Fachbereich Holzingenieurwesen
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Holztechnik B.Eng.
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Institute
Drei verschiedene Phenolharze wurden auf ihre Witterungsbeständigkeit in
einem künstlichen Bewitterungstest in Anlehnung an DIN 927-6:2006 geprüft.
Die Phenolharze wurden aufgrund des unterschiedlichen Molekulargewichts
ihrer Vorkondensate ausgewählt. Anschließend wurde Buchenfurnier (Fagus
sylvatica) mit den Phenolharzen bei 100 mbar Unterdruck für 24 h getränkt
und anschließend bei 150 °C ausgehärtet. Zur Bewertung der Photostabilität
der phenolharzimprägnierten Proben wurde über den Prüfzeitraum die Farbänderung
aufgenommen. Des Weiteren wurde zur Bewertung der chemischen
Stabilität der Phenolharze in der Holzsubstanz ATR FT-IR Messungen
durchgeführt. Die Phenolharze mit niedrigem und mittleren Molekulargewichts
zeigten verbesserte Witterungsstabilität im Vergleich zu dem Phenolharz höheren
Molekulargewichts.
Three various phenol-formaldehyde resins with varying molecular weight were investigated in respect to their potential to protect wood surfaces against photooxidative degradation. Thin beech veneers (Fagus sylvatica L.) were impregnated with the phenol-formaldehyde resins under vacuum conditions for 24 h. Subsequently, the phenolic pre-polymers were cured for 1.5 h at 150 °C. The weathering test was performed in an artificial weathering test according to the European standard EN DIN 927-6 (2006). After the test period of 960 h of UV irradiation the specimens were evaluated in respect to their colour changes, their chemical stability and the structural changes on the specimen surfaces. Thus, CIELab colour measurements, infrared spectroscopy and field emission environmental scanning electron microscope investigations were applied. It can be concluded, that phenol-formaldehyde resins with low and medium molecular weight indicating increased resistance against weathering compared to high molecular weight phenol-formaldehyde resin. However, for all investigated wood properties each treatment displayed significant increased performance compared to the untreated control.
The following study examines old and new pine wood (pinus sylvestris) with regard to its tensile strenght, using load tests and deep-penetrating visualisations via computed tomography. The Technical University of Berlin carried out tensile tests on new, commercially available lamellas and old lamellas from a building demolition, which had a documented history of 100 years´ use as ceiling beams. The tensile strength of the old pine lamellas was found to be reduced by approximately 25%. Both batches were categorised and examined in CT scans, with seven pairs compared on the basis of their tensile strength and fracture patterns. Analysis of the data revealed a breakdown of the hemicelluloses, wich was manifested by a less dense appearance of these in the CT scans due to deacylation and the associated increase in porosity and reduction in electron density, as indicated by the HU values. The cellulose structures, on the other hand, became more crystalline, leading to a denser appearance in these areas in the 3D scan visualisations. Furthermore, a greater decrease in tensile strength was observed at low force-fiber angles than at higher force-fiber angles. Accordingly, the old wood samples tended to break in front of knots, in the solid wood, whilst the new samples broke through knots slightly more often, suggesting a greater density dependence in the areas of parallel fiber loading than in the areas with loading at an angle to the fiber.
The aim of this study is to examine the treatability of European beech (Fagus sylvatica L.) with phenol-formaldehyde and their potential as plasticizing agent of beech for moulding applications. Conclusions were drawn from swelling of the wood in combination with dynamical mechanical thermal analysis. Swelling of the cell walls can serve as indicator for plasticizing effects and provides information about the impregnability of the veneers with phenol-formaldehyde (PF) resins. Thus, macroscopic swelling of beech blocks and microscopic swelling of single cell walls was measured after PF impregnation and compared to water-saturated dimensions. Additionally, the bulking effect of the treated specimens was measured after curing of the specimens to evaluate the potential of PF to penetrate the cell walls. Dynamic mechanical thermal analyses (DMTA) were employed to assay the plasticizing effect of the PF after impregnation during heating. Therefore, three commercially available PF resols were used, representing a low molecular weight, a medium molecular weight and a high molecular weight resin. It was found that low and medium molecular weight prepolymers induced slightly increased swelling values compared to water saturated beech wood, indicating an improved plasticizing effect on beech wood. DMTA studies demonstrate the influence of temperature on the wood plasticization. The measured storage modulus E’ and the loss factor tan δ suggest a different potential of the used PF for veneer plasticization. With decreasing swelling values, plasticizing effects are also decreasing. Furthermore, the study indicates, that swelling of wood might be not the determining factor to evaluate plasticizing of wood.
Three phenol-formaldehydes with varying molecular weight are investigated according to their suitability to improve the dimensional stability of beech wood (Fagus sylvatica L.). Focus of the work is to investigate the relation of the bulking effect of the wood substance caused by the varying phenolformaldehydes treatments to the corresponding dimensional stability on macroscopic level of small wood blocks compared to microscopic level of single cell walls. Therefore, beech blocks are treated with the certain phenol-formaldehydes, bulking is determined and subsequently the wood is exposed to three soaking-drying cycles. The evaluation was performed according to the anti swelling efficiency and leaching of the phenol-formaldehydes. On microscopic level, changes of the transverse area of the cell walls due to phenol-formaldehyde treatment and subsequent water soaking is estimated. The obtained results indicate, that penetration into the cell walls are the decisive factor for dimensional stability of the treated wood. Phenol-formaldehyde residuals in the cell lumina contribute negligibly to the dimensional stability.
The aim of the study is to examine the potential of Phenol/formaldehyde for a plasticizing effect of wood veneer and the treatability of thin veneer stripes (0.6 mm thickness) with Phenol/formaldehyde for moulding applications. Swelling of the cell walls can serve as indicator for plasticizing effects. Thus, swelling of cell walls of beech veneer (Fagus sylvatica L.) was measured after Phenol/formaldehyde impregnation and compared to water saturated cell wall dimensions. Furthermore, the potential of Phenol/formaldehyde uptake in respect to the dimensional stability of the treated wood was examined on cell wall level. In addition Phenol/formaldehyde resin penetration into veneers was evaluated in respect to the veneer moisture content and the impregnation parameter time. Therefor three commercially available Phenol/Formaldehydes prepolymers were used, representing a low molecular weight, a medium molecular weight and a high molecular weight resin. It was found that low and medium molecular weight prepolymers caused similar swelling values like water indicating a comparable plasticizing effect on wood. Moisture contents up to 10% of the untreated veneer affect the resin uptake only at negligible values. Full penetration of veneers was attained only for low molecular weight PF during short term impregnation.
Certain Basidiomycetes that cause brown rot are capable of precipitating copper oxalate from wood impregnated with wood preservatives. This study investigates the precipitation of copper oxalate from the water-soluble, copper-based wood preservative copper sulphate in association with the Basidiomycete Poria placenta on Pinus sylvestris L. The focus of the investigations is on light and scanning electron microscopic analyses with regard to the in situ precipitation of copper oxalate.
The background of the project is to evaluate the colour changes of wood not only as a function of time. Space-time is the fourth dimension (in addition to the three spatial dimensions). However, time is not the real factor affecting the wood surface, but the electromagnetic radiation from the sun. Light is the visible (VIS) part of the electromagnetic spectrum, in addition to other components such as ultraviolet (UV) and infrared (IR) radiation. UV-, VIS- and IR-radiation have an energetic effect on the wood surface as part of the solar radiation spectrum. What are the effects of the solar radiation spectrum on the wood surface? This is an important question, as the evaluation in terms of irradiation intensity has not been the subject of detailed investigation. Radiation intensities are reported in many studies. However, this information usually has the status of metadata. Irradiance is the term for the total power of incoming electromagnetic energy striking a surface, related to the size of the area, measured in watts per square metre [W/m²].
Die steigende Nachfrage nach Holz als nachwachsendem Rohstoff erfordert eine zunehmend
effiziente Nutzung der verfügbaren Ressourcen. Gleichzeitig führen klimatische Verän-
derungen und biotische Schadfaktoren zu einer erhöhten Variabilität der Holzqualität. Vor
diesem Hintergrund untersucht diese Arbeit den Einfluss ausgewählter Wuchsmerkmale auf
den Biege-Elastizitätsmodul von Kiefernholz (Pinus sylvestris L.) und Fichtenholz (Picea abies
Karst.). Dafür werden die Wuchsmerkmale Ästigkeit, Faserneigung, Jahrringbreite sowie die
Rohdichte gemäß DIN 4074-1 erfasst.
Die mechanischen Eigenschaften werden anhand von Vierpunkt-Biegeprüfungen in An-
lehnung an DIN EN 408 ermittelt. Die statistische Auswertung erfolgt auf Basis kontinuierlicher
Messwerte und unter Berücksichtigung von Prüfkörpern ausschließlich mit Ästen in der
konstanten Biegemomentzone. Eine Einordnung in Sortierklassen erfolgt nicht.
Die Ergebnisse zeigen einen holzartspezifischen und unterschiedlichen Zusammenhang
zwischen Elastizitätsmodul, Ästigkeit und Faserneigung. Während bei Fichte ein deutlicher
negativer Zusammenhang zwischen Ästigkeit und Elastizitätsmodul besteht, zeigt sich bei
Kiefer ein entgegengesetzter Trend. Zudem erweist sich bei Kiefer die Faserneigung als
stärkerer Einflussfaktor als die Ästigkeit, während bei Fichte die Ästigkeit dominiert.
Die Rohdichte zeigt bei beiden Holzarten lediglich moderate Zusammenhänge mit dem
Elastizitätsmodul. Die Untersuchung verdeutlicht, dass strukturelle Wuchsmerkmale den
Einfluss der Rohdichte überlagern können und eine holzartspezifische Betrachtung
erforderlich ist. Die Ergebnisse liefern Hinweise für eine differenzierte Bewertung von Bauholz
und bilden eine Grundlage für weiterführende Untersuchungen mit größeren Stichproben.
