Wood porosity is of great interest for basic research and applications. One aspect is the cell wall porosity at total dry state. When water is absorbed by wood, the uptake of water within the cell wall leads to a dimension change of the material. A hypothesis for possible structures that hold the water is induced cell wall porosity. Nitrogen and krypton physisorption as well as high pressure hydrogen sorption and thermoporosimetry were applied to softwood and hardwood (pine and beech) in dry and wet state for determining surface area and porosity. Physisorption is not able to detect pores or surface area within the cell wall. Krypton physisorption shows surface area up 5 times lower than nitrogen with higher accuracy. With high pressure sorption no inaccessible pore volumes were seen at higher pressures. Thermoporosimetry was not able to detect mesopores within the hygroscopic water sorption region. Physisorption has to be handled carefully regarding the differences between adsorptives. The absence of water-induced mesopores within the hygroscopic region raise doubts on existing water sorption theories that assume these pore dimensions. When using the term “cell wall porosity”, it is important to distinguish between pores on the cell wall surface and pores that exist because of biological structure, as there are no water-induced mesopores present. The finding offers the possibility to renew wood-water-sorption theories because based on the presented results transport of water in the cell wall must be realized by structures lower than two 2 nm. Nanoporous structures in wood at wet state should be investigated more intensively in future.

An investigation of simultaneous dynamic mass and length change measurement for wood is presented. In contrast to the equilibrium in moisture content and swelling and shrinking, where extensive data exists for different wood species, less information is available for the dynamics of moisture changes in direct comparison to the related dimensional changes during the sorption process. This is due to a lack of methods. A gravimetric sorption system, equipped with a high resolution camera and an automated image evaluation, is used to examine simultaneous effects of water vapour sorption dynamics and dimensional change. This method proves a strong correlation between mass and dimensional change, which is in contrast to other investigations. Equilibrium moisture content as well as swelling and shrinking data is in good agreement with literature and manual measurements. The method enables the possibility to determine swelling and shrinking values in-situ without disturbing the targeted climatic conditions. The system is applicable for the investigation of natural wood, modified wood, wood composites or other lignocellulosic materials.

Research Highlights: When investigating the sorption of water on lignocellulosic materials, the sorption or mixing enthalpy is an interesting parameter that, together with the sorption isotherms commonly measured, can be used to characterize and understand the sorption process. We have compared different methods to assess these enthalpies. Additionally, we propose a sorption nomenclature. Background and Objectives: Sorption enthalpies are non-trivial to measure. We have, for the first time, measured sorption enthalpies on the same materials with four different methods, to be able to compare the method’s strengths and weaknesses. Materials and Methods: The following four methods were used on beech and Scots pine wood: isosteric heat, solution calorimetry, sorption calorimetry, and RH perfusion calorimetry. Results: The results for beech and pine were similar, and were in general agreement with the literature. We do not recommend one of the methods over the others, as they are quite different, and they can therefore be used to elucidate different aspects of the interactions between water and, for example, novel biobased materials (modified woods, cellulose derivatives, and regenerated cellulose).

Detailed imaging of the three-dimensionally complex architecture of xylary plants is important for studying biological and mechanical functions of woody plants. Apart from common two-dimensional microscopy, X-ray micro-computed tomography has been established as a three-dimensional (3D) imaging method for studying the hydraulic function of wooden plants. However, this X-ray imaging method can barely reach the resolution needed to see the minute structures (e.g. pit membrane). To complement the xylem structure with 3D views at the nanoscale level, X-ray near-field nano-holotomography (NFH) was applied to analyze the wood speciesPinus sylvestrisandFagus sylvatica. The demanded small specimens required focused ion beam (FIB) application. The FIB milling, however, influenced the image quality through gallium implantation on the cell-wall surfaces. The measurements indicated that NFH is appropriate for imaging wood at nanometric resolution. With a 26 nm voxel pitch, the structure of the cell-wall surface inPinus sylvestriscould be visualized in genuine detail. In wood ofFagus sylvatica, the structure of a pit pair, including the pit membrane, between two neighboring fibrous cells could be traced tomographically.

Replacing greenhouse gas-intensive building materials with wood products from sustainable forestry contributes to the implementation of current climate conventions such as the Paris Agreement. Hardwood products, such as laminated veneer lumber made of beech (e.g., BauBuche), are an alternative to conventional building materials. For the application of wood products in the construction sector, a precise knowledge of the mechanical and physical properties is essential. Therefore, the aim of the present study was to investigate the sorption behavior and associated dimensional changes of the product BauBuche. This was done by applying a manual testing procedure (climatic chamber, balance and caliper) as well as a dynamic vapor sorption analyzer equipped with a camera. During initial moistening after production, due to the irreversible spring back (approximately 2 mm at 50 mm; i.e., 4%), Baubuche shows an extremely strong swelling in the radial direction. Once the maximum spring back is reached, Baubuche shows sorption behavior in the radial and tangential direction, which is comparable to that of solid beech wood in the radial direction. Consequently, the dimensional changes caused by moisture changes must be taken into account in the dimensioning of Baubuche components in order to avoid damage to building structures.

Formaldehyde-free, bio-based wood modification processes are gaining importance. One well-developed method is modification with citric acid and sorbitol as a copolymer (SorCA). However, some studies report that citric acid (CA) alone can be used for modification. In our previous paper, we showed that although the addition of sorbitol leads to a higher weight percent gain (WPG), the cell wall bulking (CWB), chemical fixation and impact bending strength were only marginally different compared to modification with pure citric acid. This study compares water-related properties, namely (1) dimensional stability and (2) swelling and shrinking behaviour, (3) water vapour uptake, (4) liquid water absorption and release and (5) capillary water uptake (CWU). Modification with pure citric acid resulted in higher dimensional stability compared to modification with SorCA. Dynamic vapour sorption (DVS) measurements showed that the modification with CA leads to a higher reduction in water vapour uptake. Liquid water uptake and release tests on the other hand revealed that, after leaching, SorCA-modified samples exhibited significantly lower water absorption. In summary, this study identified that while the addition of a copolymer has little effect on dimensional stability and water vapour sorption, it exerts a clear positive influence on liquid water absorption.