Nitrogen molecules have a larger diameter than oxygen.
Nitrogen - Wikipedia, the free encyclopedia
"Nitrogen molecules are less likely to escape from the inside of a tire compared with the traditional air mixture used. Air consists mostly of nitrogen and oxygen. Nitrogen molecules have a larger effective diameter than oxygen molecules and therefore diffuse through porous substances more slowly."
Why Inflating Tires With Nitrogen Makes Sense
" Over time the pressurized air inside a tire slowly migrates and permeates its way into and through the tire. Air contains moisture. So in addition to reducing the tire’s inflation pressure, the oxygen and moisture in the air reacts with the rubber compounds in the tire, causing them to break down and lose their strength and durability. A chart is available illustrating that nitrogen is the slowest of all gases to flow through a permeable barrier such as a tire. For a copy of the chart please contact us at the number or email address shown below.
An underinflated tire is much more prone to premature failures. That’s because when underinflated, as a tire rolls, it flexes more than it was designed to. This flexing bends the tire’s rubber and steel (used within the rubber to provide additional operating characteristics) and generates heat. Heat is a tire’s worst enemy and accelerates tire wear dramatically. There is a direct correlation between how much a tire is underinflated and how much faster it wears.
Since air, which contains oxygen, is not an inert gas, it is affected by changes in temperature, which affects the rate of air loss from a tire. The air inside a tire expands when heated and contracts when cooled. More air is lost in hot weather. The consensus is that for every 10-degree Fahrenheit change in temperature, there will be a one psi (pound per square inch) change in the pressure of a tire. Nitrogen will not fluctuate as much. Being an inert gas - not readily changed by chemical reaction, nitrogen provides constant pressure and is less susceptible to accelerated diffusion caused by changing temperatures.
Nitrogen inflation minimizes moisture and oxygen in a tire so there is less rubber degradation and no corrosive properties as found in compressed air. A reduction in rubber oxidation slows a tire’s “aging,” improving the casing’s structural durability, lengthening its useful life and yielding a higher proportion of retreadable casings that can survive more retread cycles. All of this helps lower operating costs. Some fleet managers, who had been dead set against retreads, are now willing to use retreads with nitrogen inflation.
Because nitrogen molecules are slightly larger and less permeable than oxygen and all the other gases in air, it migrates considerably slower through a tire. It might take a truck or bus tire inflated with nitrogen about three months to lose two psi, whereas even a well-maintained tire inflated with compressed air will lose, on average, about two psi per month. "