Sliding friction tests on photochemically cured acrylate coatings with bio-based versus fossil-based monomers

Bio-based UV-curable acrylate coatings are emerging as sustainable alternatives to conventional petrochemical systems, but their tribological performance remains scarcely studied. This study investigates the sliding friction behaviour of photochemically cured acrylate coatings with alternative bio-based and fossil-based monomers. Friction measurements were performed using a high-precision Basalt N2 tribometer under reciprocating sliding against an elastomeric O-ring simulating a rubber contact against wood coatings. Testing was conducted at room temperature under controlled normal loads, sliding speeds, and multiple measurement locations to account for coating heterogeneity. The coefficient of friction was evaluated as a function of coating monomer composition and crosslinking functionality. Results revealed substantial local variation in friction across the coated surfaces, highlighting the importance of spatially resolved tribological characterization. Therefore, an appropriate tribological testing protocol was developed to account for statistical variation of friction over different locations. Despite this variability, clear differences between coating formulations were observed. In particular, a 20 to 30% higher coefficient of friction was experienced on coatings with bio-based monomers as compared to the fossil-based analogues, while the coefficients of friction gradually decreased for a higher functionality of the acrylate monomer. These findings agree with compositional analysis that demonstrated the presence of a residual oligomer fraction at the surface of bio-based coatings. Moreover, their glass transition temperature is a few degrees lower than that of fossil-based coating analogues. Therefore, the latter phase likely acts as a mobile viscoelastic phase that causes reduced mechanical hardness and greater deformation under load in parallel with enhanced energy dissipation. The observed frictional response is consistent with previously reported improvements in ductility and reductions in abrasive wear for bio-based formulations.