Environmental Degradation of Tire-Wear Particles

Abstract

Abstract Each of the three analysis methods contributes to an understanding of the degradation processes which occur in environmentally exposed tire-wear particles. Extraction-pyrolysis-GC is the most reliable method since it treats the whole sample rather than a few particles. In 16 months, 52% of the polymer in tread-wear particulate was degraded in soil samples. In glass beads, only 36% of the polymer in tread-wear particulate degraded, perhaps because soil microorganisms were not available to degrade sulfur linkages in the vulcanized portion of the polymer. Fresh tread particles of the same size showed no degradation. The pyrolysis-GC results showed wider scatter, presumably because only a few rubber particles could be analyzed and all the particles did not degrade at the same rate. However, this method showed that the unsaturated bonds of polybutadiene undergo oxidative degradation more rapidly than do the aromatic bonds of styrene units. These results strongly suggest that one important mode of degradation of tread-wear paniculate is atmospheric oxidation. The TGA results showed that biodegradation did not reduce the total carbon content of the tread-wear particulate in this 16-month study. However, more than half of the extender oil was oxidized sufficiently to increase its vaporization temperature to the polymer temperature range. Biooxidation may have detoxified some of the polynuclear aromatic hydrocarbon portion of the oil. Overall, these results suggest that microbial attack of tread-wear particulate is less significant than attack by-atmospheric oxygen. To match the amount of rubber observed at the California freeway site, the removal rate by all mechanisms must be 0.67% per day. For this high removal rate, the amount of rubber at the roadside would reach a steady-state value within the first two years of a freeway's use at a level equivalent to five months of tire-wear. The rate of environmental degradation found in this study is 0.15 per day, or 22% of the total removal rate found in the California study. Wind erosion and water runoff probably also contribute to the total removal rate. Most importantly, we have shown that tread-wear particles degrade at a faster rate than tread-rubber itself, and that environmental degradation plays an important role in the fate of tire-wear particulate.