I think most of us already know that our hair is made up of a strong fibrous protein called keratin. And deep within the structure there are linkages and cross-bonds that create a network of strength that reinforce the hair fiber. This protein contains many copies of the amino acid cysteine, which is special because it has a sulfur containing chemical group that allows it to form strong bonds with other sulfur containing molecules.
There are three types of hair bonds: hydrogen bonds, salt bonds and disulfide bonds. Collectively, these bonds are also known as side bonds. Side bonds link chains of the hair’s amino acids together and each type of side bond contributes approx. 33% to the hair’s overall hair strength. (Hair Structure and Chemistry Simplified. "Chemical Composition of Hair” in Texas Collaborative: A Closer Look at the Properties of Hair and Scalp.
Disulfide bonds are also known as Cystine bond, Sulfur bond or S bond. These bonds are the strongest and the protein structures of the hair shaft are held together by chemical bonds called disulfide and hydrogen bonds. The more disulfide that occurs in the fiber, the curlier and kinkier the hair. These bonds cannot be broken by water or heat manipulation. Only chemicals agents can break these bonds, like perms, relaxers or color. So if a person with curly hair wants straight hair, you would need to use chemicals that can break up or “relax” these bonds. After the disulfide bonds break, relaxers “cap” the bonds so that they cannot reform again. It’s the reason why relaxers cannot be stripped or undone. Once broken, disulfide bonds cannot be reformed.
According to chemist Tonya McKay, “Hair elasticity is heavily dependent upon two key factors:
- Hydrogen bonding between water molecules and keratin strands and
- Disulfide bonds between adjacent cysteine amino acid groups, both of which are dependent upon preservation of the protein structure and hydration of the cortex. The best approach to ensure excellent elasticity is to maintain an intact protein structure inside the cortex and an adequate level of hydration”.
(Hair Structure and Chemistry Simplified. “Chemical Composition of Hair” in Texas Collaborative: A Closer Look at the Properties of Hair and Scalp. “Fundamental Properties of Afro-American Hair as related to Their Straightening/Relaxing Behaviour”. Naturallycurly.com)
These bonds are abundant throughout the cortex. Salt bonds account for nearly 1/3 of the hair's strength. They are broken by pH changes in the hair in both acid and alkaline direction. It is easily broken when a substance of a pH of 5.5 or greater is applied. Readjusting the hair’s pH will reform and stabilize these bonds. Salt bonds are formed when the positive end of an amino chain links to another amino acids negative end.
(Hair Structure and Chemistry Simplified.“Chemical Composition of Hair” in Texas Collaborative: A Closer Look at the Properties of Hair and Scalp. “Fundamental Properties of Afro-American Hair as related to Their Straightening/Relaxing Behaviour”)
These bonds are the most flexible. I have touched on this in my 3 part ‘Frizz’ series. Hydrogen bonds are easily broken in the presence of water and heat. They are the primary bonds responsible for changing our hair’s overall shape. They are responsible for up to 30% of the strength and up to 50% of its elasticity. When the hair is wet by either shampooing/conditioning or in the presence of humidity, the molecules that enter the fiber move in and break up the hair’s preset hydrogen bonds and form new ones.
Hydrogen bonding allows our hair to change shape temporarily and produces a strong hold. Here is an example of hydrogen bond manipulation resulting in an altered appearance of the hair: setting your hair in rollers. Hair is usually set in rollers while wet. The hair is then held in position until it dries. As the hair dries, hydrogen re-bonding occurs, but in the new “shape”.
When we wet our hair, these bonds are temporarily disrupted and we’re able to take advantage of the hair’s flexible hydrogen bonding arrangement to restyle our hair with more definition. So the hair will remain in the new “shape” until it’s presented with water again and that can either occur through shampooing/conditioning or humidity. Hydrogen bonding is the reason why hair frizzes and curls fall and why we’re able to manipulate our hair into a range of hairstyles. Other examples of hydrogen bond manipulation: twists outs and using a flat iron.
(Hair Structure and Chemistry Simplified. "Chemical Composition of Hair” in Texas Collaborative: A Closer Look at the Properties of Hair and Scalp. “Fundamental Properties of Afro-American Hair as related to Their Straightening/Relaxing Behaviour”. The Science of Hair Care).