Paper is hygroscopic, and that is the whole problem
Corrugated board absorbs moisture from the air until it reaches equilibrium with its surroundings. As moisture content rises, the hydrogen bonds between cellulose fibres weaken, and the board's compression strength falls. This is not a defect; it is what paper is.
The relationship is steeper than most people expect. As relative humidity climbs, corrugated board loses a large share of its compression strength, and ordinary summer conditions across much of the southeastern United States are enough to matter.
Where this bites
Unconditioned warehouses in the Gulf states and the lower Mississippi valley routinely sit above seventy percent for months. So do cold-storage anterooms, wash-down areas in food plants, and any dock with an open door in August.
The failure mode is not dramatic. Stacks lean. Bottom units bulge slightly. Then one unit buckles, usually on a Monday after a humid weekend, and a thousand pounds of product arrives on the floor.
What to do about it
The first move is to find out what your actual conditions are. A hygrometer is inexpensive. Put it on a rack upright at pallet height, not in the office, and read it weekly for a month across the worst season you have.
With that number in hand, three responses are available, in order of cost.
- Reduce stack height. Free, immediate, and the right first response in a humid building.
- Specify one construction up — double wall to triple, triple to five wall — for the affected area only.
- Shorten dwell time. Compression loss under sustained load compounds with humidity; a container stored four days is in a different world from one stored four months.
- Condition the space. Expensive, and almost never justified by packaging alone, but sometimes the product requires it anyway.
The cold-storage special case
Cold itself does very little harm to corrugated. The damage happens in the transition: a container moving from a 0°F freezer into a 75°F dock at eighty percent humidity will condense moisture across its entire surface within minutes.
Rules that work: specify one construction up, keep liners gathered rather than sealed so the interior can breathe, and do not stack anything during the first twenty-four hours after a temperature transition. Plants that follow those three rules stop having cold-chain packaging failures almost entirely.
Why used stock comes in ranges
People occasionally ask why used stock is described by a strength range rather than a single number. Humidity is a large part of the answer. A board's measured strength depends on the moisture it is carrying at the moment of the test, which depends on where it has been.
What can be checked per unit is the construction — wall count and flute combination, measured with a caliper. The strength range follows from the construction. A single certified number on recovered fiber is a measurement from a different box on a different day.