Roast my hypothetical anhydrous sunscreen formulation?

Hello everyone! This is my first post here, so hopefully I’m not exposing myself to be too much of a fool.


Essentially my question is this: is there something (obvious) wrong with the following hypothetical sunscreen formation?


The goal would be to create an anhydrous, mattifying sunscreen with okay-ish skin feel and broad-spectrum coverage. I’m not sure how much of the C12-15 alkyl benzoate would be necessary for adequate pigment dispersion, and I understand I may also need something to aid with pigment suspension. I may also have to add a tint to reduce the white cast.


Still, aside from all of that I sense that I’m missing some very major reason(s) this would fail. Any thoughts? Thanks in advance!


C12-15 alkyl benzoate

...~25??

Zinc oxide, nano, coated (HP1 Z-Cote)

~25%

TiO2, nano

~10%

Silica

...~1%?

Dimethicone/vinyl dimethicone crosspolymer

to 100%


Roughly, the procedure would involve heating the C12-15 alkyl benzoate and dimethicone/vinyl dimethicone crosspolymer, adding the zinc oxide and TiO2, homogenizing (as best I can with a vortex mixer, that is..), adding silica, and mixing again. I’d check homogenization with a light microscope.

Comments

  • AgateAgate Member
    edited December 2020
    Disclaimer, I have no personal experience formulating sunscreens, so I hope that others will comment.

    I decided not to bother for now because from what I read, the efficacy of sunscreen formulas is very hard to predict theoretically, and testing for UV protection is one of the most expensive tests in the cosmetic realm. (See here: The Trouble With Making Your Own Sunscreen | Realize Beauty (wordpress.com))
  • @gilac I work with sunscreens. For suspension of 35% pigment, you might want to go with polyhydroxy stearic acid (1.2%) to prevent agglomeration and SPF enhancement. Silica to thicken Oil Phase might affect the texture of finished product, you might want to use Ethylcellulose as thickener.
    What is your target SPF 30 or 40?
  • chemicalmattchemicalmatt Member, Professional Chemist
    First off: you are not a fool, and there are no judges in this forum ( if they lurk they need to leave), so you came to the right place. Which silica are you using? If the Aerosil (Evonik) range is employed, I would double the amount to 2%. Great suspension, no need to change. I would also add another wetting/dispersion ester (there are many) to go with Finsolv TN and more of both; decrease the cross-polymer accordingly.  Then there is your technique. Mix the silica into the esters at high sheer for a long time to absorb and disperse, then add the other ingredients, cross polymer last.  You have a lot of TiO2 and ZnO in there too. More than SPF 30. Why go any higher?
  • emma1985emma1985 Member
    edited December 2020
    As someone who is first and foremost, a skincare consumer, I'm thrilled to see that you are using 25%, the maximum allowable concentration, of Zinc Oxide, in addition to a high concentration of Titanium Dioxide. 

    It is pretty well known in skincare communities that though Zinc at lower concentrations than 20% will impart UVB protection, it does not provide adequate UVA protection. 

    See this write up.

    https://medium.com/gethealthy/what-are-the-best-sunscreens-to-protect-against-skin-aging-5a13e0e66159

    I also disagree with the argument that because SPF 30 blocks 96.7% of UVB, that a higher SPF sunscreen is not warranted. This is an incomplete analysis because what this argument neglects completely is UVA.

    UVB causes melanoma and sunburn, UVA causes 80-90% of visible skin aging AND also plays a role I the development of skin cancer.

    What I can tell you is that informed skincare consumers are more concerned with protecting their skin against UVA, because UVA is the frequency that causes the overwhelming majority of day to day sun damage. 

    For example, UVA penetrates glass, so we are essentially always exposed to UVA (unless you live in a basement with no windows.) UVB does not. Additionally, UVA penetrates much deeper in the skin than UVB, destroying DNA by activating matrix metalloproteinases. Lastly, UVB is much stronger in the summer and much weaker in the winter, whereas UVA is more constant and its "strength" does not fluctuate as much as UVB through the seasons.

    The higher the SPF and Zinc Oxide content, the better the UVA protection.

    See this write up by a microbiology Ph.D. about why SPF 50 is always preferable to SPF 30.

    http://cyrillelaurent.com/2018/12/01/spf30-or-spf50/#:~:text=An SPF30 transmits 3.3% of,burn the skin, the better.

    All that being said, unfortunately I cannot help with the actual formulation. But I wanted to jump in and say how happy I am to see you using 25% Zinc Oxide. Most sunscreens on the market use less than 10-15%. I have had such a hard time finding high concentration Zinc Oxide sunscreens that I eventually switched to chemical (organic) sunscreens. So in my opinion there's definitely a lack of high Zinc Oxide concentration sunscreens on the market.
  • ngarayeva001ngarayeva001 Member
    edited December 2020
    You need a film former (trimethylsiloxysilicate or similar) to make sure it doesn’t rub off and stays in place. Also interested to know which silica is that and whether it thickens alkyl benzoate.
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