Effects of Wood Drying Schedules on Fluid Flow in Paulownia Wood

Effects of three drying schedules on fluid flow were studied in the sapwood and heartwood of Paulownia wood (Paulownia fortunei). Boards with a commercial thickness of 5 cm were randomly dried to a final moisture content of 8 ± 2% using a mild (T₆E₃), a moderate (T₆E₄), and a severe (T₇E₄) drying schedule. Permeability measurement was carried out when specimens reached the final moisture content. Results showed a significant difference in the specific gas permeability as well as liquid permeability of the boards dried under the three drying schedules. Furthermore, a significant difference was observed in the gas permeability of sapwood and heartwood, dried under the different schedules, but not much significant difference was seen in the liquid permeability between sapwood and heartwood. T₆E₃ had the highest liquid permeability; furthermore, it was reported to have resulted in the lowest warping and most homogeneous moisture profile. This mild schedule is therefore recommended for commercial drying of Paulownia wood when further preservation and impregnation processes are planned for the dried boards. In the meantime, it is concluded that the age and drying schedule have significant effects on the formation of tyloses, significantly affecting gas and liquid permeability in Paulownia wood.     

Effects of Drying Schedules on Physical and Mechanical Properties in Paulownia Wood

The effects of six drying schedules on physical and mechanical properties of Paulownia wood (Paulownia fortune Seem.) were studied. Three schedules were based on the recommendations by Forest Product Laboratory (FPL), while the other three were established based on diffusion theory. FPL schedules consisted of a mild (T₆E₃), a moderate (T₆E₄), and a severe (T₇E₄) drying schedule; diffusion schedules consisted of three initial moisture contents (MC) of 113% (Dif-1), 75.5% (Dif-2), and 53.5% (Dif-3). Boards with a commercial thickness of 5 cm were randomly dried to the final moisture content of 8 ± 2% in all six schedules. Results indicated that drying under diffusion schedules had the most improved properties due to the beneficial effect of heat upon wood plasticity. However, lower plasticity effect in FPL schedules, due to lower temperatures, resulted in lower properties in most cases.     

The Quantitative Effect of Drying on the Surface Color Change of Reaction Woods: Spruce Compression Wood (Picea abies L.) and Poplar Tension Wood (Populus nigra L.)

The surface color change of compression wood in spruce (Picea abies L.) and tension wood in poplar (Populus nigra L.) due to drying was investigated using CIE LAB and CIELCh colorimetry techniques. The results showed that the compression wood was lighter, yellower, less red, and more statured in color with a deeper hue after drying. Similar changes were also seen with the tension wood, except that it was less yellow and less statured in the dry condition. The color change (ΔE) of compression wood was found to be more remarkable than that of tension wood. Overall, the difference in the colorimetric parameters between the reaction woods and their corresponding normal woods was less significant after drying.     

Effect of steam-drying treatment on moisture content,drying rate,color, and drying defects in juvenile wood of Tectona grandis from fast-growth plantations

Tectona grandis is the second most important species for reforestation in Costa Rica, and any improvement in its industrialization process is important, especially the drying process. Lumber obtained from a plantation of 12-year-old T. grandis trees was used to evaluate three drying schedules that include integrated steaming processes to determine the combined effect of the steam-drying system on the dried lumber. The variations in the moisture content (MC) and drying rate (DR) with drying time (DT) both before and after the steaming process were modeled mathematically for boards that have a flat and quarter grain pattern. For both types of boards that were dried with and without steaming, a relationship between MC and DT was observed. There was inflexion at 40%, and two mathematical models were computed. Additionally, the relationship between DR and DT for boards with flat and quarter grain patterns was not affected by the steaming process, and an inflection point was observed at 30?h (40% MC for the lumber). Moderate changes were observed in the CIELAB color system and dried lumber became darker. The steaming process, which was integrated in the drying schedule for boards with the flat and quarter grain pattern, decreased the incidence or magnitude of drying defects depending on the grain pattern and drying schedule used.     

Color Changes of Birch Wood During High-Temperature Drying

In low-temperature drying, a high drying rate has been found to be adequate for silver birch (Betula pendula) and European white birch (B. pubescens) timber as far as wood colour is concerned. During high-temperature drying, however, birch timber darkens significantly if steam is used as the drying medium. The objective of this research was to study the effects of drying force (wet-bulb depression), timber thickness, and initial moisture content on the color of high-temperature-dried birch wood. The reflectance spectrum of wood was measured and transformed to the CIEL?a?b? color scale.

The increase in drying force increased the lightness and decreased the redness and the yellowness of wood. At the same time, the difference in color between the surface layer and the interior of boards increased. Increase in thickness and in initial moisture content increased the difference in color between the surface and the interior of boards. Pretreatment of timber with water soaking decreased the difference in color between the surface layer and the interior of boards when low drying force was used, but this difference was increased when high drying force was used.     

Drying Characteristics of Thick Lumber in a Laboratory Radio-Frequeocy/Vacuum Dryer

ABSTRACT

A series of forty two drying runs of two wood species and two cross-sectional dimensions of wood squares were carried out in a laboratory radio-frequency/vacuum (RF/V) dryer. The experimental temperature, pressure and moisture content levels as functions of space and time are presented. The results showed that western red cedar and western hemlock can be dried to a final moisture content of 15% in about 24 and 32 hours, respectively. The quality of the dried specimens was exceptional- Detailed analysis revealed the absence of internal drying stresses, internal and external checking and surface discoloration. Furthermore, evaluation of moisture content distribution in the longitudinal and transverse direction showed minimum variation compared to conventional kiln drying. The experiments also revealed that RF/V drying rates are directly affected by the level of the electrode plate voltage. Drying rates decreased with time when the voltage remained constant throughout the drying cycle thus resulting in long drying times. That was because of the changing dielectric properties of wood due to moisture content reduction during drying. Raising the voltages with time though, resulted in constant drying rates and shorter diying times.     

Optimization of Solar Kiln for Drying Wood

Abstract

Theoretical investigation of the physical process of solar drying of timber based on conventional heat and mass transfer equations is presented. The governing equations and boundary conditions of the mass diffusion through the wood timbers are derived; also the governing equations of the components of the solar kiln are presented. The finite difference technique is used to solve the set of these equations by means of a simulation program that is based on object-oriented approach. The simulation program is used to investigate the effect of several design parameters on the drying rate and duration of the wood timbers in order to accomplish the drying process with minimal drying defects. These parameters include the ventilation conditions that control the drying schedule inside the solar kiln, wood volume as a ratio to the solar kiln absorber area, wood timber thickness, season of drying, the drying air velocity, and the stresses that formed on the timber boards due to drying with these several parameters, leading to derive the limit of damage for a selected local wood type. The selected local wood type is Casuarina, which is common in Egypt, and it is commonly used in many simple industries.     

材性法制定木材干燥基准的探索

提出了一种新的木材干燥基准制定法——材性法。与常用的百度法相比,材性法更省时、简易、准确。采集简单易测的待干木材材性数据,如基本密度、厚度、初含水率等,材性法可以快速地制定出木材干燥基准。介绍了材性法的理论基础、制定方法及试验验证。     

Drying behavior and decay resistance of rubberwood from steamed log and presteamed lumber

The objective of this study was to evaluate the effect of log steaming and steaming prior to drying singly and both treatments together on the drying behavior and decay resistance of Hevea brasiliensis wood (rubberwood). Logs with a diameter of 34.6?±?4.4?cm were used. Half of the logs were kept in their original condition, and the other half were steamed at 90°C for 36?h. Later, the logs were cut into flat sawn boards. Half of the boards were kept in their original condition, and the other half were presteamed at 90°C for 3?h after 1?h of heating-up. These boards were dried in a drying kiln. The drying defects, drying time, drying rate, and decay resistance of wood to the Pycnoporus sanguineus fungus were determined. The results showed that the steaming is not suitable for decreases in the time and the drying rates of this kind of wood, and it neither reduces the drying defects of boards. Furthermore, these treatments adversely affected the decay resistance of rubberwood to P. sanguineus fungi.     

Kiln Drying of Acacia mangium Willd Wood: Considerations of Moisture Content before and after Drying and Presence of Wet Pockets

Acacia mangium is an important plantation species cultivated in Costa Rica and other tropical countries worldwide. However, wood uses have been limited due to drying-related problems such as high initial moisture content (MC_i) and high variability in final moisture content (MC_f). The objective of this study was to investigate the causes of these problems. Climatic conditions where trees grow, tree height, grain pattern, drying schedules, distance from pith, and sapwood or heartwood presence were considered. Results showed an average MC_i of 127% ranging from 58 to 186%. MC_i variation was influenced by climatic conditions, tree height, and grain pattern. Average MC_f was 19%, ranging from 9 to 52%. Lack of MC_f uniformity after drying is influenced by tree height, drying schedule, and the interaction of both factors. Wet pockets were also found to develop during drying. Lumber from trees growing in humid tropical climates subjected to a low relative humidity drying schedule as well as rift-sawn or double rift-sawn lumber was likely to develop wet pockets.     

Establishing Kiln Drying Schedule for Beech (Fagus orientalis) at 5-cm Thickness from the Neka Region

To establish a kiln drying schedule for beech (Fagus orientalis) lumber, 5-cm-thick boards were kiln dried down to a final moisture content of 8%. Three replications were made utilizing three kiln schedules of T5-C3, T5-C4, and T6-C4. With due attention to the effect of thickness on wood drying intensity, the t-test showed no significant difference between the thicknesses of the three drying schedules at a significance level of 99%. Therefore, the results of this study can be applied for 5-cm-thick boards.

The primary dry bulb temperature in each of the three schedules was adjusted to 41°C and the final dry bulb temperatures were adjusted to 71, 71, and 82°C, respectively. The schedule offering the shortest drying time for the desired quality was chosen. Specific gravity and dry specific gravity were measured as 0.52 and 0.61, respectively. Longitudinal, radial, tangential, and volumetric shrinkage were 0.46, 5.8, 10.2, 16.48%, respectively. The extent of defects including crook, bow, twist, and three longest surface checks of the lumber was determined for each drying schedule. Quality control graphs were used to analyze the lumber defects in order to determine the best drying schedule.

Analysis of the results indicates that with either of three kiln schedules the extent of defects before and after drying was not statistically different. However, the distribution of defects in the third schedule (T6-C4) was more uniform with respect to the average line compared to other two schedules. At the end of this schedule, a 17-h equalization and 24-h conditioning treatment is recommended.     

Optimization of Solar Kiln for Drying Wood

Theoretical investigation of the physical process of solar drying of timber based on conventional heat and mass transfer equations is presented. The governing equations and boundary conditions of the mass diffusion through the wood timbers are derived; also the governing equations of the components of the solar kiln are presented. The finite difference technique is used to solve the set of these equations by means of a simulation program that is based on object-oriented approach. The simulation program is used to investigate the effect of several design parameters on the drying rate and duration of the wood timbers in order to accomplish the drying process with minimal drying defects. These parameters include the ventilation conditions that control the drying schedule inside the solar kiln, wood volume as a ratio to the solar kiln absorber area, wood timber thickness, season of drying, the drying air velocity, and the stresses that formed on the timber boards due to drying with these several parameters, leading to derive the limit of damage for a selected local wood type. The selected local wood type is Casuarina, which is common in Egypt, and it is commonly used in many simple industries.     

The impact of drying and steaming processes on surface color changes of tension and normal beech wood

Abstract

Beech wood has a high frequency of defects such as red heartwood, reaction wood (tension wood). For the experimental measurements four logs without visible defects like red heartwood, which can noticeably affects the measurements' results were qualified. The drying medium temperature in the first phase of the process before the moisture content of the specimens fell below the FSP was maintained at 45?°C. Then, the temperature was gradually increased to the maximum value 65?°C. The results showed that different initial wood moisture content does not affect to final value of tension and normal wood. The colorimetric parameter L* was much higher for tension wood. After steaming, there were greater differences in the colorimetric parameters, respectively, a* and b*. After drying the difference of all colorimetric parameters between tension and normal wood was significantly less. The color changes were only noticed in the surface layers of specimens. Differences were small, and thus, the impact of the tension wood on the color changes was not confirmed. However, since the lightness of tension wood plays a key role for its visual detection, it may be adequate to only measure the lightness parameter (L*).     

木材干燥基准的研究现状及展望

木材干燥基准是控制木材干燥过程中干燥介质的参数表。介绍了木材干燥基准的重要意义、类别、制定方法、研究现状等方面内容，指出了未来干燥基准的研究方向。     

NUMERICAL AND EXPERIMENTAL STUDIES ON INFLUENCE OF COMPRESSION WOOD TIMBER DISTORTION

ABSTRACT

In timber exposed to moisture variations, drying distortions is a serious problem that might result into sawn timber and other wood products unsuitable for construction purposes. Two characteristics of wood are that its behaviour is strongly orthotropic and that it is very sensitive to variations in moisture content. In addition, wood is characterised by variation in its properties from pith to bark. A further important property of wood, which affects its behaviour, is its spiral grain. For timber containing much compression wood the drying distortions are also highly dependent upon where the compression wood is located in the sawn boards. The present study concerns an experimental investigation of density, grain angles, shrinkage parameters and longitudinal elastic modulus in a number of spruce boards containing much compression wood. On the basis of the data obtained, numerical simulations have been carried out in order to determine the deformations that developed in the boards during changes in moisture content.     

Variation in Drying Behavior and Final Moisture Content of Wood during Conventional Low Temperature Drying and Vacuum Drying of Betula pendula Timber

Conventional and vacuum drying experiments were conducted on Betula pendula timber, which was sawn from trees felled during three different seasons. The influence of the wood procurement season on drying behavior differed, on the one hand, between the drying phases above and below 30% moisture content in the conventional drying, and, on the other hand, between the conventional and vacuum drying methods. During the first steps of the conventional drying process, relative humidity in the kiln, as well as drying time and drying rate, varied according to the felling season. Variations in environmental conditions outside the kiln and the seasonal variation in the physical properties of the wood were presumed to be the reasons for differences in drying behavior. The difference in moisture content gradient, i.e., the difference in final moisture content between the inner wood and the surface layer of boards, was greater in conventionally dried timber than in vacuum-dried timber. In conventionally dried timber there was a clear seasonal variation in the gradient of final moisture content, which was greatest for winter-felled wood. The premature drying of the surface layer during the first steps of the conventional drying process of winter-felled wood was the reason for the higher gradient of moisture content. Storage of wood as logs decreased the standard deviation of the final moisture content.     

Flame retardance of insolubilized silica inside of wood material

Beech wood boards were studied with a cone calorimeter. As the specific density of these materials is greater, the total amounts of CO and CO₂ produced were bigger than values measured on spruce specimens. Also, the shape of the heat release rate (HRR) curve was different as its second peak was bigger than the one observed on spruce wood. It was shown that impregnation of wood with water solutions of H₃BO₃ resulted in decreased HRR values with increasing acid concentrations. Mass loss and CO and CO₂ production rates also decrease in the same way. Impregnation with water‐soluble NaOH‐silica resulted in improved data, when compared to untreated specimens, but were not better than those obtained with H₃BO₃. Simple mixing of both fire retardants gave no improvement, nor was the two‐step impregnation with drying effective after the first step. When the second step‐treatment with acid was followed without drying after impregnation with NaOH‐silica in the first step, then improvement was evident. This is explained by precipitation of the silica content inside the material when the NaOH was neutralized with H₃BO₃, which could penetrate inside when the material was not dried before the second step. The improvement resulted in lower smoke production and longer times to ignition. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1948–1952, 2005     

Dynamic Mechanical and Thermal Properties of Modified Poly(propylene) Wood Fiber Composites

Dynamic mechanical and thermal properties of poly(propylene) (PP)/wood fiber composites have been studied using Dynamic Mechanical Analysis (DMA). In order to modify the PP matrix maleated poly(propylene) (PPMA) and poly(butadiene‐styrene) rubber were used as compatibilizer and impact modifier, respectively. tan δ peak temperature of the compatibilized systems was found to increase in comparison to that of composites without coupling agent, indicating improved adhesion and interaction between PP matrix and wood fibers. The storage modulus (E′)‐temperature (T) relationship of all composites is characterized by two transition points. The E′ of compatibilized composites exhibits higher values than those of the uncompatibilized ones at low temperatures (up to the β‐relaxation). In the temperature interval from β‐transition to 60 °C, the composites containing PPMA have lower modulus, and above 60 °C the E′‐T curves tend to converge. DSC indicates that the wood fibers act as nucleating agent for PP. Maleated poly(propylene) slightly retards the crystallization rate, resulting in a composite structure, composed mainly of large spherulites, with a higher crystallinity index. Fourier Transform Infrared (FT‐IR) microscopy was also applied to explore the interface between wood fibers and PP matrix. The strong absorption band at 1 738 cm^?1 in the IR spectrum scanned at the interfacial region between the fiber and matrix indicated that PPMA had probably reacted either by formation of ester bonds or hydrogen bonding with hydroxyl groups from cellulose.

Optical micrograph of PPWF composite in polarized light.     

Determination of moisture content and moisture content profiles in wood during drying by low-field nuclear magnetic resonance

In this article, the low-field nuclear magnetic resonance was used to measure the moisture content (MC) and MC profiles in poplar wood during real-time drying. The T₂ distribution curve at each drying stage measured using the Carr–Purcell–Meiboom–Gill pulse sequence provided detailed information in the dynamic change of free water as well as bound water of the whole wood sample. In addition, a new developed SE-SPI pulse sequence was first used to evaluate the spatially resolved T₂ distribution of the successive nondestructive sliced layer of wood. Combined with the area integration method, the moisture content in each layer was calculated, and the change of MC profiles within wood at the MC above and below fiber saturation point was well reflected.     

MOISTURE MOVEMENT ON DRYING SOFTWOOD BOARDS AND KILN DESIGN

ABSTRACT

This paper provides an overview of present understanding of how moisture can move through softwood boards, as a basis for determining kiln-seasoning strategies. Moisture in green wood is held essentially unbound, whereas below fibre saturation it is bound to a variable extent to the fibre walls. Sapwood, which is that part of the timber used for the transport of liquid nutrients, contains more moisture than physiologically inactive heartwood. Sawing the felled log creates a moisture-denuded layer at the damaged exposed surfaces. These features have a profound influence on the way that moisture can be removed on drying. Superimposed are differences arising from seasonal variations in the growth of wood between earlywood and latewood, which have different moisture permeabilities. When the width of the annual growth ring is relatively large compared with the board dimensions, moisture movement and the development of drying stresses depend markedly upon the sawing orientation relative to the grain direction. Quarter-sawn boards dry more uniformly (in the direction normal to the drying surfaces), but more slowly than flat-sawn boards. Most timber boards are stacked and then dried in box-shaped kilns. The uniformity of drying depends on the goodness of this stacking and on a uniform airflow being presented to the inlet face of the stack. Some non-uniformities can be mitigated by periodic reversals of the airflow direction through the stack and by overdrying the majority of boards to reduce wet spots, but there are limits, while overdrying reduces kiln capacity. Attention to aspects of the kiln geometry can reduce the fan-energy requirements and shorten the drying time, with a more uniform moisture content through out the kiln load.