Structural mechanics of wood composite materials 11: Ultrasonic propagation mechanism and internal bonding of particleboard

Ultrasonic nondestructive evaluation (NDE) methods have been successfully applied for grading lumber and veneer at the in-plant level. To expand this application in wood composite production, further research is needed to elucidate the effect of differences of component elements within wood composite panels on the behavior of ultrasonic waves traveling through them. The objective of this study was to investigate the effects of the internal bonding of particleboard specimens containing component chips of different geometry on ultrasonic velocity. Commercial chips screened at four sizes were used to produce particleboard specimens with different internal bonding by controlling their out-of-press thickness at (a) a constant thickness for boards made of each chip size, and (b) four different thicknesses for boards made of the same chip size. Twenty-four boards were made with phenol-formaldehyde (PF) resin at 8% solid resin content in our laboratory. After the velocities of the waves traveling through the thickness of the boards were recorded, the internal bond strengths were tested. Results showed the density, internal bond state, and constituted chip geometry were the main factors influencing the velocity. NDE using ultrasonic waves is an available method to evaluate the internal bonding of particleboard with a density less than about 0.75g/cm³. With densities over that value, no significant changes of the velocity were found.Part of this paper was presented at the 48th annual meeting of the Japan Wood Research Society, Kochi, April 1998     

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     

Effects of wood species,particle sizes and dimensions of residue obtained from trimming of wood–cement composites on physical and mechanical properties of cement-bonded particleboard

Abstract

The present work was conducted to study the effects of wood species, particle size and residue particle size obtained from trimming of wood–cement composites on physical and mechanical properties of cement-bonded particleboard (CBPB). Particleboard was manufactured with a wood/cement ratio of 1:3 and specific gravity 1200 kg m^?3. After manufacturing, the boards were tested. The second order plan was used to test the significant difference between factors and levels. It was shown that slenderness and compaction ratio increased and bulk density and specific surface decreased with the increase of particle size. With the increase in slenderness ratio and compaction ratio and decrease in bulk density and specific surface, thickness swelling and mechanical properties improved, but water absorption by the board increased. The addition of 6% of 5/3 fraction size of particle obtained from trimming of boards improved significantly the properties of the boards. The optimized panel properties, obtained using poplar particles with a fraction size of 7pass/on5, exceeded the BISON type HZ and EN Standard for Wood Particleboard. CBPB made of alder or poplar particles with 5/3 fraction size of residue exceeded the BISON type HZ. All CBPB with 5/3 fraction size of residue showed lower mean values of thickness swelling, well below the maximum requirements of both standards. In addition, wood species, fraction size of particles and residue size are believed to have been the main cause of change in the properties of the boards.     

Structural variability of vascular bundles and cell wall in rattan stem

Summary Anatomical investigations on six species ofCalamus demonstrate that fibre wall thickness and fibre percentage within the vascular bundles vary more than the vascular bundle size and number per unit area. Both fibre wall thickness and fibre percentage decrease consistently from the basal (order) to the top (younger) internodes of the stem and from the periphery to the centre at a given internode. The increase of wall thickness with age is more pronounced in fibres than in cortical and ground parenchyma. The thickening of the fibre wall with increasing stem density results from the deposition of additional lamellae. This appears to impart stiffness and determines the breaking behaviour of rattan both within the stem and among the species. The fracture mode of the fibres depends on the fibrillar orientation, which differs between broad and narrow lamellae.The first author is grateful to the Director Kerala Forest Research Institute (KFRI) for approving the collaborative research programme with the Institut für Holzbiologie und Holzschutz der Bundesforschungsanstalt für Forst und Holzwirtschaft. He is also indebted to the International Development Research Centre, Canada, for sponsoring the visiting programme under the project: Rattan (India) 3-P-86-0236: KFRI/109/1987. Thanks are due to Mrs. G. Weiner for her cooperation and Mr. P.K. Thulasidas (KFRI) for technical assistance in field collection     

Gas permeability of fiberboard mats as a function of density and fiber size

ABSTRACT

Gas permeability and structure of fiberboard mats are essential properties because of their impact on mat internal gas pressure, moisture content and temperature evolution during the hot-pressing process. The objectives of this work were to determine the effect of fiber size and mat density on the intrinsic gas permeability of the mat. For the study of these mat properties, panels with a homogeneous density profile through the thickness were manufactured at five density levels (200, 400, 600, 800 and 1000?kg m^?3) for three different fiber sizes. Fiber refining was performed in a disk refiner at three plate spacings. Gas permeability was measured with an in-house built apparatus. The results showed that the fiber sizes studied had no significant effect on the intrinsic permeability. This was likely due to a more significant impact of the internal porous structure of the mat compared to fiber size. Besides, the intrinsic permeability decreased significantly when the panel density increased from 198 to 810?kg m^?3. This suggests that the decrease of the intrinsic gas permeability during the last seconds of press closure plays an essential role in the bulk moisture mass transfer through the fiberboard mat.     

Water removal of wet veneer by roller pressing

High moisture content, flat sawn Japanese cedar (Cryptomeria japonica D. Don) veneer was compressed using a roller press to mechanically remove water. The amount of water removed depended on the amount of compression applied. At 60% compression, 400kg/m³ of water was removed. The process was not dependent on the size of the wood, the degree of compression, or the feed speed of the specimen. After compression, the remaining water contents were evenly distributed throughout the veneer regardless of the length of the specimen. The specimens did not completely recover to original thickness. High compression ratio and low temperature intensified the reduction of thickness. The bending strength after compression decreased in an inversely proportional manner with the thickness of the specimen and the compression degree.Part of this study was presented at the 50th Annual Meeting of the Japan Wood Research Society, Kyoto, April 2000     

Mode I interlaminar fracture property of moso bamboo (Phyllostachys pubescens)

Bamboo is a unidirectional fiber-reinforced bio-composite. Once having cracks, the delaminating propagation is not controlled by the strength but by the interlaminar fracture toughness. In this paper, the behaviors of Mode I (crack opening mode) interlaminar fracture parallel to grain of moso bamboo (Phyllostachys pubescens) were studied. Based on energy theory, the Mode I interlaminar fracture toughness, G _IC, was measured using the double cantilever beam specimens, and the fracture surfaces were examined under scanning electron microscope. The results show that: (1) the interlaminar fracture toughness of Mode I is the basic characteristic of bamboo material. The mean value of G _IC = 358 J/m² (coefficient of variation = 16.88%) represents the resistance arresting crack propagation. No significant difference was found for G _IC among the specimens located at different heights of the bamboo. (2) Due to the low G _IC of bamboo, the crack propagation parallel to grain developed easily. The crack was a self-similar fracture without fiber-bridging. On the fracture surfaces, smooth fibers and plane ground tissue were found at the extended area of Mode I fracture along the longitudinal direction. Under scanning electron microscope, it could be seen that the crack propagation developed along the longitudinal interface between fibers or ground tissue. It indicates that the longitudinal interface strength was weak among bamboo cells.     

Properties of flat-pressed wood plastic composites as a function of particle size and mixing ratio

This paper presents the effects of particle size and mixing ratio on the properties including physical, mechanical, and decay resistance of wood plastic composites (WPCs). In addition, it also presents the effects of immersion temperatures on water absorption (WA) and thickness swelling (TS) of the WPCs. WPCs with a thickness of 6 mm were fabricated from Albizia richardiana King & Prain wood particles and recycled polyethylene terephthalate (PET) by the flat-press method. To prepare the WPCs, two different wood particle sizes (0.5–1.0 and 1.01–2.0 mm) were used along with four different mixing ratios (w/w). Subsequently, the physical properties include density, moisture content, WA, and TS, and mechanical properties include modulus of elasticity (MOE) and modulus of rupture (MOR) of the produced WPCs was evaluated. Furthermore, decay resistance was evaluated by the weight loss percentage method. Moreover, the effects of immersion temperatures on WA and TS of WPCs after 24 h of immersion in water at three different temperatures, i.e., 25, 50, and 75 °C were investigated. Results showed that the wood particle size had impact on WPC’s density (only 6% decreased with the increase of particle size); however, the density decreased by 29% when the wood particle content increased from 40 to 70%. The WA and TS gradually increased with the increase of particle content and decrease of particle size. In addition, WA and TS increased proportionately with increasing immersion temperature from 25 to 75 °C. Furthermore, the highest MOE (2570 N/mm²) was found for the WPCs fabricated from large wood particles having the ration of 50:50 (wood particle:PET). For decay resistance, WPCs consisted of larger particles and higher PET content showed greater resistance against decay. Therefore, it is comprehensible that fabrication of the WPCs from 50% large particles and 50% PET is technically feasible and further improvement of WPC performance like enhancement of MOE and reduction of density using coupling agent and agricultural waste fibers, respectively, in the WPC formulation is recommended.     

Variations in bark thickness and sapwood density of <Emphasis Type="Italic">Calophyllum inophyllum</Emphasis> provenances in Australia and in Sri Lanka

Sapwood density and bark thickness of Calophyllum inophyllum L. (a multipurpose durable timber species) were studied in various locations in Northern Australia and in Sri Lanka. Measurements were taken non-destructively by using core sampling and bark gauge. From each provenance, 4-15 mature trees having girth at breast height over bark (GBHOB) at 100-150 cm were selected on the basis of the population size. Significant (p<0.05) hemispheric and provenance variations in bark thickness were found. Variations in the bark thickness are influenced by environmental variables. Variations in sapwood density were less pronounced compared to that of bark thickness. Variations in sapwood density are likely to be governed by genotypic variations.     

石灰石粉施用量对重庆酸雨区马尾松林下木荷生长的影响

重庆是我国酸雨发生最早且其污染最严重的地区之一。由于长期遭受酸雨影响,这里的森林受害明显,经济和环境损失巨大。马尾松(Pinus massonianaLamb.)和木荷(Schima superbaGardn.etChamp.)都属于重庆的重要乡土树种,马尾松在造林中被广泛使用,木荷是优良的生物防火树种,并因     

Mode III fracture energy of wood composites in comparison to solid wood

A simple experimental setup for mode III and mixed mode (I?+?III) fracture tests with anisotropic materials under steady state crack propagation has been developed. Load-displacement curves can be recorded up to the complete separation of the specimen. From the load-displacement curves several mechanical material parameters can be derived. The tests have been performed for solid wood and different wood composites, being PARALLAM^® PSL in different orientations, particleboard and INTRALLAM^® LSL, and the fracture behaviour is characterised by the specific fracture energy.     

Regeneration in experimental gaps of subalpine <Emphasis Type="Italic">Picea abies</Emphasis> forest in the Slovenian Alps

In 1998, we analysed regeneration success in four 20 year old medium-sized (600–800 m²) and four small experimental gaps (100–200 m²) in subalpine Norway spruce forest in Triglav National Park, Slovenia. We assessed the influence of site (depressions, slopes), gap size, and position within gap (centre, gap south, and north edge) on regeneration success and interactions between regeneration density, height, height increment, direct and diffuse light, ground vegetation, soil depth, thickness of humus horizons, presence of woody debris, and microrelief. Seedling density was higher in depressions, where more seedlings developed in central positions of gaps in both gap sizes, and in microsites with deeper soils, less competition from ground vegetation and convex microsites. On slopes, a higher seedling density was found in small gaps, and at the northern edge of gaps regardless of gap size. Here, seedling density was negatively influenced by diffuse radiation and positively by soil depth. The results suggest that regeneration should be initiated from medium to large gaps in depressions and small gaps on slopes. Development of established seedlings was enhanced by higher radiation levels on both sites, therefore favourable extension of the gaps would be lateral to northwards.     

Effect of flake geometry on properties of cement-bonded particleboard from mixed tropical hardwoods

Summary Laboratory scale cement-bonded particleboards were made from mixed particles of three tropical hardwoods. Boards were three-layered comprising of 2 mm thick sawdust face and 4 mm thick core layers made from flakes of three lengths-12.5 mm, 25.0 mm and 37.5 mm and two thicknesses of 0.25 mm and 0.50 mm. The panels were fabricated at three density levels of 1,050 kg/m³, 1,125 kg/m³ and 1,200 kg/m³. From the statistical factorial analysis carried out, flake length, flake thickness and board density had significant effects at 1% level of propability on the properties of the tested panels. Mean MOR ranged from 5.22 to 11.15 N/mm²; MOE-2,420 to 4,820 N/mm²; water absorption and thickness swelling following 144 hours soak in water, 32.95 to 46.00% and 0.35 to 5.47% respectively. The longer and thinner the flakes, the stronger, stiffer and more dimensionally stable the experimental cement-bonded particleboards. Similarly, the higher density panels generally exhibited higher strength values in terms of MOR and MOE and were more dimensionally stable. MOR, MOE, water absorption and thickness swelling were found to be highly correlated with flake length, flake thickness and panel density. Correlation coefficients (R) for these relationships were 0.888 to 0.986 for the combined variables; and 0.574 to 0.992 for the individual factors. In all the cases tested, the regression relationships were linear.     

筇刺竹生物学特性的研究

本文根据作者在云南南部筇刺竹集中分布地区所做的调查观测,对筇刺竹的生物学特性,包括形态特征、生态习性、个体结构规律和发笋率与竹丛结构因子的关系进行了分析研究。研究结果表明：筇刺竹是我国南部热区的适生竹种,尤其适于在河谷地带、盆地边缘和低山丘陵地区发展;筇刺竹秆形指标（ｆ１．３）为０．６９５,明显大于其它相近竹种或树木;竹秆最大直径和最长节间出现于秆之中部;胸径与秆高呈指数函数关系,可用Ｈ＝２．２４６４７３Ｄ０．９２６７２５来表达;胸径与秆重呈正线相关,可用Ｗ＝０．１９３３９５（Ｄ２Ｈ）０．６９０４０４来表达;竹丛发笋率、成竹率与秆丛密度和竹丛平均秆径有较密切的关系。本文对竹笋采收指数与老秆采伐年龄的确定进行了探讨     

Non-destructive estimation of Pinus taeda L tracheid morphological characteristics for samples from a wide range of sites in Georgia

Tracheid coarseness, specific surface, wall thickness, perimeter, and radial and tangential diameter from 119 radial strips of Pinus taeda L. (loblolly pine) trees grown on 14 sites in three physiographic regions of Georgia (USA) were measured by SilviScan. NIR spectra were also collected in 10 mm increments from the radial longitudinal surface of each strip and split into calibration (9 sites, 729 spectra) and prediction sets (6 sites, 225 spectra). NIR spectra (untreated and mathematically treated first and second derivative and multiplicative scatter correction) were correlated with tracheid properties to develop calibrations for the estimation of these properties. Strong correlations were obtained for properties related to density, the strongest R ² being 0.80 (coarseness), 0.78 (specific surface) and 0.84 (wall thickness). When applied to the test set, good relationships were obtained for the density-related properties (R _p ² ranged from 0.68 to 0.86), but the accuracy of predictions varied depending on math treatment. The addition of a small number of cores from the prediction set (one core per new site) to the calibration set improved the accuracy of predictions and, importantly, minimized the differences obtained with the various math treatments. These results suggest that density related properties can be estimated by NIR with sufficient accuracy to be used in operational settings.     

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.     

Assessment of temperature and relative humidity conditioning performances of interior decoration materials

The purpose of this study was to explore conditioning effects on wood panels (used as interior decorative materials). We examined hourly the temperature and relative humidity (RH) in a living environment based on the data of average values from 1974 to 1990 in the Taipei area. Thirty-six interior finish materials attached to one inside surface of a 35³ cu cm simulation aluminum container were used in this study. An A/V ratio (surface area/volume) of 2.86 (m^–1) or various other values and the panel thickness had no significant effect on the room temperature changing ratio. The hygroscopic conditioning performances of these decorative materials were classified into four types in accordance with b values: type I (b > 0.0200) included four solid woods (unfinished), seven wood-based materials, three composite materials, and one inorganic material. Type II (0.0170 <b < 0.0199) included one solid wood, five wood-based materials, and two inorganic materials. Type III (0.0070 <b < 0.0169) included nine wood-based materials, and four inorganic materials. The RH changing ratio decreased curvilinearly with increasing interior decorative panel thickness and A/V values in a sealed container, whereas theb values increased with increasing interior decorative panel thickness and A/V values in a sealed container.Part of this report was presented at IUFRO All Division 5 Conference, Washington State University, Pullman, WA, USA, July 7–12, 1997     

Bond durability of kenaf core binderless boards II: outdoor exposure test

An outdoor exposure test was conducted on kenaf core binderless boards (pressing temperatures 200°, 180°, and 160°C; pressing pressure 3.0 MPa, time 10 min, target board thickness 5 mm, target board density 0.8 g/cm³) to estimate their bond durability. Modulus of rupture (MOR), modulus of elasticity (MOE), internal bonding strength (IB), thickness change, weight loss, Fourier transform infrared (FTIR) spectra, and color difference (ΔE*) by the CIE L^*a^*b^* system were measured at various outdoor exposure periods up to 19 months. These values were then compared with those of a commercial medium-density fiberboard (MDF; melamine-urea-formaldehyde resin; thickness 9.0 mm, density 0.75 g/cm³). Generally, dimensional stability and the retention ratios of MOR, MOE, and IB after the outdoor exposure test increased with increased pressing temperature of binderless boards. The MOR retention ratio of the kenaf core binderless boards with a pressing temperature of 200°C was 59.5% after 12 months of outdoor exposure, which was slightly lower than that of the MDF (75.6% after 11 months of outdoor exposure). Despite this, the bond durability of the kenaf core binderless boards should be viewed as favorable, especially when considering the fact that the retention ratio of 59.5% was achieved without binder and without obvious element loss. Part of this report was presented at the International Symposium on Wood Science and Technology, IAWPS2005, November 27-30, 2005, Yokohama, Japan     

Elastic moduli and stiffness optimization in four-point bending of wood-based sandwich panel for use as structural insulated walls and floors

Several wood-based sandwich panels with low-density fiberboard core were developed for structural insulated walls and floors, with different face material, panel thickness, and core density. The elastic moduli with and without shear effect (E _L, E ₀) and shear modulus (G_b) were evaluated in four-point bending. Generally, the stiffer face, thicker panel, and higher core density were advantageous in flexural and shear rigidity for structural use, but the weight control was critical for insulation. Therefore, optimum designs of some virtual sandwich structures were analyzed for bending stiffness in relation to weight for fixed core densities, considering the manufactured-panel designs. As a result, the plywood-faced sandwich panel with a panel thickness of 95 mm (PSW-T100), with insulation performance that had been previously confirmed, was most advantageous at a panel density of 430 kg/m³, showing the highest flexural rigidity (E _L I = 13 × 10⁻⁶ GNm²) among these panels, where E _L, E ₀, and G _b were 3.5, 5.5, and 0.038 GN/m², respectively. The panel was found to be closest to the optimum design, which meant that its core and face thickness were optimum for stiffness with minimum density. The panel also provided enough internal bond strength and an excellent dimensional stability. The panel was the most feasible for structural insulation use with the weight-saving structure.     

Culling phenotypically inferior trees in seed production area enhances seed and seedling quality of <Emphasis Type="Italic">Acacia auriculiformis</Emphasis>

Improvement in seed and seedling quality of Acacia auriculiformis after culling phenotypically inferior trees was studied in a 6-year old seed production area (SPA). A 5-ha plantation was identified, of which 2.3 ha was converted into SPA. The initial stocking, 1 612 trees·ha⁻¹, was thinned down to 982 trees·ha⁻¹ based on growth characteristics. The following fruiting season, seeds were collected from 10 randomly selected trees in culled and non-culled stands, and seed physical characters, germination and seedling traits were assessed. Seed weight, seed thickness and percentage germination increased by 32.1%, 4.43% and 22.37%, respectively in the culled stand compared to the non-culled stand. Culling also increased the speed of germination, seedling dry weight and seedling vigor index. Heritability values were high for seed weight (0.974) and seed thickness (0.948) while medium values were observed for percentage germination (0.577) and total dry weight (0.534). Predicted genetic gain was 11.13% and 11.22% for seed weight and percentage germination, respectively. The actual gain was 32.1, 51.9 and 22.9% for seed weight, percentage germination and total dry matter, respectively. In conclusion, SPAs established by culling inferior trees could serve as sources of good quality seeds for reforestation programs until genetically improved seeds are made available.