Understanding bluish undertone in black tones

 
This work is to analyze the influence of the particle size of carbon black on the undertone in deep black coatings. The undertone determines the perception of the coated object. A blue undertone makes the object look brilliant; and a brown undertone leads to a warm color impression. Furthermore, this raises the question of how small the particles must be in the overall particle size distribution for the undertone, of the otherwise colorless coatings system, to appear blue. It is also important to clarify whether the phenomenon is an effect of light scattering or light absorption. The relationship between particle size and light scattering and/or absorption has already been calculated by Gustav Mie. It is therefore necessary to investigate whether the theory can also be applied to the behavior of carbon black. To answer this question, five carbon blacks, which differ in their particle size and aftertreatment, are examined in a waterborne polyurethane clearcoat system. Consideration of the influence of the dispersion time on the particle size distributions and the resulting undertones revealed that the agglomerates of the carbon blacks are continuously reduced in size as the dispersion time increases. The finer particles in the final coating film caused a bluer undertone. It also turned out that even the color of the undertone can be changed from brown to blue based on the dispersion time only and the resulting particle size distribution. In a further investigation, where the dispersing additive concentration was also changed in addition to the dispersion time, further findings were obtained. It was shown that for the dispersing additive concentrations considered, the dispersion time has a greater effect on the particle size and the undertone. This was particularly evident in the coarsest carbon black. In the fine carbon blacks, a difference was observed due to the aftertreatment. While a higher dispersing additive concentration still led to smaller particles and a bluer undertone in the non-aftertreated carbon black, the opposite was the case in the aftertreated carbon black. To clarify the question of whether the undertone is a scattering or absorption effect, the scattering and absorption coefficients were calculated using the Kubelka-Munk color strength equation and plotted against the particle sizes. This showed good agreement with the data from the literature, and it was possible to determine values for the particle size distribution from which short-wavelength light is preferentially scattered, resulting in a blue undertone. Further calculations were carried out to confirm that the undertone is a scattering effect. For this purpose, reflection spectra were generated from the particle size distributions determined using the Mie scattering equations. These clearly showed that depending on the particle sizes in the distributions, the scattering changes wavelength-dependently. On this basis, it is possible to achieve a specific scattering of the incoming light via the particle size distribution of the carbon black and to adjust the undertone if the rest of the coating system does not make a significant contribution to the coloring.