What makes an ice pack useful is the heat its water draws from the surroundings as it melts; when it freezes again in the freezer, the water releases that heat. The other substances inside tune that job in two ways: one keeps the liquid thick, the other pulls the melting point to the wanted temperature.
What is the liquid inside an ice pack, gel or water?
In a gel ice pouch the liquid is both gel and water: a gelling agent such as sodium polyacrylate or hydroxyethyl cellulose thickens the water. Many products also contain propylene glycol to lower the freezing point and a blue dye for appearance. In a eutectic plate the water becomes a brine with salt.
| Product family | Contents | Melting point |
|---|---|---|
| Gel pouch | Water and a little gelling agent; in the Cryopak example, less than 2% sodium polyacrylate | Around 0 °C; lower in products with propylene glycol |
| Eutectic plate | Salt water (brine) | Depends on salt type and ratio; GEBHARDT plates -3, -14, -18 or -21 °C |
| PCM pack | Special blends based on salts, paraffin or oils | By model; the va-Q-tec range spans -67 to +37 °C |
What the gelling agent does
Pure water runs quickly out of a punctured pouch. A gelling agent keeps the liquid thick, so when the pouch is pierced the contents largely stay put, which makes cleaning and product safety easier. The cold still comes from the water: the gelling agent adds no capacity, it only keeps the water in place.
How the melting point is set
Dissolving salt in water lowers its freezing point; it is the same effect as salting roads in winter. In a eutectic plate the type and ratio of salt are chosen so that the filling freezes and melts at a single temperature. That is why a -18 or -21 °C plate for frozen goods and a -3 °C plate working close to chilled goods carry different fillings. A plate only freezes below its melting point; GEBHARDT requires the freezer to be at least 5 °C below that point. The difference between the melting point and the product's temperature is covered in the eutectic solution: why it melts at exactly that temperature.
What the safety data sheets say about the liquid
The safety data sheet (SDS) of one Cryopak gel pack does not classify the mixture as a hazardous substance and rates the contents non-toxic, but it also notes that swallowing may cause gastrointestinal irritation. The sheet of another gel pack states that no health effects are expected under normal use. These two sheets apply only to those products; instant cold packs use chemicals such as ammonium nitrate. What to do after a leak or contact is covered in is ice-pack filling harmful.
Food contact and punctures
If a pack is punctured, its contents can reach the product. For food transport, food-contact suitability of the shell and of the filling goes into the specification as separate lines; a safety data sheet describes composition and safety measures and does not replace a food-contact declaration. For that reason food that has touched a punctured pack is separated and not used.
- a damaged pack is not repaired, it is withdrawn
- product it touched is separated and recorded
- check the local waste rule before pouring the filling down a drain; the Cryopak gel pack data sheet says it can go to landfill
- the plastic shell is collected separately; rigid plates last long, so the waste load is small