Rubber materials are composed of polymers, which are long, large, flexible molecules that have many repeating units. To make tires, a rubber solution is first prepared that consists of rubber polymer chains and additional reagents and chemicals to allow these polymer chains to overlap and intermingle in a randomized pattern with each other, similar to cooked spaghetti noodles sitting in a large pot. When the rubber solution is exposed to heat for curing, a free radical chemical reaction is initiated which allows these polymer molecules to crosslink and connect with each other, forming strong irreversible covalent bonds. In return, this process results in a robust highly interpenetrating matrix between all of these connective rubber polymer chains, thus making the rubber into a flexible solid material that are found in tires.
Due to this chemical reaction and the formation of the resulting rubber material, this process can introduce small between polymer chains which usually cannot be seen by eye. Consequently, this is the culprit which causes gas leaks in tires due to long-term usage. Similarly, this is the same reason why party balloons deflate overtime. Rubber and compressed air perform antithetically, which result in smaller molecules to leak through rapidly. In fact, it has even been reported that rubber tires loose air at a rate of 1-3 psi per month! Additionally, air can leak through these tires more rapidly if there is damage to the tires or the steel wheel, or even due to frequent changes in temperatures. To address this problem, it is highly advised to maintain your tires properly to minimize chances of any damages for optimal function. Another strategy to maintain automobile tires is to seek other alternatives to fill them other than compressed air.