One-Component polyurethane adhesive filled with milled carbon fibre and its effect on hydrothermal stability of beech-glued joints

The effect of fibres of various lengths, thicknesses, origins, and quantities on mechanical and other properties of polymer matrices is well-known, including carbon fibres. These were used in the form of milled carbon fibres (MCF) to fill a one-component polyurethane (1C-PUR) commercial adhesive to determine hydrothermal resistance of beech-bonded joints tested in shear. The effect of MCF (1%, 3%, 5%, and 7%) was first tested on thin adhesive films before and after exposure to cold water (20°C) and boiling (wet state test and after drying). A good bond of 1C-PUR to MCF was demonstrated, with tensile mechanical properties significantly increased after the addition of 3% and 5% MCF in all tested environments. The aqueous environment, especially boiling water, led to a deterioration in mechanical properties, but after drying, there was a recovery and, in many composite films, an improvement. This was due to a change in adhesive’s domain structure, as demonstrated by differential scanning calorimetry. An increase in hydrothermal resistance of glued wood joints with MCF (1%, 3%, and 5%) was not demonstrated based on tensile shear strength. However, considering the achieved wood failure, it was proven that MCF has a positive effect on stress transfer into the wood adherend.