Hair integrity protected during chemical processes

Preservation of the integrity of hair fibres requires an understanding of the structural and chemical composition of hair and the chemistry involved to counteract the damaging effects of the bleaching and color treatment processes.

The hair structure is composed of the cuticle, cortex, and medulla.1 The cuticle is the outer protective layer of the hair fibre, which provide the sensory and shine characteristics of hair. The cuticles overlap giving the appearance similar to roof shingles, thereby protecting the hair from chemical, physical, and environmental treatments.  The cortex is the major component of the hair and is responsible for the mechanical strength and is responsible for the pigment colour of hair.  The innermost layer is the medulla, which may be present or absent throughout the hair fibre.

The structural integrity of hair is due to hairs’ specific chemical composition. The hair is primarily composed of proteins, lipids, and water. Human hair is composed of 65-95% protein, which influence hair textures such as curly, wavy, kinky or straight. The main component of human hair is keratin proteins, which are complex natural compounds that contribute to its physicochemical properties. Human hair keratin is unique to other types of keratin because of its higher content of cysteine residues relative to skin keratin (7.6% and 2.9%, respectively).2 A compositional increase of cysteine residues in hair leads to a higher amount of inter- and intramolecular disulfide linkages, translating to a durable structure due to covalent bonds.3 Hair is also comprised of 1-9% lipids, which contribute to enhanced conditioning properties, such as flexibility, surface gloss and lubricity of hair.3 Lipids in the internal part of the hair provide structural reinforcement and rigidity. Water is another major component that can be found up to 32% by weight of hair.4 Water supports the formation of a network of hydrogen bonds with proteins, thereby influencing the tensile strength, swelling, flexibility and shape of hair, as well as, the formation of salt-bridges. 

During the bleaching process and colour treatment, it is crucial to introduce a product designed for improving hair integrity. These damaging processes affect the physical, mechanical, and surface properties of hair that leads to hair becoming dry and straw-like, frizzy, difficult to style and easily broken.5 During chemical treatments, much of the hydrophobic lipids are stripped from the surface of the hair, leading it to be easily tangled due to increased friction and increased surface roughness. The hair bleaching composition and method has been discussed thoroughly in the patent literature.6,7 Briefly, one or more cycles of applying high concentrations of oxidising agents with strong alkaline agents are applied onto hair to permanently decolour the hair. The bleaching process is often highly alkaline, (pH 9-13) resulting in a dramatic degradation of the external and internal structural integrity of the hair over repeated cycles. Hair lacks regenerative properties therefore it is important introduce components during the bleaching process and colour treatment to preserve and maintain the hair’s structural integrity and chemistry that reinforces the hair structure. 

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