11.2 Oxidation & Dye Chemistry

Oxidation & Dye Chemistry

Quick Answer: Permanent hair colour works by an oxidation reaction: small dye intermediates (such as para-phenylenediamine, PPD) and couplers penetrate the cuticle, and hydrogen peroxide (the developer/oxidizer) triggers a reaction that joins them into larger colour molecules trapped inside the cortex. Four dye categories — temporary, semi-permanent, demi-permanent, and permanent — differ by whether they use a developer, whether they lift, and how long they last.

How Oxidation Dyes Work

Permanent and demi-permanent hair colours are oxidation dyes. They rely on a two-part chemical system:

1. Intermediates (also called primary intermediates or para dyes) — small, colourless molecules such as para-phenylenediamine (PPD) and related compounds (para-aminophenol, para-toluenediamine). These are small enough to penetrate through the cuticle into the cortex when the hair is in an alkaline state.
2. Couplers — colourless molecules that link with the oxidized intermediates to form larger, coloured indamine and indoaniline dye molecules. The combination of different intermediates with different couplers produces the full palette of colour tones (brown, ash, gold, red, violet).

The reaction only proceeds when an oxidizer is present. That oxidizer is hydrogen peroxide, supplied by the developer. The peroxide does two things at once:

• Oxidizes the intermediates so they can couple and form the colour molecules.
• Lifts (bleaches) natural melanin by releasing oxygen that breaks melanin pigment into smaller, colourless fragments.

Once the new colour molecules form inside the cortex, they are too large to wash out easily — which is why permanent colour lasts until the hair grows out or is recoloured.

The Four Dye Types

The exam expects you to distinguish four categories of hair colour by chemistry, longevity, and effect on the hair.

Temporary colour

Temporary colour is an acid stain. It uses large, pre-formed colour molecules that are too big to penetrate the cuticle, so they simply coat the outside of the hair shaft. No developer is used, no oxidation occurs, and there is no lift. The colour rinses out in 1-2 shampoos. It is used to refresh colour between services, for a temporary tone shift, or for fun/event colour.

Semi-permanent colour

Semi-permanent colour contains pre-formed dye molecules small enough to lightly penetrate just under the cuticle but not deep into the cortex. No developer is used, and there is no lift. It deposits only, so it cannot lighten hair — it can only darken, intensify, or tone. It gradually fades over roughly 4-6 weeks and is used to enhance natural colour, refresh faded ends, or tone without commitment. It does not effectively cover gray.

Demi-permanent colour

Demi-permanent colour uses oxidation dye chemistry (intermediates + couplers) with a low-volume developer — typically around 10 volume or a dedicated low-strength demi developer. The low peroxide volume is enough to oxidize and form colour molecules but not enough to lift natural pigment. Demi deposits colour deeper than semi and lasts longer, fading gradually over roughly 6-8 weeks. Because it does not lift, it does not produce a harsh line of demarcation as it fades, which is why it is the standard choice for blending gray (soft, natural-looking coverage without a regrowth line) and for toning and glossing.

Permanent colour

Permanent colour is the only category that both lifts natural pigment and deposits new colour in the same process. It uses oxidation dyes plus 20 volume or higher developer, which provides enough peroxide to oxidize the dye and to lighten melanin. Because it lifts and deposits at once, it can lighten natural hair, cover gray completely, and produce a permanent colour change that only disappears with regrowth or recolour. The trade-off is that the higher developer volume and alkaline pH cause more cuticle stress and cumulative damage with repeated use.

pH and the Cuticle

Hair colour and lightener products are formulated alkaline (pH above 7, commonly 9-11 for permanent colour and higher for lightener). The alkalinity — classically from ammonia (ammonium hydroxide) or a milder alkaline substitute such as monoethanolamine (MEA) — does two things:

1. Opens (lifts) the cuticle so the dye intermediates and peroxide can penetrate into the cortex.
2. Activates the peroxide to release oxygen and drive the oxidation reaction.

This is the same principle covered in Chapter 4: alkaline conditions open the cuticle, acidic conditions close and seal it. After any permanent or demi service the hair should be brought back toward its natural acidic pH (4.5-5.5) with an acidic conditioner or post-colour sealer to close the cuticle, lock in colour, and restore shine.

Why This Matters for the Exam

Red Seal questions on dye chemistry most often test:

• Which dye type uses a developer (demi, permanent) vs which does not (temporary, semi)
• Which dye type lifts (permanent only) vs which does not (temporary, semi, demi)
• Which dye type is appropriate for a given goal (gray coverage → permanent or demi; tone without lift → demi or semi; refresh between services → temporary or semi)
• The role of hydrogen peroxide (oxidizer + lifter)
• The role of ammonia/alkalinity (open cuticle + activate peroxide)

Know these five facts cold and you will answer the majority of the chemistry questions correctly.