The thermal conductivity of wood fiber insulation boards is significantly influenced by the microstructure of the fiber network and in general, the efficiency of wood fiber insulation boards increases with porosity. For higher raw densities, the raw density is a good predictor for the thermal conductivity. For lower raw densities however, this simple relation does not hold anymore. Here, structural information gained from 3D computed tomography images at several scales, modeling of the microstructure, and numerical simulation of the thermal conductivity are combined to get deeper insight into which and how microstructural features influence the thermal conductivity. The model-based simulation as described here shows that the presence and orientation of wood fiber clusters impact the thermal conductivity significantly.

Gravimetric vapor sorption experiments were performed on beech wood samples to determine the directional permeability, diffusion and sorption coefficients in the three orthotropic wood directions. Dynamic Vapor Sorption (DVS) experiments allowed for the direct evaluation of the diffusion coefficient from the analysis of the kinetic sorption profile using a double stretched exponential model with values ranging from 0.10 × 10⁻¹⁰ to 1.52 × 10⁻¹⁰ m²/s and depending on the wood direction of the sample and the RH-values. Moisture sorption isotherms (MSIs) were constructed and fitted to a modified Guggenheim-Anderson-de Boer and a Sorption Site Occupancy model, which allowed for the calculation of the sorption coefficient which was found to be between 2.4 and 3.0 mol/(m³ Pa). Dynamic Vapor Transport (DVT) experiments were performed to calculate the permeability coefficient from the vapor flow rate and it ranges between 0.56 × 10⁻¹⁰ and 4.38 × 10⁻¹⁰ mol/(m s Pa) as a function of the flow direction and RH conditions. These results indicate that such an experimental approach is suitable for determining wood–moisture interactions.

Today, using one-component polyurethane (1c-PUR) adhesives in the manufacturing of engineered wood products from spruce is common practice. However, the use of other wood species can require the application of a primer to fulfill normative requirements. Previous research shows the primers’ effectiveness, especially in moist environments. However, the primers’ exact mode of action remains not yet fully understood. We hypothesize a reduction in the hygroscopic behavior of the primer-treated wood—intensity and kinetics—that could reduce the formation of stresses in the bond line region. To test this hypothesis, two commercially available primers, based on Polysorbate 20 and poly(ethylene glycol), and the hydroxymethylated resorcinol (HMR) primer are examined with wood from beech, birch, larch, and Douglas fir. Swelling experiments show that of each primer a portion infiltrates and swells the wood cell walls, affecting the wood’s hygroscopic and mechanical properties. In stepwise sorption experiments, it is seen that the primers influence differently the amount of moisture uptaken by the wood (adsorption). The rate at which the moisture spreads within the wood (diffusivity) also changed differently for the primers, while the rate at which the moisture moves through the wood (permeability) remains unchanged. The application of all primers improves the bulk flow behavior and thus the void penetration of the adhesive into the lumina in the interphase region, which in turn leads to a reduced bond line thickness. All three primers improve the tensile shear strength. The hygroscopic changes caused by the primers appear too small to be claimed as the sole and primary cause of their functionality, whereas more relevance is seen in the primers’ cell wall infiltration and the increased adhesive’s void penetration.

When producing load-bearing timber products with one-component polyurethane adhesives and certain wood species, e.g., beech or larch, primers can be used to obtain stronger and more durable bonds. The active chemicals of three existing primer systems, hydroxymethylated resorcinol formaldehyde (HMR), polysorbate 20 (PS20), and poly(ethylene glycol) (PEG), were used to impregnate beech wood. Young’s modulus (E) in the radial direction was determined in tensile mode. A modified E can influence the deformation and stress distribution at the wood-glue interface. Compared to the untreated wood, the average E was reduced by 7% for the water-treated reference and the HMR treatment. With the PS20 treatment, the average E was reduced by 16%, compared to the untreated wood, and by 45% with the PEG treatment.

For the utilization of silver birch (Betula pendula Roth) in load-bearing engineered wood products (EWPs), reliable bonding in production is a prerequisite. The current knowledge regarding the bonding of birch in EWP applications is limited. Extractives are considered a general factor of attention when securing bonding quality. Thus, in this study, the effects of hydrophilic extractives on several adhesion-related bulk and surface properties of silver birch wood were studied, e.g., vapor sorption, swelling behavior, microstructure, wettability, and mechanical properties. The extraction procedure slightly affected vapor sorption causing a reduction in swelling pressure. The extraction also led to a lower Young's modulus, as seen by compression tests. Control experiments with vapor-treated specimens, however, indicated that the effects were originating from the water imbibition and not due to the removal of extractives per se. This was supported by X-ray diffraction results, which were similarly affected by both vapor and extraction treatment. Therefore, the results indicate that the hygric history of the specimens was affecting the wood due to plasticization, increasing mobility, and thereby likely allowing biopolymer reconfiguration and subsequent quenching during re-drying, even though surface-free energy and wettability were not considerably affected. The extent to which these changes appear permanently or temporarily remains an open research question.