Sunscreen stability is the discipline that separates a product with a printed SPF from a product that still delivers that SPF in July. Two degradation paths run at once: photodegradation, where UV breaks the filter or the film, and thermal fatigue, where the emulsion separates, the viscosity drifts and the tube stops dispensing a consistent dose.
1. Photostability protocol
Apply the product to a substrate at the labelled rate, irradiate at a defined dose – typically the equivalent of one to two hours of midday summer sun – then measure absorbance or SPF before and after. Avobenzone alone loses a large share of its UVA absorbance within an hour unless it is stabilised with octocrylene, a hindered amine or a dedicated photostabiliser. A formula with unstabilised avobenzone fails on the beach, not in the chamber.
2. What photodegradation looks like
- A drop in UVA-PF of more than 20 percent after irradiation.
- Visible yellowing of a cream that was white at release.
- Odour change, usually described as plastic or rancid, from oxidised esters.
- Loss of film integrity, seen as a patchy residue after washing.
3. Emulsion fatigue
An oil-in-water sunscreen is a thermodynamically unstable system held together by an emulsifier blend. At 40 degrees Celsius the droplets coalesce faster, the emulsifier migrates and the product creeps toward separation. Signs are a watery layer at the tube shoulder, a change in gloss, loss of the uniform matte finish and a viscosity drop above 15 percent. Three months at 40 degrees Celsius is the standard screen; six months at 30 degrees Celsius is the realistic retail condition.
4. Viscosity drift and the dose problem
A sunscreen is formulated to deliver 2 mg per square centimetre at a certain dispensing force. If viscosity falls by a fifth, the same squeeze delivers more product and the tube empties faster, which the customer reads as a smaller fill. If viscosity rises, the product will not come out at all at low temperature. Measure viscosity at 5, 25 and 40 degrees Celsius at every stability pull, with the same spindle and speed – a change of spindle invalidates the comparison.
5. Preservative and microbiological control
Water-based sunscreens need a validated preservation system per ISO 11930, and microbial limits at release per ISO 17516. Sunscreens are used at the beach, left in a hot car and applied with wet hands, so the in-use challenge is harsher than the label assumes. An airless or a tube pack with a small orifice reduces contamination; a jar does not.
6. A test schedule that matches the claim
| Test | Timing | Acceptance |
|---|---|---|
| Photostability | Before launch | Under 20% UVA-PF loss |
| Accelerated stability | 3 months at 40C | No separation, viscosity within 15% |
| Ambient stability | 12 and 24 months | SPF within label tolerance |
| Freeze-thaw | 3 cycles | No phase change |
| Compatibility | 30 days inverted | No interaction with the tube |
7. What to do when it fails
A UVA loss after irradiation is a filter and stabiliser problem; an emulsion split is an emulsifier and thickener problem; a viscosity drift is usually a polymer and neutraliser problem. None of them are fixed by adding more filter. See the sunscreen range and the formulation guide before reformulating.
