Formulating Superior Metal-Oxide-Based Sunscreens

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Boy wearing sunscreen - learn about formulating sunscreens with metal oxides in the Prospector Knowledge Center.
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Metal oxide-based sunscreen formulations have become very popular in recent years as alternatives to “chemical” sunscreens and are globally approved as sunscreens. Micronized Titanium Dioxide, Zinc Oxide and chemical-based sunscreens all work in the similar manner by absorbing UV radiation, typically with Titanium Dioxide acting as the UVB absorber and Zinc Oxide as the UVA absorber. Read on to learn more about formulating sunscreens with metal oxides.

 

 

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Using only Titanium Dioxide, it is difficult to achieve a critical wavelength of 370 nm and high UVA protection factors without excessive skin whitening. Using only Zinc Oxide as an active results in needing to use significantly high amounts unless combined with Titanium Dioxide to achieve the desired SPF.

Chemical sunscreens like Octinoxate are also used with Titanium Dioxide or Zinc Oxide to formulate beauty/blemish balm (BB) creams and pigmented products with SPF. This is because the US sunscreen monograph doesn’t permit the use of Avobenzone with metal oxides due to chemical reactivity.

Advantages of using micronized metal oxides in sunscreens

  • Better positioned as “natural”
  • Globally approved
  • Less irritating and penetrating than chemical sunscreens
  • Often positioned for babies and sensitive skin
  • More SPF efficient than chemical sunscreens (~3 versus ~2 SPF units/1% sunscreen)
  • Very efficient at boosting SPF when combined with chemical sunscreens.

Disadvantages of using micronized metal oxides in sunscreens

  • Formulations are very process sensitive
  • SPF can decrease over time due to agglomeration of metal oxide particles. Need to confirm SPF on heat-aged and freeze/thaw samples. The problem is usually caused by using powdered metal oxides that haven’t been properly dispersed into the formulation.
  • High levels of metal oxides can cause a dry skin feel and whitening
  • Zinc Oxide formulations need to be at a pH>7 to avoid solubilization of the Zinc Oxide in the water phase
  • Need to use coated Titanium Dioxide and Zinc Oxide to reduce photo/chemical reactivity and improve skin feel
  • Nano PR issues in Europe: Particles in formulations < 100nm are considered to be nano and need to be labeled as such. Since the aggregate particle size in formulations is always over 100 nm, technically nanoparticles don’t exist.
  • Cannot be used in the US with Avobenzone
  • Difficult to formulate facial day wear SPF products with good skin feel using more than 3% metal oxide

Metal oxide particle size

The most important property of metal oxides is their particle size. In formulations, metal oxide particles never exist as primary particles or their manufactured particle size but as clusters called aggregates and agglomerates.

An aggregate consists of primary particles fused together and can’t be reduced without using very high energy ball milling. The typical agglomerate contains clusters of aggregate particles held together by weak forces which can be reduced with roto stator mills and the use of good dispersing agents.

The primary particle size will normally determine the UV-absorbing and skin-whitening properties since it correlates with the aggregate size. Research performed by Kobo Products suggests:

  • The optimum primary particle size for Titanium Dioxide UVB absorption with minimal skin whitening is 10-15 nm or an aggregate size of 110-130 nm.
  • For maximum UVA absorption, the primary particle should be 35-60 nm or an aggregate size of 150-180 nm.
  • For Zinc Oxide, the maximum UVA absorption is achieved using a primary particle of 20-60 nm or an aggregate size of 160-200 nm (1).

Micronized metal oxides need to be coated to reduce their chemical reactivity, especially for Titanium Dioxide. Uncoated Titanium Dioxide can form highly reactive free radicals when exposed to UV and can react with formulation ingredients causing discoloration and off odors. The coatings also improve the dispersibility of metal oxides in the water or oil phase of emulsions and improve wash resistance (for hydrophobic coatings).

The best coatings for Zinc Oxide are Silica and Triethoxycaprylylsilane and for Titanium Dioxide Silica, Alumina, with or without Dimethicone. However, there are numerous marketed Zinc Oxide formulations that use uncoated Zinc Oxide.

SPF boosters can be useful to significantly increase the SPF efficiency of metal oxides and primarily work by increasing UV absorption or increasing UV light scattering. All the top-performing marketed metal oxide-based sunscreens use SPF boosting technology (based on SPF efficiency). SPF boosters include:

  • SunSpheres™ (Styrene/Acrylates copolymer-Dow Chemical, 90% active powder) are hollow 350 nm particles that can scatter UV. Can increase SPF by up to 68%.
  • Hallbrite® BHB (Butyloctyl Salicylate-Hallstar) can increase UVB absorption
  • SolaStay® S1 (Ethylhexyl Methoxycrylene-Hallstar) can boost the UVA protection factor by up to 96%
  • Synoxyl® HSS (Triethoxybenzylidene Pentanedione-Sytheon) has been shown to boost Zinc Oxide SPF by up to 57 percent
  • SunBoost ATB™ (Argania Spinosa Kernel Oil (And) Tocopheryl Acetate (And) Bisabolol-Kobo Products) may also work as a cosmetic soothing agent
  • Caprylyl Glycol as a preservative

Recommended metal oxides

Supplier-made dispersions are preferred. If you make your own dispersions, they should be made as a separate premix that is added to the finished formulation using powdered metal oxides, dispersing agents, and high shear processing.

  • TX-85AQ (TiO2, Silica-Sunjin, 30nm primary particle)
  • MT-100SA (Titanium Dioxide, Silica, Alumina-Tayca, 15 nm primary particle)
  • HBP45TEL (Butyloctyl Salicylate, Titanium Dioxide, Alumina, Silica, Stearic Acid, Polyhydroxystearic Acid-Kobo Products,39-43% active, non nano)
  • Solaveil™ CT-100 (Titanium Dioxide, C12-15 Alkyl Benzoate, Polyhydroxystearic Acid, Stearic Acid, Alumina-Croda 47% active)
  • Zano® M Plus (Zinc Oxide, Triethoxycaprylylsilane-EverZinc/Ultra Chemical, optimized for UVA)
  • ZinClear® XP (Zinc Oxide–Antaria)
  • ZinClear® XP65COCO (Zinc Oxide (and) Coco-Caprylate/Caprate (and) Polyglyceryl-3 Polyricinoleate (and) Isostearic Acid, 65% active, non nano-Antaria)
  • HBTN65HP1 (Zinc Oxide, Butyloctyl Salicylate, C12-15 Alkyl Benzoate, Triethoxycaprylylsilane-60.5-63.5% active-Kobo products)

Click to view all metal oxide products in Prospector.

Suggested oil-soluble dispersing agents include:

  • Polyhydroxystearic acid
  • Polyricinoleic acid
  • Polyglyceryl-3 Polyricinoleate
  • Polyglyceryl-6 Polyricinoleate

Further reading:

References

  1. Borges I., Formulating High SPF Products with Inorganic UV Filters. In-Cosmetics Brazil September, 2014.

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6 Responses to “Formulating Superior Metal-Oxide-Based Sunscreens”

  3. This was a great article, thanks for posting it.
    Do you have any references/suggestions for cleaning equipment where zinc oxide/titanium dioxide formulas are produced?

  4. It depends whether the formulation is an oil in water or water in oil formulation. Oil in water metal oxide formulations clean easily with detergent and water. You may need to add some isoparaffin solvent to the water in oil may residue in the kettle before add water and detergent. You also could use steam and a detergent solution..

    George.

  5. George –

    Thank you for another very informative article! If people are using TiO2 and ZnO to position “Natural” products wouldn’t an SPF booster based on natural Bentonite and Wood Extract (Polargel UV) be a good option? These can boost SPF of any active from 50-100% while providing antioxidant benefits.

    Craig Ford

Comments are closed.