Every child knows the disappointment of seeing party balloons deflate overnight. And every driver knows—or should know—that the same thing happens to tires, even though it’s more gradual and less noticeable. In fact, low tire pressure is not always obvious without a pressure gauge.
So, why does this happen? How can a balloon hold water indefinitely (until it’s dropped, at least!) but not air? Why is it that even brand-new tires lose pressure?
Rubber is composed of polymers: long, flexible molecules that have many repeating units. To make a tire, a rubber solution is prepared that consists of polymer chains and reagents (chemicals that cause certain reactions to occur under given conditions, e.g. heat). The polymer chains overlap and intermingle in a randomized pattern, like cooked spaghetti noodles sitting in a pot. When the solution is exposed to heat for curing, a free radical chemical reaction is initiated in which the polymer molecules crosslink and connect with each other, forming strong, irreversible covalent bonds. This process results in a robust, highly interpenetrating matrix of polymer chains, creating the flexible, solid material that is found in tires.
While the rubber looks, feels and behaves like a solid, there are microscopic pores between the polymer chains, invisible to the naked eye but large enough for gas molecules to leak out. These tiny pores are why party balloons deflate over time and why vehicle tires gradually lose pressure.
It has been reported that rubber tires inflated with compressed air deflate at a rate of 1-3 psi per month. Frequent changes in temperature can exacerbate pressure loss, as can damage to the tire or the steel wheel. At the same time, driving on tires that are not properly inflated can shorten the life of the tires and affect how the vehicle handles in various road conditions. (Under-inflation and over-inflation can both be problematic, although under-inflation is more common.) Motorists are strongly advised to maintain proper tire pressure, checking at least once a month and re-inflating as necessary. An additional tactic is to inert tires with nitrogen instead of compressed air. Tests have shown that inerted tires maintain pressure longer than those filled with compressed air.