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Sunscreen “Milk” - Stability
I am working on a sunscreen fluid (SPF 30) with ZnO as the only active - 17.5%
The viscosity is low, 300-500cps and it is meant to be packaged with a shaker ball in a tottle.
After 9 weeks at 45C we see phase separation that is easily brought back to homogeneous with less than 5 shakes.
My question is for anyone who has formulated these low viscosity sunscreens, would this be considered a failure? Even though it is meant to be “shake well before use”. I would love to hear some perspective/opinions.
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6 Replies
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You won’t get a very good consumer reaction with any cosmetic that has to be shaken unfortunately. MAYBE if it’s in a pump/spray application.
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Hi Kewell,
Ideally if the product separated at 45C within 3 months is considered fail. But with 17.5% ZnO and that low viscosity, I doubt the product will be stable. I am glad it survived 8 weeks.
If the product is to be shaken before use and it going in opaque package, might not be a problem, but entirely depends on the client.
See if you could get finer ZnO to get more stable emulsion. You can also check if the formula spreads well after shaking and gives you the SPF you desire.
I formulated SPF-20 whch was low viscosity but was going in a tube with a shaker ball. The client didn’t care about viscosity.
I stabilized it with Crodafos CS-20 (Cetearyl alcohol (and) Ceteth-20 phosphate (and) Decetyl phosphate) @ 2%. But the viscosity was higher around 2000cP.
Good luck! -
does it also separate at lower temperatures?
and if so, how long does it take to separate?
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also: does it separate if you put it through several freeze/thaw cycles?
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You may need good suspending agent because of higher percentage of ZnO. You can use micronised ZnO but it is very costly.
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There are protocols that take into account high-temperature instability - but you’d be better off adding more suspending agent and/or reducing particle size.
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