Stiffness modelling of particles in the core layer for the manufacturing of wood-reduced particleboard

A great demand for wood material has led to an increase in the price of industrial wood in the past and will raise it even further in future. Industry and research have reacted to this increasing price of wood, for example, by developing weight-reduced particleboard. One approach to achieve this reduction in weight is to use suitable chip geometries in the core layer instead of lightweight, resource-saving filling materials. The geometry of the chips in the core layer must have a high compression resistance here to reach the desired apparent density profile. For this purpose, particle geometries were analysed with regard to this property within a project for the development of weight-reduced particleboards. After determining the curve of the compression resistance, it was possible to establish an equivalent spring stiffness model for different chip geometries to characterise the geometry during compression. Merely due to a low bulk density for chips in the core layer, it is not possible to increase the spring stiffness and hence to reach a required apparent density profile. Only compression of the chips leads to a rapid increase in spring stiffness.     

Some investigations concerning density profile,internal bond and relating failure position of particleboard

Testing machines for the measurement of density profiles of wood based panels have become a wide-spread tool in production laboratories. New equipment for the in-line measurement of density profiles was presented recently. The high significance of the density profile for the properties of particleboard ticleboard is well known for some years. Maximum densities in the outer zones of the profile are closely related with MOE, bending strength, water uptake and swelling. Minimum density is closely related with shear modulus (G _(xz , (G _(yz ), shear strength and compression strength. However, results about the correlation of the minimum density with internal bond are contradictory, and statements concerning the failure positions relating to the internal bond-test are published rarely. For five different thicknesses of furniture grade particleboard the relation between internal bond, parameters derived from the density profile and the respective failure position were determined. Indeptendent of panel thickness the failure position was found to be in a range of the 25-75%-panel thickness. A correlation between the failure position and parameters derived from the density profile could not be found. There appear to be three predetermined failure lines (35, 50, 65% of panel thickness). The correlation between mean density and internal bond is high for the total sample (r=0.81). The correlation between minimum density and internal bond is lower, especially with regard to results relating to a single panel thickness.     

Properties of mixed particleboards based on white birch (Betula papyrifera) inner bark particles and reinforced with wood fibres

The use of white birch (Betula papyrifera) inner bark particles as a raw material for panel manufacture is more profitable than their use for energy production. The objective of this study was to manufacture three layers mixed particleboards based on white birch inner bark particles in the core and reinforced with wood fibres in the face. Two manufacturing factors were taken into account: the percentage of wood fibres in the surface and the percentage of wood fibres added to white birch inner bark particles in the core. The panel with 25% fibres in the surface and 9% fibres added to white birch inner bark particles in the core had the best mechanical properties for M-1 grade of particleboards and 120 grade of medium density fiberboards. Only panel with 22% fibres in the surface and 5% fibres added to white birch inner bark particles in the core passed the standard for the linear expansion.     

Evaluation of low density hybrid panels using expandable granules: effect of granules diameter and content

Low density hybrid panels (16 mm thickness) were produced using wood fiber as face layers and a mixture of coarse wood particles and expandable polystyrene (EPS) as core layer. The EPS was foamed (in-situ foaming) in the core layer during the pressing stage. The effects of granules diameter (0.4, 0.8, 1.15, 1.5, and 2.2 mm) and granules content (5, 10, and 15%) on the physical and mechanical properties were evaluated. The results showed that both variables had significantly influenced the panels’ properties. Mechanical properties mostly depended on the level of mechanical inter-locking of EPS with wood particles, while the physical properties were mostly influenced by the EPS distribution. Stronger mechanical inter-locking of EPS with wood particles was achieved by both larger diameter and higher amount of EPS granules (2.2 mm and 15%). A better EPS distribution between the wood particles was observed with the smaller EPS granules and higher amount of EPS (0.4 mm and 15%).     

Measurement of wood and bark particles acidity and their impact on the curing of urea formaldehyde resin during the hot pressing of mixed panels

In the manufacture of panels, pH value is an important property to measure, especially when a pH-dependent resin like urea-formaldehyde (UF) is used. It has been proven that UF cures very well and faster in an acidic environment. In this study the acidity (pH value and buffering capacity) of both outer and inner white birch bark particles as well as that of wood particles or fibres used together with those barks as reinforcement for the manufacture of three layers particleboards was measured. The results show a significant difference between the acidity of white birch bark and wood. Barks have a lower pH as well as higher acid and alkaline buffering capacities. It was also found that the pH value of inner white birch bark is lower than that of the outer part. The significant differences observed enabled to realize that inner bark should be separated from outer bark and used separately in mixed panels manufacture. The platen temperature should be reduced when the outer bark is used in the surface of mixed panels to avoid pre-cure or over-cure. This study confirms the linear relationship between the pH and the absolute and relative buffering capacities.     

Untersuchungen zu Diffusionsvorgängen in mehrschichtigen Massivholzplatten

Multilayer solid wood spruce panels with different glueing and structure type were tested for water vapor diffusion for a wet climate (relative humidity ranging from 100% to 65% at 20°C) and for a dry climate (relative humidity ranging from 0% and 65% at 20°C). Results showed that the water vapor diffusion resistance for the dry climate is distinctly higher in comparison to the wet climate. The water vapor diffusion resistance is stronger related to the number of glue layers per panel thickness than to the type of adhesive. The average moisture content of the panels in a wet climate was 20%. If one considers a water saturation point of 28% (spruce wood) on the wet side, and 12% moisture content on the dry side (normal climate) of the panel a linear moisture gradient can be expected. Since the average moisture content of the solid wood panels was about 11.3% in a dry climate, and thus corresponding approximately to the equilibrium moisture content of spruce wood exposed to normal climate, the partial pressure gradient on equilibrium caused water uptake corresponding to the normal moisture content for spruce wood.     

Influence of face-to-core layer ratio and core layer resin content on the properties of density-decreased particleboards

As a response to increased costs and a shortage in wood supply it is a current approach to reduce the amount of material in the manufacturing process of particleboard (PB). However, the production of lightweight PB by simply reducing density results in decreased panel properties. Thus, investigations to re-engineer the panel’s core layer are required in order to achieve density-reduced panels which meet minimum property requirements (e.g., EN 312), edge processability and surface coatability. The intention of the present paper is to investigate the influence of potentially occurring changes in the face-to-core layer ratio (35/65…57/43) and core layer resin content (8 %…22.3 %) on panel properties when reducing the density from 650 to 400 kg/m³.     

Untersuchungen an Holzwerkstoffen für Möbel-Einlegeböden

The bending deflection of six different wood based materials used as shelves in the manufacture of furniture (unlaminated particleboard, melamine laminated particleboard, veneered particleboard, block-board, plywood and solid wood panel) of approximately equal thickness was investigated. Deflection data were obtained by applying loads to the board surfaces for a period of 28 days followed by a period of unloading of equal length. For all types of particleboards the deflections appeared to be greater than for the other wood based panels. The lowest bending deflection was found for the solid wood panel while plywood performed better than blockboard. Surface treatments improved the performance of particleboards but veneering appeared to be more advantagous than laminating with melamine. The bending deflection of the tested panels is strongly related to their bending strength and modulus of elasticity.     

Rohdichtebestimmung von Holzwerkstoffen mittels Computertomographie

In this work the vertical density profiles of three different commercial wood based panels (particleboard, MDF, OSB) were investigated using a conventional X-ray method as well as an industrial computer tomography (CT) method. The reconstructed gray level images of the obtained CT data were used to calculate a standardized density profile of each panel. The results showed that the measured (X-ray) and the calculated (CT) density profiles of the panels fit well in the core layer of the boards; in the most outer surface layers some anomalies in the results can occur in both methods. The advantage in the implementation and evaluation of the more complicated CT measurement compared to the conventional X-ray procedure is the three-dimensional data structure and the opportunity to do appropriate structural analysis of the investigated wood based panels.     

Indoor wood dust analysis during machining with an original experimental device

Particles generated by wood machining have a proven impact on the health of users and woodworkers. The aim of this study was to quantify and describe wood particles in solid and gas phases to reliably and reproducibly characterise these emissions. First, we developed an experimental device that produced particles from solid wood and wood-based panels using portable machine tools. The objective was to study the particles emitted by wood machining while avoiding ambient pollution. Based on own technical specifications, the experimental system was defined and composed of various elements that integrated treatment of inlet air through wood machining to the analysis section that allows solid and gas phases. The first experiments were carried out in sanding and sawing modes on materials used in construction, including solid wood (spruce) and composite panels (particle board (PB), oriented strand board (OSB), and medium density fibreboard (MDF)). Wood-based panels showed more emissive behaviour than solid wood, both for the solid phase and the gas phase. These tests validate the feasibility of generating and measuring particles and emissions of volatile organic compounds (VOCs). Further modifications to the experimental device would enable us to integrate additional devices, such as toxicological ones, to better understand the impact of these wood particles on the health of woodworkers.

     

Investigation of physical and mechanical properties of oil palm wood core sandwich panels overlaid with a rubberwood veneer face

Low-density sandwich panels consisting of an oil palm wood core overlaid with a rubberwood veneer face were manufactured. Effects of two types of grain orientation of the oil palm wood core (parallel and perpendicular to board surface) and three different veneer thicknesses (0.7, 1.8 and 2.7 mm) and core densities (223 ± 14, 301 ± 35 and 418 ± 33 kg/m³) on some physical and mechanical properties of the boards were investigated. Results showed that higher core density increased the values of thermal conductivity, screw withdrawal resistance, modulus of rupture and modulus of elasticity but decreased the value of water absorption without effect on thickness swelling of the boards. Boards with the core grain direction oriented perpendicular to panel’s surface possessed lower value of thickness swelling but higher values of thermal conductivity and strain at fracture when the board failed in a mode of core shear under bending test than those of the others. Finally, the relationship between board density and the measured physical and mechanical properties of the oil palm wood core sandwich panels overlaid with a rubberwood veneer expressed as mathematical equations could be used to predict and design the expected properties of this type of sandwich board.     

碎料板弹性模量微观力学的发展综述

通过研究刨花板、重组木弹性模量的微观力学求解理论的发展,对建立几个板种的弹性模量微观力学模型的新方法进行了总结;对相应的数学模拟计算的理论与方法进行了概述;对计算机仿真计算软件的发展提出了新见解。通过这些理论可以建立碎料材种、几何尺寸、胶种、铺装角及其误差、碎料力学参数、压缩比、容重等因素和板材弹性模量的定量数学关系,实现新型板种弹性模量的理论预测方法的应用。     

Analysing orthotropy in the core layer of wood based panels by means of fracture mechanics

In this study the orthotropic properties in the core layer of wood based panels are analyzed by means of the newly developed double cantilever I beam testing system. Four different wood based panels (i.e., OSB, particle board, particle board containing recycling chips and MDF) were tested in-plane, in longitudinal and lateral orientation. Specific fracture energy numbers yielded significant differences between the longitudinal and the lateral orientation for OSB, while the stress intensity factor analysis showed significant differences for OSB and particle board containing recycling particles.     

Influence of material origin on the size distribution of wood particles for wood-plastic composite (WPC) manufacture

This paper presents results from sieve analysis and image analysis-based particle size measurement (FibreCube) of wood particles for the manufacture of wood-plastic composites. Wood of different origin (virgin Norway spruce, post-industrial particleboard residues, mixed waste wood) was used for particle manufacture. It was found that size distributions were different, although milling conditions were equal.     

Effect of incising on the long-term biodeterioration resistance of alkaline copper quaternary (ACQ) treated wood

The aim of this study was to investigate whether making incising for injecting alkaline copper quaternary (ACQ) into wood extends the wood’s biodeterioration resistance. The biodeterioration resistance was checked by comparing long-term residual ACQ in wood with a toxic threshold to fungi. The residual ACQ in wood was estimated by leaching ratios and a long-term emission model which was derived by leaching tests of full-sized ACQ treated wood. The initial retention and the amount of ACQ leached of incised specimens were higher than those of non-incised specimens. However, the initial retention had a larger impact on leaching ratio than amount of ACQ leached. As a result, the leaching ratios of incised specimens were lower than those of non-incised specimens. The results indicate that incised wood can be expected to have the biodeterioration resistance for a longer period of time than non-incised wood.     

Zur Bedeutung von schnellwüchsigen Baumarten als Rohmaterial für die Holzwerkstoffherstellung unter besonderer Berücksichtigung von Pappelholz für Spanplatten

In the Federal Republic of Germany increasing attention is paid to the possibilities of using midi- and minirotation poplar wood as raw material for wood-based panels. In the article a brief account is given on the state of knowledge regarding the behaviour of poplar wood in particleboard manufacture. Debarked poplar chips are easily bondable with conventionel organic and inorganic binders. The use of a high amount of bark is deleterious to the physical-mechanical properties of boards, however, bark decreases the formaldehyde release of wood-based panels significantly. Due to its low density, poplar wood can be used for the production of high strength particleboards at low density level. The paper shows that no comprehensive data is available on the behaviour of tension wood towards bonding. Moreover, no detailed information is accessible on the gluability of juvenile wood. In the paper experimental results are reported concerning the properties of particleboards from 5-year old balsam poplar trees using UF- and PF-resins as binders. The results indicate clearly that there are no significant differences in the bonding strength between different clones. Using UF-resins boards with acceptable quality can be made from chips containing about 25% bark. The PF-bonded boards showed acceptable dry bonding strength. However, the wet strength was fast above or fast below the value required by the standard.     

Investigation of mechanical properties of sandwich panels made of paper honeycomb core and wood composite skins by experimental testing and finite element (FE) modelling methods

In this study, lightweight sandwich panels with different Kraft paper honeycomb core structures and wood composite skins were constructed. The influence of structural parameters, including core shape, cell size, core density, core and web thickness, and material properties of the core and skin layers on the mechanical behavior of these lightweight sandwich panels were studied by experimental testing and finite element modeling methods. The panels were subjected to compression and shear loadings. Test and simulation results indicated that core density and core shape mainly affected the panel stiffness under out-of-plane loading conditions (e.g. E _z , G _xz and G _yz ). Material properties of the skin layer affected the panel stiffness both under in-plane and out-of-plane loadings if the skin layer was orthotropic.     

脲醛树脂废胶液制造胶合板工艺及性能研究

研究将脲醛树脂废胶液与水混合、中和处理、异味处理、磨浆、过滤得到混合浆液,将混合浆液按一定比例添加至酚醛树脂中,用于Ⅰ类胶合板及竹木复合集装箱底板用胶合板生产.研究结果显示,混合浆液添加比例8％和15％条件下,7层胶合板和21层竹木复合集装箱底板用胶合板胶合强度、浸渍剥离性能均满足Ⅰ类胶合板指标要求;混合浆液添加比例为...     

Utilization of bark flours as additive in plywood manufacturing

Increasing demand for wood based panel products and shortage of wood as raw material have triggered many efforts to utilize residues generated annually by the forest industries including a large portion of bark in panel production. In this study, the effects of using bark flours as additives obtained from different wood species (walnut, chestnut, fir and spruce), having much polyphenol content, on some physical and mechanical properties and formaldehyde emission of plywood panels were examined. Wheat flour, which has been used widely as additive in plywood manufacturing, served as control. Plane tree (Platanus orientalis) logs were obtained for veneer manufacturing. Urea formaldehyde (UF) resin with 55 % solids content was used as adhesive. The bonding shear strength, bending strength, modulus of elasticity (MOE), density, equilibrium moisture content and formaldehyde emission of plywood panels were determined according to related standards. It was found that the use of flours obtained from the barks of chestnut and fir trees in the glue mixture decreased the formaldehyde emission of panels. The bonding strength values of the test panels made using the glue mixture including the flour of walnut and spruce barks as additive were lower than those of the panels with adhesive containing the flour of fir and chestnut barks. The panels manufactured with adhesives including the flour of fir bark gave the highest bending strength and modulus of elasticity values.     

Gas pressure measurements during continuous hot pressing of particleboard

Knowledge about the development of the internal gas pressure during hot pressing of wood-based composites is important for the optimization of panel properties and production speed. The gas pressure heavily affects the thermodynamic conditions inside the wood furnish mat, and a too high maximum value at press opening might cause an impairment of the panel properties. In this paper, gas pressure and temperature measurements inside a particle mat while passing through a continuous hot press are presented for the first time. The measurements were performed with a transportable system, consisting of a steel tube attached to a miniature pressure transducer and a data logger. The particleboards had a target thickness of mainly 16 mm, but also of 28 mm and 38 mm, respectively. The measurements show a distinct horizontal gas pressure distribution in both directions, in production direction and across the mat’s width. In contrast, cross-sectional gas pressure gradients were only visible inside the panels after leaving the press. By comparing the gas pressure curves measured for particleboard with those for medium density fiberboard (MDF), characteristic differences became evident. Overall, the gas pressure is higher in MDF compared to particleboard. Finally, a comparison between the gas pressure levels measured for three different panel thicknesses showed a clear relation between panel thickness and gas pressure, with a decreasing panel thickness resulting in an increase in gas pressure. The results of this paper will contribute to our understanding about the events inside wood furnish mats during continuous hot pressing.