pa_pyro Posted October 22, 2006 Share Posted October 22, 2006 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? Link to comment Share on other sites More sharing options...
cplmac Posted October 22, 2006 Share Posted October 22, 2006 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. Link to comment Share on other sites More sharing options...
Chemguy Posted October 22, 2006 Share Posted October 22, 2006 Well, according to This site, It uses a 'supercorodable' iron magnesium alloy. That acts as a cathode and anode in salt waterThis probably wont be quite so easy to do.However apparantely there are other mixtures that work, but this is the best. Link to comment Share on other sites More sharing options...
Mumbles Posted October 22, 2006 Share Posted October 22, 2006 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. Link to comment Share on other sites More sharing options...
pa_pyro Posted October 22, 2006 Author Share Posted October 22, 2006 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? Link to comment Share on other sites More sharing options...
Mumbles Posted October 22, 2006 Share Posted October 22, 2006 It probably serves the same purpose as sawdust, reaction rate modifier. It may also help to insulate the heat from the environment. Link to comment Share on other sites More sharing options...
cplmac Posted October 23, 2006 Share Posted October 23, 2006 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. Link to comment Share on other sites More sharing options...
h0lx Posted October 24, 2006 Share Posted October 24, 2006 Yes, but only with anhydrous CaCl2, the hydrate cools down as it dissolves. Link to comment Share on other sites More sharing options...
dbcpyrotechnics Posted January 24, 2007 Share Posted January 24, 2007 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. Link to comment Share on other sites More sharing options...
FrankRizzo Posted January 25, 2007 Share Posted January 25, 2007 Seach US patent #5611329 IIRC, the magnesium/iron alloy used is made by only one company (Dymatron). Link to comment Share on other sites More sharing options...
Chemguy Posted January 25, 2007 Share Posted January 25, 2007 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. Link to comment Share on other sites More sharing options...
Boomer Posted January 30, 2007 Share Posted January 30, 2007 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. Link to comment Share on other sites More sharing options...
Mumbles Posted January 30, 2007 Share Posted January 30, 2007 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. Link to comment Share on other sites More sharing options...
FrankRizzo Posted January 30, 2007 Share Posted January 30, 2007 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. Link to comment Share on other sites More sharing options...
mormanman Posted June 29, 2007 Share Posted June 29, 2007 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. Link to comment Share on other sites More sharing options...
FrankRizzo Posted June 30, 2007 Share Posted June 30, 2007 How's this for easy. Vinegar + Baking Soda = Sodium Acetate + CO2 Link to comment Share on other sites More sharing options...
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