did not significantly affect the shear strength of specimens bonded with PF adhesive. In beech assemblies bonded with LW shear strength decreased significantly with increased density, whereas in bonded spruce specimens decrease of shear strength was not significant. It was found that degree of densification and bonding process used in the study were not appropriately chosen for spruce wood specimens, since major deformations after the bonding process occurred. Wettability changed significantly after densification. Contact angle of water and LW increased after densification, whereas contact angle of PF showed inverse trend and decreased after VTC process. Furthermore, the degree of densification had a minor effect on the wettability.     

Mechanical behaviour of Québec wood species heat-treated using ThermoWood process

Finnish wood heat treatment technology, ThermoWood, was recently introduced to Québec, Canada by Ohlin Thermo Tech. Subsequently, a large number of initial trials were conducted on five commercially important Québec wood species, spruce (Picea spp.), pine (Pinus spp.), fir (Abies spp.), aspen (Populus spp.), and birch (Betula spp.). These species were thermally-modified in different batches at temperatures of 200 °C or higher. The static bending and hardness of the thermally-modified wood were examined. Decreases of 0% to 49% were observed in modulus of rupture of heat-treated spruce, pine, fir, and aspen depending on species and treatment schedules used; modulus of rupture of birch increased slightly after the heat treatment. The decrease in modulus of elasticity of heat-treated spruce and pine ranged from 4% to 28%; but the modulus of elasticity of heat-treated fir, aspen, and birch increased except one trial for fir. Hardness of the heat-treated wood increased or decreased depending on the species, test directions (radial, tangential, and longitudinal), and treatment schedules.     

Mechanical and physical properties of thermally modified Scots pine wood in high pressure reactor under saturated steam at 120, 150 and 180 °C

Scots pine sapwood and heartwood were thermally modified under saturated steam at 120, 150 and 180 °C in a high pressure reactor. Mechanical properties such as dynamic and static modulus of elasticity (MOE), static modulus of rupture (MOR), Brinell hardness and impact toughness were evaluated. The static MOE for sapwood did not decrease substantially (approximately 1 %), not even with a high mass loss of more than 12 %, when the wood was modified at 180 °C. Static MOE of the wood increased approximately 14 %, when modified at 150 °C. Surprisingly, MOR increased by 15 %, when modified at 150 °C with mass loss of 2.3 %. Whereas impact strength and hardness decreased somewhat, when modified at 180 °C. Moreover, high anti-swelling efficiency values were obtained (60 % for sapwood and 52 % for heartwood) when modified at 180 °C.     

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.     

A study of strength,hardness and deformation of acetylated Scandinavian softwoods

In this work, properties such as bending strength (expressed as modulus of rupture and modulus of elasticity), hardness, and deformation under cyclic relative humidity have been studied for acetylated Scandinavian pine and spruce. The acetylation was performed with a limited amount of liquid acetic anhydride without addition of catalyst or organic cosolvent. The weight gain due to acetylation was 19.1% for the pine samples. Two kinds of spruce samples were acetylated to 18.2% (narrow annual rings) and 23.3% (broad annual rings), respectively. The results indicated that only small changes in strength were obtained. For pine, modulus of rupture decreased with about 6%, and increased with about 7% for spruce. The modulus of elasticity also decreased for pine but increased for spruce. For practical applications, these properties can be considered unaltered. Acetylated pine wood showed increased Brinell hardness. Acetylated spruce wood did not tend to deform as much as unmodified wood when exposed to moisture cycled between 40 and 90% relative humidity     

Beitrag zur Prüfung der Leimbindefestigkeit von schutzmittelgetränktem Holz

A specimen for testing shear strength under compression was dimensioned to evaluate the glue bonding strength of impregnated timber. Upon investigating the influences of the course of annual rings, thickness and height of the specimen and the amount of glue used, optimum dimensions for the specimen were fixed. One oil-borne and one water-borne preservative were tested for their effects on the glue bonding strength in pine and spruce specimens. For pine values for shear strength under compression were the same for impregnated and control specimens; failure in the wood also showed corresponding values in all three test groups. For spruce impregnated specimens had considerably lower shear strength values, whereas failure in the wood remained constant. This suggested an evident reduction in shear strength in spruce wood due to impregnation.     

Thermo-mechanical densification combined with thermal modification of Norway spruce (Picea abies Karst) in industrial scale – Dimensional stability and durability aspects

Heat-treatments of wood to improve selected wood properties, e.g. durability and dimensional stability, are well established industrial processes. However, the main drawbacks of thermally modified timber are the reduced strength properties. In a previous study, thermo-mechanically densified wood with increased initial strength was successfully applied to an oil-heat treatment (OHT) in laboratory scale to overcome the problem of reduced strength properties. Consequently, the up-scaling of processes to industrial scale was the objective of this study. Therefore, Norway spruce (Picea abies Karst.) was thermo-mechanically densified in laboratory scale at 140 °C, 160 °C, 180 °C, and 200 °C for 0.5 h, 1 h, 2 h, and 4 h and afterwards modified by a laboratory OHT-process at 180 °C, 200 °C, and 220 °C for 2 and 4 h. Swelling properties and biological properties were investigated on matched samples to identify suitable combinations of densification and OHT for use in outdoor application. Further on, the process-parameters assessed from laboratory scale were taken over for industrial scale production. The results show that compression-set recovery of densified and oil-heat treated spruce was almost completely eliminated by an OHT at temperatures above 200 °C, as demonstrated in laboratory tests and after 30 months natural weathering. Thus, with respect to the dimensional stability and improved durability, the industrially densified and oil-heat treated spruce timber appears to be suitable for weathered application.     

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.     

Influence of pressing parameters on dimensional stability and density of compressed beech wood

European beech (Fagus sylvatica) wood specimens were subjected to thermo-mechanical densification, and the influence of pressing parameters on dimensional stability, mean density and density profiles was evaluated. The specimens with two initial moisture contents, 18% and the fibre saturation point, were compressed by 20 and 40% at temperatures of 160, 180, 200, and 220 °C for several times, specified in the Methods, ranging from 6 to 12 min. Dimensional changes were observed immediately after the removal from the hot press and subsequent conditioning (20 °C, 65% relative humidity). The properties of the compressed beech wood were influenced by all the pressing parameters. This influence, however, varied based on the factor and property. The analysis showed as most appropriate: an initial moisture content ranging from 17 to 20%, a pressing temperature?≥?180 °C and pressing time?≥?10 min for the given thickness. As for the compression degree, it is necessary to consider the purpose of compression and the wood properties required.     

Beitrag zur Torsionselastizität von Hölzern in Abhängigkeit von der Holztemperatur und der Belastungszeit

This paper is concerned with the dependency of the torsion modulus of spruce, oak and beech on temperature and time. The torsional clasticity of these species was investigated in a temperature range from 20° to 70°C, and additionally for spruce and oak, the creep behaviour under torsion was determined within the first 10³ seconds. The properties mentioned were measured at prismatic wood specimens by means of static torsion tests.     

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.     

Fertigung and Eigenschaften von schnell abbindenden,zementgebundenen Probekörpern,hergestellt durch Begasung mit Kohlendioxid

A mixture of spruce or larch wood particles, a lime rich cement, and water were pressed into specimens using an enclosed mold. The mold enabled connection to a vacuum pump or introduction of pressurized gas. The filled mold was first of all evacuated, and subsequently pressurized carbon dioxide was introduced. The influences of the following parameters were examined: carbon dioxide pressure, water/cement ratio and addition of calcium hydroxide on the maximum hydration temperature, the total hydration time, the surface soundness, the spring-back, the carbon dioxide content, the calcium hydroxide content, and the weight increase of the specimens. Moreover the influence of the storage time (under moist conditions) on the properties of the specimens were determined. Optimum fabrication conditions are: carbon dioxide pressure 7 bar, water/cement ratio 0.3 and addition of calcium hydroxide 10%. Under such conditions the binder solidifies within one to three minutes, and the specimens can be removed directly from the press. The specimens show already 75% of the ultimate strength properties five hours after pressing. Both spruce and larch wood particles are suitable for fabricating of stable specimens.