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Hair color change by dye application is a common procedure among women. Hair dyes are classified, according to color resistance, into temporary, semipermanent, demipermanent and permanent. The first two are based on molecules which are already colored. Temporary dyes act through dye deposition on cuticles, but semipermanent may penetrate a little into the cortex and so the color resists up to six washes. Demipermanent and permanent dyes are based on color precursors, called oxidation dyes, and the final shade is developed by their interactions with an oxidizing agent, but they differ from the alkalizing agent used. In oxidation systems, there is an intense diffusion of the molecules into the cortex, what promotes a longer color resistance. Dyes and color precursors present differences related to chromophore groups, hair fiber affinity, water solubility, and photo stability. The aim of this review is to discuss the differences among hair dye products available in the market and their action mechanisms, molecular structures, application methods, and some aspects of formulations.
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Cosmetics 2015, 2, 110-126; doi:10.3390/cosmetics2020110
cosmetics
ISSN 2079-9284
www.mdpi.com/journal/cosmetics
Review
Types of Hair Dye and Their Mechanisms of Action
Simone Aparecida da França, Michelli Ferrera Dario *, Victoria Brigatto Esteves,
André Rolim Baby and Maria Valéria Robles Velasco
School of Pharmaceutical Sciences, University of São Paulo, 05508-000 São Paulo, Brazil;
E-Mails: simone1002af@uol.com.br (S.A.F.); victoria.brigatto@gmail.com (V.B.E.);
andrerb@usp.br (A.R.B.); mvrobles@usp.br (M.V.R.V.)
* Author to whom correspondence should be addressed; E-Mail: michelli.dario@usp.br;
Tel.: +55-11-3091-3623; Fax: +55-11-3815-4418.
Academic Editor: Enzo Berardesca
Received: 18 February 2015 / Accepted: 2 April 2015 / Published: 22 April 2015
Abstract: Hair color change by dye application is a common procedure among women.
Hair dyes are classified, according to color resistance, into temporary, semipermanent,
demipermanent and permanent. The first two are based on molecules which are already
colored. Temporary dyes act through dye deposition on cuticles, but semipermanent may
penetrate a little into the cortex and so the color resists up to six washes. Demipermanent
and permanent dyes are based on color precursors, called oxidation dyes, and the final
shade is developed by their interactions with an oxidizing agent, but they differ from the
alkalizing agent used. In oxidation systems, there is an intense diffusion of the molecules
into the cortex, what promotes a longer color resistance. Dyes and color precursors present
differences related to chromophore groups, hair fiber affinity, water solubility, and photo
stability. The aim of this review is to discuss the differences among hair dye products available
in the market and their action mechanisms, molecular structures, application methods, and
some aspects of formulations.
Keywords: hair; dye; temporary; semipermanent; permanent; action mechanism
OPEN ACCESS
Cosmetics 2015, 2 111
1. Introduction
The use of cosmetics in order to change hair color, such as hair dye products, occurs with high
frequency, mostly among the female population [1]. However, these hair dyes, due to their action
mechanisms, may cause serious damage to the hair fiber structure [2].
Throughout human history, many people have wished to change the appearance of their hair
because it was a way to differentiate the social status. Hair dye has been used since Ancient Egyptian
times when Rameses II reinforced red hair color using henna. In Ancient Greece, the hair was bleached
with a rinse of potassium solution and rubbed with a type of ointment made of yellow flower petals and
pollen [3]. Nowadays, hair dyes are in an important phase of development and since the Second World
War, great progress in discoveries and applications of new synthetic dyes has occurred.
Brazil is a country that, because of its high miscegenation, presents almost all the hair types.
Furthermore, because of the great importance that women give to their hair treatments, Brazil is now
the world leader in hair dye products [4]. Nevertheless, the dye market has focused on exports, mainly
to South American countries.
2. Composition and Morphology of Hair Fiber
Hair is an annex of the epidermis and covers the external tissues of most mammal. It is also
considered an adornment. It works as a thermal regulator and protects the head and the skin from the
sun due to the presence of melanin [5]. Humans have between 90 and 150 thousands of hair fibers on
the scalp that grow 1 cm/month (0.37 mm/day), and the normal amount of hair lost is between 50 and
100 fibers per day. The hair diameter varies from 15 to 110 μm, depending on the race [6]. Caucasian
hair is usually thin and fine, may have waves, and is circular under the cross-section view (ellipticity of
1.25). The African hair type (wavy to curly) has a larger diameter, with a slightly oval cross-section
(ellipticity of 1.75). Lastly, Mongolian hair also has a larger diameter but varies from flat to wavy with
a cross-section similar to Caucasian hair (ellipticity of 1.35) [7,8].
Hair or fur is composed of dead skin cells which pass through a keratinization process, derived from
hair or hair follicles that are invaginations that protrude into the dermis or hypodermis [9]. Keratin,
the main protein found in the hair fiber, is produced by the keratinocytes of the epithelial tissue
invagination. Small amounts of water-soluble substances are also present, such as pentene, phenols,
uric acid, glycogen, glutamic acid, valine and leucine [10].
The hair shaft is divided into four main distinct structures: cuticle, cortex, cell membrane complex
(CMC), and the medulla [11].
The cuticles (which consists of amorphous and protein material) are the most external part of the
hair strand and ensure chemical resistance. These cuticles carry out the function of regulating the
amount of water in the hair structure, which keeps its physical properties. It contains six to ten layers
of overlapping cells in the longitudinal direction of the fiber [12]. The damage to the cuticle can be
caused by weather or mechanical friction such as combing and brushing. The excessive use of
shampoo and other inappropriate cosmetics may damage hair [13]. Each cuticle cell contains an
external thin membrane (5.0 to 10.0 nm) probably formed by a layer of fatty acid connected to the
Cosmetics 2015, 2 112
protein layer through thioester bonds, which generates cysteine residues responsible for the apparent
hydrophobic character of the fiber [12].
The cuticle contains three important layers: the A-layer (120 nm) with a high content of cysteine
and highly cross-linked; the exocuticle (B-layer), also rich in cysteine and occupying about the half of
the cell volume; and, finally, the endocuticle, a layer with a low content of cysteine and relatively high
levels of basic (lysine, arginine) and diacids (aspartic and glutamic acids) amino acids [7,14].
The cortex is the principal component of the hair, consisting of cylindrical cells of about 1 to 6 μm
of thickness and 100 μm of length. It forms the matrix where other proteins and keratin are located,
and composes the larger part of the fibrous mass of human hair, being formed by intracellular and
intercellular material [15]. The cortex represents 90% of its total weight and consists of cells filled
with keratin, with an organization that provides mechanical properties to the fibers [16]. The cortical
cells, adjacent to the cuticle, are flatter and contain less sulfur than the cells inside the cortex, which are
rich in cystine (two cysteine), amino acids, lysine and histidine, in addition to the melanin granules [7].
The matrix comprises the major structure of the hair and contains a high concentration of disulfide
bonds. It presents considerable swelling when in contact with water and forms a lightly cross-linked
gel structure. Although there are amorphous regions, the matrix presents small parts with structural
organization [17]. It exhibits keratin macrofibrils aligned in the direction of the hair strand and melanin
granules which are responsible for the hair color and its photo protection.
The CMC is an important layer in the hair structure consisting of cell membranes and adhesive
material that “glue” or link the cortical and the cuticle cells. Chemically, CMC is composed of
proteins, polysaccharides, and ceramides. It is also responsible for the hair’s natural moisture, making
it bright, transparent, and hydrated [6]. Its outer lipid layer forms the epicuticle and the inner lipid
layer is located between the cuticle cells which consist of the δ-layer, formed by proteins with a low
content of cystine (<2%) and richer in polar amino acids (12% basic and 17% acid). The CMC and the
endocuticle are usually referred to as non-keratinized regions because they have a low level of sulfur
amino acids and studies have proven that they are important pathways to the diffusion of molecules to
inner regions of the hair fiber [7].
The medulla is the innermost region and its presence along the hair is usually discontinuous or even
absent and does not interfere with the hair structure [11]. The medulla can be empty or filled with
sponge keratin, can serve as a pigment reservoir, and can contribute to the brightness of the hair.
The lipid concentration inside the medulla is bigger than anywhere else in the hair [18].
3. Hair Dyes
Hair dying systems can be divided into two main categories, oxidative or non-oxidative, and also
according to the color durability after the application on hair strands: temporary, semipermanent,
demipermanent and permanent [10,19].
Many studies established the diffusion path of the dye molecule to the inner hair fiber. It involves
the permeation of the molecules into intercuticular regions, passing through non-keratinized regions of
the endocuticle and the intracellular cement. In later stages, it migrates to keratinized regions and,
eventually, reaches the macrofibrils, before being incorporated into the matrix [7].
Cosmetics 2015, 2 113
The temporary and semipermanent non-oxidative dyes are based on colorful molecules, named dye
deposition, because the dye molecule only interacts with the hair cuticles. When there is a small
penetration of the molecules into the hair cortex, they are named semipermanent products and can be
resistant up to six washes. The demipermanent and permanent oxidative are based on precursors,
named oxidation dyes, whose color characteristics are developed by means of the interaction with an
oxidizing agent, and present longer lasting color [20].
3.1. Temporary Non-Oxidative Hair Dyeing
The temporary non-oxidative dyeing has a reduced permanence time on the fiber, leaving the hair
after the first shampoo washing because dye presents high molecular weight and deposits on the hair
surface without the capacity of penetrating the cortex [15]. This type of dye does not have the power of
whitening the hair strand and, therefore, it is indicated only to add new nuance and not to change its
color [1]. In white, blond or bleached hairs, it is possible to add a new color with a more noticeable
effect because the hair strand’s background color allows the visualization of the new applied color.
The temporary dye can be used for specific purposes such as adding colorful reflections, removing
the yellowish effects of the white hair, and covering a small quantity of white hair [21]. It allows the
dyeing of hair containing up to 15% white hair, due to their ability to deposit on the hair strands.
These dyes, that present acid characteristics [22] usually have high molar mass, according to the
structures presented in Table 1. They contain anionic characteristics and are selected to allow the
maximum solubility in water and the minimum penetration in hair so it is removed in the first
washing [17]. They are presented as shampoo, gel, emulsion and solution (liquid) with two different
forms of application: continuous application (progressive) or single application, with one wash at the
end of the application process to remove the unabsorbed dye excess on the hair strand.
Table 1. Structural and molecular formulas of acid dyes used in temporary non-oxidative
dye formulation [23]. Legend: INCI, International Nomenclature of Cosmetic Ingredients;
CAS, Chemical Abstract Service.
INCI: Acid Yellow 23 INCI: Acid Orange 7
CAS No. 1934-21-0
EINECS No. 217-699-5
Formula: C16H12N4O9S2·3Na
Chemical Classification: pyrazol
CAS No. 633-96-5
EINECS No. 211-199-0
Formula: C16H12N2O4S·Na
Chemical Classification: monoazo
Cosmetics 2015, 2 114
Table 1. Cont.
INCI: Acid Yellow 1 INCI: Acid Red 33
CAS No. 846-70-8
EINECS No. 212-690-2
Formula: C10H6N2O8S·2Na
Chemical Classification: nitro
CAS No. 3567-66-6
EINECS No. 222-656-9
Formula: C16H13N3O7S2·2Na
Chemical Classification: monoazo
INCI: Acid Red 92 INCI: Acid Violet 43
CAS No. 18472-87-2/4618-23-9
EINECS No. 242-355-6
Formula: C20H4Br4Cl4O5·2Na
Chemical Classification: xanthene
CAS No. 4430-18-6
EINECS No. 224-618-7
Formula: C21H15NO6S·Na
Chemical Classification: anthraquinone
INCI: Acid Blue 9 INCI: Acid Black 1
CAS No. 3844-45-9
EINECS No. 223-339-8
Formula: C37H36N2O9S3·2Na
Chemical Classification: triphenylmethane
CAS No. 1064-48-8
EINECS No. 213-903-1
Formula: C22H14N6O9S2·2Na
Chemical Classification: diazo
O-
S
-O
O
O
N+
-OO
N+
O-
O
Na+
Na+
Cosmetics 2015, 2 115
Frequently, two to five substances are necessary to reach the desired hair color because just one
substance does not achieve natural shades. Some formulations use two molecules to remove the
yellowish effect in white hair and also four to five substances are mixed to reach the red, brown, and
black shades [17].
The temporary non-oxidative formulations as single applications, present higher dye concentrations,
ranging from 0.1% to 2.0% (w/w) and have the purpose of promoting a stronger dyeing effect.
However, the limit of deposition must be always respected because this type of application will not
cover gray hair satisfactorily in people with more than 30% of white hair fibers. The formulation must
get in contact with hair for about 30 min and results will occur immediately. It is suitable for those
who wish for fantasy colors [24]. It resists from three to six washes when applied to bleached hair,
like semipermanent dyeing.
3.2. Semi-Permanent Non-Oxidative Hair Dyeing
These formulations contain basic or cationic dyes with low molar mass, which has a high affinity
for hair keratin and resists from three to six washes [10]. The hair dyeing process does not involve
oxidation reaction; the application is simple and lasts from 10 to 40 min, followed by rinsing [1,25].
Several products are available in the market: lotions, shampoos, mousses and emulsions. These
cosmetic forms must have the ideal viscosity so that they do not flow during the application [20]. Dyes
with low molar mass penetrate slightly in the cortex, especially because of the high pH value of the
product that promotes the cuticles opening [15].
Demipermanent hair products promote major hair color durability (resistance up to 20 washes)
because they consist of a mix of semipermanent molecules with oxidation dye precursors, applied with
hydrogen peroxide (H2O2) [20].
Another option of formulation involves mixing nitro aniline dyes with basic or acid dyes which aim
for a better color result and a bigger resistance to washes, considering the high affinity of the two
families of dyes. The hair space not filled with the basic dyes will be occupied by nitro anilines,
thus promoting a much more uniform color in the first application. Table 2 contains the molecules
allowed and used in cosmetic formulations for semipermanent hair dyeing.
The nitro anilines are molecules comprised of a neutral aromatic amine or anthraquinone derivatives
and all are classified as highly polar and present mono, di, or tri nuclear rings. These dyes are diffused
through the hair fiber and are retained by weak Van de Waals bonds [17]. Under similar conditions, the
larger molecules with tri aromatic rings are removed more slowly from hair than the smaller,
mononuclear ones [26].
The cationic dyes shown in Table 3 are used in both temporary and semipermanent dyeing.
They permit reflective effects and are excellent for instantaneous color effects. The similarities
between the size of the cationic molecules grant substantivity to the hair in a homogenous way,
ensuring the color reproducibility and the resistance to washing uniformly. In other words, all dyes are
removed simultaneously during this process [24].
Cosmetics 2015, 2 116
Table 2. Structural and molecular formulas of nitro anilines used in semipermanent hair
dyeing formulation [23]. Legend: INCI, International Nomenclature of Cosmetic
Ingredients; CAS, Chemical Abstract Service; HC, Hair Color.
INCI: HC Yellow No. 2 INCI: HC Red No. 3
CAS No. 4926-55-0
EINECS No. 225-555-8
Empirical Formula: C8H10N2O3
Chemical Classification: nitro aniline
CAS No. 2871-01-4
EINECS No. 220-701-7
Empirical Formula: C8H11N3O3
Chemical Classification: nitro aniline
INCI: 4-hydroxypropylamino-3-nitrophenol INCI: N,N'-bis-(2-hydroxyethyl)-2-nitro-
phenylenediamine
CAS No. 92952-81-3
EINECS No. 406-305-9
Empirical Formula: C9H12N2O4
Chemical Classification: nitro aniline
CAS No. 84041-77-0
EINECS No. 281-856-4
Empirical Formula: C10H15N3O4
Chemical Classification: nitro aniline
INCI: HC Blue No. 2
CAS No. 33229-34-4
EINECS No. 251-410-3
Empirical Formula: C12H19N3O5
Chemical Classification: nitro aniline
OH
N+
N
H
HO
O
-O
Cosmetics 2015, 2 117
The good performance of semipermanent dyes is directly related to their great water solubility.
In general, the nitro anilines are not soluble in water and require a glycol or glycol derivative, such as
glycerin, to be solubilized in the formulation. Specific solvents, such as mixtures of quaternary salts of
high molecular weight, such as Quaternium-80, benzyl alcohol, and glycols are used to ensure not only
their solubility in the formulation but also during application and product storage [24,27].
Cationic dyes exhibit excellent affinity for damaged hair, because positive sites of the dye molecule
bind to negative sites on the hair fiber by an ionic bond [28]. They allow greater resistance to washing
when compared to nitro anilines.
Table 3. Structural and molecular formulas of cationic dyes used in semi-permanent hair
dyeing formulation [23]. Legend: INCI, International Nomenclature of Cosmetic
Ingredients; CAS, Chemical Abstract Service.
INCI: Basic Red 51 INCI: Basic Red 76
CAS No. 77061-58-6
EINECS No. 278-601-4
Empirical Formula: C13H18N5·Cl
Chemical Classification: direct dye
CAS No. 68391-30-0
EINECS No. 269-941-4
Empirical Formula: C20H22N3O2·Cl
Chemical Classification: direct dye
INCI: Basic Brown 16 INCI: Basic Brown 17
CAS No. 26381-41-9
EINECS No. 247-640-9
Empirical Formula: C19H21N4O·Cl
Chemical Classification: direct dye
CAS No. 68391-32-2
EINECS No. 269-940-0
Empirical Formula: C19H20N5O3·Cl
Chemical Classification: direct dye
Cosmetics 2015, 2 118
Table 3. Cont.
INCI: Basic Blue 99 INCI: Basic Yellow 57
CAS No. 68123-13-7
EINECS No. 268-544-3
Empirical Formula: C19H20BrN4O2·Cl
Chemical Classification: direct dye
CAS No. 68391-31-1
EINECS No. 269-943-5
Empirical Formula: C19H22N5O·Cl
Chemical Classification: direct dye
The cationic dyes are water-soluble; however, they also require the addition of solvents to ensure
the homogeneity of the color and prevent recrystallization during storage because, in recrystallized
form, the molecule does not provide the cationic sites to bind to hair strands [24].
The pH control is essential for color stability. A weak base such as mono ethanolamine must be
added to achieve a pH of 9.0, and then a weak acid such as 10% citric solution is used to lower the pH
value to 6.0. This way, a buffer system which ensures the pH maintenance of the finished product
during shelf life is formed [24].
Other dyes, such as metallic and vegetables derivatives, are also considered to be semipermanent
dyes and can be used in hair dyeing. Henna is the most widely used vegetable dye for hair, promoting
reddish orange color shades [1,25]. In some commercial products, it is mixed with other dyes to
increase the range of color. It consists of the dried leaves of the Lawsonia alba plant, growing in North
Africa, in the Midwest, and in India [19,20]. Its coloring properties are due to the presence of the
substance 2-hydroxy-1,4-naphthoquinone, soluble in hot water and substantive to hair keratin in
pH 5.5 [10,29].
Another vegetable dye commonly used to obtain yellow shades is chamomile [1] that promotes
greater light reflection. Of all the species of chamomile, only Anthemis nobilis (Roman Chamomile)
and Matricaria chamomillae (German chamomile) have cosmetic applications, and both are
substantive to hair. The active ingredient of the flowers is 1,3,4-trihydroxyflavone, also known as
apigenin [10].
The metallic dyes are derived from silver salts, lead, and bismuth and are traditionally used by men
because the dyeing effect is not immediate and does not promote 100% of the white hair coverage.
The darkening of the hair strands occurs gradually, promoting a more natural appearance, which
satisfies the general public. The use is limited to the number of gray hair strands [1]. Therefore, hair
with a lot of white fibers cannot have a satisfactory result because of the leak of homogeneity in the
final color.
Products containing diluted lead acetate are applied to the hair daily and do not require washing,
so the metal salt is exposed to air oxygen and also reacts with sulfur from keratin. These reactions
OH
NH
NH2
O
H
N
Br
N+
Cl-
Cosmetics 2015, 2 119
generate a mixture of metallic oxides and insoluble sulfides [10], responsible for gradual darkening of
gray hair [20].
A disadvantage of the metallic dyeing process is the lack of control in the color progression because
of the composition of hair, which is based on keratin. The keratin reduction will result in different
colors, so after the first applications, hair can present greenish or yellowish shades. However, the final
result is natural because the color develops progressively to more usual shades, such as brown and
black, and coverage of 100% of white hair strands is impossible.
3.3. Permanent Oxidative Hair Dyeing
The permanent hair dyes are commonly used [1] because this category provides greater efficacy of
permanent dyeing, resistance to shampoo washes and other external factors, such as drying, friction,
light, and others. This category represents about 80% of the sold hair dyes [10] and gets any shade,
covering up to 100% of white hair strands. Also, it is possible to have dark and light natural hair color [21]
due to the combination of the oxidizing agents with the ammonia hydroxide. The principal difference
between the demipermanent hair dye in comparison with a permanent one is the alkalizing agent used
because, in the first, monoethanolamine with low color lightening power is used [28].
Color formation happens upon mixture and involves complex reactions between precursors in the
presence of an oxidizing agent [20]. The precursors can be classified into two categories: oxidation
basis or primary intermediaries, and the couplers or reaction modifiers [10].
The reaction occurs in an alkaline medium that promotes the opening of the cuticles that allows the
penetration of the dyes’ molecules into the cortex. The oxidizing agent permits the beginning of the
reaction that occurs in the cortex and results in a colorful complex with high molar mass, which avoids
the exit of molecules formed in the hair. Part of the reaction also happens on the cuticles and the
molecules are removed in the first washes [30,31].
The ammonia hydroxide and ethanolamines are the most alkalizing agents used. A mixture of
surfactants and solvents is used to disperse the dye molecules and ensure the hair wetting. A small
amount of reducing agent is added to prevent auto-oxidation of the dyes during storage of the finished
product [20], which may be formulated as an emulsion, gel, solution and powder.
The reactions involved in the formation of permanent dyes are redox types and require four major
components: the aromatic amine with substitutions at positions ortho or para (hydroxy or amino) as
the coupling bases; the reaction modifiers; an alkalizing compound; and an oxidizing agent.
3.3.1. Coupling Bases
Bases are aromatic compounds derived from benzene, substituted by at least two electron donor groups
such as NH2 and OH in para or ortho positions to confer the property of easy oxidation [10], acting as a
color developer [28]. Two major compounds are used such as p-phenylenediamine (Scheme 1) and
p-aminophenol (Scheme 2).
Cosmetics 2015, 2 120
Scheme 1. Structural formula of p-phenylenediamine (PPD). INCI name: phenylenodiamine.
CAS No. 106-50-3. EINECS No. 203-404-7. Formula: C6H8N2. Chemical classification:
aromatic amine [23]. Legend: INCI, International Nomenclature of Cosmetic Ingredients;
CAS, Chemical Abstract Service.
Scheme 2. Structural formula of p-aminophenol (PAP). INCI name: p-aminophenol.
CAS No. 123-30-8. EINECS No. 204-616-2. Formula: C6H7NO. Chemical classification:
substitute phenol [23].
3.3.2. Reaction Modifiers
Reaction modifiers, also called couplers, are aromatic compounds derived from benzene and
substituted by groups such as NH2 and OH in the meta position, which does not present easy oxidation
by H2O2 [10]. They do not produce significant color alone but can modify them when used as primary
intermediaries and oxidants [28]. There are many reaction modifiers available in the market, and some
of the most important are shown in Table 4.
Cosmetics 2015, 2 121
Table 4. Molecular and structural formulas of reaction modifiers [23]. Legend: INCI,
International Nomenclature of Cosmetic Ingredients; CAS, Chemical Abstract Service.
INCI: 4-chlororesorcinol INCI: 2,4-diaminophenoxyethanol HCl
CAS No. 95-88-5
EINECS No. 202-462-0
Empirical Formula: C6H5ClO2
Chemical Classification: halogenated phenol
CAS No. 66422-95-5
EINECS No. 266-357-1
Empirical Formula: C8H12N2O2·2HCl
Chemical Classification: aromatic amine salt
INCI: 2-amino-hydroxyethylaminoanisole sulfate INCI: 4-amino-2-hydroxytoluene
CAS No. 83763-48-8
EINECS No. 280-734-8
Empirical Formula: C9H14N2O2·H2O4S
Chemical Classification: aromatic amine salt
CAS No. 2835-95-2
EINECS No. 220-618-6
Empirical Formula: C7H9NO
Chemical Classification: aromatic substitute
INCI: m-aminophenol INCI: Resorcinol
CAS No. 591-27-5
EINECS No. 209-711-2
Empirical Formula: C6H7NO
Chemical Classification: substitute phenol
CAS No. 108-46-3
EINECS No. 203-585-2
Empirical Formula: C6H6O2
Chemical Classification: phenol
3.3.3. Alkalizing Compounds
The addition of alkalizing compounds is necessary for the process of hair dyeing to promote the
proper pH value for the beginning of the oxidation reaction. The most commonly alkalizing
compounds used are ammonia, in the form of ammonium hydroxide, and monoethanolamine, when the
formulation contains water, or sodium silicate when it is in solid form (powder).
2
OHH2N
Cosmetics 2015, 2 122
When ammonia is used, it is possible to cover 100% of white hair and to remove the natural
pigments present in hair as the melanin. This type of dye is considered permanent because, after the
formation of the colored polymer in the inner of the cortex, its complete removal is not possible.
However, some polymers formed from the reaction between couplers and precursors can be eliminated
by reducing agents such as sodium hydrosulfite [10].
The addition of monoethanolamine (MEA) is required to maintain optimum alkaline pH for the
reaction. However, this substance does not oxidize the melanin. Thus, products containing MEA
instead of ammonia hydroxide are suitable for maintenance of similar shades or to dark hair [24].
3.3.4. Reducing Agents
Reducing agents are added to oxidative dye formulations to retard the reaction between bases and
reaction modifiers and to prevent the initiation of the reaction in the packaging tube during the storage
time. One of the molecules most used for such applications is sodium metabisulfite (MBS).
3.3.5. Antioxidants
Antioxidants are necessary to avoid the reaction beginning before the addition of the oxidant itself.
It is recommended to use a water-soluble antioxidant because the manipulation of bases and reaction
modifiers could initiate the oxidative reaction, which may interfere with the final color of the product.
One of the molecules most frequently used for this purpose is the erythorbic acid (AEB). It is also
recommended to use an oil-soluble antioxidant when emulsion is used as a vehicle for hair dyes
because this avoids the yellowing of wax and the oxidation of bases and reaction modifiers. One of the
most used molecules is T-butylquinone (TBQ) [24].
3.3.6. Oxidants
There are basically two types of oxidants used: hydrogen peroxide, when the vehicle is water, and
sodium persulfate, when it is a powder. The peroxides are very unstable, requiring the use of stabilizers
such as sodium stannate and the pentasodium pentetate [24]. They are usually used in the form of
emulsion, so-called “creamy hydrogen peroxide”.
3.3.7. Vehicles
The oxidative dye in the form of emulsion is the highest selling product in the market, but other
carriers are available such as gels, solutions (liquid), and powders. The preparation of the emulsion
begins with adding the dye mixture to the reducing agents, antioxidants, and ammonium hydroxide in
20% of the aqueous phase. The other 80% of water is added in a manufacturing tank and heated to
70 °C under constant agitation. After reaching the temperature, all the wax and the emulsifying agents
are added, maintaining a constant stirring until cooling to 40 °C, when the remaining 20% of the water
previously prepared is added. It is recommended to measure the ammonia content at the end of the process
and to ensure that the amount of alkalizing is sufficient to complete the reaction after 24 h. Because it is
very volatile, a small amount of ammonia can be lost during the process, so an adjustment is necessary
before packaging [24].
Cosmetics 2015, 2 123
3.3.8. Hair Color Formation
Bandrowski’s base (Figure 1) may be formed during the color formation process by the reaction of
p-phenylenediamine (PPD) coupling base in alkaline medium containing hydrogen peroxide [32].
It is a compound that reacts preferentially with the modifiers presented in Table 4 for the formation of
colored compounds which rise gradually. This reaction occurs in stages until the final formation of the
color, so the amounts of coupling bases and modifiers vary with the desired final color [33]. About 3%
to 5% of PPD becomes a Bandrowski’s base, which is present in most oxidation reactions but does not
affect the final hair color [34].
Figure 1. Bandrowski’s base. CAS No. 20048-27-5. Formula: C18H18N6 [33].
The intermediate compounds have similar sizes and, therefore, an easy and uniform penetration
occurs inside the hair [34]. The critical diameter size of the molecules for this penetration to occur is
6.0 Å, because the intermediates, in most cases, vary from 4.7 to 5.6 Å [35]. The color formation is
based on a series of oxidation and coupling reactions, divided into three main stages:
(a) Quinonamines formation: is the oxidation of low reactivity bases with hydrogen peroxide
under alkaline conditions with the formation of monamines from para and ortho-aminophenols, and
diamines from p-phenylenediamine and o-phenylenediamine. The PPD is especially oxidized in a
reactive intermediate, the quinoamine that, in the presence of the reaction modifier, will generate a
colorful polymer [33].
(b) Diphenylamines formation: cations of quinones formed in the first stage receive an addition of
couplers to form a substituted p-phenylenediamine.
Nucleophilic compounds including meta couplers and para non-oxidized bases act as couplers
because they react with the nitrogen atom of amine-quinones. As an example, there is the formation of
diphenylamines from the reaction of p-phenylenediamine with m-phenylenediamine [24]. Thus, a full
range of substituted diphenylamines from other para bases amine-quinones and other non-oxidative
bases may be formed.
Cosmetics 2015, 2 124
(c) Color formation: Diphenylamines formed can be considered as new oxidation bases, in which
one of the benzene rings are tri substituted (1,2,4 or 1,2,5 positions) by electron donor groups. Because
of this, they have the same potential, possibility of oxidation, and coupling capacitance that the
original para bases from which they are derived [10].
The intermediate compound formed in these p-phenylenediamine trimers is Bandrowski’s base
which is considered a primary intermediate in the color formation. The process occurs at a slow rate in
the presence of hydrogen peroxide (30 to 45 min), which is interesting for the penetration of these
intermediates in the cortex. The molecules are initially small in size and are then transformed into
Bandrowski’s base with greater size than those that do not have a satisfactory penetration [35].
Various parameters may affect the color formation in the hair dyeing process, such as pH, pause
time, hair keratin, and purity of the dye molecule, amongst others.
The variation of the pH value directly influences the reaction rate because a more alkaline pH
favors the reaction and facilitates the cuticle opening, allowing the penetration of molecules into the
cortex [28].
The pause time is essential for a complete reaction between the bases and reaction modifiers to
occur. According to the manufacturer’s guidelines, the product must be in contact with the hair from
30 to 45 min after application because it is then possible to ensure color reproduction and durability to
washing. The removal of the product in a shorter time can stop the reaction before it is complete,
creating a variance in the final color [24].
4. Conclusions
Among the various options of hair dyes, it is interesting to know the application features and their
affinity for the hair fibers in order to select the best option for each hair type and to provide a satisfactory
effect, as a good covering power of gray/white hair, good color resistance to shampoo washes, and high
durability of color. The challenge is to find options that provide security in the application and allow
these benefits to occur without generating very aggressive damage to the hair strands.
Acknowledgments
We thank Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for financial support
(FAPESP Process: 2013/16070-8).
Author Contributions
Simone Aparecida da França and Michelli Ferrera Dario wrote the paper; Victoria Brigatto Esteves
drew all images; André Rolim Baby and Maria Valéria Robles Velasco reviewed the paper.
Conflicts of Interest
The authors declare no conflict of interest.
Cosmetics 2015, 2 125
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© 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/4.0/).
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... Thus, the phrase "oxidative hair dye" was coined. 3,11 There are two key components within an oxidative hair dye formulation: the precursor and the coupler. Quinone diimine intermediates are intermediate compounds generated when combined with hydrogen peroxide (developer). ...
... The developer is in one vial, whereas the precursor and couplers are in another vial. 1,11 p-Phenylenediamine (PPD) is a common chemical used as a permanent hair dye. PPD has the chemical formula C 6 H4 (NH 2 ) 2 , indicating that it is an organic substance. ...
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