Frozen gel packs - a critical component in the performance of temperature controlled packaging. By regulating the absorption of heat, frozen gel packs in a well designed system are able to help keep the product within a specified temperature range regardless of its exposure to fluctuating external temperatures.
But how frozen do the gel packs have to be?
Is there a risk of freezing the product payload?
Will conditioning gel packs to various frozen temperatures have an impact on the performance of a package?
Is there an optimum temperature for conditioning frozen gel packs?
What temperature conditioning should be used for designing and qualifying temperature controlled packages?
The answers to these questions can be very valuable information to have when designing a temperature controlled package.
How frozen do the gel packs have to be?
When the gel packs are removed from the freezer they immediately begin to transition to their phase change temperature, roughly 0°C in the case of water-based gel packs, those most commonly used. If the gel packs are removed from the freezer and immediately confined to a hermetically sealed insulated package, there is often a "thermal shock" that occurs, evidenced by a brief but significant dip in internal air temperature within the package. The lower the temperature to which the gel packs are conditioned the more dramatic the dip in temperature. The effect is only compounded with the addition of the insulation. This can be verified by placing a battery operated data logger monitoring device within the package or directly, by probing the actual product using thermocouple wire integrated with a temperature data logger.
Recordings of this phenomenon have been the cause of countless deviations, non-conformances and subsequent explanations among quality organizations within the pharmaceutical industry.
Is there a risk of freezing the product payload?
Yes. One elegant method employed by companies to avoid this danger is to shed the shock by allowing the gel packs to relax for thirty minutes or so before packing or sealing the packages closed. By allowing the gel packs to near their phase change temperature, the risk of thermal shock, if not eliminated entirely, is significantly reduced by the time the product is placed into the package with the gels so as to stay within the acceptable range, (2-8°C for example).
Not all companies however, have the ability to do this within their daily operation. Others don't employ this solution due to an inability to control and document the process. Still others allow for the dip to occur and invest the time and expense to design and qualify their packages accordingly.
Will conditioning gel packs to various frozen temperatures have an impact on the performance of a package?
By linking to the following graph, you can see that the temperature at which the frozen gels are conditioned have a direct and variable influence on the package's performance and longevity.
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.
Practitioners of insulated package design attempt maximum utilization of the phase change portion of the graph while minimizing the effects of what happens before and after (the curves on either end of the graph).
Is there an optimum temperature to condition frozen gel packs?
Beyond performance differences, there are mechanical considerations when it comes to optimizing the freezing of gel packs. One of the more remarkable properties of water is that it has the highest specific heat of any common substance, 1 calorie/gm °C = 4.186 J/gm °C. It's cheap, abundant and easy to work with - which is what makes it such an attractive source of coolant in insulated packaging systems. It does however, require a significant amount of energy to freeze. The table below, developed by Amgen, calculates the trade-offs in costs for freezing 1 Kilogram of gel. The most efficient temperature is generally considered to be around -18°C.
Source: Amgen Process Development Dept.
What temperature condition should be used for designing and qualifying temperature controlled packages?
It is important to specify and document the temperature your gel packs are conditioned to for design and qualification tests. Make certain that the conditioning temperature and tolerances of the freezers are commensurate with the freezers used for conditioning gel packs within your commercial operation. Otherwise, you may see a difference in performance between the package configurations you qualified and those you send to your customers.
For a related topic, see the posting Effects of Refrigerated Storage on Assembled Insulated Packaging in the January Archives.