NHTSA Tire Aging Test Development Project Phase 1 - Phoenix, Arizona Tire Study; Report 2: Peel Adhesion of Light Vehicle Tires as Purchased New and After Retrieval from Service in Phoenix, Arizona USA (2009)

Phase 1 of the National Highway Traffic Safety Administration (NHTSA) Tire Aging Test Development Project consisted of the analysis of six different tire models as purchased new and after collection from service on privately owned vehicles in the Phoenix, Arizona USA metropolitan area. Since the rate of degradation of tire rubber components increases with temperature, NHTSA expected that the “worst case” in-service tires in the US would be found in the southern states. (The various rationales by which Phoenix was selected as the tire collection location are documented in the project’s Phase 1, Report #1.) This study was conducted to provide a better understanding of service-related tire degradation and to help serve as a “real-world” baseline for the possible development of a laboratory-based accelerated service life test for tires (often referred to as a “tire aging test”). In this portion of the study, component material properties from a total of 45 on-vehicle tires retrieved from Phoenix of varied ages and mileages were compared to 23 new, unused versions of themselves. Tires removed from service in Phoenix exhibited statistically significant decreases in peel force for both the wedge and skim region of the tire when compared to new tires of the same type and model. As tires accumulated more age and mileage, the peel force values tended to fall into lower statistical groupings based on peel adhesion. This study indicates that peel adhesion can be successfully used to monitor changes that take place in the compounds of tires during service. The physical and chemical changes attributed to thermo-oxidative aging of tire compounds are correlated with decreases of measured peel force for tires retrieved from Phoenix, Arizona service as compared to new tires of the same brand and model. For all tire types the adhesion decreased at a relatively constant rate to approximately one-half of the original value in 2.5 to 4 years and 50,000 to 75,000 km of service. Beyond that time the rate of decrease in peel force was much less. Differences between tire types tended to be maintained throughout the study. The minimum values found in this study varied by over 130% depending on tire type for tires that were currently in service on vehicles. Since all on-vehicle tires in the study were performing adequately in service at the time they were collected, the results indicate that tires can experience large decreases in their original inter-belt peel adhesion levels during their first years of service and still retain structural integrity. Since the peel adhesion between the steel belts of a radial tire is non-recoverable and finite, adhesion levels should stabilize after the observed initial drops to prevent compromising structural durability. Therefore one part of an effective accelerated service-life test for tires may be to simulate the loss of inter-belt peel adhesion observed in real world tires, a mechanism that is likely non-linear with variables such as age and mileage.