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MRE heaters


pa_pyro

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I have recently gained an interest in handwarmers/chemical heating devices and of course came across MRE heaters. I found out the chemicals involved but I have no clue in what ratios they are used. The chems are magnesium, iron, and salt water. The salt water exothermically "rusts" the magnesium and iron creating heat. I tried out a mixture of semi coarse Mg and Fe and added a drop of saturated salt water and waited...and waited, and no noticable heat was produced. Does anyone know the proper ratios of chemicals to used?
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I've used hundreds of these things, and taken a couple apart to look. I can tell you without question I have no clue. Those things get real hot though, you can burn yourself real easy.
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Well, according to This site, It uses a 'supercorodable' iron magnesium alloy. That acts as a cathode and anode in salt water

This probably wont be quite so easy to do.

However apparantely there are other mixtures that work, but this is the best.

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The ones we have say they just use iron powder. They also have CaCl2, carbon, and sawdust in there I do believe. I am sure the sawdust is more of a heat regulator and reaction slowing agent than anything. The fact of it being an alloy, or at least possibly an alloy, doesn't suprise me at all though. A lot of the ones I've seen lately say they have "activated heat granules" or something to that effect.
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Well, I don't think I'll be making the Mg Fe alloy...I'll try the reaction again with a calcium chloride solution with fresh Mg and see if it makes a difference. Small disposable heaters I've seen use just iron to keep the heat down. Any idea what vermiculite helps in the reaction?
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It probably serves the same purpose as sawdust, reaction rate modifier. It may also help to insulate the heat from the environment.
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Calcium Chloride gets pretty warm by itself with water. It's used to melt snow and it is also added to wet concrete to speed the curing process by generating more heat more quickly than standard concrete mixes.
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  • 3 months later...
The vermiculite helps to absorb the water and keep the heat in place. Kinda like an MRE heating blanket. Still working on the aexact compounds inside the heaters such as exact weights. Still no luck after 2 dozen phone calls.
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dbc, don't bump these old topics if they have no posts in for 3 months it means that it has been usually figured out, just a pointer.
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Now that it's bumped...

 

Any idea how these hand warmers work that contain a clear gel, you bend a metal platey inside and they crystallize and get hot. You can re-activate them by melting again in hot water (microwave?):

 

I know *what* happens, heat of crystallisation, supersaturated solution etc etc.

 

But how is it started *reliably* by bending that metal strip? Ultrasound from the "klick" ? We once built a unit that did this, cause crystals in supercooled liquid by ultrasonic shocks. BUT it was hard to get right, even using a 100W piezo crystal /amplifier combo.

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Well. the salt is Sodium Acetate as I am sure you know. No idea how the metal thing works reliably though. Took a try or two with one I got.
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But how is it started *reliably* by bending that metal strip? Ultrasound from the "klick" ? We once built a unit that did this, cause crystals in supercooled liquid by ultrasonic shocks. BUT it was hard to get right, even using a 100W piezo crystal /amplifier combo.

Yep, that's exactly it. The "click" from the spring causes a minute shockwave that changes a few of the supercooled molecules to their solid form and that crystal seeds the rest of crystallization process. Sodium acetate was chosen for these packs because it is very good at supercoooling...they were engineered to be easy.

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  • 4 months later...

I don't think that the supplier review would have where to get Sodium Acetate and I was wondering if there was a cheapier place than united nuclear. (Just PM me and I'll move this to the random thread I just thought sence this is the chemistry section.)

Thanks guys.

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