The porous structure of solid wood can be compressed to obtain a higher density material. This modification method is referred to as densification and is performed by compressing solid wood samples in the radial direction between heated metal plates (~150°C). The purpose is to increase the mechanical properties of wood, as most of them are directly related to density. The deformation under compression should take place at temperatures exceeding the glass transition temperature (Tg) of the cell wall components to enable wood cell walls buckling without breaking the cell-walls. It is also possible to densify only the surface of the wood by heating only one plate in the press to improve especially the surface properties of wood, such as hardness. Surface densified wood would be beneficial to use especially for products with mainly one surface exposed to use, such as flooring boards or worktops.
When densified wood is subsequently exposed to moisture, the sample may partly or almost completely return to its original dimensions. This phenomena is commonly referred to as set-recovery. The elimination of the set-recovery phenomena usually requires additional post-treatment after densification, thermal modification being the most typical treatment. Thermal modification is a commonly known modification method to improve the dimensional stability and decay resistance of wood. On the other hand, this treatment reduces the mechanical properties of wood. By combining the densification and thermal modification processes it is possible to improve both mechanical properties and dimensional stability and obtain a high standard material. The interesting viewpoint is that this process is utilising only heat, moisture and mechanical compression and no chemicals are added to the products, which might be beneficial when it comes to recycling and reusing the material.
Keywords: densification, set-recovery, thermal modification, hardness
Authors
Laine K.
Department of Forest Products Department, Aalto University, Espoo, FINLAND
Rautkari L.
Department of Forest Products Department, Aalto University, Espoo, FINLAND
Joint Research Institute for Civil and Environmental Engineering, School of Engineering and the Built Environment, Edinburgh Napier University, UK
Hughes M.
Department of Forest Products Department, Aalto University, Espoo, FINLAND
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