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.

Sun 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 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 than1, 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).

Figure 1: The representative formulation used in the EP protocol challenge test

The challenge test results show that a non-polar system based on phenoxyethanol, with a Log P of 1.12, is insufficient to protect such a non-polar high SPF formula. At the same time, polar preservatives such as methylisothiazolinone with a Log P value less than 1 (-0.83) demonstrated good results (see Figure 2).

Figure 2: Challenge test results

We can visualise the comparison of one gram-negative bacteria as a benchmark, to four alternative preservative systems. With the non-polar preservatives, from inoculation and as the test continues, the bacteria continue to grow, resulting in a failed challenge result.

However, the polar preservatives show excellent results as the challenge test continues.

A NEW LINE OF PRESERVATIVE BLENDS

Once we understand that it is key to use the Log P of a preservative to determine its efficacy in sunscreen products, all we have to do then is select a polar preservative. Yet, there is another challenge: the list of polar preservatives with good consumer perception is in fact very short to non-existent.

This understanding initiated a search for a polar preservative system which is free from traditional and undesired ingredients, supports broad-spectrum protection and offers good compatibility with various UV filters. In addition, the non-pH-dependent characteristic is important, as sunscreen formulations are mostly at a neutral pH, which eliminates organic acids as an option.

The subsequent R&D process resulted in a line of preservative blends known asSharoSENSE Plus. Available in South Africa from CJP Chemicals, the SharoSENSE Plusline is based on the active ingredient maltol, which was identified as a key ingredient in this concept. It is a naturally occurring organic compound, found in several plants. Maltol is used as a flavour enhancer (food additiveE636), as an intermediate in pharmaceuticals and sometimes in fragrances.

It has interesting antimicrobial activity, but not sufficient to be considered a preservative. In the quest to enhance the properties of maltol, the addition of a cationic surfactant component within a specific range was found to be the best solution. A series of cationic surfactants were tested and through an optimisation process, a specific ratio was determined as the optimal way to achieve high-efficacy, presenting synergistic effects with maltol.

A ROBUST AND EFFECTIVE SOLUTION

The patented SharoSENSE Plusline demonstrates high solubility, with no need to use a solvent to carry it into the final product. This solvent-free approach offers immense benefit to the formulator, answering clean beauty trends, which can be achieved through the use of a minimalistic approach and green ingredients. It is also interesting to note the activity of maltol in a high pH-range. Many of the popular preservative solutions such as organic acids are suitable for pH of up to 5.5, leaving less preservation options for high pH-products. Maltol, however, demonstrates increased activity at a pH of 7 to 8.

SharoSENSE Plus has the ideal polarity profile for sunscreens with a Log P of0.09. This system was used as a single preservative system in the same formula shown earlier, as can be seen in the comparison noted in Figure 3. Being a polar preservative, SharoSENSE Plus performed well in the same high SPF formulation. Looking at the full challenge results, Figure 4 shows that at a level of use of only 0.7%, full broad-spectrum protection is achieved.

Figure 3: Comparison results

Figure 4: Full challenge test results

SUSTAINABLE AND SUNSCREEN-FRIENDLY

In conclusion, SharoSENSE Plus products have a proven ability to preserve sunscreen formulations at a level of use lower than 1% and as a sole preservation system in a formulation.

When we think about a sunscreen, we take into account the safety of the consumer, their skin, and most recently, environmental safety. Today we understand the safety of the product itself is being questioned, which can only be addressed through adequate protection.

With SharoSENSE Plus, formulators now have a solution that will not only address the technical challenges unique to formulations containing UV filters but also the regulatory changes pertaining both to UV filters and preservative systems, while meeting consumers’ needs for green, clean and transparent ingredient lists.

Ultimately, the essence of assembling a successful sunscreen formula lies in the sensitive balance of these complexities. •

CJP Chemicals – www.cjpchemicals.co.za Sharon Laboratories – www.sharon-labs.com