Porous polymeric microparticles foamed with supercritical CO2as scattering white pigments.

Nowadays, titanium dioxide (TiO2) is the most commercially relevant white pigment. Nonetheless, it is widely criticized due to its energy-intensive extraction and costly disposal of harmful by-products. Furthermore, recent studies discuss its potential harm for the environment and the human health. Environment-friendly strategies for the replacement of TiO2as a white pigment can be inspired from nature. Here whiteness often originates from broadband light scattering air cavities embedded in materials with refractive indices much lower than that of TiO2. Such natural prototypes can be mimicked by introducing air-filled nano-scale cavities into commonly used polymers. Here, we demonstrate the foaming of initially transparent poly(methyl methacrylate) (PMMA) microspheres with non-toxic, inert, supercritical CO2. The properties of the foamed, white polymeric pigments with light scattering nano-pores are evaluated as possible replacement for TiO2pigments. For that, the inner foam structure of the particles was imaged by phase-contrast x-ray nano-computed tomography (nano-CT), the optical properties were evaluated via spectroscopic measurements, and the mechanical stability was examined by micro compression experiments. Adding a diffusion barrier surrounding the PMMA particles during foaming allows to extend the foaming process towards smaller particles. Finally, we present a basic white paint prototype as exemplary application.

Analysis of the optical properties of the silvery spots on the wings of the Gulf Fritillary, Dione vanillae.

The ventral face of the wings of the butterfly Dione vanillae is covered with bright and shiny silvery spots. These areas contain densely packed ground- and coverscales with a bright metallic appearance reflecting more than 50% of light uniformly over the visible range. Our analysis shows that this optically attractive feature is caused by the inner microstructure of the scales located in these areas. Electron microscopy of cross sections through the scales shows that upper and lower lamina, supporting trabeculae, and topping ridges can be approximated by a 'circus tent'-like geometry. By simulating its optical properties, we show that a moderate disorder of this geometry is important for the uniform reflection of light resulting in the silvery appearance.