Retail ice substitute packs: formulation and label claims

The pack came back in a returns bag with a photo taped to it: a picnic cooler, four cans, and a gel pack that had clearly been liquid by lunchtime. The label said “keeps contents cold for 12 hours”. On the same shelf, a competitor’s pack said 8 hours and cost 40 percent more. The returned pack had been selling well for two seasons, which tells you something about how the claim was being tested — not at all.

Ice substitute packs are a formulation product more than an absorbent product. The polyacrylate is the thickener that stops the water sloshing and gives the pack its body, but the freezing behaviour, the thaw time and the label all depend on what else is in the bag.

What is actually in the bag

A typical formulation is 92 to 96 percent water, 1 to 2 percent polyacrylate, 2 to 5 percent propylene glycol or glycerol to depress the freezing point, a preservative, and a dye. The glycol matters more than most people expect: pure water freezes at 0 degrees and forms ice that is sharp, expands, and thaws abruptly. A 5 percent glycol mix freezes around minus 2 and produces a softer, more gradual thaw, which is what you want against a bottle of milk.

The polymer does three jobs at once. It raises viscosity so the pack does not bulge at one end when you lay it down, it holds water in a matrix so the thaw is slower and more even, and it stops the pack turning into a bag of loose ice crystals after a few freeze-thaw cycles. Above about 2 percent, the mix becomes difficult to fill through a 12 mm nozzle and the pack feels stiff at room temperature.

Freeze-thaw cycling is where cheap packs die

A pack that is frozen and thawed weekly for a year goes through 50 cycles. Water alone separates: ice crystals grow, the polymer network is strained, and by cycle 20 you can feel hard lumps through the film. A well-formulated pack at 1.5 percent polymer and 4 percent glycol still feels uniform at cycle 50.

Run the test yourself before you commit to a claim: ten packs, ten cycles, weighed and squeezed each time. Record the weight loss as well, because film permeability shows up as a pack that quietly loses 3 percent of its mass over a season and then stops performing. A 2 g loss on a 400 g pack is a 0.5 percent error in the freeze curve — not much, but it is a leading indicator of a film that will fail at the seal.

The label is a legal document, not a wish

“Keeps contents cold for 12 hours” is a performance claim, and in most markets it has to be substantiated with a stated test condition: ambient temperature, starting temperature of the pack, ratio of pack mass to payload, and the temperature threshold used to define “cold”. A claim tested at 18 degrees ambient with a 1:1 ratio and a 10 degree threshold is a very different promise from the same words tested at 30 degrees with a 1:4 ratio.

The wording that survives scrutiny is specific. “Tested at 25 °C ambient with 2 packs to 6 kg of chilled food, contents below 8 °C for 10 hours” is defensible and reads better than a bare number. If the marketing team wants a big number, the place to find it is the ratio — doubling pack mass relative to payload buys more hours than any formulation tweak.

Three claims to avoid

“Non-toxic” is the first. The contents are not acutely toxic, but a burst pack in a cooler with unwrapped food is a contamination event and a child can bite through the film. Say “contents are not for consumption” instead, and put it where it can be read.

“Reusable” is the second, unless the film and the seal have been tested for it. Most packs fail at the seal after 20 to 30 cycles, and a reusable claim turns a wear item into a warranty liability. “Refillable” and “reusable” are not the same word.

“Food safe” is the third, and it is a materials question about the film, not the gel. If the pack is intended to sit against food, the film needs the relevant food-contact documentation, and the gel needs to be formulated so that a leak is not a poisoning event.

If you are starting from a powder and building the formulation, the formulation notes for super absorbent powder give the viscosity and swell figures for the 1 to 2 percent range, which is where most ice packs live.