Analysis of Tire Wear Airstrip Particles (TWAP)

doi:10.21203/rs.3.rs-1761709/v1

Download PDF

Research Article

Analysis of Tire Wear Airstrip Particles (TWAP)

https://doi.org/10.21203/rs.3.rs-1761709/v1

This work is licensed under a CC BY 4.0 License

Version 1

posted

You are reading this older preprint version

Read the latest preprint version →

Every aircraft leaves tire wear particles on the ground of airports at take-off and landing. To maintain good grip properties, the pavement must be regularly cleaned up. The gathered Tire Wear Airstrip Particles (TWAP) normally get disposed by burning. As they are very similar to rubber powder the question is if they might be used as a secondary raw material to keep them longer in the loop in the sense of circular economy.

In this study TWAP gathered by the cleaning of the runway of an airport in Germany are compared to ambient ground rubber powder from truck tire treads. Both are analyzed microscopically with regard to their morphology, particle size distribution and chemical composition by calcination with following elemental characterization of the ash by Energy-Dispersion-X-ray Analysis (EDS) in the Scanning Electron Microscope (SEM) and Thermal Gravimetric Analysis (TGA). The particle size distribution of TWAP is broader than rubber powder containing also much smaller particles. The mineral content is about 60% of the gathered TWAP. These minerals can be residues of the pavement or break wear and to a smaller amount from the rubber ingredients. Although the TWAP on their own are similar to rubber powder, the impurities they contain may have an impact with regard to potential applications.

tire wear

rubber powder

analysis

secondary raw material

circular economy

Tire Wear Particles (TWP) account for a large proportion of microplastics in the environment. However, since this cannot be completely avoided, the recycling of the TWP should be considered in addition to their reduction. To maintain the grip properties, at airports the airstrips are cleaned up and a large part of the Tire Wear Airstrip Particles can so be recovered. Up to now these have only been recycled thermally, that means they are simply burnt. According to estimations from the Fraunhofer UMSICHT study, the emissions of tire wear particles into the environment amounts to nearly 100,000 tons in Germany alone [1]. There is no proper data regarding the contribution of airplane tires abrasion to this available. But Frankfurt Airport alone reported a volume of 83 tons of tire wear in 2019 [2]. In order to come closer to the ideal of a circular economy, the use of TWP/TWAP would serve greatly as an additional source of secondary raw material, next to that of used tires. The recycling of scrap tires into rubber powder and granulate amounted to 251,000 tons in Germany in 2019 [3]. Another recycling strategy is pyrolysis, used for the purposes of recovering oil, gas and carbon black [4]. This could also be considered for recycling TWP/TWAP.

The friction between the tire and the pavement during driving causes particles to be worn out from both the tire and the pavement. These particles are then carried out into the environment as abrasion. The formation of TWP/TWAP depends on the properties of the tire, the road surface, the vehicle and the driving behavior. Most studies on this have been conducted on car tires, both on the road and in laboratory simulations [5, 6]. G. Kim and S. Lee produced and analyzed abrasion particles in a laboratory tire wear simulator under various conditions such as load, speed, and driving behavior. These TWP had particle sizes smaller than 5 µm [7]. In contrast, tire abrasions from the environment have particle sizes up to 350 µm [8]. This can be due to the fact that they agglomerate fast after building. The particles produced by friction often take an elongated shape [7–9]. The chemical composition shows particularly high carbon and silicon contents as well as sulfur and heavy metals [7, 10]. The presence of zinc in the abrasion particles, next to other organic compounds such as vulcanization accelerators, can be used as an indicator for better distinguishing them from other microplastic particles [11, 12]. Abraded organic compounds and zinc can migrate out of the particles into the environment and further contribute to environmental pollution [6, 13]. In addition to abrasion from tires, particles from road markings, brakes, and asphalt can also be found in the abrasion samples [6, 14].

The TWAP were made available by Volkmann Strassen- und Verkehrstechnik GmbH and originate from the airstrip cleaning of a german airport. Larger contaminants such as leaves or sheet/packaging residues were removed beforehand. For comparison, the rubber powder Raumehl Type I from Regupol BSW GmbH was used. This rubber powder is produced by ambient grinding and originates from truck tire treads. For comparison samples of a truck tire tread provided by Gummi Berger GmbH and an aircraft tire tread provided by the University of Twente were used for thermogravimetric analysis.

Microscopic images were taken using Olympus SZX10 stereomicroscope. The particle size distribution was measured with the Mastersizer 3000 from Malvern Panalytical. Both the rubber powder and the TWAP were first sieved with the AS 400 control planar sieve machine using a sieve with a 500 µm mesh size to exclude larger particles. Thermogravimetric analyses were carried out with the STA 409 PC from Netzsch, using the measuring parameters mentioned in Table 1. The amount of mineral residues of the TWAP was determined by calcination of five specimens in the high temperature oven K1253A from Heraeus at a temperature of 900°C according to the TGA. The elemental compositions of the starting products and the ash from TGA were subsequently analyzed by means of EDS analysis with the scanning electron microscope SU5000 from Hitachi and EDS sensor X-Max 80 from Oxford Instruments. With the elemental analyzer for carbon (C), nitrogen (N), hydrogen (H) and sulfur (S) Vario Macro Cube from Elementar the percentual amount of these elements was measured in TWAP and rubber powder. Polycyclic aromatic hydrocarbon (PAH) analysis was examined in accordance to AfPS GS 2019:01 - PAK by TÜV Rheinland LGA Products GmbH.

Table 1

Measuring parameters TGA
Mode	Temperature	Heating rate	Gas
Start	30°C		N₂ (30 ml/min)
Dynamic	600°C	10 K/min	N₂ (30 ml/min)
Dynamic	900°C	10 K/min	N₂, O₂ (20 + 10 ml/min)

4.1 Particle size distribution

Despite the sieving process, large particles can still be found in both the TWAP and the rubber powder. The TWAP showing particle sizes of up to 750 µm and the rubber powder even up to 1 mm (see Fig. 1). Reagglomeration after sieving and elongated particles which pass the mesh with their smaller width are leading to higher particle sizes. The particle size distribution of TWAP is significantly wider than that of the rubber powder and also exhibits much smaller particles. The abrasion process produces finer particles of the different materials than the grinding of rubber. This is favorable for the use of TWAP as a secondary raw material similar to rubber powder in rubber compounds.

4.2 Calcination

Calcination of the TWAP yields a quantity of approx. 62% mineral constituents. This agrees well with the observations made by TGA (s. 4.4, also for the chemical composition by elemental analysis). Further information with regard to the composition of the TWAP and the rubber powder is given by the TGA.

4.3 Thermogravimetric Analysis

The thermogravimetric analysis of rubber powder is shown in Fig. 2. From room temperature to approximately 330°C a mass loss of 5.24% can be seen. This is related to volatile matter, softener and other low-molecular additives. The next big mass change of 56.17% from 330°C to 480°C is due to degradation of polymers. The degradation temperatures of natural rubber and butadiene rubber, which are used typically in truck treads, lie around 370°C to 480°C. At 600°C the atmosphere is changed from pure nitrogen to nitrogen and oxygen. From this point on Carbon Black as well as other residuals consisting of carbon are oxidizing. This results in a further mass change of 29.93%. A residual mass of 9.77% is obtained which consists of minerals. The composition of the ash will be shown in the elemental analysis.

Figure 3 shows the thermogravimetric analysis of TWAP. In the first range up to 330°C a mass loss of 2.72% can be seen. Again, this can be related to volatile and low-molecular matter. In contrast to the TGA of rubber powder the graph of TWAP shows no clear change in degradation rate going from the degradation temperature range of volatile matter to the range of polymers. A mass loss of 12% up to 480°C is seen and a further 2.12% up to 600°C. Both can be related to polymers. While rubber powder consists mainly of NR and BR, a mixture of more polymers can be expected for TWAP. These other polymers can origin from road markings and asphalt. Again, the change of atmosphere leads to oxidation of carbon which leads to a mass loss of 16.17% for the TWAP. A residual mass of 68.61% is obtained.

To compare TWAP and rubber powder with their origin from tires, TGA-analyses of a truck tire tread respectively an aircraft tire tread were examined (see Fig. 4). The results of rubber powder and truck tire tread are similar. Differences of them can be derived from varying compound recipes of different tire producers. As TWAP contain a high amount of mineral residues, the mass changes for all TGA-measurements where normalized. The sum of all mass changes was set as 100% in order to eliminate the influence of the residual mass. The results of the normalized mass changes can be seen in Table 2. Again, the rubber powder and truck tire tread are very similar. The aircraft tire tread reveals a significantly higher portion of volatile and low-molecular matter as well as lower polymer content than rubber powder and truck tire tread. The amounts of Carbon Black are comparable.

In contrast, TWAP show a mass loss of volatile and low-molecular matter nearly like the aircraft tire tread. But a difference of 15–20% less polymer content in comparison to the aircraft tire tread respectively rubber powder / truck tire tread can be seen. Also, TWAP have nearly 50% carbon content which is much higher than in the aircraft tire tread. This may be due to additional organic matter coming from the pavement abrasion that contains bitumen.

Table 2

Normalized mass changes of thermogravimetric analyses
	Rubber powder	Truck Tire Tread	TWAP	Aircraft Tire Tread
Up to 330°C	5.74%	5.73%	8.24%	9.15%
Up to 600°C	61.50%	62.19%	42.77%	57.39%
Up to 900°C	32.77%	32.08%	48.99%	33.46%

4.4 Light microscopy and SEM

The microscopic image shows that the rubber powder is free from other particles. The particles have different sizes and morphology. The rubber powder has a rough, jagged surface (see Fig. 5) which originates from the ambient grinding process. Here, the grinding is done at room temperature, where the rubber is soft and elastic.

The TWAP, on the other hand, consists of particles of much more different sizes and composition. The dark particles come mainly from aircraft tire abrasion agglomerated with other particles. During landing, driving and braking, particles are released from the tread due to friction on the pavement. Here, temperature can come close to 200°C [15] Sand grains (1) and glass beads (2) from pavement and road markings are found. In the backscattered electrons (BSE) image they differ in brightness due to the different chemical composition (see Fig. 6).

The SEM images of rubber powder and TWAP also show the different particle sizes. The morphology is clearer in these images. The ambient grinding process of rubber powder from a tire leads to particles with a rough surface (see Figs. 6 and 7). On the surface of the rubber powder are some small particles found which reveal a higher density (brighter color in SEM). TWAP show some particles with a smooth surface like the glass beads (2) and some mineral particles (1, B) (see Figs. 6 and 7). The agglomerates of tire, break and pavement wear (C) are forming bigger particles (see Fig. 7). Rubber from tire wear alone is rarely found in the TWAP (A). The rubber particle shows an elongated shape and is smaller than rubber powder particles obtained by grinding. The abrasion process takes place under higher temperatures and loadings than ambient grinding. Due to their small size most particles form directly after abrasion agglomerates with other wear particles which adhere strongly.

4.5 Elemental Analysis

EDS-Measurements taken before and after the TGA measurements of TWAP and rubber powder show the elemental compositions of the starting materials and the remaining ashes. The different positions of the elemental analyses are marked in the SEM images before.

The elemental analysis shows a very high carbon content (C) for the rubber powder. This is logical for a rubber containing Carbon Black (pure carbon) as a reinforcing filler, polymers and plasticizers (consisting of hydrocarbons). Zinc (Zn) as zinc oxide is a typical ingredient in rubber compounds for activating the vulcanization process. In addition, the crosslinking agent sulfur (S) is also present. Traces of silicon (Si), calcium (Ca), sodium (Na) and potassium (K) are found (see Fig. 8 left). The mineral residue obtained after calcination consists mainly of silicon and oxygen. These may originate from the pavement and road dust that adheres to the tread. Again, the rubber compound ingredients sulfur and zinc as zinc oxide are found. (see Fig. 8 right). The elements calcium, sodium, potassium and aluminum are detected in small amounts and may originate from break wear [16–19].

Much like the spectrum of the rubber powder, the elemental analysis spectrum A of the TWAP shows a high carbon (C) content as well. In addition to sulfur (S), silicon (Si), oxygen (O), sodium (Na) and calcium (Ca), more aluminum (Al) and other metals such as iron (Fe), magnesium (Mg), potassium (K) and titanium (Ti) are also found (see Fig. 9). These may originate from silicon oxide (SiO₂) as reinforcing filler in rubber compounds and brake wear that adhere to the rubber particles. Brake pads contain a huge variety of different materials [16–19]. Traces of chlorine (Cl) are found, which may be present as sodium chloride for deicing the pavement [20].

Spectrum B shows the elemental composition of a mineral particle. The main elements are silicon (Si) and oxygen (O). This indicates that the particle consists mainly of silicon oxide like a sand grain. Aluminum (Al), iron (Fe), calcium (Ca) and sodium (Na) are found (see Fig. 8). These are elements used in brake pads. Some brake wear particles may be agglomerated to the bigger particle. Carbon (C) can originate from diverse organic resources.

The elemental spectrum C) shows the composition of an agglomerate of different particles. Carbon (C), silicon (Si) and oxygen (O) are the main elements found which is consistent with spectra A) and B). Zinc (Zn) from the vulcanization activator and sulfur (S) as crosslinking agent are found. Again, aluminum (Al), iron (Fe) calcium (Ca), sodium (Na), magnesium (Mg), potassium (K) and titanium (Ti) indicate the presence of break wear particles. Traces of phosphorus (P) are measured. Phosphorus is used as phosphate in fertilizer and may originate from soil dust [20].

After calcination, silicon (Si) and oxygen (O) are still the most common elements found. These originate not only from the silica in the rubber particles, but also from sand and glass beads used in road markings (see Fig. 10). As seen in the spectra before calcium (Ca), titanium (Ti), magnesium (Mg), iron (Fe), sodium (Na), magnesium (Mg), aluminum (Al), potassium (K) and manganese (Mn) is found which may originate from brake pads.

Table 3

Weight percent of C, N, H and S in rubber powder and TWAP
	C	N	H	S	other
Rubber powder	82.39%	0.42%	8.19%	1.29%	7.73%
TWAP	26.47%	0.21%	2.79%	0.61%	69.95%

As the previous elemental analyses cannot give quantitative values and is not able to measure light elements, the CNHS-analysis was used to confirm the previous results. Rubber powder reveals approximately 90% of carbon and hydrogen (see Table 3) which is reasonable because it consists of pure rubber filled with Carbon Black. This is consistent with the mass loss in TGA of volatile matter, polymers and Carbon Black. The low amounts of nitrogen can be part of vulcanization accelerators, the sulfur as mentioned before is used as curing agent. All other elements have a quantity of about 8%. This summed up with the amount of sulfur agrees well with the measured residual mass in the TGA.

The amounts of C, N, H and S in TWAP differ significantly from rubber powder. It contains only about 30% C and H, this is also in agreement with TGA-measurement. The low amount of nitrogen may also come from vulcanization accelerators and the sulfur from the curing agent. Other elements sum up to 70%, which presumably consist mainly of silicon and oxygen as seen in SEM/EDS analyses. This amount is also consistent with the residual mass and calcination results.

4.6 PAH analysis

The European Union regulation REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) set maximum amounts of PAHs for different products [21]. As the further aim of this study is to use TWAP as a secondary raw material for new products these limits have to be considered.

Table 4

PAH amounts of TWAP and rubber powder measured according to AfPS GS 2019:01 PAK
	TWAP	Rubber Powder
	mg/kg	mg/kg
Phenanthrene	0,2	4
Pyrene	0,7	14
Anthracene	< 0,2	0,4
Fluoranthene	0,3	5,4
Naphthaline	< 0,2	1
Benzo(a)pyrene	< 0,2	0,5
Benzo(e)pyrene	< 0,2	1,2
Benz(a)anthracene	< 0,2	< 0,2
Benzo(b)fluoranthene	< 0,2	0,3
Benzo(j)fluoranthene	< 0,2	< 0,2
Benzo(k)fluoranthene	< 0,2	< 0,2
Chrysene	< 0,2	0,3
Dibenz(ah)anthracene	< 0,2	< 0,2
Benzo(ghi)perylene	0,5	2,7
Indene (1,2,3-cd)pyrene	< 0,2	0,2
Sum 15 PAH	1,7	30,0

TWAP exhibit significantly less PAHs than rubber powder. This is an advantage of TWAP versus rubber powder with regard to their use as a secondary raw material. One possible reason is that TWAP contain over 60% mineral residues and reveals significant less C- and H-content than rubber powder. But as the difference is more than these amounts can explain, there are more influencing factors.

The morphology and chemical composition of the TWAP are consistent with the findings of other studies. The gathered TWAP contain approximately 60% mineral residues, which partly strongly adhere to the rubber particles. With regard to use as a secondary raw material separation, esp. of the bigger particles (sand, glass beads) might be favorable. Ambient ground rubber powder and abrasion particles from tires appear to be morphologically similar. PAH analysis reveals significantly less PAHs for TWAP which facilitates the use in new products with regard to REACH. As such, the next step is to use TWAP in rubber compounds in the same way rubber powder is used today. This will allow to investigate the influence of the adherent metallic and other constituents on the compound properties. Further investigations on PAH and the materials itself may reveal the reasons for the huge difference in PAH-content.

Acknowledgement

We thank Volkmann Strassen- und Verkehrstechnik GmbH, Regupol, Gummi Berger and University of Twente for providing the samples, Pyrum Innovations AG for performing the particle size analyses and our colleagues Sebastian Bogdahn for the CNHS- and Simon Öchsner for carrying out the SEM/EDS analyses. Thanks also to Prof. Dr. Viktoriia Wagner and Sebastian Bogdahn from TH Köln for fruitful discussions with regard to the PAH analysis. We thankfully acknowledge TH Köln for the financial support that made this work possible.

Authors Contribution:

All authors contributed to the study conception and design. Danka Katrakova-Krüger acquired the funds, procured the different specimen and supervised this work. Material preparation, data collection and analysis were performed by Vanessa Spanheimer. The first draft of the manuscript was written by Vanessa Spanheimer and Danka Katrakova-Krüger commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Funding:

During the preparation of this manuscript a start-up financing and transfer fund from TH Köln were received.

Competing interest:

The authors have no relevant financial or non-financial interests to disclose.

Data availability:

The dataset generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Bertling J., Hamann L., Bertling R. (2018) Kunststoffe in der Umwelt. Kurzfassung der Konsortialstudie, Fraunhofer UMSICHT http://dx.doi.org/10.24406/UMSICHT-N-497117
Environmental Management in our Group – Waste data 2019 (2019) Fraport AG, Frankfurt am Main. https://www.fraport.com/en/environment/environmental-management.html. Accessed 17 May 2022
Altreifen nach Art der Verwertung in Deutschland im Jahr 2019 (2019) Statista. Available: https://de.statista.com/statistik/daten/studie/1099423/umfrage/altreifen-nach-verwertungsart-in-deutschland/. Accessed 20 Jan 2022
Antoniou N., Zabaniotou A. (2013) Features of an efficient and environmentally attractive used tyres pyrolysis with energy and material recovery. Renew. and Sustain. Ener. Rev. 20:539–558
Baensch-Baltruschat B., Kocher B., Stock F., Reifferscheid G. (2020) Tyre and road wear particles (TRWP) - A review of generation, properties, emissions, human health risk, ecotoxicity, and fate in the environment. The Sci. of the total environ. 733:137823 https://doi.org/10.1016/j.scitotenv.2020.137823
Andersson-Sköld Y., Johannesson M., Gustafsson M., Järlskog I., Lithner D., Polukarova M., Strömvall A. (2020) Microplastics from tyre and road wear - A literature review. VTI rapport 1028A
Kim G., Lee S. (2018) Characteristics of Tire Wear Particles Generated by a Tire Simulator under Various Driving Conditions. Environ. Sci. & technol. 52:12153–12161 https://doi.org/10.1021/acs.est.8b03459
Kreider M. L., Panko J. M., McAtee B. L., Sweet L. I., Finley B. L. (2010) Physical and chemical characterization of tire-related particles: comparison of particles generated using different methodologies. The Sci. of the total environ. 408:652–659 https://doi.org/10.1016/j.scitotenv.2009.10.016
Cadle S. H., Williams R. L. (1978) Gas and Particle Emissions from Automobile Tires in Laboratory and Field Studies. J. of the Air Pollut. Control Associ. 28:502–507 https://doi.org/10.1080/00022470.1978.10470623
McAtee B., Gustafsson M., Blomqvist G., Gudmundsson A., Sweet L., Panko J. Finley B. (2009) Physio-chemical analysis of airborne tire wear particles. Toxicol. Lett. 189:S205 https://doi.org/10.1016/j.toxlet.2009.06.621
Klöckner P., Reemtsma T., Eisentraut P., Braun U., Ruhl A. S., Wagner S. (2019) Tire and road wear particles in road environment - Quantification and assessment of particle dynamics by Zn determination after density separation. Chemosphere 222:714–721 https://doi.org/10.1016/j.chemosphere.2019.01.176
Klöckner P., Seiwert B., Eisentraut P., Braun U., Reemtsma T., Wagner S. (2020) Characterization of tire and road wear particles from road runoff indicates highly dynamic particle properties. Water Research https://doi.org/10.1016/j.watres.2020.116262
Councell T. B., Duckenfield K. U., Landa E. R., Callender E. (2004) Tire-wear particles as a source of zinc to the environment. Environ. Sci. & Technol. 38:4206–4214 https://doi.org/10.1021/es034631f
Panko J., Kreider M., Unice K. (2018) In: Non-Exhaust Emissions, 1st edn., Academic Press, London
Alroqi A. A., Wang W., Zhao Y. (2017) Aircraft Tyre Temperature at Touchdown With Wheel Prerotation. J. Aircraft 54:926-938 https://doi.org/10.2514/1.C033916
Hays M. D., Cho S.-H., Baldauf R., Schauer J. J., Shafer M. (2011) Particle size distributions of metal and non-metal elements in an urban near-highway environment. Atmos. Environ. 45:925–934 https://doi.org/10.1016/j.atmosenv.2010.11.010
Wahlström J., Olander L., Olofsson U. (2010) Size, Shape, and Elemental Composition of Airborne Wear Particles from Disc Brake Materials. Tribol. Lett. 38:15–24 https://doi.org/10.1007/s11249-009-9564-x
Kumar S., Ghosh S. K. (2020) Particle emission of organic brake pad material: A review. J. of Automobil. Eng. 234:1213–1223, https://doi.org/10.1177/0954407019879839
Timothy N., Williams E. T. (2019) Environmental Pollution by Heavy Metal: An Overview. Int. J. of Environ. Chem. 3:72-82 https://doi.org/10.11648/j.ijec.20190302.14.
Skorbiłowicz M., Skorbiłowicz E. (2019) Content of Calcium, Magnesium, Sodium and Potassium in the Street Dust from the Area of Białystok (Poland). J. Ecol. Eng. 20:125–131 https://doi.org/10.12911/22998993/113145
European Parliament and Council (2006) Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH): Nr. 1907/2006

No competing interests reported.

Download PDF

Version 1

posted

You are reading this older preprint version

Read the latest preprint version →

Analysis of Tire Wear Airstrip Particles (TWAP)

Archived Versions:

Version 1

Abstract

Figures

Introduction

Experimental

Results And Discussion

4.1 Particle size distribution

4.2 Calcination

4.3 Thermogravimetric Analysis

4.4 Light microscopy and SEM

4.5 Elemental Analysis

4.6 PAH analysis

Conclusion and Outlook

Declarations

References

Additional Declarations

Archived Versions:

Version 1