Lauri Rautkari

The water vapour sorption properties of thermally modified and densified wood

The water vapour sorption behaviour of Scots pine (Pinus sylvestris L.) and Scots pine that was densified, thermally-modified, or subjected to a combination of thermal modification and densification has been investigated. It was found that all modifications resulted in a decrease in the equilibrium moisture content of the wood samples throughout the hygroscopic range. The water vapour sorption isotherms were reproducible for the unmodified wood samples, but changed between the first and subsequent sorption cycles for the densified, thermally-modified and for wood subjected to a combination of the two treatments. This is the first time that changes in the sorption isotherm between the first and subsequent cycles have been reported for thermally-modified wood. Irrespective of the wood treatment the difference between the adsorption and desorption isotherm loops (sorption hysteresis) was the same and greater than that observed for the unmodified wood sample. After the first sorption cycle, the hysteresis decreased to the values observed for the unmodified wood, even though the isotherms were different. The sorption kinetic behaviour was also investigated and found to be accurately described using the parallel exponential kinetics (PEK) model. The PEK model describes the dynamic sorption behaviour in terms of a fast and slow kinetic process and this has been interpreted in terms of two Kelvin-Voigt elements coupled in series (i.e. relaxation-limited kinetics).

What is the role of the accessibility of wood hydroxyl groups in controlling moisture content

It has often been claimed that the equilibrium moisture content (EMC) of wood is controlled by the available hydroxyl group content and their accessibility, but this has not been proven. In the present study, the accessibility of the hydroxyl groups were analysed by deuterium exchange in a dynamic vapour sorption apparatus, and generally poor correlation with the EMC and hydroxyl group accessibility was found. Therefore, the role of the accessibility of wood hydroxyl groups in relation to exerting sole influence on the EMC is disputable. It is concluded that there has to be an additional mechanism to exercise control over the EMC in addition to hydroxyl group accessibility.

Properties and set-recovery of surface densified Norway spruce and European beech

The chemistry and wetting behaviour of surface densified wood were investigated using FT-IR spectroscopy and contact angle analyses. Furthermore, set-recovery of the surface under conditions of fluctuating humidity was measured and quantitative microscopy analyses were undertaken. FT-IR indicated that no significant chemical changes took place during the densification process. However, the wettability of the densified surfaces was significantly lower than unmodified surfaces. Following several high humidity-oven dry cycles, it was found that this densification process was almost completely reversible, i.e., there was full set-recovery.

High‐Strength Composite Fibers from Cellulose–Lignin Blends Regenerated from Ionic Liquid Solution

Composite fibres that contain cellulose and lignin were produced from ionic liquid solutions by dry-jet wet spinning. Eucalyptus dissolving pulp and organosolv/kraft lignin blends in different ratios were dissolved in the ionic liquid 1,5-diazabicyclo[4.3.0]non-5-enium acetate to prepare a spinning dope from which composite fibres were spun successfully. The composite fibres had a high strength with slightly decreasing values for fibres with an increasing share of lignin, which is because of the reduction in crystallinity. The total orientation of composite fibres and SEM images show morphological changes caused by the presence of lignin. The hydrophobic contribution of lignin reduced the vapour adsorption in the fibre. Thermogravimetric analysis curves of the composite fibres reveal the positive effect of the lignin on the carbonisation yield. Finally, the composite fibre was found to be a potential raw material for textile manufacturing and as a precursor for carbon fibre production.

Surface modification of wood using friction

The potential of linear vibration friction as an innovative means of producing increases in both surface density and surface hardness was explored. The influence of processing pressure and time on the degree of surface densification, surface hardness and surface elasticity was investigated. It was found that surface hardness (measured as Brinell hardness) was positively correlated with densification ratio. Furthermore, surface elasticity, that is the ability of the surface to recover elastically after indentation during the Brinell hardness test, could be increased by up to 33% depending on the degree of surface densification. The temperature rise due to friction was also studied. During processing, it was found that the temperature rise on both the radial and tangential surfaces was positively correlated with the processing pressure and time.

Sorption behaviour of torrefied wood and charcoal determined by dynamic vapour sorption

AbstractThe most important variable of biomass-based fuels is moisture, because it affects the entire logistic chain by creating problems related to transport, handling, storage, and combustion. Recently, there has been a growing interest in thermal pre-treatment of biomass-based fuels by torrefaction. Torrefaction is intended to overcome the moisture-related problems by significantly reducing hygroscopicity and thus the logistics of solid biofuels could be improved. In order to gain a deeper insight on the changes induced by torrefaction, the sorption properties of Finnish birch and spruce wood were investigated. The sorption isotherms, hysteresis, accessibility, and surface area were investigated with dynamic vapour sorption. Also the particle size distributions and the clustering behaviour of water molecules were examined. As a result of the thermal pre-treatment, accessibility, adsorption of water vapour, and hysteresis were reduced. Particle size distribution was shifted towards smaller particles and the surface area measured with the BET method was reduced. Hysteresis was also reduced, which may be linked to the overall reduction in material’s hygroscopicity, as well as changes in porosity. The particle size affects sorption by increasing the surface area, but has a dual effect on accessibility, as it also blocks access to pores through agglomeration. In practice, the most important result is the increased hydrophobicity, but also the increased porosity and reduced particle size are to be considered as they can affect the handling and storage properties of torrefied and charred material negatively.

A study by X-ray photoelectron spectroscopy (XPS) of the chemistry of the surface of Scots pine (Pinus sylvestris L.) modified by friction

No abstract available

Effect of initial moisture content on the anti-swelling efficiency of thermally modified Scots pine sapwood treated in a high-pressure reactor under saturated steam

Abstract The effects of initial moisture content (MC) on anti-swelling efficiency (ASE), mass loss (ML), and equilibrium MC (EMC) at 65% relative humidity of thermally modified Scots pine sapwood under saturated steam (TMSA) has been investigated. ML during the TMSA process was higher with the specimens containing moisture before modification compared with initially dry specimens. Surprisingly, the EMC of the modified specimens with initial moisture was higher than that observed with the modified wood that was initially dry before the modification process. Higher initial MC before thermal modification results in a lower ASE.

FEM simulation of the hygro-thermal behaviour of wood under surface densification at high temperature

Surface densification of solid wood increases the density on the surface, when compressed by single side heated press. A recent experimental study has pointed out the influence of the process parameters on the development of the density profiles in the modified samples. Numerical modelling can help to optimize the experimental work which is often time consuming and laborious due to the large amount of experiments required to check the influence of the pressing parameters. In the present work, a FEM simulation of the hygro-thermal behaviour of wood under surface densification is proposed by using a three-dimensional hygro-thermal model based on earlier literature approaches. The model is implemented in a user subroutine of the FEM code Abaqus starting from the definition of a weak form of the governing hygro-thermal equations of the problem. The numerical profiles of moisture content and temperature during the wood densification process are simulated for some wood specimens tested in a previous study. Conclusions are given on the relationship between these profiles and the experimental density profiles due to different process parameters.

Cupping behaviour of surface densified Scots pine wood: the effect of process parameters and correlation with density profile characteristics

Surface densification is a process by which the mechanical properties of the wood surface can be improved, increasing the attractiveness of low density wood for applications such as flooring. The purpose of this study was to investigate the development of undesired cupping deformations in surface densified solid wood. The effects of process parameters (compression ratio, temperature, holding time, closing time, and initial moisture content) on cupping were studied, and correlation analysis was used to determine the strength of association. Correlation analysis was also performed to explore the potential dependence of cupping on the density profile characteristics of surface densified wood. Correlations of moderate strength were found between cupping and the process parameters, as well as between cupping and the density profile characteristics. Compression ratio was found to be the most influential process parameter: samples with a high compression ratio showed distinctly different cupping behaviour than samples with a low compression ratio. Density profile characteristics were considered a probable contributor to cupping, but the influence of other effects was also considered likely. Interestingly, certain combinations of parameters were found to result in very small cupping deformations. Therefore, the results of this study indicate that cupping can be minimised by optimisation of process parameters.