Ice pack
Encyclopedia
An ice pack or gel pack is a plastic sac of ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

, refrigerant
Refrigerant
A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase change from a liquid to a gas. Traditionally, fluorocarbons, especially chlorofluorocarbons, were used as refrigerants, but they are being phased out because of their ozone depletion...

 gel
Gel
A gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state...

 or liquid, or, in an emergency, even frozen vegetables. The refrigerant, usually non-toxic
Toxicity
Toxicity is the degree to which a substance can damage a living or non-living organisms. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...

, can absorb a considerable amount of heat, since its enthalpy of fusion
Enthalpy of fusion
The enthalpy of fusion is the change in enthalpy resulting from heating one mole of a substance to change its state from a solid to a liquid. The temperature at which this occurs is the melting point....

 is high. It is commonly used as a cold compress to alleviate the pain of minor injuries or in cooler
Cooler
A cooler, cool box, portable ice chest, chilly bin , or esky most commonly is an insulated box used to keep food or drink cool. Ice cubes are most commonly placed in it to help the things inside stay cool...

s or insulated shipping container
Insulated shipping container
Insulated shipping containers are a type of packaging used to ship temperature sensitive products such as foods, pharmaceuticals, and chemicals. They are used as part of a cold chain to help maintain product freshness and efficacy...

s 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 (5–73.9 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
Enthalpy of fusion
The enthalpy of fusion is the change in enthalpy resulting from heating one mole of a substance to change its state from a solid to a liquid. The temperature at which this occurs is the melting point....

 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
Hydroxyethyl cellulose
Hydroxyethyl cellulose is a gelling and thickening agent derived from cellulose. It is widely used in cosmetics, cleaning solutions, and other household products....

 (Cellusize) or vinyl
Vinyl
A vinyl compound is any organic compound that contains a vinyl group ,which are derivatives of ethene, CH2=CH2, with one hydrogen atom replaced with some other group...

-coated silica gel
Silica gel
Silica gel is a granular, vitreous, porous form of silica made synthetically from sodium silicate. Despite its name, silica gel is a solid. It is a naturally occurring mineral that is purified and processed into either granular or beaded form...

.

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
Instant cold pack
An instant cold pack is a device that consists of two bags, one containing water, which is inside a second bag containing ammonium nitrate. When the inner bag is broken by squeezing the package, the ammonium nitrate is allowed to dissolve in the water in an endothermic reaction...

 use an endothermic reaction
Endothermic
In thermodynamics, the word endothermic describes a process or reaction in which the system absorbs energy from the surroundings in the form of heat. Its etymology stems from the prefix endo- and the Greek word thermasi,...

 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, or ammonium chloride
Ammonium chloride
Ammonium chloride NH4Cl is an inorganic compound with the formula NH4Cl. It is a white crystalline salt that is highly soluble in water. Solutions of ammonium chloride are mildly acidic. Sal ammoniac is a name of natural, mineralogical form of ammonium chloride...

dissolving in water.

The reusable hot cold pack 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.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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