Reformulation briefs in sun care today routinely stack four or five requirements that pull against each other: reef-safe, broad-spectrum SPF 50, no white cast, elegant finish across all skin tones. A team swaps oxybenzone and octinoxate for a newer organic absorber and surface-treated zinc oxide, bench SPF passes, six-week stability looks clean... then field results show chalking on darker skin tones, photoprotection drop-off after two hours of solar exposure, and zinc flocculation at the bottom of the bottle. The formulation has to be rebuilt with three months gone and a launch window closing.

Sunscreen development used to be a comparatively bounded problem because formulators had a stable palette of approved UV filters, predictable interactions, and decades of accumulated empirical knowledge. Today, the palette is shifting under multiple pressures at once. Regulators in Hawaii, Palau, Thailand, and parts of Mexico and the Caribbean have restricted specific organic filters over coral reef concerns. The European Commission’s SCCS has issued tighter exposure recommendations on filters like homosalate and octocrylene. Consumers demand higher SPF with lighter sensorial profiles, no visible residue on melanin-rich skin, and ingredient transparency that often excludes nanoparticles or specific synthetic actives. Brands trying to balance all of this end up reformulating around a moving target.

The global sun care market is approaching $15 billion in annual sales, and sunscreen represents roughly two-thirds of that volume. The category continues to grow as awareness of UV-induced photoaging and skin cancer rises, but growth is happening alongside the tightest reformulation constraints the industry has ever faced. Three physicochemical properties sit at the center of whether a next-generation sunscreen succeeds: shear-thinning rheology, surface tension and interfacial behavior, and colloidal stability. Filter chemistry constraints such as photostability and transparency couple back through these properties rather than standing apart from them.

Trends Driving Reformulation

Several pressures are converging at once, and they all push sunscreen design toward a new generation of UV filter technologies:

Regulatory restrictions on legacy filters

Bans and reviews of oxybenzone, octinoxate, and several other widely used organic absorbers have removed a substantial portion of the toolkit formulators relied on for decades. Replacement filters rarely match the protection profile of what they replace, and the SPF mathematics has to be rebuilt. New high-performance organic filters such as bemotrizinol, bisoctrizole, and phenylene bis-diphenyltriazine are approved in Europe and Asia but not yet in the United States, which fragments the global formulation strategy for international brands.

Reef-conscious and biodegradable filters

Concern about coral bleaching and broader aquatic toxicity has pushed brands to formulate without the filters most strongly associated with reef harm. The technical question is what replaces them. Mineral filters like zinc oxide and titanium dioxide are often positioned as reef-friendlier alternatives, but they bring their own challenges in dispersion, transparency, and rheology. New organic filters with improved biodegradation profiles are entering the market, but their UVA performance and photostability vary widely.

Photostability and broad-spectrum coverage

Avobenzone remains the most widely used UVA1 filter in the U.S. despite well-documented photoinstability that requires stabilizing partners like octocrylene. As octocrylene itself faces scrutiny, formulators are looking for organic absorbers that resist degradation without a stabilizer, or for hybrid mineral-organic systems. Meeting the strict UVA/UVB ratio requirements set by ISO 24443 has become a primary design constraint rather than a checkbox at the end.

Transparency on darker skin tones

Mineral sunscreens have always struggled with white cast, and this issue has become unavoidable now that brands are expected to perform across all skin tones. Engineered particle morphology, surface treatments, and refractive-index matching with surrounding emulsion phases are all being deployed to reduce visible residue. The trade-off is that smaller particle sizes can improve transparency but may also reduce UV scattering efficiency and raise regulatory questions about nanomaterial classification.

Sensorial elegance at higher SPF

Consumer expectation has shifted toward sunscreens that feel like a moisturizer or primer. This is especially demanding at SPF 50+ where high filter loads can easily produce heavy, occlusive, or tacky films. Achieving 18-25% total filter content while delivering a fast-absorbing, breathable finish requires careful attention to emulsion design, polymer choice, and rheology engineering.

Encapsulation for compatibility and skin tolerance

Silica shells, polymeric capsules, and lipid carriers reduce direct skin contact, mitigate irritation, and can stabilize otherwise photolabile absorbers. The trade-off is that the encapsulated filter no longer interacts directly with the surrounding emulsion or with skin, which can shift the SPF response in non-intuitive ways.

The Properties That Matter

Three physicochemical properties determine whether a next-generation sunscreen meets its targets: shear-thinning rheology, surface tension and interfacial behavior, and colloidal stability. These are the levers a formulator can engineer directly through composition, structure, and processing, and they are the properties that predictive modeling can characterize quantitatively. The other constraints that dominate sunscreen development, including photostability, optical transparency, and regulatory acceptability, are real, but they cascade through these three properties rather than acting independently of them.

Shear-Thinning Rheology

How a sunscreen flows determines whether mineral filters stay suspended in the bottle, whether the product spreads easily under the shear of rub-in, and whether the resulting film deposits uniformly on skin. Shear-thinning behavior is the central design target. At rest the formulation should hold structure to prevent sedimentation of zinc oxide or titanium dioxide particles and to keep encapsulated organic filters uniformly distributed. Under the higher shear rates of dispensing and rubbing, viscosity should drop sharply so the product spreads cleanly, then recover enough structure on the skin to form a continuous, even film without slumping or pooling.

Yield stress and the rate of structural recovery after shear are as important as the viscosity values themselves. A formulation with adequate steady-shear viscosity but slow recovery can leave streaks or uneven films, while one with too much recovery feels tacky or drags during application. Designing for the right rheological signature becomes more difficult as filter loads climb toward the 18-25% range required for SPF 50+ products, where the dispersed phase fraction itself drives much of the bulk rheology and leaves less room to tune behavior through polymer additives alone.

Rheology modifiers must coexist with the surface chemistry of mineral dispersions, with electrolyte content, and with the polar-nonpolar balance of the oil phase. Switching from a traditional carbomer to a natural polymer such as xanthan or sclerotium gum, often required for clean-beauty or sustainability claims, frequently changes the entire suspension behavior of the formulation. Sprayable sunscreens add another constraint, since the formulation must shear-thin enough to atomize while retaining enough structure to suspend particles in the cartridge or bottle.

Surface Tension and Interfacial Behavior

Surface tension at the air-water interface governs how a sunscreen wets and spreads on skin. Skin has a relatively low surface energy, and a film that does not spread readily leaves bare patches that compromise SPF delivery. Lower equilibrium surface tension improves spreading, but the dynamic surface tension, meaning the value during the first fractions of a second after a new interface is created, often matters more for sunscreens because application happens under fast deformation. Surfactants and emulsifiers with slow adsorption kinetics can give acceptable equilibrium readings while still leading to poor wetting during actual use.

Interfacial behavior also drives several manufacturing and packaging issues that show up late in development. Bubble formation during high-shear mixing or filling depends on surface tension and on the elasticity of the interfacial film, with persistent foam slowing fill lines and creating dose variability in pumped or sprayed products. Emulsion stability over shelf life, which depends on oil-water interfacial tension and on the structure of the interfacial film around dispersed droplets, is the difference between a formulation that holds together for 24 months and one that creams within weeks under temperature stress.

The shift toward bio-surfactants, silicone-free systems, and natural emulsifiers changes both equilibrium and dynamic interfacial behavior compared to the legacy systems that have been optimized for decades. Sucrose esters, polyglyceryl esters, and bio-based alkyl polyglucosides typically have larger molecular structures and slower adsorption kinetics than synthetic emulsifiers, so achieving the same wetting performance often requires reformulating the entire surfactant package rather than substituting one ingredient. Treated mineral particles also interact with the air-water interface, and their effective surface activity depends on the surface treatment chemistry, which couples interfacial behavior back to dispersion design.

Colloidal Stability

Sunscreens are colloidal systems. Mineral UV filters are dispersed particles, encapsulated organic filters are core-shell particles, and the emulsion itself is a dispersed phase. Colloidal stability refers to whether these dispersed phases remain uniformly distributed over the shelf life of the product and is among the most frequently cited failure modes in late-stage sunscreen development, where aggregation, flocculation, and creaming can all emerge after months of apparently stable bench results. Aggregation, flocculation, and creaming change SPF efficiency, optical appearance, and consumer perception simultaneously, and once aggregation begins, it is rarely reversible.

Colloidal stability depends on the balance of attractive van der Waals forces and repulsive electrostatic or steric forces between particles. Zinc oxide and titanium dioxide are typically supplied as pre-dispersed concentrates with surface treatments such as silica, alumina, dimethicone, stearic acid, or triethoxycaprylylsilane, each tuned for compatibility with a different polar-nonpolar balance in the surrounding formulation. A surface treatment optimized for nonpolar oil dispersion will flocculate in a more polar carrier, and switching surface treatments often requires re-engineering the dispersant and emulsifier package together.

Stability that looks acceptable at six weeks can degrade rapidly at three months or under temperature stress, especially in systems combining bio-based emulsifiers, natural rheology modifiers, and high mineral loads. Aggregates scatter visible light more than well-dispersed primary particles, producing the white cast consumers reject, and they also reduce the effective UV-blocking surface area. The same colloidal forces that determine particle dispersion stability also govern the integrity of encapsulated organic filters, where capsule walls must survive processing shear and storage without releasing payload prematurely.

The Coupling Problem

The three predictable properties do not vary independently, and they couple tightly to the filter chemistry constraints that drive next-generation reformulation. A change made to improve one property almost always affects several others, sometimes in non-intuitive ways, and the levers that solve a photostability or transparency problem typically propagate back through rheology, interfacial behavior, and colloidal stability.

Increasing zinc oxide concentration to improve UVA protection is a useful starting example. The higher dispersed-phase volume fraction raises bulk viscosity, shifts the shear-thinning profile, and changes the optical scattering behavior of the film. A formulation that hits the SPF target through higher mineral loading can simultaneously become harder to spread, more prone to settling in the bottle, and more visibly white on darker skin tones. Compensating with an organic absorber may restore the rheological and aesthetic targets, but the SPF balance and photostability of the system have to be reverified. Avobenzone suffers significant photodegradation within minutes of solar exposure and typically requires a stabilizing partner such as octocrylene, while newer photostable absorbers like bemotrizinol and bisoctrizole dissipate UV energy through internal molecular pathways and retain over 90% of their absorbance after extended solar simulator exposure.

Particle morphology choices made for transparency cascade into rheology and colloidal stability. Reducing mineral particle size reduces white cast, but smaller particles have higher specific surface area, increasing demand on the dispersant package and altering how the suspension responds under shear. Engineered shapes such as rods, plates, or controlled aggregates pack differently than spherical primaries, shifting yield stress and recovery behavior. Refractive-index matching with the surrounding emulsion phase improves perceived transparency but constrains the choice of carrier oils, which in turn affects emulsion stability and interfacial tension. Particles below approximately 30 nm enter the regulatory definition of nanomaterials in many jurisdictions, so morphology and surface treatment have become the preferred levers for transparency rather than further size reduction.

Encapsulating an organic filter to improve photostability or reduce skin penetration ties the three predictable properties together even more tightly. The capsule shell adds a new dispersed phase that must remain colloidally stable through processing shear, temperature cycles, and the ethanol common in sun care. The interfacial chemistry of the capsule surface determines how it sits at the oil-water interface and how it interacts with treated mineral particles, and the capsules also contribute to the dispersed-phase fraction that drives bulk rheology. The microenvironment inside the capsule influences the filter molecule’s absorbance spectrum, so the SPF contribution at a given nominal concentration may differ from that of the non-encapsulated filter.

The variables that affect next-generation UV filter performance include filter type and concentration, surface treatment chemistry, dispersant level, emulsifier system, oil-phase composition, polymer rheology modifier, pH, electrolyte content, and temperature. Even a modest exploration of three filters at five concentrations each, with two surface treatments and three rheology modifiers, generates 90 distinct formulations before considering pH or temperature. Realistic sunscreen design spaces span thousands of viable combinations, and traditional design of experiments scales poorly into this territory.

Where Predictive Tools Fit

The traditional approach to sunscreen reformulation is empirical. Propose a formulation, prepare a bench sample, measure SPF, run stability, evaluate sensorial properties, and iterate. Each cycle is expensive in time and materials, in vivo SPF testing has long lead times and regulatory complications, stability testing requires months at multiple conditions, and aesthetic evaluation depends on panel testing that is difficult to standardize.

The complexity of the coupled design space compounds the problem. With multiple filters interacting in non-linear ways, with surface treatments and dispersants that affect both optical and rheological properties, and with regulatory constraints that vary by region, the number of plausible formulations is enormous relative to what any team can test experimentally. Sustainability and reformulation pressure have accelerated the iteration count, so development teams are being asked to do more iterations in less time with smaller benches of viable options.

Predictive modeling offers a way to narrow the design space before committing to experimental work. If the relationships between filter chemistry, dispersion conditions, and resulting properties can be captured computationally, formulators can identify the most promising candidates virtually and focus experimental resources on validation rather than blind iteration. The point isn’t to replace experiments. Sunscreen safety, efficacy, and consumer perception are too important to rely on unsupported predictions. The point is to identify the right experiments to run.

Building such models requires chemistry-aware approaches trained on physicochemical data, not generic machine learning applied to formulation problems. UV filter behavior is driven by molecular absorbance spectra, intermolecular interactions in emulsion phases, particle-particle interactions in mineral dispersions, and the rheological response of complex multicomponent systems. Standard machine learning approaches struggle with the small datasets and chemical complexity typical of cosmetic formulation; chemistry-aware models that understand the underlying physics generalize better and require fewer training samples to produce useful predictions.

FastFormulator has developed this capability through a combination of proprietary data generation and purpose-built models that map directly to the three predictable properties. The Virtual Viscometer predicts shear-thinning rheology and yield behavior across shear rates for emulsions with varying filter loads, surface treatments, and rheology modifiers. The Virtual Surface Tensiometer predicts equilibrium and dynamic surface tension for surfactant and emulsifier systems, capturing the interfacial behavior that governs wetting, spreading, and emulsion formation. The Virtual Stability Chamber forecasts colloidal stability under temperature and humidity stress, which is particularly relevant for systems combining mineral dispersions, encapsulated organics, and natural rheology modifiers. Beyond these core instruments, FastFormulator can also develop custom models for other properties relevant to a specific sunscreen program, whether that means film uniformity, spray atomization behavior, or product-specific sensorial endpoints. Used together, these tools allow formulators to explore the sunscreen design space computationally and to prioritize the formulations most likely to meet the full set of SPF, stability, sensorial, and regulatory targets.

Takeaways

The shift toward reef-conscious, photostable, broad-spectrum sun protection with elegant skin feel and full-spectrum compatibility has made UV filter selection central to sunscreen success. Shear-thinning rheology, surface tension and interfacial behavior, and colloidal stability are no longer secondary considerations to be addressed late in development. They are the primary design variables that determine whether a formulation can deliver its SPF claim, hold together over shelf life, and meet consumer expectations on feel and appearance, and they are the physicochemical levers through which downstream constraints like photostability and transparency actually get satisfied. Reformulation failures in any of these areas can stall product launches, force costly reworks, and erode the trust that brands have built with consumers around sun safety.

The difficulty is that these properties are interconnected through the underlying chemistry and physics of UV filter systems. Filter concentration affects both protection and rheology, while surface treatment of mineral particles influences both optical appearance and dispersion stability. Encapsulation can solve a photostability problem but introduce new compatibility constraints; switching to a reef-safer filter may improve the environmental profile but require rebuilding the SPF mathematics of the entire product. Optimizing a next-generation sunscreen requires navigating this coupled design space, and traditional empirical approaches scale poorly against the speed and breadth of current reformulation pressures.

Predictive tools built on chemistry-aware modeling can help by providing early insight into how formulation choices affect multiple properties simultaneously. By identifying promising regions of the design space before extensive experimentation, these tools allow development teams to focus their bench resources on candidates most likely to succeed across the full set of performance, stability, regulatory, and aesthetic targets. The result is faster development, fewer failed iterations, and a more reliable path from reformulation brief to launch-ready sunscreen.

References

• Osterwalder, U., Sohn, M., and Herzog, B. (2014). Global state of sunscreens. Photodermatology, Photoimmunology & Photomedicine, 30(2-3), 62-80.
• Diffey, B.L. (2009). Sunscreens: expectation and realization. Photodermatology, Photoimmunology & Photomedicine, 25(5), 233-236.
• Herzog, B., Wehrle, M., and Quass, K. (2009). Photostability of UV absorber systems in sunscreens. Photochemistry and Photobiology, 85(4), 869-878.
• Smijs, T.G. and Pavel, S. (2011). Titanium dioxide and zinc oxide nanoparticles in sunscreens: focus on their safety and effectiveness. Nanotechnology, Science and Applications, 4, 95-112.
• Schneider, S.L. and Lim, H.W. (2019). Review of environmental effects of oxybenzone and other sunscreen active ingredients. Journal of the American Academy of Dermatology, 80(1), 266-271.
• Manaia, E.B., et al. (2013). Inorganic UV filters. Brazilian Journal of Pharmaceutical Sciences, 49(2), 201-209.
• ISO 24443:2021. Determination of sunscreen UVA photoprotection in vitro. International Organization for Standardization, Geneva.
• Scientific Committee on Consumer Safety (2021). Opinion on Homosalate, SCCS/1622/20. European Commission, Brussels.
• Grand View Research (2024). Sun Care Products Market Size, Share & Trends Analysis Report, 2024-2030.
• Lionetti, N. and Rigano, L. (2017). The new sunscreens among formulation strategy, stability issues, changing norms, safety and efficacy evaluations. Cosmetics, 4(2), 15.