Development of high-performance strand boards: engineering design and experimental investigations

Strand-based engineered wood products such as oriented strand boards enjoy great popularity in structural engineering and are widely used for a variety of applications. To strengthen their competitiveness and to enlarge their range of utilization particularly in the load-bearing sector, the mechanical properties of these products need to be improved. This motivated the research efforts to use large-area, slender veneer strands for the production of strand boards with increased stiffness and strength. Target-oriented development of these products requires comprehending the effects of the relevant (micro-)characteristics, such as wood quality, strand geometry, and strand orientation and compaction during the production process, as well as layer assembly and density profile, on the mechanical properties of the finished strand boards. Comprehensive test series, in which these effects on tension, bending and shear properties of the boards have been studied individually, are presented in this paper. The obtained results provided insight into the microstructural load-carrying mechanisms and, thus, yielded valuable knowledge for product optimization and further improvement of custom-designed strand-based engineered wood products.     

Properties of strand boards with uniform and conventional vertical density profiles

Randomly oriented strand boards with both uniform and conventional vertical density profiles (VDP) were manufactured, and their properties were evaluated and compared. The bending modulus of elasticity (MOE) of conventional strand boards was predicted using the laminated beam theory and the MOE-density regression equation from the uniform strand boards. The results showed that the predicted MOE of conventional strand boards was close to the measured MOE with a difference of less than 10%. The internal bond strength values of uniform strand boards were found to be higher than conventional strand boards while no significant difference was found in water-related properties. Compared with uniform strand boards, MOE values of conventional strand boards were improved only at higher density level. About 10% of improvement in MOE can be obtained for the strand boards investigated by manipulating the VDP. Steeper VDPs were predicted to be required for thinner boards than for thick boards in order to achieve the same improvement in MOE.     

Effects of strand length and layer structure on some properties of strandboard made from bamboo

Strandboard panels were experimentally produced from moso bamboo (Phyllostachys pubescens) using various strand lengths and layer structures to evaluate the effects of manufacturing parameters on panel properties. The strandboard was fabricated in a laboratory using diphenylmethane diisocyanate (MDI) resin and laboratory-made strands of four lengths and four different structures. Strand alignment distributions and concentration parameter (k) values were greatly affected by strand length. A linear correlation was found between the value of k and the modulus of rupture (MOR), with correlation coefficients of 0.81 and 0.93 for unidirectional boards and three-layer boards, respectively. This correlation may be used to predict the strength properties of boards. Bending properties were significantly affected by both the strand length and the layer structure of the bamboo strandboard tested. Elasticity data from unidirectional boards and random boards can be used to predict the elastic properties of three-layer boards. The linear expansion (LE) of the random boards increased with decreasing strand length. The difficulty in mat forming and resin distribution for longer strands could cause deviation in modulus of elasticity (MOE) and LE, especially in strand lengths around 80 mm.     

The mechanical and physical properties of strand boards treated with preservatives at different stages of manufacture

The physical and mechanical properties of boards treated with a preservative at different points during the manufacture process were evaluated to determine the best stage for the application of preservative. A copper boron tebuconazole amine water-based preservative was used in 3% PF-bonded strand boards to achieve five different retentions. Preservative addition was examined at different stages of the manufacture cycle, namely, green strand diffusion, dry strand vacuum treatment, glue-line spray addition, heat and cold quench of manufactured board, and by post-manufacture vacuum treatment. The treatment methods had marked effects on the mechanical properties of some of the boards when the boards with the highest preservative retention were compared with their respective untreated controls. The best results were achieved where the preservative was applied by vacuum treatment of dry strands or by diffusion of green strands before board manufacture. Increasing preservative retention had minimal effects on board properties with these two methods but significant deterioration was noted when the preservative was applied by spraying dry strands or by post-board-manufacture heat and cold quench. An increase of pressing temperature resulted in significant improvements to the mechanical properties of the spray-treated boards. Post-manufacture vacuum treatment of boards caused excessively high losses in internal bond strength.     

Effect of board density and layer structure on the mechanical properties of bamboo oriented strandboard

This study examined the effects of density and layer structure on the mechanical properties and dimensional stability of strandboard manufactured from moso bamboo (Phyllostachys pubescens). The strandboard was fabricated in a laboratory at five densities and three different structures including a randomly oriented homogenous board, a unidirectionally oriented homogenous board, and a three-layered board with a cross-oriented core layer (BOSB). Bamboo strand alignment distribution could be predicted using the von Mises distribution function. Bending properties increased with increasing density and were affected by layer structure. The modulus of rupture (MOR) of the threelayered board in the parallel direction increased remarkably compared with the random board MOR; in the perpendicular direction, it exhibited less strength reduction. Elastic properties of the three-layered board could be predicted using elastic constants of the unidirectional board. Internal bond strength (IB) was greatly affected by density, but the layer structure effect did not appear in IB. Linear expansion per unit moisture change ranged from 0.017 to 0.022 for random and three-layered boards; these values are comparable with or lower than values for commercial board.     

Production and properties of Japanese oriented strand board I: effect of strand length and orientation on strength properties of sugi oriented strand board

Oriented strand boards (OSB) were made using sugi wood strand with different lengths at different free fall distance conditions. Strand alignment and mechanical properties of sugi OSB were evaluated. Results obtained can be summarized as follows. The alignment angle distribution was greatly affected by both free fall distance and strand length. It was found that the standard deviation of the angles can be a measure for predicting the distribution when employing the von Mises distribution function with concentration parameter. The Monte Carlo simulation showed an agreement between the theoretical considerations and the experimental results on the strand alignment. The mechanical properties as affected by both strand length and layer structure were determined. Bending properties could be equal in both directions at 25% face layer ratio. Young's modulus obtained by the in-plane vibration method showed almost linear relation to the face layer ratio. No significant differences or only a slight difference was observed for the internal bond strength, plate-shear modulus, and nail resistance properties. Further studies are necessary.     

Effect of Ratio of Face to Core Particles on Mechanical and Physical Properties of Particleboard Manufactured from Ethiopian Highland Bamboo

The study was conducted to evaluate effect of ratio of face to core particles on mechanical and physical properties of oriented strand board produced from Ethiopian highland bamboo.Three-layered oriented particleboards were manufactured with 4 proportions of face to core particles at 750 kg/m~3 target density.Ten percent urea formaldehyde resin was used as a binder.Strength and dimensional stability performances of all boards were assessed based on ISO standards.The results showed that modulus of rupture...     

Image analysis and bending properties of model OSB panels as a function of strand distribution,shape and size

The technique of image analysis has been used to assess the quality of model oriented strand board panels by investigating the relationships between shape and size of strands, the distribution of strands and bending properties. A batch of commercial strands was analysed by image analysis and the distribution of the shape and size of strands was quantified. The strands were categorised into five strand types as a function of size and aspect ratio. In general, strand shapes were observed to be mostly rectangular and there was also a wide variation in strand dimensions in commercial material. Bigger area strands had low aspect ratios and small strands had high aspect ratios. Half of the commercial strands were longer than 100 mm.Model OSB panels were manufactured in the laboratory by hot pressing strand mats formed from each of the five strand types. Strands were laid up by hand into the forming mat and following pressing the orientation and shape of strands was evaluated by image analysis and the panels were tested in a three point bending. Large area (type 3) strands with high aspect ratios produced model panels with optimum strand orientation and mechanical properties.Type 3 panels were also fabricated from strands dropped through a slotted forming device in order to simulate the delivery of strands to the forming line under factory conditions. As the height of strand delivery increased from 0 to 100 to 200 mm the disorientation of strands in the pressed panels progressively increased and as a result mechanical properties in bending were reduced.Image analysis is therefore a powerful tool for evaluating the distribution of commercial strand shapes and the relationship between strand geometry, strand orientation and the mechanical properties of oriented strand board.     

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…     

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。     

铜基防腐剂对竹材定向刨花板性能的影响

选用季铵铜(ACQ)、铜唑(CuAz)及环烷酸铜(CuN)3种铜基防腐剂,考察防腐剂添加对竹材定向刨花板物理力学性能的影响。研究表明,3种防腐剂处理材的物理力学性能,均满足LY/T 1580-2000《定向刨花板》中OSB/3规定;其中,CuAz及CuN各载药量处理材的性能均超过OSB/4的要求,并且综合力学性能优于ACQ处理材。添加2.21 kg/m3的CuAz制作的板材性能略低于对照试板外,其余防腐剂或载药量对板材性能无不利影响。     

Composite behavior of laminated strand lumber

The mechanical properties of laminated strand lumber are dependent on the orientation of strands and on the variability of strand alignment in the production process. A model is proposed to predict the in-plane properties and their statistical distributions to allow manufacturers to set target reliability levels for their products. The model is based on the theory of mechanics of composites and assumes homogeneity in each panel layer to allow for multiple-layer panels to be simulated. To verify the model, five types of panels are fabricated using aspen strands with the following stacking sequences: (a) fully-oriented (0° throughout); (b) fully-random (R throughout); (c) random core/oriented surfaces (0°/R/R/0°); (d) random surfaces/oriented core (R/0°/0°/R); and (e) eight oriented layers (0°/+45°/-45°/0°/0°/-45°/+45°/0°). In-plane elastic moduli and ultimate strengths (in tension, compression, and shear) are determined for each panel type. Model predictions match well with experimental results. Properties are shown to be dependent on the degree of strand alignment in each panel type. Using the first-order reliability method, statistical distributions on the properties were predicted and found to compare well with experimental results. A method is proposed for dealing with misalignment of strands based on the von Mises distribution of strand angles to assist manufacturers with production process optimization.     

定向结构生物质复合板研究进展与发展前景

将生物质材料加工成定向结构复合板材是提高其承载性能的重要途径,也是木材工业发展的主要方向,符合国家生态文明建设、经济转型发展和建设小康社会的目标。从木材、竹材和农作物秸秆三类主要原材料入手,分析了国内在定向结构复合板材领域的研究进展、发展方向和存在的问题,以期为相关科研工作提供借鉴。     

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.     

热压工艺对竹重组板材性能的影响

探讨了以竹材为主要原料的竹重组板材热压工艺的优化,研究了热压工艺对竹重组板材力学性能的影响,讨论分析了热压压力、热压时间、热压温度对竹重组板材吸水厚度膨胀率、耐沸水性、静曲强度、弹性模量、耐磨性、耐化学腐蚀性、浸渍剥离率和甲醛释放量等性能的影响。通过正交试验,得出的优化热压工艺为:①热压压力2.0MPa、热压温度145℃、热压时间1.7min/mm,热压压力对竹重组板材耐酸性、静曲强度和弹性模量等影响显著,对耐沸水性、耐碱性、耐盐性、耐磨性和浸渍剥离率等影响不显著。②热压时间对竹重组板材静曲强度有显著影响,对其他试验指标影响不显著。③热压温度对竹重组板材各试验检测指标均有一定的影响,但不显著。     

Effect of particle shape on linear expansion of particleboard

Hinoki (Japanese cypress) strand with 0.6mm thickness was hammer-milled to prepare three types of particles that had the same thickness but different lengths and widths. Screen analysis and image analysis were conducted to evaluate the shape and distribution of the particle dimensions. Laboratory-scale particleboards were fabricated at three density levels using these particles. To determine the true effect of particle shape on the linear expansion (LE) of the board, these boards exhibited almost the same temperature behavior during pressing, the same density profile, and the same bending properties at each board density level. LE at 40°C and 90% relative humidity of the board was found to be affected by the particle shape. The board composed of small particles showed a larger LE at the same density level. It was considered that the out-of-plane orientation angle of the particles affected the LE of the boards. Thickness swelling and internal bond strength were also affected by the particle shape.Part of this report was presented at the 50th annual meeting of the Japan Wood Research Society, Kyoto, April 2000     

Development of structural composite products made from bamboo I: fundamental properties of bamboo zephyr board

A study was conducted to determine the suitability of zephyr strand from moso bamboo (Pyllostachys pubescens Mazel) for structural composite board manufacture. Thirty-two 1.8×40×40cm bamboo zephyr boards (BZB) were produced using four diameters of zephyr strand (9.5, 4.7, 2.8, and 1.5mm) and four target densities (0.6, 0.7, 0.8, and 0.9g/cm³). Results indicate that BZB exhibits superior strength properties compared to the commercial products. The size of the zephyr strand and the level of target density had a significant effect on the moduli of elasticity and rupture, internal bond strength, water absorption, and thickness swelling, but they did not have a significant effect on linear expansion. With regard to the physical properties, BZB exhibited less thickness swelling and exhibited good dimensional stability under dry-wet conditioning cycles.Part of this research was presented at the 48th annual meeting of the Japan Wood Research Society, Shizuoka, April 1998; it was reported at the 4th Pacific Rim Bio-Based Composite Symposium, Bogor, Indonesia, November 1998     

Duration-of-load and creep effects in strand-based wood composite: a creep-rupture model

In order to investigate the creep and duration-of-load (DOL) effects in thick strand-based wood composite products, a creep-rupture model is proposed linking the accumulated damage to creep deformation. Results from long-term constant load tests have been interpreted by means of this creep-rupture model, which is capable of representing the time-dependent deflection and time-to-failure data at different stress levels. The predictions of the model have been verified using results from ramp load test at different rates of loading. The creep-rupture model incorporates the short-term strength of the material, the load history and predicts the deflection history as well as the time-to-failure. As it is a probabilistic model, it allows its incorporation into a time-reliability study of wood composites’ applications.     

Densification,screw holding strength,and Brinell hardness of European beech and poplar oriented strand boards

In this study, selected mechanical properties of beech and poplar oriented strand board such as screw holding strength (SH) and Brinell hardness (BH) as well as the relationship of stress and strain under compression for both species under two temperatures (180°C and 220°C) were examined. Poly methylene diphenyl diisocyanate resinated laboratory-scale boards in different design and density of both species (650 and 720?kg/m³) were prepared. The results showed that density has a positive effect on SH and BH of panels. It was also observed that wood species (beech or poplar) and size of strands (normal or fine size) also effect on SH. Both species showed a specific behavior regarding the lapse of stress and strain during compression.