The Physical Properties of Hair
In this section we look at the ways in which hair can change or be changed. It can be stretched, bent and curled. It can absorb moisture or lose it. Its behaviour can alter when it is wetted or when it is brushed. We look at the special properties of hair, such as its elasticity, porosity and texture.
Understanding these properties is important for all of us as we cope with our hair under different circumstances. It is especially important for the hair stylist who has to decide on styles and to suggest continuing hair care regimes for clients.
Elasticity
This is one of the most important properties of hair. Because of its elasticity, hair can resist forces that could change its shape, its volume or its length. Its elasticity lets it spring back to its original form without damage.
When healthy hair is wetted and stretched, it can increase in length by up to 30% and still return to its original length when it is dried. Stretching it more than this will tend to damage it, however, leading to permanent lengthening and even breaking.
The elasticity of hair depends on the long keratin fibres in the cortex. Chemical treatments of hair such as perming and bleaching can alter the cortex after repeated damage, and change the hair's elasticity. Hair with poor elasticity will stretch only to a limited extent. It will not curl, it will break easily when it is groomed and it cannot be permed satisfactorily.
Both natural sunlight and artificial ultraviolet light break down chemicals in the hair and damage its elasticity in the same way that bleaching does, though to a much lesser degree.
A stylist must always examine the elasticity of a client's hair before deciding on any chemical procedure.
Static electricity
When dry hair is rubbed, as it is whenever it is brushed or combed, static electric charge builds up on the hairs. This is especially noticeable in hot, dry weather. The charges tend to push each other apart, and as a result charged hairs can never lie smoothly against each other. The result is 'fly away' hair, which stands out from the head and looks unmanageable.
Conditioners, and shampoos with high levels of conditioning agents in them, leave the surfaces of the hair smooth. There is therefore less friction when the hairs are rubbed: hence less static electricity builds up on them, and 'fly away' is reduced.
Moisture content
The moisture content of hair is greater when the atmosphere is moist and humid, and less when the air is dry. The reason why hair 'collapses' in hot, humid atmospheres is summed up by:
heat and humidity -> more moisture
-> less static electricity
-> collapse
In dry conditions:
heat and dryness -> less moisture
-> more static electricity
-> more volume (body)
When hair is wet the cortex swells and the edges of the cuticle scales tend to lift. The hair surface temporarily loses its smoothness.There is therefore more friction when wet hair is rubbed than when it is dry. This is what can lead to matting and tangles developing during over-vigorous shampooing (there is not greater static charge on wet hair than on dry).
These tangles are one of the reasons why many children hate having their hair washed, and the problem is easy to avoid.
Hair diameter
The elastic properties of both wet and dry hair are related to the diameter of the hair shaft. The thicker the hair, the more it will tend to resist stretching.
Porosity
In a normal, undamaged hair shaft, very little water can get either into or out of the cortex. This is because the cuticle covering the cortex is intact, and is then almost (but not quite) waterproof. Shampoos do not damage the cuticle. When hair is permed or tinted, however, the chemicals have to penetrate the cortex in order to react with the keratin inside it. Increasing the temperature, or applying an alkaline lotion, separates the scales of the cuticle enough to allow the chemicals to pass through. After the processing is finished the scales gradually close up again.
Particularly thick, strong hair: the hairs grow densely and each has a wide diameter
But if hair is processed too many times the cuticle scales may never return to their original tightness and the protection they once offered is lost. The cuticle can also be damaged in the same way by too much blow drying, curling irons that are too hot, and the effects of wind and sun. The hair becomes increasingly porous, and water can then pass in and out of the cortex.
Over-porous hair is dry, and tends to develop split ends. The damaged cuticle is fragile, and the damage worsens as time goes by. The greater the damage, the more the cortex swells with water whenever the hair is washed, but the more water it loses when it dries. The repeated wetting and drying of the cortex gradually weakens the hair.
Texture
Much of the attraction of a beautiful head of hair lies in its texture, or feel. The texture of hair depends on several things.
The first is the average diameter of the individual hairs. We have seen that these vary widely. The larger the hair diameter, the coarser it will feel.
Secondly, different people's hair naturally feels different: some hard and others soft, some silky and others wiry. The reasons underlying these differences are still a matter for scientists to argue over.
Thirdly, the texture is affected by the degree of weathering of the hair.
Finally, hair texture is affected by what has been put on it. Repeated lavish applications of hair spray gives hair a different feel from that of hair that has been freshly washed and conditioned. Conditioners make hair feel soft and smooth. Conditioners that contain silicones even give a slightly different feel from those that don't (most manufacturers do put silicones into conditioners nowadays, however, as they protect the hair cuticle). Contrary to popular belief, this altered feel is not a sign of build-up.
