Crystallising Sunscreen Formulations

Perhaps now the greatest cause of SPF failure in sunscreen testing, the growth of crystals of actives in sunscreen formulas has become more prominent for reasons set out below. Oxybenzone, 4-Methylbenzylidene camphor and Avobenzone and Diethylamino Hydroxybenzoyl Hexyl Benzoate (Fig 1) above are examples of commonly used active UV absorbers, which are all solids and thus need to be well dissolved in the sunscreen formulation. They will not function as solids in solution, or in the thin film applied to the skin. In order to perform their protective function, they need to be kept in solution not only in the formula itself, but also right up to the point of product dry down on the skin following application.

The three main reasons for an apparent increase in incidence of this issue are...

Higher SPF

The first reason for this resulting lack of efficacy is the move to higher SPFs, which typically requires the inclusion of higher levels of these crystalline actives. The most commonly seen crystals in samples we examine in our laboratory are Avobenzone, as it is the most commonly used UVA absorber. In SPF 50 sunscreens, up to 5% is needed in order to active the now required UVA balance – Broad Spectrum for Australia, for example.

Formulation Elegance

A second reason for crystallisation becoming more prominent as an issue is the trend towards more aesthetic appeal in the formulation. Typically, this involves the use of lower levels of the oils and esters which are needed to act as solvents for the solid actives.

Use of more “Natural” excipients.

If synthetic esters have been replaced by fixed oils , then, as these are almost universally poor solvents for the actives. Heating will help during manufacture, but precipitation on cooling is very likely. For example, 5% Avobenzone would require at least 60 % of Fixed Oil to be solubilised. Fig 2 above is an example of this effect.

Solubility in other Actives

Among the best solubilisers are other sunscreen actives - Ethylhexyl salicylate, Homosalate and Ethylhexyl Methoxy cinnamate. These can have around 4 to 5 times as much solubilising capacity as fixed oils, but for each percent of solid active, at least 2 or 3 times as much “solvent” is needed.

Challenge the Formula

There are several steps that can be taken to screen formulations before sending for solar testing. The first is temperture looping – fridge to 40oC for 4 or 5 cycles. The second is to prepare the formulation without the water phase and place in the fridge overnight. Then check microscopically.

Several suppliers of cosmetic emollient esters provide guidance on the solubility of specific actives in their cosmetic excipients. Often, a mix of these will work better. Fixed Oils and silicones are particularly poor solvents for these actives.

Apart from when the formulation includes high levels of inorganic actives or other oxides, it is usually easy to identify which active/s are at issue.

Crystallisation is not just an issue for actives that have been in long term use. The manufacturer of Bemotrizinol has reported (1) the challenge of including more than around 3 percent of this newer active in sunscreen products.

The advice of this author is to invest in microscope for the R & D lab. It is surprising how many labs do not use this cost effective tool. This simple check can reduce the disappointment when SPF testing does not reach expectations.

Ref. https://www.carecreations.basf.com/science-excellence/studies- articles/news-detail/2013/9/10/solubility-of-uv-absorbers-for-sunscreens-is- essential-for-the-creation-of-light-feel-formulations