Thirty Days at Sea: Why Gel Arrives Caked Inside the Container

Thirty-one days after leaving port, a 40-foot container was opened in a yard at 34 °C. The bags at the door were fine. Eleven rows back, against the container wall, six pallets had fused into blocks you could stand on: 25 kg of polymer in a shape that no longer fit any hopper, metering cup or mixer. The buyer weighed one block at 27.4 kg. The extra 2.4 kg was water, and it had arrived free of charge.

Polymer is hygroscopic. It will pull moisture out of air at 60 percent relative humidity until it reaches equilibrium, and once the surface granules swell they stick to each other and to the liner. The load is not ruined, but the dosing accuracy that the whole application depends on has just walked out of the container.

Where the water comes from

Not from a leak, usually. A steel container crossing the tropics can swing from 12 °C at night to 45 °C under deck during the day. Warm air holds moisture; when the walls cool at night, that moisture condenses on the underside of the roof and drips onto the top layer of bags. Meanwhile the product itself is releasing moisture it picked up at the factory, and in a sealed box that moisture has nowhere to go.

Two details make it worse. Pallets loaded hard against the corrugated walls sit in the condensation path. And bags stacked 15 high compress the bottom layer, which grinds granules against each other and creates the fines that later absorb moisture fastest. A caked block is usually a fines problem wearing a water problem’s clothes.

What to specify before the order ships

Most of this is decided at the packing line, months before anyone opens the door.

• Inner liner of at least 0.08 mm polyethylene, heat sealed or taped at the top, inside a woven or paper outer bag.
• Moisture at packing below 7 percent, tested by oven at 105 °C, with the result on the lot record rather than only on the certificate.
• Pallet stacking no higher than the bag manufacturer’s limit, and never more than 12 high for 25 kg units.
• A 100 mm air gap from the container walls, with pallets secured so they cannot shift into that gap.
• One 1 kg desiccant bag per pallet for humid routes, or a container desiccant strip rated for the voyage length.

For short sea routes or road freight the desiccant is optional. For anything crossing the equator in a dry container, it is cheaper than the caking.

Receiving: check the two ends and the middle

Open the doors and sample before you sign. Three bags at the door, three from the deepest row you can reach, three from the middle of the stack. Squeeze each one: a bag that crunches is dry, a bag that feels like a beanbag has started to cake, and a bag that will not deform has fully fused. Weigh ten bags at random. Anything above nominal plus 1.5 percent is holding water.

Photograph the container interior before unloading, including the roof and the wall-side pallets. If you intend to claim, most carriers want notice within 14 days and they want the container number, the seal number and a weight record. A photo of a fused block with no container context is hard to argue with a surveyor who was not there.

The counterintuitive part

Caked gel usually still absorbs. We have tested fused blocks that came back at 210 g/g in tap water, well inside specification. The loss is not chemical, it is operational: you cannot meter a block, you cannot dry-blend it, and a crew that has to break it up with a mattock will either under-dose or give up and over-dose. On one roadside job the caked pallets went into the hydroseeder as lumps, blocked the strainer three times, and the crew thinned the slurry to keep it moving, which put the dose 40 percent under target on the last 800 m².

So treat caking as a dosing failure with a slow fuse, not as a bad batch. Grinding a caked block back through a 4 mm screen is possible, but the fines you create will dust and clump again.

For the next shipment, add four lines to the purchase order: liner thickness, packing moisture, maximum stack height, and desiccant per pallet. Then put a moisture meter and a set of scales in the receiving bay so the check happens before the forklift leaves. The storage and handling notes on our water retaining polymer product page give the target moisture range and the bag construction options to quote against.