A new look at rubber recycling and recreational surfaces: The inorganic and OPE chemistry of vulcanised elastomers used in playgrounds and sports facilities

We revisit current understanding of the chemical complexity of different kinds of particulate vulcanised elastomers widely used in playgrounds and sports fields, adding new data on trace element and organophosphate ester contents of used tyre and EDPM crumb rubber. Enrichments in elements such as Zn, S, Co, Bi and Nd relate to the vulcanised mixtures created during manufacture. Zinc concentrations vary across an order of magnitude, being highest in our used tyre particulate samples (up to 2.4 %). In contrast, other trace element variations are due to coloured pigments causing increases in Cu and Zr (blue), Cr (green), and Fe and Sn (red and yellow) concentrations. The use of pale fillers such as CaCO3 and clay minerals (rather than carbon black) strongly influences Ca and Al concentrations, which are much higher in the coloured EPDM than in black used tyre crumb rubber. Representatives from all three of the main organophosphate ester (OPE) groups, namely chlorinated (e.g. TCEP, TCIPP, TDCPP), alkyl (e.g. TEHP, TEP, TNBP, TBOEP) and aryl (e.g. TPHP, EHDPP, TCP) were identified, confirming how these chemicals are commonly used in modern rubber compounding as flame retardants and plasticisers. Elevated concentrations of TEHP (up to 117 μg/g) in coloured-coated used tyre turf infill crumb rubbers were traced to the coating rather than the crumb rubber itself. The presence and weathering behaviour of OPEs in recreational crumb rubber materials deserve closer investigation. The ecological "onehealth" impact of potentially toxic substances present in recreational crumb rubbers will depend on if and how they are released during play and sports activities, maintenance procedures, and natural weathering. We argue that detailed chemical data on these materials should be available to buyers, as manufacturers strive to reduce ecotoxin content as part of the quest towards sustainable use and recycling of vulcanised elastomers.

This work was supported by the ACS Foundation and the Spanish Ministry of Science and Innovation (Projects EXPOPLAS PID2019-110576RB-I00 and REPLAY EUR2022-134037) and by the Generalitat de Catalunya (Consolidated Research Groups 2021 SGR00447 and SGR01150). IDAEA-CSIC is a Centre of Excellence Severo Ochoa (Spanish Ministry of Science and Innovation, Project CEX2018-000794-S). Thanks to MB. Montell, JA. Navarro, L. Zapata, I. Martí and A. Millas from the Institut Municipal Medi Ambient i Serveis Urbans-Ecologia Urbana - Ajuntament de Barcelona for providing the EPDM and black SBR crumb rubber samples currently used in playgrounds in the city. We are also grateful to Jose Carvalho of Genan for supplying colour-coated granulates for synthetic turf infill produced from recycled rubber granules from end-of-life tyres, and to Wes Gibbons for discussion and improving the text.

Peer reviewed