Sun care is the primary defense against the damaging effects of sun exposure. With heightened consumer awareness and knowledge about the harmful effects of UV rays, especially with growing concerns about premature photoaging and skin cancer, global demand for sunscreen has increased considerably in recent years. In 2015, the sun care market was valued at $14.8 billion and is expected to reach $24.9 billion by 2024.¹

Dynamically as well, SPF products with the inorganic UV filters of zinc oxide and/or titanium dioxide have become more popular than ever in recent years and continue to be an application for brand development and formulation innovation. This growing approval for mineral SPF has not only progressed from the trend for more naturally-derived and organic personal care products, but also from the recent regulatory comments from the FDA, other personal care product authorities, and even state legislatures such as Hawaii.

These statements include discussions on safety and toxicity concerns over the organic UV filters with zinc oxide and titanium dioxide deemed as GRASE by the FDA. With this movement toward sun care products with mineral UV filters, demand remains substantial for elegant, non-whitening, stable and innovative application of SPF formulas. The difficulties of inorganic UV filter formulation, either by aesthetics or by chemistries and processing, have encouraged formulators and companies to develop new methods, ingredients and techniques to ensure the consumers’ requirements are met and have favorable reactions to mineral sun care.

Formulating Concepts
Consider these general formulation and ingredient concepts when formulating mineral SPF products:

Particle Size Distribution
The desire for “non-nano” inorganic UV filters has become ubiquitous with mineral SPF products. Even though various scientific studies have shown little to no evidence that nano zinc oxide/titanium dioxide are systemically absorbed at concerning amounts, consumers and brands still prefer the claim of “non-nano” inorganic UV filters.

Definitions and regulations for non-nano and nano zinc oxide/titanium dioxide can be convoluted and vary, especially if the discussion includes aggregation and agglomeration. While the FDA is still finalizing its opinion on the matter and posed a broad definition and guidance in 2014,² the EU defines non-nano as “the primary particle size is greater than 100nm” and Australia defines non-nano as “more than 90% of particles are above 100nm.”

Regardless of this lack of clarity, there are still options to obtain a more transparent and less whitening mineral SPF formulation and still follow the multiple definitions of non-nano technology. Increasingly, suppliers of zinc oxide and titanium dioxide are ensuring the primary particle size of their product is restricted with a narrow range. The primary particle size is still within the guidance of the EU, while the particle size distribution is more defined and controlled to be as close to 100nm as possible. This helps mitigate the increased whitening attributed to larger particles while adhering to the available, yet ambiguous, regulations. Suppliers should be able to provide information on particle size distribution.

Coated SPFs
For a variety of reasons, including photoreactivity concerns, ease of formulation and additional properties, many mineral products are coated with an inert substance. Alumina and silica are most commonly deposited on titanium dioxide due to reactivity concerns. Some surface treatments for zinc oxide include triethoxycaprylylsilane, jojoba esters and stearic acid. The specific coating utilized can add performance benefits to the formulation, including improvement of feel/aesthetics and increased pigment wetting, which can lead to enhanced transparency and potentially boost SPF values. Some newer surface-coated treatments are offering enhanced SPF values based on the specific coating technology.

Surface-treatments can be helpful depending on needs and requirements and it is often worthwhile to experiment with different surface treatments to develop formulation preference.

Dispersions, Dispersants & Choice of Emollients
Mineral dispersions are perfect options for formulators who either do not have the technical capabilities to make dispersions in-house or want formulation ease in terms of an optimized system. Dispersions can offer effective transparency, aesthetics and even formula stability. Simplicity and ease for production is a further advantage of mineral dispersions. However, if in-house capable, making your own mineral dispersions can be preferable in terms of creativity and for variance in formulation type and skin feel.

Adding a dispersant is essential to your SPF product formulation for a uniform dispersion that often helps create a smooth formulation with reduced whitening. Dispersants often perform as a pigment wetter and thus, help lower surface tension and increase spreading. This is essential for mineral SPF formulations as a uniform and even application is needed for favorable SPF results. Polyhydroxystearic acid is often used in mineral formulations, but emollient esters, such as isononyl isononanoate, ethylhexyl olivate, and various alkanes also provide pigment wetting functionality. Film formers can help obtain a uniform, even application.

Many marketed SPF booster ingredient technologies are based on the concept of solvent polarity. In regard to inorganic UV filters, more polar emollients can help refine the formulation’s overall cosmetic aesthetics. They can function to increase spreadability and moisturization, which is very important in mineral SPF formulations due to the drying and often viscid feel of zinc oxide and titanium dioxide.

Anti-inflammatory & Antioxidant Properties
Due to the challenges of inorganic UV filters, it is also beneficial to reduce, as much as possible, the level of minerals to achieve the desired SPF value. Ingredients with anti-inflammatory and antioxidant properties can function in SPF products to help reduce UV-induced erythema and potentially increase SPF values. SPF is measured via a ratio of Minimal Erythema Dose (MED), or the amount of radiation required to produce perceptible redness. Thus, by formulating with anti-inflammatory and antioxidant ingredients, there is potential to reduce erythema and raise SPF value. Antioxidants are also important in SPF formulations to further protect the skin from UV-induced damage. Possible ingredients include licorice root extract and derivatives (dipotassium glycyrrhizate), bisabolol, tocopherol, tocopheryl acetate, and ingredients rich in natural polyphenols, such as resveratrol, rosemary oil and argan oil.

The challenges of zinc oxide and titanium dioxide are manifold, but these formulation tips make it possible to create effective and consumer-preferred mineral SPF products. 

References:

1. https://www.prnewswire.com/news-releases/sun-care-market-is-expected-to-reach-us-24-9-billion-by-2024-growing-awareness-of-personal-care-to-boost-global-market-says-tmr-882935452.html
2. https://www.fda.gov/regulatory-information/search-fda-guidance-documents/guidance-industry-safety-nanomaterials-cosmetic-products

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Marisa Plescia
Bell International Laboratories

Marisa Plescia is an R&D chemist at Bell International Laboratories in Eagan, MN. She earned a BS in biochemistry from Santa Clara University and earned an MS in pharmaceutical science, with an emphasis in cosmetic science from the University of Cincinnati. She continued her cosmetic science education internationally and enrolled in ISIPCA (Institut Superieur International du Parfum de la Cosmetique et de’l Aromatique) for its European Fragrance and Cosmetic Master program, earning a dual degree: M.Sc. European Fragrance and Cosmetics and a Master of Business and Management (MBM) specializing in the fragrance and cosmetic industry. Marisa is additionally passionate about STEM education and has a M.Ed. in Secondary STEM Education from University of Southern California. She can be reached at m.plescia@bellintlabs.com; Tel: 612-206-3420