Hair stylists apply numerous chemical principles as they create curly or straight hair.
It seems to be the nature of people that those with curly hair wish for straight hair, and people with straight hair wish for curly hair. A considerable amount of chemistry is involved in trying to fulfill these wishes.
Hair curls due to two different types of chemical bonds in the keratin of the protein that constitutes hair. The first is hydrogen bonding, which helps create the coil or curl of the hair. Hydrogen bonding can be disrupted simply by getting hair wet, which is why curly hair may become straighter when it is wet, and relatively straight hair may become curlier in high humidity. When hair stylists reshape hair using hair dryers, rolling hair over hot curlers or curling irons, or straighten hair with hot, flat plates they are affecting the hydrogen bonding of the hair. But these treatments only last for at most a day.
For longer-term results, chemists created hair perms and hair relaxers that focus on the second type of chemical bonding, which links pairs of sulfur atoms at different points along the protein chain to make disulfide bonds. In this process, the first step is to break these disulfide bonds. Sodium hydroxide, or lye, is used on extremely curly or extremely coarse hair, but it can damage both a person’s hair and scalp. Two alternatives, Guanidine hydroxide, (no-lye relaxer) and ammonium thioglycolate (thio-relaxer) aren’t as harsh; respectively, they provide intermediate and mild solutions. All three of these solutions change the pH of hair, making it more basic – between 9 and 10. Stylists use these solutions while the hair is either wrapped around curling rods or pulled straight. A second step relies on hydrogen peroxide to neutralize the first solution, lowering the pH, and reforming disulfide bonds in the desired positions, making new curls or new straight hair relatively permanent.
Additional considerations for relaxing and perming hair can be found at http://www.hairboutique.com/tips/tip086.htm
Leslie Boisvert of Newington, CT contributed to the background for this topic.