W HAT’S THAT STUF F? DESIGNER: TYPE INSIGHT OR HINDSIGHT AFTER COLOR BAR IN WHITE
hand warmers Small packets of warmth work through a simple EXOTHERMIC REACTION
“You want to make this thing act quickly because people like to open up the packet and feel warm right away.” WWW.CEN-ONLINE.ORG
36
JANUARY 2 5, 20 10
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 heatreleasing 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. AMANDA YARNELL/C&EN (BOTH)
FOR WINTER SPORTS enthusiasts, hand warmers can mean the TOASTY Hand difference between calling it a day warmers can extend the early and playing outside for as winter fun. long as possible. In fact, anyone who braves cold temperatures might be tempted to try the little pouches that emit warmth within seconds of being exposed to air. Hand warmers date back centuries to when the Japanese would use hot stones to warm their hands, says Keiko Ishikawa, a marketing manager of handwarmer maker Mycoal USA. Portable hand warmers filled with hot ash were the version that followed, she says. These days, disposable hand warmers turn up the heat more complicated. “You want to make this in your mittens by means thing act quickly because people like to open of an exothermic reaction up the packet and feel warm right away, but that, in essence, just creyou also want it to last a long time,” says Joe ates rust. Each pouch Vergona, manager of engineering and prodtypically contains uct development for Grabber Performance iron powder, Group, a Grand Rapids, Mich.-based comsalt, water, pany that sells hand warmers. For example, an absorbent some hand warmers last seven hours, and material, and others can last more than 24 hours. activated carbon. To lengthen the time a hand warmer When the pouch is lasts, some companies opt to increase the removed from its outer amount of iron in the packet, Vergona says. packaging, oxygen drifts Another strategy is to experiment with across the pouch’s permethe iron powder. “If you vary the raw maable covering. With salt and terials in the warmer, you can change how water present, the oxygen requickly the reaction happens or how much acts with the iron powder located inside to of the warmer is reacted at one time,” form iron oxide (Fe2O3) and release heat. Vergona adds. For example, the greater the The absorbent material can be vermicusurface area of the iron, the more it can relite, pulverized wood, or a superabsorbent act with oxygen to produce heat, he says. polymer such as polyacrylate. It helps reThe pouch material also affects the tain the moisture so that the reaction can performance of the hand warmers. “It’s a occur. The activated carbon helps to evenly balance of the ingredients inside the pack disperse the heat produced, which can avand the performance characteristics of the erage 135 °F. pouch itself,” Vergona says. The iron powAlthough the chemistry of disposable der and other ingredients are contained hand warmers is simple, their engineering is in a blended nonwoven material that has
BESIDES WARMING hands and feet, the
technology has other applications. For example, Grabber sells heavy-duty warmers that can be used to transport tropical fish. The company is also expanding into the medical and therapeutic fields. Hand warmers can even be used to teach exothermic reactions, as Kathy Ceceri, a mom who homeschools her children, discovered. The packaging instructions might have “said not to open the packet, but I immediately opened it and poured the contents into a glass jar,” she says, noting that the iron powder began to smoke. By completely exposing the iron to air, she accelerated the exothermic reaction. Ceceri—and anyone else who’s used hand warmers—has discovered that there’s nothing like simple chemistry to help turn up the heat.—LINDA WANG
