Quebracho Tannin Bio-Based Adhesives for Plywood.

Wood-based products are traditionally bonded with synthetic adhesives. Resources availability and ecological concerns have drawn attention to bio-based sources. The use of tannin-based adhesives for engineered wood products has been known for decades, however, these formulations were hardly used for the gluing of solid wood because their rigidity involved low performance. In this work, a completely bio-based formulation consisting of Quebracho (Schinopsis balancae) extract and furfural is characterized in terms of viscosity, gel time, and FT-IR spectroscopy. Further, the usability as an adhesive for beech (Fagus sylvatica) plywood with regard to press parameters (time and temperature) and its influence on physical (density and thickness) and mechanical properties (modulus of elasticity, modulus of rupture and tensile shear strength) were determined. These polyphenolic adhesives presented non-Newtonian behavior but still good spreading at room temperature as well as evident signs of crosslinking when exposed to 100 °C. Within the press temperature, a range of 125 °C to 140 °C gained suitable results with regard to mechanical properties. The modulus of elasticity of five layered 10 mm beech plywood ranged between 9600 N/mm2 and 11,600 N/mm2, respectively, with 66 N/mm2 to 100 N/mm2 for the modulus of rupture. The dry state tensile shear strength of ~2.2 N/mm2 matched with other tannin-based formulations, but showed delamination after 24 h of water storage. The proposed quebracho tannin-furfural formulation can be a bio-based alternative adhesive for industrial applicability for special plywood products in a dry environment, and it offers new possibilities in terms of recyclability.

Fire Retardancy of Cementitious Panels with Larch and Spruce Bark as Bio-Admixtures.

The aim of this study is to investigate the production of fire-resistant panels made out of bark from spruce (Picea abies), larch (Larix decidua Mill.) and cement. This research included test panels produced from bark, cement, water and cement-bonded recycling material aiming for the target density of 750 kg/m3. The physical (density, dimension stability, thickness swelling) and mechanical properties such as tensile strength and compressive strength together with fire resistance were tested. Considering the results, appealing values have been achieved: max. compressive strength: 3.42 N/mm2; max. thickness swelling: 5.48%; and density: 515 to 791 kg/m3. In principle, the properties of the produced panels depend not only on the density, but also on the hydration and, above all, on the compaction and the composition of the boards. The fire tests demonstrated that the produced panels have an enormous potential in terms of fire resistance and could be utilized for fire-retardant applications.

Mechanical Properties of Cellulose and Flax Fiber Unidirectional Reinforced Plywood.

This research presents the influence of two different cellulose (hydrophobic pretreated/non-pretreated) and one flax-fiber unidirectional nonwoven low areal weight fiber reinforcements on the mechanical properties of urea-formaldehyde bonded five layered beech (Fagus sylvatica L.) plywood as an alternative to commonly used synthetic fiber reinforcements. The results display divergent trends regarding the improvement of the mechanical properties-modulus of elasticity, modulus of rupture, tensile strength, shear strength, and screw withdrawal resistance. The non-treated cellulose and flax reinforcing nonwoven fabrics revealed similar mechanical behaviors. The hydrophobic pretreatment of cellulose nonwovens improved the performance of plywood regarding tensile strength (10-11%), shear strength (7-16%), screw withdrawal resistance (11-15%), and modulus of rupture (0-2%), but lowered modulus of elasticity (2-3%) compared to the reference.

Efficiency of High-Frequency Pressing of Spruce Laminated Timber Bonded with Casein Adhesives.

This study identifies the importance of reducing press times by employing high-frequency pressing of spruce-laminated timber bound with sustainable casein adhesives. Spruce lamellas with dimensions of 12 × 10 × 75 cm were bonded into five-layered laminated timber and then separated into single-layer solid wood panels. Three types of casein (acid casein from two sources and rennin) were used. To compare the effectiveness of the casein formulation, two control samples bonded with polyvinyl acetate (PVAc) adhesive were pressed at room temperature (20 °C) and also with high-frequency equipment. The tests included compression shear strength, modulus of rupture, modulus of elasticity and screw withdrawal resistance on the wood panel surface and in the glue line. The average values of casein-bonded samples compression strengths ranged from 1.16 N/mm2 and 2.28 N/mm2, for modulus of rupture (MOR) were measured 85 N/mm2 to 101 N/mm2 and for modulus of elasticity (MOE) 12,200 N/mm2 to 14,300 N/mm2. The screw withdrawal resistance (SWR) on the surface of the wood panels ranged from 91 N/mm to 117 N/mm and in the adhesive line from 91 N/mm to 118 N/mm. Control samples bonded with PVAc adhesive did not perform better for compression shear strength, MOR and MOE, but for SWR in the adhesive line with 114 N/mm. Casein-bonded spruce timber pressed with HF equipment represents a sustainable new product with reduced press times, hazardous emissions and improved workability.

Selected Properties of Cement Bound Spruce and Larch Bark Bio-Aggregates.

The aim of this study is to investigate the suitability of spruce and larch bark for the production of cement-bonded composites. At the beginning of this research, the curing behaviour of the admixtures was quantified with temperature profiles when testing spruce, larch, pine and poplar bark, to determine the compatibility between the components of the bio-aggregates, to analyse the cement curing and to establish which bark species should be successfully included in cement bonded composites. Considering the results, it was observed that the average densities of 600-700 kg/m3 of bio-aggregates are 40-55% lower than that of established products on the market, although spruce and larch bark are in a similar range. The situation is different for the compressive strength, as larch bark showed up to 30% higher values than spruce bark. This study revealed also different hardening characteristics of the two cement types used as binders for spruce and larch bark. The results of this study demonstrated that tree bark of Picea abies and Larix decidua Mill. can be successfully utilized for the production of a cement-bonded composite material.

Influence of Adhesive Systems on the Mechanical and Physical Properties of Flax Fiber Reinforced Beech Plywood.

In order to improve the acceptance of broader industrial application of flax fiber reinforced beech (Fagus sylvatica L.) plywood, five different industrial applicated adhesive systems were tested. Epoxy resin, urea-formaldehyde, melamine-urea formaldehyde, isocyanate MDI prepolymer, and polyurethane displayed a divergent picture in improving the mechanical properties-modulus of elasticity, modulus of rupture, tensile strength, shear strength and screw withdrawal resistance-of flax fiber-reinforced plywood. Epoxy resin is well suited for flax fiber reinforcement, whereas urea-formaldehyde, melamine urea-formaldehyde, and isocyanate prepolymer improved modulus of elasticity, modulus of rupture, shear strength, and screw withdrawal resistance, but lowered tensile strength. Polyurethane lowered the mechanical properties of flax fiber reinforced plywood. Flax fiber reinforced epoxy resin bonded plywood exceeded glass fiber reinforced plywood in terms of shear strength, modulus of elasticity, and modulus of rupture.

Thermophysical Properties of Larch Bark Composite Panels.

The effects of using 100% larch bark (Larix decidua Mill) as a raw material for composite boards on the thermophysical properties of this innovative material were investigated in this study. Panels made of larch bark with 4-11 mm and 10-30 mm particle size, with ground bark oriented parallel and perpendicular to the panel's plane at densities varying from 350 to 700 kg/m3 and bonded with urea-formaldehyde adhesive were analyzed for thermal conductivity, thermal resistivity and specific heat capacity. It was determined that there was a highly significant influence of bulk density on the thermal conductivity of all the panels. With an increase in the particle size, both parallel and perpendicular to the panel´s plane direction, the thermal conductivity also increased. The decrease of thermal diffusivity was a consequence of the increasing particle size, mostly in the parallel orientation of the bark particles due to the different pore structures. The specific heat capacity is not statistically significantly dependent on the density, particle size, glue amount and particle orientation.

Binderless Thermal Insulation Panels Made of Spruce Bark Fibres.

Tree bark is a by-product of the timber industry available in large amounts, considering that approximately 10% of the volume of a tree stem is bark. Bark is used primarily for low-value applications such as heat generation or as mulch. To the best of our knowledge, this study is the first one that scrutinises thermal insulation panels made from spruce bark fibres with different densities and fibre lengths manufactured in a wet process. The insulation boards with densities between 160 and 300 kg/m3 were self-bonded. Internal bond, thermal conductivity, and dimensional stability (thickness swelling and water absorption), together with formaldehyde content, were analysed. The thermal properties of the boards were directly correlated with the density and reached about 0.044 W/m*K, while the internal bond was rather influenced by the fibre length and was relatively low (on average 0.07 N/mm2). The water absorption was high (from 55% to 380%), while the thickness swelling remained moderate (up to 23%). The results of this study have shown that widely available bark residues can be successfully utilised as an innovative raw material for efficient eco-friendly thermal insulation products.

Surface Modification of Spruce and Fir Sawn-Timber by Charring in the Traditional Japanese Method-Yakisugi.

The traditional Japanese method of wood surface charring was studied. To perform the surface charring, three sawn Norway spruce and Silver fir wood boards of dimension 190 × 24 × 4000 mm3 were tied together to act as a chimney and charred in a short time (3-4 min) with open flame at a temperature above 500 °C. Temperature inside the chimney was recorded on the three different positions during the charring process. Surface temperature of spruce increased from 0 °C to 500 °C in approx. 120-300 s while fir increased in approx. 100-250 s. The thickness of the charred layer and the resulting cupping effect were investigated at the different heights of the chimney to evaluate its variability. Temperature achieved during the charring process was sufficient to get a significant charred layer of 2.5 and 4.5 mm on average for spruce and fir samples, respectively. The analyzed samples showed a significant cupping effect to the charred side with no difference between the annual ring orientation of sawn boards. Spruce exhibit a more significant cupping effect when compared to fir, i.e., 3.2-6 mm and 2.2-4.5 mm, respectively. Furthermore, the pH values of charred samples increased significantly, which could be an indication of improved resistance against wood-decay fungi. For better insight into the traditional charring method, further studies should be carried out to execute the charring process in a consistent quality and therefore fully exploit its potential.

Changing in Larch Sapwood Extractives Due to Distinct Ionizing Radiation Sources.

Wood extractives have an influence on different material properties. This study deals with the changes in wood extractives of larch sapwood due to two different low doses of energy irradiations. Electron beam irradiation (EBI) and γ-ray irradiation treatments were done by using two industrial processes. After the different modifications the extractions were performed with an accelerated solvent extractor (ASE) using hexane and acetone/water. The qualitative and quantitative chemical differences of irradiated larch sapwood samples were analysed using gas chromatography-mass spectrometry (GC-MS) and Fourier-transform infrared spectroscopy (FT-IR) vibrational spectroscopy methods. The yields of the quantitative extractions decreased due to the two different irradiation processes. While the compounds extracted with nonpolar solvent from wood were reduced, the number of compounds with polar functionalities increased based on the oxidation process. Quantitatively, resin acids and polyphenols were highly affected when exposed to the two irradiation sources, leading to significant changes (up, down) in their relative amount. Furthermore, two new substances were found in the extracts of larch sapwood samples after EBI or γ-ray treatments. New insight into the different effects of larch sapwood and wood extractives by EBI and γ-ray was gained in this study.

Oversized Planer Shavings for the Core Layer of Lightweight Particleboard.

Planer shavings (PS) are side-products generated during the processing of solid wood, typically used for heating, packaging, or insulation purposes. PS has been used for decades in particleboard manufacture, particularly in the core layer. The aim of this research is to investigate the use of PS with a length over 4 mm in low-density one-layer particleboard manufacturing with a thickness of 10 mm, as an option to reduce the raw material demand for wood-based panels. Correlations towards the mechanical properties of the particleboards, fabricated at a density of 475 kg/m3, could be drawn by analyzing the effects of different urea-formaldehyde adhesive contents (6%, 9%, and 12%). Two methods of adhesive application (pouring and spraying) and two types of blending of PS with adhesive (plowshare mixer and drum mixer) were investigated, with the aim that PS will have controlled resin application. The difference between the adhesive application methods was examined by analyzing the mechanical properties as an internal bond, modulus of rupture, and modulus of elasticity as well as indirectly by visualizing the adhesive distribution by adding a green pigment to the adhesive before application. PS demonstrated reduced bending properties in comparison with the EN 312 standard requirements of particleboards for internal use in dry conditions (type P2), due to the low density. The internal bond strength in the case of the particleboard without pigment application (up to 0.5 N/mm2) was higher compared to the P2 requirements (0.4 N/mm2), and significantly lower (0.15 N/mm2) in combination with the pigment (2.5% based on the board weight, compared to 0.1%, specific for such industry applications), but still superior to the values of the reference panel manufactured with wood particles.

Potential of Brewer's Spent Grain as a Potential Replacement of Wood in pMDI, UF or MUF Bonded Particleboard.

Brewer's spent grain (BSG) is the richest by-product (85%) of the beer-brewing industry, that can be upcycled in a plentiful of applications, from animal feed, bioethanol production or for removal of heavy metals from wastewater. The aim of this research is to investigate the mechanical, physical and structural properties of particleboard manufactured with a mixture of wood particles and BSG gradually added/replacement in 10%, 30% and 50%, glued with polymeric diisocyanate (pMDI), urea-formaldehyde (UF) and melamine urea-formaldehyde (MUF) adhesives. The density, internal bond, modulus of rupture, modulus of elasticity, screw withdrawal resistance, thickness swelling and water absorption were tested. Furthermore, scanning electron microscopy anaylsis was carried out to analyze the structure of the panels after the internal bond test. Overall, it was shown that the adding of BSG decreases the mechanical performance of particleboard, due to reduction of the bonding between wood and BSG particles. This decrease has been associated with the structural differences proven by SEM inspection. Interaction of particles with the adhesive is different for boards containing BSG compared to those made from wood. Nevertheless, decrease in the mechanical properties was not critical for particleboards produced with 10% BSG which could be potentially classified as a P2 type, this means application in non-load-bearing panel for interior use in dry conditions, with high dimensional stability and stiffness.

Investigation of 3D-Moldability of Flax Fiber Reinforced Beech Plywood.

The current work deals with three dimensionally molded plywood formed parts. These are prepared in two different geometries using cut-outs and relief cuts in the areas of the highest deformation. Moreover, the effect of flax fiber reinforcement on the occurrence and position of cracks, delamination, maximum load capacity, and on the modulus of elasticity is studied. The results show that designs with cut-outs are to be preferred when molding complex geometries and that flax fiber reinforcement is a promising way of increasing load capacity and stiffness of plywood formed parts by respectively 76 and 38% on average.

Larch Bark as a Formaldehyde Scavenger in Thermal Insulation Panels.

The aim of this study is to investigate the formaldehyde content and emissions of bark-based insulation panels bonded with three types of adhesives: urea formaldehyde, melamine urea-formaldehyde, and tannin-based adhesives. These panels were produced at two levels of density-300 and 500 kg/m3-and a thickness of 20 mm, and the influence of the adhesive amount and type on the formaldehyde emissions and content was measured. Other mechanical and physical properties such as modulus of rupture, modulus of elasticity, internal bond, and dimensional stability were also scrutinized. With one exception, all the panels belonged to the super E0 classification for free formaldehyde content (perforator value ≤1.5 mg/100 g oven dry mass of panels). The measurements using the desiccator method for formaldehyde emissions assigned all the testing specimens in the F **** category for low-emission panels according to the Japanese International Standards.

Bark Thermal Insulation Panels: An Explorative Study on the Effects of Bark Species.

Tree bark is a byproduct of the timber industry which accrues in large amounts, because approximately 10% of the volume a log is bark. Bark is used primarily for low-value applications such as fuel or as a soil covering material in agriculture. Within the present study, thermal insulation panels made from larch, pine, spruce, fir and oak tree bark with different resins (urea formaldehyde, melamine formaldehyde, Quebracho, Mimosa) as a binder are discussed. Also, the properties of panels made from larch bark mixed with industrial popcorn are investigated. The physical-mechanical properties of the panels, which are dependent on panel density, bark species, resin type, resin content and particle size, are analyzed. The bark species has a minor effect on the mechanical characteristics of the panels, while the compression ratio is important for the panel strength, and hence, barks with lower bulk density are preferable. Under laboratory conditions, panels made with green tannin resins proved to have adequate properties for practical use. The addition of popcorn is a means to lower the panel density, but the water absorption of such panels is comparably high. The bark type has a minor effect on the thermal conductivity of the panels; rather, this parameter is predominantly affected by the panel density.

Water-Resistant Casein-Based Adhesives for Veneer Bonding in Biodegradable Ski Cores.

The aim of this study is to investigate the performance of casein-based adhesives for the bonding of ash (Fraxinus spp.) veneers for the manufacture of biodegradable skis. Different formulations containing casein powder, water, lime, sodium silicate, and various glue amounts were tested for shear strength after water storage, modulus of rupture and modulus of elasticity, water absorption, and thickness swelling. Two other classic wood adhesives, namely epoxy and polyvinyl acetate (PVAc) type D4 were used as control. The highest efficiency of both mechanical and physical properties was recorded for the samples glued with caseins and an increased amount of lime. There was also an affinity between casein adhesive distribution and physical and mechanical plywood performance. Moreover, the developed casein-based glues were also used to bond the plywood for ski cores and tested in real-life winter conditions.

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.

Analysis of Larch-Bark Capacity for Formaldehyde Removal in Wood Adhesives.

Ecofriendly wood-based materials are required by consumers at present. Decorative panels are part of a large group of wood-composite materials, and their environmental properties must not be neglected. More environmentally friendly decorative panels can be achieved by various methods. This paper describes a method of production from larch bark. Tree bark, as a byproduct of the wood industry, is one of the research topics that have gained interest in the last decade, especially for its applications in biobased lignocomposites, with regard to the shrinkage of wood resources. In the present work, the formaldehyde content of decorative boards based on larch bark (0.6 g/cm³) was analyzed when bonded with five different types of adhesive systems: urea-formaldehyde, polyvinyl acetate, the mixture of 70% urea-formaldehyde + 30% polyvinyl acetate, polyurethane, and tannin-based adhesive. A self-agglomerated board was also analyzed. The formaldehyde content of the larch-bark samples was determined with the perforator method (EN 120:2011), and findings showed that all tested samples reached the E1 classification (≤8 mg/100 oven dry). Moreover, 75% of the values of the corrected formaldehyde content were included in the super-E0 class (≤1.5 mg/100 oven dry). In the case of boards bonded with tannin-based adhesive, this natural polymer acted as a formaldehyde scavenger.