Physical and chemical characterisation of tyre and road wear particles

Abstract

Tyre and road wear particles (TRWPs) are recognised as a significant global pollution threat. The abrasion of tyres during driving generates particles containing tyre and road material, hence termed TRWPs. An increasing number of TRWPs, due to the electrification of cars, potentially pose risks to humans and the ecosystem. Despite the omnipresence of TRWPs, there are uncertainties regarding their quantification and the validation of analytical methods for their identification. This thesis aims to integrate critical reviews and seek to establish a standardised analytical framework for the comprehensive analysis of TRWPs.

A lack of a suitable marker for TRWPs remains a significant obstacle in accurately determining their concentration in environmental settings. Here, a marker refers to a unique characteristic of TRWPs that can act as a detectable indicator of their presence in the environment. A thorough review of existing literature shows that relying on a single chemical component as a marker, such as zinc or N-(1,3-dimethylbutyl)-N’-phenyl-p-phenylenediamine (6PPD), may lead to an overestimation of TRWPs.

This is because zinc has been linked to brake wear, and 6PPD has been detected in other industrial applications, such as dyes, which limits the specificity of chemical additives as unique markers. The sausage-shaped assembly is distinctive to TRWPs. A suite of physical and chemical characteristics (including organic and elemental constituents), therefore, is recommended to improve their identification in the environment. This approach could serve as an international standard for quantifying TRWPs.

Given the heterogeneity of TRWPs, the use of complementary analytical techniques is recommended. This includes microscopy and spectroscopy to examine surface characteristics such as size, shape, and elemental composition. Thermoanalytical methods are recommended to analyse organic additives in TRWPs. Validating these techniques requires environmentally relevant TRWPs. The reference standard for TRWPs was generated using a pin-on-disc tribometer (PoD), providing a reproducible standard for future studies.

The critical review established in this thesis emphasises the importance of consistent sampling principles to promote standardisation in future research. This involves employing multiple field blanks (quality control measures) to prevent cross-contamination and careful consideration of the sampling matrix to ensure representativeness. These findings highlight that the reliable identification of TRWP (other than appropriate markers) is intrinsically linked to a standardised approach/workflow.

Based on these insights, micro-computerised tomography (µ-CT) was established as a robust analytical method for the analysis of physical properties of TRWPs. 23,540 TRWPs from three commercial tyre brands (Bridgestone, Michelin, and Hankook), comprising approximately 50,000 slices, were assessed.

Each particle was reconstructed and analysed through single particle analysis. Here, slices refer to cross-sectional images of TRWPs generated via µ-CT, enabling comprehensive three-dimensional characterisation from all angles/sides. Quantitative analysis shows significant heterogeneity in the shape of TRWPs, with sphericity ranging from 0.25 to 0.75 and aspect ratio from less than 1 to greater than 6. The implications of these findings suggest that TRWPs with an aspect ratio less than 1, due to increased aerodynamic drag, may undergo secondary motion in air. The increased drag may prolong the suspension of TRWPs in air, facilitating their transport to distant locations. These findings show the suitability of µ-CT for the comprehensive physical analysis of TRWPs and considering the physical properties to determine fate and transportation of TRWPs in the environment.

A novel analytical framework consisting of a scanning electron microscope with energy-dispersive Xray spectroscopy (SEM-EDS) and two-dimensional gas chromatography-mass spectrometry (2DGC/MS) was established for comprehensive chemical analysis. The presence of per- and polyfluoroalkyl substances (PFAs) in the polymer structure of the particles highlights a potential for long-term risks from the presence (and potential release) of these toxic, persistent components. The employed analytical techniques may help future studies in the quantification of TRWPs present in the environment and their associated risks.

Overall, this thesis not only established a methodological foundation but also laid the groundwork for harmonised monitoring efforts for future research. The developed analytical methodologies and frameworks are a significant contribution to the field, emphasising the importance of this research in the study of TRWPs.

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Identifiers

ORCID for Zainab Tariq: orcid.org/0000-0002-4965-5812

ORCID for Ian Williams: orcid.org/0000-0002-0121-1219

ORCID for Andy Cundy: orcid.org/0000-0003-4368-2569

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