6PPD
6PPD is an organic chemical widely used as stabilising additive (or antidegradant) in rubbers, such as natural rubber, styrene-butadiene and butyl rubber; all of which are common in vehicle tires. Although it is an effective antioxidant, it is primarily used because of its excellent antiozonant performance. It is one of several antiozonants based around p-phenylenediamine. It has been used in rubber since the late 1970s, but has been the subject of increasing scrutiny since 2021, when it was determined that its oxidation product (6PPD-Q) causes pre-spawn mortality in coho salmon.
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| Names | |
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| Preferred IUPAC name
N1-(4-Methylpentan-2-yl)-N4-phenylbenzene-1,4-diamine | |
| Other names
N-(1,3-dimethylbutyl)-N'-phenyl-1,4-benzenediamine
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| Identifiers | |
3D model (JSmol)
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| ChEMBL | |
| ChemSpider | |
| ECHA InfoCard | 100.011.222 |
| EC Number |
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PubChem CID
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| UNII | |
| UN number | 3077 |
CompTox Dashboard (EPA)
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| Properties | |
| C18H24N2 | |
| Molar mass | 268.404 g·mol−1 |
| Appearance | brown or violet solid powder |
| Density | 1.07 |
| Melting point | 45 °C (113 °F; 318 K) |
| Boiling point | 260 °C (500 °F; 533 K) |
| log P | 3.972 |
| Hazards | |
| GHS labelling: | |
  
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| Danger | |
| H302, H317, H360, H410 | |
| P201, P202, P261, P264, P270, P272, P273, P280, P281, P301+P312, P302+P352, P308+P313, P321, P330, P333+P313, P363, P391, P405, P501 | |
| Flash point | 204 °C (399 °F; 477 K) |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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6PPD is an organic chemical widely used as stabilising additive (or antidegradant) in rubbers, such as natural rubber, styrene-butadiene and butyl rubber; all of which are common in vehicle tires.[1] Although it is an effective antioxidant, it is primarily used because of its excellent antiozonant performance. It is one of several antiozonants based around p-phenylenediamine.[2]
It has been used in rubber since the late 1970s,[3] but has been the subject of increasing scrutiny since 2021, when it was determined that its oxidation product (6PPD-Q) causes pre-spawn mortality in coho salmon.[4][5]
Manufacturing
[edit]6PPD is prepared by reductive amination of methyl isobutyl ketone (which has six carbon atoms, hence the '6' in the name) with phenyl phenylenediamine (PPD).[6] This produces a racemic mixture.
Application
[edit]6PPD is a common rubber antiozonant found in vehicle tires. It is mobile within the rubber and slowly migrates to the surface via blooming. On the surface it forms a "scavenger-protective film" that reacts with the ozone more quickly than the ozone can react with the rubber.[7] This process forms aminoxyl radicals[8][9] and was first thought to degrade only to the quinone diimine, but has since been understood to continue to oxidize to quinones, amongst other products.[10] Despite 6PPD being used in tires since the mid 1970s, its transformation to quinones was first recognized in 2020.[11][12] The oxidized products are not effective antiozonants, meaning that 6PPD is a sacrificial agent.
The tendency of 6PPD to bloom towards the surface is protective because the surface film of antiozonant is replenished from reserves held within the rubber. However, this same property facilitates the transfer of 6PPD and its oxidation products into the environment as tire-wear debris. The 6PPD-quinone (6PPD-Q, CAS RN: 2754428-18-5) is of particular and increasing concern, due to its toxicity to fish.
Environmental impact
[edit]6PPD and 6PPD-quinone enter the environment through tire-wear particles and are sufficiently water-soluble to enter river systems via urban runoff. From here they become widely distributed (at decreasing levels) from urban rivers through estuaries, coasts and finally deep-sea areas.[13]
6PPD-quinone is of environmental concern because it is toxic to coho salmon, killing them before they spawn in freshwater streams.[14][15][16]
A 2022 study also identified the toxic impact on species like brook trout and rainbow trout.[17] The published lethal concentrations are:[17][18]
- coho salmon: LC50 = 0.095 μg/L
- brook trout: LC50 = 0.59 μg/L
- rainbow trout: LC50 = 1.0 μg/L
It is not known why the ozone-oxidised 6PPD is toxic to coho salmon, but has been suggested that the large differences in lethal dose between species may relate to their ability to rid themselves of 6PPD-Q via glucuronidation.[19] The Nisqually and nonprofit Long Live the Kings installed a mobile stormwater filter at a bridge in the Ohop Valley in 2022. The Washington Department of Ecology, Washington State University and the US Tire Manufacturer's Association are working on regulation and education.[20]
6PPD itself is deadly to rotifers, especially in combination with sodium chloride, though not at the level generally found in the runoff from road salt.[21] A small-scale biomonitoring study in South China has shown both 6PPD and 6PPDQ to be present in human urine; concentrations were low but the health implications are unknown.[22] A synthetic route to the 6PPD-quinone has been posted on ChemRxiv.[23]
See also
[edit]- N-Isopropyl-N'-phenyl-1,4-phenylenediamine (IPPD), a related antiozonant
- N,N'-Di-2-butyl-1,4-phenylenediamine, a phenylenediamine-based antioxidant used as a fuel additive
References
[edit]- ^ U.S. Tire Manufacturers Association (July 15, 2021). "Statement of Sarah E. Amick Vice President EHS&S and Senior Counsel U.S. Tire Manufacturers Association". Committee on Natural Resources Subcommittee on Oversight and Investigations United States House of Representatives.
- ^ Krüger, R H; Boissiére, C; Klein-Hartwig, K; Kretzschmar, H-J (2005). "New phenylenediamine antiozonants for commodities based on natural and synthetic rubber". Food Addit Contam. 22 (10): 968–974. doi:10.1080/02652030500098177. PMID 16227180. S2CID 10548886.
- ^ Ashworth, B. T.; Hill, P. (1979). "Chapter 7: Protective Agents". Developments in Rubber Technology, Volume 1. London: Applied Science Publishers. pp. 227–239. ISBN 0853348626.
- ^ Tian, Zhenyu; Zhao, Haoqi; Peter, Katherine T.; Gonzalez, Melissa; Wetzel, Jill; Wu, Christopher; Hu, Ximin; Prat, Jasmine; Mudrock, Emma; Hettinger, Rachel; Cortina, Allan E.; Biswas, Rajshree Ghosh; Kock, Flávio Vinicius Crizóstomo; Soong, Ronald; Jenne, Amy; Du, Bowen; Hou, Fan; He, Huan; Lundeen, Rachel; Gilbreath, Alicia; Sutton, Rebecca; Scholz, Nathaniel L.; Davis, Jay W.; Dodd, Michael C.; Simpson, Andre; McIntyre, Jenifer K.; Kolodziej, Edward P. (8 January 2021). "A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon". Science. 371 (6525): 185–189. Bibcode:2021Sci...371..185T. doi:10.1126/science.abd6951. PMID 33273063.
- ^ Chadwick, Julie (12 July 2024). "How researchers uncovered the coho killing tire toxin 6PPD-Q". The Discourse. Retrieved 26 July 2024.
- ^ Hans-Wilhelm Engels et al., "Rubber, 4. Chemicals and Additives" in Ullmann's Encyclopedia of Industrial Chemistry, 2007, Wiley-VCH, Weinheim. doi:10.1002/14356007.a23_365.pub2
- ^ Lattimer, R. P.; Hooser, E. R.; Layer, R. W.; Rhee, C. K. (1 May 1983). "Mechanisms of Ozonation of N-(1,3-Dimethylbutyl)-N′-Phenyl-p-Phenylenediamine". Rubber Chemistry and Technology. 56 (2): 431–439. doi:10.5254/1.3538136.
- ^ Cataldo, Franco; Faucette, Brad; Huang, Semone; Ebenezer, Warren (January 2015). "On the early reaction stages of ozone with N,N′-substituted p-phenylenediamines (6PPD, 77PD) and N,N′,N"-substituted-1,3,5-triazine "Durazone®": An electron spin resonance (ESR) and electronic absorption spectroscopy study". Polymer Degradation and Stability. 111: 223–231. doi:10.1016/j.polymdegradstab.2014.11.011.
- ^ Cataldo, Franco (January 2018). "Early stages of p-phenylenediamine antiozonants reaction with ozone: Radical cation and nitroxyl radical formation". Polymer Degradation and Stability. 147: 132–141. doi:10.1016/j.polymdegradstab.2017.11.020.
- ^ Seiwert, Bettina; Nihemaiti, Maolida; Troussier, Mareva; Weyrauch, Steffen; Reemtsma, Thorsten (April 2022). "Abiotic oxidative transformation of 6-PPD and 6-PPD quinone from tires and occurrence of their products in snow from urban roads and in municipal wastewater". Water Research. 212 118122. Bibcode:2022WatRe.21218122S. doi:10.1016/j.watres.2022.118122. PMID 35101694. S2CID 246336931.
- ^
Tian, Zhenyu; Zhao, Haoqi; Peter, Katherine T.; Gonzalez, Melissa; Wetzel, Jill; Wu, Christopher; Hu, Ximin; Prat, Jasmine; Mudrock, Emma; Hettinger, Rachel; Cortina, Allan E.; Biswas, Rajshree Ghosh; Kock, Flávio Vinicius Crizóstomo; Soong, Ronald; Jenne, Amy; Du, Bowen; Hou, Fan; He, Huan; Lundeen, Rachel; Gilbreath, Alicia; Sutton, Rebecca; Scholz, Nathaniel L.; Davis, Jay W.; Dodd, Michael C.; Simpson, Andre; McIntyre, Jenifer K. (3 December 2020), "A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon", Science, 371 (6525): 185–189, Bibcode:2021Sci...371..185T, doi:10.1126/science.abd6951, PMID 33273063, S2CID 227281491,
... existing TWP [tire wear particle] loading, leaching, and toxicity assessments are clearly incomplete. ... Accordingly, the human health effects of such exposures merit evaluation. ... It is unlikely that coho salmon are uniquely sensitive ... ( in print 8 Jan 2021)
- ^ Also an erratum to this paper published in Science vol. 375, No. 6582, 18 Feb 2022 doi:10.1126/science.abo5785 reporting the updated toxicity estimates, as referenced below.
- ^ Zeng, Lixi; Li, Yi; Sun, Yuxin; Liu, Liang-Ying; Shen, Mingjie; Du, Bibai (31 January 2023). "Widespread Occurrence and Transport of p -Phenylenediamines and Their Quinones in Sediments across Urban Rivers, Estuaries, Coasts, and Deep-Sea Regions". Environmental Science & Technology. 57 (6): 2393–2403. Bibcode:2023EnST...57.2393Z. doi:10.1021/acs.est.2c07652. PMID 36720114. S2CID 256458111.
- ^ "Pollution from car tires is killing off salmon on US west coast, study finds". The Guardian. 3 December 2020.
- ^ "Scientists solve mystery of mass coho salmon deaths. The killer? A chemical from car tires". Los Angeles Times. 3 December 2020.
- ^ Johannessen, Cassandra; Helm, Paul; Lashuk, Brent; Yargeau, Viviane; Metcalfe, Chris D. (February 2022). "The Tire Wear Compounds 6PPD-Quinone and 1,3-Diphenylguanidine in an Urban Watershed". Archives of Environmental Contamination and Toxicology. 82 (2): 171–179. Bibcode:2022ArECT..82..171J. doi:10.1007/s00244-021-00878-4. PMC 8335451. PMID 34347118.
- ^ a b Markus Brinkmann; David Montgomery; Summer Selinger; Justin G. P. Miller; Eric Stock (2022-03-02), "Acute Toxicity of the Tire Rubber-Derived Chemical 6PPD-quinone to Four Fishes of Commercial, Cultural, and Ecological Importance", Environmental Science & Technology Letters, vol. 9, no. 4, pp. 333–338, Bibcode:2022EnSTL...9..333B, doi:10.1021/acs.estlett.2c00050, S2CID 247336687
- ^ Tian, Zhenyu; Gonzalez, Melissa; Rideout, Craig; Zhao, Hoaqi Nina; Hu, Ximin; Wetzel, Jill; Mudrock, Emma; James, C. Andrew; McIntyre, Jenifer K; Kolodziej, Edward P (11 January 2022), "6PPD-Quinone: Revised Toxicity Assessment and Quantification with a Commercial Standard", Environmental Science & Technology Letters, 9 (2): 140–146, Bibcode:2022EnSTL...9..140T, doi:10.1021/acs.estlett.1c00910, S2CID 245893533
- ^ Montgomery, David; Ji, Xiaowen; Cantin, Jenna; Philibert, Danielle; Foster, Garrett; Selinger, Summer; Jain, Niteesh; Miller, Justin; McIntyre, Jenifer; de Jourdan, Benjamin; Wiseman, Steve; Hecker, Markus; Brinkmann, Markus (19 December 2023). "Interspecies Differences in 6PPD-Quinone Toxicity Across Seven Fish Species: Metabolite Identification and Semiquantification". Environmental Science & Technology. 57 (50): 21071–21079. Bibcode:2023EnST...5721071M. doi:10.1021/acs.est.3c06891. PMID 38048442. S2CID 265658590.
- ^ Lena Beck (17 May 2022). "Your car is killing coho salmon". The Counter.
- ^ Klauschies, Toni; Isanta-Navarro, Jana (2022-07-10). "The joint effects of salt and 6PPD contamination on a freshwater herbivore" (PDF). Science of the Total Environment. 829 154675. Bibcode:2022ScTEn.82954675K. doi:10.1016/j.scitotenv.2022.154675. PMID 35314241. S2CID 247577987 – via Dynatrait.
- ^ Du, Bibai; Liang, Bowen; Li, Yi; Shen, Mingjie; Liu, Liang-Ying; Zeng, Lixi (13 December 2022). "First Report on the Occurrence of N -(1,3-Dimethylbutyl)- N ′-phenyl- p -phenylenediamine (6PPD) and 6PPD-Quinone as Pervasive Pollutants in Human Urine from South China". Environmental Science & Technology Letters. 9 (12): 1056–1062. Bibcode:2022EnSTL...9.1056D. doi:10.1021/acs.estlett.2c00821. S2CID 253828438.
- ^ Agua, Alon; Stanton, Ryan; Pirrung, Michael (2021-02-04). "Preparation of 2-((4-Methylpentan-2-Yl)amino)-5-(Phenylamino)cyclohexa-2,5-Diene-1,4-Dione (6PPD-Quinone), an Environmental Hazard for Salmon" (PDF). ChemRxiv. doi:10.26434/chemrxiv.13698985.v1. S2CID 234062284.