I’m surprised I haven’t seen this before. I’m one of the few researchers out there who have worked with these materials to lower solar gain in commercial building applications. My research similarly landed on 8 micron diameter glass microspheres as the ideal diameter for reducing solar gain. We ran into two problems that ultimately prevented us from commercializing the research:
-
We got the best solar reflectance with high volume fraction of spheres to acrylic binder in the coating formulations. The concentration of spheres was so high that we were ‘crowding out’ the plastic resin in the coating, which significantly affected the mechanical properties (such as tensile strength, abrasion resistance, elongation, and the like). We even investigated certain chemical additives, such as zinc oxide nanoparticles and silane coatings applied to the microspheres to promote enhanced mechanical properties.
-
We got the best results with hollow microspheres which were on the order of 8 microns in diameter. The problem with manufacturing these spheres is that we end up with a particle size distribution, where the majority of spheres are approximately 7-9 microns in diameter, however a fair amount (>20%) are under 4 microns and do not contribute to enhanced reflectivity. We would sink these out by floating the spheres in water before adding them to the coating, which wasted a lot of material. It may be possible for the manufacturer to optimize the manufacturing process to result in more of the spheres falling in the target size range.
We also tried using diatomaceous earth, which are fossilized plankton organisms with a glassy amorphous silicateous microstructure, some of which are in the correct particle size range for this to work. These materials did exhibit high infrared reflectance when incorporated in to polymer coatings, however the challenge was finding the right grades of diatom materials from various countries that also exhibited good visible light reflectance (or transmission for this matter). A lot of these materials tend to have a sandy color in the visible spectrum.
If anyone is interested in collaborating on this, I am available to discuss this topic further. My previous research was funded by the EPA and sponsored by Dow Chemical (maker of acrylic coating raw materials) and PQ Corporation (maker of glass microspheres).
Finally, here is a scanning electron microscope image of one of our 8 micron glass hollow spheres in some acrylic binder goop (left) and some similarly sized diatomaceous earth particles in a similar acrylic coating, colorized for fun.
