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June 1, 2005
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The topochemical distribution of phenolic extractives in steamed and kiln-dried beechwood with discolourations was investigated on a cellular level by using scanning UV microspectrophotometry (UMSP). For the chemical characterisation of accessory compounds, acetone and methanol extracts of the discoloured beechwood were separated by accelerated solvent extraction (ASE) and analysed with high performance liquid chromatography (HPLC). The UV microscopic investigations reveal that the accessory compounds responsible for the discolouration of beechwood are mainly restricted to the longitudinal and ray parenchyma cells and the lumen of vessels. The detected extractives are characterised by high UV absorbance values and an absorbance maximum in a wavelength range between 280 and 290 nm. The separation of the acetone and methanol extracts of discoloured beechwood shows the presence of different low molecular phenols such as catechin and 2,6-dimethoxybenzochinon, which are transformed into high condensation compounds during steaming and kiln-drying.
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June 1, 2005
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We developed an effective detection method for wood-decaying fungi by hybridization of immobilized Sequence-Specific Oligonucleotide Probes with florescent-labeled PCR-amplified fungal rDNA internal transcribed spacer sequences. This method takes advantage of both the sequence specificity of Southern blot hybridization and the sensitivity of the previously reported PCR-based fungal species identification methods. Both in vitro cultured fungal strains and naturally decaying wood samples were used to demonstrate that this method is robust and practical for detection of incipient wood-decaying fungi. It can be a useful tool for microbial ecology, plant pathology, protection of wood products in service, preservation efforts for high-value furniture and wood-based art and DNA fingerprinting for tracking the source of contamination of wood decay fungi.
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June 1, 2005
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This paper examines the contribution of various xylem proteins to wood formation in Eucalyptus gunnii . Proteins were extracted from differentiating xylem harvested from a crooked tree, separated by high-resolution two-dimensional polyacrylamide gel electrophoresis, visualised by silver nitrate staining and analysed with a computer-assisted system for single protein spot quantification. Growth strain measurements allowed xylem samples to be classified quantitatively from tension wood to normal wood. Regression of lignin content on growth strain showed that a decrease in lignin content corresponded to decreasing growth strain values, i.e. , presence of tension wood. Out of the 140 studied protein spots, 12 were significantly associated with growth strain: 7 being less abundant in tension wood and 5 being more abundant in tension wood. A clustered-correlation analysis was performed to study protein expression simultaneously along the gradient of gravistimulated stressed xylem tissue. Proteins were found to form “expression clusters”.
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June 1, 2005
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The phenolic and lipophilic extractives in the heartwood of knots from seven Scots pine trees were analysed by GC, GC-MS and HPSEC. The knots contained large amounts of phenolic stilbenes, 1–7% (w/w), and lignans, 0.4–3% (w/w), while the stemwood contained around 1% (w/w) of stilbenes and no detectable lignans. In young trees without stem heartwood the stilbene content in the knots was up to 200 times that in the stem. Some in-tree and between-tree variation was seen in the content of phenolic compounds in the knots. The ratio of pinosylvin monomethyl ether to pinosylvin was higher in the knots than in the stemwood. The most abundant lignan was nortrachelogenin, but also matairesinol, secoisolariciresinol and liovil were present in small amounts in the knots. The knots also contained a complex mixture of lignan-like compounds, here called oligolignans. The flavonoid pinocembrin was present in both stemwood and knots in amounts below 0.02% (w/w). The stilbene concentration in the radial direction, from the pith to the outer branch, decreased or was on the same level inside the stem, while it decreased markedly in the outer branch. The lignan concentration was on the same level or decreased slightly inside the stem, while it decreased markedly in the branches and became almost non-existent within 10 cm out in the branches. The knots contained large amounts (4.5–32% (w/w)) of lipophilic extractives, mainly resin acids. Some in-tree and between-tree variation was seen for the resin acids. The abietane-type resin acids dominated over the pimarane-type acids and abietic acid was the most abundant resin acid in the knots and in stem heartwood. The amount of resin acids in the radial direction decreased or was on the same level inside the stem, while a clear decrease was detected in the branches. The profile of the distribution of resin acids and phenolic compounds was similar. The knots also contained up to 0.5% (w/w) of diterpenyl aldehydes.
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June 1, 2005
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June 1, 2005
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The chemical structure of lignin in normal and compression wood of spruce has been studied. Thioacidolysis using both methanethiol and ethanethiol as nucleophiles was used for degradation. The results showed that these nucleophiles work equally well in thioacidolysis, but that methanethiol gives a somewhat higher yield than ethanethiol. In spruce compression wood, several of the monomeric, dimeric and trimeric products were structurally elucidated using gas chromatography-mass spectrometry analysis of the desulphurated thioacidolysis products. Some of the oligomeric structures contained a p -hydroxyphenyl unit, but most of the material seemed to be present in non-condensed β-O-4 structures. A catechol structure was found and the results also indicated the presence of β-5 stilbene structures in native spruce lignin.
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June 1, 2005
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Three biomimetic compounds were evaluated for their ability to preferentially degrade lignin in the presence of carbohydrate using two water-soluble polymeric model compounds: lignosulfonate and hydroxyethyl cellulose (HEC). The three biomimetic systems studied were FeSO 4 , Fe-EDTA and hemoglobin, each in the presence of hydrogen peroxide. When both polymeric substrates were present, a high molecular weight product was observed to form upon addition of H 2 O 2 . This high molecular weight product is believed to be the result of a condensation reaction between lignosulfonate and HEC. The condensation product was also observed to form in the absence of biomimetic catalyst. For all reactions, the molecular weight of the condensation product was observed to decrease with increasing reaction time. By altering the ratio of lignosulfonate to HEC, a limit was observed in the relative amount of condensation product formed. The formation of this condensation product is believed to limit the effectiveness of acidic bleaching systems.
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June 1, 2005
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Sufficient stress relaxation within a medium density fibreboard (MDF) mat during hot pressing is critical for producing a panel with acceptable mechanical properties and dimensional stability. This paper describes a series of load relaxation tests of wood fibres conducted at temperatures and moisture contents similar to those within an MDF mat during the hot pressing process (temperatures ranging from 80°C to 100°C with relative humidity levels ranging from 9% to 68%). The results clearly show an increase in relaxation when either the temperature or relative humidity is increased. The relaxation behaviour of MDF is also measured and the trends observed are comparable to those of the wood fibres. Taguchi experimental design is employed to analyse the effects of temperature, moisture content and initial stress on the viscoelastic behaviour of MDF.
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June 1, 2005
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Wood fibre length of Scots pine ( Pinus sylvestris L.) was measured in wood sticks and 5-mm increment cores. The aim was to evaluate whether fibre length estimates from such small-diameter cores could be used to calculate genetic parameters, in spite of the increased amount of cut fibres produced at boring. The correlation between mean fibre lengths obtained from cores and sticks, with substantially fewer cut fibres, was high ( r = 0.87, n = 53) and of the same magnitude as the correlation between samples from varied positions in the same tree ( r = 0.87, n = 46). As regards evaluation of genetic tests and ranking for selection purposes, values from non-destructively sampled 5-mm cores from 0.5 m tree height appear to serve well. Fibre length development along annual ring classes started to differentiate between trees at annual rings 13–15, and after ring 16 there was a slight tendency towards stabilisation which may be interpreted as a reasonably advanced transition from juvenile wood to mature wood.
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June 1, 2005
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The occurrence of log-end cracks, due to the release of growth stress pre-existing in the standing tree, causes severe damage at the early stage of wood transformation. A mechanical model based on Griffith's theory for elastic-fragile materials has been developed to explain the observed patterns: a crack can only progress when the elastic energy release rate (G) exceeds the toughness (G c ) of the material for the given fracture mode and orientation. At each stage of the crack propagation, G was calculated using the finite-element method. The influence of various parameters related to the rigidity components, the initial growth stress field or the crack geometry has been investigated, based on a set of experimental data gathered on a population of Eucalyptus. In all cases the high G values just after crack initiation are followed by a marked decrease until the periphery has been reached. Their order of magnitude for a typical log is similar to G c values measured independently on similar material, thus supporting the validity of the approach.
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June 1, 2005
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This study develops a method to theoretically calculate ovendry softwood resistance and resistivity based on block transformation theory. The resistance of a single cell was calculated as the sum of a combination of serial and parallel connections of the two end cell walls, four side cell walls and cell cavity. One cell was considered as a resistor, and the wood as a network of single cell resistors. Resistance was calculated for a group of four resistors. After several iterations, the total resistance of wood was obtained. To test the model, experiments were conducted on Masson pine specimens. The resistances of fifty specimens were measured and their resistivities calculated. A comparison of the theoretical calculations and the measured results revealed a difference between the measured average resistivity of specimens and the model calculations of less than 8%. The resistivity of softwood was found to relate to the size of wood cells, amount of ray cells and the percentage of latewood.
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June 1, 2005
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The strength of larch compression wood specimens in longitudinal shear in the radial plane was determined and compared to normal wood. Fracture surfaces were examined with a scanning electron microscope. Compression wood showed higher shear strength than normal wood. The difference persisted after correction of the strength values for density. Scanning electron microscopy revealed clear differences in the pattern of failure in normal wood compared to compression wood. While transwall and intrawall fracture predominate in normal wood, intercell fracture at the middle lamella occurs in compression wood. An explanation of this change in fracture behaviour is proposed in terms of microfibril angle and lignification of the cell wall.
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June 1, 2005
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In this study the influence of oscillating climates on the drying of timber is investigated theoretically. First, the effect of oscillations superimposed on an ordinary drying schedule is examined by solving the corresponding partial differential equations. It is shown that the average drying rate is not affected by the oscillations. Second, properly selected oscillation amplitudes and frequencies can result in enhanced mechano-sorptive creep in the board surface layer, which enables increased external driving forces with an unchanged risk of checking. This method is analysed by using a drying simulation model. It is shown that substantial reductions in the drying time can be obtained in theory. From a practical point of view the benefit is probably rather marginal.
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June 1, 2005
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This study examined the effects of physical (moisture content, water content and wood density) and chemical properties (concentration of phenolics and resin acids) on the electrical properties of Scots pine ( Pinus sylvesteris L .) wood specimens. Complex impedance was measured from heartwood and sapwood specimens using frequencies between 5 kHz and 1 MHz. Significant correlation between density and electrical properties was found at high frequencies with sapwood specimens in which the extractive content was low. Moisture content had an effect on electrical properties over the whole frequency range. Electrical properties of heartwood samples with high extractive content were differently affected by the chemical and physical properties. Electrical properties were sufficient to distinguish between the samples from the brown-rot resistant and susceptible Scots pine trees.
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June 1, 2005
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The use of wood as a structure-giving material may be the key to producing temperature-resistant ceramics featuring high and directed porosity combined with necessary strength. The objective of this study was to develop a simple process to convert the evolutionarily optimized material wood into highly porous ceramics. Beech and pine, known to be relatively permeable, were pyrolyzed in a nitrogen atmosphere. The carbon-templates formed were infiltrated with various kinds of silica sol (SiO 2 ). The resulting SiO 2 /C composite was transformed into a SiC-ceramic (silicon carbide) via carbothermal reduction. Through the described process the macroscopic pore-structure of wood was transformed exactly into SiC. The SiC-ceramic produced proved to be thermo-resistant. It remained stable in oxygen atmosphere at 1200°C, after a SiO 2 coating around the SiC had been formed. This study focused on the alteration of the cell wall microstructure during the conversion of wood into SiC. Furthermore, the optimization of the individual process steps, pyrolysis, infiltration and ceramization along the most efficient route was pursued.
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June 1, 2005
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Fiberboard was manufactured without synthetic resins from spruce and beech fibers activated by treatment with Fenton's reagent (H 2 O 2 /FeSO 4 ) and the formation of radicals in the fibers by the action of Fenton's reagent was studied. The treatment resulted in a strong improvement of board strength and a large increase in fiber free radical content. The improved adhesion is probably largely due to interfiber bonds formed by reactions of radicals or other reactive groups generated in the fibers by Fenton's reagent. The reactivity of spruce fibers in terms of radical formation increased as defibration temperature increased. The thickness swell of the boards depended more on the amount of sizing agent or other additive than on the defibration temperature or fiber reactivity toward Fenton's reagent.
