Cycle Testing Reusable Gel Packs: What 50 Freeze-Thaw Rounds Actually Show

Every reusable pack supplier claims 100 cycles. Almost none of them can show you the data, and the few who can usually tested with water rather than gel, at -18°C rather than -25°C, and without the wash step that a real return loop imposes. Fifty cycles is a reasonable first milestone, and the results are more interesting than a single pass or fail number.

What degrades, and in what order

Three things go wrong, at different rates. Water loss comes first: a pack with a imperfect seal loses 1–3% of its mass per cycle to evaporation through the film, and by cycle 20 the gel has thickened, the pack no longer fills the pouch, and contact with the box wall drops. Film fatigue comes second: creases form along the seal line where the pack was stacked, and at cycle 30–40 you get pinholes at the corners. Polymer breakdown comes last, and it is slower than folklore suggests. A cross-linked polyacrylate in deionised water at 0.8% loses maybe 8–12% of its gel strength over 50 cycles, mostly from freeze concentration — ice formation pushes dissolved salts and polymer into a shrinking liquid phase, and repeated concentration stresses the network.

So the failure you see at cycle 40 is usually mechanical, not chemical. That matters, because it means you can extend life with film and fill decisions rather than a more expensive polymer.

The test that takes a season but settles the argument

Take 20 packs from a production run. Number them. Each cycle: freeze at -20°C for 20 hours, thaw at 30°C for 4 hours, drop each pack 1 m onto concrete on a different face, weigh it, and record. Every tenth cycle, cut two packs open and measure gel strength with a simple penetrometer or a drain test in a 2 mm mesh basket for 10 minutes. Record the drained mass as a percentage — fresh gel drains under 3%; a pack at 10% is releasing water in service.

Typical results for a 500 g pack in 90 micron coex PE: 2% mass loss by cycle 10, 5% by cycle 30, first corner leaks between cycles 35 and 50, drained mass rising from 2% to 7%. Switch to 120 micron film with a 15 mm seal and the leak point moves past 70 cycles for about $0.03 more per pack. That is a better return than reformulating.

Sanitising is the hidden cycle killer

Return loops wash packs. Chlorine at 200 ppm for 5 minutes is common in food logistics, and hypochlorite attacks the polymer backbone as well as the film. Twenty wash cycles with 200 ppm chlorine can cost more gel strength than 50 freeze cycles. If the pack contacts food packaging, you still need the wash — so either accept a shorter life and price it in, or specify a pack with a laminated food-contact film and a formulation tolerant of oxidisers, and test the wash separately from the freeze.

Setting an honest cycle claim. Claim the cycle count at which 5% of your sample has leaked, not the average. If that is 45, say 40 and add a visual inspection step at cycle 20 and 35: squeeze the pack, look for creases at the corner seals, weigh five packs from each batch — anything more than 4% below nominal has lost water and should be retired. A 40-cycle claim that holds is worth more than a 100-cycle claim that a customer disproves in a quarter.

Freeze concentration is the one factor you can influence from the formulation side, and a higher molecular weight SAP tolerates it better than a cheap high-swell grade. Test both before you commit to a cycle claim.