1,721,018 research outputs found
Photodegradation of thermally-modified Scots pine and Norway spruce investigated on thin micro-veneers
No full textThe exposure to ultraviolet light results in surface photodegradation of uncoated wood during exterior application. When using thermally-modified wood in exterior applications, chemical changes that occur during the modification process might affect a subsequent photodegradation. This study investigates the impact of artificial weathering on the photodegradation of thin micro-veneers of thermally-modified Scots pine and Norway spruce by means of FT-IR spectroscopy and micro-tensile strength testing. FT-IR spectra revealed photodegradation reactions of lignin that resulted in the disappearance of the peak at 1508 cm(-1) after 144 h, irrespective of the thermal modification process. Loss in micro-tensile strength by photodegradation was higher for finite- than for zero-span micro tensile strength as a result of the loss of amorphous carbohydrates. Although strength loss of unweathered veneers was evident for thermally modified wood, the rate at which strength loss occurs during artificial weathering was considerably decreased by the thermal modification process. It was concluded that the amorphous carbohydrates were less degraded during artificial weathering as they were already pre-degraded during the thermal modification process.International ThermoWood Association (Helsinki, Finland
Influence of process conditions on hygroscopicity and mechanical properties of European beech thermally modified in a high-pressure reactor system
No full textEuropean beech (Fagus sylvatica L.) was thermally modified in a closed reactor system under various process conditions. Sorption cycles, dynamic vapor sorption (DVS) measurements, and a three-point bending test were performed on thermally modified wood (TMW) to assess hygroscopicity and mechanical properties. As a function of mass loss (ML), the initial equilibrium moisture content (EMC) measured at 20 degrees C/65% relative humidity (RH) directly after the process was strongly influenced by the RH during the process. This effect is explained by realignments of amorphous polymers in the cell wall ultra-structure in the course of thermal modification (TM). However, the EMC of TMW gradually increased after sorption cycles consisting of conditioning over liquid water and water-soaking. This increase was most distinct for TMW modified at low RH, which is an indication for reversible ultra-structural realignments. Results of the bending test suggest that structural realignments also hindered the plastic flow of amorphous cell wall polymers, thereby reducing inelastic toughness and inelastic deflection, while other bending properties were solely affected by ML alone. Process conditions in a closed reactor systems have a profound impact on resulting wood properties, and thus, the partial reversibility of these property changes need to be considered during the application
Wood moisture content during the thermal modification process affects the improvement in hygroscopicity of Scots pine sapwood
No full textElevated wood moisture contents during the thermal modification process have been shown to adversely affect the improvement in dimensional stability and hygroscopicity. This study tested the hypothesis that the effect of elevated wood moisture content is based on the impact of water on chemical reactions which determine the cell wall matrix stiffness. Samples of Scots pine sapwood (Pinus sylvestris L.) were thermally modified in saturated water vapor at different peak temperatures and durations starting either in oven-dry or in water-saturated state. For a given mass loss caused by the modification process, the improvement in maximum swelling and equilibrium moisture content was stronger for oven-dry samples. After removal of water-soluble degradation products, which caused a cell wall bulking effect, the maximum swelling even increased after modification in water-saturated state. Based on dynamic vapor sorption measurements, it was evidenced that the modification in oven-dry state increased the cell wall matrix stiffness which improved dimensional stability and hygroscopicity. Enhanced bond formation in the polymeric network, i.e., via condensation and cross-linking reactions during the treatment of oven-dry wood, is suggested as a cause for this increase in matrix stiffness. In contrast, the modification in water-saturated state enhanced the flexibility of the cell wall matrix, which increased the cell wall swelling and limited the improvement of hygroscopicity to the reduction in OH groups by removal of hemicelluloses. This enhanced matrix flexibility was potentially caused by predominant hydrolytic cleavage of bonds in case of water-saturated samples, evident from the chemical analysis of soluble degradation products, which increased the free volume between adjacent matrix polymers
Thermally modified Scots pine and Norway spruce wood as substrate for coating systems
No full textThermally modified wood (TMW) is increasingly used in exterior applications as an alternative to tropical hardwoods or wood impregnated with biocides. Despite its enhanced biological durability and dimensional stability, a surface treatment of TMW with coating systems can be required in certain applications. This study assessed material characteristics of Norway spruce and Scots pine wood that was thermally modified according to the ThermoWood(A (R)) process and their effect on the performance of commercially available coating systems: a solventborne oil, a waterborne alkyd-reinforced acrylate paint and a waterborne acrylate paint. Residual extractives and remaining degradation products found in TMW, carry the risk of causing discoloration or of interfering with the curing reactions of coating systems. The penetration of coating systems into TMW was not found to differ from unmodified wood, although an excessive penetration of solventborne oil was found occasionally for TMW. The adhesion strength of waterborne coatings depended on the system that was used. While one system performed sufficiently on TMW, the other coating systems showed a considerable reduction in adhesion strength already after a mild treatment (< 200A degrees C). This reduction could not be attributed to the increase in hydrophobicity of TMW that was evident from contact angle measurements, but was rather related to the mechanical interaction of the specific substrate/coating system.International ThermoWood Association (Helsinki, Finland
Wood defects during industrial-scale production of thermally modified Norway spruce and Scots pine
No full textAnalyzing the UF resin distribution in particleboards by confocal laser scanning microscopy
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Quality control methods for thermally modified wood
No full textThermally modified wood (TMW) is currently produced commercially by a range of processes across many countries. A prerequisite of the commercial success is an efficient quality control (QC), and methods with this regard are discussed in this review. When direct measurement of the key attribute of the material is not feasible, QC is based on a suitably chosen physical or chemical "marker". A critical evaluation of currently applied markers reveals that most of them only provide data for comparative purposes for a particular species and/or over a narrow process range. Such markers do not allow making an objective judgment of quality, which is independent of process information or reference samples provided by the manufacturer. On the other hand, they can be very useful for monitoring product variability in the TMW factory and wood during the heat treatment. Recommendations for future development are the general validation of (combinations of) known TMW markers for different wood species and processes, resulting in (1) a reliable and fast laboratory QC method for given samples of unknown origin, (2) a simple and fast indicative QC test for end users, and (3) in-line product markers for feedback-controlled production
Wood degradation affected by process conditions during thermal modification of European beech in a high-pressure reactor system
No full textThe degradation of beech wood during a thermal modification process in a high-pressure reactor system using steam as medium was investigated. The wood was modified at different peak temperatures (150-180 A degrees C), peak durations (1-6 h) and maximum water vapor pressures (0.14-0.79 MPa), while wood mass loss and wood moisture content as well as soluble degradation products were analyzed. Wood degradation was found to be predominantly determined by the maximum pressure, rather than the peak temperature applied. However, accumulation of degradation products, i.e., carbohydrates and furfural, in wood modified at elevated pressure had to be considered when using mass loss as a marker for wood degradation. Mass loss and mass loss rate increased with the maximum pressure until reaching saturation at mass losses above 20 %, due to the limited amount of amorphous carbohydrates within the wood. Several factors have been discussed with regard to their impact on accelerated degradation reactions at elevated water vapor pressure, such as a better heat transfer in a compressed gas atmosphere, reduced evaporative cooling, the accumulation of organic acids as well as the presence of water in the wood during the process. However, none of these individual factors were completely consistent with the observed mass loss progression, which leads to the conclusion that the impact of elevated water vapor pressure, rather, is a combination of several factors that apply simultaneously. The application of elevated pressure might enable an effective process technique to generate sufficient wood degradation to upgrade dimensional stability and biological durability of wood at a low temperature range
Effect of curing conditions on the water vapor sorption behavior of melamine formaldehyde resin and resin-modified wood
No full textImpregnation modification of wood with melamine formaldehyde resin reduces the adverse effects caused by moisture uptake, but the underlying modes of action are not fully understood. The present study showed that it is crucial to understand the sorption behavior of the pure resin when interpreting the behavior of resin-modified wood. Furthermore, the applied heat-curing conditions had a significant effect on the moisture uptake of resin-modified wood. At the same resin loads, dry curing conditions were more effective in causing a cell wall bulking effect than wet curing conditions. This reduced the water-accessible cell wall pore volume in dry cured wood and counterbalanced the moisture uptake by the resin. Deuterium exchange measurements suggested that the occupancy of cell wall pores reduced the number of simultaneously active sorption sites. However, there was no evidence that a swelling restraint or reduced mechanical relaxation affected the water sorption of resin-modified wood significantly.Peer reviewe
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