Sunscreen Formulation FAQ

White cast is scattering; grey cast is usually hue. They take different fixes, so diagnose before reformulating.

White cast comes from visible-light scattering by particles whose size is comparable to the wavelength. Reduce it three ways: a smaller primary particle — 15 nm scatters less visible light — a lower loading, or better dispersion. Dispersion is the one most often skipped: an agglomerate behaves like a much larger particle, scatters hard and scatters unevenly, so a badly dispersed batch looks whiter than the same formula dispersed properly.

Grey cast on skin is a hue problem. The film scatters and the skin tone underneath shows through unevenly, so the result reads ashy. The standard answer is a trace of iron oxide — yellow and red, at fractions of a percent — to bring the film back toward skin tone, which is what most Japanese sunscreens do.

Two things to check before reformulating: that the dispersion really is good, measured by fineness and microscope, and that the assessment was made on a range of skin tones rather than one panelist.

Usually the interface, not either product on its own.

A mineral sunscreen built on a W/O emulsion leaves a film that a water-based foundation then sits on. The two films shear against each other as they are rubbed, and the top one rolls up. The same sunscreen alone can be perfectly fine.

Three contributors. Film-former incompatibility across the two products is the common one. Too high a solids load with too little elastomer or oil to bind it leaves loose powder at the surface, which pills readily. And over-rubbing during application sets the film and then breaks it up.

Fix from both sides: reduce solids or add a film former on the sunscreen side, add a small amount of spherical filler or boron nitride for slip so less rubbing is needed, and on the foundation side avoid a film former that is incompatible with the one underneath.

Test the way customers use it — full skincare routine, then sunscreen, then base, on the same panel at a fixed dose. A single-product test will not reproduce it, which is why this complaint arrives from the field rather than the lab.

It rises, then flattens, and past the knee it can fall. Do not size the loading from a linear calculation.

More filter means more shielding per unit area, so SPF climbs. Then the marginal return drops away: particles pack, dispersion degrades, the film stops being uniform, and efficiency falls back. Meanwhile the cost shows up as white cast, heavy feel and instability.

Particle size does not move it monotonically either — different sizes are efficient in different bands, so equal loadings at different sizes give clearly different SPF. Dispersion matters more than either: agglomerates cut the effective shielding area and land you well below theory.

Build a curve instead of a point: 5%, 10%, 15% at fixed everything else, measured in vitro, and read the knee off the graph. Keep film thickness and substrate consistent with real use, or the numbers will not transfer.

If the knee sits below the SPF you need with mineral filters alone, the honest options are a smaller particle size, a ZnO and TiO2 combination that covers UVA separately, or a small amount of organic filter — each with its own consequences for what you can claim.

All of it, by mass. A pre-dispersion is a raw material that brings oil with it, and that oil is in the formula whether or not it was counted.

Our dispersions come in several carriers: C12-15 alkyl benzoate (NanoT 4015, 4513, Z-6060), dicaprylyl carbonate (NanoT 4528) and dimethicone (NanoT 2355, Z-6050). Each one lands in the oil phase at the dispersion's carrier fraction.

The arithmetic: work out the filter solids you need, divide by the dispersion's solids content to get the dispersion weight, then subtract the carrier mass from the oil phase. Skipping that step is the usual reason a converted formula comes out greasier and less stable than the powder version it replaced.

Two checks afterwards. Compatibility: the carrier has to be miscible with the main oil phase or it will separate. And skin feel: carriers differ in spread and residue, so the swap is felt even when the numbers match.

Where the formula already uses an ester, choosing a dispersion in the same carrier removes most of the rebalancing. Where it does not, pick a compatible carrier and re-run stability rather than assuming.

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.

Powder form, oil structure, film former — in that order. Cutting the filter load is the last resort, because it costs protection.

Powder first. A pre-dispersion spreads more evenly than powder dispersed in situ and removes the agglomerates that read as drag. A treated grade — silane or silicone — disperses better in the oil phase and lowers oil absorption, so viscosity drops and the film feels lighter at the same solids.

Then the oil phase: less high-viscosity oil, more light esters such as C12-15 alkyl benzoate or dicaprylyl carbonate. Dimethicone gives slip but reads as a film if overused.

Then the film former, which is what actually decides how occlusive the product feels. Choose one with good breathability and an appropriate glass transition rather than the thickest film available.

Only then consider lowering total solids, and if you do, replace the lost UVB with ZnO for UVA or a small amount of organic filter — both change what you can claim. Judge the result on a sensory panel scoring spread, tack, residue and shine; instrument data will not tell you whether people will wear it.

They settle, because the particles are denser than the carrier. That is physics; the job is to keep it redispersible and to dose by solids.

How fast depends on three things: particle size, since agglomerates settle quickly; carrier viscosity, since thicker settles more slowly; and storage temperature, since heat thins the carrier and speeds it up. A well-dispersed nano grade settles slowly into a loose layer that stirs back in. A flocculated one settles into a hard cake.

Store cool, avoid heat and freeze-thaw, use first in first out, and stir with a paddle rather than shaking. Confirm by sampling top, middle and bottom for solids content before the batch is charged.

Consistency comes from three habits: verify the drum is homogeneous before weighing; dose by effective filter solids rather than by dispersion weight; and retain a standard sample for colour and fineness comparison.

If the drum has hard lumps that survive stirring, the flocculation is irreversible. Do not use it — the consequence is lower protection and more white cast, and it will not show up until the batch is finished.

The inhalation route is the whole question, and it has to be settled before the formula is developed, not before launch.

A spray creates an inhalable aerosol. Nano TiO2 or ZnO reaching the lung is a different risk assessment from the same material on skin, and several markets restrict or add requirements for sprays containing nanomaterials. Check the target market first: if the answer is no, no amount of formulation work helps.

Technically, a spray needs low viscosity and clean atomisation, which means the dispersion must stay stable over the shelf life — otherwise the actuator clogs and the dose per actuation drifts. Powder settling in the can is the other failure mode.

Test spray pattern, dose per actuation and the particle-size distribution of the aerosol, not just formula stability. A product that delivers 0.6 mg/cm² per pass will not be used at the labelled rate whatever the bottle says.

Where the regulatory position is tight, the workable route is to position the product as a sprayable lotion that still needs rubbing in, and to put clear usage directions on the pack — do not spray directly at the face.