Zinc Oxide USP-2 vs USP-3 for concealer formulation

Which is likely better? USP-2 has mean particle size 330 nm, USP-3 220 nm.

Want to optimize for blendability (no mineral clumps in the formula) whilst retaining good opacity.
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  • BobzchemistBobzchemist Member, PCF student
    Try them both and see.
    Robert Zonis, Sr. Formulation Chemist, Beaumont Products "All opinions and comments expressed are my own, have no relation to Beaumont Products, are fully copyrighted, and may not be used without written permission."
  • BobzchemistBobzchemist Member, PCF student
    Sorry for the too-quick answer.

    Theoretically, pigments produce more opacity/intensity from using the smallest possible particle size that's above the reflective wavelength of the visible light that they reflect, and from having the largest number of individual particles in a given formula.

    In theory then, all other things being equal, the USP-3 should be significantly more opaque/reflective than the USP-2. But...there's a catch.

    ZnO powder loves to agglomerate, and it does so very strongly. One of the reasons that coated ZnO has been so successful is that the powder treatment inhibits re-agglomeration. So, if you're using untreated ZnO, and you mill it strongly enough to de-agglomerate it down to its primary particle size, you might also be breaking down the primary particles also. In that case, you wouldn't see a difference between -2 and -3, because all the particles would be the same size. Sadly, you'd get the same results if your milling/grinding process wasn't strong enough to get down to the primary particle size - for both grades, you would be milling down to the same agglomerate size, and you wouldn't be able to see a difference.

    So, the answer to your question is that it depends on how strongly and completely you can mill the ZnO. If you have the batch time and the milling power available to actually get down to the primary particle size, then USP-3 will be better.  But that kind of milling isn't easy, or cheap, or quick. And the milled material will want to re-agglomerate, so you will need a way to keep that from from happening.  It may turn out that you only have the time and power to mill down to 2 microns - which is twice as small as lipstick pigments are typically milled down to - but is still 2,000 nanometers. In that situation, since both ZnO grades will be the same size agglomerate, the only difference between the two grades will be the porosity of the agglomerate. Porosity controls oil absorption, so it will take more oil to wet out the USP-3, but you won't see any other difference.
    Robert Zonis, Sr. Formulation Chemist, Beaumont Products "All opinions and comments expressed are my own, have no relation to Beaumont Products, are fully copyrighted, and may not be used without written permission."
  • Bobz you should really do an article on colour cosmetics / pigments for Chemistcorner! 

    How does reflective wavelength work? Is there some sort of coefficient you multiply particle size with to see what parts of the visible spectrum it reflects? Allegedly USP-1 with a 110 nm avg particle size is less opaque and less suited for this use.

    In my case I have a oil/hydrocarbon based formula, I'm going to try a TiO2 and perhaps Boron Nitride combo, but perhaps you could comment on these strategies to ensure good opacity without excessive agglomeration:

    Ideas to get around ZnO agglomeration
    1. Switch to TiO2 exclusively.
    2. Use coated ZnO; Are there oil soluble coatings?
    3. Use a pre milled ZnO/TiO2 slurry.
    4. Mill minerals with Lortone ball mill using 1/4" stainless steel balls
    5. Lapidation of the ZnO/TiO2 in paste form
    6. Outsourcing of the milling?
    Thanks!


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