In my previous articles, we looked at the non-oxidative hair dyes, and the oxidative demi-permanent hair dyes; what they are and how they work. In this article we will take a deeper look the chemical reaction behind the dye formation in oxidative permanent hair coloring, and the damage that one should expect.
About permanent hair coloring
Permanent hair dyes are commonly used because they provide greater efficacy of dyeing, and are resistant to shampooing (despite the gradual fading) or hair styling. Once the color is formed, it cannot be removed easily. For example, brushing your hair will not remove these hair dyes as compared to semi-permanent or demi-permanent hair dyes. This type of hair dye represents about 80% of the sold hair dyes and can be formulated into any shade, covering up to 100% of grey hair strands.
The primary intermediates and coupling agents
Unlike the other hair dye categories, permanent hair dyes do not start off from colored hair molecules. Instead they consist of 2 groups of colorless dye molecules: the primary intermediates and the coupling agents. Upon reaction with each other through an oxidation process, they form a wide range of large colored molecules (dyes) that are then trapped into the hair due their size. The image below was obtained from compoundchem.com:
This shows the different colors obtained when the primary intermediates react with the couplers. While in theory, this is a straightforward reaction, formulating the actual product is actually more challenging since it requires a delicate balance of the right primary intermediates and couplers in the right ratios in order to achieve the desired shade.
The role of hydrogen peroxide and ammonia
In order for the dye precursors to react inside hair, there are two key components that are needed: hydrogen peroxide and ammonia.
The ammonia’s pimary role to provide a high pH environment for the cuticles to swell and open.
Hydrogen peroxide on the other hand has two important primary functions: to ‘lift’ the color of the hair (lighten the hair by breaking down/diffusing the melanin) and to facilitate the oxidation of the dye precursors. In order for the peroxide to be reactive, it requires a high pH, which the ammonia provides. Depending on the starting color and the final desired shade, the concentration of the hydrogen peroxide can be adjusted in the ‘developer’ to provide the desired level of lift. Usually the strength of the hydrogen peroxide is referred to in terms of ‘volumes’ such as 10V or 40V (high reactivity).
A note about hair damage
It is worth noting that the higher the lift, the more internal damage the hair gets. Thus for someone changing shades from black hair to light brown or blonde, the volume of peroxide used will probably be around 40V and there will be low color deposition. On the other hand, from blond to auburn, would require only a small peroxide volume typically 20V, and will result with more color deposition. Either way the peroxide is needed for the oxidation reaction of the dye precursors to occur in the hair, but the concentration of peroxide used can make a big difference in terms of damage.
As far as external damage goes, regardless of the peroxide volume, it is inevitable that there will be some cuticle damage. The fact that the cuticles are forced to swell and open at such high pH makes them vulnerable. This is why most permanent coloring processes will end with conditioning, which brings the pH back to neutral and allows the cuticles to close. More importantly, the conditioner will then act as a temporary protective barrier against further damage from grooming or other.