Sodium Acetate reusable heating packs -- anyone know about them?

Factual Questions
1

A church acquaintance told me this morning about Sodium Acetate reusable heating packs – that she’d bought some, how wonderful they were, and so on. Frankly, what she describe sounded almost like magic so I’m pretty skeptical, but I’ve known her slightly for years and she’s always seemed pretty grounded in reality, so…

Here’s what she described:

The packs she’d bought (from Amazon if it matters) were made of some flexible mostly clear pale green vinyl-ish plastic, about 5X9 inches, full of some sort of viscous goo (presumably the Sodium acetate?) The brand she bought was called “Heat Wave”, though there are probably others, and she says there were a couple other sizes for sale, too.

Set into one corner of the pad was a round metal ‘button.’ When you press the button it immediate starts heating up and the goo turns into a white solid, holding the shape of whatever you might be holding it against. (Her achy arthritis wrists in her case.) She says it gets quite hot, like water right out of a hot water faucet level – so hot she wears gloves under the pad to keep her skin safe over time if she falls asleep with them, and they stay hot hot for several hours then slowly cools off.

So far, magical enough, but I’m familiar with the pocket hand warmers you can buy which, I think, work with iron somehow?

Anyway, the real difference is that these are reusable, supposedly for a hundred times or more! The guy who’d told her about them claims he has some he’s used over and over for over a decade! To ‘reload’ them, you put the pad in a pot of boiling water for about ten minutes, and then leave it to simmer for 20 minutes more. The crystal melts (or something) and turns back into the goo form. Then you fish it out of the pot and leave it sitting on a towel on a table or some such, and leave it to cool to room temperature and put it away.

And that’s it. The next time you need the heat, just put the pad against whatever and click the button again, and the entire turns solid white and heats up thing happens again.

Frankly, it sound sort of like perpetual motion. These pads cost considerably more than the disposable ones I’m familiar with, like she paid around $30 for a two pack, while the disposable ones come around $5 for ten, but if you can use them long term…

Anyway, does anyone know about these? How can they have been on sale for over a decade without anyone ever mentioning them?

2

The rigamarole with the heating and simmering creates a supercooled liquid inside. The ‘click’ is a shock wave that crystallizes the supercooled liquid. Crystallization releases heat. When supercooled water instantly crystallizes that releases heat too.

I think they’re not more popular because of how hard it is to “reset” them. Definitely not perpetual motion or getting energy from nothing.

3

I used to have some. Mine worked as she describes. I used them for outdoor activities.

The thing that seems to be magic-like are these.. I put one on in the morning and they are still warm. when I get home. 8-10 hours. They don’t get super hot but they are perfect for easing sciatica pain at work. If not a tad pricy at over $4 a go.

4

They used to be really big with skiers. I had several, and they worked pretty well, but were always kind a PITA to use, and I finally abandoned them.

5
6

Yes, these have been around for well over 30 years. I got mine back in the early 90’s, and I assume they were around before then.

Sodium acetate decahydrate is a chemical that has water molecules trapped in the crystal matrix. When you heat it up, the crystals literally dissolve in their own water. As they cool, you would think they would crystalize out, but this material is pretty neat in that it cools to room temperature without crystalizing. All that heat from warming up the material in a microwave or pot of water is stored in this unstable…really wanting to become solid again, but can’t… material. Give it a shock and it will become solid in seconds. You can watch the crystals form across the package. Upon forming the solid it jumps back up to its melting/freezing point; which is well above room temperature.

You may have seen this effect with pure water bottles that are in the freezer but still liquid. That water is also supercooled to well below the freezing point (32F, 0C). But when you shake them, it freezes and the temp jumps back up to the melting/freezing point.

7

I used to use them, but found they were only “reusable” up to a point. After a few times boiling them, they’d leak/tear/not work any more.

This was many years ago so maybe they are better made now.

8

Yeah, there’s no fundamental limit to how many times you can re-use them, just “until they get damaged”. For which quality of manufacturing would certainly matter.

9

Yes. The ones I am familiar with are, indeed, plastic bags containing a supersaturated solution of sodium acetate.

These have been available for well over “a decade”.

Well, yes.

They worked okay, unless you damaged them, but IIRC they were not that great if you needed something that would stay warm for over an hour.

10

That’s not my experience (someone had some at AfrikaBurn this year).

They work very well off the bat, and are a great science illustration, but yeah, the ones my friend had didn’t stay warm as long as the iron-based disposables. But much more in keeping with the Burn ethos, though.

How so? You’re constantly refreshing the system (by boiling water) so it’s in no way breaking any physical laws.

11

Thank you for the replies! It’s always true: SOMEBODY on the Dope knows about it, whatever it is.

(I especially liked the vid Joey_P linked to – a little too long and repetitive, but very clear and covered the subject well.)

Despite the negatives people pointed out, especially that the heat lasts a shorter time than my church friend said, I still think I may end up investing in at least one set of these pads. My problem is that I have become super sensitive to cold, which for me is anything below eighty or so, and it gets worse the older I get. Yes, I’m one of those women who wear jackets to the grocery store year round, because otherwise I simply cannot bear the dairy and frozen food aisles long enough to get what I need.

My doctor is always after me about getting out into the open air and walking around to be exposed to fresh air/sunlight/light exercise – but here in New England, most of fall and spring outdoor weathers are painful. Maybe tucking a couple of hot pads against my torso would make a significant difference for me? I’m really not looking to go walking for more than an hour or so at a stretch anyway. And as for the hassle of recharging them? It’s a rare day I don’t spend at least a stretch or two in the kitchen. If I need to have a pot of water boiling during one of them? Eh. No biggie.

OTOH, my timing is bad. We’ve finally had a couple of days that reached eighty! So no point doing anything about it until September. As we’ve always said in my family, hey, I might be dead by then anyway. :wink: