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Why does lip balm dent as it cools?

If you are not sure what I’m talking about then this picture might help:

NB: This type of packaging HAS to be hot-filled. I added this comment after publishing after it became obvious that fact wasn’t well made before 🙂


Lip balms, lipsticks and gloss sticks are a mixture of waxes and other stuff – typically oils either hydrogenated or not and sometimes a butter/oil combo . Some contain colourants such as this one I’ve made which has mica added while others contain nothing but the greasy stuff.


A perennial problem with this type of formula is in getting it to stop doing the above and leaving you with a dented top. That’s actually quite tricky to do.


Lipsticks, which these are based on, are typically made by pouring hot waxy stuff into a metal mould, cooling that rapidly then popping the lipstick out and putting it into the container. Lip balms are poured straight into the container and that’s why we have a problem and it’s all to do with heat transfer.


You may notice if you pay enough attention, that liquid wax/butter/oil combos take up more space than solid wax/butter/oil combos. This is quite normal and is to do with the way the molecules of stuff move about and combine. You could try an experiment with your own body if you like. Dance around the room a bit and see how much space you take up. Then lie flat on the floor and measure that. Of course, you actually take up the same amount of space but the space you influence is smaller when you stop moving. When you dance you need not only the space you actually take up but also the space you potentially take up so you effectively create a ‘dance zone’ around you where other things can’t come in close because you are spinning and moving too much and the other things don’t want to get hit.

OK weird analogy but it is somewhat like that with chemistry.


Hot waxy stuff = dancing and creating dance zones around them.


Cold waxy stuff = knackered and asleep on the floor so other stuff can pile on top, next to and underneath thus fitting more stuff per unit of space.


So what’s with the shrinking?


Physics, it’s physics!


So as things are cooling off in the side, the wax is shrinking and as heat rises the top of the stick stays hottest for the longest. As well as heat going to the top it also goes into the middle. It’s like the heat is trying to get away from the cold spots so it goes inwards and upwards as much as it can until finally it gives up and cools. The only trouble is that by the time the super hot core has run out of oomph and starts cooling, everything else has started to set around it which forms a barrier to its integration. So, instead of just becoming part of the rest of the waxy tube the core cools on its own. The bigger the difference between the cooling time of the outside vs the cooling time of the inside, the larger the crater.


Can we prevent this?


It is quite tricky to prevent this as there is a limit to the temperature you can fill a tube – go too cool and while you might solve the hole in the top problem you may find it messy and difficult to get a good fill and this may make your sticks look lumpy instead of smooth. Fill too hot and the problem is back again.


I ran a little experiment to see if I could demonstrate different fill and cool procedures visually for you. Here are the results:




Conditions:

100C, 75-80C, 65-68C pour freezer cooled.

100C, 75-80C, 65-68C pour bench cooled, lid on.

100C, 75-80C, 65-68C pour bench cooled, lid off.


Explanation:


100C is the maximum you’d want to go to with most oils. At that temperature, you get a nice quick fill as the product is typically very fluid and easy to pour. Also, you get a long time to pour because it takes a while to start setting. At this temp it’s unlikely that air bubbles will form in the tube as any air will escape before the product sets. That leads to a much more robust and dense balm with greater oxidative stability.


75-80C is a good temperature to fill when your formula contains more delicate natural oils. At this range, you are unlikely to start oxidising the oils and changing their colour but you can still pour for a while before gelling starts. The only downside here is that air bubbles might get trapped if you are not careful with your mixing. This is because the bulk sets quite quickly on pouring and can set before any bubbles escape.


65-68C is much harder to work with but is helpful in other ways. This is the temperature when gelling is just about to start (with my formula anyway). The bulk is getting thicker but is yet to be set. The lower energy level going into the tube makes it much less likely that large shrinkage will happen but it is more likely that air bubbles will get trapped. It is possible to perfect the pour in this temperature range but it takes a bit of doing.


Lid vs no lid =. The lid will trap heat in for longer so I wanted to see how that would impact the denting.


Bench cool vs freezer cool = Freezer is rapid cool and I wanted to see how that impacted the stick. Bench cooling times will vary depending on your lab temperature.


Results.


My first round trial didn’t produce a satisfactory result with regards to indentation but it did help to see how the different temperatures and conditions affected things. However, in terms of the stick formation, I found both the freezer and the 75-80C fill to produce the best-looking sticks.


Experiment part 2.


Next, I tried again with the same batch but this time I filled to the end of the twisty middle but not to the top and used a fill temp of 75C-80C as that seemed the best performer of the first round. Once cooled I topped the tube up and let them stand on the bench without the lid. Doing this I was able to produce two lipsticks that didn’t indent on cooling – We have a winner!

A look at the body of the lipstick shows mixed results. Maybe this needs some work…

Discussion of these results.


The lipstick poured well at 75C. I left it for a few minutes to really set then added the rest. I made these two using the same method but as you can see one looks in much better shape than the next. I am not the neatest person in the world (as you can see from my writing and other bits) but from this I’d say that the pour, leave then top up method at approx. 75C is worthy of a bit more tweaking and investigation. Also now we know we have two parameters to measure 1) the dent on the top and how to avoid it and 2) the smoothness of the lipstick shaft.


Conclusion on pour method.


For my formula, a filling method of filling the tubes to 1/2 or 3/4, letting it cool and then topping up. Keeping the lid off and cooling at room temp (around 25C at present) was best. My formula had an expected gel temperature of around 60-65C based on the waxes, butters and mica contribution.


Further discussion and next steps.


Different formulations will have different melting points, this one is a little high to be honest and the resulting stick is a little hard to transfer onto the lips. It is likely that the higher the melting point of the stick, the harder it will be to pour and avoid indentation or other problems. However, the lower the melting point, the softer the stick and that might then not hold up. It is likely that the ideal temperature is between 52-58C so I could play with my formula to see if the same variables in terms of filling gave me a different result when I had a lower melting point wax blend. As with many things scientific, just because the scientific theory stacks up it doesn’t mean we will be able to see anything different in practice. It may be that the change in melting point possible for a lipstick isn’t large enough to make much of a difference and that the pouring method is always the best chance of gaining control over the product's appearance.


As you can see from the above, something as simple as stopping your lipstick from indenting is actually quite a scientific endeavour. As a consultant chemist, it is my job to solve these types of problems for brands and optimise their Intellectual Property (formulations). One of the big reasons that you almost never find the exact (best) answer you need for your chemistry question on an online forum or blog (like this) is that everyone’s formula, manufacturing process, packaging and expectations are different. While there is some learning that is transferable (such as an experimental method), the detail is always somewhat personal to you and that’s why it’s all so interesting.


I hope that helps and if that all looks too daunting, do give us a call as we'd be happy to test your formula out and find the best method for you.


Amanda x

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