Home Cosmetic Science Talk Formulating Zinc rash cream

  • Zinc rash cream

    Posted by geepee on November 18, 2022 at 12:43 am

    Hi I am formulating a zinc cream for rash application and it is a great consistency, however it will only emulsify at lower temperatures ~50C and above that is split into water and oil.  In stability testing it has split when heated after a couple of weeks (not immediately) but has not split in freeze thaw or centrifuge testing. 
    I have never experienced something not emulsifying until it reaches a low temp so feel a bit out of my depth here.  The HLB is balanced with an overall HLB of around 8.
    An outline of the formula is as follows:

    Water <30%
    Zinc Oxide 15%
    Castor Oil <15%
    Apricot Kernel Oil <10%
    Glycerin ~5%
    Glyceryl Stearate Citrate SE 4%
    Mango Butter <5%
    Vegan Lanolin <5%
    Calendula infused oil (olive oil), borage seed oil, isoamyl laurate, aloe vera oil, MCT oil <10%
    Sorbitan Olivate 3%
    Pomegranate Extract <5%
    Rice Bran wax <5%
    Silica <5%
    Plantaserv A 1%
    Glyceryl Caprylate <1%
    I am wondering a few things:

    • Do i add some xanthan to the water phase to thicken it to help stop splitting?
    • Do i try adding a liquid solubiliser like Polysorbate 60 or Soya Lecithin?
    • What could be causing it to only emulsify at low temp - is it due to stopping mixing as it cools or water and oil phase at different temps?
    • Should i ignore the splitting in stability testing in an oven (~40C) as it wont be exposed to that heat for weeks on end (note it did not split until week 3 in an oven, all other samples at room and fridge are perfect after over a month) and i have other samples at room temp that have been fine for over 6 months
    • The product needs to be filled at below 50C so i cannot wait until it is completely cool to fill which is another issue.
    ketchito replied 1 year, 4 months ago 7 Members · 12 Replies
  • 12 Replies
  • paprik

    Member
    November 18, 2022 at 1:27 am

    If I am not mistaken Glyceryl Stearate Citrate is anionic high HLB emulsifier - O/W. 
    But it looks like you are trying to formulate W/O emulsion [Sorbitan Olivate, High lipid input, Lower water, zinc balms are usually W/O] …. Or if you are not trying to formulate W/O you should consider it (?)

    Or add high HLB non-ionic emulsifier with little less Sorbitan Olivate. 

    I think it only holds together when cooled down due to the waxy material that solidify. But there is not enough high HLB emulsifier to hold all that lipid. 

    - Rheology modifier (gum, polymer) is also good for stability. For example XG for O/W or Polyamide-3 for W/O. 
    -Accelerated stability testing is a double-edged sword. As you mentioned, in RT it is ok. Sometimes things that are happening in the over won’t happen in real life. But it is a good indicator. Freeze/thaw is also very good - go for 6 cycles. 

    Hope this helps a little bit, hopefully someone will have another idea what is going on :) 

  • geepee

    Member
    November 18, 2022 at 1:43 am

    Thank you, I was feeling that this might be the issue in regards to the glyceryl stearate citrate.  
    What if I:

    • Added 0.5% xanthan - help thicken the water phase
    • Scrapped GSC and swapped it for another nonionic emulsifier with an hlb ~9 at 5% and took sorbitan olivate down to 2%
    • HLB drops to ~7.5

    Do you think the higher HLB emulsifier would be better at holding all of the lipid at the higher temp?

    100% accelerated stability is frustrating, it’s passed 6 rounds of freeze thaw but when you see the results from the oven it’s disheartening and makes me question the stability of the formula when I am sure it is fine.

    Should i be worried about only emulsifying in the cool down phase?  Is it an issue if it all comes together in the end? - I’m completely stumped here as i have never experienced this.  

  • Shams

    Member
    November 18, 2022 at 2:11 am

    Glyceryl Stearate Citrate is anionic emulsifier.
    Zinc + ions don’t prefer anionic.
    For ZnO creams , it is better to use non-ionic emulsifiers 

  • geepee

    Member
    November 18, 2022 at 3:08 am

    Thank you! 
    I have solved it now 🙂

  • graillotion

    Member
    November 18, 2022 at 4:03 am

    I have had tons of experiments that won’t go together until the temp cools a bit.  NONE of those are EVER stable.

    Generally, I fixed those concerns with using a minimum of 2% of a 165 type emulsifier (blended together with various other partners), or using almost entirely 165 type.

    I should say….I do not work with Zinc.

    Bottom line….if the emulsion does not go right together…you will have trouble later on….in stability.  Just my observance.

    Your emulsion doesn’t seem to have much in the way of builders….maybe some gum or a fatty alcohol?  C10-18 Triglycerides in the oil phase is also a lot of fun.

  • graillotion

    Member
    November 18, 2022 at 4:15 am

    Paprik said:

    If I am not mistaken Glyceryl Stearate Citrate is anionic high HLB emulsifier - O/W. 
    But it looks like you are trying to formulate W/O emulsion [Sorbitan Olivate, High lipid input, Lower water, zinc balms are usually W/O] …. Or if you are not trying to formulate W/O you should consider it (?)

    Or add high HLB non-ionic emulsifier with little less Sorbitan Olivate. 

    I think it only holds together when cooled down due to the waxy material that solidify. But there is not enough high HLB emulsifier to hold all that lipid. 

    - Rheology modifier (gum, polymer) is also good for stability. For example XG for O/W or Polyamide-3 for W/O. 
    -Accelerated stability testing is a double-edged sword. As you mentioned, in RT it is ok. Sometimes things that are happening in the over won’t happen in real life. But it is a good indicator. Freeze/thaw is also very good - go for 6 cycles. 

    Hope this helps a little bit, hopefully someone will have another idea what is going on :) 

    I think there is a ‘typo’ in the original post:

    Glyceryl Stearate Citrate SE 

    No such thing that I am aware of ….. There is Glyceryl Stearate SE, and there is : Glyceryl Stearate Citrate  …. but have you ever heard of a GSC SE???

    I would guess …they were using Glyceryl Stearate SE ….which is, hmmmh…. :)  (I don’t think so…category.)

  • paprik

    Member
    November 18, 2022 at 7:53 am

    @Graillotion, good point! yeah I wasn’t really sure about the SE either :D
    That is why I didn’t write it, but you could be right with Glyceryl Stearate SE! In both cases, it would be high HLB anionic emulsifier. 

    Anyhow, looks like @geepee has solved it? :) 

    PS. you should always evaluate your samples the next day. Or even 2 days after. Something like Polawax GP200 takes couple days to fully thicken. 

    Good stuff, happy formulating!  

  • ketchito

    Member
    November 18, 2022 at 11:07 am

    I believe Glyceryl stearate is a non ionic molecule. It sounds like a salt, but it’s in fact an ester  :)

  • PhilGeis

    Member
    November 18, 2022 at 11:32 am
  • paprik

    Member
    November 19, 2022 at 5:21 am

    ketchito said:

    I believe Glyceryl stearate is a non ionic molecule. It sounds like a salt, but it’s in fact an ester  :)

    But he wrote Glyceryl Stearate SE. Which should make it anionic? 

  • markbroussard

    Member
    November 19, 2022 at 2:28 pm

    @geepee

    Your problem is that you are using Glyceryl Caprylate … it blows the viscosity of emulstions.  You can replace it with Sodium Benzoate and keep the Plantaserv A … that will give you good preservation coverage.

    Also, if you are using Glyceryl Stearate Citrate, you will get viscosity build from the emulsifier.  If you are using Glyceryl Stearate SE, you won’t get much visocisity build

  • ketchito

    Member
    November 21, 2022 at 11:36 am

    Paprik said:

    ketchito said:

    I believe Glyceryl stearate is a non ionic molecule. It sounds like a salt, but it’s in fact an ester  :)

    But he wrote Glyceryl Stearate SE. Which should make it anionic? 

    @Paprik My apologies, I didn’t read carefuly. Yes, Glyceryl Stearate SE has some sodium/potassium stearate (soap), which is actually the anionic emulsifier.

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