Take two sunscreens with the same filter package, the same SPF 50 on the label and the same 2 mg/cm² application. Immerse both for 40 minutes under the same protocol. One comes back at 70% of its original protection; the other comes back at 35%. Nothing in the filter list explains the difference, because nothing in the filter list is what the water removed. Water does not dissolve an organic UV filter out of a molecule; it lifts, swells, cracks or re-emulsifies the film the filter is sitting in. Water resistance is therefore a polymer and emulsion property, and it is bought on the film-former line of the formula, not on the filter line.
This matters commercially because the claim is regulated in three incompatible ways, and because the arithmetic used in two of them penalises exactly the high-SPF products that marketing wants to sell.
The film is thinner than you think
The test application is 2 mg/cm², which for a face and neck is roughly 2.5 mL. At a formulated density of 0.9 to 1.1 g/cm³ that works out to a wet film of 18 to 22 µm — a fifth to a third of the thickness of a human hair. Everything the product promises is carried in that layer: the filters, the emollients, the film former, the preservative. A 20% loss of film thickness is a 20% loss of protection, and it is invisible on the skin.
It also explains why vehicle studies repeatedly find that the same filter concentration gives different SPF in different formats: an O/W lotion, a W/O cream and an alcohol spray do not lay down films of the same thickness or continuity, and the thicker continuous film wins. When a formulator reports a disappointing in-vivo result, the first question is not which filter to add but how thick and how continuous the film actually was.
Three claim regimes, three different questions
The same words — water resistant — mean three different things depending on the market the product is sold into. The table below is the one worth keeping next to the spec sheet.
| Item | United States: 21 CFR 352.76 | ISO 16217:2020 + ISO 18861:2020 | Australia / New Zealand: AS/NZS 2604 |
|---|---|---|---|
| Immersion protocol | Indoor fresh water pool at 23–32 °C; 20 min activity in water + 20 min rest, no towelling; two cycles for 40 min, four cycles for 80 min | Water at 30 ± 2 °C, flow 0.02–0.05 m/s, conductivity ≥500 µS/cm, pH 6.5–7.5; 20 min immersion + 5–20 min drying per cycle; two or four cycles; final dry ≥15 min | Immersion per ISO 16217:2020 |
| What is measured | SPF is re-measured after immersion and that number becomes the label SPF | Static SPF and post-immersion SPF on the same subjects; percentage of water resistance is calculated | Residual SPF after the claimed duration, plus the standard SPF and broad-spectrum requirements |
| Pass criterion | No retention threshold at all — only truthful labelling of 40 or 80 minutes | 90% one-sided lower confidence limit of %WR ≥50%; report the mean %WR | Maximum claim duration is capped by SPF tier: SPF 8–14 up to 40 min, 15–29 up to 2 h, 30 and above up to 4 h |
| On pack | Water Resistant (40 minutes) or Water Resistant (80 minutes); waterproof and sweatproof are prohibited | Percentage of water resistance, with the method and duration stated | Duration in minutes or hours, consistent with the tier |
Two consequences fall out of this table. First, the US regime is a labelling regime, not a performance regime: a product may carry a 40 minute claim with modest retention provided the lower SPF is what appears on the label. The consumer sees the post-immersion number directly. Second, the ISO regime is the only one that publishes a number for retention, and that number carries a 90% one-sided lower confidence limit — a statistical floor, not the mean. A mean of 55% with a wide confidence interval will fail a 50% floor.
The subtraction of one
ISO 18861 defines percentage of water resistance as the retained fraction of the protection above bare skin, not the retained fraction of the SPF number:
%WR = (SPF after immersion − 1) ÷ (SPF before immersion − 1) × 100
That −1 is the unprotected baseline. It looks like a technicality until you run the numbers, at which point it becomes a commercial constraint.
| Static SPF | SPF needed for 50% WR | SPF needed for 70% WR | What a drop to SPF 20 scores |
|---|---|---|---|
| 15 | 8.0 | 10.8 | 136% (above 100% — within method noise) |
| 30 | 15.5 | 21.3 | 66% |
| 50 | 25.5 | 35.3 | 39% |
| 50 (measured 60) | 30.5 | 42.3 | 32% |
Three readings follow. A product labelled SPF 50 needs to hold SPF 25.5 after immersion to clear the 50% floor — which is what the shorthand "half the SPF" implies, but only because 25.5 happens to round close to half of 50. At SPF 30 the same 50% floor only requires 15.5, so an identical absolute loss of 14.5 SPF units reads as 50% at SPF 30 and as 71% at SPF 50. High-SPF products are therefore judged more harshly for the same film behaviour, and the higher the label, the more of the retention budget has to be spent on the film rather than on extra filter loading.
There is a calibration anchor that makes this concrete: in ISO 18861 the reference sunscreen P2 is accepted with a post-immersion SPF of 11.5 (range 9.0–15.0) and a mean water resistance of 68.1% (acceptance range 50–85%). The reference standard itself gives up roughly a third of its protection. A formulator chasing 85% is chasing the top of what the reference material achieves, not a comfortable target.
Four ways a film actually fails
- Re-emulsification. An O/W emulsion carries its own emulsifier. Immersion gives that emulsifier water on both sides, and the film can re-emulsify from the surface inward. This is why W/O systems and anhydrous or alcohol-based systems are inherently more water resistant than the same filters in a lotion, and why adding a film former to an unstable O/W emulsion buys very little.
- Swelling and leaching. The polymer network takes up water, loosens, and releases soluble species. Water-soluble filters are the extreme case: benzophenone-4 (CAS 4065-45-6) and disodium terephthalidene dicamphor sulfonate are ionic and water-soluble, so unless they are locked inside an oil phase or a hydrophobic network they leave with the water. A water-soluble UVA filter in an unprotected O/W film is a design contradiction, not a formulation detail.
- Mechanical removal. The FDA protocol forbids towelling precisely because rubbing removes film, and real users towel. Films with high glass transition temperatures and no plasticiser crack along skin flex lines and then flake; the crack pattern is visible under UV imaging long before the user notices anything.
- Electrolyte and pH shock. Pool water, sweat and seawater bring salts. Carbomer-thickened gels collapse or lose viscosity when the ionic strength jumps, and pH-sensitive thickeners behave the same way. A gel that thins on contact with salt water will not hold a film regardless of how much film former it contains.
Five film-former families, ranked by what they survive
The families below are not interchangeable at equal percentage, and their order of water resistance is almost exactly the inverse of their order of removability. That trade-off, not the INCI length, is the purchase decision.
| Family | Representative INCI | Water resistance | Film character | Removal | Formulation constraint |
|---|---|---|---|---|---|
| MQ / T silicone resins | Trimethylsiloxysilicate; silicone acrylate such as acrylates/polytrimethylsiloxymethacrylate copolymer (CAS 31807-55-3) delivered in isododecane | Highest | Hard, tack-free, transfer resistant; the silicone acrylate version stays flexible | Needs an oil cleanser | Requires a volatile carrier; poor compatibility with high-polarity esters at high load |
| Olefin / acrylate long-chain | C30-38 olefin/isopropyl acrylate copolymer | Very high | Rich, water-repellent, slightly heavy | Oil cleanser | Oily systems only; limited in low-viscosity sprays |
| Acrylate copolymers | Acrylates/octylacrylamide copolymer; acrylates/dimethicone copolymer | High | Transparent, flexible, good adhesion | Soap and warm water | Octylacrylamide grade is a powder for alcohol systems; needs a neutraliser in aqueous systems unless supplied as a ready dispersion |
| VP / vinyl copolymers | VP/eicosene copolymer; VP/hexadecene copolymer | Medium | Soft, slightly tacky, good pigment wetting | Ordinary cleanser | Sticky above roughly 3%; plasticiser needed |
| PVP and water-soluble polymers | PVP; PVP/VA copolymer | Low to medium | Glossy, water-removable | Water alone | Re-dissolves on immersion; suitable for daily wear, not for sport claims |
Two data points worth carrying into a formulation review. A VA/butyl maleate/isobornyl acrylate copolymer was reported to raise measured UV absorbance from 98.5 to 117.1, an increase of about 19%, and to hold that gain after immersion — the film former is not only a glue, it changes the optical path through the layer. And supplier work on SPF 50+ systems tested at 80 minutes reports that roughly 1% of a high-efficiency acrylate dispersion matches the water resistance of about 2% of older polymers, which is a reminder to compare on a solids basis rather than on an as-supplied percentage.
Formulating for retention, not for SPF
- Start from the emulsion type. Decide W/O, silicone-in-water, anhydrous or alcohol-based before choosing a film former. Adding 2% polymer to a film that re-emulsifies is money spent on a symptom.
- Use 1–3% film former on solids, and state the solids content when you quote the number; a 60% dispersion at 3% is 1.8% polymer.
- Plasticise deliberately. A film that cracks has failed even if it has not dissolved. Trimethylolpropane tricaprylate/tricaprate (CAS 11138-60-6), trimethylolpropane triethylhexanoate (26086-33-9) and C12-15 alkyl benzoate (68411-27-8) lower the effective film-forming temperature and keep the layer continuous across skin flex; the benzoate additionally solvates crystalline filters.
- Respect the volatile carrier. Isododecane and cyclopentasiloxane leave the film by evaporation, which is why silicone-acrylate dispersions deliver a hard film from a thin liquid; they also demand that the package be tight and the filling line be solvent-rated.
- Check the crystalline filters. Butyl methoxydibenzoylmethane (CAS 70356-09-1) is the usual offender. Crystallisation inside the film creates UV-transparent channels that no amount of extra filter fixes; solvent choice and film flexibility decide whether it stays dissolved through drying and immersion.
- Do not exceed what the film can hold. High filter loading with a weak film gives a high static SPF and a poor post-immersion one, which is the worst possible combination under a percentage-based regime.
What to put on the purchase order
- INCI plus CAS plus carrier: for dispersions, the carrier is part of the product. Acrylates/polytrimethylsiloxymethacrylate copolymer in isododecane (CAS 31807-55-3) is not the same purchase as the dry polymer.
- Solids content and viscosity, with the test method, because dosing is calculated on solids.
- Acid value and neutralisation requirement for acrylate film formers; it decides whether the grade can go into a pH 5.5 emulsion at all.
- Glass transition or minimum film-forming temperature, so the plasticiser level is an engineering decision rather than a guess.
- Water resistance evidence with the standard named: a statement of 40 or 80 minutes per 21 CFR 352.76, or a %WR per ISO 18861 with the confidence interval. A claim without the method is not evidence.
- Compatibility statement for ethanol, isododecane and the ester plasticiser actually used in the formula.
- Residual monomer and regulatory pack (EU 1223/2009 Annex, China IECIC listing, US TSCA status), since the same polymer family can be listed differently by grade.
Supply
For the film side of a water-resistant system we supply acrylates/polytrimethylsiloxymethacrylate copolymer in isododecane (CAS 31807-55-3), a silicone-acrylate dispersion that dries to a flexible water-resistant film; isododecane and dimethicone crosspolymer (CAS 31807-55-3; 213629-14-2) and cyclopentasiloxane and dimethicone crosspolymer (541-02-6; 63394-02-5) for elastomer-network texture and matte finish; and dimethicone 350 cSt (CAS 63148-62-9) for spreading and water-repellent skin feel.
For plasticisers and spreading esters: trimethylolpropane tricaprylate/tricaprate (CAS 11138-60-6), trimethylolpropane triethylhexanoate (26086-33-9), trimethylolpropane triisostearate (68541-50-4) and C12-15 alkyl benzoate (68411-27-8).
For the filter phase: butyl methoxydibenzoylmethane (CAS 70356-09-1), benzophenone-3 (131-57-7), benzophenone-4 (4065-45-6, the water-soluble one), homosalate (118-56-9), ethylhexyl salicylate (118-60-5), DHHB (302776-68-7), drometrizole trisiloxane (155633-54-8) and butyloctyl salicylate (190085-41-7), the last of which doubles as a spreading and solubilising ester.
For the absorption side of filter selection see our organic UV filter formulating guide; for what each market will let you put on the pack, the five-market compliance guide; for why molecular size and photostability decide whether a filter stays where you put it, the molecular weight and photostability guide; and for the stick format, where film continuity is even harder to achieve, our SPF lip balm guide. All grades are available for B2B supply with sample quantities shipped worldwide — requests for film-former compatibility data, retention test support or COA interpretation are welcome through the enquiry form on any product page.