Heat Processes of Hair

 

Heat Processes of Hair

There are a range of different structural changes caused by heat but the factors crucial to determine extent of these changes are firstly the temperature that the hair gets to, and secondly the cumulative time of exposure to this temperature. Many people can use heat implements without noticeable damage if they take care with frequency of use and moderate the temperature of the device.

However, the benefits sought with heat implements, e.g. curl formation or straightening, and are correlated with the level of heat used, so it can often be a tradeoff between level of damage and desired style achievement. For some people, heat implements are used simply to dry the hair but the majority use heat implements as a tool to style hair. Heat will evaporate water from between hair fibers (capillary water) and from inside hair (Internal Water). It is internal water that is crucial for style achievement, as removing this water will create hydrogen bonds between neighboring protein chains, effectively temporarily locking the desired style in place, the more water that is evaporated (i.e. the higher the temperature or the longer hair is dried for), the stronger these hydrogen bonds links will be and the longer the style will last. Of course, during water evaporation hair has to be styled in the final shape, which is what curling tongs or flat irons do effectively by either creating a curl or straightening while heating (Figure 1.30).

Figure 1.30 

 

Blow dryers are the most common form of heated used, and in the US ~50% of women use a blow dryer regularly. The air flow temperature is typically up to 100 ⁰C but hair will only get close to this temperature once capillary and internal water has evaporated.

While hair is wet, heat will be absorbed by water while it is evaporating. No significant changes to the hair structure occur at these temperatures, but physical cracking of cuticles can occur at these temperatures if hair is dried very rapidly. These cracks occur as the rapidly drying cuticle contracts around a still swollen cuticle. Once cracks are formed the cuticle more easily removed by grooming.

Chemical degradation of hair due to heat will only occur if hair is heated above 150 ⁰C. These temperatures can readily occur if heated straightening or curling irons are regularly used. With these implements on the highest setting, hair can reach temperatures of above 220 ⁰C, especially of sections of hair are repeatedly straightened or hair is slowly pulled through the flat iron. In addition, a critical degradation of protein takes place above 190 ⁰C which will involve cysteine disulfide bond breakage. At very high levels of exposure to this temperature, melting of keratin can even occur; severe keratin degradation and hair breakage occur when hair is flat ironed for 3 minutes at 210 ⁰C.

These exposures are extreme and are unlikely to occur, but excessive heat damage can be seen in increased incidence of broken hair for some women who use high heat implements on a regular basis. These broken fibers will then lead to poor shine and smoothness, directly indicating poor hair health.

However, used with care, heated implements can also be used to increase perception of healthy hair. Blow dryers and flat irons can be used to straighten and align hair, giving it shine and making it easier to comb. In addition, creating a good “wet-set” by removing internal water will increase resistance of style to humidity, thus reducing frizz.