How do I test whether a mineral sunscreen will oxidise in the bottle?

Accelerated light exposure plus peroxide value. The coating specification tells you the risk; the test tells you the outcome.

The mechanism: nano TiO2 and ZnO generate electron-hole pairs under UV, which produce radicals that attack oils, fragrance and actives. You see it as shade shift, off odour and viscosity drift, and it often appears only after weeks on a shelf.

Test it directly. Expose the finished product under xenon or UV for 24–72 hours, then read ΔE, peroxide value and odour against an unexposed control. A parallel 40°C sample catches the non-photochemical side.

Choose the coating with this in mind. Alumina with stearic acid and silica with triethoxycaprylylsilane are not equivalent here; the silica composite grades are the ones to look at first when stability is the constraint, and our range is specified by coating type so you can select on it.

Assume the coating reduces photocatalysis rather than removing it. At high solids and long exposure something will still move, so build an antioxidant system into the formula and check metal ions, since iron and copper catalyse the same chemistry.