Occurrence and Oxidation Kinetics of Antioxidant p-Phenylenediamines and Their Quinones in Recycled Rubber Particles from Artificial Turf

N,N′-Substituted p-phenylenediamines (PPDs) are widely utilized as rubber antioxidants and emitted into the environment with their transformation products (TPs) including the highly toxic 6PPD-quinone. However, the occurrence of PPDs and TPs in inconspicuous rubber products and their oxidation kinetics under natural aging conditions remains unclear. Herein, we performed a field survey of these compounds in crumb rubbers from 40 school artificial turfs. Twelve PPDs and TPs were frequently detected in rubber particles, dominated by the well-known N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) and N-isopropyl-N′-phenyl-1,4-phenylenediamine (IPPD), as well as their PPD-Qs. One pristine rubber material was chosen to simulate the aging process and examine the oxidation kinetics. The concentrations of all antioxidants in rubber powder decreased > 50% within 4 days under natural aging, much faster than that in particles, suggesting the size effect on oxidation. Different PPDs had distinct oxidation degrees under both field and simulated experiments. Besides ozone, we found that high temperature could induce a significantly faster decay of PPDs within rubber particles than UV exposure. In sum, our field and simulated investigations reveal that the transformation of PPDs to PPD-Qs is ubiquitous and chemically specific in artificial turf, which could be affected by particle size and temperature as key factors in controlling oxidation kinetics.