Once formed, the hair fibre is no longer nourished by the living tissue which made it. The end of a 30 cm long hair (about two years of growth) is subjected to a series of attacks (sun, drying, washing, brushing, humidity, pollution, etc.) and therefore ages like any material which wears with time.
Hair is remarkably resistant
The organisation of keratin in the cortex makes it possible for a hair to support a weight of about 100 g. A lock of 100 hairs can thus withstand a weight of 10 kg. As to the average head of hair, it could withstand 12 tons, if the scalp was strong enough! The hair shaft thus has great mechanical resistance due its elastic properties. The elasticity allows the hair shaft to stretch when pulled and then return to its previous length or shape.
The use of an extensiometer, which gradually stretches the hair at a speed of 1 cm per minute, allows precise study of the modifications that occur before it breaks. When stretched by up to 5% the hair is elastic. The structure of keratin is what endows hair with this property: an alpha helix in its natural state, stretching rearranges it into a sinusoidal keratin â structure. When the stretching stops it returns to its initial form like a spring and the hair enters a so-called flowing zone: it can thus be stretched by more than half its length before breaking. In a moist atmosphere, water penetrates and breaks the keratin hydrogen bonds. Thus keratin fibres elongate much more easily than in dry conditions. The tension needed for hair deformation is then smaller — a property exploited in hairdressing.
The hair’s surface
The condition of the surface of hair depends on the cuticle and affects other properties, such as electrical properties. Keratin has insulating qualities which decrease when the water content increases. If hair is rubbed it produces a static electrical charge. Frictional properties are also affected: hair resists rubbing because of the microscopic scales on its surface and their orientation. The architecture of the scales ensures high resistance to being rubbed the “wrong” way, i.e. from the end of the hair towards the root. Finally hair has cosmetic properties which combine shine, softness, ease of untangling and combing. These properties depend on the condition of the surface of the hair fibre: the smoother and more regular the surface, the shinier and softer it is. Damaged hair on the other hand is dull, the scales are chipped and the hair is harsher to the touch.