1. The more common white pigment titanium white is not good for cooling something exposed to sun, because it strongly absorbs ultraviolet light. (Besides generating heat, this ultraviolet absorption also causes photo-chemical reactions that degrade in time the organic paint binders.)
2. So they have chosen barium sulfate, which reflects not only visible light, but also ultraviolet and near infrared.
3. Barium sulfate not only reflects solar light, but it also emits efficiently in the far infrared, which cools the painted surface.
4. Barium sulfate was traditionally considered a worse white pigment, because it has a lower refractive index than other pigments, like titanium white.
5. The Purdue University team has overcome this disadvantage of barium sulfate by designing a manufacturing process for the pigment that not only ensures a very small average pigment particle size but also a broad distribution of the particle sizes, which results in good reflection properties over the entire solar spectrum.
https://arxiv.org/abs/2011.01161
The main points are:
1. The more common white pigment titanium white is not good for cooling something exposed to sun, because it strongly absorbs ultraviolet light. (Besides generating heat, this ultraviolet absorption also causes photo-chemical reactions that degrade in time the organic paint binders.)
2. So they have chosen barium sulfate, which reflects not only visible light, but also ultraviolet and near infrared.
3. Barium sulfate not only reflects solar light, but it also emits efficiently in the far infrared, which cools the painted surface.
4. Barium sulfate was traditionally considered a worse white pigment, because it has a lower refractive index than other pigments, like titanium white.
5. The Purdue University team has overcome this disadvantage of barium sulfate by designing a manufacturing process for the pigment that not only ensures a very small average pigment particle size but also a broad distribution of the particle sizes, which results in good reflection properties over the entire solar spectrum.