Silicone Fluids in Sunscreen: Functional Overview
Sunscreen formulation has increasingly converged on silicone-based vehicles as the preferred sensory platform. Linear polydimethylsiloxane (PDMS), cyclic silicones (D4/D5), and specialty-modified silicones address three distinct formulation challenges: sensory modification, UV filter delivery, and water resistance maintenance. For formulators evaluating silicone options for sunscreen development, the selection logic centers on volatility, viscosity, UV filter compatibility, and regulatory status in the target markets.
Dimethicone (linear PDMS) and cyclomethicone (cyclic silicones) are the two primary categories. Phenyl silicone (ZEMSIL PH series) is a niche but growing option for high-SPF formulations where refractive index matching with organic UV filters improves filter efficiency.
Sensory Modification: PDMS as Skin-Feel Modifier
Low- to medium-viscosity PDMS (5–100 cSt, ZEMSIL DM-5 to DM-100) is the workhorse skin-feel modifier in sunscreen emulsions and anhydrous formulations. Key sensory contributions:
- Dry, non-greasy after-feel: PDMS spreading coefficient is lower than most organic oils, meaning it does not leave the thick, occlusive film associated with mineral oil or vegetable oil systems. At 1–5% in O/W emulsions, it replaces a portion of the emollient oil phase.
- Slip and spreadability: reduces application friction, allowing thin, even coverage — directly relevant to SPF reproducibility in consumer use, where uneven application is the primary cause of SPF underperformance.
- Compatibility with inorganic filters: TiO₂ and ZnO particles treated with PDMS or dimethicone co-polyol dispersants show reduced agglomeration. PDMS at 5–10% in the oil phase improves distribution of coated mineral filter.
Viscosity selection guideline for sunscreen:
| Viscosity | Form | Typical Loading | Best Application |
|---|---|---|---|
| 5–20 cSt | Low-viscosity fluid | 2–8% | Lightweight fluid emulsions, clear gels |
| 100–350 cSt | Medium fluid | 3–10% | SPF lotions, body milk |
| 1,000–12,500 cSt | High fluid | 1–5% | Emollient-rich creams, SPF sticks |
| Cyclomethicone D5 | Volatile | 10–30% | Dry-touch sprays, mousse sunscreens |
UV Filter Carrier Performance
Organic UV filters (oxybenzone, avobenzone, octinoxate, Tinosorb S/M) have limited solubility in conventional organic solvents at the concentrations required for high-SPF products. PDMS at 100–1,000 cSt provides a stable solvent medium for several important UV absorbers:
- Avobenzone (butyl methoxydibenzoylmethane) dissolves well in PDMS and shows reduced photodegradation compared to ethanol or ester-based vehicles, likely due to the non-reactive, thermally stable silicone environment.
- Tinosorb S (bis-ethylhexyloxyphenol methoxyphenyl triazine) is a solid filter requiring solubilization; PDMS blended with C12-15 alkyl benzoate provides an effective co-solvent system.
- Phenyl silicone (ZEMSIL PH-100 to PH-200, RI 1.43–1.46) improves optical clarity in clear sunscreen gels where filter crystallization is a stability concern.
Cyclic silicones (D5 cyclomethicone) are valued as volatile UV filter carriers in spray-format sunscreens: they dissolve organic filters, provide a light, fast-drying skin feel, and evaporate after application, leaving the filter deposited on the skin surface without the greasy residue of non-volatile oils. D5 volatility also reduces the dilution of SPF by the vehicle, allowing the filter to be presented at higher effective concentration.
Fumed Silica as Thickener in Anhydrous Stick Sunscreens
Anhydrous sunscreen sticks (SPF lip balms, stick-format body sunscreen) require a semi-solid, waxy vehicle that holds organic UV filters in uniform dispersion without phase separation. Hydrophobic fumed silica — ZEMSIL FH-150 or FH-200 (surface-treated with HMDS or PDMS) — thickens the oil phase through a particle network mechanism (physical gel), providing:
- Thixotropic consistency: flows under stick application shear, recovers immediately on cessation — necessary for smooth application without dragging or tearing.
- Uniform filter suspension: prevents sedimentation or phase separation of dense mineral filters (TiO₂, ZnO) during shelf storage at 45 °C/3 months.
- Transparency in stick wax systems: unlike crystalline wax thickeners, fumed silica at 0.5–2.0% does not contribute haze in clear, wax-based stick formulations.
Loading: 0.5–2.0% hydrophobic fumed silica in the oil phase, pre-dispersed with a Cowles blade or rotor-stator mixer before incorporation of wax components. Pre-dispersion is essential — dry fumed silica added to a completed wax melt will agglomerate and create visible particles.
Water Resistance and PDMS Film Formation
Water resistance (as defined by FDA sunscreen monograph and ISO 16217) requires that ≥50% of labeled SPF be retained after two 20-minute water immersion periods. PDMS-containing sunscreens consistently outperform purely aqueous emulsion bases in water resistance testing because:
- PDMS forms a continuous, hydrophobic film at the skin surface during emulsion deposition and drying
- The film repels water ingress and retains the organic UV filter in the film rather than allowing it to wash off
- High-viscosity PDMS (1,000–12,500 cSt) contributes to cohesive film formation more than low-viscosity grades
For maximum water resistance, formulators combine PDMS with film-forming polymers (Acrylates/Octylacrylamide Copolymer, VP/Eicosene Copolymer) and acrylate-silicone graft copolymers. The silicone-acrylate combination creates a hybrid film that combines the hydrophobicity of silicone with the film cohesion of acrylic polymer — the preferred approach in high-performance sport sunscreen formulations.
Related Reading
Silicone Oil — DM Series · Phenyl Silicone — PH Series · Hydrophobic Fumed Silica — FH Series · Applications: Cosmetic Emollient · Applications: Hair Care Conditioner