PRESERVATIVES
Solving the sunscreen preservation challenge Sunscreen products are known to be challenging to preserve. Yet, the reasons are diverse and not fully addressed. Naama Eylon, VP personal care at Sharon Laboratories, looks at the influence of the unique composition and features inherent to sunscreen formulations and how they affect preservation, as well as new alternatives that solve the antimicrobial challenges of this changing segment.
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un care is considered one of the most important categories in personal care. Formulating sunscreens requires specific knowledge, as many different aspects should be taken into account. An optimum sunscreen formula should first provide broad-spectrum skin photoprotection. However, as a sunscreen is a cosmetics product, it should also hold sensorial appeal and good skin feel. Over the years, the preservative challenge for sunscreens has remained a known fact, but it is one that is not fully explained. As a result, it has become common practice to concede a quantitative answer by raising the level of use of the preservatives, creating issues such as incompatibility with other ingredients, instability of the formula, odour and colour impact, potential skin irritation and more. In discussions with sunscreen formulators, we hear the same struggle: a high SPF formula with common or popular preservatives fails a challenge test. At this point, the product development team will likely add another booster, and sometimes another system, to reinforce the antimicrobial protection. We also often see the use of traditional preservatives, which are no longer in favour, such as parabens and formaldehyde-donors like diazolidinyl urea, for the sole reason that they provide the protection needed, even if they go against the requirements of a clean INCI or the clean beauty marketing perception.
BACK TO THE BUILDING BLOCKS To uncover the reasons why sunscreen formulations require special preservation attention, we need to start with the core building blocks of SPF formulations – UV filters. A sunscreen can be classified depending on its mode of action. UV filters absorb, reflect or scatter harmful UV rays, preventing them
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FEBRUARY 2021 // WWW.PHARMACOS.CO.ZA
Did you know? SharoSENSE Plus systems have the right polarity to ensure the preservation of sunscreen formulations while using a clean INCI. The maltol-based line is not pH-dependent and is also readily biodegradable, offering an additional benefit in terms of sustainability.
from penetrating the skin. A formula would contain either a physical UV filter such as zinc oxide or titanium dioxide, or an organic UV filter like benzophenone, octyl salicylate, or more commonly – a combination of organic and mineral. Blending of various UV filters at low percentages is sometimes preferred. Processing oily-soluble UV filters into a stable fine emulsion dictates the use of a greater dosage of solubilisers and humectants, resulting in a significantly higher oily-phase formula. Unlike other cosmetic emulsions, which are composed of both polar and non-polar phases evenly dispersed in one another, in sunscreen emulsions, the non-polar phase has a greater organic share.
THE SIGNIFICANCE OF POLARITY Defining polarity is important to understand the key to the solution. Each molecule has a polarity value that can be scaled in many ways. One of the most popular polarity scales is Log
P (or the partition coefficient). Preservatives also have a Log P value that can anticipate their distribution in a cosmetic emulsion. Non-polar preservatives, which can be defined by a Log P value that is greater than 1, will have a tendency to migrate into the organic phase. This characteristic will result in the preservative ‘deserting’ the water phase. As antimicrobial activity originates in the water phase, a poorly protected water phase is surely going to pose a problem. As we review the polarity index of various preservatives, we find that many popular and accepted preservatives such as phenoxyethanol-based systems are in fact non-polar, have a Log P value greater than 1 and are likely to demonstrate low performance in sunscreen formulas. This assumption was taken to a comparative test. Several preservatives with different polarity indexes were chosen and challenged in an EP protocol test in the same representative formula (see Figure 1).
