The substituted p-phenylenediamines (PPDs) represent a suite of effective antioxidants broadly applied in rubber industries. However, knowledge of their environmental occurrences and fate remains extremely limited. Herein, we explored the occurrence of six major PPD antioxidants and one newly defined transformation product in dust particles from different environments, including roads, underground parking lots, vehicles, and houses. The majority of the PPDs exhibited ubiquitous occurrence in these environments. Median concentrations of total PPDs were determined to be 226 ng/g in road dust, 232 ng/g in parking lot dust, and 156 ng/g in vehicle dust, orders of magnitude greater than those in house dust (14.0 ng/g). Different composition profiles of PPDs were also found between house dust and vehicle-related dust, likely indicating the influence of vehicle tires or other rubber products. In addition, a major ozonation product of N-(1,3-dimethylbutyl)-N′-phenyl-1,4-phenylenediamine (6PPD), 6PPD-qunione, was also identified in dust with levels (median range of 32.2–80.9 ng/g) comparable to that of 6PPD except in house dust. To the best of our knowledge, this is the first systematic investigation of the occurrence of major PPD antioxidants and 6PPD-qunione in various dust matrices. Our findings would attract attention to their environmental fate and ecological and human health risks.

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In this study, the effect of different dimple shapes on the tribological performance of surface texture has been investigated. First, a numerical model was developed to study the effect of dimple shapes on hydrodynamic pressure generation. The selected dimple shapes include circle, square, and ellipse, and the flow direction of lubricant is perpendicular to the major axis of elliptical dimples or the sides of square dimples. The results showed that the dimple shape can be optimized for greater hydrodynamic pressure generation. Then, the reciprocating sliding tests were carried out under oil lubrication and face-contact conditions. Tests were conducted for rotational speeds in the range 50-500 r/min and test loads of 200 and 400 N. Surface texture patterns with different dimple shapes were used, the dimple shapes and orientations were the same as those used in the numerical model, and the dimple depth and dimple area ratio were varied under fixed dimple area conditions. The test results indicated that a better friction reduction effect compared with untextured specimens can be obtained by selecting a suitable dimple area ratio and dimple depth for each dimple shape. Comparing the friction reduction effect, the elliptical dimples showed the best performance, the square dimples showed the second best results, and the circular dimples performed worst. However, as the test load increased, the friction reduction effect of all dimple shapes decreased; moreover, the differences between the shapes became smaller.

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Reduction of hematite fines and magnetite concentrates containing carbon using microwave energy