p-Phenylenediamine (PPD): What It Is and Why It's Used

Origin

p-Phenylenediamine was synthesised in the 1870s and its use as a hair dye was established before the end of the 19th century — it was among the first synthetic organic dyes applied to hair. Its capacity to cause allergic contact dermatitis was documented almost as early, and regulatory attention to it dates back to the early 20th century. It remains the primary oxidative dye intermediate in most permanent and semi-permanent hair colour products.

Why It's Used

PPD works through an oxidative coupling reaction: in the presence of an oxidant (typically hydrogen peroxide), it combines with a coupler to produce large, coloured molecules that become trapped in the hair shaft. The resulting colour is permanent until the hair grows out. The same oxidative chemistry that produces the colour is the mechanism of its allergenic potential — PPD is one of the most common causes of allergic contact dermatitis from personal care products.

Elsewhere

p-Phenylenediamine and its derivatives are used in the production of polymers, including aramid fibres such as Kevlar. Related amine dye chemistry is used in the textile and leather industries.

What the Framework Sees

a regulatory mechanism genuinely different from anything else in this series.

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