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Materials derived from renewable resources are highly desirable in view of more sustainable manufacturing. Among the available natural materials, wood is one of the key candidates, because of its excellent mechanical properties. However, wood and wood-based materials in engineering applications suffer from various restraints, such as dimensional instability upon humidity changes. Several wood modification treatments increase water repellence, but the insertion of hydrophobic polymers can result in a composite material which cannot be considered as renewable anymore. In this study, we report on the grafting of the fully biodegradable poly(ε-caprolactone) (PCL) inside the wood cell walls by Sn(Oct) 2 catalysed ring-opening polymerization (ROP). The presence of polyester chains within the wood cell wall structure is monitored by confocal Raman imaging and spectroscopy as well as scanning electron microscopy. Physical tests reveal that the modified wood is more hydrophobic due to the bulking of the cell wall structure with the polyester chains, which results in a novel fully biodegradable wood material with improved dimensional stability.

A study with synchrotron radiation X-ray tomographic microscopy (SRXTM) of PUR, PVAC, and UF adhesive bond lines in beech wood, bonded under various growth ring angles is presented. After determining the hardening characteristics of the adhesives, we evaluate the bond line morphologies, and the adhesive penetration into the wood structure. We find distinct bond line imperfections for the different adhesive systems. To describe the adhesive distribution inside the bond line we propose the saturation of the pore space instead of the commonly used maximum penetration depth. The results are the basis for a penetration model of hardening fluids into hardwood (part II).

A "grafting-from" polymerization approach within and at the complex and heterogeneous macromolecular assembly of wood cell walls is shown. The approach allows for the implementation of novel functionalities in renewable and functional wood-based materials. The native wood structure is retained and used as a hierarchical multiscale framework for a modular two-step polymerization process. The versatility and potential of the approach is shown by a polymerization of either hydrophobic or hydrophilic and pH-responsive monomers in the wood structure. Characterization of the modified wood reveals the presence of polymer in the cell wall, and the new properties of these wood materials are discussed.

Water transport in wood is vital for the survival of trees. With synchrotron radiation Xray tomographic microscopy (SRXTM), it becomes possible to characterize and quantify the 3D network formed by vessels that are responsible for longitudinal transport. In the present paper, the spatial size dependence of vessels and the organization inside single growth rings in terms of vessel induced porosity was studied by SRXTM. Network characteristics, such as connectivity, were deduced by digital image analysis from the processed tomographic data and related to known complex network topologies.

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Tensile shear tests according to EN 302-1 for load-bearing timber structures were performed on European beech wood (Fagus sylvatica L.) and Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] bonded by means of a one-component polyurethane adhesive (1C PUR). Results reveal a substantial loss of tensile shear strength (TSS) and wood failure percentage (WFP) at the wet stage compared to the dry stage. As can be seen from microscopic images, this is accompanied by a loss of adhesion at the boundary layer. Therefore, the aim of this work was to find a priming fluid that improves the load transmission between adhesive and adherend at the wet stage without introducing formaldehyde into the gluing process. A substantial improvement of TSS and WFP was achieved by means of the hygroscopic organic solvent N,N-dimethylformamide (DMF). In addition, contact angle measurements were carried out, revealing that DMF heavily enhances the wettability of the joining surface. Furthermore, it was attempted to integrate the outcomes into the swelling strain model stated by Frihart in 2009. By way of comparison a hydroxymethylated resorcinol coupling agent, a mixture of diphenylmethane-4,4 0-diisocyanate isomers and water were also tested as priming fluids. The data confirm that TSS and WFP of 1C PUR bonded wooden joints do not correlate, whilst WFP is mostly not normally (at wet stage often bimodally) distributed.

Wood exhibits anisotropic swelling and shrinking upon changes of wood moisture content (MC). By manufacturing bi-layered structures with adapted grain orientation in the two bonded layers, humidity-driven actuators are generated, which have the potential to be used for autonomous climate adaptive building with tile. The present study deals with design principles for upscaling the size of the bilayers and for increasing the rate of MC change and, thus, rate of shape change. Wood bilayers with widths of up to half a meter were subjected to changes of relative humidity (RH). Moisture and curvature changes were recorded. Bilayers with different widths showed curvature exclusively along their length. Next to this, the performance was compared between bilayers with and without milled-in grooves. These grooves lead to shorter diffusion paths along fibre direction for increasing the rate of MC change. The highest rates of MC change were visible for the samples with the smallest width within the first hours after change of RH. Later on, all samples showed similar rates. The milling of grooves increased the moisture change rate substantially compared to the non-milled samples resulting in a higher rate of curvature change. The increase is especially pronounced for cyclic changes of RH. This study shows that, by applying material specific design principles, the shape change of wood bilayers can be adapted and the rate of the MC change can be increased by keeping diffusion paths short along fibre direction. These principles may facilitate the use of large-scale wood bilayers as lamellae in shading systems.