The temperature at which the frozen gels are conditioned have a direct and variable influence on the package's performance and longevity. Example:

The graph is an average culled from multiple tests and illustrates 1.) equilibrium at various frozen temperatures, 2.) time to attain phase temperature of 0°C +/-1°C, and 3.) the consequent longevity of the gel pack. In each test, identical 16 oz., 0°C phase change gel packs were probed with thermocouples and conditioned simultaneously in freezers set at -10°C, -20°C, -30°C and -80°C. They were removed simultaneously after 48 hours and placed into a controlled temperature chamber at 23°C +/- 1°C. Data were then logged at 15 minute intervals over a 48 hour period.

There is evidence of an obvious performance difference. The lower the temperature, the more exaggerated the effects of thermal shock. It is more pronounced and of longer duration, decreasing the air temperature within the package and increasing the threat of pulling the product temperature out of specification, possibly below 0°C. Attempting to artificially increase the longevity of the package by freezing the gel packs at lower temperatures extends the thermal shock curve but does nothing to extend the duration of the heat of fusion. Each gel pack contains a finite amount of energy, called enthalpy. For instance, all 16 ounce 0°C phase change gel packs contain roughly 179 kilojoules of energy. Regardless of what temperature the gel packs were frozen, once they reached their phase change temperature, the graph shows that they all performed the same and melted essentially at the same rate, if not the same time. 

You can find more on this topic in the following blog archives:

March 2009

April 2006