Colour responses from wood,thermally modified in superheated steam and pressurized steam atmospheres

Abstract

In this study, two different methods were used to produce thermally modified wood. One was carried out in a typical kiln drying chamber using superheated steam (SS) and the other used pressurized steam in an autoclave cylinder (PS). Overall, both processes followed the same principles and the wood was not treated with any chemicals. Two wood species were studied, Scots pine (Pinus sylvestris) and Norway spruce (Picea abies). Treatments in the autoclave were carried out under pressure using temperatures of 160°C, 170°C and 180°C. Temperatures of 190°C and 212°C were used in treatments in the chamber at normal air pressure. The colour was measured using L*C*H colour space. Results for both species showed that similar L* (lightness) can be reached at lower (20–30°C) temperatures using PS compared with SS treatment. The hue angle of PS-treated wood was smaller than that of SS-treated wood. No significant difference in C* (chroma) was detected. The difference in E value between PS- and SS-treated wood was smaller for Norway spruce than for Scots pine. The residual moisture content was about 10% higher in wood treated by the PS process compared with the SS process.     

Changes in structural and chemical components of wood delignified by fungi

Summary Cerrena unicolor, Ganoderma applanatum, Ischnoderma resinosum and Poria medulla-panis were associated with birch wood that had been selectively delignified in the forest. Preferential lignin degradation was not uniformly distributed throughout the decayed wood. A typical white rot causing a simultaneous removal of all cell wall components was also present. In the delignified wood, 95 to 98% of the lignin was removed as well as substantial amounts of hemicelluloses. Scanning and transmission electron microscopy were used to identify the micromorphological and ultrastructural changes that occurred in the cells during degradation. In delignified areas the compound middle lamella was extensively degraded causing a defibration of cells. The secondary wall, especially the S₂ layer, remained relatively unaltered. In simultaneously white-rotted wood all cell wall layers were progressively removed from the lumen toward the middle lamella causing erosion troughs or holes to form. Large voids filled with fungal mycelia resulted from a coalition of degraded areas. Birch wood decayed in laboratory soil-block tests was also intermittently delignified. Selective delignification, sparsely distributed throughout the wood, and a simultaneous rot resulting in the removal of all cell wall components were evident. Scanning electron microscopy appears to be an efficient technique for examining decayed wood for fungi with the capacity to selectively delignify wood.The authors would like to thank Kathy Zuzek for technical assistance and Dr. M. Larsen, Forest Prod. Lab., Madison, for identifying the sporophores of Poria medulla-panis. This research was founded in part by a grant from the USDA Forest Service, Forest Products Laboratory and from the Graduate School, University of Minnesota     

Classification of the hot air heat treatment degree of larch wood using a multivariate analysis of near-infrared spectroscopy

In this study, hot air heat treatments of larch (Larix kaempferi) wood specimens were conducted at various temperatures (160–220 °C) and times (1–12 h) to classify the degree of hot air heat treatment using near-infrared (NIR) spectroscopy. NIR reflectance spectra were acquired from specimen cross-sections and were then preprocessed using the standard normal variate. Hierarchical clustering analysis (HCA) using the complete linkage and squared Euclidean distance was conducted to classify the three degrees of heat treatment. A principal component score plot of the NIR spectra was well grouped by the HCA grouping result, and the first component reflected the cluster analysis grouping well. A partial least squares discriminant analysis was performed to develop the discriminant regression model of the three heat treatment degrees. The R² and root mean square error of validation were 0.959 and 0.191, respectively. NIR is considered to be a good candidate to routinely measure the degree of hot air treatment for larch wood.     

Effect of steam explosion treatments on drying rates and moisture distributions during radio-frequency/vacuum drying of larch pillar combined with a longitudinal kerf

This study was carried out to investigate the effects of steam explosion treatment on drying rates, moisture distribution, shrinking, and checking during radio-frequency/vacuum drying of Korean larch pillar combined with a longitudinal kerf. All the pillars except the nonkerf/control with high initial moisture content (MC) could be dried within 89 h from the green condition to about 15% MC. In pillars with high initial moistures or in the early stage of drying, the drying rates were sharply accelerated by steam explosion treatments. The final moisture gradients along the transverse direction were gentler for the steamexploded pillars and for pillars with a longitudinal kerf, respectively, than for the unexploded pillars and for pillars without a longitudinal kerf. The moisture gradients along the longitudinal direction on all the layers were gentler for the pillars with a longitudinal kerf than for those without the kerf. Formation of checking was significantly controlled in the pillars with a longitudinal kerf. All the steam-exploded pillars except the kerf/1-cycle, however, were more severely damaged by checks than the unexploded pillars. A prong test revealed an extremely low level of residual stress for all the dried pillars.     

Changes of decay and termite durabilities of Japanese larch (Larix leptolepis) wood due to high-temperature kiln drying processes

We investigated the effects of high-temperature drying schedules (120°–130°C) on decay and termite feeding of Japanese larch timbers. Thermogravimetric analysis was conducted to investigate changes of the wood components. Decay and termite feeding tests showed that specimens dried under high-temperature schedules were susceptible against a decaying fungus Fomitopsis palustris and attacks from termites Coptotermes formosanus and Reticulitermes speratus. These drying schedules changed chemical components, which were suggested by the thermal analytical result compared to the control sample. The results of this study indicated that the acceleration of termite feeding takes place even under temperatures that are comparatively lower than that used in our previous research in which 170°C steaming treatment was applied to Japanese larch wood. Decay durability against a brown rot fungus also decreased, possibly from production of low molecular weight fragments when hemicellulose decreased during the high-temperature drying processes.     

Changes in chemical characteristics of bamboo (Phyllostachys pubescens) components during steam explosion

Chemical changes in cell wall components of bamboo internode during steam explosion process were analyzed to investigate self-binding mechanism of binderless board from steam-exploded pulp. More than 30% of xylose on initial mass, which is a major hydrolyzate of bamboo hemicelluloses, was lost after steam explosion treatment. Bamboo lignin is characterized by the presence of ester- and/or ether-linked p-coumaric acid to lignin. The content of phenolic hydroxyl groups of lignin isolated from steam-exploded pulp was characterized 2.3 times higher than those of the extract-free bamboo internode due to the cleavage of β-O-4 linkages. Alkaline nitrobenzene oxidation of the bamboo lignin gave vanillin, syringaldehyde and p-hydroxybenzaldehyde as major products. The content of p-hydroxybenzaldehyde decreased after steam explosion treatment, indicating the cleavage of ester- and/or ether-linked p-coumaric acid. The total yield of erythronic and threonic acids in ozonation products of the extract-free bamboo internode lignin was 268 mmol (200 g lignin)⁻¹, while those of lignins in the steam-exploded pulp and powdery fraction were 96 and 129 mmol (200 g lignin)⁻¹, respectively, suggesting the significant cleavage of β-O-4 linkages during steam explosion treatment. The cleavage of β-O-4 linkages was also confirmed by ¹H- and ¹³C-NMR spectroscopic observations.     

Effect of steam pretreatment on wood moisture content and characteristics of vacuum drying

The effects of high temperature steam pretreatment on the change in wood moisture content (MC) and characteristics of vacuum drying were investigated in this study. Poplar and manchurian walnut woods were pretreated with high temperature steam at 100°C and 140°C, prior to vacuum drying. A comparison of the characteristics of vacuum drying between steam pretreated wood and untreated wood was carried out. The results show that during steam pretreatment, the MC of wood decreased within a few hours. The reduction of MC varied with the temperature; the higher the temperature, the faster the MC dropped. During the vacuum drying stage, the rates of drying of pretreated samples were higher than those of untreated samples when MC was below the fiber saturation point. Furthermore, the total drying time of samples treated at a steam temperature of 140°C was lower than that of untreated samples. Therefore, a vacuum procedure after steam pretreatment can effectively shorten the drying time when drying wood.     

Changes of crystallinity in wood cellulose by heat treatment under dried and moist conditions

The different effects of heat treatment on wood, especially on the cellulose crystallites of wood under ovendried and highly moist conditions, were investigated by X-ray diffractometer. Heat was found to increase significantly the crystallinity of wood cellulose; moreover, almost twice as much crystallization was observed after heat treatment of spruce and buna under a highly moist condition than under the oven-dried condition. In pure cellulose almost the same crystallization was observed under both the conditions, whereas more crystallization occurred in wood cellulose than in pure cellulose. Absolute crystallization was observed for the wood species and pure cellulose under both conditions, considering the thermal decomposition of the amorphous region in addition to crystallization. Our results suggested that other components accomparying wood cellulose were involved in the increase of crystallinity by heat treatment, and that wood cellulose contained more quasicrystalline regions than pure cellulose. Moreover, calculated apparent activation energies revealed that crystallization and decrystallization in pure and wood cellulose under heat treatment of highly moist condition were some-what easier than those under the oven-dried condition. The behavior of the piezoelectric modulusd ₂₅ almost paralleled that of crystallinity.     

AN ANALYSIS OF HEAT AND MASS TRANSFERPHENOMENON IN WOOD DURING DRYING

INTRODUCTIONTheconventionaltwingmethodisgener-all3usedduringwoocldryinginChinaandothercountriesatpresent.Fluidvelocity,tem-peratUreandrelativehumidityofdryingmediaarecontrolledmainlybymoisbecontentofwood'dunngwtingprocess.Soitisveryim-portanttounderstandthephenomenonofheatandmasstransferinwoodduringdIyingproc-essformakingreasonablythewtingtechnoI-ogy-controlwtingprocess,heightenwtingquality'.decreasewtingenergyconsumptionanddiminishwtingtime.Thephenomenonofheatandmasstrans-ferinwooddurin…     

Experimental determination of the convective heat and mass transfer coefficients for wood drying

The knowledge of the convective heat and mass transfer coefficients is required for the characterization of the boundary conditions of the heat and mass transfer equations of a wood drying model based on water potential. A new experimental method for the determination of the convective mass transfer coefficient is presented. This method is based on the measurement of the moisture content, and indirectly the water potential, at the surface of a wood specimen at different drying times. Drying experiments were performed on red pine (Pinus resinosa Ait.) sapwood from nearly saturated to dry conditions at 56 °C, 52% relative humidity and air velocities of 1.0, 2.5 and 5.0 m s⁻¹. The results show that the convective mass transfer coefficient is constant until the wood surface moisture content reaches about 80% and then decreases more or less gradually as the moisture content decreases further. The convective mass transfer coefficient increases with air velocity. A regression analysis shows that there is no significant improvement in considering the water potential gradient near the wood surface when the difference in water potential between the surface and the surrounding air (ψ_s − ψ_∞) is used to determine the convective mass flux at the surface. Also, ψ_s − ψ_∞ is more appropriate than the water vapour pressure difference (pv_s − pv_∞) as the responsible driving force of the moisture flux leaving the wood surface. The convective heat transfer coefficient was determined during the same experiments. A plateau is observed at high values of moisture content corresponding to the constant drying rate period. Received 27 February 1998     

木材干燥产生缺陷的预防措施及补救方法

缺陷对木材的加工及使用影响很大 ,本文对木材干燥产生的缺陷及产生的原因进行了探讨 ,提出干燥引起缺陷的预防措施。     

Relations between various extracted basic densities and wood chemical components in Eucalyptus globulus

Relations between various extracted basic densities and wood chemical components were investigated by their within-tree variations in Eucalyptus globulus for assistance in the prediction of the properties of wood or wood-derived products. Extraneous compounds affect the relations between various basic densities and wood chemical components such as holocellulose and the lignin syringyl/guaiacyl ratio. We also discuss the relation of various densities, the molar composition of neutral sugars constituting hemicellulose, and fiber morphology.     

Mass and heat transfer mechanism in wood during radio frequency/vacuum drying and numerical analysis

The mass and heat transfer mechanisms during radio frequency/vacuum(RF/V) drying of square-edged timber were analyzed and discussed in detail, and a new one-dimensional mathematical model to describe the transport phenomena of mass and heat during continuous RF/V drying was derived from conservation equations based on the mass and heat transfer theory of porous materials. The new model provided a relatively fast and efficient way to simulate vacuum drying behavior assisted by dielectric heating. Its advantages compared with the conventional models include:(1) Each independent variable has a separate control equation and is solved independently by converting the partial differential equation into a difference equation with the finite volume method;(2) The calculated data from different parts of the specimen can be displayed in the evolution curves, and the change law of the parameters can be better described. After analyzing the calculated results, most of the important phenomena observed during RF/V drying were adequately described by this model.     

Optimization and simulation of drying processes using diffusion models: application to wood drying using forced air at low temperature

This paper discusses the suitability of four diffusion models to describe the drying of wood. The first model refers to the analytical solution of an infinite slab with boundary condition of the first kind. In the last three models, the boundary condition is of the third kind and the analytical solutions refer to the following geometries: infinite slab, rectangle, and parallelepiped. The analytical solutions were coupled to an optimizer based on inverse method, which enables to calculate process parameters for an experimental data set. Once the process parameters were determined, the drying kinetics of wood was simulated through each model proposed. The obtained results lead to the conclusion that the first model does not represent the drying kinetics properly. The last three models adequately describe the drying kinetics, but statistical indicators enable to affirm that the best of them is the three-dimensional model.     

The modelling of time-dependant deformation in wood using chemical kinetics

Summary The development of rheological models to predict creep has led to the derivation of quite complex equations that can predict creep reasonably accurately. However, these models are conceptual and are not based on a fundamental understanding of the actual deformation processes occurring within the material. The concept of modelling creep using a chemical kinetic approach is one that attempts to understand creep in wood at a molecular level and, from this, to develop models that more accurately predict creep deflections.This paper presents two models developed from chemical kinetic theory, that describe the time-dependant deformation of wood. The validity of applying these models to experimental data has been assessed by stress relaxation tests on thin samples of Sequoia sempervirens. Two stages of experimentation were carried out. In stage 1, both models were applied to the results of stress relaxation tests on 6 samples. Similar values of activation energy and activation volume were calculated by both models and a single energy barrier was found to dominate the deformation process.In stage 2, the effect of varying the initial applied stress on activation energy and activation volume was assessed by carrying out stress relaxation tests at stress levels of 25%, 30% and 35% of the short-term strength. Values of activation energy and activation were found to increase as the applied stress level decreased.Both models describe the time-dependent behaviour of wood well, however their ability to predict long-term creep deflections may be limited. Future work will develop these models further in order to improve long-term creep prediction and then apply them to the results of both creep and stress relaxation tests at a variety of stress levels and moisture contents in order to test their validity.     

Effect of intermittent heat treatment on crystallinity in wood cellulose

The effect of intermittent heat treatment on cellulose crystallites in wood was studied to evaluate quantitatively the changes of crystallinity induced by intermittent and continuous heating. The changes in crystallinity were found to be strongly affected by the intermittent heat treatment. The increased crystallinity, the width of the crystals, and the piezoelectric properties were the same for the first intermittent heating as for continuous heating. Further intermittent heating for the same time duration and temperature had no effect on the above properties, probably due to the stopping of the thermal reaction during the interval. Our results suggested that intermittent heat treatment has certain critical cooling temperatures that stop the thermal reaction and are closely related to the duration of the interval. Samples once exposed to a certain duration of heat treatment and then cooled need more time, about twice that of the first duration of intermittent heating compared with continuous heating, to reach maximum crystallinity in wood cellulose.     

Changes of chemical composition and crystalline of compressed Chinese fir wood in heating fixation

The changes in relative crystalline, chemical composition and internal structure of compressed Chinese fir wood after different heating fixations were found strictly related to fixation conditions. The compressed wood powders were fixed either by heating at different temperatures all resulting in a 10% recovery, or by incubating at 180℃ for different periods with subsequent recovery levels. Both X-ray diffraction and infrared absorption of those samples have been measured. Relative crystalline increases at early stage of heating fixation, and then decreased gradually. Hemicellulose and lignin decomposition were induced by the fixation process, especially at 180℃, and lignin was degraded actively. Furthermore, absorbed water was lost after heating, but cellulose did not change markedly. Although different fixation pathways can result in the same recovery level, the major chemical reactions underlying them vary, which is consistent with the difference of fixation mechanisms.     

杨木制备纤维乙醇过程中化学成分变化的分析

杨树是我国重要的速生树种。以杨木为原料制备生物乙醇是解决当今能源问题的一项新的尝试。对杨木原料蒸汽爆破预处理后以及菌处理后的化学成分及化学性质进行了测定分析。通过对比其化学成分(纤维素、木素、半纤维素、抽出物、灰分及蛋白质)的变化,研究了以杨木为原料制备生物乙醇生产过程中化学成分变化机理,旨在为指导杨木制备木质纤维乙醇的生产,并为发酵后副产物的回收再利用提供理论依据。     

Changes in wettability of wood surface using electron beam irradiation

ABSTRACT

This study deals with the influence of electron beam irradiation (EBI) on the wettability of Norway spruce surface samples. To evaluate this possible effect in detail, the changes in chemistry and wood structure were analysed using the methods of IR spectroscopy and Scanning Electron Microscopy (SEM). The surface wettability was determined by the contact angle measurement method. The results of infrared spectra show decreased relative numbers of available hydrophilic groups (e.g. hydroxyl and carbonyl groups) in the wood surfaces due to the electron beam irradiation. This behaviour is consistent with the values from the surface wettability measurements. Structural differences of the wood surfaces after electron beam irradiation were not detectable. Based on these findings pre-treatment of wood surfaces for industrial usages can be a potential field of application of this technology.     

Changes in the microstructure of birch wood after hydrothermal treatment

Birch wood (Betula pendula) samples were treated in a thermal regime (140, 160, 180 °C) for 1 h and investigated by means of scanning electron microscopy (SEM). SEM microimages of the wood cross-section were taken from one and the same place before and after the thermal treatment (magnification 100–2,000×). The results of measurements of areas and linear sizes of the birch wood cells show significant changes, which depend on the thermal treatment conditions and the type of the cell: libriform, tracheid, vessel and ray. After the treatment at 180 °C, the integrity of wood morphological structure begins to break up. Voids and cracks are formed between fibres, thus leading to a decline in the mechanical properties of the wood.