Inorganic Sunscreen Actives

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There are two kinds of sunscreen actives. One is inorganic sunscreen actives and the second is organic sunscreen actives. If you are not familiar with the technical term of organic, it refers to any material that is based on the element carbon. Inorganic sunscreen actives (do not contain carbon) consist of titanium dioxide (TiO2) and zinc oxide (ZnO). Inorganic sunscreen actives are also called physical sunscreens actives because of the mechanism used to protect your skin from UV rays. Physical sunscreen actives work by a combination of absorption and scattering the UVA and UVB radiation. Titanium dioxide protects against UVB rays but not the full spectrum of UVA rays. Zinc Oxide, on the other hand, protects against the entire spectrum of UVB and UVA rays.

There are two kinds of TiO2, anatase and rutile. Rutile TiO2 is more stable than anatase TiO2. TiO2 and ZnO both are powders.

Here are some of the differences between TiO2 and ZnO:

                                                      Anatase TiO2 Rutile TiO2       ZnO
Molecular Weight (MW)                       79.87              79.87              81.4
Density, g/cc                                            3.9                  4.2                 5.7
Refractive Index                                     2.52                2.76               1.99

As you can see from the chart, ZnO is heavier and more dense than TiO2. ZnO has a lower refractive index than TiO2 which means it is not as whitening as TiO2. If you have ever compared TiO2 and ZnO on your hand, you will see how much more whitening TiO2 really is! TiO2 is part of the reason why some sunscreens are very whitening on your skin (particle size is another reason). Going along with refractive index, rutile TiO2 has a higher refractive index, hence more whitening,  than anatase TiO2. It is important to note that because rutile TiO2 has a higher refractive index and is more whitening, it does NOT mean that rutile TiO2 is more opaque than anatase TiO2.

TiO2 can feel draggy. ZnO always feels grainy. ZnO is actually antimicrobial. TiO2 is easier to work with because it is inert is almost all applications. It is only soluble in very strong acid. ZnO, on the other hand is amphoteric, meaning it is able to react with both base and acid. Too avoid issues, it is recommended that you use zinc oxide in W/O (water in oil) or W/Si (water in silicone) emulsions. You have to be careful if formulating ZnO in an O/W emulsion because at pH values <6, the solubility of zinc oxide increases and ZnO may migrate from the oil phase to the water phase causing pH drift and change in color in the emulsion. For this reason, it is best to add ZnO to the oil phase of the formulation. This can best be achieved by using hydrophobic coated ZnO in a oil/ester/silicone/or hydrocarbon based dispersion. The hydrophobic coating will prevent the ZnO from migrating to the water phase of the formulation. Additionally, one can avoid the use of or minimize the use of anionic emulsifiers and thickeners such as carbomers.  Another sunscreen formulating issue that you may run into is using TiO2 and vitamin C. Vitamin C itself is not stable and it reacts with TiO2.

The max concentration of TiO2 you can use is in a sunscreen is 25%. Japan has no limits. The max concentration of ZnO that you can use is 25%. The exception is Australia where you can only use 20% ZnO. Japan also does not have a limit for ZnO.  Inorganic sunscreens start protecting you immediately after application.

In the next post I will discuss organic sunscreen actives.