Volume 69, Issue 7 - Highlight Issue: COST Action FP0904 “Thermo-Hydro-Mechanical Wood Behaviour and Processing” Selected Papers Guest Editors: Parviz Navi and Lennart Salmén

The correct prediction of the behavior of wood components undergoing environmental loading or industrial process requires that the hygrothermal and mechanical (HTM) behavior of wood is considered in a coupled manner. A fully coupled poromechanical approach is proposed and validated with neutron imaging measurements of moist wood specimens exposed to high temperature. This paper demonstrates that a coupled HTM approach adequately captures the variations of temperature, moisture content, and dimensions that result in a moist wood sample exposed to one-side heating.

The correct prediction of the behavior of wood components undergoing environmental loading or industrial process requires that the hygric, thermal and mechanical (HTM) behavior of wood are considered in a coupled manner. A fully coupled poromechanical approach has been used to perform a parametric study on wood HTM behavior, and the results have been validated with neutron imaging measurements on a moist wood specimen exposed to high temperature. Further, HTM behavior of wood during welding has been simulated by the model. For such a simulation, proper material properties are needed, as some of them, for example thermal conductivity, have a significant influence on the local and temporal behavior of the material.

The mechanical properties of thermally modified wood are discussed with regard to toughness. The molecular origins of the mechanical properties and, in particular, the role of the hemicelluloses are considered. The important role of water and its interaction with the cell wall components is also examined. The properties are discussed from the point of view of composite theory, with the three main macromolecular components acting as reinforcement, matrix and interfacial coupling agent. The important role that hemicelluloses play as a coupling agent between the cellulosic microfibril reinforcement and the lignin-rich matrix is highlighted. Destruction of the hemicelluloses during the thermal modification process has a profound effect upon the mechanical behaviour.

Thermo-hydro-mechanical (THM) treatment is a combined action of temperature, moisture, and mechanical force, which leads to modified wood (THMW). Various types of eco-friendly THM processes have been developed to enhance wood properties and generate new materials, such as welding, densification, molding, bending, profiling, artificial aging, panel manufacture, and surface densification. The various transformation processes in the course of THM bring about positive effects in terms of the mechanical and physical properties as well as the biological durability. To the negative effects belong the loss in strength and fracture toughness, and one of the challenges is to minimize these negative aspects. The present paper reviews the chemical transformations processes during THM treatment in a closed processing system and presents the relationship between processing parameters and THMW properties. The discussion includes the problems associated with eliminating the set recovery of densified wood by THM posttreatments and the chemical origin of the relaxation of internal stresses induced by densification.

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.

This paper presents the state of the art of different wood densification processes as one emerging process technology. The main principles for the processes are discussed, such as bulk and surface densification, bending, moulding of shells and tubes, as well as methods for reducing the shape memory effect of densified wood. The main challenges are in the field of scaling up to industrial applications. To provide a better understanding with this regard, some relevant scientific results are presented. Furthermore, the discussion considers the contribution of thermo-hydro and thermo-hydro-mechanical processes to a sustainable and low-carbon economy.

Thermal modification is an environmentally friendly process that enhances the lifetime and properties of timber. In this work, the absorption of water in pine wood ( Pinus sylvestris ) samples, which were modified by the ThermoWood process, was studied by magnetic resonance imaging (MRI) and gravimetric analysis. The modification temperatures were varied between 180°C and 240°C. The data shows that the modification at 240°C and at 230°C decreases the water absorption rate significantly and slightly, respectively, while lower temperatures do not have a noticeable effect. MR images reveal that free water absorption in latewood (LW) is faster than in earlywood (EW), but in the saturated sample, the amount of water is greater in EW. Individual resin channels can be resolved in the high-resolution images, especially in LW regions of the modified samples, and their density was estimated to be (2.7±0.6) mm -2 . The T 2 relaxation time of water is longer in the modified wood than in the reference samples due to the removal of resin and extractives in the course of the modification process.

Bamboo was thermally treated at 180°C and 200°C, and the micromechanical properties of its cell walls were investigated by means of quasi-static and dynamic nanoindentation experiments. With increasing treatment temperatures, the average dry density and mass of the bamboo decreased, whereas the already reduced elastic modulus at 180°C of the fiber cell walls did not change, but the hardness showed increasing tendencies. Dynamic nanoindentation revealed reduced storage modulus (E′r)$({E'_{\rm{r}}})$ and loss modulus (E″r )$({E''_{\rm{r}}}\,)$ for the thermotreated bamboo cell walls compared with the untreated bamboo fibers in all frequency regions. Moreover, E′r, E″r,${E'_{\rm{r}}},{\rm{ }}{E''_{\rm{r}}},$ and loss tangent (tan δ) of treated bamboo decreased with increasing treatment temperature.

Tensile wood creep has not been measured previously in the temperature region 135°C–150°C for a range of moisture content (MC) up to fiber saturation point due to equipment and measurement challenges. Yet this is the region where the most dramatic softening effects can be observed. The aim of this study was to develop specialized equipment to measure tensile solid wood creep over 70°C–150°C for a range of MC and loads. Creep displacement was successfully able to be isolated from elastic and mechanosorptive strains and statistically modeled by regression analysis. This proved more accurate than complex series expansions consisting of spring- and dashpot-type components. The best creep displacement relationship was a power law with a strain root mean square error of 0.28%. The amplitude of the power law was nonlinear with respect to stress and temperature and changed by a stress factor of up to 0.9 for a stress of up to 1.2 MPa and by a temperature factor 1.2–6.3 over the investigated range. The creep relationship provides a valuable tool for predicting time-dependent distortion and internal stress of wood during drying or thermohygromechanical modification below 150°C.