Compound Interest: The Chemistry of Permanent Hair Dyes (2024)

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Today, hair dyes are widely used, either to cover up grey hairs, or simply by those wanting to change their natural hair colour. The chemistry behind how they change the colour of hair can actually get pretty complicated, but this graphic tries to boil it down to the key classes of chemicals involved, and an overview of the process that produces the dye molecules.

Before talking about the chemistry of hair dyes, it’s worth giving a mention to the molecules that cause our hair to be coloured in the first place. These are pigments called melanins, of which there are two types: eumelanin, and pheomelanin. Eumelanin causes shades ranging from brown to black, whilst pheomelanin gives colours in the range of blond to red. Darker hair, therefore, contains more eumelanin; generally, eumelanin is often the more abundant of the two types, though red hair contains primarily pheomelanin. Differing hair colours are merely the consequence of different balances in concentrations of these two pigments, and blond hair is often aresult of a low concentration of melanin in general.

There are a number of ways of dyeing hair, but we’ll be examining permanent (oxidative) dyes. These dyesare based on an observation made over 150 years ago, that a chemical called paraphenylenediamine (PPD) produces brown shades when exposed to oxidising agents. Today, PPD is still one of the primary chemicals used in hair dyes as a primary intermediate, and, despite a number of online claims to the contrary, its use in these dyes is not banned in any country, though its levels are regulated. Its ubiquity, along with that of similar related compounds, is due to the fact that even in the 150 years since the discovery of its potential, we’ve still yet to find any better alternative for permanent hair dyeing. The PPD derivative 2,5-diaminotoluene orp-aminophenols are also sometimes used as alternative primary intermediates.

Primary intermediatesonly produce dye molecules when exposed to an oxidising agent. For this reason, hydrogen peroxide is included in almost all hair dyes. Hydrogen peroxide is a strong oxidising agent, and can oxidise the natural melanin pigments in hair, removing some of the conjugated double bonds that lead to their colour, and making their moleculescolourless. More commonly, of course, we refer to this as bleaching the hair. The peroxide also oxidises the primary intermediate molecules, producing a reactive species which can then go on an react and form the dye molecules.

Of course, just being able to dye hair a dark brown colour wouldn’t be all that much use if dark brown isn’t the colour you’re after. For this reason, other compounds are also added into the hair dye mixture. Known as couplers or coupling agents, whilst these compounds are not coloured themselves, they can react with molecules of theprimary intermediates to produce a range of different coloured dyes. Hair dyes will typically include a range of different couplers in varying concentrations in order to achieve the exact shade required, so it’s rarely the case of it being one particular dye that causes the hair colour, but a mixture. The couplers are broadly grouped into three categories: blue couplers, red couplers, and green couplers.

The reactions which produce the dyes are carried out at an alkaline pH, and in many cases this is provided by the presence of ammonia in the formulation. The ammonia causes the cuticles of the hair to swell, which then allows the dye molecules to pass into the hair and induce permanent colouring. This process can, however, damage the hair, particularly if you frequently dye it. For this reason, many companies have produced ammonia-free hair dyes, using substitutes such as ethanolamine. This is a milder agent, but also doesn’t cause the cuticle to swell as much as ammonia, meaning it has a few aesthetic drawbacks: it often washes out after a certain time period, unlike permanent colourings which merely grow out, and isn’t as effective at lightening hair.

If you’re a regular user of permanent hair dyes, you might be wondering if there are any health concerns surrounding them. Some componentsin hair dyes have been categorised as ‘sensitisers’ – that is, that after an initial exposure, a repeat exposure could lead to an allergic reaction. This is the reason that some hair dyes recommendperforming an allergy test 48 hours before applying the formulationto the hair, in order to guard against severe allergic reactions. Whilst life-endangering reactions are rare, they are not unheard of – only this month, a woman experiencedpotentiallydeadlyanaphylaxis, and another woman died in 2011 after a similar reaction.

Cancer risk from some of the chemicals in hair dyes has also been debated. Whilst studies have yet to establish a strong link, somestudies looking at people who use hair dyes regularly at work discovered a correlation with a minor increase in rates of bladder cancer. As a result, they recommend that gloves are always used when applying the dyes.Studies on those who merely dye their own hair at home found no relation between development of bladder cancer and use of the dyes, though future studies may help to clarify whether this also poses a minor risk. If you do use hair dyes,gloves should be used, and the formulation shouldn’t be left on the hair for longer than the recommended time period.

There’s a lot more to the complex chemistry of hair dyes – there’s an excellent review paper on the matter here. I’m also grateful to Rebecca Guenard for her helpwith the research forthis graphic. You can read her in-depth discussion of the chemistry and associated risks of hair dyes for Mosaic Science here.

Note: the graphic originally showed m-phenylenediamine as a couplerthat could produce a dye giving a blue-black colouration. Whilst this is correct, m-phenylenediamine has since been phased out for toxicological reasons. It’s been replaced in this updated version of the graphic.

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Compound Interest: The Chemistry of Permanent Hair Dyes (2024)

FAQs

What is the chemical formula for permanent hair dye? ›

The majority of permanent hair dyes are a blend of various chemicals. Paraphenylenediamine (PPD) is the main chemical. Formula is C6H4(NH2)2.

What is the compound in hair dye? ›

To achieve a permanent color, many hair dyes work using a system of ammonia (or ethanolamines in the case of some ammonia-free products), hydrogen peroxide, and p-phenylenediamine. The ammonia pulls apart layers of the hair's proteins, so that the dye can access the hair shaft.

How does permanent hair dye work chemistry? ›

Colorful reaction. Hair color chemists create permanent dyes by starting with p-phenylenediamine (PPD)or a related primary dye precursor. When oxidized with hydrogen peroxide, PPD forms quinonediimine, which is then reacted with a coupler, such as resorcinol, to produce a colorless leuco-dye intermediate.

What is the composition of permanent hair dye? ›

Permanent hair dyes require three components: (1) precursor agents, that is, primary intermediates comprised of ortho- (o-) and para- (p-) aromatic amines substituted with amino groups and/or hydroxides; (2) coupling agents that are formed by aromatic compounds meta- (m-) substituted with electron-donating groups, such ...

What makes permanent hair dye permanent? ›

Because permanent hair color modifies the structure of your hair, it lasts until you re-dye or cut your hair. It won't wash out with a few shampoos, and it won't fade with time.

What type of chemical reaction is hair dye? ›

The chemical ingredients of hair dyes vary among formulations that involve oxidative reactions to achieve coloring and those using nonoxidative processes. Temporary and semipermanent hair dyes typically rely on nonoxidative processes, while permanent hair dyes rely on oxidative reactions (Table 1).

What molecules are in permanent hair color? ›

Permanent hair coloring

The dyes used are actually dye precursors. These small molecules are monomers which are able to penetrate into the hair all the way to the cortext. Common compounds used include p-phenylenediamine and p-aminophenol.

What is the process of permanently coloring hair? ›

Ammonia and hydrogen peroxide act together to open up the cuticle, interact with melanin and keratin (those are responsible for color and texture), and change the structure in order to deposit dye. Permanent hair color can lighten, darken, or change the tone of your hair; and it'll last about six to eight weeks.

What is the chemistry behind hair? ›

The overall chemical composition of hair is 45 % carbon, 28 % oxygen, 15 % nitrogen, 7 % hydrogen and 5 % sulphur. The hair shaft is essentially composed of keratin. Hair keratin is hard, compact and strong.

What does permanent hair dye contain? ›

Permanent hair color contains ammonia and is mixed with developer in various volumes in order to permanently change hair color. The ammonia is an alkalizer that opens the cuticle layer, allowing the color molecules and developer to penetrate into the cortex.

What are the worst chemicals in hair dye? ›

Some harmful chemicals found in hair dye include ammonia, hydrogen peroxide, and parabens. These chemicals can cause scalp irritation, hair damage, and in some cases, allergic reactions. It's important to choose hair dye products that are free from these toxic ingredients for safer hair coloring options.

What is the chemical composition of dye? ›

In addition to chromophores, most dyes also contain groups known as auxochromes (colour helpers), examples of which are carboxylic acid, sulfonic acid, amino, and hydroxyl groups.

What is the chemical formula for hair? ›

The overall chemical composition of hair is 45 % carbon, 28 % oxygen, 15 % nitrogen, 7 % hydrogen and 5 % sulphur. The hair shaft is essentially composed of keratin. Hair keratin is hard, compact and strong.

What is the PPD chemical in hair dye? ›

Many permanent and some semi-permanent hair dyes contain a chemical called paraphenylenediamine (PPD), which is a known irritant and allergen. Darker coloured dyes contain higher level of PPD. PPD is the cause of most reactions to hair dye.

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