Chelating Agents for Sunscreen Photostability and SPF

Sun care is the one cosmetic category where the label makes a number-based promise. A product declared at SPF 30 should deliver something close to that on the skin, after storage, after a warm warehouse, and after the tube has been open for a season. Trace metals are one of the quieter reasons a formula drifts away from that promise, and a chelating agent is one of the cheaper ways to slow the drift.

Why iron and copper matter more in sun care than in other cosmetics

Most cosmetic formulas tolerate a little iron. A lotion or a cream carries it through the shelf life without much visible consequence, because nothing in the product is actively trying to react with it.

A sunscreen is different in one specific way: it is designed to absorb ultraviolet light and convert that energy into heat. The energy is present in quantity, for hours, on skin. Any transition metal in the film has a continuous supply of activation energy to work with, and iron and copper are both competent at catalysing radical reactions once they are in the right oxidation state.

The metals arrive from raw materials rather than from the tap. Iron at a few parts per million is common in mineral thickeners, clays, pigment dispersions, some emulsifiers and even in water used for the aqueous phase. Copper can come from plant-derived actives and from processing equipment. Neither is a defect in the supplier's material at those levels, and both are enough to matter in a formula that spends its working life in sunlight.

Photostability of avobenzone and the metal pathway

Avobenzone, the most widely used UVA filter in Europe, is the filter most often discussed in this context, and the discussion is usually about its intrinsic photolability. It absorbs strongly across the UVA range and, under irradiation, moves through a ketone-enol equilibrium that opens degradation routes. Formulators stabilise it with other filters, with antioxidants, and with careful pH control.

The metal pathway runs alongside. Iron and copper catalyse the breakdown of hydroperoxides that form in the oil phase during irradiation, and the resulting radicals attack the filter and the emollients around it. The practical consequence is that two formulas built on the same filter package can behave differently after a season in a warm warehouse, and the difference often tracks the trace metal content of the raw materials rather than anything visible on the specification sheet.

A chelant does not fix intrinsic photolability. It removes the catalytic accelerant, which is the part a formulator can actually control.

Colour drift and the complaint that follows

The visible symptom is a formula that leaves the factory pale and arrives slightly beige. In tinted products the shift is worse, because the pigment system and the filter system both respond to the same radical chemistry.

Consumers read that shift as spoilage even when the UV performance is intact. In a category where repurchase depends on trust, a visible colour change is expensive out of proportion to the chemical change behind it. Iron chelation removes the most common driver, and it does so without touching the emulsifier or the film former.

Where the loss shows in testing

SPF is measured in vivo to ISO 24444, and the UVA protection factor to ISO 24443 in vitro. Neither method is designed to catch a slow photodegradation that happens during use, and both are run on freshly prepared samples.

The gap is filled by the stability programme: an accelerated storage study, an irradiation study on the finished product, and a test on the product after simulated use. In those, a formula with unmanaged trace metal typically shows a fall in the UVA protection factor, a change in the absorption spectrum near 350 to 400 nanometres, and a rise in peroxide value in the oil phase. A chelant present at a low dose moderates all three.

For the regulatory file under the EU Cosmetics Regulation, the product information file needs evidence that the claimed protection holds through the declared shelf life. Trace metal control is part of that evidence, and the documentation is easier to assemble when the chelant was specified from the start rather than added after a failed stability run.

Mineral filters and the photocatalysis question

Zinc oxide and titanium dioxide are semiconductors. Under UV, they generate electron-hole pairs that produce radicals at the particle surface, and that is why they are used in self-cleaning coatings as well as in sunscreens. In a sunscreen the effect is moderated by coating the particles and by dispersing them well.

A chelant does not change the semiconductor physics, and it would be dishonest to suggest otherwise. What it can do is intercept the metal ions that the particle surface releases into the surrounding oil or water phase, and reduce the secondary damage that dissolved ions cause to the organic filters nearby. In a hybrid formula that combines mineral and organic filters, that secondary route is worth controlling.

Choosing a chelant for a leave-on product

Leave-on sun care sets a different bar from a rinse-off detergent. The dose is low, the product stays on skin for hours, and the ingredient appears on the label in a category where consumers read them.

Four practical criteria apply. The agent should be effective at low dose, typically a few hundredths of a per cent, because adding more destabilises the emulsion and raises the electrolyte load. It should carry an established INCI name and a documented safety assessment, since it becomes part of the cosmetic product safety report. It should be compatible with the emulsifier system, which in practice means adding it to the water phase before emulsification. And it should not introduce its own colour or odour, which rules out technical grades with residual iron.

The biodegradable agents in the GLDA, MGDA and IDS families are water-soluble sodium salts and behave predictably in an oil-in-water emulsion at low dose. Their INCI entries are established, and their documentation is familiar to European safety assessors, which shortens the review.

Compatibility with filters, emulsifiers and film formers

Most organic UV filters are oil-soluble and sit in the internal phase, so direct interaction with a water-phase chelant is limited. The interactions that do occur are at the interface: the chelant changes the ionic environment, which can shift emulsifier performance, and it can affect the swelling of some thickeners.

Film formers are the ingredient class worth checking. Many are acrylic or urethane dispersions whose stability depends on pH and electrolyte, and adding an ionic chelant to a formula that already sits near a stability boundary can push it over. A simple bench check, adding the chelant at twice the intended dose and watching for viscosity change and particle size drift over a week, catches most of these before they reach a batch record.

Specifications and documentation for European supply

For a sun care brand the document set matters as much as the chemistry: INCI name and CAS number, REACH registration status, specification with assay and trace metal limits, safety data sheet, microbial limits, and a statement on cosmetic suitability. Where the brand makes a biodegradability or reef-related claim, the OECD 301 evidence should be available before the claim is drafted rather than after.

Ask also for a batch certificate rather than a generic specification. Trace metal content varies between lots, and the certificate is the only document that tells you what was actually in the drum that went into the batch.

Frequently asked questions

Will a chelant raise the SPF number? No. It slows the loss of protection during storage and use. It does not add absorbance.

How much is needed in a sunscreen? Usually a small fraction of a per cent. The aim is to bind trace metal, not to complex the whole formulation.

Can it be used in a mineral-only sunscreen? It can, to control ions released from the particle surface and from raw materials. It does not change the photocatalytic behaviour of the particles themselves.

Is it acceptable in a natural-certified formula? It depends on the standard. Some allow biodegradable synthetic chelants, others restrict them, and the certification body's list is the reference rather than a general claim.

Does it affect the preservative system? It usually helps, by removing the metal that inactivates some preservatives, but the challenge test still has to be run on the final formula.

For a sample to run through your own photostability protocol, or a documentation pack for the product information file, write to [email protected] or call +86-537-3739818. The biodegradable chelating agent range is listed at yuanlianchem.com, with formulation notes at yuanlianchelating.com.