Brake Wear Debris Research Articles

Alteration of Hordeum vulgare and Sinapis alba germination and early growth in response to airborne low-metallic automotive brake wear debris

Road transportation significantly contributes to environmental pollution, both in terms of exhaust and non-exhaust (brake wear) emissions. As was proven, brake wear debris is released in a wide variety of sizes, shapes, and compositions. Although studies confirming the possible adverse health and environmental impact of brake wear debris were published, there is no standardized methodology for their toxicity testing, and most studies focus only on one type of brake pad and/or one test. The lack of methodology is also related to the very small amount of material released during the laboratory testing. For these reasons, this study deals with the mixture of airborne brake wear debris from several commonly used low-metallic brake pads collected following the dynamometer testing. The mixture was chosen for better simulation of the actual state in the environment and to collect a sufficient amount of particles for thorough characterization (SEM, XRPD, XRF, chromatography, and particle size distribution) and phytotoxicity testing. The particle size distribution measurement revealed a wide range of particle sizes from nanometers to hundreds of nanometers, elemental and phase analysis determined the standard elements and compounds used in the brake pad formulation. The Hordeum vulgare and Sinapis alba were chosen as representatives of monocotyledonous and dicotyledonous plants. The germination was not significantly affected by the suspension of brake wear debris; however, the root elongation was negatively influenced in both cases. Sinapis alba (IC50 = 23.13 g L−1) was more affected than Hordeum vulgare (IC50 was not found in the studied concentration range) the growth of which was even slightly stimulated in the lowest concentrations of brake wear debris. The plant biomass was also negatively affected in the case of Sinapis alba, where the IC50 values of wet and dry roots were determined to be 44.83 g L−1 and 86.86 g L−1, respectively.

Challenges pertaining to particulate matter emission of toxic formulations and prospects on using green ingredients for sustainable eco-friendly automotive brake composites

This review provides current insights on designing brake friction composites and states the extent of deviation since its historical evolution, the nuances of braking-induced contact dynamics, the adverse effect of brake wear debris followed by the contemporary bio-fiber/ naturally derived composition alternatives based on life cycle analysis implications and formulation design strategy assessment in view of the toxicity consequences of metallic constituents in brake friction composites. To reduce this adverse environmental impact, bio-based resinous binders, plant fibers, and naturally derived additives have been highlighted with their roles and contribution to tribological performance attributes particularly elucidated in terms of brake composites. Moreover, the role of bio-based natural friction additives and degradable solid waste as fillers in controlling braking and sliding-induced contact dynamics has been elaborated. Thus the paper comprehensively analyses the compositional influence pertaining to bio-based/naturally occurring/degradable solid waste as alternative materials to sustainably design new friction compositions while addressing the objective of minimizing braking-induced particulate matter emission and microplastic generation. The review further explores the life cycle analysis of brake friction composition of the ideologically appropriate one to that of the conventional composites in the broader realms of SWOT analysis and strategy assessment to explore the incorporation of green ingredients as a class of functionally integral components into brake composites.

Rapid generation of aged tire-wear particles using dry-, wet-, and cryo-milling for ecotoxicity testing

Strict environmental laws have been enacted to regulate the emission of exhaust particulate matter (PM), which is one of the most hazardous pollutants that reduce air quality and pose a serious risk to the human health. In addition, non-exhaust PM, such as road wear, tire wear, and brake wear debris, is a significant source of airborne pollutants. Road dust less than 100 μm in size may include tire wear particles (TWPs), which are broken down into finer particles with sizes on the order of tens of micrometers because of weathering. TWPs can be transported to water bodies via runoff, potentially contaminating water systems and negatively affecting aquatic ecosystems. Therefore, ecotoxicity tests using reference TWPs are required to investigate the impact of TWPs on the human health and environment. In this study, aged TWPs were produced using dry-, wet-, and cryo-milling methods, and the dispersion stability of TWPs in dechlorinated water was evaluated. Aged TWPs prepared by dry- and wet-milling had an average particle size of 20 μm, whereas pristine TWPs had an irregular shape and average particle size of 100 μm. The capacity of the ball-milling cylinder and excessively long 28-d generation time constrain the amount of aged TWPs that can be produced through conventional milling. In contrast, cryo-milling reduces the particle size of TWPs at the rate of −275.0 μm/d, which is nine times higher than that upon dry- and wet-milling. Dispersed cryo-milled TWPs had a hydrodiameter of 2.02 μm and were more stable in the aqueous phase in relation to the other aged TWPs. The results of this study suggest that cryo-milled TWPs can be used for aquatic exposure assessments as controls for real-world TWPs.

Airflow Simulation and Measurement of Brake Wear Particle Emissions with a Novel Test Rig

Particle emissions generated by the braking systems of road vehicles represents a significant non-exhaust contributor. Fine particles such as these are transported through airborne routes. They are known to adversely affect human health and currently there are no policies in place to regulate them. Before this issue can be addressed, it is important to characterise brake wear debris which is the purpose of this study. A newly-developed test rig consisting of a closed but ventilated enclosure surrounds a brake dynamometer equipped with a cast iron rotor. A sampling probe was made in accordance with the isokinetic principles in order to withdraw a representative aerosol sample from the outlet duct. Measurements of real-time particulate numbers and mass distributions are recorded using a Dekati ELPI<sup>®</sup>+ unit and the brake materials were tested under drag-braking conditions. Prior to measurements, Computational Fluid Dynamics (CFD) simulations were performed to investigate the most suitable sampling points used in the experiments. Preliminary experimental results show that there is a noticeable increase in particle numbers, compared to background levels, with a corresponding change in the mass distribution; coarser particles become more prominent during these braking events. These results provide confidence in the performance of the test rig and its ability to measure airborne brake wear debris in order to compare emissions from various friction pairs.

Determination of Oxidative Potential Caused by Brake Wear Debris in Non-Cellular Systems.

Wear debris from automotive brake systems represents a major source of non-exhaust emissions from road traffic and its production increases with number of cars worldwide. However, impact of brake wear debris on the environment and organisms is still not clear. One of the most possible ways by which these particles may affect living organisms is oxidative stress. Production of reactive oxidative species may cause damage of basic cell components, lipids, proteins, etc. Aim of this study is to perform characterization of airborne and nonairborne fractions of brake wear debris generated during standard dynamometer tests and evaluation of its potential to induce oxidative stress via lipid peroxidation and carbonylation of proteins in non-cellular system. Elemental and phase composition were determined by scanning electron microscopy, Raman microspectroscopy, and X-ray powder diffraction analysis. Carbon in amorphous form and graphite, copper, and iron in form of oxides were identified as major components in both studied fractions. Characteristic size of studied wear particles was evaluated by dynamic light scattering. Both airborne and nonairborne samples showed ability to induce oxidative stress which results from determination of carbonylated proteins.

Biological response of an in vitro human 3D lung cell model exposed to brake wear debris varies based on brake pad formulation

Wear particles from automotive friction brake pads of various sizes, morphology, and chemical composition are significant contributors towards particulate matter. Knowledge concerning the potential adverse effects following inhalation exposure to brake wear debris is limited. Our aim was, therefore, to generate brake wear particles released from commercial low-metallic and non-asbestos organic automotive brake pads used in mid-size passenger cars by a full-scale brake dynamometer with an environmental chamber simulating urban driving and to deduce their potential hazard in vitro. The collected fractions were analysed using scanning electron microscopy via energy-dispersive X-ray spectroscopy (SEM–EDS) and Raman microspectroscopy. The biological impact of the samples was investigated using a human 3D multicellular model consisting of human epithelial cells (A549) and human primary immune cells (macrophages and dendritic cells) mimicking the human epithelial tissue barrier. The viability, morphology, oxidative stress, and (pro-)inflammatory response of the cells were assessed following 24 h exposure to ~ 12, ~ 24, and ~ 48 µg/cm2 of non-airborne samples and to ~ 3.7 µg/cm2 of different brake wear size fractions (2–4, 1–2, and 0.25–1 µm) applying a pseudo-air–liquid interface approach. Brake wear debris with low-metallic formula does not induce any adverse biological effects to the in vitro lung multicellular model. Brake wear particles from non-asbestos organic formulated pads, however, induced increased (pro-)inflammatory mediator release from the same in vitro system. The latter finding can be attributed to the different particle compositions, specifically the presence of anatase.

Automotive airborne brake wear debris nanoparticles and cytokinesis-block micronucleus assay in peripheral blood lymphocytes: A pilot study

Motor vehicle exhaust and non-exhaust processes play a significant role in environmental pollution, as they are a source of the finest particulate matter. Emissions from non-exhaust processes include wear-products of brakes, tires, automotive hardware, road surface, and traffic signs, but still are paid little attention to. Automotive friction composites for brake pads are composite materials which may consist of potentially hazardous materials and there is a lack of information regarding the potential influence of the brake wear debris (BWD) on the environment, especially on human health. Thus, we focused our study on the genotoxicity of the airborne fraction of BWD using a brake pad model representing an average low-metallic formulation available in the EU market. BWD was generated in the laboratory by a full-scale brake dynamometer and characterized by Raman microspectroscopy, scanning electron microscopy, and transmission electron microscopy showing that it contains nano-sized crystalline metal-based particles. Genotoxicity tested in human lymphocytes in different testing conditions showed an increase in frequencies of micronucleated binucleated cells (MNBNCs) exposed for 48h to BWD nanoparticles (NPs) (with 10% of foetal calf serum in culture medium) compared with lymphocytes exposed to medium alone, statistically significant only at the concentration 3µg/cm2 (p=0.032).

Laboratory testing of airborne brake wear particle emissions using a dynamometer system under urban city driving cycles

To measure driving-distance-based mass emission factors for airborne brake wear particulate matter (PM; i.e., brake wear particles) related to the non-asbestos organic friction of brake assembly materials (pads and lining), and to characterize the components of brake wear particles, a brake wear dynamometer with a constant-volume sampling system was developed. Only a limited number of studies have investigated brake emissions under urban city driving cycles that correspond to the tailpipe emission test (i.e., JC08 or JE05 mode of Japanese tailpipe emission test cycles). The tests were performed using two passenger cars and one middle-class truck. The observed airborne brake wear particle emissions ranged from 0.04 to 1.4 mg/km/vehicle for PM10 (particles up to 10 μm (in size), and from 0.04 to 1.2 mg/km/vehicle for PM2.5. The proportion of brake wear debris emitted as airborne brake wear particles was 2–21% of the mass of wear. Oxygenated carbonaceous components were included in the airborne PM but not in the original friction material, which indicates that changes in carbon composition occurred during the abrasion process. Furthermore, this study identified the key tracers of brake wear particles (e.g., Fe, Cu, Ba, and Sb) at emission levels comparable to traffic-related atmospheric environments.

Influence of the Automotive Brake Wear Debris on the Environment - A Review of Recent Research

Influence of the Automotive Brake Wear Debris on the Environment - A Review of Recent Research

Water suspended nanosized particles released from nonairborne brake wear debris

The paper addresses characterization of nanosized particles which may be potentially mobilized from nonairborne brake wear debris when being suspended in the water environment. Wear particles were generated in a fullscale automotive brake dynamometer simulation using a model low metallic friction composite and a gray cast iron disc. The three types of samples were analyzed: (a) nontreated wear debris, (b) wear debris particles dissolved in water and (c) filtered suspension of dissolved wear debris particles. Samples were subjected to a combination of analyses (XRD, TEM, SEM with EDS, Raman microspectroscopy and vibration magnetometry). Raman microspectroscopy revealed the presence of Fe2O3, Fe3O4, BaSO4, CaCO3, MoS2, graphite and amorphous carbon in the wear debris samples. The TEM analysis of particles in water suspension proved that the nanosized particles are released into water. Barite particles were detected in the water suspension by Raman microanalysis most often. The prepared filtered suspension exhibited magnetic character which is probably related to the presence of magnetic forms of iron and iron oxides.

Assessment of methods for collecting fallout brake pad wear debris for environmental analysis

Three methods for collecting or generating fallout brake pad wear debris for environmental analysis were assessed: collection from wheels or hubs of automobiles (natural), generation from an inexpensive sanding process (sanded), and collection of fallout debris from dynamometer tests using the Los Angeles City Traffic protocol (LACT). Brake wear debris was collected from four automobiles with semimetalic brake pads and analyzed for physicochemical properties. For automobiles where all three types of debris were collected, bulk copper mass fractions ranged from 22–23% in sanded particles and 24–27% in LACTparticles, but were reduced to 1–6% in natural debris. The smaller copper mass fraction in natural debris was attributed to contamination with road dust, which was found to comprise 37–97% of the natural particles. The ratio of surface to bulk copper mass fraction was up to five times larger for natural than LACT debris, suggesting that copper may leach into stormwater faster and to a greater extent for natural particles. While the LACT method appears best for collecting only fallout particles, significant differences in copper distributions in the natural and LACT debris suggests that metal distribution in LACT debris may not be representative of fallout particles generated under actual driving conditions, where airborne road dust may play a role. Although dynamometer tests have been the preferred method for generating debris for assessment of metal dissolution from brake particles, data from this study indicate that such samples may result in biased estimates of metal leaching.

Airborne Asbestos Concentrations Associated with Heavy Equipment Brake Removal

Asbestos-containing brake linings were used in heavy-duty construction equipment such as tractors, backhoes, and bulldozers prior to the 1980s. While several published studies have evaluated exposures to mechanics during brake repair work, most have focused on automobiles and light trucks, not on heavy agricultural or construction vehicles. The purpose of this study is to characterize the airborne concentration of asbestos to workers and bystanders from brake wear debris during brake removal from 12 loader/backhoes and tractors manufactured between 1960 and 1980. Asbestos content in brake lining (average 20% chrysotile by polarized light microscopy) and brake wear debris [average 0.49% chrysotile by transmission electron microscopy (TEM)] was also quantified. Breathing zone samples on the lapel of mechanics (n = 44) and area samples at bystander (n = 34), remote (n = 22), and ambient (n = 12) locations were collected during 12 brake changes and analyzed using phase contrast microscopy (PCM) [National Institute for Occupational Safety and Health (NIOSH) 7400] and TEM (NIOSH 7402). In addition, the fiber distribution by size and morphology was evaluated according to the International Organization for Standardization method for asbestos. Applying the ratio of asbestos fibers:total fibers (including non-asbestos) as determined by TEM to the PCM results, the average airborne chrysotile concentrations (PCM equivalent) were 0.024 f/cc for the mechanic and 0.009 f/cc for persons standing 1.2-3.1 m from the activity during the period of exposure ( approximately 0.5 to 1 h). Considering the time involved in the activity, and assuming three brake jobs per shift, these results would convert to an average 8-h time-weighted average of 0.009 f/cc for a mechanic and 0.006 f/cc for a bystander. The results indicate that (i) the airborne concentrations for worker and bystander samples were significantly less than the current occupational exposure limit of 0.1 f/cc; (ii) approximately 2% of respirable fibers were >20 microm in length; and (iii) approximately 95% of chrysotile in the brake linings degraded in the friction process. The industrial hygiene data presented here should be useful for conducting retrospective and current exposure assessments of individuals, as well as hazard assessments of work activities that involve repairing and replacing asbestos-containing brakes in heavy construction equipment.

Low-pressure decomposition of chrysotile as a function of time and temperature

Chrysotile from Thetford, Quebec, was heated at constant temperature in quench furnaces from 200 to 1000 °C for 4 to 720 h, and the products were analyzed by XRD and optical microscopy. XRD patterns for chrysotile from Thedford, Jeffrey Quebec, and New Idria, California, heated for up to 8 h at constant temperatures from 400 to 800 °C in hydrothermal diamond-anvil cells, were obtained at 300 s intervals by using synchrotron radiation. The studies show that differences in the decomposition temperatures of chrysotile reported in the literature can be explained by differences in the temperatures, time at temperature, particle size, and partial pressure of water. Chrysotile heated for 30 days is destroyed between 475 and 500 °C, whereas chrysotile heated to 800 °C survives for only minutes. A prograde heating experiment, consistent with the literature, shows chrysotile destruction beginning at 600 °C and completed at 750 °C, demonstrating that such experiments should not be used to establish temperatures of thermal stability. Forsterite forms readily at 600 °C when water is retained transiently in the decomposition products, but does not form after 4 h of heating samples of small fiber size and high surface area from which water is readily lost. When water is retained in the sample chamber during heating to 800 °C over 4 h, a rapid destruction of chrysotile and formation of forsterite was observed, followed by the appearance and later destruction of the intermediate reaction products talc, a tridymite-like phase, and anthophyllite; enstatite appearance coincides with the destruction of the tridymite-like phase and talc. In quench experiments, a 10+ A phase is observed between 500 and 600 °C, and talc is observed between 587 and 800 °C. The optical properties of the run products between 600 and 900 °C are highly variable, with a range in the index of refraction perpendicular to length of up to 0.06 and a minimum index of refraction 0.03 below the starting material in fibers in the 650 °C run product; the variability reflects the inhomogeneity in products and reactants in this temperature range. In quench experiments, the intensity of the XRD patterns of chrysotile decreased on heating between 200 and 400 °C, and increased at 450 °C, demonstrating recrystallization of chrysotile. A gradual heating experiment showed a similar pattern with the lowest intensities at 250 °C and maximum intensities at 550 °C. The two types of chrysotile reported in the literature may reflect simply the presence of chrysotile of different particle sizes. Chrysotile may also possess a spectrum of stabilities due to variable strain energies of curvature. The presence of a tridymite-like phase supports the formation of a Si-rich dehydroxylate II. The absence of forsterite and chrysotile in brake-wear debris reflects the high temperature and disaggregated fiber bundles resulting from friction.

Effects of organic ligands and pH on the leaching of copper from brake wear debris in model environmental solutions.

Copper leaching from a disc brake wear debris sample was examined in a variety of aqueous solutions to simulate potential leaching processes during rain events and in surface waters. Synthetic rainwater leached 40% of the total copper present in the brake wear debris into solution after 18 h in batch reactors, which was approximately three times more copper than that extracted by the US Environmental Protection Agency's Synthetic Precipitation Leaching Procedure. Formate and acetate were responsible for the enhanced copper leaching, as demonstrated by higher average amounts of leached copper in synthetic rainwater with- versus without the organic acids (40 versus 31% recovery). This observation suggests leaching tests that do not incorporate the appropriate types and concentrations of organic ligands present in rainwater will likely underestimate copper mobilization from brake wear debris during rain events. Leaching of copper from the brake wear debris ranged from 23 to 40% in solutions containing 3 to 15 mg C L(-1) dissolved humic substances, and was higher still in solutions containing relatively high concentrations of the synthetic metal chelating agent ethylenediaminetetraacetic acid. Static pH tests demonstrated that copper leaching from brake wear debris is highly pH dependent, with more leaching occurring at lower solution pH values. Leaching rate studies revealed that equilibrium generally was not attained within 48 h in the model solutions, indicating that additional copper can be expected to be released in environments where brake wear debris is exposed to long-term leaching processes.

Airborne brake wear debris: size distributions, composition, and a comparison of dynamometer and vehicle tests.

Particle size distributions of light-duty vehicle brake wear debris are reported with careful attention paid to avoid sampling biases. Electrical low-pressure impactor and micro-orifice uniform deposit impactor measurements yield consistent size distributions, and the net particulate matter mass from each method is in good agreement with gravimetric filter measurements. The mass mean diameter of wear debris from braking events representative of urban driving is 6 microm, and the number-weighted mean is 1-2 microm for three currently used classes of lining materials: low metallic, semimetallic, and non-asbestos organic (NAO). In contrast, the wear rates are very material dependent, both in number and mass of particles, with 3-4 times higher emissions observed from the low metallic linings as compared to the semimetallic and NAO linings. Wind tunnel and test track measurements demonstrate the appearance of micron size particles that correlate with braking events, with approximately 50% of the wear debris being airborne for the test vehicle in this study. Elemental analysis of the wear debris reveals a consistent presence of the elements Fe, Cu, and Ba in both dynamometer and test track samples.

Copper leaching from brake wear debris in standard extraction solutions.

Quantification of the copper content of and copper leaching from a disc brake wear debris sample was performed using microwave-assisted acid digestion, the Federal Toxicity Characteristic Leaching Procedure (TCLP), and the State of California Waste Extraction Test (WET). The brake wear debris tested was a composite sample obtained from a brake dynamometer test of one brake pad source material. Comparative digestion studies demonstrated that a modified aqua regia matrix (HNO3:HCl:H2O2 = 1:3:0.5) optimized the digestion effectiveness for determining the total copper content in the brake wear debris. No significant sample heterogeneity within the brake wear debris was observed, based on statistically indistinguishable total copper content results for subsamples with a wide range of sample masses. Upon pooling all subsample results, an overall total copper content for the composite brake wear debris sample was determined to be 10.8% (g/g), with a 95% confidence limit of +/- 0.5% (g/g). Copper leaching increased with decreasing solid-to-liquid ratios in TCLP tests, but was unaffected by the solid-to-liquid ratio in the WET. For a 1:10(4) (g/g) solid-to-liquid ratio, 85% and 99% of the total mass of copper present in the composite brake wear debris sample was leached into solution during the TCLP and WET, respectively. Rate studies also demonstrated that the WET resulted in a faster rate and higher extent of copper leaching relative to the TCLP. Compared to reference copper-containing materials, the composite brake wear debris sample exhibited relatively higher TCLP and WET copper leaching characteristics. The higher copper leaching exhibited by the brake wear debris sample may have resulted from its higher specific surface area and/or from changes in the chemical form of copper that occurred during the braking process.