Aleš Ugovšek

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.

Thermal modification of wax-impregnated wood to enhance its physical, mechanical, and biological properties

Abstract Thermal modification is the most important commercial modification procedure. Thermally modified (TM) wood has improved durability, but its performance does not meet expectations predominately under moist conditions. To reduce water uptake of TM wood, Norway spruce specimens were treated with suspensions of a natural wax by dipping impregnation (DipI) or by vacuum-pressure impregnation (VPI). Wax-treated specimens were subsequently TM at 185, 200, 215, and 230°C. Control specimens were heated up to 100°C only. Contact angle (CA), short-term and long-term water uptake, bending strength, and performance against wood decay fungi of the resulting material were determined. The results show that a combination of wax treatment and thermal modification have a synergistic effect that considerably improves hydrophobicity, reduces liquid water uptake, slows down water vapor uptake, and improves the resistance against fungal decay of the treated material.

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.

Short-term performance of wooden windows and facade elements made of thermally modified and non-modified Norway spruce in different natural environments

ABSTRACT Thermally modified wood is becoming an increasingly popular material for different applications in buildings. Laboratory tests indicated a positive effect of thermal modification on durability, dimensional stability and thermal conductivity of wood. Therefore, windows and facade elements made of thermally modified Norway spruce and non-modified Norway spruce were tested in the field and installed in different test objects which were exposed at five locations in Europe (Slovenia, Germany, Sweden, and Spain). Results from monitoring showed that elements and windows made of thermally modified spruce (TMS) had considerably lower wood moisture content compared to the ones made of non-modified spruce and that wax further positively influenced moisture performance. Colour changes of TMS were more intensive compared to non-modified spruce but were successfully retarded by adding pigments to the wax. Mould and stain growth was largely dependent on the location, amount of precipitation and relative humidity.

Upogibna trdnost in togost slojnatega furnirnega lesa (LVL) iz termično modificirane in nemodificirane bukovine / Bending strength and stiffness of laminated veneer lumber (LVL) made from thermally modified and unmodified beech veneer

The aim of the research was to define and develop a wooden composite with a thin cross-section and which could be used as reinforcement material in oversized wooden window profiles. An additional limitation was to bond the composite at room temperature. Based on a review of the literature, laminated veneer lumber (LVL) was chosen as the best option. The researched LVL samples were made of 0.5 mm thick, cut, beech (Fagus sylvatica L.) veneer, bonded with polyurethane adhesive (Purbond HB 440). Half of the samples were made of thermally modified veneers, and other half of unmodified. Bending strength and stiffness were determined with a three-point bending test. Thermally modified samples had on average 19 % lower bending strength compared to the unmodified samples, but the modulus of elasticity (stiffness) did not change significantly. The bending strength of up to 150 MPa was satisfactory, but the modulus of elasticity of 13 GPa was far below expectations. This is attributed to the selection of too thin veneer and too low bonding temperature, which does not enable densification of the composite.

Bending performance of 3-layer beech (Fagus sylvativa L.) and Norway spruce (Picea abies (L.) Karst.) VTC composites bonded with phenol–formaldehyde adhesive and liquefied wood

Low quality beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) were densified with viscoelastic thermal compression (VTC) process to two different degrees of densification, and lamellas were used to manufacture different types of 3-layer VTC composites. Bending properties of 3-layer VTC composites bonded with phenol formaldehyde (PF) adhesive and liquefied wood (LW) were determined and compared to 3-layer composites produced with undensified beech or spruce wood lamellas. Morphology of VTC spruce wood of higher density was analysed with fluorescent microscopic technique. All composites produced with densified beech lamellas and bonded with PF adhesive had significantly higher values of modulus of rupture (MOR) and modulus of elasticity (MOE) than composites produced with undensified lamellas. Densified spruce lamellas contributed to better bending performance of 3-layer VTC composites bonded with PF adhesive to some extent. Furthermore, composites bonded with LW had significantly lower MOR and MOE values compared to composites bonded with PF adhesive. Study of VTC spruce wood microstructure showed that densification caused non-uniform deformation of cell wall structure, in which cell wall fractures were observed.ZusammenfassungBuchen- (Fagus sylvatica L.) und Fichtenholz- (Picea abies (L.) Karst.) Lamellen geringer Qualität wurden mittels viskoelastischer thermischer Verdichtung (VTC) in zwei Verdichtungsgraden verdichtet und zur Herstellung unterschiedlicher dreilagiger VTC-Verbundwerkstoffe verwendet. Die Biegeeigenschaften der dreilagigen, mit Phenol-Formaldehydharz (PF) und Flüssigholz (LW) verklebten VTC-Verbundwerkstoffe wurden mit den Eigenschaften von dreilagigen Verbundwerkstoffen aus unverdichteten Buchen- oder Fichtenholz-Lamellen verglichen. Die Morphologie von VTC-Fichtenholz höherer Dichte wurde mittels Fluoreszenzmikroskopverfahren untersucht. Die Biegefestigkeit (MOR) und der Elastizitätsmodul (MOE) aller aus verdichtetem Buchenholz hergestellter und mit PF-Klebstoff verklebter Verbundwerkstoffe waren signifikant höher als bei den Verbundwerkstoffen aus unverdichteten Lamellen. Die verdichteten Fichtenlamellen trugen in einem gewissen Maße zu einem besseren Biegeverhalten der dreilagigen, mit PF-Klebstoff verklebten VTC-Verbundwerkstoffe bei. Daneben wiesen die mit Flüssigholz verklebten Verbundwerkstoffe signifikant niedrigere Biegefestigkeits- und Elastizitätsmodulwerte auf als die mit PF verklebten Verbundwerkstoffe. Eine Untersuchung der Mikrostruktur von VTC Fichtenholz zeigte, dass die Verdichtung zu einer unregelmäßigen Verformung der Zellwandstruktur führte, bei der Schädigungen der Zellwand auftraten.

Bending performance of 3-layer beech (Fagus sylvativa L.) and Norway spruce (Picea abies (L.) Karst.) VTC composites bonded with phenol–formaldehyde adhesive and liquefied wood@@@Biegefestigkeit dreilagiger, mit Phenol–Formaldehydharz und Flüssigholz verklebter VTC Buchen- (Fagus sylvatica L.) und Fichtenholz- (Picea abies (L.) Karst.) Verbundwerkstoffe

Low quality beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) were densified with viscoelastic thermal compression (VTC) process to two different degrees of densification, and lamellas were used to manufacture different types of 3-layer VTC composites. Bending properties of 3-layer VTC composites bonded with phenol formaldehyde (PF) adhesive and liquefied wood (LW) were determined and compared to 3-layer composites produced with undensified beech or spruce wood lamellas. Morphology of VTC spruce wood of higher density was analysed with fluorescent microscopic technique. All composites produced with densified beech lamellas and bonded with PF adhesive had significantly higher values of modulus of rupture (MOR) and modulus of elasticity (MOE) than composites produced with undensified lamellas. Densified spruce lamellas contributed to better bending performance of 3-layer VTC composites bonded with PF adhesive to some extent. Furthermore, composites bonded with LW had significantly lower MOR and MOE values compared to composites bonded with PF adhesive. Study of VTC spruce wood microstructure showed that densification caused non-uniform deformation of cell wall structure, in which cell wall fractures were observed.ZusammenfassungBuchen- (Fagus sylvatica L.) und Fichtenholz- (Picea abies (L.) Karst.) Lamellen geringer Qualität wurden mittels viskoelastischer thermischer Verdichtung (VTC) in zwei Verdichtungsgraden verdichtet und zur Herstellung unterschiedlicher dreilagiger VTC-Verbundwerkstoffe verwendet. Die Biegeeigenschaften der dreilagigen, mit Phenol-Formaldehydharz (PF) und Flüssigholz (LW) verklebten VTC-Verbundwerkstoffe wurden mit den Eigenschaften von dreilagigen Verbundwerkstoffen aus unverdichteten Buchen- oder Fichtenholz-Lamellen verglichen. Die Morphologie von VTC-Fichtenholz höherer Dichte wurde mittels Fluoreszenzmikroskopverfahren untersucht. Die Biegefestigkeit (MOR) und der Elastizitätsmodul (MOE) aller aus verdichtetem Buchenholz hergestellter und mit PF-Klebstoff verklebter Verbundwerkstoffe waren signifikant höher als bei den Verbundwerkstoffen aus unverdichteten Lamellen. Die verdichteten Fichtenlamellen trugen in einem gewissen Maße zu einem besseren Biegeverhalten der dreilagigen, mit PF-Klebstoff verklebten VTC-Verbundwerkstoffe bei. Daneben wiesen die mit Flüssigholz verklebten Verbundwerkstoffe signifikant niedrigere Biegefestigkeits- und Elastizitätsmodulwerte auf als die mit PF verklebten Verbundwerkstoffe. Eine Untersuchung der Mikrostruktur von VTC Fichtenholz zeigte, dass die Verdichtung zu einer unregelmäßigen Verformung der Zellwandstruktur führte, bei der Schädigungen der Zellwand auftraten.

Bending performance of 3-layer beech (Fagus sylvativa

Low quality beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) were densified with viscoelastic thermal compression (VTC) process to two different degrees of densification, and lamellas were used to manufacture different types of 3-layer VTC composites. Bending properties of 3-layer VTC composites bonded with phenol formaldehyde (PF) adhesive and liquefied wood (LW) were determined and compared to 3-layer composites produced with undensified beech or spruce wood lamellas. Morphology of VTC spruce wood of higher density was analysed with fluorescent microscopic technique. All composites produced with densified beech lamellas and bonded with PF adhesive had significantly higher values of modulus of rupture (MOR) and modulus of elasticity (MOE) than composites produced with undensified lamellas. Densified spruce lamellas contributed to better bending performance of 3-layer VTC composites bonded with PF adhesive to some extent. Furthermore, composites bonded with LW had significantly lower MOR and MOE values compared to composites bonded with PF adhesive. Study of VTC spruce wood microstructure showed that densification caused non-uniform deformation of cell wall structure, in which cell wall fractures were observed.ZusammenfassungBuchen- (Fagus sylvatica L.) und Fichtenholz- (Picea abies (L.) Karst.) Lamellen geringer Qualität wurden mittels viskoelastischer thermischer Verdichtung (VTC) in zwei Verdichtungsgraden verdichtet und zur Herstellung unterschiedlicher dreilagiger VTC-Verbundwerkstoffe verwendet. Die Biegeeigenschaften der dreilagigen, mit Phenol-Formaldehydharz (PF) und Flüssigholz (LW) verklebten VTC-Verbundwerkstoffe wurden mit den Eigenschaften von dreilagigen Verbundwerkstoffen aus unverdichteten Buchen- oder Fichtenholz-Lamellen verglichen. Die Morphologie von VTC-Fichtenholz höherer Dichte wurde mittels Fluoreszenzmikroskopverfahren untersucht. Die Biegefestigkeit (MOR) und der Elastizitätsmodul (MOE) aller aus verdichtetem Buchenholz hergestellter und mit PF-Klebstoff verklebter Verbundwerkstoffe waren signifikant höher als bei den Verbundwerkstoffen aus unverdichteten Lamellen. Die verdichteten Fichtenlamellen trugen in einem gewissen Maße zu einem besseren Biegeverhalten der dreilagigen, mit PF-Klebstoff verklebten VTC-Verbundwerkstoffe bei. Daneben wiesen die mit Flüssigholz verklebten Verbundwerkstoffe signifikant niedrigere Biegefestigkeits- und Elastizitätsmodulwerte auf als die mit PF verklebten Verbundwerkstoffe. Eine Untersuchung der Mikrostruktur von VTC Fichtenholz zeigte, dass die Verdichtung zu einer unregelmäßigen Verformung der Zellwandstruktur führte, bei der Schädigungen der Zellwand auftraten.