Wood-based Composites Research Articles

Influence of moisture content on the mechanical properties of selected wood-based composites

Wood quality depends on many circumstances, as it is sensitive to changing properties, depending on the environment. This work evaluates the influence of moisture content of selected wood-based composites on their basic mechanical properties, i.e., modulus of rupture and modulus of elasticity. The selected panels were divided by application in construction materials and furniture materials, which demand specific conditions during service-life. The increase of moisture content in different types of wood-based panels resulted in a slight reduction of the modulus of rupture and the modulus of elasticity. Boards for use in dry conditions, mainly in the furniture industry, were more sensitive to lowering their modulus of elasticity with higher board moisture content compared with those designed for humid conditions, mainly from the building industry.

Sound-Absorption Coefficient of Bark-Based Insulation Panels

The objective of this study was to investigate the sound absorption coefficient of bark-based insulation panels made of softwood barks Spruce (Picea abies (L.) H. Karst.) and Larch (Larix decidua Mill.) by means of impedance tube, with a frequency range between 125 and 4000 Hz. The highest efficiency of sound absorption was recorded for spruce bark-based insulation boards bonded with urea-formaldehyde resin, at a level of 1000 and 2000 Hz. The potential of noise reduction of larch bark-based panels glued with tannin-based adhesive covers the same frequency interval. The experimental results show that softwood bark, an underrated material, can substitute expensive materials that involve more grey energy in sound insulation applications. Compared with wood-based composites, the engineered spruce bark (with coarse-grained and fine-grained particles) can absorb the sound even better than MDF, particleboard or OSB. Therefore, the sound absorption coefficient values strengthen the application of insulation panels based on tree bark as structural elements for the noise reduction in residential buildings, and concurrently they open the new ways for a deeper research in this field.

Enhancing the performance of low molar ratio urea–formaldehyde resin adhesives via in-situ modification with intercalated nanoclay

ABSTRACT Low molar ratio urea–formaldehyde (UF) resin adhesives are primarily used for the reduction of formaldehyde emission (FE) from wood-based composites, at the expense of poor reactivity and adhesion. This study aimed to enhance the performance of low molar ratio UF resins via in-situ modification, via the addition of octadecylamine (ODA)-modified bentonite (ODA–BNT) nanoclay. The molecular weights, curing behavior, chemical groups, and crystallinity of modified UF resins with different levels of ODA−BNT were characterized by gel permeation chromatography, differential scanning calorimetry, Fourier-transform infrared spectroscopy, and X-ray diffraction (XRD). The ODA–BNT/UF resins showed better reactivity and lower activation energy compared with neat resins. The XRD results clearly showed that ODA–BNT in modified UF resins had been intercalated. A 5% ODA−BNT addition resulted in a decrease in the crystallinity of the modified UF resins, leading to better adhesion strength and lower FE compared with neat UF resins. These results suggest that intercalation of ODA–BNT in low molar ratio UF resins improve their cohesion through the formation of a larger branched network in their cured state.

Biomimetic water-in-oil water/pMDI emulsion as an excellent ecofriendly adhesive for bonding wood-based composites

In this study, a novel and environmental strategy inspired by mussels is reported for the construction of a high-performance water-in-oil water/pMDI adhesive with strongly adherent catechol groups. It is found that the design of the biomimetic water-in-oil emulsion significantly increases the apparent viscosity and storage stability of the adhesive, which in turn influences the handling and bonding performance in practical application. Additionally, although the biomimetic emulsion design consumes part of active isocyanate groups in the pMDI, the contained catechol can serve as a reactive platform to induce secondary crosslinking interactions with the wood substrate to further improve the mechanical and adhesion performances of the modified pMDI adhesives. Consequently, compared to the pristine pMDI sample, the wet shear strength of the biomimetic water-in-oil water/pMDI adhesive is increased by 129.7 %, exhibiting that the obvious optimization of adhesion and water resistance properties. Overall, our findings provide new insights into exploiting novel and superior wood adhesives, and the constructed high-performance adhesive presents potential applications for sustainable wood products.

Converting crystalline thermosetting urea–formaldehyde resins to amorphous polymer using modified nanoclay

Thermosetting urea–formaldehyde (UF) resins as the most common adhesives for wood-based composites emit formaldehyde, which forces producers to lower formaldehyde/urea (F/U) molar ratio for the UF resins synthesis. However, low-molar-ratio (below 1.0) UF resins have low formaldehyde emission at the expense of poor adhesion, which is responsible for the formation of crystalline domains as a result of hydrogen bonds between linear molecules. For the first time, this study reports the conversion of crystalline UF resins to amorphous polymers by blocking the hydrogen bonds, using transition metal ion-modified bentonite (TMI-BNT) nanoclay through in situ intercalation. The modified UF resins with 5% TMI-BNT showed an almost amorphous structure, faster curing and higher cross-linking density compared with those of neat resins, and resulted in 56.4% increase in the adhesion strength and 48.3% reduction in the formaldehyde emission. Thus, blocking hydrogen bonds in low F/U molar ratio UF resins with TMI-BNT converted crystalline UF resins to almost amorphous ones, resulting in a significant improvement in their adhesion with a low crystallinity.

Development of low-tech laminated bamboo esterilla sheet made of thin-wall bamboo culm

Bamboo sheets, called pelupuh, made from thin-wall bamboo culm and isocyanate adhesive, were used to produce laminated bamboo esterilla sheets (LBES) based on a low-tech method. The present work investigated the fabrication technique and the flexural properties of LBES. The study results support the idea of using a low-tech method for producing bamboo-based composites with flexural properties comparable to those of wood-based composites. From the perspective of flexural strength property, the LBES made of thin-wall bamboo culm has the potential to act as a substitute for timber and as an alternative to bamboo composites produced using high-tech processing technology.

Non-destructive testing of thin birch (Betula pendula Roth.) veneers

In technical applications of wood-based composites, the predictability of elasticity and strength is important. The aim of this study was to predict the static modulus of elasticity and tensile strength of thin (0.55 mm ± 0.05 mm) birch veneers. Based on the dynamic modulus of elasticity estimated via means of wave transmission time the observed dynamic modulus of elasticity was on average 14% lower than the corresponding static modulus of elasticity. This difference could be explained by a decreased measuring area during the tensile testing or by defects within the samples. The dynamic modulus of elasticity correlated well with the static modulus of elasticity (r = 0.821). Therefore, using wave transmission time to non-destructively predict the elasticity of veneers proves to be a promising tool. The dynamic modulus of elasticity showed a significant and positive correlation with the tensile strength (r = 0.665), but this correlation was weaker than with the static modulus of elasticity. Therefore, the wave transmission time or the static modulus of elasticity must be combined with additional strength-influencing properties, such as fiber angle or density, to allow for a highly accurate prediction of tensile strength.

Moisture Transport in Wood-Based Structural Panels under Transient Hygroscopic Conditions

Abstract Development of moisture gradients within wood and wood-based composites can result in irreversible moisture-induced damage. Accelerated weathering (AW), generally employing harsh environmental conditions, is a common tool for assessing moisture durability of wood composite products. Use of milder AW conditions, such as cyclic changes in relative humidity (RH), may be of interest to the wood-based composites industry in assessing moisture durability under more realistic conditions. The primary objective of this study was to determine whether moisture profile development in oriented strand board and plywood during cyclic RH changes could be reasonably predicted with a simple moisture transport model, which may be practical for wood-based composite industry members seeking to develop new AW protocols. The diffusion model based on Fick's second law with empirically determined moisture transport parameters fits the experimental data reasonably well for the purpose of screening RH parameters.

Wood Functional Modification Based on Deposition of Nanometer Copper Film by Magnetron Sputtering

Abstract It can be helpful for selected applications to improve the functionality of wood by compounding nano-metal materials with wood, endowing the wood surface with certain physical properties, for example, metallicity, electrical conductivity, and hydrophobicity. Therefore, in this study, a thin copper film was deposited on the surface of Pinus sylvestris L. var. mongholica Litv. veneer by magnetron sputtering. The film was applied at both room temperature and 200°C to obtain nano-copper–wood composites. The physical properties of wood-based nano-metal composites were characterized. The results indicated that the wood veneer metallization had no effect on the crystallization zone of wood; there were still wood cellulose characteristic peaks, but the intensity of the diffraction peak decreased. At the same time, there were characteristic diffraction peaks of copper. The mechanical properties of the wood veneer surface changed greatly; the surface of copper-plated wood veneer had good electrical conductivity and the wettability of the wood surface transformed from hydrophilic to hydrophobic. When the base temperature was 200°C, not only was the sheet resistance of the sample with coating time of 15 minutes about 4.6 times that of the sheet resistance of the sample at room temperature, but also the quality of the copper film on the wood surface was better than that at room temperature. The copper film was mainly composed of small particles with a compact arrangement.

Sustainable wood-based composites for microwave absorption and electromagnetic interference shielding

In this review, wood-based composites for efficient EMI shielding and MA are comprehensively introduced.

An overview on development of environmental friendly medium density fibreboard

Medium density fibreboard (MDF) is extensively used in the construction and furniture industry, especially as building materials, household furniture, fencing and flooring because of its desirable properties such as being low cost, having excellent mechanical and physical properties and being isotropic. However, the sustainability of wood supply has become a serious environmental issue. Therefore, it is necessary to find alternative strategies in order to reduce the wood demand for producing wood-based composites such as MDF. This article reviews the various alternative materials such as wheat straw, kenaf, corn stalk, sisal and bagasse to reduce the dependency on wood in the related industries. Moreover, this article also reports on the development of environmental friendly adhesives and non-adhesive MDF to reduce the emission of formaldehyde which is a carcinogen agent and may lead to various health problems.

Effect of Elevated Temperature Exposures on Shear Properties of Sheathing Panels

Abstract Structural wall sheathing such as oriented strand board (OSB) and plywood have been heavily used in residential and commercial timber frame construction. The response of these wood-based composites under elevated temperatures between 100°C and 200°C (herein referred to as elevated temperatures) and exposure time needs to be characterized to assess residual strength of the materials in the existing structures. The main objective of this work is to study the effect of temperature and exposure time on shear strength and shear modulus of plywood and OSB. A total of 110 test specimens was tested in shear after exposure to five different temperatures and two exposure durations, followed by cooling to ambient temperature. The results indicated that the plywood and OSB behaved differently after exposure to elevated temperatures and exposure duration. Plywood showed a consistent degradation of shear strength with elevated temperature and time, while OSB did not exhibit a clear picture of thermal degradation. The results further indicated that the shear modulus of plywood and OSB remained unaffected after exposure to elevated temperatures.

THE INDUSTRIAL SIGNIFICANCE OF NEW TECHNOLOGY IN THE PROCESS OF ASYMMETRICAL VENEERING OF WOOD-BASED COMPOSITES

THE INDUSTRIAL SIGNIFICANCE OF NEW TECHNOLOGY IN THE PROCESS OF ASYMMETRICAL VENEERING OF WOOD-BASED COMPOSITES

Formalizing shape-change: Three-dimensional printed shapes and hygroscopic material transformations

Shape-changing materials have become increasingly popular among architects in designing responsive systems. One of the greatest challenges of designing with these materials is their dynamic nature, which requires architects to design with the fourth dimension, time. This article presents a study that formalizes the shape-changing behavior of three-dimensional printed wood-based composite materials and the rules that serve to compute their shape-change in response to variations in relative humidity. In this research, we first developed custom three-dimensional printing protocols and analyzed the effects of three-dimensional printing parameters on shape-change. We thereafter three-dimensional printed kirigami geometries to amplify hygroscopic material transformation of wood-based composites.

Mold and larvae resistance of wood-based composites incorporating sodium fluoride

The efficacy of particleboards manufactured with sodium fluoride against mold fungi and Hylotrupes bajulus (L.) larvae was tested. Laboratory-size particleboards were manufactured from untreated wood particles with inclusion of fine sodium fluoride (NaF) at the 1, 1.5, and 3% levels relative to total particle weight. Sodium fluoride was introduced as powder during the manufacturing process just before blending resin with wood particles. The laboratory mold test indicated that even the lowest level of retention of NaF with or without leaching significantly reduced mold growth on particleboard surfaces when compared to the untreated control specimens. The increased retention of NaF from 1% to 3% further suppressed mold growth towards lowest levels even on leached specimens. The laboratory Hylotrupes bajulus larvae tests revealed that the particleboard environment with or without NaF is not appropriate for larvacidial activity. While the tested biocide, NaF, tested positive against mold growth on particleboard surfaces, no effect was determined relative to larval deaths.

Experimental testing of elastic properties of LayWood pyramidal cores

Sandwich panels most commonly used in the furniture industry are layered structures composed of a hexagonal cell paper core. The use of wood-based composites in modelling truss and pyramidal cores of layered furniture panels is rather scarce. The effect of geometry in the auxetic truss core on the mechanical properties of manufactured wood-based materials was primarily explored in this study. Moreover, the need to conduct further studies was also stressed to determine the elastic properties of cells and cores manufactured from wood filaments using 3D printing. The aim of this study was to determine the effect of the type of filament used in 3D printing and the geometry of pyramidal core cells on elastic constants in cores with identical relative density. This paper presented analytical models of manufactured cells, results of numerical calculations performed using the finite element method, as well as experimental tests determining elastic constants of the cores. Digital image analysis was used and showed that cell geometry had a considerable effect on elastic properties of the core while maintaining identical relative density of these structures. The angle of cell arms had a particularly marked effect on these properties.

Effect of the initial F/U molar ratio in urea-formaldehyde resins synthesis and its influence on the performance of medium density fiberboard bonded with them

The formaldehyde to urea molar ratio (F/U) plays an important role on the properties of urea formaldehyde (UF) resins and of the medium density fiber board (MDFs) bonded with them. This work presents a hypothesis that besides the final F/U molar ratio in UF resin preparation, the initial F/U affects both the formaldehyde emission levels and the physico-mechanical properties of the boards. Three initial molar ratios F/U of 1.9, 2.1 and 2.3 in resin synthesis have been examined in this work. The structural changes and thermal curing behavior of UF resins were tracked by 13C NMR and DSC, respectively. At parity of final F/U molar ratio, the resin with initial F/U = 2.1 yielded the highest proportion of linear methylol groups, resulted in the best internal bond strength and lowest thickness swelling of the board. A resin synthesized with the initial F/U = 1.9 provided the highest proportion of total methylene linkages, most methylene ether linkage and lowest free formaldehyde, consistently yielding the lowest formaldehyde emission. These results partially explained why UF resins with lower F/U molar ratios showed relatively poor adhesion when used to manufacture wood-based composites. MDFs bonded with a UF resin having the initial F/U = 2.3 had a relatively good performance with the exception of free formaldehyde, but also showed the best storage stability compared to the other resins having lower initial F/U molar ratios.

Evaluation of screw pulling and flexural strength of bamboo-based oil palm trunk veneer hybrid biocomposites intended for furniture applications

Screw withdrawal and flexural strength were evaluated for Dendrocalamus asper and Gigantochloa levis bamboo species to explore the possibility of their use as structural material in place of wood. Dry bamboo strips and 4-mm-thick oil palm trunk veneer (OPTV) were processed into thin laminates and hot-pressed using urea formaldehyde resin to produce bamboo-OPTV hybrid biocomposites. Bamboo furniture is far more resistant to damage than traditional hardwoods. Bamboo is even used in cutting boards for this reason. Even though there have been some reports on the mechanical enhancement of the bamboo-based composites, so far there has been no comprehensive study on the screw pulling and flexural strength of bamboo-based hybrid composites. The results revealed a stronger correlation of the bamboo hybrid under screw withdrawal and flexural strength, but there was a weaker correlation in the mechanical properties of the bamboo hybrid due to the random selection of laminate from different bamboo species. Furthermore, test results clearly showed that bamboo-OPTV hybrid biocomposites can be used as an alternative to wood and wood-based composites for furniture applications.

3D Microscale Heat Transfer Model of the Thermal Properties of Wood-Metal Functional Composites Based on the Microstructure.

This study presents a model for simulating the microscopic heat transfer processes in a wood-metal composite material. The model was developed by analyzing the microstructure of experimental samples comprising a melted alloy impregnated in a wood matrix. According to the thermal parameters of the materials and the boundary conditions, an analytical model of microscale heat transfer was established using Abaqus finite element analysis software. The model was validated experimentally by comparing temperature curves obtained via simulation and experiments; the resulting correlation coefficient was 0.96557. We then analyzed the temperature distribution of the composite material with different cell geometries and heat transfer conditions (heat transfer direction and applied temperature). The thermal properties of the unit cell models were in good agreement with the general trends predicted by several heat transfer equations. This study provides a method for analyzing the microscale heat transfer process in wood-based composites. In addition, the model framework characteristics can be used to evaluate the heat transfer mechanism of impregnated modified wood.

Some properties of composite panels manufactured from peach (Prunus persica) pits and polypropylene

In this research, the use of peach pits in polypropylene-based composites has been investigated. Specimens manufactured with 50% and 60% peach pits were evaluated and compared with a control sample manufactured with 60% radiata pine wood and polypropylene. Both composites filled with peach pits presented improved physical properties, and higher porosity than radiata pine wood-based composites. In addition, the peach pits-based composites showed lower mechanical properties than those made from pine particles. This difference increased when adding higher amounts of peach pits to the composites. The lower mechanical properties were attributed to the higher amount of extractives of the peach pits compared with pinewood.