Mode I cohesive law of birch wood-biobased adhesive systems

The development and assessment of bio-based wood adhesives face challenges due to the limitations of conventional test procedures, particularly in predicting adhesive performance in real-world applications. This study investigated the mode I cohesive law of two birch wood-biobased adhesive systems, comparing them with conventional fossil-fuel-based systems. The experimental approach, developed during the ‘90s at Lund University, involves direct measurement of the traction-separation relation and allows evaluation of post-peak behaviour and fracture mechanical property characterisation. SEM analysis confirmed fracture development within the bond line for all tests. The birch–fish-adhesive bonds demonstrated peak stress of approx. 4 MPa, with serrated fracture surfaces and favourable fracture properties (specific fracture energy approx. 370 Nm/m2, brittleness approx. 40 GPa/m). Birch wood bonded with lignin-based adhesive showed lower, yet reasonable, levels of peak stress (approx. 3 MPa) but less favourable fracture properties (specific fracture energy approx. 110 Nm/m2, brittleness approx. 90 GPa/m), suggesting room for further optimisation.