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Wood under thermo-mechanical densification behaves differently depending on the cross-sectional growth ring orientation (GRO) relative to the direction of compression. This influences the degree of cell damage, but also the shape-memory effects occurring when the compression load is released (spring-back) and when the timber is re-moistened (set-recovery). To study how the GRO influences the shape-memory effects, Scots pine specimens were separated into three distinct groups of GRO (Flat, Inclined, Hybrid) and then thermo-mechanically surface-densified. Spring-back and set-recovery were determined by thickness measurements and by digital image correlation. A GRO parallel to the densified surface, resulted in a low spring-back and a high set-recovery which were uniform over the width of the specimen. Specimens with a GRO between 15 and 45° to the densified surface showed high spring-back and low set-recovery, indicating cell-wall damage. Spring-back mainly occurred in the non-plasticised region immediately below the heated surface region and elasto-plastic rolling-shear deformation along individual growth rings occurred. The GRO of softwood subjected to thermo-mechanical densification determines if an applied load results in rolling shear-deformation or radial compression. This in turn determines where in the cross-section and when in the process the cells deform and if this deformation occurs below or above the glass-transition temperature. [ABSTRACT FROM AUTHOR]

The density profile of surface-densified wood has a major influence on the indentation resistance of the material. A method that can predict the density profile in surface-densified wood from measurements of the indentation in a hardness test was established. The combined information of hardness and density profile is expected to better assess the performance of surface-densified wood. Density profile and hardness test data for surface-densified Scots pine have been subjected to a partial least squares analysis to determine the relationship between the indentation depth measured during a hardness test and the density profile measured by X-ray densitometry. Among seven different hardness tests, which varied in test force and indenter geometry, the Brinell method according to the EN 1534 standard showed the highest correlation between the indentation-versus-time curve and the density profile. The mean absolute error for the prediction of density profiles in an external test set was 5–10%, indicating that the method proposed in this study can be used to replace X-ray densitometry in process control and process design. [ABSTRACT FROM AUTHOR]

The established methods for testing the hardness of wood are of questionable value for assessing the performance of surface-densified wood, since the density profile beneath the densified surface is an important property that needs to be considered. The purpose of this study was to evaluate the influence of the density profile of surface-densified wood and the hardness test parameters, such as indenter geometry and applied load on the measured hardness. The influence of the density profile varied considerably depending on the hardness test parameters. This can make a comparison of hardness values of surface-densified wood prone to misinterpretation. The selection of hardness test parameters should either be product-specific, or the density profile itself should be used to evaluate the hardness of surface-densified wood. A strong influence of the density profile on the indentation depth development during the hardness tests indicates the possibility of predicting the density profile based on the hardness test methods. [ABSTRACT FROM AUTHOR]

Compressing the surface of sawn timber results in a substantial increase in hardness, and this opens up new market opportunities of using low-density timber species as the raw material for high-value wood products. Unfortunately, widespread commercialisation is hindered by the lack of an industrially viable surface densification process, the major obstacle being the set-recovery (SR) of the densified wood cells upon exposure to moisture. Our hypothesis is that partial dissolution of the crystalline cellulose during densification will largely prevent the SR of densified wood. We therefore evaluated the effect of ionic liquid (IL) or organic superbase pre-treatment on the elastic spring-back (SB), SR and Brinell hardness (HB) of surface-densified wood. Specimens of Scots pine were treated with solutions of ILs or superbases, and then densified in a hot press at temperatures between 200°C and 270°C. The SR was reduced from 90% for the control group to only about 10% for the treated materials. The treated and densified specimens exhibited a higher HB than their untreated and densified counterparts. The method presented in this study is a precursor to the development of a continuous densification process adapted for an open system. Further studies are needed to understand the underlying mechanisms of the pre-treatment. [ABSTRACT FROM AUTHOR]