Comparison of the pressing behaviour of wood particleboard and strawboard

To improve the understanding of strawboard manufacturing processes, mat pressing behaviour of wood particleboard and strawboard bonded with urea formaldehyde resins were experimentally investigated and compared in terms of mat compressibility, transverse permeability, mat pressure, core temperature, core gas pressure and vertical density profile. The results have shown that straw particles are much more compressible and therefore require less platen pressure for pressing. Compared to wood particle and refined straw particle mats, hammer milled straw mats have low permeability and subsequently show high core gas pressure and high maximum core temperature during hot pressing, in addition to large differential densities between surface and core layers in the final pressed boards. It is recommended that a slower press closing rate and longer press opening time be used to develop the strawboard pressing schedule.     

Flake compression behavior in a resinless mat as related to dimensional stability

The purpose of this study was to evaluate compression and swelling characteristics of individual furnish elements sampled through the thickness of lab panels pressed without resin. Commercial southern pine OSB furnish was used to press resinless mats so individual flakes could be removed from the panel after pressing and evaluated for compression behavior. 19 flake sets, each set consisting of 15 southern pine flakes with 0.65% wax, were marked and measured for thickness and mass. One set of marked flakes was randomly distributed in one layer of a mat which consisted of 19 total layers; each of the 19 layers had 15 marked flakes randomly distributed in the layer. After hot pressing each marked flake was removed from the mat. After achieving equilibrium at 35%, 65% and 98% relative humidity, each flake was again remeasured for thickness and mass. Experimental results include flake compaction ratio and its distribution through the mat thickness, flake thickness swelling under different RH environments, compaction ratio-thickness swelling relationship as well as individual flake compaction ratio and thickness swelling variations. Comparison is made to adsorption/desorption behavior of pressed flakes. Flakes from surface layers exhibited compression of 25 to 37%, about double that of flakes in core layers. As expected, flakes from surface layers showed much greater thickness swell than core flakes and the response was accentuated with higher EMC conditions. Received April 26 1999     

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     

竹木混合水泥刨花板工艺初步研究

研究了热压法制造竹木混合水泥刨花板工艺并对影响板材性能的因素进行探讨。结果表明:竹刨花水煮预处理后所制备的板材性能较好;本实验范围内,竹木混合水泥刨花板较合适的工艺参数为竹木比1∶3、灰木比4∶1、热压时间1～2min/mm板厚、CaCl2用量为灰重的5%、水灰比0.4。     

Effect of microfibril angle on the longitudinal tensile creep behavior of wood

In this report, we undertook studies of the viscoelastic properties of wood from the viewpoint of the fine structure and properties of the constituent materials in the wood cell wall. To measure the mechanical properties of the wood as the behavior of the cell wall, it is required to perform the longitudinal tensile test using a homogeneous specimen. In this study, microtomed specimens of sugi (Cryptomeria japonica D.Don) earlywood were used for the creep test, which were conducted at the fiber saturation point. The substantial creep compliance of the cell wall was simulated using a simplified viscoelastic model consisting of a Voigt element and an independent spring in series. Based on the experimental results, the values of the parameters were optimized. The results were as follows: (1) the longitudinal tensile creep deformation tends to increase with the elapsed time, similar to the bending creep behavior; (2) the magnitude of the longitudinal creep function increases with MFA; and (3) each parameter in the simplified viscoelastic model is markedly affected by the MFA. Based on these results, the mechanism of the longitudinal tensile creep deformation of wood is discussed.     

The variable parameter rheological model of wood

How to establish the rheological model to simulate creep behavior of wood and wood-based composites under change-load has not been solved in research of wood rheology. This paper presents a new model—variable parameter rheological model. The bending creep behavior of small clear poplar specimens under different constant load levels were examined. The load levels within 50% of rupture load of the specimens, and the experimental creep behavior were simulated by the variable parameter Maxwell model. The results show that using only one model of variable parameters may simulate the creep behavior of wood under different constant load levels very well. Applying the generalized Boltzmanns superposition principle, the variable parameter rheological model can be used to simulate the creep behavior of wood under change-loads conveniently and accurately.     

Recent research and development work on wood composites in Japan

Summary This paper highlights a number of articles on research and development on wood composites. The coefficient of variation in the mechanical properties of wood components tends to decrease when the thickness of the wood elements decreases. This finding was utilized in the manufacture of hollow cylindrical columns of laminated veneer lumber (LVL) from Cryptomeria plantation thinnings. In the future, the marketing of wood composite boards will tend toward two types: thick boards with low density properties and thin boards with fiber orientation. For the production of thick boards, fundamental parameters and application of steam-injection pressing have been studied, and continuous steam-injection pressing has been developed. For thin board production, generation principles of aligning torque in high-voltage system and the application has been studied. A new oriented mat former with electrodes positioned only at the reverse side of a forming belt has been developed. Various synthetic resins of low-molecular weight have been applied to improve dimensional stability of laminated products, such as LVLs and particleboards. Acetylation and formalyzation of fibers and particles were investigated to provide stabilized panels. High pressure steam treatment during pressing of wood composites has been studied and the process has been found effective and promising.The authors wish to thank Mr. Dwight A. Eusebio for his help in the preparation of this paperThis paper was presented at the IUFRO All Division 5 conference in Nancy, France, on August 15, 1992     

基于ATmega128的试验压机控制器设计

设计了一种新型试验压机智能控制器,并给出系统的软件程序设计及硬件的选取.系统采用嵌入式微控制器和触摸屏串行通信的方法,实现了智能监测、自动过程控制和实时显示、手动与自动、升降速度可调等多项功能.试验证明:系统能精确按照设定压力自动执行,热压参数控制精度高,稳定性好.该控制器尤其适用于噪声、电磁干扰大、环境污染严重的场合.     

Effects of mat moisture content and press closing speed on the formation of density profile and properties of particleboard

Isocyanate resin-bonded 0.5 and 0.7 g/cm³ lauan (Shorea sp.) particleboards were produced from mats with uniform and distributed moisture content (MC) distributions, using three hot press closing speeds. The effects of these processing variables on the formation of density profile in particleboard and board properties were analyzed statistically. A definition of the density profile was introduced, and the correlations among the various defining factors were established. The results are summarized as follows. (1) The peak density (PD) of particleboard could be increased, with a slight reduction in the core density (CD), using mats with different MC distributions. (2) In a conventional density profile, CD and PD are highly dependent on the board mean density (MD); and the gradient factor (GF), peak distance from the faces (Pdi), and peak base (Pb) are significantly correlated to each other, at the 99% significance level. (3) Greater press closing speed reduces Pdi and Pb, with an increase in GF. (4) Greater press closing speed could increase the PD in board of low MD, with minimal effect on CD. (5) The modulus of elasticity (MOE) of particleboards from mats with high MC near the faces were consistently higher than those from mats with uniform MC, irrespective of the press closing speed, whereas their modulus of rupture (MOR) became indifferent at higher MD under slow and fast closing speeds. (6) Sanding does not improve the MOR and MOE of particleboard significantly.     

平面密度分布对刨花板内结合强度的影响

平面密度变异是非单板类木质人造板所共有的一种结构现象 ,其变异程度和分布特征对板材性能有一定影响。本文利用单板条模拟 4种具有不同平面密度分布的刨花板 ,讨论了随机铺装刨花板坯模型的平面密度分布规律及试件大小与密度分布的关系 ,分析了密度以及平面密度分布对非单板类木质人造板内结合强度的影响。结果表明 :随机铺装刨花板的平面密度遵循正态分布规律 ;内结合强度受密度影响较大 ,密度增加有助于内结合强度提高 ,但密度过高将导致内结合强度下降 ;试件大小影响试件之间密度变异程度 ,适当增加试件尺寸 ,可降低内接合强度测试结果的离散性。     

基于纳米压痕技术的木材胶合界面力学行为

【目的】研究木材胶合界面的静态和动态力学行为,探讨树脂渗透对木材管胞壁层力学性能的影响,为木质复合材料制造工艺优化和增强改性提供理论依据。【方法】采用纳米压痕静态和动态力学测试技术(Nano-DMA),对针叶材火炬松与酚醛树脂(PF)、脲醛树脂(UF)胶黏剂所形成胶合界面区域各相材料的静态弹性模量、硬度、蠕变性能以及储能模量和损耗模量等力学行为进行分析。【结果】静态力学行为方面,在界面区域,PF和UF渗透进入管胞壁层后,木材管胞壁的弹性模量( E r)和硬度( H )提高;经PF渗透后,木材管胞壁的 E r和 H 分别增加7%和26%;Burgers蠕变力学模型可有效描述胶合界面区域管胞壁的纳米压痕蠕变特性,经树脂渗透后,木材管胞壁的瞬时弹性模量增加,黏弹性模量和黏性系数减小;在保载初期,PF界面区域木材管胞壁的蠕变柔量约下降60%,UF界面区域木材管胞壁的蠕变柔量约下降58%。动态力学行为方面,随着加载频率增加,界面材料的储能模量( E ′ r)逐渐增大,而损耗模量( E ″ r)和损耗因子(tan δ)呈减小趋势;当加载频率为10 Hz时,PF和UF树脂渗透使得管胞壁层的储能模量分别增加16%和29%。【结论】胶合界面区域胶黏剂进入管胞壁层,对木材管胞的静态力学性能具有增强作用,同时胶黏剂可提高管胞壁的短期抗蠕变能力;木材管胞壁具有较高的储能模量和损耗模量,而树脂的储能模量和损耗模量较低,经树脂渗透后,木材管胞壁的储能模量增加,但损耗模量和损耗因子呈下降趋势,可能对界面传递和分散应力产生不利影响。     

刨花板厚度方向变形研究Ⅰ.尺寸稳定之化学热力学研究初步

本文从化学热力学角度出发 ,理论上分析了人造板尺寸稳定的一些普遍性规律 ,初步证明人造板热压后在变形恢复时 ,蠕变和松弛将会同时发生 ;人造板的尺寸稳定过程是通过释放非膨胀功耗能量和膨胀功耗能量而实现的 ,只有使体系以非膨胀功耗能量释放 ,才能对人造板的尺寸稳定性有利 ;同时为热堆放处理有利于人造板尺寸稳定从化学热力学找到理论根据     

Fundamentals of flakeboard manufacture: wood-moisture relationships

Summary A procedure is presented to estimate the relative vapor pressure, equilibrium moisture content, average flake temperature, and average flake moisture content in a flakeboard mat during hot-pressing. This method is based on measurements of temperature and total gas pressure in the mat during hot-pressing. A heat and mass transfer model was adapted from the literature to predict the temperature and moisture content inside an individual flake. Significant moisture gradients are predicted to develop within flakes. Convective heat transfer appears to control the change of moisture content within a flake. Thermodynamic equilibrium between the gas phase and the wood component is not achieved during hot-pressing.This work was funded by the USDA Competitive Grant Program for Forest and Rangeland Renewable Resources, Project No. 85-FSTY-9-0106 and 87-FSTY-9-0253     

Effects of Horizontal Density Distribution on Internal Bond Strength of Flakeboard

Along with the worldwide decrease of round woodwith high quality and large diameter day by day, theoutput of lumber and plywood used in constructionis declining year by year. But the market demand forcomposites keeps rising. Non-veneer woodcomposites (Oriented Strand Board, Flakeboard,Medium Density Fiberboard) will be the mainstreamof composites in the future. At present, OrientedStrand Board and structural flakeboard have to a largedegree replaced structural plywood in buildingconstructi…     

杉木积成材制造工艺研究

研究以杉木制材板皮为原料，采用梳解加工法制造杉木积成材的制板工艺。结果表明，密度为0．6g／cm^3，厚为16mm的杉木积成材，其较佳制造工艺为木束条含水率6％-9％，施胶量8％-9％，热压压力2．0MPa，热压温度160℃。热压时间10min。     

Cantilever experimental setup for rheological parameter identification in relation to wood drying

Wood exhibits a pronounced time dependent deformation behavior which is usually split into ‘viscoelastic’ creep at constant moisture content (MC) and ‘mechano-sorptive’ creep in varying MC conditions. Experimental determination of model rheological parameters on a material level remains a serious challenge, and diversity of experimental methods makes published results difficult to compare. In this study, a cantilever experimental setup is proposed for creep tests because of its close analogy with the mechanical behavior of wood during drying. Creep measurements were conducted at different load levels (LL) under controlled temperature and humidity conditions. Radial specimens of white spruce wood [Picea glauca (Moench.) Voss.] with dimensions of 110 mm in length (R), 25 mm in width (T), and 7 mm in thickness (L) were used. The influence of LL and MC on creep behavior of wood was exhibited. In constant MC conditions, no significant difference was observed between creep of tensile and compressive faces of wood cantilever. For load not greater than 50% of the ultimate load, the material exhibited a linear viscoelastic creep behavior at the three equilibrium moisture contents considered in the study. The mechano-sorptive creep after the first sorption phase was several times greater than creep at constant moisture conditions. Experimental data were fitted with numerical simulation of the global rheological model developed by authors for rheological parameter identification.     

Development of an air-injection press for preventing blowout of particleboard I: effects of an air-injection press on board properties

An air-injection press was developed to prevent particleboard from blowing out during the manufacturing process. The air-injection press, which has holes punched in the heating plates, injects high-pressure air into the board through the holes of one plate and releases the air through the holes of the other plate. The high-pressure air forces out vapor trapped within the board, thus preventing blowout. The newly developed press reduced the pressing time required for manufacturing board from high-moisture-content particles. However, the manufactured boards exhibited mechanical properties and dimensional stability inferior to conventionally manufactured boards.     

Phenol-formaldehyde resin curing and bonding under dynamic conditions

Summary To better understand the curing and bonding behavior of phenol-formaldehyde (PF) resin under dynamic conditions, flakeboards were manufactured either by conventional pressing at 7% or 12% mat moisture content or by steam injection pressing with 10 or 20 seconds steaming duration. Resin-impregnated glass-cloth samples and lap-shear tension specimens were embedded in the core of each flakeboard. After the flakeboards were pressed for various periods of times, the samples and specimens were quickly retrieved. The degree of resin cure was determined on the resin-impregnated glass-cloth samples by dynamic mechanical analysis. The bond strengths were measured from the lap-shear tension specimens on a mechanical testing machine. The results of resin curing and bonding were then correlated to the performance of the resin-bonded boards, which were evaluated by internal bond, modulus of rupture, modulus of elasticity, and thickness swelling. Resin curing and lap-shear bonding did not proceed simultaneously. In conventional pressing, the mat moisture content (MC) at 12% favored resin curing, but slightly retarded lap-shear bonding, as compared to 7%MC. In steam injection pressing, the rates of resin curing and lap-shear bond strength development were much faster for 20 seconds than for 10 seconds of steaming duration. Longer press times were needed to obtain boards with maximum strength in the 12%MC conventional pressing and the 20-s steam duration steam injection pressing than in those conditions where moisture content was lower or steam time was less. The need for longer press times must be attributed to higher internal vapor pressures and/or lower wood and resin component strengths, rather than to incomplete cure or bonding.This material is based on work supported by the Ministry of International Affairs, Quebec Government, the Natural Sciences and Engineering Research Council of Canada, and Laval University (Quebec City). The work was also supported by the U.S. Department of Agriculture under research joint venture agreement FP-92-1835