Hair straighteners work by breaking down the hair's hydrogen bonds found in the hair's cortex, which cause hair to bend and become curly. Once the bonds are broken, hair is prevented from holding its original, natural form, though the hydrogen bonds can re-form if exposed to moisture.
In 1872, Marcel Grateu used heated rods to straighten or style hair. patented a hair straightener in 1906 that comprised metallic teeth that are combed through the hair, while in 1909 Isaac K. Shero patented a hair straightener composed of two flat irons that are heated and pressed together., however the hair straightners, as we know them today, are credited to Scottish heiress, Lady Jennifer Bell Schofield, who refined and combined previous efforts into a hinged, two plated heated iron in 1912.
Years later, Sharrell invented heated metal hair-care implements, such as hot combs. The hot metal straightener slid easily through the hair, causing less damage and dryness than previously-used techniques. Later, ceramic and electrical straighteners were introduced, allowing for adjustment of heat settings and straightener size.
High-end modern hair straighteners have ceramic heating elements and are more effective due to their constant heat, with a very fast heat up time (less than 10 seconds). A higher quality plate is the most desirable asset of any flat iron. Better plates straighten the hair more easily and cause much less damage. Very hard ceramic coatings that are sprayed in layers and baked on, tend to do the best. Less expensive units usually do not have ceramic heating elements and some may have a paint-like coating, that gives the appearance of ceramic plating. These cheaper coatings wear off very quickly and cause more damage to the hair.
There are two types of damage all flat irons create. Heat damage and mechanical damage. Heat damage is unavoidable since the hotter a flat iron is, the better it works (up to 450 degrees Fahrenheit). Less mechanical damage is achieved through smoother plates. Most plates are aluminum and coated with something, such as a hard ceramic coating. As cheaper coatings wear off, the levels of mechanical damage increases.
Daily or very frequent use of a flat iron will damage the hair. Less frequent straightening - every other day, for example - will reduce this damage.
Depending on the hair type, repeated use of hair straighteners, like most other heat styling tools, can cause varying amounts of damage such as split ends, thinning of the hair shaft and even causing hair to become brittle and prone to breakage. Generally, after extensive damage, hair will show signs of dryness and display an unhealthy appearance. However, many companies have began to manufacture heat protection sprays, shampoos and cream which aim to reduce the amount of damage that is caused by the use of straighteners.
Hair straighteners should only be used on dry hair, unless they are specially made with vents to let the moisture out. These sort of irons are usually called "Wet Dry" irons.