Field Aging of Tires, Part II

Abstract

Abstract In an attempt to better understand the mechanism of rubber aging in tires, used tires were retrieved from customer cars. Consumers were contacted in six cities located throughout the United States (Detroit, MI; Hartford, CT; Phoenix, AZ; Miami, FL; Denver, CO; and Los Angeles, CA) and interviewed with regards to driving and maintenance habits. A total of approximately 2,500 tires were obtained from the field and of those, around 1,500 were dissected and analyzed. Some of the tests performed were tensile and elongation properties of the rubber at the belt edges, swelling ratio measurements across the belt skim rubber, and the peel adhesion force required to separate the two steel belts. This paper is concerned with the determination of aging mechanisms based on the change in rubber properties within the tire. Analysis of rubber property evolution with respect to time and mileage are the subject of another paper. Three analytical approaches were taken to investigate the mechanism of aging in field retrieved tires. The first approach, the ‘Ahagon’ analysis, showed that for the three tires collected from the vehicle class SUV/Minivan all tires aged oxidatively in the field, regardless of geographic location. The second approach explored the relationship within the skim rubber between the swelling ratio and the peel strength. The results showed that a significant correlation exists, and that as the swelling ratio decreases, the peel strength also decreases. The third methodology employed investigated the link between swelling ratio of the skim rubber and the strain ratio at break of the wedge rubber, which also showed a significant correlation between the two properties. Spare tires obtained from the same vehicles (2 out of the 3 vehicles came equipped with a full size spare) were then compared to the road tires using the same analytical techniques. In every case, the spare tire data was statistically identical to the road tire data, meaning that mechanical fatiguing does not impact the aging process with regards to property change of the rubber. This also means that statically aging a tire (either as a spare or in an oven) completely duplicates the oxidative aging seen in road tires. Oven aged tires of different constructions were compared to the spare tire data and shown to be similar. Future work will be to oven age tire constructions that field data is available for, so direct comparisons can be made.