Packaging specifications are usually written and tested in the conditions the writer works in. The boxes then travel somewhere else. Polypropylene is more temperature sensitive than most people expect, and the difference between a rating measured at 20 °C and behaviour at 50 °C inside a container in summer is not a rounding error.
What heat does to the board
Stiffness drops and creep accelerates. The load a wall will carry for a month at 20 °C it will carry for a much shorter time at 50 °C, and the deflection under the same load is larger from the first day. Neither effect is visible in a specification that quotes a single figure without a temperature.
Where the heat comes from
Shipping containers in direct sun reach temperatures well above ambient. Trailers standing on a yard do the same. Warehouses without climate control in southern Europe exceed 30 °C for weeks at a time. None of this is exotic; it is the ordinary condition of a summer supply chain, and it is usually absent from the test report.
Humidity is the gentler variable
Polypropylene does not absorb water in any meaningful amount, which is one of the reasons it is used at all. Moisture matters indirectly: it changes friction between surfaces, it condenses on parts and on the inside of the load, and it affects labels and any adhesive in the pack. The board survives it; the load does not always.
Metal parts and condensation
A load of steel parts moved from a cold store into a warm humid environment will collect water on the cold surface, inside the box. That is a corrosion problem rather than a packaging failure, but it lands on packaging to solve. Ventilation, a desiccant, and avoiding the temperature shock in the first place are the three levers, in that order of preference.
What to specify for hot routes
Ask for the load rating at the temperature the load will actually see, or for the de-rating curve. Where a supplier cannot give one, apply a margin and reduce the stack height for the summer months. It is a blunt instrument and it works.
Checking after a hot leg
Measure the bottom box before and after a hot journey and look for permanent set. Where it has taken a set, the rating was measured at a lower temperature than the load experienced. One fewer layer, or a box with more vertical reinforcement at the corners.
What to do about it
Reduce the layer count for the hot months, or specify a box with vertical reinforcement that holds its stiffness at temperature. The first costs nothing and works; the second costs tooling and also works. Where the lane is hot year-round, the second is usually worth it.
Ventilation helps where the problem is condensation rather than heat. Where the problem is the board softening, ventilation does nothing and the answer is structure. Knowing which of the two is happening is worth the five minutes it takes to check whether the bottom box has deformed or simply got wet.
What we can do
We build PP corrugated turnover boxes and we will quote the stack rating with the temperature it was measured at, so a hot lane can be specified honestly. Tell us where the boxes will sit in July and we will work from that. Turning that into a layer count is set out in working out the permitted layer count.