Stability of Sunscreen - Chemists Corner

Formulating

Stability of Sunscreen

Posted by ssdc on October 5, 2015 at 9:02 am

Dear All,

Below is a formulation i made in the lab.

Phase A

Avobenzone- 5%

Octoocrylene - 4%

Ethylhexyl Methoxycinnamate- 7.5%

Benzophenone-3 - 1%

Ethylhexyl Salicylate - 5%

Uvinul A Plus - 2%

Triethylhexanoin -10%

Emulsifier- 8.8%

Phase B

Water- 56%

Hydroxyethyl cellulose- 0.2%

Phase C

Preservative - 0.5%

The calculated SPF is 30, however after in vitro test the SPF value I got is only 7.

Could this be due to the instability of the formulation.

If yes, how can I improve on the stability?

Thank you

Best Regards

Stephanie

ssdc replied 9 years, 3 months ago 6 Members · 13 Replies
13 Replies

Bobzchemist

Member
October 5, 2015 at 2:08 pm

The answer to your question is sort of yes and sort of no.

Formula instability will not affect in-vitro results. However, sunscreen photo-instability will, It’s likely that the UV from your solar simulator is breaking down your sunscreens.

Do some research on sunscreen photo stabilizers. Hallstar makes some good ones, but so do a lot of other people. Then add enough to your formula that you get the right SPF results.

(https://www.hallstar.com/product/solastay-s1/)
Bobzchemist

Member
October 5, 2015 at 2:12 pm

Please also remember that you won’t be able to sell a sunscreen that is unstable - from a formula point of view, because it will not be able to pass the required drug stability testing that you need to establish an expiration date.
Bill_Toge

Professional Chemist / Formulator
October 5, 2015 at 5:59 pm

the SPF achieved in practise is also very dependent on the nature and rheology of your emulsion, i.e. whether or not it readily breaks down on rubbing and forms an even film

for the amount of filters you have in that formula, 7 is a very low SPF indeed; however, I’d personally describe it as “currently unfit for purpose” rather than “unstable”
Bobzchemist

Member
October 5, 2015 at 10:32 pm

Bill makes a very good point - there are a number of ingredients billed as “SPF enhancers” that work primarily by enhancing the film-forming properties, in one way or another, of your emulsion. Other than the HEC, I don’t see any of those ingredients in your formula.

Do some research, talk to suppliers - we’d be happy to give you suggestions - and I think that you’ll fix your problem without too much trouble.
MichelleReece

Member
October 6, 2015 at 12:02 am

@ssdc

Have you considered doing an W/O emulsion or encapsulating the AIs in nanotechnology? The latter tends to have a better feel, though it can get pricey depending where you live.
ssdc

Member
October 6, 2015 at 1:04 am

However I did the SPF test on the oil phase alone and got the expected SPF value (SPF 30)

Oil Phase

Avobenzone- 5%

Octoocrylene - 4%

Ethylhexyl Methoxycinnamate- 7.5%

Benzophenone-3 - 1%

Ethylhexyl Salicylate - 5%

Uvinul A Plus - 2%

Triethylhexanoin -10%

It is when I make it into an emulsion my SPF value runs very far from the actual calculated SPF. Could this still be due to photostability?
pma

Member
October 6, 2015 at 2:55 pm

If you add silicones your SPF can be boosted a lot.

And I wouldn´t use AVO with OMC - this combination can be very photounstable even in presence of AVO stabilizers like OCT.
Bill_Toge

Professional Chemist / Formulator
October 6, 2015 at 9:36 pm

@ssdc if your formula were not photostable, you’d see a gradual loss of SPF over time rather than an immediate drop; this is because your batch hasn’t been exposed to a significant amount of light (unless you’re keeping your batches on a windowsill for several months after you manufacture them)
ssdc

Member
October 9, 2015 at 9:34 am

I checked the SPF on the following combination

Oil phase + water + emulsifier :SPF=30 (like calculated)

Oil phase + emulsifier :SPF 30 (like calculated)

With this results can i say hydrolysis occur in the formulation?

Please advise
Bill_Toge

Professional Chemist / Formulator
October 11, 2015 at 5:49 pm

@ssdc you can’t say that with any truth, because:

1. most of your sunscreens don’t have any functional groups that can be hydrolysed

2. the pH of your formula is nowhere near extreme enough, and the amount of free acid or base in your formula is nowhere near high enough to cause any hydrolysis (I hope!)

3. even if any hydrolysis did happen, there is no way you could tell for sure unless you used HPLC combined with several other analytical techniques, which I’m guessing you haven’t

as I said before, the problem lies with the structure of your emulsion, and the quality of the film it forms when it’s applied to a surface

the fact you’re achieving the predicted SPF when you test the oil phase on its own, but not achieving it when the oil phase is part of an emulsion, only goes to proves this point

personally, my advice would be to try using different emulsifiers and/or rheology modifiers
ssdc

Member
October 12, 2015 at 12:05 am

@Bill_Toge

Thank you for your opinion

Sorry I made a mistake on the previous statement

Oil phase + water + emulsifier :SPF=7

Oil phase + emulsifier :SPF 30 (like calculated)

I will try to change the emulsifer used. Currently I am using an anionic emulsifier

The reason I am suspecting hydrolysis because ethylhexyl methoxycinnamete (OMC) has an ester bond and one of my emulsifier has an acid value of 30.

I do not expect film formation to be the problem because my emulsion is only applied on PMMA plates and did not undergo any irradiation. The SPF measured is the actual SPF before irradiation and water resistant test.
nedazirakzadeh

Member
October 30, 2015 at 3:34 pm

we couldn’t get an SPF value based on the amount of active ingredients alone .the SPF value depends on the formulation, for example film formers provide film on the skin carrying the active ingredients on the skin that increase the SPF value and also You need to identify incompatibilities with your formula.
ssdc

Member
December 8, 2015 at 7:33 am

@nedazirakzadeh how do i define the incompatibilities with my formula