At Cruise Altitude the Pouch Swells: Film Stress in Air-Freighted Packs

The packs left Singapore flat and firm. They landed in Frankfurt with rounded, pillow-shaped pouches, a few with gel sitting in the corner and a faint damp patch on the carton. Nothing had leaked in the conventional sense. The film had simply been asked to hold a pressure difference for seven hours and had crept.

Air cargo holds are pressurised, but not to sea level. Typical cabin altitude is 6,000 to 8,000 feet, which is about 0.75 to 0.78 atmospheres. That is a 25% reduction, and for a sealed pouch containing air, it means the trapped gas expands by roughly a third.

What that does to a pack

A gel pack is not a rigid container. It has a headspace, and that headspace contains air. At 0.75 atm, that air expands, the pouch balloons, and the film stretches. Three things follow.

The seal goes into peel mode. A seal is strong in shear and weak in peel, and ballooning converts internal pressure into peel force at the seal edge. A seal that holds 12 N/15 mm in a lab test may see the equivalent of 4 N/15 mm of peel load for seven hours.

The film creeps. Polyethylene and PLA both creep under sustained load, especially warm. A pack at 25 °C in a cargo hold with a 0.25 atm differential will deform permanently over a few hours. It does not recover on landing.

The gel redistributes. A ballooned pack lets the gel slide to one end, which puts a concentrated mass against one seal and leaves the other end with a thin film and no support. That is the pack that fails on the next leg.

Where the air comes from

Three sources, and only one is obvious.

The headspace is the obvious one. A pack filled to 88% has 12% air by volume. Reduce the fill headspace and you reduce the problem proportionally.

Dissolved air is the second. Water at 20 °C holds about 18 mg/l of dissolved oxygen and more nitrogen. When the pack is sealed and then cooled or agitated, that air comes out of solution as microbubbles, which add to the internal pressure and create foam against the film.

Air trapped in the gel structure is the third. A batch mixed with a vortex, or pumped with a centrifugal pump, carries entrained air. It looks like a slightly opaque gel and it behaves like a sponge full of gas at altitude.

The fixes, in order of effect

Reduce headspace to 3–5%. Fill to 95–97% and squeeze the pouch flat before sealing. This is the single biggest lever, and it costs nothing but a small change at the filler.

Deaerate the gel. A vacuum deaerator, or simply letting the batch stand for 30 minutes before filling, removes most entrained air. If you fill immediately after mixing, you are sealing foam.

Use a film with better creep resistance. Nylon laminates and thicker coextruded PE resist sustained pressure far better than a thin monolayer. The cost is 2–4 cents per pack and it is cheap against a claim.

Widen the seal and cool it properly. A 10 mm seal with a controlled cooling cycle, rather than a hot bar pulled off while still molten, gives a much better peel resistance. Seal strength measured immediately after sealing is misleading; measure it after 24 hours.

The test you can run in a vacuum jar

You do not need an altitude chamber. A 5 litre vacuum jar with a hand pump, the kind used for degassing resin, will do. Put five filled packs in, pull to 0.7 atm, hold for four hours at 30 °C, release, and inspect.

• Any permanent deformation of the pouch is a fail.
• Any gel in the seal zone is a fail.
• Any pack that does not return to flat within a minute is marginal.
• Weigh before and after to catch weeping through the seal.

Four hours at 0.7 atm is harsher than a typical flight, which makes it a useful accelerated screen. If a pack survives it, the air leg is not your risk.

The reversal

An exporter switched to a thicker film to stop the ballooning and saw more leaks, not fewer. The thicker film was stiffer, so it did not conform to the gel, and the headspace at the top became a rigid bubble that concentrated all the pressure on a small area of seal. The thinner, more compliant film had distributed the load across the whole seal length.

Compliance beat strength. That is not intuitive, and it is why testing beats specifying.

What to ask before an air lane

Ask for the seal strength after 24 hours at 30 °C, the film’s creep behaviour under 0.3 atm differential, and the headspace percentage on the fill spec. If any of those is unknown, run the vacuum jar test on the first carton before the pallet flies. SAP powder for air freight packs is selected for low entrained air and fast deaeration, which removes one of the three sources of internal pressure.