Milan Sernek

The mechanical properties of densified VTC wood relevant for structural composites

AbstractThe mechanical properties of densified wood relevant for structural composites were studied. Low density hybrid poplar (Populus deltoides × Populus trichocarpa) was densified using the viscoelastic thermal compression (VTC) process to three different degrees of densification (63, 98, and 132%). The modulus of rupture (MOR) and the modulus of elasticity (MOE) of the control (undensified) wood and of the VTC wood were determined. The bonding performance of the control and VTC wood, using two phenol-formaldehyde (PF) adhesives, was studied. Four different 3-layer composites were also prepared from undensified and VTC wood, and tested in four-point bending. The results showed that the bending properties of the VTC wood (MOR and MOE) were significantly improved due to the increased density. The bonding performance of VTC wood with PF adhesives was comparable with or better than in the case of the control wood. Increased density of the face layers in the 3-layer VTC composites was advantageous for their mechanical performance. ZusammenfassungDie für tragende Verbundwerkstoffe relevanten mechanischen Eigenschaften von verdichtetem Holz wurden untersucht. Das Holz der Hybridpappel (Populus deltoides × Populus trichocarpa), einer Holzart mit geringer Dichte, wurde mit viskoelastischer thermischer Verdichtung (VTC) in drei verschiedenen Graden (63, 98, und 132%) verdichtet. Die Biegefestigkeit und der Elastizitätsmodul einer Referenzprobe und des VTC Holzes wurden bestimmt. Das Verklebungsverhalten mit Phenolformaldehyd (PF)-Harz einer Referenzprobe und des VTC Holzes wurden untersucht. Vier verschiedene dreilagige Schichthölzer wurden aus unverdichtetem und aus VTC Holz hergestellt und anschließend im 4-Punkt-Biegeversuch geprüft. Die Biegefestigkeit und der Elastizitätsmodul des verdichteten Holzes wurden durch die Verdichtung signifikant verbessert. Das Verklebungsverhalten des VTC Holzes mit PF-Harz war vergleichbar oder besser als das der Referenzprobe. Die erhöhte Verdichtung der Deckschicht des dreilagigen VTC Schichtholzes wirkte sich vorteilhaft auf die mechanischen Eigenschaften aus.

Limitation of XPS for analysis of wood species containing high amounts of lipophilic extractives

Chemical composition of Norway spruce and pine, two softwood species, has been investigated by X-ray Photoelectron Spectroscopy (XPS). Contrary to results previously obtained with beech wood, which allow to obtain information on bulk chemical composition from surface composition analysis, XPS analysis appears to be unsuitable for the characterisation of Norway spruce and pine wood chemical composition. Indeed, chemical compositions calculated from XPS data differ strongly from those obtained from microanalyses which are in good agreement with theoretical composition described in the literature. XPS analysis of both the softwood surfaces indicated high carbon contents explained by migration of lipophilic extractives to the surface under the influence of the vacuum necessary for XPS analysis. Nonvolatile extractives contained in wood were extracted and deposited on glass plates and analysed. Survey and detailed C1s spectra indicated similar signals to those recorded on wood surfaces. This phenomenon was not observed when samples had been previously extracted before analysis. These results strongly evidenced that extractives present in both species are able to migrate through resin canals from the bulk of the sample to the surface when put into ultra high vacuum. XPS presents, therefore, some limits in the case of the analysis of softwood species containing extractives able to migrate to the surface during analysis. This behaviour, difficult to control, could lead to erroneous interpretations due to extractives enrichment of the surface under the effect of vacuum.

Microscopic analysis of the wood bond line using liquefied wood as adhesive

The bonding of beech (Fagus sylvatica L.) with liquefied wood (LW) causes deterioration of the wood surface, resulting in a high percentage of wood failure at a relatively low bond shear strength. Light microscopy, scanning electron microscopy, FT-IR micro-spectroscopy and elemental carbon, nitrogen and sulphur (CNS) analysis techniques were used to investigate the formation of such bonds. It was assumed that the degradation of lignin, hemicelluloses and parts of the cellulose occurred in the cells of the wood surface where the LW had been applied. At the elevated temperatures occurring during the bonding process, the deteriorated cells were carbonised to some extent. The weak boundary layer of the bond was determined to be a layer of delignified cells located between the zone of partly carbonised cells on the one side and the cells of the undamaged wood of the adherend on the other side. The bonds which formed during the bonding of wood with LW were found to be very untypical compared to bonds formed by synthetic wood adhesives. No adhesive film was formed, the adhesive-adherend interface was not clear and the cells of the adherend subsurface were damaged.

Effect of pressing parameters on the shear strength of beech specimens bonded with low solvent liquefied wood

Liquefied wood (LW) is a naturally based product which has the potential to be used as an adhesive. It can be used as a part of a polymer formulation, as a part of an adhesive mixture with commercial adhesives, or as an independent material for wood bonding. In this study, wood was liquefied at 180 °C using ethylene glycol as the solvent and sulphuric acid as a catalyst. In the first part of research, LW with different pH values was used for the bonding of solid wood at 200 °C for 15 min. In the second part, LW with an optimal pH value was used for bonding at different press temperatures for 15 min. In the third part, the minimum pressing time at the optimal pH value and at the optimal press temperature was determined. Unmodified LW with a negative pH value, a press temperature of 180 °C, and a pressing time of 12 min was determined to be optimal (based on highest shear strength) for the bonding of 5 mm thick wood lamellas with the LW used in this study. At these conditions bonds exhibited shear strength of around 7 N/mm2 which was too low to attain standard requirements. Despite this, high wood failure (100%) was observed as a consequence of low pH value and high press temperature which caused damage of the part of beech lamellas where LW was applied.

Erratum to: The wettability and bonding performance of densified VTC beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) bonded with phenol–formaldehyde adhesive and liquefied wood

The influence of viscoelastic thermal compression (VTC) on surface wettability and bonding performance of wood was evaluated. Low quality beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) were densified with the VTC process to different degrees of densification. Control and densified strips were bonded with phenol–formaldehyde (PF) adhesive and liquefied wood (LW). Shear strength of bonded assemblies was determined after 1 week of conditioning at 20 °C and relative humidity of 65 %. Wettability was determined on the basis of the contact angle of water, PF adhesive, and LW using the Wilhelmy method. Results showed that densification of beech and spruce wood did not significantly affect the shear strength of specimens bonded with PF adhesive. In beech assemblies bonded with LW shear strength decreased significantly with increased density, whereas in bonded spruce specimens decrease of shear strength was not significant. It was found that degree of densification and bonding process used in the study were not appropriately chosen for spruce wood specimens, since major deformations after the bonding process occurred. Wettability changed significantly after densification. Contact angle of water and LW increased after densification, whereas contact angle of PF showed inverse trend and decreased after VTC process. Furthermore, the degree of densification had a minor effect on the wettability.

Use of wood powder and adhesive as a mixture for 3D printing

In recent years there has been much development in the field of additive manufacturing technologies, but only a few attempts have been made to use natural materials like wood for 3D printing. In this research different ratios of wood powder were used as a component in adhesive mixtures for 3D printing. Polyvinyl acetate and urea–formaldehyde adhesives were used as binders, and the optimum mixture was determined by measuring the corresponding extrusion forces. Simple blocks were 3D printed and the bending properties of these blocks were investigated. The bending strength depended on the amount of wood powder in the mixture and on the type of adhesive.

Fungal decay of viscoelastic thermal compressed (VTC) wood

Low-density hybrid poplar (Populus deltoides × Populus trichocarpa) was densified with the viscoelastic thermal compression (VTC) process to three different degrees of densification (63%, 98%, and 132%). Durability of the VTC treated specimens was assessed by exposure to two white rot fungi (Pleurotus ostreatus and Trametes versicolor) for four or eight weeks. After incubation, mass loss in grams and percentage mass loss were determined. The results showed that VTC densification did not change decay resistance to Pleurotus ostreatus and Trametes versicolor. The mass losses caused by Trametes versicolor were more extensive than those by Pleurotus ostreatus. As expected, longer exposure time resulted in higher mass loss. Furthermore, the degree of densification did not influence susceptibility to fungal degradation.

The effect of the heat treatment of spruce wood on the curing of melamine–urea–formaldehyde and polyurethane adhesives

The effect of the heat treatment of spruce wood on the curing of melamine–urea–formaldehyde (MUF) and polyurethane (PUR) adhesives was monitored by measuring their rheological properties by means of a rheometer. Instead of the standard aluminium discs, wooden discs, made from heat-treated wood with different degrees of thermal modification and conditioned in different climates, were used. The wooden discs provided more realistic curing of the adhesives compared to the real-life bonding of wood, because of solvent absorption. The results of the rheological measurements suggested that the modified wood inhibited the curing of MUF and PUR adhesives. The curing of the MUF adhesive was slower because of the reduced absorption of water from the adhesive. The curing of the one-component PUR adhesive was affected by the lower moisture content (MC) of the modified wood.

Bonding of Heat-Treated Spruce with Phenol-Formaldehyde Adhesive

The modified chemical, physical and structural properties of wood after heat treatment can affect the bonding process with adhesives. The objective of this research was to determine to what extent the degree of heat treatment influenced the bonding of treated wood with phenol-formaldehyde (PF) adhesive. Spruce (Picea abies Karst) lamellas were heat treated at 180°C and 220°C and then bonded. The shear strength and wood failure of the differently pretreated specimens were determined. Wettability and penetration of the adhesive were also investigated. The results showed that the shear strength of the PF adhesive bond was influenced by the heat treatment of the spruce wood, and depended on the type of pretreatment of the wood specimen prior to testing. The observed reduction in the shear strength of the PF adhesive bond was ascribed to a decrease in the wood strength itself, caused by the heat treatment, and also to other effects induced by exposure of the wood to elevated temperatures. The changes in wettability and adhesive penetration did not significantly influence the bonding process of the heat-treated wood with the PF adhesive.

Properties of liquefied wood modified melamine-formaldehyde (MF) resin adhesive and its application for bonding particleboards

In this study, we modified melamine-formaldehyde (MF) resin adhesive with liquefied wood (LW) and determined the properties of MF–LW adhesive mixtures. Furthermore, we produced particleboards using prepared MF–LW mixtures and evaluated their mechanical and physical properties. Results showed that with increasing content of LW in the adhesive mixture gel time and peak temperature increased while reaction enthalpy decreased. With increasing substitution of MF resin adhesive with LW the thermal stability of adhesive mixture reduced, namely thermal degradation started at lower temperature and weight loss increased. Properties of particleboards improved with increasing amount of LW in the adhesive mixture up to 20% and then deteriorated. Nevertheless, the properties of particleboard with 30% LW in the adhesive mixture were comparable to the properties of particleboard without LW while they worsen at greater portion of LW. Consequently, MF resin adhesive with 30% LW substitution could be used to produce particleboards with suitable mechanical properties and reduced formaldehyde release content.