6PPD

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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. 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. 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). This produces a racemic mixture. 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. This process forms aminoxyl radicals 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.