Sara Beth Horton

 Many different styles  Two main types ◦ Iron Oxidation ◦ Salt Crystallization  How do they work?  What are the differences?

 Produce low level heating  Sustain temperature for long periods of time  Convenient packaging

 Utilizes exothermic reaction of oxidizing iron (making rust) ◦ 4Fe + 3O 2  2Fe 2 O 3  Irreversible reaction: one time use  Produces temperatures from °F  Can last from 5-24 hours  Wide range of styles ◦ Hand warmers ◦ Toe warmers ◦ Therapeutic heating pads

 Contain 7 ingredients ◦ Iron ◦ Oxygen ◦ Water ◦ Vermiculite / sodium polyacrylate ◦ Activated carbon ◦ Salt ◦ Cellulose/sawdust  Can change the properties of the heat packs by altering the components

 Iron ◦ Powdered works best, more surface area for reaction ◦ More iron means more heat  Oxygen ◦ Outer package is airtight to prevent reaction while in storage ◦ Heat pack wrapping is air permeable, transfer of air is generally not restricted  Water ◦ Not present in overall reaction, but required to initially reduced oxygen

 Salt ◦ Catalyzes reaction – promotes ion formation  Vermiculite/Sodium polyacrylate ◦ Reservoir for water ◦ Acts as insulation  Activated carbon ◦ Disperses heat throughout pack  Cellulose/Sawdust ◦ Insulation and filler

 Utilizes exothermic phase change of crystallizing salt ◦ Releases heat of fusion  Reversible process: reusable heat packs  Generally use Sodium Acetate ◦ Produces temperatures of ~130°F ◦ Last from 15 minutes – 3 hours

 While changing phase, compounds remain at their boiling/melting points until all of the product is fully converted  If a salt solution has a melting point of 130°F: ◦ Heat solution to 150°F to make liquid phase ◦ Cool slowly down to room temperature ◦ When the system is perturbed, crystallization is initiated and the temperature instantly increases to the melting point of 130°F

 (Specifically tri-hydrate form)  Melting point is 130°F  Very stable when sub-cooled  But still easy to initiate crystallization  Non-toxic, used as a food preservative

 Temperature ◦ Melting point of tri-hydrate is 130 ◦ Can use less concentrated solution for lower average temperature ◦ Outer casing will provide insulation for lower surface temperature  Duration ◦ Size is only design variable

 One-time-use vs. Reusable  Variability vs. Consistency