Introduction to thermo-hydro-mechanical (THM) wood processing

Wood is the ultimate renewable material. It possesses qualities that have made it a material of choice for millennia, these qualities further enhanced by its recognised carbon sequestration. However, as a biological material it suffers the same fate as any natural material, namely degradation. There are ways in which wood can be enhanced, including eco-friendly methods. One of the emerging eco-friendly methods is the combined use of temperature, moisture and mechanical action – so-called Thermo-Hydro-Mechanical (THM) treatments. THM processing can improve the intrinsic properties of wood, to produce new materials and to acquire a form and functionality desired by engineers without changing its eco-friendly characteristics. There are numerous THM processing techniques and the number of these processes is growing continuously. THM processing can be divided into two major categories; Thermo-Hydral treatments (TH) and Thermo-Hydro-Mechanical treatments (THM). TH is usually used to enhance wood properties, and is of importance in increasing stress relaxation during drying under high temperature, in the formation of wood-based composites or veneer products and in the artificial ageing of wood. THM on the other hand, is employed in the producing of new materials by densification, shaping by moulding, welding of wood by friction, embossment, bending of wood, wood fusion, and chip-less manufacturing. During THM treatment, wood undergoes large deformation, stress relaxation and chemical degradation, but with strong shape memory depending on parameters like temperature, moisture content, applied forces, processing time and the type of wood. Several TH and THM processes have recently been developed in Europe, Japan, the USA and Canada, but only some of them have been scaled-up industrially. However, to overcome the problems associated with TH/THM wood during processing at the laboratory scale, during scale up and controlling the end use properties, a detailed knowledge of these phenomena becomes important. This requires a close collaboration between experts in wood chemistry, wood mechanics and material science from both academia and from industry. With this report we would like to give a brief state-of-the-art of the important field of THM-processing. The aim of this work has not been to cover the complete field of THM-processing, just give an introduction