Every seal has a temperature limit
How elastomers change with heat and cold, and what that does to sealing.
Every seal has a temperature limit because rubber and the other elastomers used to make a squeezed seal only behave like rubber, soft, elastic, and able to spring straight back to its original shape once released, across a fairly narrow band of temperature, and stepping outside that band in either direction leaves the same material unable to do the one job a seal actually exists to do.
The physics of a rubber's glass transition
An elastomer is built from long, tangled polymer chains free, at room temperature, to wriggle and slide past one another as they are stretched and released, and that molecular motion is exactly what lets a squeezed O-ring flow into the tiny irregularities of its groove and then recover its shape the instant a gap opens back up. Cooling removes the energy those chains need to keep moving, and below a specific temperature, called the glass transition, the wriggling essentially stops and the chains lock into place, turning a soft, rubbery material into something closer to a rigid plastic, still the same substance but no longer capable of the flow that made it useful as a seal at all. Heat fails the same material by an entirely different route, since sustained high temperature does not merely speed the chains up, it slowly breaks and rearranges the chemical crosslinks holding the network together, so that a ring held hot for long enough stops springing back after being squeezed and settles into whatever shape it was last compressed into, a change no amount of cooling afterward can undo. A working seal sits between two genuinely different failure mechanisms, a reversible loss of flow on the cold side and an irreversible chemical change on the hot side, and the temperature range printed on a datasheet is really a description of how far a given compound can be pushed in either direction before one of those things happens.
The rubber-band comparison
An ordinary rubber band left in a freezer overnight demonstrates the cold failure directly, since pulling it out and trying to stretch it produces not the familiar give but a sharp snap, the band having crossed its own glass transition and turned brittle enough to fracture at the first flex rather than deform and recover the way it would at room temperature. The same band left for months on a sunny windowsill demonstrates the opposite failure just as clearly, losing its snap gradually, developing a faint tackiness, and eventually stretching without ever fully returning to its original length, evidence of the same kind of chemical breakdown happening slowly at a temperature far below anything that would visibly melt or scorch the material. A ring squeezed into a housing carries precisely the same two risks, only with the stakes raised, since the seal is expected to work rather than merely survive: a ring chilled below its rated range can turn stiff enough that it no longer conforms fully into its groove at the exact moment a joint is closed, leaving a leak path the same ring would never have allowed warmer, while a ring held too hot for too long takes a permanent set and stops recovering its shape once pressure or dimension changes around it, a failure that shows up not as a sudden leak but as a seal that quietly stops doing its job.
The number that matters here
Crossing a compound's glass transition does not degrade its stiffness gradually, it multiplies it, and a rubber compound cooled through that narrow band can stiffen by two or three orders of magnitude within a span of only a few tens of degrees, turning a ring that was supple enough to conform to a groove into something closer to a hard plastic almost overnight as the temperature keeps falling. That sharpness is what makes the cold failure so unforgiving, since a seal sitting comfortably above its rated minimum offers no warning that it is approaching a cliff edge rather than a gentle slope, and equipment that sealed perfectly on a mild afternoon can fail outright on a cold morning only a handful of degrees further down.
Why this matters in practice
Because the two failure routes are chemically distinct, no single elastomer suits every environment, and picking the compound is as much a part of specifying a seal correctly as cutting the groove to the right depth or choosing the right squeeze, both covered earlier in this set. Silicone tolerates cold far better than the nitrile rubber used in most everyday O-rings, staying flexible at temperatures that would already have stiffened a nitrile ring solid, but it gives up wear resistance to get there, which is why it turns up in static gaskets exposed to genuine cold rather than in dynamic seals that slide or rub. Fluorocarbon compounds sit closer to the opposite end, tolerating sustained heat that would slowly cook a nitrile ring into a permanent set, at the cost of stiffening earlier once the temperature drops. None of this shows from inspecting a ring visually or testing it once at room temperature, which is exactly why a seal chosen on a supplier's generic part number, without checking its rated range genuinely covers the coldest and hottest conditions the finished product will actually see, is a seal whose real limit has never been tested.
Where this stops being true
The risk shrinks sharply for equipment that spends its working life comfortably inside the middle of its seal compound's rated range, since both failure routes need a real excursion toward one edge or the other before they start doing any damage at all, and a seal that never gets near either edge can go through years of ordinary temperature swings without ever encountering the glass transition or the slow chemical breakdown described above. It matters most for anything stored at a genuine extreme, left in an unheated shed through a hard winter, or run hot for long stretches without a break, conditions that push a seal toward the very edge of a range most equipment never visits. Dust wedged into a closing gap, pressure cycling forcing air back and forth through a microscopic imperfection, and a seal material carried outside its own working temperature are three separate ways the same rubber ring can stop doing its job without ever being cut, torn, or squeezed incorrectly in the first place.