Ice pack

An ice pack or gel pack is a plastic sac of ice, or of refrigerant gel or liquid. Both the ice pack and the non-toxic gel (which is mostly water) can absorb a considerable amount of heat due to the high enthalpy of fusion of water. These packs are commonly used to keep food cool in coolers for consumption later in the day; or as a cold compress to alleviate the pain of minor injuries; or in insulated shipping containers to keep products cool during transport.

Ice packs are used in coolers to keep perishable foods (especially meats, dairy products, eggs, etc.) below the 41 - 165 F danger zone when outside a refrigerator or freezer. The amount of ice needed to cool a given mass of food varies greatly, depending on: the initial temperature of the food; the temperature, solidity, and mass of the ice used; the insulating value of the container both are put into; the ambient temperature around the container; and whether the container is exposed to direct sunlight or kept in the shade.

Water has an unusually high enthalpy of fusion and a convenient melting temperature (one accessible by household freezers). Additives to improve the properties of water are often used. For example, substances can be added to prevent bacterial growth in the pack, or to prevent the water from solidifying so it remains a thick gel throughout use.

Gel packs are often made of non-toxic materials that will remain a slow flowing gel, 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.

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.

Safety Concerns
Gel packs have been made with diethylene glycol and ethylene glycol both of which can cause illness if ingested in large amounts. Neither one of these substances should be used in gel packs intended for use with food. Their use, in the United States, has led the Consumer Product Safety Commission to force their recall.