Mixing things with different melting points

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Envelope Generator
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Mixing things with different melting points

Postby Envelope Generator » Thu Jan 28, 2016 6:51 pm UTC

This is surely a very elementary thing but I have no knowledge of physics at all! I need to make a mixture of two waxes that have different melting points somewhere in the 40-80°C range. Melting them down and stirring the resulting liquid is something that even I can manage, but how do I ensure that, once cooled down, the resulting solid has an even consistency? Won't the wax with the higher melting point solidify first and separate from the wax with the lower melting point?
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Tyndmyr
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Re: Mixing things with different melting points

Postby Tyndmyr » Thu Jan 28, 2016 6:54 pm UTC

Seperation takes time. Yeah, if you keep the temperature between the melting points for a while, you can seperate out stuff, but generally it's not hard to cool something down fast enough for separation to be an issue.

Note that this is partially a function of size, since surface area scales more slowly than volume. The bigger the project, the more interesting this gets.

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Envelope Generator
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Re: Mixing things with different melting points

Postby Envelope Generator » Thu Jan 28, 2016 7:06 pm UTC

You probably mean "for separation not to be an issue"? Thanks, that was quick!
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slinches
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Re: Mixing things with different melting points

Postby slinches » Thu Jan 28, 2016 7:25 pm UTC

What you're looking for is called a "eutectic system". With waxes, the melting points and thermal properties are probably close enough that uniform solidification can occur at room temperature across a relatively wide range of mixture ratios, but the principle still holds. If you wanted to be all sciency about it, you could mix up a bunch of samples at different ratios to melt and let cool so that you can build your own phase diagram.

BedderDanu
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Re: Mixing things with different melting points

Postby BedderDanu » Thu Jan 28, 2016 7:40 pm UTC

So, I'm more familiar with metals, but basically when you melt two solids, mix them, and solidify them again, the melting/freezing point tends to dip below the actual melting point of the two items as separate, pure solids.

For example, look at the yellow gold table here. Pure gold has a high meting point, and impure gold has a high melting point, but the alloys in between have lower points than either of the ends.

Again, not to familiar with wax itself, but in theory, if you had Wax A at 40C and Wax B 80C melting point, your chart will probably look something like this:

%B: MeltPoint
0: 40C
5: 37C
10: 38C
25: 45C
50: 58C
75: 70C
90: 75C
95: 78C
100: 80C


I believe that you will also find that if you do it slowly enough, you are going to get a melting RANGE, not just a melting point. Basically, cooling your product to between the melting points will freeze out more B than A. This will cause the melting point to shift because of the change in composition of the liquid. So the next layer will have more A than B in it (shifting the melting point down lower, closer to your current temperature) And this will continue, with a gradiant of A and B forming in the material. I believe that the faster you cool it, the less of an effect there is, but rapid cooling can have other effects on your material as well.

So your wax won't separate into two distinct layers, but the outside may have slightly different properties than the inside, most likely that will be a minor effect, though.

(It's been a while since my last Materials Engineering class. You either rapidly cool it or you hold JUST under the freezing point of the mixture for a long time. One of these strategist helps with the above problem.)

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Re: Mixing things with different melting points

Postby KarenRei » Fri Jan 29, 2016 1:55 am UTC

The short answer is, it varies greatly depending on what you're mixing together. More to the point, some things won't truly mix at all, while others will form eutectics (mixtures). Even more to the point in your wax example, your wax *is* a mixture of different compounds (paraffins) with different melting points ;) I'm sure you know this from real-world experience - think of all of the chemicals in, say, fruit juice - if you spill some juice and it dries does it form countless tiny layers as each chemical condenses out individually?

Basically, as a rough guideline, if you have multiple chemical species, and they're both A) capable of forming a true mixture in the liquid state, and B) compatible into solidifying into a structure wherein they're intermingled with each other, then when you cool them, they'll do just that - condense out into an intermingled structure with its own chemical properties (including melting point), usually somewhere in-between the different chemicals (although often with a lower melting point, as was mentioned above). But if they're incompatible in either the liquid or the solid state, then you'll tend to have them condense out individually.

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Re: Mixing things with different melting points

Postby PM 2Ring » Fri Jan 29, 2016 7:18 am UTC

Mixtures tend to have broad melting points. Waxes are generally mixtures of compounds, and even if your wax sample is supposed to be a pure compound it's likely to consist of a mixture of isomers with slightly different melting points. A conventional way of dealing with this problem when assigning a single number as the melting point for a wax is to use its slip melting point:

Wikipedia wrote:The Slip melting point (SMP) or "slip point" is one conventional definition of the melting point of a waxy solid. It is determined by casting a 10 mm column of the solid in a glass tube with an internal diameter of about 1 mm and a length of about 80 mm, and then immersing it in a temperature-controlled water bath. The slip point is the temperature at which the column of the solid begins to rise in the tube due to buoyancy, and because the outside surface of the solid is molten.

This is a popular method for fats and waxes, because they tend to be mixtures of compounds with a range of molecular masses, without well-defined melting points.


Making a mixture of waxes with such widely-separated melting points should not be a problem. Your waxes are probably soluble in each other over a wide range of proportions, so you shouldn't get much separation (unless you're doing something weird like mixing a silicone wax with a normal hydrocarbon wax). But if you're concerned, just stir it a bit while it's setting.

FWIW, I've successfully mixed beeswax with linseed oil (and a dash of lavender oil) with no separation issues (I was making furniture polish for my harp).


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