It happens in minutes rather than hours, and it is entirely a barrier question.

Two attacks at once

Hot chips sitting directly on board push oil into the fibres by capillary action. At the same time the food is giving off steam, which condenses on the cooler board around it.

Board that is wet on one side and oiled on the other has lost most of its stiffness. That is the translucent limp patch under the food, and it is why the tray fails in the middle first.

What a barrier does

A grease-resistant treatment or coating stops the oil entering the fibre structure, so the board keeps its stiffness and the tray keeps its shape for the length of a meal.

Moulded fibre needs it more, not less. Untreated pulp is effectively a sponge and will soften under hot oily food faster than uncoated board will.

Why quotes are vague about it

Because “coated” covers everything from a light grease-resistant treatment to a full plastic laminate, and the difference in cost is large.

Ask for the treatment by name and ask what it means for recycling and composting claims. Anyone who will only say “coated” is describing the cheapest option.

What else helps

A rolled rim, because a tray that is already stiffer tolerates a softened middle for longer. And a compartment layout that keeps the wettest item off the largest span.

But neither of those is a substitute for the barrier. If chips go straight onto bare board, no amount of structure saves it.