How to optimise sprayable sun care formulations

The sprayable format within the sun care category has grown considerably in popularity, mainly due to consumer preference for a convenient product format that can be sprayed directly onto skin and the desire for a non-greasy application. In this article, Alchemy Ingredients explores the optimisation of sprayable oil-inwater emulsions formulated with organic sun filters.

The formulation of sprayable sun care products can take different forms, including water-in-oil and oil-in-water emulsions, oil-based products and volatile solvent-based formulas (aerosol or non-aerosol).

To reach a high SPF, a relatively high-level of organic or inorganic (mineral) sun filters must be used. Certain sun filters are more effective at absorbing in the UVB range, while others perform better in the UVA range. Ultimately, the formulator needs to select a suitableblend of filters to obtain the correct UVB/UVAprotection ratio to prevent aging and skin cancer. However, this fine balance is beyond the scope of this article.

The maximum permitted levels of some organic sun filters are outlined in Table1. Approvals for sun filters vary greatly throughout the world and are constantly changing. There are some online tools available for calculating the predicted SPF of a product by entering the level of different sun filters used. For example, by adding 5% of Homosalate, 10% of Octylmethoxycinnamateand 5% of Octyl Salicylate to a formulation, you can achieve an SPF of 15. If you add a further 10% of Octocrylene, you can reach an even higher SPF of 26.

However, these tools are only an indication and further testing is required to determine the actual in vitro or in vivo SPF.

The aim of the formulator is to develop formulas that not only have the required SPF, but also feel good on skin and are light and non-greasy. It is helpful for a sun care formulation to have additional characteristics such as waterproofing, anti-aging properties and favorable rheology. This means other formulation additives should be selected with care. Some of the commonly used sun care additives and their functions are detailed in Table 2.

KNOWLEDGE ON EMULSIFIERS

One of the most desirable characteristics of an oil-in-water emulsion is the fine, smooth texture. This can be achieved by obtaining a small oil droplet size. A smaller, more uniform droplet size will also lead to greater stability as Ostwald ripening and flocculation will be minimised.

Certain emulsifiers are better at producing a small particle size than others, so different emulsifiers need to be investigated to see what works well. Another factor to consider is that the emulsifier should not add a lot of viscosity to the sprayable formula, or it will become too thick to spray.

The rheology of the product should be carefully considered – formulations that are Newtonian or shear thickening may have issues when packaged in a sprayable format. The ideal type of rheology for this formulation is thixotropic, as the force applied when the material is sprayed causes it to temporarily thin down. The structure reforms once it has been applied on skin, preventing run-off. This also helps with stability, as the thixotropic thickening system prevents phase separation.

RECOMMENDED EMULSIFIER SYSTEM

An emulsifier system that works well in sprayable oil-in-water products is Sucragel AP V2 (INCI: Glycerine, Aqua, Sucrose Laurate) from Alchemy Ingredients, represented locally by Carst & Walker. This emulsifier effectively emulsifies high levels of oil while remaining at a low viscosity. It also produces a small droplet size due to the unique shape of the sucrose laurate molecule, which allows tightly-packed micelles.

A special method is recommended when using Sucragel as an emulsifier to obtain optimum texture and stability. Firstly, the oil phase must be incorporated into Sucragel to make a gel. This forms a concentrated emulsion where the oil phase is approximately 85% and droplets are around 300nm in size.

The second step involves dilution of the gel or concentrated emulsion with water to make the emulsion and increase the size of the external phase. The degree of dilution can be varied depending on the level of oil required. Viscosity will remain low regardless of the amount of oil; however, it can be manipulated using rheology modifiers in the diluting external phase.

SUCRAGEL CONCEPT IN SUN CARE

A study was carried out to establish whether a Sucragel emulsifier system would be suitable for a sprayable sun care product and whether it gives an enhanced SPF compared to the online calculator. It was predicted that using Sucragel / Sucrathix VX would result in a better than predicted SPF due to a small particle and even dispersion of sun filter.

In these two formulations, featured in tables 3 and 4, Sucragel AP V2 was used in conjunction with various sun filters and other ingredients to create a low-viscosity, thixotropic, stable sun lotion with an elegant skin feel. The sprays were also tested to determine SPF. Testing was carried out independently at Helioscreen SAS, France, using a Labsphere UV2000c in vitro method. The use of another additive from AlchemyIngredients, Sucrathix VX (INCI: MicrocrystallineCellulose, Cellulose Gum, Xanthan Gum), proves useful in optimising stability in these formulations. Sprayable formulas are very susceptible to separating due to their low viscosity – Sucrathix VX allows the structure to remain fully sprayable while providing thixotropy. It also gives a light, non-sticky skin feel.

CONCLUSION

Sucragel, in conjunction with Sucrathix VX, provides a stable, sprayable system for sun care products. It fulfils the brief for a light, non-greasy product, with thixotropic rheology, ensuring the product remains on the skin until rubbed in.

Due to the small droplet size, the SPFis maximised and minimal skin whitening occurs. Sucragel AP V2 and Sucrathix VX are classified as natural according to theCOSMOS standard, meaning that the natural index of formulations can be increased when using these ingredients. In addition, a major benefit is the enhanced SPF benefit of this system as a result of the small particle size and excellent dispersion of sun filters in the formulation. •

Alchemy Ingredients – www.alchemy-ingredients.com

Carst & Walker – www.carst.co.za