36 SUN CARE
Crystallisation of solid organic UV filters As already discussed, solid organic UV filters are easily identified under polarised light microscopy. In formulating cosmetic products, it is imperative that these solid organic UV filters are solubilised in the relevant phase and refrain from crystallizing over the course of their shelf life to uphold maximum efficacy. Under polarised light, crystals of solid UV filters can be non-uniform and can have different sizes and shapes, their shape often indicative of specific chemical characteristics. Crystallisation can impact SPF efficacy and the UVB/UVA efficacy of the formulation due to reduced UV filter levels within the emulsion, as their effectiveness at UV absorption hinges on proper solubilization. Intense crystallization not only jeopardizes SPF performance but can also lead to an undesirable tactile experience, manifesting as a gritty sensation akin to broken glass on the skin. Figure 11 shows the characteristic appearance of some of the most used organic UV filters in sunscreen emulsions when they crystallise. In addressing challenges related to
solubilization, formulators often explore the option of modifying the emollients employed in the formulation. The emollient quantity may be insufficient for adequate solubilization, or the choice of emollient may not be optimal for this purpose. Most organic UV filters are oil soluble. Some of these are liquids and as such are relatively easy to incorporate into formulations. The solid types however require suitable solvents to be incorporated into the oil phase to ensure that they are well dissolved and remain in solution. If the solvent properties of the oil phase
are poor, the filters can re-crystallise during storage, thus dramatically reducing efficacy. If recrystallisation does occur, SPF can be adversely affected by a poor solvent system. To determine the optimal emollient for solubility, conducting a straightforward solubility study can be conducted, where the maximum concentration of a UV filter that a specific emollient can dissolve (referred to as the saturation point) is determined. Table 1
UNPOLARISED LIGHT
Figure 9: Diagram comparing unpolarised and polarised light
shows a Croda study of emollient solubility, encompassing various organic UV filters and a range of Croda emollients. It is worth noting that the study,
while providing valuable insights, utilizes concentrations beyond the maximum permitted levels. This deliberate choice is made to underscore and highlight the optimal emollients for the solubilization of each UV filter, offering valuable guidance in formulating cosmetic products. The results are based on a blend of the organic UV filter and the emollient only, not a finished a sunscreen formulation. Another factor contributing to the
persistence of undissolved UV filters may stem from the manufacturing process itself. It is essential to ensure that the manufacturing process reaches high temperatures necessary for reaching the melting point of solid organic UV filters. While this critical information is typically
available on the raw material data sheet, it is worth noting that the specified temperature could be as high as 85°C. This attention
POLARISED LIGHT
to manufacturing parameters reinforces the meticulous considerations required to guarantee the thorough solubilization of UV filters in cosmetic formulations. Lastly, pH of organic filters can also be crucial. Phenylbenzimidazole Sulfonic Acid (PBSA) is a water-soluble organic UVB filter whose solubility can be improved by adding a base (e.g. NaOH or TEA) to the formulation. In formulation, the pH needs to be buffered to pH6-7, as at lower pH, PBSA can revert to its acid form, resulting in precipitation (hence the appearance of crystals), shown in Figure 12. Recrystallisation of UV filters is a major risk
in formulation and must be avoided at all costs. Crystalline UV actives must be kept in solution throughout the life cycle of the product, as recrystallisation will result in a lower protection factor, a reduction in product stability and a deterioration in the appearance of the product. Tracking recrystallisation as a function of
temperature and storage time is necessary over the whole process of stability and studies have shown it is possible to record the nucleation
Figure 10: Miscroscope images of an O/W emulsion conatining cellulose as a sensory modifier, the left image is without polarised light and the right image is with polarised light
PERSONAL CARE February 2024
www.personalcaremagazine.com
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