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SUN CARE
The study found unsafe levels of oxybenzone and homosalate in test samples, which has led to setting limits of 2.2% for oxybenzone (6% limit in the USA) and 1.4% for homosalate (15% limit in the USA). Studies reported by the FDA have shown levels of oxybenzone, ocitnoxate, octisalate, octocrylene, homosalate and avobenzone have been absorbed into the body after only one application and can be found in the body for weeks thereafter.5 Mineral-based UV filters have much better
health and safety data and are sometimes marketed as ‘naturally sourced’. The ‘whiteness’ appearance of a mineral sunscreen and the ease with which it is rubbed in, leaving no white tint on the skin, is largely determined by the particle size of either the titanium dioxide or zinc oxide. The larger this is, the greater the tendency for the sunscreen to appear white.
Regulation of UV filters In addition to human safety and efficacy, sunscreens are evaluated based on their environmental impact. One concern is the impact of the UV filter on the health of coral reefs. In response to this concern, in the past few years, the State of Hawaii and the City of Key West, Florida, have banned sunscreens that contain certain UV filters. The Hawaiian regulation bans the use of
oxybenzone and octinoxate in sunscreens, effective from 1 January 2021, and is evaluating adding avobenzone and octocrylene to the ban, effective from 1 January 2023. The Key West ban targets the same UV filters. Other locales have enacted similar sunscreen bans. Haereticus Environmental Laboratory (HEL)
offers a Protect Land & Sea certification for sunscreens.9
HEL maintains a list of materials that
should not be included in sunscreens due to their risk of environmental harm (Table 3). It should be noted that non-nano zinc oxide is not included as an environmental hazard on any of these lists. A broad-spectrum sunscreen, providing
protection from both UVA and UVB rays, is a highly desired product. Given that sunscreens with a zinc oxide filter have broad-spectrum protection
TABLE 3: HEL LIST MATERIALS THAT SHOULD NOT BE INCLUDED IN SUNSCREENS Oxybenzone
Octinoxate
Para-aminobenzoic acid (PABA)
Butyl paraben
Any form of microplastic sphere or beads.
and that zinc oxide in the non-nano form is environmentally friendly and safe, the preferability of zinc oxide as a UV filter is obvious. In the USA, the FDA allows the use of up to 25% zinc oxide in a formulation. Zinc oxide is typically sold as a solid powder.
As a fine powder, it can be difficult to wet and disperse. Therefore, it is sometimes coated to increase dispersibility when formulated in an aqueous dispersion. Coatings used on zinc oxide can range from medium- to long-chain fatty acids, such as caprylic acid or stearic acid, or silanes, such as triethoxycaprylylsilane. Both coated and uncoated zinc oxides are available in nano and non-nano forms.
Sunscreen components The critical ingredient in sunscreens is the UV filter that provides the sunscreen with the desired sun protection factor. Depending on the desired SPF claims, a combination of UV filters may be required.4 Sunscreens typically have the following
components in addition to a UV filter. First and foremost, they have consumer-facing ingredients and sensory enhancers, such as emollients, moisturising ingredients and fragrances. These provide a sunscreen with a desired fragrance and a luxurious feel when applied to the skin. These performance properties create the customer’s desire to use a certain product over another. The second components are formulation
stabilisers, such as emulsifiers, dispersing agents, chelating agents, pH stabilisers, anti-oxidants and viscosity modifiers. These keep it stable
Methyl paraben Benzyl paraben
Octocrylene
Ethyl paraben Triclosan
Any nanoparticles like zinc oxide or titanium dioxide.
and homogeneous and allow the sunscreen to be spread evenly on skin. They also provide the sunscreen with the desired shelf life.4
Trial formulation A starting point sunscreen formulation using zinc oxide as the UV filter is shown in Table 4. This formulation was designed to obtain a broad- spectrum (UVA and UVB) protection with an SPF of approximately 15. It should also be noted that the composition of this sunscreen is primarily derived from natural ingredients (>90%)’ after ‘primarily. To create sunscreen, Sequence 1 ingredients
are mixed at ambient temperature with a Silverson mixer until smooth and homogeneous. The zinc oxide is slowly added and mixed until it is fully dispersed and homogeneous. Sequence 3 ingredients are mixed together at ambient temperature with standard propeller mixing at medium speed. The pH adjuster is added to the Sequence 3
ingredients as needed to ensure that the pH is between 6.9 and 7.2. The combined sequence 3 and 4 materials are slowly added into the combined Sequence 1 and 2 mixture and then are mixed until smooth and uniform at ambient temperature. The product is then packaged in a suitable container. An advantage of using zinc oxide as the UV
filter is that you do not have to use thickeners because the dispersion of zinc oxide provides plenty of viscosity to the system. In addition to the formulation, the particle size of the zinc oxide impacts the final viscosity of the sunscreen.
Experimental A series of three sunscreens were prepared using three different grades of zinc oxide. The particle size of the zinc oxide was determined using a Bettersize model 9300S laser light-scattering detector.
The D50 value, the median particle size in the distribution (where 50% of the particle in the distribution are smaller and 50% of the particles are larger), value for each lot of zinc oxide was 800, 3440 and 5175 nm. The viscosities (25ºC, LVT #2, 12 rpm for 1 minute) of the three sunscreens produced with these three lots of zinc oxide were 2025, 1687 and 1120 cps, respectively. As shown in Figure 1, there is a strong
Figure 1: Correlation between sunscreen viscosity & zinc oxide particle size ZnO D50 particle size (nm)
PERSONAL CARE April 2022
correlation between the final viscosity of the sunscreen and the particle size of the zinc oxide, so a skilled formulator can utilise the particle size of the zinc oxide to create a sunscreen with the desired viscosity. It has also been found that by exposing particles of zinc oxide to high shear mixing for extended periods of time, it is possible to reduce the initial size of the zinc oxide particles.
www.personalcaremagazine.com
4-methylbenzylidene camphor
Propyl paraben
Viscoisty (cps)
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