Cross-linkers and redox chemistry all in the name of beauty

Human always want what they don't have. When they have naturally curly hair, they want it to be straightened, when they have naturally straight hair, they want it to be curled. Well, no matter which style you want to wear, theres always chemistry involved! 

Our hair is made up mainly by a protein called keratin.The keratin arranged themselves in straight bundles, which is held together by the covalent disulphide (-S-S-) bonds. However, hydrogen bond is also responsible for the structure of our hair. That is why our hair becomes weak when it is wet because the hydrogen bonds found in the hair is being broken down by water but the hydrogen bond will reform again when the hair is dried.

The idea of hair straightening or perming is to break up the bond and re-shape it. Both hair straightening and perming can be done temporary by using a hair-blow dryers, which helps to rearrange the hydrogen bonding arrangement but the arrangement will looses it shape when it gets contact with water.

Hence, to achieve a permanent straightening or perming effect, a chemical straightener is what you need. The most common compound found in a chemical straightener is ammonium thioglycolate, the perm salt. The chemical formula for this salt is (HSCH₂CO₂NH4).  The -SH group which is also known as thiol group will replace one of the sulphur atom in the disulphide group during chemical reaction.

Keratin-S-S-keratin + 2HS-CH₂CO₂NH₄  --> -HO₂CH₂CS-SCH₂CO₂H + 2NH₃ + 2HS-keratin

Since the S-S bond is broken, the keratin bundles break apart, and hair is weakened, allowing it to be re-styled.

How to get your hair re-styled?

1) Wash your hair clean and remove any hair products or debris on your hair.

2) Apply the perm salt onto the hair for a short while. ( The ammonia released helps to loosen the hair so that glycolate can seep through)

3) For perming, tie your hair with curlers around to get the desired curl/ For straightening, press the hair firmly among flat irons until it becomes straight.

4) When the hair is shaped to your satisfaction, it needs to be strengthened so that the style stays permanent. An oxidation lotion is applied for this. The lotion contains hydrogen peroxide(H₂O₂), to the form new disulphide bonds.

2Keratin-SH + H₂O₂ --> Keratin-S-S-keratin + 2H₂O

However, all these processes have to be done

in a controlled way as the hair fiber can be damaged irreversibly if any of the processes is overdo. It is also important to handle the hair with extra care and treatment after straightening or perming as hair gets fragile after the processes. Oil and trim your hair frequently, choose the right shampoo and conditioner for your hair may also help to maintain the healthiness of your hair.