In this blog post, we will take a closer look at the structure of hair, the scientific principles of hair dyeing, the effects of hair dye ingredients on hair, and precautions when dyeing hair.
The history of life that began in the ocean entered a new phase as it moved onto land. To survive in an environment different from that of the water, life underwent evolution, and in the process, its skin gradually became harder and began to grow hair. The hair of some life forms became long and thick with a focus on keeping warm, while the hair of some life forms took on a light and smooth shape suitable for gliding.
However, humans gradually began to shed their fur to quickly dissipate the heat generated by their enlarged brains, and most of the fur disappeared except for a few important areas. Among them, the hair on the head has come to have the function of protecting the brain, maintaining body temperature, and carrying the characteristics and messages of each individual. In particular, the meaning of this has been highlighted in modern society, where women and students shave their heads to show their will or determination, and the bright yellow and pink hair of punks well reveal their anti-establishment tendencies. They do not hesitate to invest time and money in such hairstyles as a means of “expression.”
The importance of hair as a means of expression can be seen historically. In ancient Egypt, royalty and aristocrats wore different wigs to indicate their status, and in medieval Europe, social status was expressed through hairstyles. Hair has become an important element that reveals social and cultural identity, beyond being a simple part of the body. In modern times, with the development of the fashion industry, various hair styling methods such as dyeing, perming, and cutting have been developed, and they have become a major means of expressing individuality and creativity.
So, what characteristics of hair have made these activities possible? And what principles do dyes utilize? First, let’s learn about the structure of hair.
Hair is composed of three main parts. The innermost part is the medulla, the middle part is the cortex, and the outermost part is the cuticle. Let’s talk about the medulla first. The medulla is the central part of the hair, and it has a structure in which honeycomb-shaped polygonal cells filled with cavities are arranged in the longitudinal direction of the head. Collectively, they act as a heat insulator because they contain air, and the more air they contain, the more lustrous they become. However, they are not present in all hair, and they are present in hair that is approximately 0.09 mm or thicker, depending on the thickness of the hair, but are almost invisible in hair that is 0.07 mm or thinner.
Next is the cuticle. The cuticle is the outermost part of the hair, which accounts for about 10-15% of the hair. It is composed of a hard protein called keratin and has a structure of 5-15 layers of transparent, thin cells stacked like roof tiles. It has strong chemical resistance and plays a role in protecting the inner part. If the cuticle is damaged, it is difficult to regenerate and affects the cortex and medulla, causing the hair to break.
Finally, let’s take a look at the cortex. The cortex is the part that exists between the medulla and the cuticle, and it accounts for 85-90% of the hair. In the case of hair shafts, they are hollow and the cuticle is made up of transparent, thin cells, so it does not affect the color. However, the cortex is composed of cortical cells (keratin protein) and intercellular substances, and it is the part that contains a small amount of melanin pigment in the cortical cells to express the natural color of the hair. There are two types of melanin pigments. One is u-melanin, which mainly produces colors ranging from brown to black. The other is pheomelanin, which produces red or yellow colors. Depending on the ratio of these two pigments, various hair colors ranging from black to brown to gold are produced. In addition to hair color, it is the core part of hair because it determines the overall characteristics of hair, such as elasticity, strength, and texture.
To sum up, hair is made up of the cuticle, cortex, and epidermis. Of these, the cuticle and epidermis only serve to hold the hair in place or protect it from the outside, and do not play a major role in determining the color or other characteristics of the hair. This is manifested through the composition of the hair shaft, which makes up the majority of the hair, and chemical procedures are performed to destroy or modify these components. Let’s take a look at the principle of dyeing hair based on this.
There are three main types of dyeing. One is temporary dyeing, which involves applying dye to the surface of the cuticle for a short period of time to reveal the color. In this case, the color is expressed with hair color spray or color mousse, and it is a one-time dye that disappears immediately when washed with water. The other is a semi-permanent dye that allows the dye to be slightly absorbed into the cortex and forms a thin film on the surface of the epidermis to reveal the color. Hair coating, nail polish, and waxing are all examples of this type of dye. It has the advantage of not causing damage to the hair because it does not affect the cortex much. However, because it literally only coats the surface of the epidermis, the more you wash your hair, the more the color will peel off, returning to the original hair color in about 1-2 months.
Finally, there is permanent dyeing, which destroys the melanin pigment and settles the desired color in the cortex. This dyeing, unlike the previous two, lasts permanently unless the hair is cut, but it has the risk of damaging the hair because it directly affects the cortex. Three substances are mainly used for permanent dyeing: ammonia, hydrogen peroxide, and dye. The outer skin is made of keratin protein, which is a very stable protein, and therefore has strong chemical resistance. Therefore, in order to deposit pigments into the outer skin, the epidermis must be removed or lifted to allow the pigments to enter. Ammonia acts to lift the 5 to 15 layers of thin cells that make up the outer skin. So it allows hydrogen peroxide and dyes to penetrate the hair well. Hydrogen peroxide destroys the melanin pigment in the hair. If melanin, the pigment that gives the hair its color, remains in the hair, it is difficult to express the desired color, and the pigment is difficult to penetrate, so it destroys the melanin pigment. So, the melanin pigment is destroyed, and the dye we want to put in its place is finally settled in the fur. The newly settled pigment is fixed in the fur (at this time, the dye molecules that have penetrated become larger pigment molecules through a chemical reaction), and then when hair treatment is used, the raised fur scales settle again, so the pigment does not come off easily. So, even if you wash your hair, the color will not fade and will last permanently.
There are some things to keep in mind before proceeding with this type of dyeing. When it comes to the hair dye used for dyeing, synthetic hair dyes that produce good color and coloring are commonly used. Synthetic hair dyes are divided into oxidizing dyes, direct dyes, metal dyes, and natural dyes depending on the type of dye. Direct dyes and metal dyes are used for temporary and semi-permanent dyeing and do not penetrate deep into the hair or contain ingredients that are sensitive to the skin, so there are no major precautions to be taken, except that they must be thoroughly washed off after use. However, the oxidizing dyes used in permanent dyeing contain many substances that cause allergic reactions. Diamine-based ingredients (p-phenylenediamine (PPD), toluene-2,5-diaminesulfate (TDS)), 2-methyl-5-hydroxy ethylaminophenol (MHEAP)N, N’ Bis(2-hydroxyethyl)-p-phenylenediaminesulfate (2HEPPD), p-toluenediamin (PTD), etc. are examples of such substances. Among them, p-phenylenediamine is particularly allergenic and can cause allergic contact dermatitis, scalp diseases, and hair loss. Therefore, a patch test should be conducted before dyeing to ensure that the hair dye is suitable for each individual.
Finally, after dyeing, proper care is needed to protect the hair. After dyeing, the hair is dry and prone to damage, so it is important to regularly use a moisturizing hair mask or treatment. It is also important to protect the hair from the sun by using a sunscreen-infused hair product. Since dyed hair is more sensitive and prone to damage than normal hair, it is recommended to use a soft brush and avoid using high-temperature styling tools.
We have now learned about the structure of hair and how it is dyed. Although the ratio of the structure varies from person to person, the color, thickness, and shape of hair, which is composed of 85-90% of the cortex and 10-15% of the cuticle, are determined by the ratio of the substances contained in the cortex. Among these, the color appears in various colors depending on the ratio of melanin pigments, and the process of destroying these melanin pigments is necessary for dyeing. Therefore, hydrogen peroxide is needed for hair dye, and ammonia is used to allow hydrogen peroxide and dyes to enter the cortex. However, ammonia is a strong base that can have a negative effect on the hair and scalp. Also, since the hair dye used for dyeing is an organic synthetic compound, people with sensitive skin may experience an allergic reaction. Therefore, it is necessary to conduct an allergy test on the hair dye before dyeing and be careful not to leave any dye residue after dyeing.
