Commercial Hand WarmersSara Beth Horton

Many different styles Two main types

◦ Iron Oxidation◦ Salt Crystallization

How do they work? What are the differences?

Intro

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

Goal of Hand Warmers

Utilizes exothermic reaction of oxidizing iron (making rust)◦ 4Fe + 3O2 2Fe2O3

Irreversible reaction: one time use Produces temperatures from 95-140 °F Can last from 5-24 hours Wide range of styles

◦ Hand warmers◦ Toe warmers◦ Therapeutic heating pads

Iron Oxidation

Iron Ox Heat 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 Oxidation

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

Components - Reactants

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

Components - Others

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

Salt Crystallization

Sodium Acetate Heat Pads

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

Crystallization

(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

Why Sodium Acetate?

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

Design considerations

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

Differences

Questions/Discussion?