WHY DO SOME NATURAL LIP BALMS HAVE DENT IN THE MIDDLE?

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 colorants while others contain nothing but the oils and wax.

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 mold, cooling that rapidly then popping the lip-stick 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 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.

As this may seem like a simple solution, I do not recommend going in to add a hat of liquid after the initial fill has cooled down. After refilling or topping-up with to “fix” dent, you may realize when using the lip balm, that the hat pops off after cooling down. This is because it doesn’t bind properly with the previous fill as both are filled at different temperatures.