Zinc Oxide — Frequently Asked Questions

Because the accelerators do not work properly without it. Sulphur crosslinking of rubber is far too slow to be commercial on its own, and accelerators speed it up, but they are themselves largely ineffective without an activator. Zinc oxide, usually with stearic acid to form soluble zinc stearate in situ, supplies zinc ions that complex with the accelerator and dramatically increase the rate of crosslink formation.

Because it works by sitting on the skin and interacting with light directly, absorbing across UVB and UVA and scattering visible light, rather than being absorbed into the skin and converting UV energy chemically. That makes it broad spectrum, photostable and one of very few filters approved in essentially every market, and it is why it is favoured for sensitive skin and infant products.

The French or indirect process vaporises refined zinc metal and oxidises the vapour, giving a high-purity product with controlled particle size, which is what rubber, pharmaceutical and cosmetic applications require. The American or direct process reduces zinc ore in situ and oxidises the vapour, giving a cheaper material with more impurities, used where purity matters less.

Because surface area drives performance. In rubber, finer grades disperse better and activate more efficiently. In sunscreen, particle size governs the balance between UV protection and visible whiteness, with micronised grades reducing the white cast. In ceramics and varistors, it affects sintering. Grade selection therefore depends as much on particle size and surface area as on assay.