54 SUN CARE
Using spherical silica gels to improve sunscreen
Michael Sweeney - AGC Chemicals Americas
Skincare product formulators often turn to water-in-oil (W/O) emulsions when working with inorganic sunscreen actives because of their superior sun protection factor (SPF) efficiency per unit of active. Oil-in-water (O/W) formulations have tactile aesthetic advantages but are less SPF-efficient and present formulation issues with inorganic sunscreen actives, especially zinc oxide. W/O formulations provide better SPF
efficiency in part because of the more consistent film they apply to the hydrophobic skin surface. There are other advantages as well, including the fact W/O emulsions have a degree of inherent water resistance not seen in O/W emulsions that contain hydrophilic emulsifiers.
Challenges of W/O formulations using mineral actives Despite these significant advantages, W/O formulations have always had some major drawbacks when it comes to their look and feel on the skin. They generally feel very greasy and slimy when applied. Also, they seem to sit on the surface of the skin instead of rubbing in. Apart from the aesthetic issues, W/O
formulations can be more difficult, time- consuming and tricky to produce than their O/W counterparts. When waxes are used in the formulation, there are added challenges when processing at high temperatures. A challenge for all formulations containing
inorganic sunscreen actives is achieving the desired SPF without leaving the skin with a white cast. Recent advances in silica gel technology have solved many of these issues. Now it is possible to produce W/O
formulations with mineral actives that provide the desired SPF and tactile aesthetics without the white cast on the skin. In fact, W/O formulations can now be created that rival the appeal and processing ease of O/W formulations.
Challenges of safe UV protection for daily wear applications Skincare and cosmetic brands have been working hard to develop higher-SPF broad spectrum formulations for daily wear products including moisturizers and foundations. Interest in using mineral-based actives like zinc oxide continues to rise, driven by the current regulatory climate in the US surrounding organic (chemical) UV filters. These chemical actives are increasingly targeted as unsafe for users and the environment. Zinc oxide is long recognized by consumers
PERSONAL CARE October 2022
as a safe inorganic UV filter, making it the ideal choice for sun care and cosmetic products with broad spectrum protection. Data shows that sunscreen launches in the US using only zinc oxide as the UV filter increased by about 20% from 2017 to 2018, while new sunscreen launches containing all-chemical UV filters have been on the decline since 2014. This trend continues based on the activity and discussions at the Florida Sunscreen Symposium held in September 2021. Now, some zinc oxide formulations are able to support ‘free- from’ and ‘clean’ claims. However, formulations containing enough
inorganic (mineral) actives to achieve broadband SPF status have always had qualities that are unattractive to users. They feel thick, heavy, pasty and sticky, appear white, and do not absorb into the skin. Consumers may accept these qualities for sunscreen products but certainly not for daily wear applications like moisturizers and foundations.
SPF booster options The best way to improve the aesthetic qualities of a mineral formulation is to reduce the percentage of mineral filters used to deliver the required SPF. This is achieved by raising the
efficacy of the filters. Particles with no in vitro SPF can be added
that scatter the light (UV rays) through the layer of sunscreen on the skin. These additives are called SPF boosters. Adding SPF boosters to an emulsion reduces the amount of sunscreen active required to reach a given SPF value. Several types of SPF boosters are
available as sphere-shaped particles. Waxes like beeswax can be used to optimize the rheological behavior of products and provide an even sunscreen film on the skin, which can boost the SPF of UV filters. However, they do not offer any water resistance and are difficult to process. Other SPF boosters are made of styrene/
acrylate copolymers that are built as hollow spheres. These can provide up to a 25% SPF boost when used with zinc oxide but are petroleum-based and present some processing challenges. Another type of SPF booster is microsphere- shaped silica gels. These gels are available with various degrees of porosity. Like hollow-sphere particles, silica gels do not have any inherent sun protection in W/O sunscreen formulations. For example, when tested, a formulation with
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