Tire endurance as measured by performance on the National Highway Traffic Safety Administration (NHTSA) Stepped Up Load (SUL) test is shown to be a function of both tire construction and the extent of oxidation in the skim and wedge rubber regions of the tire, as measured by peel strength or elongation to break retention. Tire constructions can be distinguished by speed rating. Tires with higher speed ratings (> S) tend to have relatively high times-to-failure (TTF) in the SUL test and are relatively insensitive to rubber oxidation. SUL TTFs for tires with speed rating of S and lower tend to be much more sensitive to rubber oxidation. For these tires, the SUL TTF decreases linearly with aging time in the field. The rate of loss of SUL performance is proportional to the rate of loss of rubber properties. The large variability observed in the SUL results from field aged tires can be explained by the natural variability in oxidation aging rates observed for these tires. For oven aged tires, the correlation between SUL and rubber oxidation is more complex. Initially, the SUL failure time does not change much with rubber oxidation. At a critical oxidation level, however, the SUL failure time begins to drop rapidly with rubber oxidation approaching the behavior of the field tires at high levels of oxidation. The reason for the difference in behavior between the oven aged and field aged tires is the lack of mechanical damage in the belt edge in the oven aged tires relative to the field aged tires.
The kinetics of aging of key tire properties both in the field and in oven exposures at different temperatures has been interpreted by using a combination of empirical models and accelerated shift factors. Crosslink density and rubber modulus increase with aging while peel strength and elongation-to-break decrease. In the case of oven aging, the rate of property change increases from 40 °C to 70 °C and then decreases. In the case of field aging, the rate of property change is greatest in hotter climates such as Phoenix and is slower in cooler climates such as Detroit. Spare tires age at a rate that is ∼70% as fast as on-road tires. Below 70 °C, the rate data for all of the aging changes can be fit to an Arrenhius relationship with an activation energy of ∼69 kJ/mole, a value that is consistent with the aging process resulting from diffusion limited oxidation. The measured acceleration factor of oven aging at 70 °C relative to on-road aging in Phoenix is independent of the property change measured confirming that it is possible to chemically age tires in ovens. It takes 6–7 weeks of oven aging at 70 °C to produce a tire that is aged 4 years in Phoenix. Field results show that the rate of tire aging varies by over a factor of 5 for the different tire types and brands studied in this work. The implications for tire durability testing are discussed.
The purpose of this research is to determine the conditions whereby a new tire can be artificially aged in an accelerated manner, in order to duplicate the actual mechanism of chemical aging observed in field-aged tires. The ultimate goal of the study is to age tires to a desired level, say equivalent to 4 years old, and then test the tires in durability, high speed, and performance tests. The previous paper described various oven aging methodologies and the data analysis techniques used. This paper will build on the previously described data analysis techniques developed for elongation at break measurements and apply them to swelling ratio data and peel strength data. By utilizing the method initially developed by Gillen and modified by this laboratory for use with tires, it has been shown that the skim rubber of tires oxidatively ages at oven temperatures between 40 °C and 70 °C when mounted and inflated with either air or a blend of 50/50 N 2 /O 2 . The methodology has been successfully extended from elongation at break data to peel strength and swelling ratio data. The calculation of the Arrhenius activation energy for diffusion of oxygen through new and aged rubber was also determined. The effect of aging on permeability is to reduce the permeability of oxygen and increase the activation energy. These results have important implications when attempting to model the diffusional aging characteristics of inflated tires. The effect of changing the partial pressure of oxygen and its concomitant effect on the acceleration of aging was also investigated. The results indicate that by doubling the partial pressure of oxygen, the rate of oxidation is increased by approximately 1.5 times. This result is entirely consistent with the theory of diffusion limited oxidation.
The purpose of this research is to determine the conditions whereby a new tire can be artificially aged in an accelerated manner, in order to duplicate the actual mechanism of chemical aging observed in field-aged tires. The ultimate goal of the study is to age tires to a desired level, say equivalent to 4 years old, and then test the tires in various durability, high speed and performance tests. The first step was to determine the aging characteristics of field-aged tires, which has been the subject of another paper. For this work, tires were statically aged in ovens. Tires were mounted, inflated, then oven aged continuously at temperatures ranging from 40 °C to 100 °C for various periods of times (from 2 weeks to 12 weeks). Both air and a 50/50 blend of N 2 /O 2 were used as the inflation media. The tires were then dissected and analyzed for tensile and elongation properties of the rubber at the end of the steel belts. The results show that as the temperature was increased from 40 °C to 70 °C, the property degradation of the steel belt rubber accelerated. Shift factors were determined based on time-temperature superposition and the results analyzed by using the Arrhenius methodology. The oven results were similar to the field data, meaning the chemical aging mechanism was the same for both. As the oven temperature increased above 70 °C, degradation reactions began to dominate and the apparent aging mechanism changed.