One of the primary indicators of the health of your hair is its elasticity. Healthy hair has a high level of elasticity, which gives it body, bounce and curl formation. Elasticity makes it possible to style hair and also is responsible for curl retention. But what exactly does the term elasticity mean? We know it has to do with the stretchiness of our hair, and we know it is a desirable property, but it may not be entirely clear what it is.
Also, what contributes to elasticity of hair, and how can we maintain or improve the quality in our own locks? These are important questions, and as always, much insight can be gleaned by an examination of the fundamental principles as well as the molecular structures that make up the hair.
What Does Elasticity Mean?
Elasticity is a term used to describe how a material responds to the application and removal of a specific type of mechanical load (pulling and/or bending). When a stress (force per unit area) is applied to a material, it stretches a certain amount beyond its original length. This deformation is dependent upon the stiffness or rigidity of the material. The ratio of applied stress to the amount of deformation/elongation that occurs is called the elastic (or Young’s) modulus.
Rigid materials, such as iron, stretch very little with an applied force, while other materials, such as synthetic rubber, can stretch many times their original length without breaking. Dry hair can stretch to approximately 1.2 – 1.3 times its original length and still return to its dimensions, while wet hair is less rigid than dry hair and can stretch up to 1.5 times its length. Curly hair can stretch even more than straight hair can, as it is highly coiled in its relaxed state.
A material is said to exhibit elastic behavior if it returns to its previous shape and size once an applied force is removed. This is called reversible deformation. Simple materials such as elemental metals typically display purely elastic behavior. These tend to stretch to a certain point and then experience sudden fracture if the stress is not removed. Materials such as these are described as being brittle.
More complicated materials such as polymers, proteins, biomaterials and some inorganic amorphous solids exhibit elastic behavior until a certain stress is exceeded (yield strength). Beyond this point, less force is required to induce further deformation, and the material is unable to recover its size and shape once the load is removed. This phenomenon is referred to as irreversible deformation, plastic deformation, or permanent set. The applied force causes something to change inside the substance at a molecular level that causes it to become fundamentally different in its physical structure. The change can be a rearrangement of crystalline lattice structure from one type to another, shifting or slippage of molecular alignment in an amorphous or semi-crystalline material, change of protein tertiary structure, or breaking of bonds in polymeric compounds. Materials with this property are referred to as being ductile or having greater toughness than brittle substances.
Plastic deformation is particularly relevant to the health hair and its appearance. If excessive force is used to style or comb hair, the yield strength can easily be exceeded, and the hair can no longer bounce back when it is pulled out of shape. This can adversely affect its ability to hold a style or retain curl and can result in shapeless, frizzy hair.
Additionally, special caution should be taken with wet hair. Hair saturated with water is fragile and can stretch much more easily than when it is dry. It is very easy to exceed the yield strength when hair is wet and permanently diminish its elasticity, or even cause breakage. For this reason, it is crucial to use extreme care when handling and combing wet hair. The use of a good conditioner helps protect wet hair from plastic deformation by decreasing combing forces (less force is required to get the comb through tangles).