The importance of rheological characterisation of innovative paint formulations for autonomous vehicles

It is critical for paint formulators to create products that have the proper flow properties, as well as delivering the desired colour, to ensure trouble-free application and consistent film formation. Recent efforts for the coloration of robust paint systems suitable for autonomous vehicles with efficient Light Detection and Ranging (LiDAR) perception pose new challenges for formulators, as pigmentation (type and quantity of pigments in use) is radically changed to colour-match conventionally formulated finishes. Solvent-borne acrylic topcoat paints containing traditional dark pigment dispersions typically exhibit low reflectance at the wavelength of 905 nm, which is the operating wavelength for LiDAR technology, and this feature needs to be improved to avoid collisions in traffic. In this study, three solid dark colours from the Reichsausschuss f & uuml;r Lieferbedingungen (RAL) 841-GL colour chart-RAL 5015, RAL 5017 and RAL 9011-were reformulated in a solvent-borne acrylic topcoat system for passenger vehicles, aiming to enhance reflectivity at 905 nm through the incorporation of dispersions containing CoAl2O4 and (Cr, Fe)2O3 complex inorganic colour pigments and coarse titanium dioxide. The rheological properties of colour-matched pairs of innovative and conventional paint formulations were compared. The thixotropic behaviour of the formulations was determined through 3-interval-time tests and the viscoelastic characteristics were revealed by amplitude sweep tests. The correlation between shear stress and shear rate was clarified using mathematical models. The shear-thinning behaviour of both conventional and innovative formulations for RAL 5015 and RAL 5017 was best described by the Herschel-Bulkley model, whereas the behaviour of RAL 9011 was best described by the Casson model.