Polypropylene changes its mechanical behaviour with temperature more than most packaging materials people are used to. It becomes stiffer and more brittle in the cold, and softer and more prone to creep in the heat. Both effects are real, both are gradual rather than abrupt, and both have practical consequences that show up as handling problems rather than as obvious failures.
Cold: stiffness and brittleness
As temperature falls, PP stiffens and its impact strength drops. A container that shrugs off a knock at room temperature can crack from the same knock at minus twenty. The practical consequence is that cold-store and winter outdoor handling needs gentler treatment, and that a container dropped from a height it normally survives may not survive it cold.
What to do about cold
Specify a grade with good low-temperature impact performance, which is a standard option and costs little. Reduce drop heights and handling severity where containers are cold. And allow containers to warm before any high-impact handling, where the process permits. Where a fleet works in a cold store, the first of those is a specification question and it should be raised at the order.
Heat: softening and creep
As temperature rises, PP softens and creeps faster. A stack that is stable at twenty degrees deforms at fifty, and a container under sustained load in a hot building takes a permanent set in weeks rather than years. Where a fleet works in a warm climate or near a heat source, the stack height and the duration have to be reduced accordingly.
The range that matters
Most polypropylene containers work comfortably from roughly minus ten to plus sixty degrees for normal handling, with the useful extremes wider than that for storage without load. Where an application sits outside that range – a deep-freeze application, or a process that puts containers through an oven – the material grade has to be chosen for it and the standard grade will not do.
What happens at the transition
The temperature change itself is often the problem rather than the temperature. A container moved from a cold store to a warm area collects condensation, as described in the condensation that follows a cold load. One moved from a hot wash to a cold rinse is stressed by differential contraction. Where a process crosses temperature bands, the transition is worth as much attention as the endpoints.
Dimensional change
PP has a relatively high coefficient of thermal expansion, so a container at sixty degrees is measurably larger than the same container at zero. For a long container on automated equipment with tight tolerances, that difference is not negligible. Where a fleet crosses a wide temperature range and runs on tight equipment, the tolerance has to be specified at the working temperature rather than at ambient.
What to ask the supplier
The working temperature range, whether the grade is specified for low-temperature impact, and what the dimensional tolerance is at the extremes rather than at room temperature. Three questions. Where a fleet works anywhere unusual – cold store, desert climate, a heated process – these are the questions that determine whether the container will last.
What to test
Take a container to the working temperature, loaded, and handle it the way it will be handled. A cold container dropped from working height, and a hot container stacked for a day, tell you more in an afternoon than any datasheet. Where an application is at an extreme, this test is worth doing before the fleet order rather than after.