কোম্পানির খবর Science Popularization | | Scientific Understanding of Hair Structure and Dyeing Principles

Science Popularization | | Scientific Understanding of Hair Structure and Dyeing Principles

The pursuit of beauty by humans has driven the continuous evolution of hair dyeing technology, spanning thousands of years from ancient Egyptian pharaohs' golden brown curls to modern diverse hair color choices. Since the first synthetic dye, aniline violet, was introduced in 1856, oxidative hair dyes have undergone over a hundred years of development and have now formed a complete industrial chain covering raw material suppliers, producers, brands, and consumers. With the rapid development of the domestic cosmetics market, the hair dye industry is gradually maturing, and the regulatory system is becoming increasingly perfect. High quality raw materials, mature technology, and consumer recognition have become the core of product competitiveness.

(一)

The Biological Structure and Chemical Essence of Hair: The Natural Basis of Hair Dyeing

The color change of hair is essentially the result of the interaction between hair dye substances, hair structure, and chemical composition. The cross-section of hair (hair shaft, which is the part that has detached from the hair follicle) is divided into three layers from the outside to the inside: hair epidermis, hair cortex, and hair medulla, and is mainly composed of alpha keratin. This structure and compositional characteristics directly determine the feasibility and efficiency of hair dyeing.

01

The three-layer structure of hair: from barrier to core functional area

(1)Hair epidermis: the "first door" of hair dye substances

The epidermis is the outermost layer of the hair shaft, composed of 6-10 layers of flat, transparent keratinocytes arranged in a "fish scale" overlapping pattern (similar to roof tiles). Its core function is to protect the hair cortex from external stimuli and control the entry and exit of substances.

For hair dyeing, the epidermis is the primary interface of hair dye substances: surfactants or alkaline ingredients (such as ammonia) in hair dye products will first wet the epidermis, causing the "fish scale" cell gaps to expand, opening up channels for subsequent dye molecules to enter the cortex. If the hair epidermis is damaged (such as frequent perming and dyeing causing scales to fall off), it can cause the dye to penetrate too quickly, which may lead to dry hair and uneven color; On the contrary, a healthy hair epidermis requires optimizing the mildness of the "opening barrier" in formula design to avoid excessive damage to hair structure.

(2)Cortical: the 'core area' of color change

The hair cortex accounts for 80% -90% of the volume of the hair shaft and is composed of parallel arranged keratin fiber bundles. The fiber bundles are filled with amorphous keratin protein and natural pigment particles. This layer is the core area where hair dye substances exert their effects.

The oxidizing agent (such as hydrogen peroxide) in permanent hair dye will first react with melanin in the hair cortex, destroying the original pigment structure (i.e. "decolorization");

Subsequently, synthetic dye molecules (such as phenylenediamine) enter the hair cortex and covalently bind with amino and thiol groups in keratin, forming stable new pigments and achieving "long-lasting coloring". The efficiency of this process depends on the size of dye molecules and their reactivity with keratin.

(3)Medulla: The 'Auxiliary Permeation Zone' of Color

The hair medulla is located at the center of the hair shaft and is composed of loosely arranged keratinocytes. It only exists in coarse hair with a diameter greater than 60 μ m (fine hair may not have a medulla). Its structure is loose and the gaps are large, allowing hair dye to further diffuse to this area, enhancing the uniformity and durability of the color. However, due to its low proportion, it is not the key area for dyeing hair, so there is no need to overly focus on product design. Only the compatibility of the formula for coarse and fine hair needs to be considered

02

Alpha keratin:

The "chemical skeleton" of hair and the "target" of dyeing reaction

The main component of hair is alpha keratin, and its molecular structure and chemical properties directly determine the mechanism of action of hair dye substances.

Structural characteristics: Alpha keratin forms a polypeptide chain in a "right helix" structure, and multiple polypeptide chains are connected by disulfide bonds (provided by cysteine, which accounts for a high proportion of cysteine in the peptide chain), hydrogen bonds, etc. to form stable fiber bundles. This structure gives hair good mechanical strength and resistance to solvents and weak acids and bases.

Reaction characteristics: Alpha keratin is sensitive to oxidants, reducing agents, and strong bases. Disulfide bonds can be oxidized and broken under the action of oxidants, and can be reduced and broken under the action of reducing agents; Meanwhile, amino (- NH ₂) and carboxyl (- COOH) groups in the peptide chain can covalently bind or electrostatically adsorb with dye molecules.

It is precisely this characteristic that makes hair dyeing possible. While oxidants destroy melanin, they can also moderately open the disulfide bond of keratin, expand the molecular gap, and facilitate dye entry; And dye molecules achieve "stable coloring" by binding to keratin groups, which is also the core chemical basis for permanent hair dyes to maintain color for a long time.

(二)

The scientific principle of dyeing hair:

From material action to process decomposition

Based on the structure and chemical nature of hair, the dyeing process can be broken down into four core steps: contact penetration reaction coloring. Each step needs to match hair characteristics and product formula design, as follows.

01

Step 1:

The "contact and wetting" between hair dye substances and hair epidermis

After applying hair dye products, the first step is to reduce the surface tension of the hair through surfactants (such as sodium dodecyl sulfate), so that the hair dye substances (dyes, oxidants, alkaline components, etc.) are evenly wetted and adsorbed on the surface of the hair epidermis. The key to this step is "wetting uniformity". If the surfactant is not selected properly, it may lead to insufficient local wetting and cause subsequent color unevenness

02

Step two:

The "opening and dye penetration" of the hair skin barrier

The alkaline components in hair dye products, such as ammonia and ethanolamine, can increase the pH value of the hair surface, causing the "fish scale" cells in the hair epidermis to swell and the intercellular spaces to expand; At the same time, oxidants (such as hydrogen peroxide) will slightly oxidize the keratin protein in the epidermis, further disrupting the barrier structure. The two work together to open a "channel" for dye molecules.

Subsequently, dye molecules (such as small molecule precursor dyes) enter the hair cortex through "concentration difference diffusion". The smaller the molecule, the higher the solubility, and the stronger the diffusion efficiency; However, macromolecular dyes (such as flavonoids in natural plant dyes) are difficult to penetrate, which is also the core reason for the poor color persistence of natural hair dyes.

Step 3:

Discoloration and Reaction in Cortical Hair

After entering the hair cortex, oxidants (such as hydrogen peroxide) first react with natural melanin: the indole structure in melanin is oxidized and broken, and the original color disappears (i.e. "decolorization"), making space for new colors. At the same time, the oxidant moderately breaks the disulfide bonds of keratin, expanding the fiber bundle gaps in the hair cortex and further promoting the diffusion of dye molecules.

Subsequently, dye molecules react with keratin: if it is a permanent hair dye, small molecule precursor dyes (such as p-phenylenediamine) will undergo oxidative polymerization under the action of oxidants, forming macromolecular pigments that covalently bind with the amino groups of keratin (with strong binding force and are not easily detached); If it is a semi permanent hair dye, dye molecules (such as acid dyes) adhere through electrostatic adsorption (binding to the carboxyl group of keratin), with weak binding force and easy detachment due to washing.

Step 4:

The diffusion and coloring stability of dyes towards the medulla oblongata

Unreacted dye molecules will further diffuse into the medulla, achieving color uniformity through physical filling; At the same time, with the rinsing of hair dye products and drying of hair, the "fish scale" cells on the hair cuticle gradually close, and the barrier function is restored, locking dye molecules in the hair cortex and medulla.

The retention of new hair color is closely related to the interaction between the hair medulla and epidermis. Cells gradually close, barrier function is restored, and dye molecules are "locked" in the hair cortex and medulla. At this point, the hair dyeing process is completed, and the hair presents a new color that is not easily shed during daily cleaning (permanent type) or slowly sheds (semi permanent type).

The core of hair dyeing technology is the scientific matching of "hair structure and chemical properties" with "the mechanism of action of hair dye substances". For practitioners, only by deeply understanding the barrier function of the hair epidermis, the core role of the hair cortex, and the response characteristics of alpha keratin, can they find a balance between "effectiveness, mildness, and durability" and develop high-quality products that meet market demand. From the 19th century to today's 21st century, every breakthrough in dye research and dyeing technology stems from a deeper understanding of hair science, which is also the core direction of future industry innovation.