Conditioning And The Hair Fiber Surface Hair Damage

In previous chapters, it has been shown that hair fibers consist of a central cortex that comprises the major portion of the fiber, surrounded by 8 to 10 layers of overlapping cells termed the cuticle. The cortex is responsible for the tensile properties of the hair [8,9], while the state of the cuticle affects a variety of consumer perceivable properties including, e.g., hair feel, shine, and combability.

A major function of conditioners is to protect the hair's structural elements, especially the cuticle, from grooming damage. This type of stress, characterized by chipping, fragmenting, and wearing away of cuticle cells, is probably the single most important source of damage to the hair surface [10-12].

A rather extreme example of combing damage can be seen in Figure 1, which shows the results of an experiment in which a tress of virgin hair was washed with a cleaning shampoo and then combed 700 times while wet. Since hair is more fragile when wet [3] and combing forces are higher [2], combing under these conditions insures maximum damage. It can be seen that damage to the cuticle was extensive with many cuticle cells lifted from the surface, while others were completely torn away by the combing process.

The ability of conditioning agents to protect the hair from this type of damage can be seen in Figure 2, which shows the results of an experiment in which a tress was washed with a high-conditioning ''2-in-1'' shampoo and then combed 700 times while wet. In this case, because the conditioning agents in the shampoo reduced combing forces, the hair surface is seen to be intact with evidence of only minor chipping and fragmenting of cuticle cells. This demonstrates the important role conditioners can play in maintaining the integrity of the hair fiber.

Figure 1 Typical scanning electron micrograph (SEM) of hair taken from a tress washed with a cleaning shampoo and then combed 700 times while wet. Note raised and chipped cuticle cells, and areas where cells have been completely torn away.
Figure 2 Typical SEM photo of hair taken from a tress washed with a high-conditioning 2-in-1 shampoo and then combed 700 times while wet. Note the minimal damage compared with Figure 1.

Hair Damage and the Cuticle Surface

The susceptibility of a hair fiber to grooming damage and the type of conditioner most effective in preventing this damage is affected to a large degree by the nature and state of the hair surface. It is therefore helpful to precede a discussion of conditioning agents with a presentation on the hair surface and how it affects conditioner requirements and deposition.

Virgin Hair Surfaces

Hair that has not been chemically treated is termed virgin hair. The cuticle surface of virgin hair in good condition is hydrophobic [13,14], in large part as a result of a layer of fatty acids covalently bound to the outermost surface of the cuticle (epicuticle) [15,16]. As a result of its protein structure, however, the hair surface has an isoelectric point near 3.67 [17], which insures that the surface will contain negatively charged hydrophilic sites at the ordinary pH levels of haircare products. This mix of hydrophobicity and hydrophi-licity affects, of course, the types of conditioning agents that will bind to the virgin hair surface.

The situation is further complicated by the fact that the negative charge density on virgin hair increases from root to tip. This is primarily a result of oxidation of cystine in the hair to cystine S-sulfonate and cysteic acid as a result of exposure to UV radiation in

sunlight [18,19]. The tip portions of the hair, being older than the root portions, will have been exposed to damaging [10] UV radiation for a longer period of time and will therefore be more hydrophilic, again affecting the nature of species that can bind to these sites.

In addition to greater UV damage, the tips of hair are also subject to greater combing damage. One reason for this is simply that, being older, the tip portions will have been exposed to more combing. In addition, the surface friction of hair tips is higher (C. Reich, unpublished data) so that combing forces increase as one moves from root to tip. Finally, the ends of hair are subject to unusually high combing stress as a result of entangling during the combing process [2]. This eventually results in destruction of the covalently bound lipid layer and a feeling of dryness at the tips. Because of this, the tip ends of hair require more conditioning than the rest of the fiber. Without sufficient conditioning, the cuticle layer is eventually lost, resulting in a split end. An example is seen in Figure 3, which clearly shows the exposed cortical cells.

Chemically Treated Hair Surfaces

Chemical treatments, perming, bleaching, and permanent dyeing, can all cause significant damage to the hair fiber [3,10,20-22]. In addition to causing tensile damage, all of these treatments, which include oxidative steps, modify the surface of the hair, introducing negative charges as a result of oxidation of cystine to cysteic acid [3,10,20,21,23]. This can result in transformation of the entire fiber surface from a hydrophobic to a hydrophilic character.

Cysteic Acid Hair

Figure 3 SEM photograph of a split end. Note the exposed cortex and the complete loss of cuticle cells on the fiber surface.

Figure 3 SEM photograph of a split end. Note the exposed cortex and the complete loss of cuticle cells on the fiber surface.

All of these treatments also increase surface friction considerably [3,4,24,25] resulting in a significant increase in combing forces. The result is hair that feels rough and dry and is subject to extensive grooming damage. Because of this, treated hair generally requires significantly more conditioning than does virgin hair.

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