Chemistry:6PPD

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Short description: Organic chemical used in rubber tires
6PPD
6PPD skeletal.svg
Names
Preferred IUPAC name
N1-(4-Methylpentan-2-yl)-N4-phenylbenzene-1,4-diamine
Other names
N-(1,3-dimethylbutyl)-N'-phenyl-1,4-benzenediamine
  • N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine
  • 6PPD
  • HPPD
  • DMBPPD
Identifiers
3D model (JSmol)
ChEMBL
ChemSpider
EC Number
  • 212-344-0
UNII
UN number 3077
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 pictograms GHS07: HarmfulGHS08: Health hazardGHS09: Environmental hazard
GHS Signal word Danger
H302, H317, H360, H410
P201, P202, P261, P264, P270, P272, P273, P280, P281, P301+312, P302+352, P308+313, P321, P330, P333+313, 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).
Infobox references

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]

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).[3] 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.[4] This process forms aminoxyl radicals[5][6] 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.[7] Despite 6PPD being used in tires since the mid 1960s, its transformation to quinones was first recognized in 2020.[8][9] The oxidized products are not effective antiozonants, meaning that 6PPD is a sacrificial agent.

6PPD ozone-oxidation overview.svg

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.[10]

6PPD-quinone is of environmental concern because it is toxic to coho salmon, killing them before they spawn in freshwater streams.[11][12][13]

A 2022 study also identified the toxic impact on species like brook trout and rainbow trout.[14] The published lethal concentrations are:[14][15]

  • 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.[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

References

  1. ↑ "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. https://naturalresources.house.gov/download/2021_07_15_written_testimony_sarah-amickpdf. 
  2. ↑ 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. 
  3. ↑ 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
  4. ↑ 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. 
  5. ↑ 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. 
  6. ↑ 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. 
  7. ↑ 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. doi:10.1016/j.watres.2022.118122. PMID 35101694. Bibcode: 2022WatRe.21218122S. 
  8. ↑ Tian, Zhenyu; Zhao, Haoqi; Peter, Katherine T.; Gonzalez, Melissa; Wetzel, Jill; Wu, Christopher; Hu, Ximin; Prat, Jasmine et al. (3 December 2020), "A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon", Science 371 (6525): 185–189, doi:10.1126/science.abd6951, PMID 33273063, "... 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)" 
  9. ↑ 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.
  10. ↑ 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. doi:10.1021/acs.est.2c07652. PMID 36720114. Bibcode: 2023EnST...57.2393Z. 
  11. ↑ "Pollution from car tires is killing off salmon on US west coast, study finds". The Guardian. 3 December 2020. https://www.theguardian.com/environment/2020/dec/03/coho-salmon-pollution-car-tires-die-off. 
  12. ↑ "Scientists solve mystery of mass coho salmon deaths. The killer? A chemical from car tires". Los Angeles Times. 3 December 2020. https://www.latimes.com/california/story/2020-12-03/coho-salmon-tire-chemical. 
  13. ↑ 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. doi:10.1007/s00244-021-00878-4. PMID 34347118. Bibcode: 2022ArECT..82..171J. 
  14. ↑ 14.0 14.1 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 9 (4): 333–338, doi:10.1021/acs.estlett.2c00050, Bibcode: 2022EnSTL...9..333B 
  15. ↑ Tian, Zhenyu; Gonzalez, Melissa; Rideout, Craig; Zhao, Hoaqi Nina; Hu, Ximin; Wetzel, Jill; Mudrock, Emma; James, C. Andrew et al. (11 January 2022), "6PPD-Quinone: Revised Toxicity Assessment and Quantification with a Commercial Standard", Environmental Science & Technology Letters 9 (2): 140–146, doi:10.1021/acs.estlett.1c00910, Bibcode: 2022EnSTL...9..140T 
  16. ↑ Montgomery, David; Ji, Xiaowen; Cantin, Jenna; Philibert, Danielle; Foster, Garrett; Selinger, Summer; Jain, Niteesh; Miller, Justin et al. (19 December 2023). "Interspecies Differences in 6PPD-Quinone Toxicity Across Seven Fish Species: Metabolite Identification and Semiquantification". Environmental Science & Technology 57 (50): 21071–21079. doi:10.1021/acs.est.3c06891. PMID 38048442. Bibcode: 2023EnST...5721071M. 
  17. ↑ Lena Beck (17 May 2022). "Your car is killing coho salmon". The Counter. https://thecounter.org/coho-salmon-pacific-northwest-car-tire-toxins-nisqually-tribe-research/. 
  18. ↑ Klauschies, Toni; Isanta-Navarro, Jana (2022-07-10). "The joint effects of salt and 6PPD contamination on a freshwater herbivore" (in en). Science of the Total Environment 829: 154675. doi:10.1016/j.scitotenv.2022.154675. PMID 35314241. Bibcode: 2022ScTEn.829o4675K. https://www.dynatrait.de/fileadmin/projects/dynatrait/PDF/2022_Klauschies_and_Isanta-Navarro_STOTEN.pdf. 
  19. ↑ 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. doi:10.1021/acs.estlett.2c00821. Bibcode: 2022EnSTL...9.1056D. 
  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" (in en-US). ChemRxiv. doi:10.26434/chemrxiv.13698985.v1. https://chemrxiv.org/engage/api-gateway/chemrxiv/assets/orp/resource/item/60c754ab842e658615db41c8/original/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. 



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