Home Cosmetic Science Talk Formulating issues with solid sunscreen stick

  • issues with solid sunscreen stick

    Posted by mitchellp on May 5, 2015 at 9:54 pm

    Hi All,

    I have been experimenting with solid sunscreen sticks in a multitude of formulas, most of which have the following base ingredients: 
    coconut oil
    other oils like jojoba, almond, or olive
    beeswax
    shea butter 
    non-nano zinc oxide ≈22%
    vitamin e oil
    Typically I will melt the oil(s), beeswax, and butters together and mix well… then add a drop or two of vitamin e oil and zinc, and mix again using a hand blender & pour into a small container.  
    After a few days, the stick’s surface starts yellowing.  Not uniformly, just here & there… but it’s very noticeable.  Has anyone experience this before or know what might be causing it?  
    Another issue is when I pour the formula into the container and it dries, there is always a hole in the center!  Not sure why.  How do the professionals avoid that?  
    Thanks :)  
    mitchellp replied 9 years, 1 month ago 4 Members · 13 Replies
  • 13 Replies
  • Bobzchemist

    Member
    May 5, 2015 at 10:03 pm

    The zinc oxide is a pigment, so it isn’t soluble - the best you can do is a suspension. The yellowing in some areas is a sign that you do not have a uniform suspension. As a result, the SPF value of your stick will be quite variable, so I wouldn’t count on it delivering sun protection, if I were you.

    The hole in the center is just physics - wax and wax blends shrink as they cool. There is no way to avoid it.
  • mitchellp

    Member
    May 5, 2015 at 10:11 pm

    Thanks Bob.  Perhaps my electric hand mixer is not enough to provide a uniform suspension in the time I have before the formula begins to dry (2-3 minutes max).

    Any idea what tool(s) would be good for this?  Something for home use rather than a commercial lab - like a stick blender?  
    /m
  • belassi

    Member
    May 5, 2015 at 11:20 pm

    Just a thought - have you tested it for melting point? 

  • Bobzchemist

    Member
    May 6, 2015 at 1:04 am

    In the US, sunscreens are OTC pharmaceuticals. As a professional, it would be irresponsible of me to help you do this (outside of a FDA registered facility), or even recommend that you do so on your own.

  • mitchellp

    Member
    May 6, 2015 at 6:28 am

    @belassi - hmmm, why do you ask?  Depending on the formula, the stick starts melting somewhere between 80-90 degrees F.  

    @bob - I respect your position, however these are for personal use & testing.  My goal is to create a formula that I approve of on my own, before having it made professionally, undergoing SPF testing, etc. 
    Hopefully that eases your moral dilemma, if not perhaps someone else will chime in. 
  • Bobzchemist

    Member
    May 6, 2015 at 3:42 pm

    It does, actually. Too many home crafters feel that they should be able to throw together a sunscreen just as easily as they can make salad dressing.

    So, I will return to your questions and hopefully be more helpful. It takes a lot of shear force to properly disperse zinc oxide, so if you can’t afford an expensive high-shear mixer, your best bet will be to use a pre-dispersed mixture. Generally speaking, using a mixture will not give you much, if any, room for other oils. You will have to melt your waxes and butters, mix, and then add the ZnO dispersion. This should get rid of your yellowing problem.
    The funnel-shaped hole in the stick can be gotten rid of by one of two methods - either fill from the back (which is how lipsticks are poured) or fill from the front, but pour in enough extra material that you can cut off the excess and have a perfectly smooth surface.
  • MarkBroussard

    Member
    May 6, 2015 at 5:12 pm

    @mitchellp:

    Take a look at AppleChem predispersed ZnO products.

    The other way to “fill in” the hole is to pour, allow to harden (but not completely) and then pour a “top layer” to fill in the hole and cover the surface (that’s if you are top filling).  As BobZ pointed out, the easiest method is to use Bottom Fill stick containers.

  • belassi

    Member
    May 6, 2015 at 5:39 pm

    “Depending on the formula, the stick starts melting somewhere between 80-90 degrees F”

    - where I live, such a stick would turn into a liquid in the summer heat (shade temp up to 45C)
  • Bobzchemist

    Member
    May 6, 2015 at 5:52 pm

    Something like this is really useful on a small scale.

  • belassi

    Member
    May 6, 2015 at 10:10 pm

    You’re tempting me, Bob.

  • mitchellp

    Member
    May 7, 2015 at 5:07 am

    @belassi - my latest batch has a higher melting point, 95-100 degrees F.  More beeswax and butter, less of the oils.  It’s a “summer” formula :)  

    @bob - from my research it seems like an entry level, high-shear mixers/homogenizer can be found for around $700-1000. 

    It may be worth it to simply work with a contract manufacturer on the formulation… I just really want to do it myself!   
  • Bobzchemist

    Member
    May 7, 2015 at 3:26 pm

    Some people consider $1,000 too high - and I’m not sure if the homogenizer will provide enough shear. Ideally, you’d use a 3-roll mill, or a ball mill, or a colloid mill. A ball milling set up can be put together for $1,000 - $2,000. Used colloid or 3-roll mills can be about the same price.

    Look at these two starting formulas for ideas:



    Note that the pour temperature for each is ~70C/160F. This is typical, because much lower than these temps and you start getting sticks melting in sun-heated cars or beach bags, etc. You’ll find that most contract manufacturers are already set up for that temp range, also. 
  • mitchellp

    Member
    May 15, 2015 at 11:45 pm
    Do you think something like this would work, despite being advertised for homogenizing tissues & brains!  

    In the meantime, I am getting quotes from some contract manufacturers in the area.  For not much more ($1000-1500,) I can have professionals assist with the formulation in a real lab.  

    Those starting formulas look nice, but I’m trying to achieve something that appeals to the all-natural, active crowd.  Only 5-6 organic ingredients + zinc, very sweat & water resistant, and sold in biodegradable packaging!  

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