An ice pack or gel pack is a plastic sac of ice, refrigerant gel or liquid, or, in an emergency, even frozen vegetables. The refrigerant, usually non-toxic, can absorb a considerable amount of heat, since its enthalpy of fusion is high. It is commonly used as a cold compress to alleviate the pain of minor injuries or in coolers or insulated shipping containers to keep products cool during transport. The simplest type of ice pack is simply a sack, bag or towel filled with cubed or crushed ice.
Ice packs are used in coolers to keep perishable foods (especially meats, dairy products, eggs, etc.) below the 41–165 °F (5–74 °C) danger zone when outside a refrigerator or freezer. If the foods and the ice packs are placed in a cooler directly from the freezer, then the equivalent of 10 to 20 pounds of ice is needed for each 24 hour period. If the foods start off warmer (for example, non-frozen food from a refrigerator), they will not be able to remain safely cool for as long.
Water (ice) has an unusually high enthalpy of fusion and a convenient melting temperature (one accessible by household freezers). However it isn't ideal for ice packs for various reasons, so additives to improve the properties of water are often used. For example, substances can be added to prevent bacterial growth in the pack, as can additives that cause the water to remain a thick gel throughout use, instead of transitioning between a solid and a free-flowing liquid like plain water. These gel packs are often made of non-toxic materials that will not liquefy, and therefore will not spill easily or cause contamination if the container breaks. Gel packs may be made by adding hydroxyethyl cellulose (Cellusize) or vinyl-coated silica gel.
These gel packs, as with ice itself, are chilled before use. The gel-pack or water is placed in a freezer or other cooling system to lower its temperature, and then it is used to keep other items cool. Ice packs are effectively a device for storing cooling capacity.
Instant cold packs use an endothermic reaction to cool down quickly. These types of ice packs are stored at room temperature rather than needing to be physically cooled before use. When one breaks a tube inside the pack, two chemicals mix or react and absorb enough energy to produce a cooling effect. Common types include solid ammonium nitrate dissolving in water.
The first hot and cold pack was introduced in 1948 with the name Hot-R-Cold-Pak and could be chilled in a refrigerator or heated in hot water.
The first reusable hot cold pack that could be heated in boiling water or heated in a microwave was first patented by Jacob Spencer of Nortech Labs in 1973 (Patent No. 3,780,537). Reusable hot cold packs differ from instant cold packs in that they can be either frozen or microwaved.
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