SUN CARE
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.
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he 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 suitable blend of filters to obtain the correct UVB/UVA protection ratio to prevent ageing 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 Table 1. 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 Octylmethoxycinnamate and 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-ageing properties and favourable 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,
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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
Table 1: A selection of permitted organic sun filters EU
US
Rest of world incl. Japan and Australia
4-Aminobenzoic Acid
Banned
15%
15%
Padimate O
Banned
8%
8%
8%
4%
Varies between 3% and 8%
Banned
3%
6%
Dioxybenzone
3%
3%
3%
Oxybenzone
10%
6%
5% to 10%
Homosalate
10%
15%
10% to 15%
Banned
5%
5%
Octocrylene
10%
10%
10%
Octyl Methoxycinnamate
10%
7.5%
10% to 20%
Octyl Salicylate
5%
5%
5% to 10%
Sulisobenzone
5%
10%
10%
Banned
12%
12%
5%
3%
5% to 10%
Ingredient
Phenylbenzimidazole Sulfonic Acid Cinoxate
Menthyl Anthranilate
Trolamine Salicylate Avobenzone
