Understanding bluish undertones in black tones

Abstract The study analyzes how the particle size distribution of carbon black influences the undertone in deep black coatings, affecting the perception of the coated object. A blue undertone creates a brilliant appearance, while a brown undertone gives a warm, muddy impression on automotives. The research investigates the particle size distribution required for a blue undertone in a colorless coating system and whether the effect is due to light scattering or absorption. Five carbon blacks with varying particle sizes and aftertreatments were examined in a waterborne polyurethane clearcoat. Results showed that increasing dispersion time reduced particle agglomerates, leading to a bluer undertone. Even the undertone color shifted from brown to blue based on dispersion time. By varying the coatings formulation, more precisely the concentration of the additive that enhances the dispersibility of the carbon black, dispersion time was found to have a greater impact on particle size and undertone, especially with coarser carbon blacks. Scattering and absorption coefficients, calculated using the Kubelka–Munk equation, confirmed that the undertone is primarily a scattering effect. Calculated reflection spectra based on Mie scattering equations demonstrated that particle sizes influenced wavelength-dependent scattering. This allows adjustment of the undertone by controlling the particle size distribution.