6PPD is an

organic chemical In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The st ...

widely used as stabilising additive (or antidegradant) in rubbers, such as NR, SBR and BR; all of which are common in vehicle tires. Although it is an effective

antioxidant Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricant ...

it is primarily used because of its excellent

antiozonant An antiozonant, also known as anti-ozonant, is an organic compound that prevents or retards damage caused by ozone. The most important antiozonants are those which prevent degradation of elastomers like rubber. A number of research projects study ...

peformance. It is one of several antiozonants based around ''p''-phenylenediamine (PPD).

Manufacturing

6PPD is prepared by

reductive amination Reductive amination (also known as reductive alkylation) is a form of amination that involves the conversion of a carbonyl group to an amine via an intermediate imine. The carbonyl group is most commonly a ketone or an aldehyde. It is considered ...

methyl isobutyl ketone Methyl isobutyl ketone (MIBK) is the common name for the organic compound 4-methylpentan-2-one, condensed chemical formula (CH3)2CHCH2C(O)CH3. This colourless liquid, a ketone, is used as a solvent for gums, resins, paints, varnishes, lacquers, and ...

with 4-aminodiphenylamine. This produces a

racemic In chemistry, a racemic mixture, or racemate (), is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds are produced industrially as racemates. ...

mixture.

Application

6PPD is a common rubber antiozonant, with a major application in vehicle tires. It is mobile within the rubber and is slowly forced to the surface via

blooming Bloom or blooming may refer to: Science and technology Biology * Bloom, one or more flowers on a flowering plant * Algal bloom, a rapid increase or accumulation in the population of algae in an aquatic system * Jellyfish bloom, a collective n ...

. Here it forms a "scavenger-protective film", reacting with the ozone more quickly that the ozone can react with the rubber. This process initially forms

aminoxyl radical Aminoxyl denotes a Radical (chemistry), radical functional group with general structure R2N–O•. It is commonly known as a nitroxyl radical or a nitroxide, however IUPAC discourages the use of these terms, as they erroneously suggest the presenc ...

s and was thought to stop at the quinone diimine, but is now known to continue to form

quinone The quinones are a class of organic compounds that are formally "derived from aromatic compounds

uch as benzene or naphthalene Uch ( pa, ; ur, ), frequently referred to as Uch Sharīf ( pa, ; ur, ; ''"Noble Uch"''), is a historic city in the southern part of Pakistan's Punjab province. Uch may have been founded as Alexandria on the Indus, a town founded by Alexand ...

by conversion of an even number of –CH= groups into –C(=O)– groups with any necessary rearrangement of double ...

s, amongst other products. Despite 6PPD being used in tires since the mid 1960s, this transformation to quinones was first identified in 2020. These oxidised products are not effective antiozonants, meaning that 6PPD is a sacrificial agent. : 6PPD ozone-oxidation overview

The tendency of 6PPD to bloom towards the surface is beneficial for the item, as the surface film of antiozonant is replenished from reserves held within the rubber. However, this same property facilitates the transfere of 6PPD and its oxidation products into the environment as tire-wear compounds. The 6PPD-quinone (6PPD-Q, CAS:2754428-18-5) is of particular and increasing concern, due to its effect on 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 Urban runoff is surface runoff of rainwater, landscape irrigation, and car washing created by urbanization. Impervious surfaces (roads, parking lots and sidewalks) are constructed during land development. During rain , storms and other precipit ...

. From here they become widely distributed (at decreasing levels) from urban rivers through to estuaries, coasts and finally deep-sea areas. 6PPD-quinone is of environmental concern, as it is toxic to

coho salmon The coho salmon (''Oncorhynchus kisutch;'' Karuk: achvuun) is a species of anadromous fish in the salmon family and one of the five Pacific salmon species. Coho salmon are also known as silver salmon or "silvers". The scientific species name i ...

, killing them before they

spawn Spawn or spawning may refer to: * Spawn (biology), the eggs and sperm of aquatic animals Arts, entertainment, and media * Spawn (character), a fictional character in the comic series of the same name and in the associated franchise ** '' Spawn: Ar ...

in freshwater streams. A 2022 study also identified the toxic impact on species like

brook trout The brook trout (''Salvelinus fontinalis'') is a species of freshwater fish in the char genus ''Salvelinus'' of the salmon family Salmonidae. It is native to Eastern North America in the United States and Canada, but has been introduced elsewhere ...

and

rainbow trout The rainbow trout (''Oncorhynchus mykiss'') is a species of trout native to cold-water tributaries of the Pacific Ocean in Asia and North America. The steelhead (sometimes called "steelhead trout") is an anadromous (sea-run) form of the coasta ...

. The published lethal concentrations are: *

: LC₅₀ = 95 ng/L *

: LC₅₀ = 0.59 μg/L *

: LC₅₀ = 1.0 μg/L It is not known why the ozone-oxidised 6PPD is toxic to coho salmon. The Nisqually and nonprofit Long Live the Kings are trying out a mobile stormwater filter at a bridge in the Ohop Valley. The

Washington Department of Ecology The Washington State Department of Ecology (sometimes referred to simply as "Ecology") is the state of Washington's environmental regulatory agency. Created in February 1970, it was the first environmental regulation agency in the U.S. predating ...

Washington State University Washington State University (Washington State, WSU, or informally Wazzu) is a public land-grant research university with its flagship, and oldest, campus in Pullman, Washington. Founded in 1890, WSU is also one of the oldest land-grant unive ...

and the US Tire Manufacturer's Association are working on regulation and education. 6PPD itself is deadly to

rotifer The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John H ...

s, especially in combination with

sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...

, though not at the level generally found in the

runoff Runoff, run-off or RUNOFF may refer to: * RUNOFF, the first computer text-formatting program * Runoff or run-off, another name for bleed, printing that lies beyond the edges to which a printed sheet is trimmed * Runoff or run-off, a stock market ...

from road salt. A small-scale

biomonitoring In analytical chemistry, biomonitoring is the measurement of the body burden of toxic chemical compounds, elements, or their metabolites, in biological substances. Often, these measurements are done in blood and urine. Biomonitoring is performed ...

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. A synthetic route to the 6PPD-quinone has been posted on

ChemRxiv ChemRxiv (pronounced "chem archive"—the X represents the Greek letter chi � is an open access preprint archive for chemistry. It is operated by the American Chemical Society, Royal Society of Chemistry and German Chemical Society. The new pre ...

References

Anilines Antioxidants Plastics additives Water pollution {{Organic-compound-stub

Manufacturing

Application

Environmental impact

See also

References