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January 12, 2007
Abstract
A preparation enriched in lignin-carbohydrate fragments (LCC-AcOH) was isolated in the course of purification of loblolly pine crude milled wood lignin (MWL). The preparation contained approximately equal amounts of lignin and carbohydrates, with high amounts of arabinose and galactose compared to their levels in wood. LCC-AcOH was investigated by 2D 1 H- 13 C (HMQC and HMBC) correlation NMR techniques and quantitative 13 C NMR. The HMQC spectra allowed direct detection of phenyl glycoside, ester and benzyl ether lignin-carbohydrates linkages in high amounts. The assignment of these structures was supported by the HMBC technique. It is noteworthy that in the ester lignin-carbohydrate linkages, a uronic acid residue was attached not to the benzylic position of lignin, as commonly believed, but to the γ-position of the side chain.
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January 12, 2007
Abstract
First-thinning Scots pine ( Pinus sylvestris ) and the outer (containing long fibers) and inner (containing short fibers) parts of its stem wood were delignified on a laboratory scale by kraft pulping followed by oxygen-alkali delignification and bleaching with D 0 (EO)D 1 (EP)D 2 . The aim was to evaluate the potential use of the bleached pulps as reinforcing material in various mechanical and chemical pulps. The physical and optical properties of the pulps indicated that only the “outer part pulp”, with rather similar properties to those of a reference softwood kraft pulp, seemed suitable for this purpose. In contrast, materials from first-thinning stem wood and its inner part resulted in lower yields as early as the kraft pulping stage. Rather mediocre strength properties were obtained, but the bleached kraft pulps prepared from the two first thinning-based materials had good optical properties.
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January 12, 2007
Abstract
Liquefied products with high wood content were prepared by pretreating wood with ozone before liquefaction. As a result, the ratio of wood to polyhydric alcohol (W/P ratio) used as solvent could be increased to 2:1. Resin blends were prepared by mixing liquefied products with ethylene glycol diglycidyl ether (EGDGE, water-soluble) and diglycidyl ether of bisphenol A (DGEBA, oily consistency). The wood content of the resin blend could be increased to 53%. The resins were cured by citric acid or triethylene tetramine (TETA), and their mechanical properties were evaluated. Dynamic mechanical measurements revealed that the former had higher glass transition temperatures than the latter. It was found that the resin with DGEBA cured by citric acid had almost the same level of tensile strength as commercial plastics.
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January 12, 2007
Abstract
There is increasing awareness of the durability, dimensional stability, and strength properties of thermally modified timber products. Appropriate standards and test methods for industrial-scale quality control are needed. The aim of this study was to investigate the suitability of CIE L*a*b* color measurements for implementation in industrial quality control. Good linear correlation was found between a cumulated measure of L* and b* data and the heat treatment intensity for spruce (Picea abies Karst.), pine ( Pinus sylvestris L.), and beech ( Fagus sylvatica L.). Wood samples homogenized by milling revealed less variation in color values compared to wood surface measurements. Thus, measurements on milled wood are recommendable to obtain results with high statistical significance. Color measurements are rapid, precise, and highly reproducible. Based on the strong correlation between color data and heat-treatment intensities, color measurements could easily be implemented for quality control of thermally modified timber.
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January 12, 2007
Abstract
The limited penetration of wood by light explains why the weathering of wood exposed outdoors is a surface phenomenon. Wood is rapidly degraded by short-wavelength UV radiation, but the penetration of light into wood is positively correlated with its wavelength. Hence, sub-surface degradation is likely to be caused by longer-wavelength light that still has sufficient energy to degrade wood. In this paper we test this hypothesis and determine the wavelengths of visible light that extend photodegradation into wood beyond the zone affected by UV radiation. Sugi ( Cryptomeria japonica ) earlywood was exposed to UV and visible light with narrow band gaps (20 nm) and the penetration of light into the wood was measured using a photodetector. Photodegradation was depth-profiled using FT-IR microscopy. There was a positive correlation between the penetration of light into sugi earlywood and the wavelength of the incident radiation within the range 246–496 nm. The depth of photodegradation also increased with wavelength up to and including the violet region (403 nm) of the visible spectrum. Blue light (434–496 nm) penetrated wood to a greater extent than violet light and was capable of bleaching the wood, but it did not significantly modify lignin, and hence it was not responsible for sub-surface photodegradation of wood. We conclude that violet light is the component of the visible spectrum that extends photodegradation into wood beyond the zone affected by UV radiation. Accordingly, surface treatments designed to protect wood used outdoors should shield wood from the effects of violet light.
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January 12, 2007
Abstract
The dynamic mechanical analysis (DMA) of wood with moisture content (MC) below 1% has not yet been described. Aiming at this low MC range, the linear viscoelastic response region (LVR) of thoroughly dried wood was studied in single-cantilever bending. The LVR limit was determined as a function of grain orientation and temperature using yellow-poplar ( Liriodendron tulipifera ) and southern yellow pine (Pinus spp.). The LVR limit for dry wood ranged from approximately 0.03% to 0.16% strain. The LVR limit was greater for bending perpendicular to the grain than for parallel to the grain, suggesting that DMA signal quality would be better in the former case. Southern pine generally exhibited a greater LVR limit than yellow-poplar. The LVR anisotropy was greater in yellow-poplar than in southern pine. These findings suggest that detailed LVR analysis might be useful for wood analysis, or at least that regular LVR analysis is required for reliable wood DMA. The effects of wood moisture changes (between 0% and 1%) were observed in low-temperature secondary relaxations, consistent with the previous findings of others. Yellow-poplar specimens exhibited a significant storage modulus increase over a 250°C temperature range when specimen moisture increased from 0% to ∼0.7%.
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January 12, 2007
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The mechanical and physical properties of modified wood were assessed experimentally. Timber studs with a cross-section of 45 mm×70 mm were modified on a semi-industrial scale by four different methods: 1) acetylation (AC) with acetic anhydride; 2) modification with methylated melamine formaldehyde resin (MMF); 3) heat treatment in vegetable oil (HT); and 4) furfurylation (FA). Sapwood of Scots pine ( Pinus sylvestris ) with test specimen dimensions of 10×10×200 mm 3 was investigated. A total of 2449 specimens were included in the study. The following properties and their correlations were studied: density, modulus of elasticity (MOE dyn ) measured dynamically, equilibrium moisture content (EMC), and swelling behaviour. The properties were measured at humidity levels of 30%, 65%, and 90% RH, while the temperature was kept constant at 23°C. Significant changes in material properties took place. AC increased the density and reduced MOE dyn , EMC and swelling strain. FA increased the density and the swelling coefficient and reduced EMC and swelling strain. HT reduced the EMC, while MMF modification increased it.
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January 12, 2007
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The thin-veneer strip technique was applied to investigate the modifying effects of 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) on the tensile strength of wood. Pinewood veneers treated with solutions of DMDHEU alone and in combination with magnesium chloride (MgCl 2 ) as a catalyst showed considerable strength losses of up to 50% in zero-span and up to 70% in finite-span testing modes. The higher strength losses in the latter case are due to hemicellulose degradation, which cannot be assessed in zero-span testing. Strength loss observed after treatment with DMDHEU and MgCl 2 was approximately as high as the sum of the strength losses determined after individual treatments with MgCl 2 or DMDHEU. Micrographs of veneers after finite-span testing revealed that catalysed DMDHEU treatment changed the predominant failure mode from interfibre fracture (in controls) to intrafibre fracture. The mechanism of strength loss resulting from treatment with DMDHEU is discussed.
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January 12, 2007
Abstract
An amino-silicone in the form of a micro- and a macro-emulsion (average particle size 40 and 120 nm, respectively) was tested in impregnation of solid wood. During vacuum-pressure impregnation, the uptake of 5% emulsions was only slightly reduced compared to water up-take. At 15% concentration, a significant reduction in emulsion uptake was observed, particularly in the radial and tangential directions of the wood. The penetration of silicone into the cell wall was dependent on the particle size of the emulsion. Applied as a macro-emulsion, a maximum of only 14% of the total silicone penetrated the cell wall, whereas 25–35% of the silicone was found in the cell wall when applied as a micro-emulsion. The degree of cell wall penetration of silicone for the micro-emulsion was confirmed by X-ray micro-analysis (SEM-EDX). The penetrated silicone caused bulking and dimensional stability. 1 H NMR relaxometry T 2 distributions were determined for dried and moisture-conditioned wood samples. The silicone-treated specimens displayed spin-spin interactions of protons from water and silicones (or their emulsifiers). Conditioning of samples at 25°C and 65% relative humidity appeared to cause a decrease in the mobility of the non-water protons of the silicone and/or emulsifier. Freezing to -5°C caused separation of protons from bound water and from the silicone at low relaxation times. It is assumed that treatment with the micro-emulsion reduced water sorption due to micro-pore blocking in the cell wall. The formation of a sandwich complex is postulated, which comprises cell wall polymers, water molecules and a layer of silicone.
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January 12, 2007
Abstract
The relation between growth stress and stress wave velocity has been investigated on standing trees of Eucalyptus nitens . Longitudinal growth stress and the stress wave velocity were measured at breast height on 155 trees. Large variation in both growth strain and acoustic velocity was observed. The initial investigation with 34 trees showed good correlation (R 2 =0.68). Measurements on all 155 trees revealed no reliable relationship (R 2 =0.05). The influence of internal stress on acoustic velocity has also been investigated on radiata pine boards loaded in bending and tension. It was observed that the resonance frequencies were influenced by the position of loaded supports in bending, but were not affected by the bending load. In boards under tensile stresses, there was no change in acoustic velocity measured by the pulse transit time. The two experimental series, one on standing trees with water-filled lumens and the other on air-dried boards, failed to reveal any dependence of the stress wave velocity on growth stress. No valid basis therefore exists for the use of stress wave velocity to determine growth stress levels in either standing trees or in seasoned wood.
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January 12, 2007
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Hybrid poplars are currently used in North America primarily for the production of pulp fibre and in the manufacture of engineered solid wood products. Recently, the deployment of poplars as a short-rotation fibre crop has been of interest to mitigate the increasing amount of plantation-grown short fibre resources (hardwoods) derived from the Southern Hemisphere, as well as in the context of global climate change, both as a means to rapidly sequester carbon and as a feedstock for potential bioenergy production. Knowledge on the utility of hybrid poplars in the value-added secondary wood-processing sector, however, is very limited. To improve this situation, the variation in kiln-drying quality of five hybrid poplar genotypes of similar age, harvested from a common site in British Columbia, Canada, was evaluated for three different kiln-drying schedules. The results clearly demonstrate that the drying schedule has a greater effect on grade recovery and the degree of deformation than the hybrid poplar genotype. Furthermore, it was shown that many of the deformations inherently associated with wood derived from fast-grown trees can be reduced or removed with drying, in particular with an aggressive drying schedule.
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January 12, 2007
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As a continuation of previous publications on a physical-mathematical model of heat and mass transfer and a structural model of mat permeability, this paper presents typical prediction results for 15 pressing variables for strand mats. A case study and complete solutions to the governing equations are provided. The results show how the heat and mass transfer is controlled by heat conduction and convection involving gas flow and phase change. The model predictions provide a comprehensive illustration of the temporal and spatial variations of basic pressing variables, including mat temperature, gas pressure, moisture control and resin curing rate. The model offers a powerful tool for simulating the effects of mat structure, pressing schedule and initial mat conditions.
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January 12, 2007
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The effects of panel density and strand size on the temperature and gas pressure inside strand mats during hot pressing has been experimentally investigated. The results show good agreement with model predictions. Strand dimensions have a strong effect on the core temperature and gas pressure when the mat/panel density is relatively high. At lower density, the temperature and gas pressure are controlled only by the panel density. Comparison between the model predictions and experimental results also reveals the need for further characterisation of the basic mat properties, especially conductivity and permeability.
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January 1, 2007
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The dielectric relaxation due to motions of water molecules adsorbed on wood treated at various temperatures up to 550°C was investigated based on the relationships between activation enthalpy (Δ H ) and entropy (Δ S ) in relaxation. The relationships indicated two straight lines with different slopes, depending on the treatment temperature. Given the same Δ S , Δ H values for water molecules adsorbed on wood treated at temperatures below 400°C were greater than for those treated above 450°C. It was considered that before heat treatment, water molecules were adsorbed mainly on hydroxyl groups by strong hydrogen bonds and formed ice-like structures. On the other hand, hydroxyl groups were not detected in wood after heat treatments above 400°C. We assume that water molecules are condensed in nanometer-scale micropores that are formed during the carbonization of wood. The relationship between Δ H and Δ S for the motion of water adsorbed on wood treated above 450°C was similar to the value extrapolated from that for bulk water at temperatures below 0°C. We suggest that water molecules are adsorbed on charcoal in a supercooled state.
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January 1, 2007
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The combined effects of the grain-angle ( θ ) and the microfibril angle (MFA) on the piezoelectric voltage ( P ) are discussed for a wood specimen subjected to unidirectional stress. The fiber axis of the specimen investigated was not parallel to the loading direction. A theoretical discussion is provided concerning fundamental elastic equations that predict the elastic strain field in the crystalline cellulose microfibril (CMF). It was assumed that an ideal wood specimen could be represented by a multi-tracheid model consisting mainly of the middle layer of the secondary wall. Experimental results revealed that plots of the piezoelectric voltage ( P ) versus the grain angle ( θ ) show concave curves with a maximum at θ =45°, regardless of MFA in the specimen. A similar tendency was obtained for the relationship predicted between the average shear strain along the CMF and θ . The results indicate that generation of a shear strain in the CMF positively contributes to the piezoelectric effect in wood.
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January 1, 2007
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January 1, 2007
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The occurrence of fungal species on pine sapwood samples obtained from an above-ground field test study was analysed by terminal restriction fragment length polymorphism (T-RFLP), cloning, and sequencing. Samples were taken from eight double-layer set-ups that were exposed to the environment at six different locations in south-west Germany. The occurrence of fungal species was correlated with decay intensity and rot types on one hand, and characteristics of the test sites, such as precipitation, average temperature and height above sea level on the other hand. In total, 62 different fungal species were found based on T-RFLP, cloning and sequencing. Of the 39 species that were found four or more times, 30 were ascomycetes, five were basidiomycetes, and four could not be classified. The most common fungus found in this study was Coniochaeta ligniaria ((Grev.) Cooke), a soft rot fungus that occurred in 87 of 152 samples (57%). No single factor at the test sites seemed to be decisive for the abundance of fungal species or decay intensity. Within the first years of this study, soft rot fungi was found more frequently in pine sapwood specimens than basidiomycetes.
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January 1, 2007
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