André Wagenführ

Alteration of the unsteady sorption behaviour of maple (Acer pseudoplatanus L.) and spruce (Picea abies (L.) Karst.) due to thermal modification.

Abstract The aim of the present investigation was to evaluate the influence of a thermal modification on the kinetics of the water vapour sorption of maple (Acer pseudoplatanus L.) and spruce [Picea abies (L.) Karst.] based on the assumption that the Fickian equation can be applied in this regard in the first approximation. The unsteady-state sorption process between two equilibria of humidity was modelled as a diffusion process. The rate of sorption was recorded by the gravimetric method, and then the diffusion coefficient was determined through inverse parameter identification. The thermal modification leads to an alteration of the unsteady sorption behaviour of both wood species. Transport of water vapour decreases with increasing degree of modification. Depending on wood moisture, the trends of diffusion coefficients include all three levels of modification. Furthermore, the diffusion coefficients decrease when the thickness of specimens decreases. The calculated diffusion coefficients showed a length dependency both for unmodified and thermally modified spruce and maple. Accordingly, the results clearly show that these woods have a non-Fickian moisture transport behaviour. The results are nevertheless useful for comparative purposes.

Alteration of the pore structure of spruce (Picea abies (L.) Karst.) and maple (Acer pseudoplatanus L.) due to thermal treatment as determined by helium pycnometry and mercury intrusion porosimetry

Abstract The pore size distribution in wood affects sorption and transport of moisture. In the present paper, the pore structure of spruce and maple was examined in untreated and thermally modified samples. The relative humidities of the specimens were 33%, 43%, 53%, and 76%. Tests were carried out by helium pycnometry and mercury intrusion porosimetry. The results clearly show that thermal treatments change the apparent density, pore structure, and pore size distribution. Measurements by the mercury intrusion porosimetry indicated that the influence of various environmental conditions (humidity, temperature) on the porosity and pore size distribution is small.

Material behaviour of veneer during multidimensional moulding

During the coating of multidimensional moulded surfaces the coating material suffers different stresses, which limit the possible mould. The behaviour of veneer as a thin coating material is discussed in the following article. The investigations were carried out with veneers of beech (Fagus sylvatica L.), sycamore maple (Acer pseudoplatanus L.) and American walnut (Juglans nigra L.) of different thicknesses. The anisotropy of the material is analysed concerning the various structure of the considered wood species. Feasibilities and limits of a three dimensional moulding are presented and conditions for a plastic deformation are discussed. ZusammenfassungBeim Beschichten mehrdimensional geformter Flächen erfährt das Beschichtungsmaterial verschiedene Beanspruchungen, die die mögliche Form begrenzen. Im folgenden Artikel wird das Verhalten von Furnier als dünnes Beschichtungsmaterial beim Verformen diskutiert. Für die Untersuchungen wurden Furniere von Rotbuche (Fagus sylvatica L.), Bergahorn (Acer pseudoplatanus L.) und Amerikanischem Nussbaum (Juglans nigra L.) unterschiedlicher Dicke verwendet. Die Anisotropie des Materials hinsichtlich der unterschiedlichen Struktur der betrachteten Holzarten wird analysiert. Es werden Möglichkeiten und Grenzen bei einer dreidimensionalen Formung aufgezeigt und Bedingungen für eine plastische Verformung diskutiert.

Untersuchungen zur vergleichenden Charakterisierung von thermisch modifizierter Fichte für Resonanzdecken von Gitarren

ZusammenfassungZiel der dargestellten Arbeit war die vergleichende Untersuchung thermisch modifizierter und unmodifizierter Zwillingsproben aus Fichtenresonanzholz für Gitarrendecken. Durch eine thermische Modifikation können die physikalisch-technischen Eigenschaften des Resonanzholzes derartig verändert werden, dass sie den Erfordernissen des Musikinstrumentenbaus besser entsprechen als unmodifizierte Proben. Vor allem sind dies die reduzierte Wassersorption, grössere Dimensionsstabilität, höhere Dauerhaftigkeit aber vor allem auch die Verbesserung von akustischen Kennwerten. Der Einfluss einer relativ milden thermischen Modifikation auf das Resonanzholz kann mit einer “künstlichen” Alterung verglichen werden. Das Holz erfährt durch die Modifikation eine Verbesserung objektiv messbarer Kennwerte, welche auf ein verbessertes Klangverhalten schliessen lassen. AbstractThe aim of this work was to compare thermally modified and unmodified twin samples of resonance spruce wood for sound boards. By a special thermal treatment physical-technical characteristics of wood can be changed in such a way that they correspond better to the requirements of wood for sound boards than unmodified woods. Above all this is the reduced moisture sorption, larger dimensional stability, better durability as well as acoustic characteristics. The effects of the relatively mild thermal treatment can be compared with an “artificial aging”. Due to the thermal modification objectively measurable parameters are improved which permit conclusions on improved sound characteristics.

Testing of set recovery of unmodified and furfurylated densified wood by means of water storage and alternating climate tests

Abstract Densification is a well-known method for improving the mechanical properties of wood. In the present study, unmodified and furfurylated wood samples were densified and submitted to cyclic water storage tests and cyclic alternating climate tests. Swelling coefficients and spring-back data were determined for the evaluation of the quality of densification. The study shows that results depend on the test method applied. Simple water storage tests do not reflect the behavior of densified wood in the high humidity range. The spring-back data of unmodified samples are more influenced by the testing method than those of the furfurylated ones.

Comparative analysis of thermally modified and native spruce loaded perpendicular to the grain

The aim of this work was to analyse the material behaviour of thermally modified and native spruce by load applied perpendicularly to the grain. Therefore, thermally modified and native samples were prepared for bending tests with the load in tangential direction. The bending strength in radial direction decreases significantly with the thermal treatment. The fracture surfaces in the tangential section of the bending samples after testing were analysed with a scanning electron microscope. For native samples, the typical failure was found in the intercellular area. For thermally modified samples, failure of the entire cell wall was observed. The material behaviour is more brittle, as demonstrated by the load-deflection curves, but also by anatomical studies of the fracture surfaces.ZusammenfassungZiel dieser Arbeit war es, das Materialverhalten thermisch modifizierter und nativer Fichte bei Beanspruchung quer zur Faserrichtung zu analysieren. Hierfür wurden thermisch modifizierte und native Proben in Biegeversuchen in tangentialer Belastungsrichtung geprüft. Durch die thermische Behandlung sinkt die Biegefestigkeit in radialer Richtung signifikant. Die Tangential-Bruchflächen der Biegeproben wurden nach dem Test mit einem Rasterelektronenmikroskop untersucht. Bei nativen Proben wurde das typische Versagen im interzellulären Bereich gefunden. Bei thermisch modifizierten Proben wurde dagegen ein Versagen der gesamten Zellwand beobachtet. Das Materialverhalten thermisch modifizierter Hölzer ist spröder als das von nativen Hölzern, wie die Last-Verformungs-Kurven, aber auch die anatomischen Untersuchungen der Bruchflächen gezeigt haben.

Emissions from thermally modified beech wood, their reduction by solvent extraction and fungicidal effect of the organic solvent extracts

Abstract The characterization and removal of odorous substances caused by thermal treatment of wood were investigated using chromatographic and spectroscopic methods. Emissions of pentanal and hexanal, probably formed by oxidation of unsaturated fatty acids, were the prevailing aldehydes in untreated wood samples. Their amount decreased during thermal treatment, while furfural and 5-methylfurfural (degradation products of hemicelluloses) accounted for almost 100% of the aldehyde fraction. Furthermore, it can be assumed that odorous emissions of thermally modified wood can be strongly reduced by solvent extraction. An ethanol–cyclohexane mixture that produces wood samples without significant odour emissions obtained good results. Certain organic solvent extractives from thermally modified beech have fungicidal properties. Methanol extracts, resolubilized in propanone, caused a retardation of fungal growth on native beech specimens. Beech specimens impregnated with these extractives could be assigned to durability class 4 (not permanent). The analysis of methanol extracts using gas chromatography coupled with mass spectrometry showed an increased share of phenolic compounds known to cause resistance effects. Their presence in organic extracts of thermally modified wood may explain the improved resistance of the impregnated specimen.

Aqueous Poly(N-Vinylformamide-co-Vinylamine) as a Suitable Adhesion Promoter for Wood Veneer/Biopolyethylene Composite Materials

Wood veneer/biopolyethylene (bio-PE) biocomposite materials were produced by using poly(N-vinylformamide-co-vinylamine) (PVFA-co-PVAm) copolymers as a phase-mediating reagent. In a preliminary step, PVFA-co-PVAm was adsorbed onto the wood veneer component from aqueous solution. In its adsorbed form, it served as an adhesion promoter and improved the compatibility between both the highly polar wood veneer and weakly polar bio-PE surface. Structural parameters and their effect on the adsorption process, such as the degree of hydrolysis (DH) of poly(N-vinylformamide) (PVFA) (30, 50, and > 90%), the molecular weight of PVFA-co-PVAm (Mw 10,000, 45,000, or 340,000 g/mol), and the pH value (4, 7, and 11) influenced the resulting wetting behavior of the PVFA-co-PVAm-modified wood veneer surface. Thus, the hydrophobizing effect of the PVFA-co-PVAm was clearly detectable because the contact angle with water was considerably increased up to 116° by adsorption of PVFA-co-PVAm 9095 at pH 11. The adsorbed amount of PVFA-co-PVAm was determined by energy-dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS). The PVFA-co-PVAm-coated wood veneers were consolidated with bio-PE in a hot press process. The modified composite materials showed remarkably improved Young’s moduli (552 MPa) and tensile strengths (4.5 MPa) compared to former composite materials produced without PVFA-co-PVAm modification.

Strategies, concepts and approaches to avoid cuttermarks on wooden workpiece surfaces

Abstract Milling is one of the most important cutting procedures in the wood industry. It allows shaping of workpiece surfaces with a high chip-removal rate. This is possible because of a fast rotary cutting motion, the possibility to place many cutting edges on the tool circumference and a fast linear feeding motion. Due to these different kinds of motions the resulting effective motion follows a prolonged cycloid which forms periodic waves on the workpiece surface. These surface patterns, called cuttermarks, may be visible on the machined surface after cutting and mainly influence the workpiece quality. Further surface machining steps are required to meet the demanded standards of surface quality. This is why researchers have been looking for possibilities to lower or avoid these cuttermarks. Beside the description and the analyses of the geometry of cuttermarks, magnitudes of disturbance, mechanical and mechatronic approaches and alternative procedures will be introduced and discussed. Especially mechatronic approaches can improve this situation by basically adjusting the cycloid motion track. In this way, the effective motion of the cutting edge is changed to a nearly linear motion by means of piezo-electric actuators or magnetic bearings. First successful set-up and experiments will be shown.

Capillary pore-size distribution and equilibrium moisture content of wood determined by means of pressure plate technique

Abstract This paper deals with the determination of the capillary pore-size distribution (CPSD) and equilibrium moisture content (EMC) of untreated and thermally modified (TM) Norway spruce [Picea abies (L.) Karst.] by means of the pressure plate technique (PPT). Desorption experiments were conducted at very high values of relative humidity (RH) in the range between 99.2% and 100%. The thermal modification of spruce results in an alteration of the CPSD, owing to the formation of intercellular cracks in the middle lamella, as a result of cell-wall compression. The desorption curves for both untreated and TM spruce show an extremely upward bend at 99.97% RH. This step reflects an EMC of 38.1% for untreated spruce and 33.8% for TM spruce. None of the samples shrunk during the PPT measurements. Following desorption experiments at 97.4% RH, all samples shrunk. This step reflects an EMC of 27.9% for untreated spruce and 21.7% for TM spruce.