One question we’re often asked here at Cooler Shock is about the packaging – why do we use our flexible bags rather than making hard-sided ice packs. The answer is pretty simple: we’ve spent a lot of time engineering the best container for the job.
Rigid ice packs usually have a shell made of HDPE (high density polyethylene) or of polypropylene. These are both fine materials: they’re cheap, they keep everything in one shape, and they contain stuff reliably. But these rigid plastics have some disadvantages, too. They also get brittle with age, and (more importantly) they have thick walls – about one millimeter thick. This is a problem, because plastic is an insulator. Insulation is good in the wall of your cooler, since it keeps the heat outside from getting inside and warming up your cold beers and just-caught fish. But it makes no sense to isolate the cold in the ice pack from the stuff in the cooler that needs to be kept cold. On top of that, once you need to put your ice pack back in the freezer to get it ready again, you don’t want to insulate the contents of the pack from the cold of the freezer. Yet, HDPE will do exactly that. Finally, they are known to crack or split from the expansion and contraction of freeze-thaw cycle causing an unrepairable leak.
We went through a few generations of bags at Cooler Shock before we came on this current, optimal design. These bags are made of aluminum, polyethylene, and nylon. Basically, the aluminum and polyethylene keep the contents of the bag in the bag, and the nylon gives added strength so that the container will last a long time and can take some abuse. Aluminum is a good conductor of heat (or cool) – the opposite of an insulator. With the nylon, the walls can be thinner than if no nylon were used. But those thin walls work really well: they help Cooler Shock to chill more quickly in the freezer, and then to transfer that chill in a cooler more effectively. If you add together the three layers of our bags, the total thickness is 8 mils, or about 0.2 millimeters. That’s about one fifth the thickness of a rigid ice pack wall.
Can you put a number on that difference in materials? Well, to be really accurate, you’d need to be working with exact specifications, and there too many different ice packs out there made with too many different things for exact comparisons. But you can certainly rough out some calculations. “Thermal Conductivity” is the measure of a material’s ability to transmit heat (1). The lower a material’s thermal conductivity, the better it acts as an insulator. HDPE has a thermal conductivity of about 0.45, nylon about 0.25, polypropylene about 0.15, and aluminum – a whopping 205 (2). Basically, polypropylene is the best of these as an insulator (but we don’t want an insulator!), and aluminum is no insulator at all. So, if the plastic of a rigid container is 5 times as thick as the total thickness of a Cooler Shock bag, and the bag is partly aluminum, then the bag will be AT LEAST five times better at passing cooling through the bag. This definitely will assist with re-freezing or with rapidly cooling the contents of a cooler.
You shouldn’t judge a book by its cover. But you really should judge an ice pack by its cover!