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Air-activated heated eye masks like the Kao Megurhythm Steam Hot Eye Mask are awesomely convenient little inventions. They heat up instantly once you open the package, so you don’t have to mess around with hot water or microwaves. Here’s the science behind how they work!
Air-activated eye masks work the same way as the air-activated heat packs called kairo that are incredibly popular in Japan, especially in winter. These heat packs work thanks to a heat-releasing (exothermic) chemical reaction that starts when you expose the porous pack to oxygen in the air.
Here’s the reaction:
4Fe(s) + 3O2(g) + 6H2O(l) → 4Fe(OH)3(s) ΔH < 0
iron + oxygen + water → iron(III) hydroxide + heat
This oxidation process is pretty much the same as what happens when iron rusts. The product itself contains iron powder and water but no oxygen, so the reaction can’t proceed until the sealed bag is open and oxygen from the air can work its way through the holes in the pack to reach the other reactants. The bags also often contain salt which acts as a catalyst to speed up the reaction, and vermiculite and activated carbon which can help distribute water and oxygen more evenly for a more sustained reaction, as well as insulate the reaction and trap the heat in the heat pack. The inside of a heat pack I bought from Daiso looks like this after 6 hours, with a pad of black activated carbon and brown iron(III) hydroxide:
Enough heat is produced to keep the heat pack warm for hours – my heat pack was still warm after 10 hours! Once the iron powder in the pack has all been converted to iron(III) hydroxide, the pack is finished and can’t be reused unless you reverse the reaction with some fiddly chemistry, which is probably not worth it.
Because the reaction starts as soon as oxygen hits the iron and water in the heat pack, you have to keep the pack sealed until you’re ready to use it. You also can’t use it in more than one go, as you can’t get the oxygen out of the bag once you’ve exposed the heat pack to air.
If you’re wondering, all of these ingredients are very safe and non-toxic, with the biggest health risk being burning yourself with the heat. However, these packs aren’t particularly environmentally friendly as they can’t be recycled or reused, so I’ll still be mostly sticking to my wheat and rice bags. And even though they’re quite cheap, using them regularly will end up being expensive compared to a reuseable product. These are very handy for travel though, and I can see myself using them to help me sleep on the plane!
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