Heat Pad

http://www.bladeforums.com/forums/showthread.php/319683-Military-Heating-Pad

...stay warm...

Thanks for the great link.Using the NSN number, visible on the photo,I found them for $4.99 each.

Still,I would like to know what makes them tick.

Any chemists in the forum that know for sure?

Thanks!

Note:

For anyone else interested the NSN is 6530-00-786-4640.

http://www.armysurpluswarehouse.com/survival-disaster/reusable-chemical-heating-pad.html

Found this....

"The ration heater contains finely powdered magnesium metal, alloyed with a small amount of iron, and table salt. To activate the reaction, a small amount of water is added, and the boiling point of water is quickly reached as the reaction proceeds.^[1]"

http://en.wikipedia.org/wiki/Flameless_ration_heater

That is an interesting link,but it is not reusable.

It is a one-time reaction.

The chemical heating pad that I refer to is reuseable for multiple times so I think

there must be a different process at work here.

Also these pads preceed the MRE heater by at least 25 years or more.

Thanks for the link,I learned something new and useful from it.

Thanks everyone!

Your links and consequent following of other possible links led me to a patent of the item in question.

It lists the chemicals used,and the proportions.

This is not the exact one I remember,becasue it was not patented until 1975,the one I remember was from the mid '60's.

Perhaps t is a newer improved model,or an improved patent on the original.

I just wonder why this is not on the commercial market.

I am aware of the Saturated Sodium Acetate heat pack,but they have to be boiled to reactivate.

These only require water.

Link follows:

https://www.google.im/patents/US3980070

HiTek: The combination you've guessed would work well- TOO DARNED WELL!!! Google " Michigan tanker explosion" to see what dilute caustic soda does to the genius that put it in an aluminum tanker. The slightest amount of moisture in there will kick start a run away reaction, and depending on how well it was sealed from air beforehand (and if the iron was ground rust) could possibly turn into a thermite reaction. Just forget the sawdust, you don't even need it. Note that you'll also be generating hydrogen....

Unless it is something that has a large potential energy, and you are only using a small amount each time until it's exhausted it has to be based on a physical reaction (EG change of state) rather than chemical. This could include heat of solution, which would require drying it out afterwards. Any thing that might indicate this?

In the patent link above,it desribes the chemical process involved,and the chemicals

used.

It IS an exothermic reaction,but it is limited by the small amount of water

introduced,and the heat eventually dries out the moisture,and the reaction stops.

Some of the chemicals are used up each time,but it is good for many cycles before

exhausted.(If you follow the directions on amount of water added).

It looks like the one I remember from the '60's but it seems to claim it is an

improved version of previous art.

I did not pursue all of the previous art links,but the

old one is probably in there some where.

I do remember if you put in more than the recommended amount of water it would

get too hot to handle.That makes sense.

It is not a state-change chemical,and does not require reheating to activate.

It is a self limiting exothermic reaction.

The supersaturated Sodium Acetate on some hand warmers can be made very simply

at home, but the purity is not as good as in the commercialy type sold as hand warmers.

By the way,did you know that Sodium Acetate is used in potato chips for the

salt/vinegar taste?

Also used in concrete to make it more resistant to water/salt damage.

Many more uses for Baking soda/vinegar mixture!

Thanks to everyone for their valuable feedback.