72 Percent Relative Humidity in March: Caking, Liners and a Sack That Gains Weight

The sack went into the warehouse at 24.9 kg in February and came out at 25.3 kg in April. Same sack, same pallet position, no spillage. The difference was 72 percent relative humidity and a liner that had a pinhole at the fold. The powder inside was not wet in any obvious way. It was caked into lumps the size of a fist at the bottom third, and the absorbency of those lumps had fallen by a fifth.

Polyacrylate is hygroscopic by design. That is the whole point of the material, and it does not switch off when the bag is closed. Storage is therefore a specification question, not a housekeeping question, and the failures are quiet until they are expensive.

What moisture does inside a sealed sack

Water vapour passes through a polyethylene liner slowly but steadily, and it passes faster through a liner that has been creased, folded, or pierced by a pallet corner. Once inside, the vapour is absorbed by the outermost granules first. Those granules swell slightly, and because they are confined by the surrounding powder, the swelling is expressed as a soft, sticky surface rather than a change in volume.

Then the sack is stacked, and the soft granules at the bottom are pressed together. Contact points dry and re-form as hard bridges. That is caking: not one big lump of wet powder, but a network of small welds between granules that turns a free-flowing product into something requiring a hammer.

The absorbency loss follows from the same mechanism. Granules that have already taken up a few percent of water have less capacity left, and their surfaces are partly sealed. A 6 percent moisture product that reaches 9 percent in storage may lose 10 to 20 percent of its free swell, and it will lose more of its absorption speed, which is often the number that matters in a plant.

Reading the moisture line correctly

Standard polyacrylate ships at 5 to 8 percent moisture, and that water is part of the specification, not a defect. It keeps the granule from becoming brittle and generating fines. A product dried to 1 percent is dustier, more brittle, and often worse to handle than one at 6.

What matters is the change after delivery, not the absolute figure. Weigh a sack on arrival, mark the weight on the bag, and weigh it again in three months. A gain of more than 0.5 percent of the net mass is a signal that the liner or the store is failing. On a 25 kg sack that is 125 g, which is small enough to be ignored and large enough to matter.

If you have a moisture balance, take a sample from the top, the middle and the bottom of the pallet and dry at 105 C to constant weight. A gradient from top to bottom tells you the moisture is coming through the bag, not from the batch. A uniform reading across all three tells you the lot arrived that way.

Liners, sacks and pallet practice

Specify a polyethylene liner of at least 0.08 mm, sealed at the top, inside a woven or multiwall outer. A liner that is merely folded over is not sealed, and a folded liner in a humid store equilibrates within weeks. Ask whether the liner is a tube or a gusseted bag; tubes have fewer seams and fewer failure points.

On the pallet, keep sacks off the floor by at least 100 mm and away from external walls by 300 mm. The floor slab in an unheated warehouse is often 3 to 5 C colder than the air, and the wall is colder still, which produces condensation exactly where the bottom row sits. Stack height matters too: eight sacks high puts about 5 kPa on the bottom bag, which is enough to compact partly swollen granules into a cake.

Stretch wrap the pallet, but leave the top slightly open if the store is dry and closed if it is humid. The wrap is there to stop draughts and dust, not to make the pallet waterproof. Water that gets under a wrap and cannot escape is worse than no wrap at all.

The cold store mistake

Moving polymer into a chilled area to protect it from heat is a common and usually wrong move. A cold sack brought into a warm packing hall sweats. Condensation forms on the liner and on the powder surface, and the resulting caking is worse than anything the warm air would have done.

If the material must move between temperature zones, let it equalise in its sealed liner for 24 hours before opening. That single rule prevents more caking incidents than any change of grade.

What to do with a caked sack

Break the lumps gently, not in a hammer mill. A mill generates heat and fines, and the fines from caked material are the dustiest fraction you will handle all year. A roller or a gentle crusher that breaks the bridges without grinding the granules is the right tool.

Then test the recovered material before using it at full rate. Sieve it, measure free swell, and compare with the retained sample from the same lot. If the absorbency is within 10 percent, use it at the normal dose and note the lot. If it is down 20 percent, use it in a less critical application or blend it at a low ratio.

The sack that gained 0.4 kg should have been the last one. Our note on storing SAP in a real warehouse covers the liner specification and the weighing routine, and it takes about ten minutes to set up.