In many industrial applications, titanium dioxide (TiO₂) is the pigment of choice when a bright white color impression is required. The effect is based on the high refractive index of TiO₂ leading to a hight contrast to the embedding medium. However, sustainability [1] and health concerns [2] on TiO₂ led to a continuous controversial debate of its usage and recently first bans were introduced in the EU. In nature, on the other hand, a bright white color impression is frequently based on sophisticated nanostructures such as porous networks [3], ellipsoidal beads [4] or air voids [5]. The later can be mimicked by foaming polymers with supercritical CO₂ (scCO₂) resulting in light scattering air voids in the nanometer range in thin films [6] and particles [7]. In many applications, however, the particle size is a crucial factor. Thus, the development towards smaller, easily integrable and polymer-based pigment alternatives is the aim of our study. For that, the utilization of a diffusion barrier in a variation of the supercritical CO₂ foaming process is a promising strategy. Thereby the particles are surrounded by a diffusion barrier during the foaming process, which hinders the fast escape of CO₂ during the essential depressurization step. Thus, more CO₂ is entrapped in the polymer particle and contributes to the formation of light scattering pores. With this modification particles of only 20 µm diameter can be foamed, by variation of the diffusion barrier material and the process parameters the particles can be further optimized for high reflectance.

References
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