Sublimation: A One-Kilo Pack That Weighs 940 Grams After Three Weeks

The packs came out of the freezer at 941 g. They had gone in at 1,002 g three weeks earlier, and nobody had opened the pallet. No leak, no puncture, no spill on the floor: the water had left as vapour, through the film, one molecule at a time.

Sublimation is the phase change from ice directly to vapour, and it happens in any freezer with low humidity and air movement. It is slow, silent, and it removes exactly the thing you paid to freeze. On a 48-hour lane, 6% mass loss is roughly three hours of protection gone.

Why it happens in a freezer

Air in a freezer is extremely dry, because most of its moisture has already condensed on the evaporator coil. That creates a vapour pressure gradient across the pouch film: saturated at the ice surface, near zero outside. Water molecules diffuse through the film along that gradient, and the ice surface recedes.

The rate depends on four things: the film’s water vapour transmission rate, the temperature difference between the pack and the freezer air, the air velocity over the packs, and how long they stay there. Blast freezers with high air movement are the worst case, because moving air sweeps vapour away and keeps the gradient steep.

A typical 90 micron PE pouch transmits 3–6 g/m²/day at −18 °C. A 100 × 150 mm pack has 0.03 m² of surface, so that is 0.1–0.2 g per day, or 2–4 g over three weeks. That does not explain 60 g.

Which means the loss is usually not through the film alone. It is through the seal, the fold, or a micro-channel at the gusset. A seal that passes a dye penetration test at 20 °C can leak vapour slowly at −18 °C because the film becomes stiff and the seal interface opens by microns.

How to measure it without a lab

Weigh ten packs from a pallet on the day they are frozen, write the number on the carton, and weigh the same ten three weeks later. Do it on a pallet that has not been opened.

• Under 1% loss in three weeks: normal, no action.
• 1–3%: acceptable for short storage, watch the trend.
• 3–8%: the seal or film is transmitting; investigate.
• Over 8%: do not ship these packs on a long lane.

Also check for frost inside the pouch. If there is ice crystal on the inner film surface above the gel line, vapour is moving inside the pack and condensing, which means the seal is the path.

What actually reduces it

Shorten the storage time. This is the most effective lever and the cheapest. A pack frozen on Monday and shipped on Wednesday loses almost nothing. A pack held for a month in a blast freezer loses enough to matter. Freeze to order where the lane allows it.

Reduce air movement over stored packs. Keep pallets out of the direct airstream of the evaporator fans, and wrap them. A stretch-wrapped pallet has a still air layer around the cartons that cuts the gradient substantially.

Improve the seal. A wider seal, 10 mm instead of 6, and a slightly higher seal temperature with a longer dwell, closes the micro-channels. Test peel strength at −18 °C, not just at room temperature, because the failure is a cold failure.

Use a barrier film where the economics allow. A metallised or EVOH layer cuts WVTR by an order of magnitude, at roughly 15–25% more film cost. On a pack that will be stored for a month before shipping, that is usually worth it.

The reversal

A distributor switched to a thicker film to stop the weight loss and found the problem got worse. The thicker film was stiffer, so it did not conform to the gel surface as the pack contracted, and it created a small void at the top of the pack. Water evaporated into that void, the gradient across the film stayed high, and the seal was stressed by the film trying to return to shape.

The fix was a thinner film with a better seal and a 95% fill, which kept the film in contact with the gel. Less material, better result.

Put a scale in the freezer

The practical control is a monthly weight check on a sealed pallet, recorded on the carton. It costs five minutes and it catches both sublimation and slow leaks before a customer does. If a pallet has lost more than 3%, ship it on a short lane or rework it, and find out whether the loss is film or seal.

For lanes over ten days, ask for WVTR data at −18 °C rather than at 38 °C, because the two numbers are not related in the way most datasheets imply. SAP for frozen gel packs is supplied with recommended film and seal combinations for extended frozen storage.