Secondary particle size is the result of a heat induced process that re-bonds lots of small particles into larger groups.
Sun Balm’s Zinc Oxide is made up of primary Nano Particles in the tens of nanometers range that form aggregates (clusters of nanoparticles with very strong bonds) that make up a nano porous structure with a secondary particle a few microns to tens of microns in size. Larger aggregates have a unique porous structure that provides a closer match between the refractive index of the particle and the refractive index of the emollient, resulting in high transparency.
In other words, for our zinc oxide, the result is that a bunch of nano-sized primary particles turn into much larger, strongly bonded micron-sized aggregates as a secondary particle. This allows the zinc to do its job, sit on the surface of the skin and yet not be white paste.
The unique aggregated porous structure provides both high UVB SPF and UVA protection.
It is this primary particle size that constitutes the requirement to label our Zinc Oxide (nano)
Our zinc has been tested for photo stability and these strong aggregates are stable under UV radiation.
Our Zinc Oxide was also tested using electron microscopy and obtained the following typical images. The clumps that you see are the secondary particles. They have a unique porous structure as shown in the SEM image below.