PPD, or p-Phenylenediamine is an organic compound. It’s a crystalline solid which darkens on exposure to the air. This process is known as oxidation and it’s critical to the hair colouring process.
The way dye colours hair varies depending on the type of dye - see The Different Types of Hair Colourant- here we’ll look at permanent colour.
Permanent hair dye consists of a group of different components which ingeniously work together to colour hair. These are the dye (the PPD), an alkaline (usually ammonia), an oxidising agent or developer (usually hydrogen peroxide) and colour couplers.
The hydrogen peroxide has a dual role: oxidising both melanin (in order to strip the colour from dark hair - necessary before re-colouring it) and the PPD, changing its state to allow it to next react with the colour couplers, which dictate the specific shades that result from the completed process. Because of the oxidising effect of hydrogen peroxide on PPD, these elements need to be kept apart until the colouring process begins.
The PPD, though, can happily be mixed with the ammonia before the process. This solution acts as a kind of door opener, the ammonia raising the pH value of the cuticle and causing it to swell. This in turn allows the various dye molecules and hydrogen peroxide to enter the cortex of the hair, where they form new, larger molecules which can’t be washed out. It’s these larger, ‘stuck’ molecules that make the dye permanent.