Developing high performing zinc oxide containing sunscreens is among the most difficult challenges facing the cosmetic chemist.
- Small formulation changes can result in large losses in SPF
- Need to provide good in use SPF/UVA protection with minimal active migration on skin over time
- Provide good wash resistance
- Provide good skin feel with minimal whitening
- Have good skin compatibility
- Prevent SPF loss on aging due to metal oxide agglomeration
- Sunscreens made using powdered metal oxides can be very processing/formulation sensitive
- Zinc oxide is a dense material and which can settle in formulations over time, causing SPF variability. This can be a problem when zinc oxide is formulated into the oil phase of a water-in-oil emulsion unless the continuous oil phase is thickened and the formulation has a high enough yield value to help suspend it.
The major factors that influence the performance of zinc oxide based formulations include:
- The type of zinc oxide used
- Particle size-primary, aggregate, and agglomerate size
- Particle porosity, surface area, surface texture, and shape
- Surface treatment-uncoated/coated, coating type
- Dispersant/wetting used-type and amount
- Type of emollient if dispersing in oil; polarity and surface tension
- Type of emulsifier (if making an emulsion)
- The type of formulation; o/w, w/o, anhydrous oil
- Process used to make the formulation
During the last 1.5 years, I’ve done extensive research on formulating high-performing zinc oxide based sunscreens using a technical modeling, single variable approach. Over 400 prototypes were made, tested in vitro for SPF, and a rigorous technical performance model was developed. Every aspect of the formulation was examined, including evaluating different types of zinc oxide powders/commercial dispersions, the impact of dispersants, emulsifiers, emollients, SPF boosters, and processing. My conclusion is that the best strategy is to use the least amount of zinc oxide possible to achieve the desired SPF; efficiency is key. Current commercial zinc oxide products have an average efficiency of approximately 1.5-2.5 SPF units/1% active, which means that an SPF 50+ could contain around 25-33% zinc oxide. My research has shown it is possible to achieve an SPF of 60+ using only 10% zinc oxide. This results in a dramatic reduction in cost, improved skin feel, and much less skin whitening. The best current commercial formulation uses 12% active to achieve an SPF of 50+.
In general, using a zinc oxide powder is preferred to using a commercial dispersion due to better SPF/UVA performance, lower cost (~60-90% less) and greater formulation flexibility. Zinc oxide commercial dispersions are typically optimized for reduced skin whitening, which results in too small an aggregate particle size and lower SPF/UVA protection. Powders, however, require significantly more expertise to formulate properly. The choice of powder is critical. In my evaluations, various uncoated powders were shown to vary from an SPF of 12 to 29 when using 15% dispersed in the oil phase of a model water-in-oil emulsion. Silicone-coated powders varied from 6 to 24. The choice of dispersant used was also critical, varying the SPF from 15 to 34 using the same powdered zinc oxide. The best performing commercial dispersants were several proprietary dispersants I developed, polyhydroxystearic acid (PHS), and polyglyceryl 6 polyricinoleate however, differences in performance were seen between suppliers.
What phase you add the zinc oxide to in a formulation is critical. Zinc oxide is very hydrophilic and prefers to be dispersed in water. When adding to an emollient, the surface energy of zinc oxide needs to be reduced to that of the emollient. This is achieved by using the proper dispersing and wetting agents. Using silicone-coated zinc oxide can facilitate this process. In my experiments, adding 15% of different uncoated powdered zinc oxides to the water phase of a water-in-oil emulsion (using a proprietary dispersant I developed) resulted in SPFs of 34 to 108. The same zinc oxides added to oil using PHS as a dispersant only provided SPFs of 15 to 24.
SPF product stability over time is a problem that I believe is not being properly addressed by most manufacturers of zinc oxide based sunscreens. Chemical sunscreen stability in product is normally not an issue. Stability samples can easily be analyzed for their sunscreen content which correlates with their performance. Since the particle size of zinc oxide in formulations is critical to achieving the claimed SPF, simply doing a chemical analysis is not enough. Only an in vitro or in vivo SPF determination can confirm whether the formulation will perform and that the zinc oxide has not agglomerated. In my experiments, I determined that the aged-6-month SPF of a formulation containing 15% zinc oxide dispersed in oil in a water-in-oil formulation can vary from -27 to +10% by just changing the concentration of dispersant used. Using different dispersants resulted in a -2 to -37% loss in 6 month SPF.
In collaboration with Chemspire, I developed a novel patent-pending polyhydroxyricinleic acid (PRA) based dispersant that outperforms the gold industry standard PHS. PRA provides the same level of SPF as PHS, is more natural, has better SPF stability, doesn’t crystalize like PHS and is cheaper.
Recommendations
- Develop in vitro SPF testing capabilities
- Always in vitro SPF test aged versus fresh batches and stability samples (top and bottom of the sample)
- Analyze production batches using in vitro SPF as a quality control test
- Powdered zinc oxide is preferred to using commercial dispersions. It reduces cost and increases formulation flexibility. Silicone coated are preferred if purchasing or making dispersions.
- Use silicone coated or uncoated zinc oxide dispersed in the oil phase or uncoated dispersed in the water phase for water-in-oil emulsions
- Use silicone-coated zinc oxide dispersed in oil for oil-in-water emulsions
- Use 6% dispersing/wetting agent based on the weight percent of zinc oxide used in the formulation
- Formulate SPF boosters in the same phase as the zinc oxide
- Use 1-2% poly C10-30 alkylacrylate or polyethylene as SPF boosters when dispersing zinc oxide in the oil phase of a water-in-oil emulsion. They also can thicken the oil phase to help stabilize the water-in-oil emulsion and keep the zinc oxide from settling and possibly agglomerating.
- Use free radical quenchers like ethylhexyl methoxycrylene or trimethoxybenzylidene pentanedione as SPF boosters when dispersing zinc oxide in the oil phase or water phase of a water-in-oil emulsion.
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Dear George,
Yes, this is one of the biggest challenges I have as a formulator.
I was wondering how much in-vivo SPF data you generated from your experimental formulations. In our experience with in-vivo SPF testing, we rarely obtain a yield of greater than 1.5/% ZnO. We have also found ZnO products that have delivered a much lower in-vivo result than the label claim. From your experimentation, do you have a preferred emulsifier system or emollient ester? Also, there is a myriad of materials claiming to be boosters. Apart from Poly C10-30 Alkyl Acrylate or EHMC which are your preferred boosters? Anyway, you have provided some handy tips that I will try to put into practice.
Thank you for another interesting and informative article.