Adhesives on the basis of urea‐formaldehyde (UF) and melamine‐urea‐formaldehyde (MUF) are extensively used in the production of wood‐based panels. In the present study, the attempt was made to improve the mechanical board properties by reinforcing these adhesives with cellulose nanofibers (CNFs). The latter were produced from dissolving grade beech pulp by a mechanical homogenization process. Adhesive mixtures with a CNF content of 0, 1, and 3 wt% based on solid resin were prepared by mixing an aqueous CNF suspension with UF and MUF adhesives. Laboratory‐scale particle boards and oriented strand boards (OSBs) were produced, and the mechanical and fracture mechanical properties were investigated. Particle boards prepared with UF containing 1 wt% CNF showed a reduced thickness swelling and better internal bond and bending strength than boards produced with pure UF. The reinforcing effect of CNF was even more obvious for OSB where a significant improvement of strength properties of 16% was found. For both, particle board and OSB, mode I fracture energy and fracture toughness were the parameters with the greatest improvement indicating that the adhesive bonds were markedly toughened by the CNF addition.

Cellulose nanofibrils were prepared by mechanical fibrillation of never-dried beech pulp and bacterial cellulose. To facilitate the separation of individual fibrils, one part of the wood pulp was surface-carboxylated by a catalytic oxidation using (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) as a catalyst. After fibrillation by a high pressure homogenizer, the obtained aqueous fibril dispersions were directly mixed with different urea–formaldehyde-(UF)-adhesives. To investigate the effect of added cellulose filler on the fracture mechanical properties of wood adhesive bonds, double cantilever beam specimens were prepared from spruce wood. While the highest fracture energy values were observed for UF-bonds filled with untreated nanofibrils prepared from wood pulp, bonds filled with TEMPO-oxidized fibrils showed less satisfying performance. It is proposed that UF-adhesive bonds can be significantly toughened by the addition of only small amounts of cellulose nanofibrils. Thereby, the optimum filler content is largely depending on the adhesive and type of cellulose filler used.

The here-introduced concept of the wet esterification enables direct modification of never-dried cellulose fibres in aqueous media and is a straight-forward and sustainable avenue towards surface-acetylated cellulose nanofibers.

Regarding mechanics, wood is one of the most efficient materials available. Due to its unique combination of composite and cellular microstructure, it exhibits superb specific mechanical properties which exceed many man-made materials. However, concerning absolute mechanical properties, wood is often inferior to metals and novel engineered composites. Since many wood properties are strongly correlated with its density, densification is a promising pathway towards improved absolute mechanics. Spruce, beech and poplar wood were densified in a two-step process. First, amorphous wood polymers were partially extracted according to an alkaline (AL) and an organosolv (OS) protocol. Subsequently, partially delignified veneers were densified by hot pressing in tangential direction. After densification, average densities increased to 1.00–1.20 g cm⁻³. FTIR analysis confirmed chemical changes, mostly in the bands attributed to hemicelluloses and lignin, of chemically treated and hot-pressed veneers. To evaluate the modification process regarding mechanics, tensile and bending properties were characterized and revealed promising results. Compared to untreated control specimens, stiffness and strength tested in tension and bending improved, regardless of wood species and pretreatment. Regarding average tensile properties, the OS treatment improved stiffness, up to 40 GPa, whereas the AL treatment improved strength, up to 300 MPa. Set-recovery tests showed, that chemically treated and densified samples exhibited a better dimensional stability compared to H₂O-soaked and compressed specimens. However, 24 h water soaking resulted in excessive thickness swelling. This might be a major drawback of partially delignified and densified wood and should be considered in a material selection process.

The mechanical reinforcement of porous materials made of microfibrillated cellulose by in situ polymerisation of furfuryl alcohol prior to freeze-drying from aqueous slurry was studied. Besides a slight improvement in the modulus of elasticity measured in compression testing, no beneficial effects of furfuryl alcohol addition on porous materials produced from microfibrillated cellulose derived from bleached softwood pulp were observed. By contrast, when microfibrillated cellulose containing substantial amounts of residual non-cellulosic cell wall polymers, termed microfibrillated lignocellulose, was used, clear mechanical reinforcement effects due to furfuryl alcohol addition were measured. By means of SEM, significantly improved wetting of the cellulosic fibrillary architecture of porous materials with furfuryl alcohol was observed for microfibrillated lignocellulose compared to microfibrillated cellulose. It is proposed that the specific surface-chemical character of microfibrillated lignocellulose enables wetting of fibrils with furfuryl alcohol, thus providing micron fibre-reinforced structures with improved strength after in situ polymerisation. Besides mechanical properties, the density and thermal stability of cellulose-based porous materials were found to increase with increasing amounts of furfuryl alcohol added to the initial reaction slurry.