Structure-scratch properties relationships of acrylate photo-polymerizable protective coatings for thermoplastic substrates

•Micro-scratch tests enable a finer differentiation of the coatings than DMTA.•The crosslinking density plays a part in the scratch resistance.•Nanoparticles did not modify the properties of the coating.•Bio-based coatings show a better scratch resistance than petro-based counterparts. The relationships between the composition and the scratch resistance of clear photo-polymerized protective coatings for thermoplastic substrates were studied in relation with their thermomechanical properties. For this purpose, dynamic mechanical analyses of free-standing films were compared to micro-scratch tests of thick or thin coatings deposited on polycarbonate. In these experiments, the depth indented by the tip, the elastic recovery of the material, the residual depth of the scratch, and the load at which the first crack appears, were analyzed. Different coatings close in formulation were studied. First, the proportion of a specific difunctional monomer featuring a hard structure was varied in order to change the crosslinking density of the polymer network. The thermomechanical properties were consequently modified at high temperature, but remained similar at 23°C, whereas at this temperature, the scratch properties of the coating evolved with its composition. The addition of 5wt% alumina or silica nanoparticles did not modify the thermomechanical properties or the scratch resistance of the coatings, even if a more concentrated filler layer was observed near the surface of the coating. Nevertheless, the consequent incorporation of a new diacrylate monomer in the polymer matrix delayed the ductile-brittle transition. Finally the substitution of petro-based monomers by slightly different bio-based compounds led to a change of the scratch behavior of the thickest coatings (150μm-thick), and increased the critical load for the thinnest coatings (15μm-thick). It comes out that micro-scratch tests allow a finer comparison of the samples.