Combined Densification and Thermo-Hydro-Mechanical Processing of Wood
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Combined
Densification and Thermo-HydroMechanical Processing of Wood Parviz Navi and Frédéric Heger
Abstract The process of heating and compressing wood to improve its properties or reform it to a new shape has been known for decades. Such improvements are usually accompanied by “shape memory,” where the deformation produced by compression is not permanent, and the material recovers when re-moistened and heated. The combination of densification and a thermo-hydro-mechanical (THM) treatment can transform wood into a new material with improved mechanical properties, decreased sensitivity to moisture, increased durability, and no shape-memory effects. This article presents the principles of combined densification and THM processing, the products and experimental results, the origin of the shape-memory effect and its elimination by THM treatment, and the potential use of THM-processed densified wood in construction applications. Keywords: construction materials, densification, shape memory, thermo-hydromechanical processing, wood.
Introduction Wood is one of the oldest construction materials used by humans. On a volume basis, more wood is used than any other engineering material except concrete. The use of wood will continue to be important because it is a natural, renewable material; on average, 4–5 billion cubic meters of wood is harvested per year. Wood exhibits many advantageous characteristics as a construction material: it has a high strength-to-weight ratio, excellent workability, and good potential for modification. However, wood also has certain undesirable characteristics: it is hygroscopic, anisotropic, and biodegradable (i.e., improving its durability is critical for cost-effective use). In order to use wood more effectively in construction, the impact of these characteristics needs to be minimized. For centuries, wood has been used in its virgin state, that is, without processing to
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improve its mechanical performance or durability. Although the basic constituents of wood have been known for a long time, the structure and properties of wood polymers, an understanding of which is a prerequisite to physicochemical modification, have only been characterized in the last 40 years. The process of heating and compressing wood (densification) has been known for decades. The main reasons to densify wood are to improve its physical and mechanical properties and to change its shape. It has been shown that densification improves the transversal shear strength of spruce and maritime pine by more than 13-fold (from 8.4 N/mm2 to 110.6 N/mm2), increases surface hardness by more than fivefold (from 12 N/mm2 to 61.8 N/mm2 for a radial cross section), and reduces the uptake of water
at equilibrium (i.e., hygroscopicity) by one-half.1 Densification reduces the long, cylindrical void spaces, called lumens, in wood cells by heat treatment combined with compression in the radial direction. This simple and cheap method (called thermomechanical densification) does not require any impregnation of the wood with resin or oth
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