Micron-sized Zinc Oxide Dispersion for Transparent High SPF Formula

Feb 1, 2010 | Contact Author | By: Paul McCormick, PhD, Antaria Ltd.; Robert Fletcher, PhD, and Charles Jones, PhD, Dow Home and Personal Care
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Title: Micron-sized Zinc Oxide Dispersion for Transparent High SPF Formula
micron-sized ZnOx transparencyx SPFx sunscreen regulationsx UVA/UVBx
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Keywords: micron-sized ZnO | transparency | SPF | sunscreen regulations | UVA/UVB

Abstract: Regulatory bodies require quantifiable, photostable UVA protection for sunscreens and while zinc oxide provides an effective option, historically, its white appearance on skin has limited its acceptance. Therefore, in the present article the authors explore a micron-sized zinc oxide dispersion technology to enable the formulation of transparent yet full spectrum sunscreen formulas.

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P McCormick, R Fletcher and C Jones, Micron-sized zinc oxide dispersion for transparent high SPF formula, Cosm & Toil 125(2) 50-57 (Feb 2010)

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Regulatory bodies require quantifiable, photostable UVA protection for sunscreens, and zinc oxide (ZnO) provides such protection, as well as a long history of safety in sunscreens and cosmetic applications. Groups such as the US Food and Drug Administration (FDA) and the Therapeutic Goods Association (TGA) also have approved ZnO for use as a UV active ingredient in OTC sunscreens. Furthermore, ZnO has Generally Recognized as Safe (GRAS) status, is the only ingredient recognized by the FDA as providing both UVA and UVB protection, and is defined by the FDA as a Category I skin protectant, inferring that it can be used safely on compromised or environmentally challenged skin.

However, a drawback to formulating with ZnO is its white appearance on skin, which creates low cosmetic acceptability and marketability. This whiteness is caused by the back-scattering of light from submicron sized ZnO particles in sunscreen formulas. By reducing the ZnO particles to nanometer size (< 100 nm), product developers have successfully decreased this scattering and developed transparent formulations but such formulas are limited to approximately 5% w/w ZnO content.

Recent FDA regulations require the quantification of UVA protection in sunscreens as well as labeling that specifies the level of UVA/ UVB protection provided by a product. In addition, proposed regulations in Australia may soon require sunscreens to provide a minimum level of UVA protection. While these new regulations are not concerned with transparency, effective sunscreens developed to meet these requirements must be transparent to be marketable.

Transparency is critical in beach and daily wear products but it holds even greater importance in liquid foundations. Increasingly, formulators are combining ZnO and titanium dioxide (TiO2) in makeup foundations to impart added UV protection, thus supporting sun care claims. However, darker colored foundations with sun protection benefits are difficult to achieve due to the skin whitening caused by conventional inorganic filters. Considering these needs, in the present article, the authors describe a micron-sized ZnO dispersion to enable transparent formulations using higher levels of ZnO for effective sun protection, as shown by transmission spectroscopy and SPF testing.

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Figure 1. Porous, micron-sized ZnO particles of controlled structure and density

Figure 1. Porous, micron-sized ZnO particles of controlled structure and density

Three sample ZnO dispersions were evaluated via transmission spectroscopy: a control sample prepared using pigment grade ZnO, a test sample including nano-sized ZnO, and another sample incorporating micron-sized ZnO manufactured to form porous particles of controlled structure and density (shown here).

Figure 2. Transmission spectroscopy

Figure 2. Transmission spectroscopy

Transmission spectroscopy showed that pigment grade ZnO interacted with the visible light; nano-sized ZnO performed somewhat better; and the micron-sized dispersion achieved the highest transmission due to its porous design.

Figure 3. Particle size distributions

Figure 3. Particle size distributions

Figure 3. Typical particle size distribution of micron-sized ZnO as evaluated by three different laboratories

Figure 4. Comparison of whitening effects

Figure 4. Comparison of whitening effects

The micron-sized ZnO showed no whitening on the dark substrate whereas the equivalent nano-sized ZnO formulation showed a white streak.

Figure 5. Sunscreens were tested in vivo on five-person panels

Figure 5. Sunscreens were tested in vivo on five-person panels

In a five-person panel, the w/o formulations outperformed the o/w formulations containing equal amounts of ZnO.

Footnotes [McCormick (125(2)]

a Z-Cote Max (INCI: Zinc Oxide (and) Dimethoxydiphenylsilane/Triethoxycaprylylsilane Crosspolymer) is a product of BASF.
b ZinClear IM (INCI: Zinc Oxide (and) C12-15 Alkyl Benzoate) is a product of Antaria and distributed by Dow Personal Care.

Formula 1. Sunscreen formulations

Formula 1. Sunscreen formulations

To demonstrate transparency, a test formula was prepared twice; one version (Formula 1a) contained a nano-sized ZnO powder while another (Formula 1b) included the micron-sized ZnO dispersion.

Formula 2. O/W SPF 30+ with ZnO

Formula 2. O/W SPF 30+ with ZnO

The first set of o/w formulas included an oil dispersion of either nano-sized ZnO or micron-sized ZnO.

Formula 3. W/O SPF 30+ cream

Formula 3. W/O SPF 30+ cream

The second set of w/o formulas included either nano-sized ZnO or micron-sized ZnO.

Formula 4. Natural sunscreen SPF 20*

Formula 4. Natural sunscreen SPF 20*

Mineral sunscreens were formulated using 30–40% w/w micron-sized ZnO dispersions (15–20% ZnO) to develop formulations with an SPF of 20–30+ (see Formulas 4, 5 and 6) and at this loading, the formulas were not found to cause skin whitening (data not shown).

Formula 5. Daily wear lotion with sunscreen SPF > 30

Formula 5. Daily wear lotion with sunscreen SPF > 30

Sunscreens were formulated using 30–40% w/w micron-sized ZnO dispersions (15–20% ZnO) to develop formulations with an SPF of 20–30+ (see Formulas 4, 5 and 6) and at this loading, the formulas were not found to cause skin whitening (data not shown).

Formula 6. Very water-resistant beach wear lotion with SPF 50

Formula 6. Very water-resistant beach wear lotion with SPF 50

Mineral sunscreens have been formulated using 30–40% w/w micron-sized ZnO dispersions (15–20% ZnO) to develop formulations with an SPF of 20–30+ (see Formulas 4, 5 and 6) and at this loading, the formulas were not found to cause skin whitening (data not shown).

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