Wim Willems

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.

Hydrostatic pressure and temperature dependence of wood moisture sorption isotherms

By expressing wood moisture content data as a function of adsorption energy, an interesting scaling capability is obtained, wherefrom the general hydrostatic pressure and temperature dependence of wood moisture content is determined. The scaling law is fully consistent with the thermodynamics of swelling. It can be used to transform room condition sorption isotherms to other temperatures and hydrostatic pressures, provided that the wood matrix is not irreversibly modified. A special procedure is suggested for the case of an irreversibly changing wood matrix, as in thermal modification and thermo-hydro-mechanical treatments. Using the present scaling theory, several fundamental aspects of wood moisture sorption are explained, such as the absence of a significant quantity of strongly bound wood moisture, the internal stress generation by sorption hysteresis in the wood cell wall, and the reason for the reversible disappearance of the sigmoid shape of the sorption isotherm at higher temperature. The results of this research may be useful (a) for transformation of known sorption data to other conditions, notably where in situ moisture measurements are difficult to perform and (b) to quantify the effects of internal stresses in the ultrastructure of the cell wall on moisture content.

A critical review of the multilayer sorption models and comparison with the sorption site occupancy (SSO) model for wood moisture sorption isotherm analysis

Abstract The wood moisture sorption (WMS) isotherm is generally considered to contain information on the water-cell wall interaction and the abundance of water sorption sites (SSs) in wood. The Hailwood-Horrobin (HH) model – as an example of the multilayer surface sorption models – is discussed for its suitability to analyze experimental WMS isotherms, to elaborate the fundamental sorption parameters. Based on multiple independent experimental and theoretical arguments, it was concluded that the basics of the surface multilayer-sorption models do not apply to wood. This is clearly illustrated by applying the analysis to the temperature-dependence of WMS isotherms, to the comparison of adsorption vs. desorption isotherms and to the quantification of SSs in wood. A sorption site occupancy (SSO) model is presented as an alternative for the HH model. It provides a comprehensive, thermodynamically consistent and quantitative basis for the analysis of WMS isotherms. The predicted SS densities are realistic and can be used to quantify sorption hysteresis and cell wall relaxation.

Thermal wood modification chemistry analysed using van Krevelen's representation

Abstract Wood compositional changes during thermal modification follow a characteristic trajectory when mapped in a van Krevelen diagram. The trajectories of widely different wood species appear to merge into a single master curve, suggesting a common thermal modification chemistry shared by these wood species. The largest effect of thermal modification on the chemical composition can be explained by dehydration reactions, followed by decarboxylation reactions. A carbon valence electron donor–acceptor model is proposed, which relates the observed compositional changes to changes in polarity and redox character, which in turn are related to the characteristic hydrophobic and fungal resistance effects on wood by thermal modification.

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.

Equilibrium thermodynamics of wood moisture revisited: presentation of a simplified theory

Abstract The equilibrium moisture content (EMC) of a wood specimen is known to be a function of the (absolute) temperature T and humidity h of the environment. In the present paper, it is directly derived from equilibrium thermodynamics that EMC is more specifically a function of the water chemical potential μ=RT ln h (Polanyi’s postulate). It is shown that wood moisture thermodynamics then becomes considerably simplified, allowing the calculation of the energy of wood-water interactions from the data of a single-temperature moisture adsorption. A critical comparative analysis on the theoretically calculated adsorption enthalpy and published data, obtained from isosteric and calorimetric measurements, is given. It is deduced from the theory that all bound moisture is non-freezing and that the heat capacities of bound and free wood moisture are equal.