Electron Microscopical Examination of Worn Tire Treads and Tread Debris

Abstract

Abstract Tire wear is an important phenomenon, and the industry needs to know as much as possible about the mechanism of rubber abrasion that results in wear. A wealth of information can be obtained by studying tread surfaces and debris particles using the scanning electron microscope, energy dispersive x-ray analysis, and transmission electron microscopy of ultrathin sections. Such an examination was conducted on the worn tread surface of an SBR compound containing 75 phr N285 carbon black and 45 phr oil. A liquid-like layer composed of an intimate mixture of degraded rubber and road silt could be seen on the worn tread surfaces. The thickness of this layer is variable but generally in the micron and submicron region, except where it had been rolled up and was about to be thrown off as a debris particle. A general examination of NR and BR tread compounds revealed a similar appearance. Several collections of rubber debris particles were also examined and found to have the same composition of degraded rubber and road silt that was observed on the tread surface. These observations are in agreement with the results of Boonstra, Heckman, and Kabaya with respect to the analysis of road silt materials on tread surfaces. Since the silt is within the thin layer of degraded rubber, it cannot be washed off the tread surface with soap and water. It can be removed with benzene swabbing. In addition, this report provides visual documentation of tire wear that serves to support previous speculations. These results emphasize the importance of considering the abrasion of tire treads as a three-bodied system: the tread compound, the road surface and the layer between the two, which is composed of an intimate mixture of degraded rubber and road silt. Previous references to the presence of road silt particles on worn tread surfaces implied their incidental nature. This study shows that road silt penetrates into and becomes a part of the substructure of worn tread surfaces. Furthermore, the internal morphology of this substructure is identical to the morphology of the tread debris thrown off from the tread surface.