PPD is short for Phenylenediamine.
The sequence of events that led to its discovery began in 1856, when William Perkin, a student at the Royal College of Chemistry (now Imperial College London) conducted a series of experiments at the behest of the college’s director, August Wilhelm von Hoffman. Hoffman was trying to synthesise an alternative to the expensive malaria treatment quinine, and to this end Perkin carried out his research. After one failed attempt, in which he oxidised a compound called aniline, Perkin swilled out his flask with alcohol to clean it, and noticed the reaction created a substance with an intense purple colour. A refined version of this substance, which Perkin called mauveine, became the first synthetic dye.
Professor Hoffman (who was presumably fairly relaxed about his student’s distraction from the quinine project!) worked with Perkin’s discovery and in turn derived the molecule called para-phenylenediamine, which darkens on exposure to air.
It wasn’t until 1907 that French chemist Eugène Schueller utilised PPD in a hair dye - since when it has been a staple of the hair colourant industry. It proved popular because it was long-lasting and imparted a natural look to dyed hair.
PPD, though, has many other uses. It can be employed as a Henna substitute in temporary tattoos, as a staining agent in microscopy, and even as an aid to the identification of lichens. A form of PPD is used in the photographic development process and a derivative is used in tyre manufacture (and in other rubber and plastic products) to protect against the effects of ozone.