TiO2 and ZnO Powders

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I am going to focus again on zinc oxide and titanium dioxide. These inorganic sunscreens come in a powder (particulate) form. Their particle size is very important. The smaller the particle size, the better the UVB protection, hence higher SPF. This is the result of a higher level of surface area which is scattering UV radiation. However, the smaller the particle size, you will lose some of the UVA protection. There is an optimum particle size for in vivo PFA testing. For a critical wavelength of 370nm or above, which is an in vitro test, you want a bigger particle size to provide more blocking (create a film on the skin).  But be careful, because as you get bigger you will be more whitening. Therefore, it is best to choose a particle size that is mid-range, not too small and not too big for both UVB and UVA protection or two use a combination of TiO2 and ZnO.

You can incorporate TiO2 and ZnO in a powder form into your sunscreen formulation. If is is untreated it is naturally hydrophilic and it will go into the water phase. However, it is beneficial to surface treat these powders so that they can be easily incorporated into the oil/silicone phase of your formulation, and also to avoid problems with zinc oxide and pH. W/O emulsions are more efficient for SPF because it leaves a thicker film. Surface treatment can either make TiO2 and ZnO hydrophobic (water hating) and/or lipophillic (oil loving). The sunscreen actives will then be in the oil or silicone phase and not be washed off easily when you come in contact with water.  Making a water in silicone sunscreen is especially nice because W/Si does better with water resistance. For a water in silicone sunscreen, you want to make sure that the sunscreen active is surface treated so that it is hydrophobic. Surface treatment is also beneficial because it will prevent unwanted chemical reactions between the untreated TiO2 or ZnO and the other ingredients in your formula. Additional, small sized TiO2 gets a grayish blue color when exposed to UV light and to prevent this color change a surface treatment of alumina on the TiO2 is extremely helpful.

It is important to note that TiO2 and ZnO are not supplied as primary particle sizes, they are supplied as  aggregates. Aggregates form due to the high surface energy of the primary particles and charges making the primary particles stick together. Furthermore, agglomerates can be formed from the aggregates sticking together. Treating TiO2 and ZnO and breaking the agglomerates down during manufacturing creates smaller TiO2 and ZnO which creates a nicer finished formula.

However, if you are using TiO2 and/or ZnO powders you must have the milling equipment to bring the particles down to a very small size, which is difficult. If you have multiple locations where your sunscreen product is being made, do all locations have the same equipment and will the same parameters be used? It is much easier to use TiO2 and ZnO dispersions which already have the particles milled to an ideal size.  Therefore, the dispersions you use will always have the same particle size and hence your product's SPF/UVA testing will be consistent. I will discuss dispersions in my next post.