Achieve efficient, elegant and safe sun care with photostabilised mineral UV filters

The photostabilisation of mineral UV filters is critical to achieve high performance, elegant and safe mineral-only or hybrid sun care products. Hallstar’s photostabilisation technology and its scientific performance measurement platform are a reliable partner to achieve this goal. Hallstar’s sun care solutions are available in South Africa from Millchem.

The importance of photostability for chemical UV filters has been well recognised in the sun care market. However, when it comes to mineral UV filters such as zinc oxide (ZnO) and titanium dioxide), there is a common misconception that they work only by reflecting and scattering sunlight and are therefore safer options. Mineral-only sunscreens are often considered natural sunscreens. These are becoming increasingly popular, despite the drawbacks such as heavy sensory, a whitening effect and a difficult to reach high UV performance.

A closer look at these mineral UV filters reveals their photocatalysis nature. Both ZnO and TiO2 are semiconducting materials capable of absorbing UV energy up to 400nm, which contributes to the most of their UV-shielding ability. After absorbing UV energy, these mineral particles are promoted to their higher energy conduction band and become photoactive, generating reactive oxygen species(ROS) that are harmful to skin. In a hybrid system, where organic and mineral UV filters are combined, the photolability of mineral UV filters accelerates the photodegradation of organic UV filters.

THE BENEFITS OF DPPH TESTING

Effectivephotostabilisationof mineral UV filters not only preventsROS generation and makes them safer, but also improves their photoprotection efficacy. As a result, a lower amount is needed to reach the same or higher SPF/PFA performance. Conventional surface treatment of mineral UV filters is not sufficient. To reveal and monitor the photoactivity of inorganic filters, Hallstar proposes a test utilising the well-studied chemistry of PPH (1,1-diphenyl-2-picrylhydrazyl). The unreacted DPPHradical has a deep violet colour in solution. When it reacts with and is neutralised by other radicals, the solution becomes pale yellow to colourless. During a typical test, ethanolic solutions of 0.01% (w/w) DPPH were used to disperse mineral UV filters or formulations. The solution was irradiated under simulated sunlight or natural sunlight. The faster the colour changes, the more the ROS generation.

Quantitative analysis of ROS generation speed can be achieved by measuring the DPPH concentration. In addition, utilising Solasure – Hallstar’s patented in vitro testing platform – UV protection performance of commonly used ZnO and TiO2 grades with or without Hallstar photostabilisers are scientifically compared at the same sunscreen film thickness. Reproducible data shows that when Hallstar photostabilisers are incorporated, ROS generation of mineral UV filters is effectively inhibited, and a higher SPF/PFA can be achieved with lower UV filter dosage, significantly reducing whitening and tackiness. Table 1 includes an example of a widely-used ZnO powder product in a sunscreen formulation.

IMPROVED SUNSCREEN PERFORMANCE

From this study, it’s clear to see that when Hallstar photostabilisers – either a combination of HallBrite BHB and SolaStay S1, or HallBriteBHB and AvoBrite – are incorporated in a ZnO formulation, higher UV protection performance is achieved with lower ZnO dosage. This is thanks to the improved ZnO absorbing efficiency and dispersion quality.

ROS generation is also effectively inhibited. The overall sensorial portfolio is much more pleasant, with less whitening and lighter skin feel. These additional benefits are also noted:

• the ROS generating rate of the formulation is reduced by 66%

• equal or better sunscreen performance is provided at 40% to 60% less ZnO

• formulation whitening and heaviness aresignificantly reduced. •

Hallstar – www.hallstar.com Millchem – www.millchem.co.za