Effects of press closing time on mat consolidation behavior during hot pressing and on linear expansion of particleboard

Mat consolidation behavior was investigated at various press closing times (PCTs) using hinoki (Japanese cypress) particle mats. The temperature in the core layer of the mat, press pressure, and platen distance were measured. At the plateau stage during hot pressing, the core temperature decreased with increasing PCT. The core temperature did not increase before the platen distance equaled the target board thickness in the PCT range of 4–50 s and rose slightly when the PCT exceeded 100s. There was a linear relation between the logarithm of PCT and the maximum press pressure. The density profile across board thickness was strongly affected by the PCT. As the PCT increased, the position of the peak density (PD) moved toward the core layer as the PD itself decreased. The effect of PCT on the linear expansion (LE) of the board is discussed in relation to the density profile. There was no difference in LE after high relative humidity treatment in the PCT range 4–50s. LE appeared to be related to the low density and the precured region of the board surface. The LE after drying treatment was around –0.1%, and the thickness swelling after high relative humidity and drying treatments increased with increasing PCT.Part of this report was presented at the 51st Annual Meeting of the Japan Wood Research Society, Tokyo, April 1988     

Formation of the density profile and its effects on the properties of fiberboard

Two main types of fiberboards were produced using lauan (Shorea spp.) fibers with an isocyanate resin as the binder; fiberboard with a flat, homogeneous (homoprofile), and typical U-shaped (conventional) density profile along the board thickness. The processing parameters included manipulation of mat moisture content distribution, press closing speed, and hot pressing method. The results are summarized as follows: (1) A larger variation was observed in the peak density (PD) and core density (CD) of fiberboards at 0.5g/cm³ mean density (MD) level than in those at 0.7 g/cm³. Generally, PD showed a greater variation than CD, irrespective of MD level. (2) Boards produced using two-step hot pressing recorded substantially higher PD with reduced CD. (3) Multiple regression analysis showed that CD and PD could be calculated based on the other profile defining factors, and a rough estimation for peak distance and gradient factor was possible. (4) Based on static bending, conventional fiberboard had a higher modulus of rupture (MOR) than the homo-profile board but a similar modulus of elasticity (MOE). (5) At 0.5 g/cm³ the MOR and dynamic MOE of fiberboard increased by up to 67% and 62%, respectively, when the PD increased from 0.5 to 1.07 g/cm³. Similarly, an increase of PD from 0.7 to 1.1 g/cm³ resulted in corresponding increases of 55% and 34% in the MOR and dynamic MOE of 0.7 g/cm³ boards. (6) The internal bond strength and screw withdrawal resistance were almost entirely dependent on the CD and MD, respectively. (7) Homo-profile fiberboards registered higher thickness swelling and water absorption than conventional fiberboards throughout the dry/wet conditioning cycle.     

Effect of manufacturing parameters on the linear expansion and density profile of particleboard

Particleboards were fabricated in the laboratory with different board densities and resin contents to evaluate linear expansion when exposed to both vapor and liquid water. Density profiles were measured to determine the relation to the elastic constants and to the dimensional properties of the boards. It was found that density profiles were affected by the board density and resin content applied. The high-density layer formed in the thickness direction affected the elastic constant measurements. A model introduced to predict the linear expansion closely matched the data during exposure to water. Under conditions of 40°C and 90% humidity, linear expansion increased with increasing board density. An increase in resin content from 6% to 12% slightly increased the linear expansion and decreased the thickness swelling. A linear relation was found between board density and linear expansion per unit of moisture content change.     

Effects of density profile of MDF on stiffness and strength of nailed joints

Nail-head pull-through, lateral nail resistance, and single shear nailed joint tests were conducted on medium density fiberboard (MDF) with different density profiles, and the relations between the results of these tests and the density profiles of MDF were investigated. The maximum load of nail-head pull-through and the maximum load of nailed joints were little affected by the density profile. However, the ultimate strength of lateral nail resistance, the stiffness, and the yield strength of nailed joints were affected by the density profile of MDF and showed high values when the surface layer of the MDF had high density. It is known that bending performance is also influenced by density profile. Therefore, the stiffness and the yield strength of nailed joints were compared with the bending performance of MDF. The stiffness of nailed joints was positively correlated with the modulus of elasticity (MOE); in the case of CN65 nails, the initial stiffness of joints changed little in response to changes in MOE. The yield strength of nailed joints had a high positive correlation with the modulus of rupture (MOR). The stiffness and the yield strength of nailed joints showed linear relationships with MOE and MOR, respectively.     

环境温度对刨花板生产质量的影响及改进方法

本项研究对气温(尤其在冬季)是如何影响刨花板生产进行了分析。通过对不同季节时的两种极端情况——较高的板坯芯层温度和较低的板坯芯层温度在热压过程中板坯芯层渴度的变化,进一步说明了芯层温度对热压时间以及刨花板质量的影响,并就如何解决这一问题提出了建议。     

Dimensional stability and strength properties of particleboard produced by a closed-press system

The purpose of this study was to reveal the effects of various levels of mat-moisture content (m.m.c.) and the closed-press system for making single- or three-layer particleboard on the density profile, thickness swelling, specific moduli of elasticity (MOE) and rupture (MOR) and internal bond strength. Internal gas pressure was measured in an enclosed frame; and the larger the m.m.c., the higher the internal gas pressure became. When rising water vapor (steam) struck particles, it plasticized them and cured the adhesive, resulting in improved interparticle contact. The vertical density gradient in the three-layer board was larger than that in the single-layer board. As for thickness swelling by cold-water soaking, the single-layer boards were less affected than the three-layer boards and showed good dimensional stability with increased m.m.c. The open-system boards swelled more than the closed-system boards. The closed-system single-layer board made at high m.m.c. returned nearly to the prime thickness by air-drying after cold-water soaking. Specific MOE and MOR were larger at 15% or 10% m.m.c. than those at other m.m.c. Considerable reductions of specific MOR and MOE of the closed-system three-layer board were observed at 20% or 25% m.m.c.Part of this report was presented at the 45th annual meeting of the Japan Wood Research Society, Tokyo, April 1995 and at the 48th annual meeting of the Japan Wood Research Society, Shizuoka, April 1998     

THE EFFECTS OF WETTING AND REDRYING CYCLE ON PHYSICAL PROPERTIES OF PARTICLEBOARD

INTR0DUCTI0NOvertheyearsmmpresearchershavefoundUFandPFparticleboardstoedsbitsimilarshorttermphysicalpropeniesatlowhumidityandtemperatUre(DeainandOstInan,l983).DifferentbehaviorpatternsbetWeentl1eresinsemefgeasrelativehumidityandtem-peratUrerise,finallycouldresultinthefailureofboards(R0dwell,l99l).AnumberofworkshavequestionedWhetherhydrolysisistheonlyfactorinvolvedinb0ndfailure.Irlereported(l986)thedifferenteffectsofwettingandredryingonthepropertiesofpureUFandPFresinfilms.Soitmaybeim…     

Behavior of acoustic emission generation during tensile tests perpendicular to the plane of particleboard II: effects of particle sizes and moisture content of boards

Particleboard specimens with various particle sizes were conditioned into two ranges: low and high moisture content. One set was investigated for internal bond (IB) strength and acoustic emission (AE) events during tensile tests perpendicular to the plane and the other for ultrasonic wave transmission characteristics in the thickness direction. The particleboard structural mechanics were changed as a result of the moisture effect. Specimens conditioning to higher moisture content had lower IB strength and lower cumulative acoustic emission event counts (T _AE). The decrease in IB strength indicated that the irreversible thickness swelling was seen when recovery forces of the particles exceed the restraining action of the adhesive. This was attributed to stress release, which resulted in internal failure of the board. The change in the internal structure caused an increased stress level at the initiation of AE generation. No events were recorded before this stress level, obeying theKaiser effect. The decrease inT _AE was not only related to the decrease in IB strength but was also affected by the transformation (attenuation) of the AE signals during IB tests according to the mesh size used.Part of this paper was presented at the 45th and 50th Annual Meetings of the Japan Wood Research Society, Tokyo and Kyoto, 1995 and 2000, respectively     

Influence of moisture content on the vibrational properties of hematoxylin-impregnated wood

The vibrational property of hematoxylinimpregnated wood was investigated from the aspect of moisture content dependence. The specific dynamic Young's modulus (E/) and loss tangent (tan) of hematoxylin-impregnated wood were determined in the relative humidity (RH) range of 0%–97%, and were compared with those of the untreated and some conventional chemically treated woods. The changes in theE/ and tan of wood with increasing RH were suppressed by acetylation and formaldehyde treatment because of a marked reduction in the hygroscopicity of the wood. Although the hematoxylin impregnation did not significantly affect the hygroscopicity of the wood, its influence onE/ and tan were similar to that of formaldehyde treatment at low RH and of acetylation at medium RH. It was supposed that at low to medium RH hematoxylin restrains the molecular motion of amorphous substances in the cell wall because of its bulkiness and rigidity. On the other hand, at high RH it seems to work as a plasticizer with adsorbed water molecules.     

Veneer strand flanged I-beam with MDF or particleboard as web material II: effect of resin type,application rate,strand dimension,and pressing time on the basic properties

Optimization of the manufacturing conditions of the veneer strand flanged I-beam invented in the previous study was investigated using different combinations of strand dimensions, resin types between web and flange, different pressing times, and different wood–resin moisture contents under conventional hot pressing conditions. The main results revealed that the strand dimensions have no effect on the bending properties of the flange part and the dimensional stability of the I-beam. Increasing the resin application rate between strands was found to improve the dimensional stability of the I-beams. The use of isocyanate (MDI) resin between web and flange significantly improved the bond strength between web and flange, the modulus of rupture of the I-beam, and the modulus of rupture of the flange part. Dimensional stability was also improved. Shortening the pressing time from 20 to 12min was found to be feasible. Using low wood-resin moisture content was found to interfere with the curing of the phenol–formaldehyde (PF) resin at the flange part resulting in poor quality beams. Of the three moisture content levels tested, 12% was found to be the optimal level for producing I-beams with balanced mechanical properties and dimensional stability.Part of this work was presented at the 53rd Annual Meeting of the Japan Wood Research Society, Fukuoka, March 2003     

Formation of the density profile and its effects on the properties of particleboard

Summary Two types of particleboards bonded with an isocyanate resin, one with uniform vertical density profile (homo-profile), and the other with conventional U-shaped profile, were fabricated to various density levels using lauan (Shorea spp.) particles. The fundamental relationships between the density profile and the board properties were determined, and the results are summarized as follows: 1. In homo-profile boards, the moduli of rupture (MOR) and elasticity (MOE), internal bond (IB) strength, and screw withdrawal resistance (SWR), are highly correlated to the board mean density. 2. The bottom limit of the board density is estimated to be ca. 0.25 g/cm³, based on the correlation regressions between mechanical properties and mean density. 3. At equal mean density level, the MOR and MOE of the conventional particleboards are higher than the homo-profile boards, due to the higher density near the faces. However, the reverse is true for IB, owing to the presence of the low density core in the former. 4. The net impact of peak density on MOR and MOE is greater at higher mean density level while raising the core density results in more pronounced improvement in IB at lower density. 5. In addition to the compaction ratio, the dimensional stability of the board is also affected by the peak area and mat moisture content. Received 9 January 1997     

Predicting moisture content and density distribution of Scots pine by microwave scanning of sawn timber

This study was carried out to investigate the possibility of calibrating a prediction model for the moisture content and density distribution of Scots pine (Pinus sylvestris) using microwave sensors. The material was initially of green moisture content and was thereafter dried in several steps to zero moisture content. At each step, all the pieces were weighed, scanned with a microwave sensor (Satimo 9,4GHz), and computed tomography (CT)-scanned with a medical CT scanner (Siemens Somatom AR.T.). The output variables from the microwave sensor were used as predictors, and CT images that correlated with known moisture content were used as response variables. Multivariate models to predict average moisture content and density were calibrated using the partial least squares (PLS) regression. The models for average moisture content and density were applied at the pixel level, and the distribution was visualized. The results show that it is possible to predict both moisture content distribution and density distribution with high accuracy using microwave sensors.     

Effects of surface laminate type and recycled fiber content on properties of three-layer medium density fiberboard

ABSTRACT

High global production of medium-density fiberboard (MDF) in recent years could generate an equal quantity of waste MDF at the end of its service life, requiring recycling of waste MDF instead of landfilling or incineration. This study investigated effects of the addition of recycled fiber (RF) obtained from surface laminated MDFs with three different materials to the properties of three-layer recycled MDF (rMDF). Three types of surface laminates such as low-pressure laminate, polyethylene terephthalate, and polyester coating were hammer milled, and then went through a patent-pending fiber recovery system to obtain the resultant RFs that were added to the core layer of rMDF. These RFs at three contents (10, 20, and 30%) were blended with 12% of urea-formaldehyde (UF) resin prior to hot-pressing. Statistical analysis showed that the best internal bonding strength, modulus of rupture, and modulus of elasticity of rMDF panels were obtained for LPL-rMDF with a 20% RF content. Thickness swelling, water absorption, and formaldehyde emission of rMDF were reduced by increasing the RF content. These findings suggest that a minimum RF content of 20% can be replaced with virgin fibers for the rMDF manufacture, indicating the feasibility of recycling waste laminated MDF into three-layer rMDF.     

稻草覆盖对果园土壤温度与含水量的影响

在洮南市北部半山区的果树试验场,按果树树冠垂直投影筑树盘,对树盘进行稻草覆盖处理,研究其对土壤温度与含水量的影响。结果表明：覆草处理能够有效地提高土壤含水量,年平均含水量比清耕地高出16．0％;经覆草处理的土壤,生长前期土温上升缓慢,后期土温下降缓慢,越冬前,土温高于清耕地2．00℃,可减少低温对根系的危害。     

Impact of moisture content on dynamic mechanical properties and transition temperatures of wood

The dynamic shear modulus and the loss modulus of Betula alba, Ulmus parvifolia, Quercus robur, Acer platanoides, Tilia cordata, Fraxinus excelsior and Pinus sylvestris wood were measured using an inverted torsion pendulum within a wide temperature range. The glass transition temperature of the lignin–carbohydrate complex and the decomposition temperature of the wood cellulose were estimated. The temperature band from 170°C to 240°С shows the transition of the lignin–cellulose complex from the glassy to the rubbery state. Mechanical properties of different types of wood are affected by moisture and anatomical differences, but glass transition and decomposition temperatures are the same. More than 5% of moisture in the wood stored at normal conditions were found. After drying, the increase of dynamic shear modulus of wood over the entire region of the glassy state was observed. The intensity of maximum peak of dynamic loss modulus is also increased due to activation of the segmental motion of macromolecules of the ligno-carbon complex. The decomposition temperature of the cellulose crystallites was unchanged for specimens containing moisture and for dried specimens.     

Predicting moisture content and density distribution of Scots pine by microwave scanning of sawn timber II: evaluation of models generated on a pixel level

The purpose of this study was to use images from a microwave sensor on a pixel level for simultaneous prediction of moisture content and density of wood. The microwave sensor functions as a line-scan camera with a pixel size of 8mm. Boards of Scots pine (Pinus sylvestris), 25 and 50mm thick, were scanned at three different moisture contents. Dry density and moisture content for each pixel were calculated from measurements with a computed tomography scanner. It was possible to create models for prediction of density on a pixel level. Models for prediction of moisture content had to be based on average values over homogeneous regions. Accuracy will be improved if it is possible to make a classification of knots, heartwood, sapwood, etc., and calibrate different models for different types of wood. The limitations of the sensor used are high noise in amplitude measurements and the restriction to one period for phase measurements.     

The effect of girdling on the moisture content of small-sized trees

In this study, the effect of girdling on the moisture content of small-sized trees for heat energy production was clarified. The moisture content was measured for Scots pine (Pinus sylvestris), Norway spruce (Picea abies), and Downy birch (Betula pubescens) during two growing seasons after girdling. The trees were girdled at breast height for around 30 cm by removing the bark, phloem, and cambium from around the stem. At the beginning of the growing season the mean moisture content of the living Scots pine (P. sylvestris) and Norway spruce (P. abies) was 60%, and for Downy birch (B. pubescens) it was 50%. During the first growing season the effect of girdling on the moisture content was low, but during the second growing season the moisture content decreased significantly. The moisture content of the Norway spruce (P. abies) (23%) and Downy birch (B. pubescens) (33%) was at its lowest point at 14 months after girdling. There were no significant changes in the moisture content of the Scots pine (P. sylvestris) in this study. The results of this study can be used in basic research and in the development of energy wood production.     

干旱胁迫对红椿盆栽土壤含水率与植株伤害程度的影响

以2年生红椿盆栽幼苗为对象,开展干旱胁迫及复水试验,对各观测日的土壤含水率、叶片伤害指数及植株萎蔫度进行分析,结果表明:(1)各时间段受试盆栽土壤含水率及叶片伤害指数差异均呈极显著差异;(2)复水一定程度上改善了胁迫伤害;(3)土壤含水率与植株受伤害程度呈极显著的负相关关系。(4)要保证2年生红椿幼苗的正常生长,必须将土壤含水率保持在25%以上。     

刚果12号桉W5无性系立木木材密度与含水率关系的初探

本文主要是对刚果12号桉w5无性系立木材密度与含水率的关系进行分析和探讨。分析结果表明两者相关程度达高度紧密程度,相关显著性达极显著水平,回归公式估测精度甚高,回归方程在本林区适用。     

竹粉含量及粒径对竹塑复合材料性能的影响

以竹粉和高密度聚乙烯为原料,通过混炼、平板热压制备竹粉/HDPE复合材料,研究竹粉含量和粒径大小对复合材料的吸湿、吸水性以及力学性质的影响.试验结果表明:随着竹粉添加量的增加,竹塑复合材料的吸湿、吸水性能也逐渐增大,同时,当竹粉粒径变小时,复合材料的吸湿、吸水性能也增大;复合材料的冲击强度随着竹粉含量的增加而减小,而拉伸强度与弯曲强度随着竹粉含量增加而增大,但当竹粉含量超过50％,这些强度反而降低;随着竹粉粒径增大,抗冲击强度逐渐降低,而拉伸强度与弯曲强度增大,但当粒径超过180μm时,这两个强度则开始下降.