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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.
Small packets of warmth work through a simple exothermic reaction
For winter sports enthusiasts, gel 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.
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.
To lengthen the time a hand warmer lasts, some companies opt to increase the amount of iron in the packet.
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,” For example, the greater the surface area of the iron, the more it can react with oxygen to produce heat.
The pouch material also affects the performance of the instant hand warmers. “It’s a balance of the ingredients inside the pack and the performance characteristics of the pouch itself,” 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.
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 reusable will last a week and die,” because oxygen can get in and spoil the product. The outside wrappers are usually made of polymers such as the plastic polyethylene.
The main difference between disposable hand warmers and some reusable hot hands 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.Warmers that can help tropical fish survive transportation through chilly climates.