Chemical Reaction : How dye works on your Hair?

From the youngest to the not so young, hair dyes have become very popular in all hairdressers. The hair coloring process has a lot to do with science and especially with chemistry. If you want to know how hair dyes work and the science behind these products, take a look at this article.

Hair dyes: how do they work?

Before we get into how hair dyes work, it is essential to understand the structure of the hair itself.

Each hair is made up of three parts:

Cuticle: It is the outermost layer of the hair and is made up of superimposed layers of keratinized cells in the form of scales. Its function is to protect the core of the hair.

Cortex: It is the middle layer of the hair. It contains the melanin that defines the color of the hair. This pigment can be of two types: eumelanin (gives dark color) or pheomelanin (light color). Depending on the proportion of each pigment that our hair has, it will be of one color or another. On the other hand, it is on this pigment that hair dyes act.

Medulla: it is the deepest part of the hair. However, in many cases this layer is non-existent.

All of this is covered by what we know as the F 18-MEA layer (18-methylicosenoic acid), whose hydrophobic nature prevents the hair from absorbing too much water, protects the hair and adds shine.

How do hair dyes work?

Mainly, there are two ways to color your hair: through permanent dyes or temporary dyes. Next, we will chemically explain how they differ and how each of these hair dyes works.

Permanent Hair Dyes

In order for the dye to change the color of our hair in a lasting way, it is necessary for it to penetrate the cortex, passing through the F layer and the cuticle. To achieve this, an alkaline medium such as ammonia or ethanolamine C2H7NO (MEA) is used.

Firstly, the alkaline medium increases the pH of the hair, in such a way that it breaks the disulfide bridges of the hair, allowing the hair dye to reach the cortex. Once the dye has gone through both layers, various chemical reactions can take place depending on the color change you want to achieve.

Permanent and Bleached Hair Dyes

When what you want is to go from dark hair to a much lighter tone, once the cuticle is open thanks to the change in pH, you have to bleach the hair. The chemical reaction that takes place in this case is oxidation. Hydrogen peroxide, that is, hydrogen peroxide is responsible for removing the pigment from the crust.

At this point, the hair is colorless and so you can choose to leave it that way or color the cortex any color you like, known as “oxidative coloring.” To re-pigment the cortex, the second step would be to add two chemical components: an activator such as Paraphenylenediamine (PPD) and the pigments.

In this second step, it must be taken into account that hydrogen peroxide acquires another function, since when it comes into contact with paraphenylenediamine (PPD), it reacts, achieving variations in the color of the dye. The dye in the dye then binds to the cortex due to its increase in molecular size.

After applying the dye mixture to the hair, an acid pH conditioner is added. This causes the new disulfide bridges that had been eliminated at the beginning with the alkaline medium to form.

Permanent hair Dyes and on Fade

On the other hand, if it is not intended to bleach the natural color of the hair itself, the amount of hydrogen peroxide used in the hair dye will be lower since it should not remove the pigment from the cortex. In this case, only the second step already explained would be carried out.

Semi-permanent or Temporary Hair Dyes

As we have already mentioned, there are different types of hair dyes on the market, including semi-permanent ones. Unlike permanent dyes that reach the cortex, semi-permanent hair dyes only penetrate the outer layer of the hair, the cuticle, and are washed out.

Although these hair dyes are not as long-lasting as permanent ones, they are less damaging to the hair. On the market you can find them both in spray and mask format and they can last one wash or even several weeks.

Natural Hair Dyes

On the other hand, many people prefer to opt for dyes of plant origin, the best known being henna and chamomile. In both cases, the reason why they add color to the hair is in chemistry.

Henna contains a chemical compound known as lawsone, which reacts with keratin, giving it a coppery hue. The dye does not penetrate the hair fiber, so it is lost as the keratinized cells regenerate. Even so, it can last up to six weeks.

In the case of chamomile, it is capable of slightly lightening hair thanks to apigenin, a chemical compound from the flavonoid family. However, its action is very light, so many chamomile-based products also include some hydrogen peroxide to enhance its effect.

Allergy to hair dye

Although all substances, natural and synthetic, are susceptible to producing allergies, in the case of allergy to hair dye the most common cause is usually paraphenylenediamine(C₆H₄(NH₂)₂), a compound that, as we have explained, plays an important role in achieving the final color. For this reason, hairdressers usually do a preliminary test to detect possible allergies to hair dyes.

Before dyeing keep in mind:

Although dyeing your hair has become a very common action, there are certain aspects to take into account before using hair dye.

For example, it is important that you know the consequences of the habitual use of bleached hair dyes. And it is that the natural structure of the hair and its sulfur bridges are altered since hydrogen peroxide not only acts on melanin, but also affects proteins and amino acids.

However, the hair will continue to grow little by little and will be replaced by the natural hair color.

(Melanin is a substance in your body that produces hair, eye and skin pigmentation.)

At this point you will already understand what happens when hair dye is used to cover gray hair. Since it is colorless, hydrogen peroxide will not bleach the hair, so it will act directly by applying the new dye to the capillary cortex.

Another important aspect to consider when choosing a hair dye is the thickness of the hair. When it comes to thick hair, the time it takes to absorb the hair dye mixture is longer. On the contrary, if the hair is dry it manages to absorb the hair dye in less time.

Now that you’ve learned how hair dye works, you may have realized the importance of chemical compounds in achieving a long-lasting, even application. And it is that science is behind our most daily actions.