Hair coloring is the practice of changing the color of hair.
Common reasons are to cover gray hair, to change to a color regarded as
more fashionable or desirable, and to restore the original hair color
after it has been discolored by hairdressing processes or sun bleaching.
In the 1661 book Eighteen Books of the Secrets of Art & Nature, various methods of coloring hair black, gold, green, red, yellow, and white are explained.[1]
The most popular way to achieve permanent hair coloring is through
the use of oxidation dyes. The ingredients of these products include
1,4-diaminobenzene (historically) or 2,5-diaminotoluene (currently), a
coupling agent, and an oxidant. The process is typically performed under
basic conditions.[2]
The mechanism of oxidation dyes involves three steps: 1) Oxidation of
1,4-diaminobenzene derivative to the quinone state. 2) Reaction of this
diimine with a coupler (more detail below). 3) Oxidation of the
resulting compound to give the final dye.
The preparation (dye precursors) is in the leuco (colorless) form. Oxidizing agents are usually hydrogen peroxide,
and the alkaline environment is usually provided by ammonia. The
combination of hydrogen peroxide and the primary intermediate causes the
natural hair to be lightened, which provides a blank canvas for the
dye. Ammonia opens the hair shaft so that the dye can actually bond with
the hair, and ammonia speeds up the reaction of the dye with the hair.
Various combinations of primary intermediates and couplers provide a
spectrum of shades of hair colors. The primary intermediates are
aromatic para compounds, such as 1,4-diaminobenzene or 4-aminophenol.
The couplers are meta-substituted derivatives of aniline. They come in three major classes based on the color that they produce when they react with the primary intermediate
Bleaching is a chemical process for removal of some or all natural or synthetic color from hair. Hydrogen peroxide and ammonium hydroxide
are common bleachants. Any color treatment to change to a lighter color
requires bleaching. Subsequent to bleaching, a light colored permanent
or semi permanent dye or toner may be applied.
Bleaching has the side effect of raising of the cuticle, making the hair more porous.
Semi-permanent hair dye has smaller molecules than temporary dyes,
and is therefore able to partially penetrate the hair shaft. For this
reason, the color will survive repeated washing, typically 4–5 shampoos
or a few weeks. Semi-permanents contain no, or very low levels of
developer, peroxide or ammonia, and are therefore safer for damaged or
fragile hair. However, semi-permanents may still contain the toxic
compound p-phenylenediamine
or other such ingredients. The U.S. Environmental Protection Agency
reported that in rats and mice chronically exposed to PPD in their diet,
it simply depressed body weights, and no other clinical signs of
toxicity were observed in several studies.[3]
The final color of each strand of hair will depend on its original
color and porosity, so there will be subtle variations in shade across
the whole head. This gives a more natural result than the solid, all
over color of a permanent dye. However, it also means that gray or white
hairs will not dye to the same shade as the rest of the hair. If there
are only a few grey/white hairs, the effect will usually be enough for
them to blend in, but as the gray spreads, there will come a point where
it will not be disguised as well. In this case, the move to permanent
color can sometimes be delayed by using the semi-permanent as a base and
adding highlights.
Semi-permanent color cannot lighten the hair.
Demi-permanent hair color
Demi-permanent hair color is permanent hair color that contains an
alkaline agent other than ammonia (e.g., ethanolamine, sodium carbonate)
and, while always employed with a developer, the concentration of
hydrogen peroxide in that developer may be lower than used with a
permanent hair color. Since the alkaline agents employed in
demi-permanent colors are less effective in removing the natural pigment
of hair than ammonia these products provide no lightening of hair's
color during dying. As the result, they cannot color hair to a lighter
shade than it was before dyeing and are less damaging to hair than their
permanent counterpart.
Demi-permanents are much more effective at covering gray hair than semi-permanents, but less so than permanents.
Demi-permanents have several advantages as compared with permanent
color. Because there is essentially no lifting (i.e., removal) of
natural hair color, the final color is less uniform/homogeneous than a
permanent and therefore more natural looking; they are gentler on hair
and therefore safer, especially for damaged hair; and they wash out over
time (typically 20 to 28 shampoos), so root regrowth is less noticeable
and if a change of color is desired, it is easier to achieve.
Demi-permanent hair colors are, in essence, permanent color and the
darker shades in particular may persist longer than indicated on the
packet
Temporary hair color
Temporary hair color is available in various forms including rinses,
shampoos, gels, sprays, and foams. Temporary hair color is typically
brighter and more vibrant than semi-permanent and permanent hair color.
It is most often used to color hair for special occasions such as
costume parties and Halloween.
The pigment molecules in temporary hair color are large and cannot penetrate the cuticle layer. The color particles remain adsorbed
(closely adherent) to the hair shaft and are easily removed with a
single shampooing. Temporary hair color can persist on hair that is
excessively dry or damaged in a way that allows for migration of the
pigment to the interior of the hair laurens shaft. It lasts for about a
few hours to 1 day
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