Donald S. Tracey

NITROGEN INFLATION OF TIRES

Inflation pressure retention (IPR as a loss rate) is a key predictive parameter to improving tire durability. Improved Tire IPR, manifested as reduced percent pressure loss per month values, has statistically been shown to be a direct result of innerliner compounds made with increasing amounts of halobutyl rubber when used as a direct replacement for natural rubber if other variables are constant. Roadwheel performance of tires is highest when using 100-phr of halobutyl rubber in the innerliner compound. This affords a tire with desirably low IPR loss rate values, desirably low tire intracarcass pressure (ICP) values, and increased tire durability as measured in hours until failure on laboratory roadwheels. The use of nitrogen gas in tires in severe service conditions is known, and a variety of benefits have been claimed for use in passenger car tires. In order to test specific claims, tires with innerliners having a 100-phr bromobutyl rubber, and 80/20 and 60/40 bromobutyl/natural rubber blends were studied using three filling gases: dry, pure nitrogen (99.4%), dry air and a 50/50 mixture of dry nitrogen/oxygen (49.8%). Performance was measured for new and oven-aged tires. It is shown that the direct benefits of using nitrogen are in reducing the Tire IPR loss rate values, and in reducing aging since oxygen gas has been initially excluded. Other claimed benefits are found to be a secondary result of the lower Tire IPR loss rate values. Use of nitrogen as the inflation gas does not benefit the measured cavity gas temperature or tire rolling resistance, both of which are shown to be tire pressure dependent, but not dependent upon the filling gas when measured at equal tire inflation pressures.