Hygroscopicity and Packaging: Why Bags Clump in Transit

A fibre drum arrives with an intact seal and an intact certificate. Inside, the polyethylene bag has been folded and taped, and the powder in contact with the fold has formed a firm block roughly the size of a fist. Loss on drying, measured before shipment, was 0.4 percent. Nothing in the file is wrong, and the material is not what the buyer expected.

Clumping is a moisture question, and moisture is a packaging question before it is a quality question. The water that caused the block did not appear in the reactor. It entered through a barrier that was chosen for cost or habit rather than for the transmission rate it needs to meet.

Where the water actually sits

Water in a crystalline powder occupies three positions, and they behave differently. Surface moisture sits on the particle surface and in the voids between particles, and it moves freely with humidity. Adsorbed water sits in the amorphous regions at particle surfaces and in lattice defects. Water of crystallisation is part of the crystal structure and only leaves when the structure changes.

Loss on drying mostly measures the first two. That is why a material can pass a 0.5 percent limit and still clump: the limit is set for chemical stability, while the handling problem begins at a fraction of that.

The barrier, not the drum

Buyers routinely specify the drum and leave the liner to the supplier. The drum is mechanical protection. It provides almost no moisture barrier. The barrier is the primary liner, and the difference between liners is measured in orders of magnitude, not percentages.

Pack format	Moisture barrier	Where it is adequate
Single polyethylene bag, taped	Low	Short inland journeys in a dry climate
Double polyethylene bag, heat sealed	Moderate	Air freight with controlled handling
Aluminium foil laminate, heat sealed	High	Sea freight, humid destinations, long dwell
Foil laminate plus inert headspace	High	Materials sensitive to both moisture and oxygen

The wording matters as much as the choice. "Packed in polyethylene bags" is not a specification, because it does not say how many, whether they are sealed or taped, or what the outer layer is. "Heat-sealed aluminium foil laminate liner inside a fibre drum" is a specification, because it can be checked at goods-in.

The heat seal is the whole point

A bag that is folded over and taped is not closed in any meaningful sense. Air exchanges through the fold with every temperature cycle, and a sea container cycles through a large temperature range twice a day. A heat-sealed bag with a continuous weld does not.

This is why the failure so often appears at the fold or at the seam, and why a buyer inspecting a delivery should look there first. A block that has formed against a fold is telling you exactly which mechanism operated.

Storage wording that can be enforced

A storage statement should describe conditions the pack can survive, not conditions the material would prefer. "Store in a dry place" cannot be audited. A temperature range, a relative humidity ceiling and a light condition can be, and each maps to something a warehouse can control or fail to control.

For most bile-acid powders the practical statement is a controlled room temperature range, protection from light, and a sealed pack until use. The third clause is the one that gets ignored, and it is the one that matters most after the drum is opened.

The repacking problem

Many buyers do not consume a full pack. They open it, weigh out a portion, and reseal with a clip. From that moment the barrier is gone, and the material is exposed to the warehouse atmosphere for the remaining months of its life.

Two practices reduce this materially. Request pack sizes matched to consumption, so that a pack is used in one or two sittings. And where that is impossible, transfer the remainder into a container with a known barrier and a desiccant, and record the opening date on the label. Neither is sophisticated. Both are routinely skipped.

What to specify, in one paragraph

Primary liner material and construction, number of layers, seal method, pack size, headspace treatment if any, and the storage condition the pack is designed for. Add the requirement that the liner be intact on arrival, and that any damage be photographed and reported before the material is sampled.

That paragraph prevents most clumping complaints. It does not prevent all of them, because no barrier survives a bag that has been cut open with a knife, which is how a surprising number of deliveries are opened.

The commercial consequence

A clumped delivery usually triggers an argument about quality, which is the wrong argument. The certificate is correct, the material is within specification, and the supplier can defend both. The real failure is in a specification that never described the pack, and in a goods-in inspection that never looked at the seal.

Fixing the specification is cheap. Fixing a batch that has been rejected and returned is not, and the return journey is usually worse for the material than the outbound one.

How to inspect a delivery in ten minutes

Look at the seal before the material. A continuous heat seal with an unbroken weld is the only thing standing between the powder and the container atmosphere. A folded and taped bag has no barrier at the fold, and the fold is where the block will have formed.

Then look at the drum for dents that indicate a drop, and at the liner for condensation. Then sample from more than one position, including any compacted region, and submit the positions separately. A single sample from the loose material at the top will tell you what you want to hear.

Three articles here cover the analytical side. Loss on drying versus water content explains why the release result may not describe the moisture you are looking at. The solubility profile explains why a small moisture change matters more in some downstream processes than others. And crystal form and crystallinity explains the third mechanism by which a powder changes on storage without any chemical change.

Where the stability expectations sit

The conditions a pack must survive are defined by the climatic zone the product is destined for, and the framework for those zones is set out in the stability guidance published by the International Council for Harmonisation. The United States Pharmacopeia publishes general chapters on packaging and on water determination. The European Pharmacopoeia carries the corresponding general methods.

None of them specifies a liner for a fibre drum. That is a commercial decision, and it is the decision that determines whether the material arrives in the condition the stability data assume.

What a moisture specification should contain

Four items: a loss on drying or water content limit with its method named, a primary liner defined by material and seal, a pack size matched to consumption, and a storage condition the pack is designed to meet. Add a requirement that liner damage be photographed and reported before sampling.

That is a paragraph, and it is the difference between a specification that describes a material and one that describes a delivery. The second is what a warehouse actually receives.

The desiccant question

Adding a desiccant sounds like a solution and is often a substitute for one. A sachet inside a sealed laminate absorbs moisture that enters the headspace during packing and any that passes the seal over time. It does not compensate for a bag that is folded rather than welded, because the transmission rate through an open fold exceeds anything a sachet can absorb.

Where the barrier is sound, a desiccant extends the margin, particularly for a pack that will be opened and resealed several times. Where the barrier is not sound, the desiccant saturates within weeks and then stops doing anything, usually without any indication that it has.

There is also a documentation question. A desiccant is a component of the pack, and a pack containing a desiccant that is not declared is a pack whose storage condition cannot be reproduced. If a desiccant is used, it belongs in the packaging specification with its type and quantity, not as an undocumented addition at the packing station.

What the buyer can control

Two things, and both are worth more than they cost. Specify the pack, in the terms described above, so that the barrier is a requirement rather than a habit. And inspect the seal on arrival, before the material is sampled and before the drum is opened wide enough to change the atmosphere inside it.

Everything else depends on the supplier's packing line, which the buyer cannot see. That is precisely why the specification has to be written in terms that can be checked from the outside: material, construction, seal type and pack size.

A buyer who specifies those four and inspects the seal will avoid most clumping complaints. A buyer who specifies only the drum will keep receiving material that meets its certificate and does not meet its process.

Frequently asked questions

Does a passing loss on drying result rule out moisture problems?

No. Loss on drying measures what leaves the sample under the specified conditions, which is mostly surface and loosely bound water. A powder can pass at 0.4 percent and still pick up several tenths of a percent on the surface during transit, enough to cause clumping without breaching the specification.

What packaging actually controls moisture ingress?

The primary liner. A polyethylene bag alone has a high moisture vapour transmission rate. An aluminium foil laminate, or a metallised film with a sealed liner, reduces it by orders of magnitude. The drum provides mechanical protection; it provides almost no moisture barrier.

Is nitrogen flushing worth asking for?

For an oxidation-sensitive material it is. For moisture alone it is secondary to the liner. Where both moisture and oxidation matter, the combination of a foil laminate and an inert headspace is the practical answer, and it should be stated as a specification rather than offered as an option.

How should storage conditions be worded?

With a temperature range, a humidity statement and a light statement, phrased as conditions the pack can survive rather than conditions the material would prefer. A statement that the product should be kept dry is not a condition, it is an aspiration.

Why does clumping matter if the assay still passes?

Because the material no longer handles the way the process assumes. Feeding, sieving, blending and compression all depend on flow. A block that must be broken up introduces a manual step, and manual steps introduce variability that the batch record does not capture.

What should we do with a clumped delivery?

Sample it properly before deciding. Take increments from more than one position, including the compacted region, and test moisture and assay separately by position. A single sample from the loose material at the top will understate the problem and support a conclusion you will regret.