Titanium dioxide (TiO₂) is the most used white pigment for consumer goods of all kinds, its production accounts for more than 90% of the global titanium mineral consumption. In recent years rising raw material costs, labor-intensive extraction with a large carbon footprint, cost-intensive disposal of harmful by-products and growing concerns that nano-sized TiO₂ impairs human health with ensuing policy measures, reinforced the search for alternatives to TiO₂.
In living nature however a bright white color is often achieved by air-filled cavities in a structure of a transparent bio-material e.g. chitin, cellulose or keratin, which ensures the necessary contrast in the refractive index for light scattering. This strategy can be reproduced by introducing small, air-filled cavities into initially transparent polymers like PMMA. With pore sizes in the range of a few hundred nanometers, all wavelengths of visible light are scattered. Foaming with non-toxic, inert CO₂ in its supercritical state can form such pores in a wide range of polymers with high CO₂ solubility. CO₂ thereby acts as a plasticizer, lowering the glass transition temperature T_glass of the polymer in dependence on the pressure. A rapid pressure quench induces the supersaturation with CO₂ and triggers nucleation and pore growth. The process can be adjusted to foam transparent microparticles of PMMA. The formation of a porous core with light scattering nanopores causes the particle to appear perfectly white to be used in paint formulations and coatings. However, the untreated PMMA microspheres loose CO₂ during foaming in the pressure release step, resulting in a several micrometer thick unfoamed layer. To reduce this effect, and thus to enable foaming of smaller particles, a diffusion barrier like water-soluble PVA, can be used. An important future step will be the foaming of bio-based and biodegradable polymers, which allows further application possibilities in cosmetics or paper industry. Promising candidates are PLA or cellulose acetate, but it can be expected that many more biopolymers are suitable for the formation of nano-pores by foaming with supercritical CO₂.