6PPD explained

6PPD is an organic chemical widely used as stabilising additive (or antidegradant) in rubbers, such as NR, SBR and BR; 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] It has been the subject of increasing scrutiny since 2020, when it was determined that its oxidation product (6PPD-Q) causes pre-spawn mortality in coho salmon.[4] [5]

Manufacturing

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

6PPD is a common rubber antiozonant, with major application in vehicle tires. It is mobile within the rubber and slowly migrates to the surface via blooming. Here it forms a "scavenger-protective film", reacting 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 1960s, its transformation to quinones was first recognized in 2020.[11] 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

6PPD and 6PPD-quinone enter the environment through tire-wear 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 to estuaries, coasts and finally deep-sea areas.[12]

6PPD-quinone is of environmental concern because it is toxic to coho salmon, killing them before they spawn in freshwater streams.[13] [14] [15]

A 2022 study also identified the toxic impact on species like brook trout and rainbow trout.The published lethal concentrations are:

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.[16] 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.[17]

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.[18] A small-scale biomonitoring study in South China has shown shown both 6PPD and 6PPDQ to be present in human urine; concentrations were low but the health implications are unknown.[19] A synthetic route to the 6PPD-quinone has been posted on ChemRxiv.[20]

See also

Notes and References

  1. Web site: U.S. Tire Manufacturers Association . 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 . July 15, 2021.
  2. Food Addit Contam . 2005 . 22 . 10 . 968–974 . New phenylenediamine antiozonants for commodities based on natural and synthetic rubber . R H . Krüger . C . Boissiére . K . Klein-Hartwig . H-J . Kretzschmar . 16227180 . 10.1080/02652030500098177 . 10548886 .
  3. Book: Ashworth . B. T. . Hill . P. . Developments in Rubber Technology, Volume 1 . 1979 . Applied Science Publishers . London . 0853348626 . 227–239 . https://books.google.com/books?id=Sa8Kq50AkvgC&dq=6PPD&pg=PA227 . Chapter 7: Protective Agents.
  4. 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. . A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon . Science . 8 January 2021 . 371 . 6525 . 185–189 . 10.1126/science.abd6951. 33273063 . 2021Sci...371..185T .
  5. News: Chadwick . Julie . How researchers uncovered the coho killing tire toxin 6PPD-Q . 26 July 2024 . The Discourse. . 12 July 2024 . en-CA.
  6. Hans-Wilhelm Engels et al., "Rubber, 4. Chemicals and Additives" in Ullmann's Encyclopedia of Industrial Chemistry, 2007, Wiley-VCH, Weinheim.
  7. Lattimer . R. P. . Hooser . E. R. . Layer . R. W. . Rhee . C. K. . Mechanisms of Ozonation of N-(1,3-Dimethylbutyl)-N′-Phenyl-p-Phenylenediamine . Rubber Chemistry and Technology . 1 May 1983 . 56 . 2 . 431–439 . 10.5254/1.3538136.
  8. Cataldo . Franco . Faucette . Brad . Huang . Semone . Ebenezer . Warren . 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 . January 2015 . 111 . 223–231 . 10.1016/j.polymdegradstab.2014.11.011.
  9. Cataldo . Franco . Early stages of p-phenylenediamine antiozonants reaction with ozone: Radical cation and nitroxyl radical formation . Polymer Degradation and Stability . January 2018 . 147 . 132–141 . 10.1016/j.polymdegradstab.2017.11.020.
  10. Seiwert . Bettina . Nihemaiti . Maolida . Troussier . Mareva . Weyrauch . Steffen . Reemtsma . Thorsten . 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 . April 2022 . 212 . 118122 . 10.1016/j.watres.2022.118122. 35101694 . 2022WatRe.21218122S . 246336931 . free .
  11. Also an erratum to this paper published in Science vol. 375, No. 6582, 18 Feb 2022 reporting the updated toxicity estimates, as referenced below.
  12. Zeng . Lixi . Li . Yi . Sun . Yuxin . Liu . Liang-Ying . Shen . Mingjie . Du . Bibai . Widespread Occurrence and Transport of p -Phenylenediamines and Their Quinones in Sediments across Urban Rivers, Estuaries, Coasts, and Deep-Sea Regions . Environmental Science & Technology . 31 January 2023 . 57 . 6 . 2393–2403 . 10.1021/acs.est.2c07652. 36720114 . 2023EnST...57.2393Z . 256458111 .
  13. News: Pollution from car tires is killing off salmon on US west coast, study finds. The Guardian. 3 December 2020.
  14. News: Scientists solve mystery of mass coho salmon deaths. The killer? A chemical from car tires. Los Angeles Times. 3 December 2020.
  15. Johannessen . Cassandra . Helm . Paul . Lashuk . Brent . Yargeau . Viviane . Metcalfe . Chris D. . The Tire Wear Compounds 6PPD-Quinone and 1,3-Diphenylguanidine in an Urban Watershed . Archives of Environmental Contamination and Toxicology . February 2022 . 82 . 2 . 171–179 . 10.1007/s00244-021-00878-4. 34347118 . 8335451 . 2022ArECT..82..171J .
  16. 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 . Interspecies Differences in 6PPD-Quinone Toxicity Across Seven Fish Species: Metabolite Identification and Semiquantification . Environmental Science & Technology . 19 December 2023 . 57 . 50 . 21071–21079 . 10.1021/acs.est.3c06891. 38048442 . 2023EnST...5721071M . 265658590 .
  17. News: Your car is killing coho salmon. The Counter. Lena Beck. 17 May 2022.
  18. Klauschies . Toni . Isanta-Navarro . Jana . 2022-07-10 . The joint effects of salt and 6PPD contamination on a freshwater herbivore . Science of the Total Environment . en . 829 . 154675 . 10.1016/j.scitotenv.2022.154675 . 35314241 . 2022ScTEn.82954675K . 247577987 . Dynatrait.
  19. Du . Bibai . Liang . Bowen . Li . Yi . Shen . Mingjie . Liu . Liang-Ying . Zeng . Lixi . 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 . 13 December 2022 . 9 . 12 . 1056–1062 . 10.1021/acs.estlett.2c00821. 2022EnSTL...9.1056D . 253828438 .
  20. 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. ChemRxiv. en-US. 10.26434/chemrxiv.13698985.v1. 234062284 .

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