Let’s start at the beginning – what do we want from a cold pack? What characteristics of a cold pack do we want to optimize for? At Minus Works, we design for a cold pack that stays cold as long as possible, meaning it absorbs as much heat as possible. We try to optimize for the Latent Heat of Fusion, a concept we talk about in Part 1 of this Series if you need a refresher.
OK, fine, so we know what we want to maximize but how do we do that? What properties affect this mumbo jumbo called “Latent Heat of Fusion”?
We at Minus Works believe that one of the major, controllable properties that influences latent heat of fusion is the quality of crystallization, a fancy way of saying the quality of the freeze.
By way of analogy, let’s say you are a medieval monarch constructing a castle. Would you prefer a castle wall that looked like this:
Or a castle wall that looked like this:
We believe B would be the better choice. The tight, densely packed structure of Wall B will take more energy to break down when defending against those barbarian hordes. In this case the structure of the castle wall is analogous to the molecular structure of a frozen cold pack. A more orderly, more densely packed freeze will require more energy to melt, thus more heat can be absorbed and we can protect those perishables longer against higher ambient temperatures.
One of the key ways to get orderly crystallization is to engineer the hydrogel material for good thermal conductivity. Good thermal conductivity allows for different parts of the cold pack to experience similar temperatures at the same time, which allows the entire cold pack to freeze at a uniform rate – producing similar sized crystals and an orderly lattice structure. We have also set up our manufacturing process to optimize for homogeneity or consistency in our hydrogel, which improves crystal size and distribution uniformity. Additionally, the mechanism through which water is bound within our hydrogel formulation works as a natural governor on crystal ice, promoting uniformity.
Another very important factor in crystallization is the rate of freeze. This is something that you the user of cold packs can influence. A more rapid freeze will create smaller, more uniform crystal sizes and a more densely packed “Wall”. The working theory here comes from the way ice crystals are formed. Nucleation (or seeding) of crystals occurs at higher probabilities at lower temperatures so when you get colder faster, you will have more crystal seeds in your cold pack that’s on its way to be frozen. On the other hand, with a slow freeze, you will have a lower nucleation rate or fewer seeds. With the slow freeze, the seeds that do occur will act as the foundation for the overall freeze so you will have fewer, larger crystals in your cold pack, which will end up looking more like Wall A. How do you speed up the rate of freeze? The easiest way is to turn the temperature down on your freezer – a colder freezer creates a higher temperature differential, which increases the rate of freeze. There are also specialty freezers (blast freezers, shock freezers) that are purpose built to achieve this.
Unfortunately, the inconsistencies and voids we see in Wall A happen frequently in lower quality cold packs, which produce a weaker product that melts rapidly and just adds weight and cost to your package. Often times it’s hard to tell high quality from low quality refrigerants. There is no substitute for field testing – we are very happy to provide sample material for you to trial in your cold chain application or run comparison tests for you with our in-house test chamber.
Send us a note, we're excited to hear from you.