Michael Altgen

Quality control methods for thermally modified wood

Abstract Thermally 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 moisture content during the thermal modification process affects the improvement in hygroscopicity of Scots pine sapwood

Elevated 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.

Influence of process conditions on hygroscopicity and mechanical properties of European beech thermally modified in a high-pressure reactor system

Abstract European 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°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 defects during industrial-scale production of thermally modified Norway spruce and Scots pine

Abstract This research investigates wood defects, particularly the formation of surface cracks, during the production of thermally modified wood and its exposure to cyclic moisture changes. Boards of Norway spruce and Scots pine originating from different steps within the production of ThermoWood® were collected and wood defects were investigated at macroscopic and microscopic scale. Subsequently, the wood was exposed to capillary wetting cycles to record its sensitivity towards cracking. After the modification process, typical anatomical defects of conventional kiln-drying became more frequent and severe, with the magnitude being to some extent depending on the presence of defects in the raw material. At microscopic scale, damages to ray parenchyma and epithelial cells as well as longitudinal cracks within the cell walls of earlywood tracheids were evident in thermally modified wood. Despite a lower water uptake and higher dimensional stability, thermally modified wood was more sensitive to surface cracking during wetting cycles than unmodified wood, i.e. at the outside face of outer boards (near bark). For limiting surface cracking of thermally modified wood during service life, the use of high-quality raw material, the exposure of the inside face of the boards (near pith) and the application of a surface coating are considered beneficial.

Comparison of EMC and durability of heat treated wood from high versus low water vapour pressure reactor systems

Abstract A large number of different heating technologies has been put into use for industrial scale thermal modification of wood. A useful classification of these processes is by the level of water vapour pressure, which ranges from vacuum to high saturated steam pressures. Only high water vapour pressure systems can maintain a finite moisture content during the heat treatment, but little is known about the water vapour pressure dependence of the thermal modification chemistry and the resulting modified wood properties. It is concluded from our analysis that the thermal wood reaction chemistry at the molecular functional group level is quite independent of the process and wood species. Wood properties that are strongly determined by wood chemical composition, such as the fungal durability and the equilibrium moisture content (EMC), can hence be equally achieved by all processes and for all wood species. This finding cannot be transferred to every other thermally modified wood property.

The effect of de- and re-polymerization during heat-treatment on the mechanical behavior of Scots pine sapwood under quasi-static load

Abstract Loss in strength and ductility is a major drawback for the heat-treatment of solid wood. Previous studies focused mainly on the de-polymerization of cell wall constituents as a cause and the importance of the preferential removal of hemicelluloses. This study tested the hypothesis that the mechanical behavior of wood is additionally affected by re-polymerization reactions within the cell wall matrix during heat-treatment. This was achieved by comparing changes in chemical composition, FT-IR spectra, and mechanical properties of Scots pine sapwood that was heat-treated in either dry state in superheated steam or in wet state using pressurized hot water. Although preferential de-polymerization of hemicelluloses was evident for both heat-treatment techniques, the analysis of the chemical composition and FT-IR spectroscopy indicated additional re-polymerization reactions within the cell wall matrix of dry heat-treated wood. The consequent formation of covalent bonds and cross-links increased the resistance against compression loads and hindered inelastic deformation during bending. This resulted in an additional reduction in bending strength and strain energy density of dry compared to wet heat-treated wood. Re-polymerization reactions during heat-treatments of wood in dry state were suggested as the main cause for the brittle failure under bending loads, while the effect of hemicellulose-removal on brittleness was much smaller than stated previously.