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Energy Storage

What’s inside disposable hand warmers?

Small packets of warmth work through a simple exothermic reaction

by Linda Wang
January 25, 2010 | APPEARED IN VOLUME 88, ISSUE 4

Credit: Amanda Yarnell/C&EN
Credit: Amanda Yarnell/C&EN

For winter sports enthusiasts, hand warmers can mean the difference between calling it a day early and playing outside for as long as possible. In fact, anyone who braves cold temperatures might be tempted to try the little disposable pouches that emit warmth within seconds of being exposed to air.

Hand warmers date back centuries to when people in Japan would use hot stones to warm their hands, says Keiko Ishikawa, a marketing manager at hand warmer maker Mycoal USA. Portable hand warmers filled with hot ash were the version that followed, she says. These days, you can buy a variety of hand warmers based on battery packs and lighter fuel, but disposable hand warmers rely entirely on chemistry.

Disposable hand warmers turn up the heat in your mittens by means of an exothermic reaction that, in essence, just creates rust. Each pouch typically contains iron powder, salt, water, an absorbent material, and activated carbon. When the pouch is removed from its outer packaging, oxygen drifts across the pouch’s permeable covering. With salt and water present, the oxygen reacts with the iron powder located inside to form iron oxide (Fe2O3) and release heat.

The absorbent material can be pulverized wood, a polymer such as polyacrylate, or a silicon-based mineral called vermiculite. It helps retain the moisture so that the reaction can occur. The activated carbon helps to evenly disperse the heat produced, which can average 135 °F.

Although the chemistry of disposable hand warmers is simple, their engineering is more complicated. “You want to make this thing act quickly because people like to open up the packet and feel warm right away, but you also want it to last a long time,” says Joe Vergona, manager of engineering and product development at Grabber Performance Group, a Grand Rapids, Mich.-based company that sells hand warmers. For example, some hand warmers last seven hours, and others can last more than 24 hours.

To lengthen the time a hand warmer lasts, some companies opt to increase the amount of iron in the packet, Vergona says.

Another strategy is to experiment with the iron powder. “If you vary the raw materials in the warmer, you can change how quickly the reaction happens or how much of the warmer is reacted at one time,” Vergona adds. For example, the greater the surface area of the iron, the more it can react with oxygen to produce heat, he says.

Credit: Amanda Yarnell/C&EN
Hand warmers can extend the winter fun.
Credit: Amanda Yarnell/C&EN
Hand warmers can extend the winter fun.

The pouch material also affects the performance of the hand warmers. “It’s a balance of the ingredients inside the pack and the performance characteristics of the pouch itself,” Vergona says. The iron powder and other ingredients are contained in a blended nonwoven material that has specific permeability characteristics. If the pouch admits more oxygen, the reaction occurs more quickly. Toe warmers, for example, use a nonwoven material that lets in more oxygen to compensate for the low-oxygen environment inside a shoe. “The level of perforation, the size of the holes—all that’s going to govern how much oxygen enters the warmer,” Vergona says.

To extend the shelf life of hand warmers, the outside wrapper is specially chosen to ensure that minimal amounts of oxygen get in and minimal water gets out. “Any old plastic, and the hand warmers will last a week and die,” because oxygen can get in and spoil the product, Vergona says. The outside wrappers are usually made of polymers such as the plastic polyethylene.

The main difference between disposable hand warmers and some reusable versions is the chemicals used to produce the heat-releasing reaction. Reusable hand warmers don’t contain iron but instead use a supersaturated solution of sodium acetate that releases heat as it crystallizes. Boiling the used packet restores the solution to its supersaturated state. Air-activated hand warmers can’t be reused.

Disposal hand warmers don’t just keep humans from getting too cold. Grabber also sells heavy-duty warmers that can help tropical fish survive transportation through chilly climates.

UPDATE: This article was modified on December 12, 2016 to refresh its information and data.



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SaffyBelleRox (January 31, 2012 12:59 PM)
I have a pair of re usable hand warmers but i don't know how long to re warm them for, does anyone know?
null (February 13, 2012 11:18 AM)
boils them in a towel for 15 min.
christine zamora (February 1, 2012 11:18 PM)
i used hand warmers for golf and that very same day I noticed micro slivers in both my hands I have tried to remove them but have not been able to do so. They are very painful when brushed against anything. I have used a magnifying glass to see the slivers but can't see anything.. Have you heard of this happening??
Bill (January 4, 2017 2:55 PM)
I suspect it's fiberglass. I didn't know any hand warmers use that, but I don't know what else it could be. By now, it should have worked it's way out of your skin.
Jay (January 4, 2017 6:01 PM)
to remove microslivers rinse or soak in cold water to dilate the pores, air dry briefly and use the sticky surface of dust tape to remove the slivers. This technique is also effective for fiberglass insulation and thistle removal.
DeLaMancha (March 6, 2017 1:59 AM)
Heat causes dilation and cold causes constriction
Cameron Peters (January 6, 2017 1:02 PM)
Do you think maybe you got the fiberglass in your hand from the sand trap rakes? I know those things are made of fiberglass sometimes and I have personally been a victim
mike (February 22, 2012 2:01 AM)
meco hand warmer, made in china, we made the best hand warmer in the world
Melody (February 24, 2012 10:26 AM)
I've used hand and toewarmers a great deal up here in the Northeast. Sometimes you get a bad batch and its probably because oxygen got in. It occured to me that it might be fun to try to make your own - as a science experiment, not to save money!
KT (January 7, 2013 2:10 PM)
Does anyone know when and who discover the use of sodium acetate to make hand warmers?
ras1341 (January 14, 2013 2:33 PM)
I mixed iron powder and salt and water but not seen any reaction.
Jessie (June 5, 2013 12:31 AM)
I was wondering if anyone knows the amount of iron in a single pouch. My dog got into one and I was wondering if the amount in one pack is enuf to kill him or if he will be ok. He definately did not eat it all, there was a huge mess left to clean up, if he ate any. Im not sure.Just need a starting point so I know if I should worry or not. thanks.
barbara hemmings (February 27, 2017 1:04 AM)
Was your dog fine? My dog also ate some but not the whole thing I did not take er to the vet as she did not have a reaction towards it she didnt vomit she is acting normal.
Theresa (October 26, 2015 11:45 AM)
Can hand warmers be used in outside cat shelters during the cold winter?
Anonymous (November 6, 2015 8:35 AM)
I use hand warmers all winter
Normal Jamon (January 4, 2017 2:16 PM)
Given that iron is an essential element, it seems unlikely that your dog ate an unhealthy amount if he only consumed part of a packet. However, if you can now pick your dog up with a large magnet, it's probably time to see a vet. :)
Stanislav Jaracz (January 13, 2017 3:01 PM)
I like the light naphtha fueled catalytic hand warmer. It can be shut off at any time and refuelled for continued use.
Robert Buntrock (January 29, 2017 5:06 PM)
Decades ago, outdoor skaters would carry freshly baked potatoes for warmers, then eat them when they cooled. For around the house, we use rice bags that my wife made, microwaved for warming bags, from the freezer for cold bags. Reusable ad infinitum.
D, Green (February 13, 2017 11:05 AM)
How do you dispose of used hand-warmers (environmentally safely, of course). Thanks
J. Baker (February 10, 2018 4:13 PM)
Just throw them away in your regular trash. Everything in the packet is completely natural (iron, salt, water, and carbon). The most environmentally unfriendly part of the product is probably the outer packaging (heavy duty plastic). If you felt okay throwing that in the trash, the rest shouldn't bother you.

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