Hair Iron’s and Hair Straightening work by changing the structure of the hair. It does this by breaking the Hydrogen Bonds in the hair. The hydrogen bonds are located in the cortex, and it is these bonds that cause the hair to curl. If these bonds are broken, then the hair can’t hold its natural form, remaining straight, or however it is styled.

To Permanently curl hair (a perm) the products used contain thioglycolic acid, a chemical that is used to break Disulfide bonds in the Cysteine units in the hair, and then reduce them to -SH groups. This separates the protein strands in the hair. A neutralizer is then used, which has the opposite effect, converting -SH groups back into disulfide bonds. If you shape/style the hair during this process (i.e. use curlers), then the hair will remain in that style.

However, Hydrogen bonds can re-form in they are exposed to moisture.