Hair coloring
Hair coloring, or hair dyeing, is the practice of changing the color of the hair on humans' heads. The main reasons for this are cosmetic: to cover gray or white hair, to alter hair to create a specific look, to change a color to suit preference or to restore the original hair color after it has been discolored by hairdressing processes or sun bleaching.
Hair coloring can be done professionally by a hairdresser or independently at home. Hair coloring is very popular, with 50-80% of women in the United States, Europe, and Japan having reported using hair dye.[1] At-home coloring in the United States reached sales of $1.9 billion in 2011 and was expected to rise to $2.2 billion by 2016.
Plant-based dyes[edit]
Henna is an orange dye commonly used as a deposit-only{explain} hair color whose active component, lawsone, binds to keratin. It is therefore considered semi-permanent to permanent, depending on a person's hair type. Most people will achieve a permanent color from henna, especially after the second dye. With repeated use the orange color builds up into red and then auburn. While "natural" henna is generally a red color, variations exist. These variations usually contain ingredients from other plants and even synthetic dyes.
Indigo is natural dye from a plant (Indigofera tinctoria, I. suffructicosa, or I. arrecta) that can be added to henna or layered on top of it to create brown to black colors in the hair. Henna is orange, and indigo is blue, so as complementaries on a standard color wheel, the two colors' combined effect is to create brown tones. Like henna, indigo may fade after one application, but it becomes permanent on the hair with repeated use.
Using a plant-based color such as henna can cause problems later when trying to do a perm or permanent synthetic hair color, as some store-bought henna in addition contains metallic salts which react to hydrogene peroxide. This reaction may lead to unpredictable results, such as unwanted greenish or blueish tones. These added metallic salts also can cause skin irritation. Pure henna and purely plant-based mixes of henna, indigo ("black henna"), walnut hulls etc. without such addition are commercially available, too.
In 2018, a system for making semi-permanent hair dyes from renewable waste blackcurrant (Ribes nigrum L.) fruit skins from the fruit pressing industry was developed. Anthocyanin pigments were extracted from the blackcurrant skin waste[11] and formulated into hair dyes. The dyes showed high buildup on hair and gave an intense blue color, which was stable to multiple washes.[12]
Temporary color[edit]
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 Christmas and Halloween costume parties.
The pigments in temporary hair color cannot penetrate the cuticle layer of the hair. The color particles remain adsorbed (closely adherent) to the surface of 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 shaft.
Some plants can be used for temporary coloring, too, e.g. bete juice.
Synthetic permanent hair coloring requires three components: (1) 1,4-diaminobenzene (historically) or 2,5-diaminotoluene (currently), (2) a coupling agent, and (3) an oxidant. The process is typically performed under basic conditions. The mechanism of oxidation dyes involves three steps:[7]
1) Oxidation of 1,4-diaminobenzene derivative to the quinone state.
2) Reaction of this diimine with a coupler compound (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 ammonia causes the natural hair to be lightened, providing a "blank canvas" for the dye. Ammonia opens the hair shaft pores so that the dye can actually diffuse inside the fiber. These dye intermediates and coupler compounds can undergo oxidation and coupling reaction as shown in the scheme below to form high molecular weight products, which are trapped in the hair matrix and cannot be readily removed through washing.
Various combinations of primary intermediates and coupler compounds provide a spectrum of shades of hair colors. The primary intermediates are aromatic para compounds, such as 1,4-diaminobenzene or 4-aminophenol. The coupler compounds (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.
The first step shows the oxidation of p-phenylenediamine to the quinonediimine (C6H4(NH)2):
This species exists in equilibrium with the monoprotonated form (C6H4(NH)(NH2)+) (not shown). The second step involves the attack of this quinonediimine on the coupler. In organic chemistry, this reaction is called electrophilic aromatic substitution:
In the third and final step, the product from the quinonediimine-coupler reaction oxidizes to the final hair dye.
It was once believed that the dye forms in the above reaction bonds to hair permanently.[7] It was later shown that the main reason that this reaction imparts a permanent color on hair is that it produces larger dye molecules, which is locked inside the hair.[33]
Legal restrictions[edit]
Hair dyes are cosmetic compounds that make contact with the skin during application. Because of this skin contact, there exists some health risk associated with use of hair dyes.[34] Thus, hair dyes are regulated in the commercial marketplace and, as new toxicity data is generated for some hair dyes and health risks are discovered, some of these hair dyes are being legally restricted from the cosmetic marketplace.
The European Union is particularly stringent with regard to health regulations. To ensure that hair dyes contain only safe substances, the European Commission adopted the Directive 2012/21/EU to restrict the use of around 45 chemicals in hair dyes.[35] The directive on dyes is part of a general and comprehensive set of regulations, the EU Cosmetics Directive 76/768/EC.
Religion[edit]
For Muslims, if a man is not of advanced age, there is no harm in his using black dye. Some of the early Muslims, including some sahabah, permitted the use of black dye.[36] As narrated in a hadith, the best thing with which to dye gray hair is henna and katam (a plant from Yemen which colors it black tinged with red).
