In this blog post, we’ll take a scientific look at the causes of curly hair and the principles behind how a straightening perm straightens it.
Hair that stands on end like a lion’s mane, frizzy and dry-looking strands, and hair that quickly reverts to its original state no matter how hard you straighten it with a flat iron or apply styling products—these are concerns that anyone with curly hair can relate to at least once. While straightening perm technology has advanced significantly in recent years, greatly reducing these inconveniences, curly hair still requires more maintenance than straight hair. While the proportion of people with straight hair is relatively high in South Korea, curly hair is known to be a polygenic trait influenced by multiple genes and is often inherited from parents. So why does curly hair occur? Also, what principles and methods make it possible to straighten curly hair with a straightening perm?
The cause of curly hair lies in the shape of the hair follicles. Straight hair has hair follicles that are relatively symmetrical and close to circular in shape. When the hair follicle is straight and well-balanced, the cross-section of the hair also becomes close to circular, allowing the hair to grow straight. In contrast, curly hair often has asymmetrical hair follicles that are closer to oval in shape. In these asymmetrical follicles, the cross-section of the hair takes on an oval or flattened shape. Because the hair follicle itself is curved, the growing hair naturally bends as well, resulting in a wavy or curly appearance. Generally, the greater the asymmetry of the hair follicle, the more pronounced the curl tends to be.
Straight hair, wavy hair (slightly curly), and coily hair (very curly) all have different hair follicle shapes, and generally, the degree of curl increases as you move to the right.
Fortunately, at a hair salon, you can get a straightening perm to make curly hair closer to straight. To understand the principle behind a straightening perm, it helps to first understand the principle of a regular perm. Let’s first examine the structure of hair. Hair is composed of a protein called keratin, which contains an amino acid called cysteine. When two adjacent cysteine molecules bond, a cystine bond—also known as a disulfide bond (S-S bond)—is formed, and this bond plays a crucial role in maintaining the hair’s shape. A perm is a process that breaks these S-S bonds, shapes the hair into the desired form, and then rebonds them.
The perming process generally proceeds in the order of reduction, shaping, and rebonding.
Permanent wave solutions generally consist of two parts: Part 1 and Part 2. Part 1 contains a reducing agent, such as ammonium thioglycolate, which breaks the S-S bonds between keratin proteins. The hair is then wrapped around rods to create the desired shape. Next, the second agent—a neutralizing agent—is applied; this contains an oxidizing agent such as hydrogen peroxide, which causes new S-S bonds to form. After undergoing this process, the hair retains its new shape even after the rods are removed.
A straight perm essentially uses a similar principle but includes one additional intermediate step. Looking a little more closely at the structure of hair described earlier, various intermolecular interactions exist even within a single keratin protein. Keratin consists of numerous amino acids linked together in a chain-like structure, and each amino acid contains a hydrogen atom and an oxygen atom. During this process, a hydrogen atom from one chain is attracted to an oxygen atom from another chain, forming a hydrogen bond. A straightening perm first breaks the S-S bonds using a perm solution and then applies heat to alter these hydrogen bonds as well. As a result, the hair becomes more flexible. At this point, the hair is straightened with a flat iron, and then a neutralizing agent is applied, forming new S-S bonds that maintain a shape close to straight hair.
We have thus examined the causes of curly hair and the principles behind a straightening perm. Curly hair results from the innate asymmetrical structure and shape of hair follicles, and these characteristics are directly reflected in newly growing hair. A straightening perm is a procedure that uses chemicals and heat to temporarily alter the bonds of keratin—the main component of hair—and then sets it into a new form. However, a straightening perm only alters the shape of hair that has already grown; it cannot change the structure of the hair follicles in newly growing hair. Therefore, as new curly hair grows over time, periodic treatments may be necessary.
