CHAPTER 31

Hair Colorants

INTRODUCTION
The coloring of hair is one of the oldest and most prevalent acts of adornment.
Early Egyptians used henna (i.e., Lawsonia inermis) preparations for imparting
reddish-orange highlights to hair, and still today henna, henna extract, and
the actual color active lawsone (2-hydroxy-l,4-naphthoquinone) are recog-
nized cosmetic ingredients. Modern usage of natural colorants is quite minor,
however, because of the vastly superior performance of synthetic colorants.
It is estimated that today up to 50% of women in the industrialized coun-
tries are users of coloring products, either in the home or at the hairdresser.
Enhancement of appearance remains the primary motivation for hair coloring,
and extends from temporary color addition through more durable color high-
lights and gray coverage to lasting color changes. Each stage has its own
set of dye types and chemistries, and discussion should begin with a general
overview of the coloring systems.
This review will stress current technologies and will not address the majority
of patent literature. The breadth of patents covering hair coloration is quite
vast, yet only a portion has been reduced to practice. Only this applied knowl-
edge is within the scope of this chapter.

HAIR COLORING SYSTEMS

Modern systems of hair coloring may be divided into three broad categories
determined by the duration of color on the hair. However, new dyestuffs
and improved formulations have somewhat blurred the distinctions between
these categories. The market share of each of these categories is, perhaps not
surprisingly, directly related to their degree of durability. The longest-lasting
products are by far the most popular.

669
670 Harry's Cosmeticology

TEMPORARY COLORING
These are fugitive colors that can be removed at the first shampooing. Commer-
cial products are frequently designated "color rinses." These products utilize
ionic colorants of a high molecular weight that are in effect deposited on the
surface of the hair without being able to penetrate the cortex. Additionally,
these are acid dyes that have minimal affinity for hair that has an overall
anionic character.
A second class of temporary colorants are cationic compounds. These are
basic dyes with at least an ionic attraction for hair, but still only surface
deposition and less durability than semipermanents.
Hair may also be colored temporarily with pigments, but usually has an
unnatural feel, and this type of temporary is least durable and most subject to
flaking and abrasion.

SEMIPERMANENT COLORING
These are colors that resist several shampooings (six to eight), but whose fast-
ness is poorer than that of permanent colors. The colors used in this case are
direct dyes of low molecular weight having a good affinity for hair keratin.
Because of this, they are capable of penetrating the cuticle and diffusing
throughout the cortex.

PERMANENT COLORING
As the name indicates, this category provides effectively permanent coloration,
resistant to shampooing and other external factors such as brushing, friction,
light, and so forth. This is the process most widely used and represents at least
80% of the hair coloring market.
In this system, uncolored precursors undergo a series of chemical reactions
to produce the desired color in situ in the hair. The process is one of oxidation
(almost always effected by hydrogen peroxide), followed by coupling and
further oxidations. Thus this system is also called oxidative coloring. This is
the only system in which the dyes are not preformed; the oxidative chemistry
involved is somewhat similar to that used in color photography.

CHARACTERISTICS OF AN IDEAL HAIR COLORANT

The ideal hair colorant should possess the following properties:

Harmlessness
(1) It should not damage the hair structure and should color the hair without impairing
the natural texture and gloss;
(2) It should possess no primary irritant action and be free from sensitizing properties.
 Hair Colorants 671

The optimization of formulas to maximize dyeing efficiency while minimizing

dye concentrations and oxidative damage have gone far to fulfill these goals.

Adequate Physical and Chemical Stability on the Hair

The color of the dyed hair should be stable to air, sunlight, friction (rubbing,
brushing), sweat, and chlorinated water (municipal supplies and pool treat-
ments).

Compatibility with Other Hair Treatments

It should not change color or bleach out on the application of any hair
cosmetic preparation such as brilliantines, fixatives, hair lacquers, hair waving
preparations, or conditioners. Shampoos should remove color completely for
temporaries, gradually and on-shade for semipermanents, and not at all for
permanents.

Stability in Solution
Colorants should be stable both chemically and physically (i.e., no precipita-
tion) in formulated products, whether liquid, cream, gel, or solid.

Absence of Selectivity
Because it is always necessary to use a mixture of dyestuffs, selectivity is quite
important. Hair is very heterogeneous both individually and in its "history"
(the ends damaged by air and sun, previous perms or other treatments, roots
compared with shaft, etc.). The ideal colorant avoids:

(1) significantly different coloration on different parts of the same hair;

(2) different fastness over time of different dyestuffs on the hair.

The problem of selectivity plays a most significant part in the technology of

hair colorants. It is probably most critical in the semipermanents, which rely
solely on physical diffusion into the hair of dyes with significantly different
molecular sizes. Oxidative dyes are somewhat less prone to selectivity issues,
since the precursors are generally small molecules and there is an evenness
after reactive formation of the larger dye molecules. Additionally, there may
be some leveling effects with peroxide.

Affinity for Hair Keratin

Hair dyeing is quite unlike the dyeing of any other fiber in terms of neces-
sary conditions. Textiles and other fibers may be treated in very stringent
environments of pH and temperature extremes, but hair must be colored in a
physiologically acceptable manner. Thus the physicochemical characteristics
of affinity of the dyestuff for, and its diffusabilty into, the hair shaft are quite
significant in light of the limitations controlling hair dyeing. These include
672 Harry's Cosmeticology

low temperature (40 °C), short time of contact (5-40 minutes), minimum dye
concentrations, and so forth. The formulator faces the difficult task of devel-
oping a product that initially solubilizes the dyestuffs, keeps them in stable
solution, then gives optimum dyeing performance under these nonstringent
conditions.
Affinity is the more significant factor for temporary colorants, which only
adhere to the exterior hair surface to impart color. These molecules are far too
large for any diffusion through the cuticle and into the cortex.
Conversely, diffusability is more critical for semipermanent and permanent
colorants. At least part of the durability of these compounds is because of
their complete distribution throughout the hair shaft; thus diffusion must be
fast, even, and comprehensive.
Ingredients such as water, organic solvents, swelling agents, alkalis, condi-
tioners, preservatives, and so forth must be well balanced to improve pene-
tration and/or modify the partition coefficient between water and hair. It can
easily be seen that the vehicle may be just as critical as the dyes for an effective
product.

THE PROCESS OF HAIR COLORING

Hair colorants provide a range of commercial products capable of coloring
the hair in various shades and tints, ranging from very light blonde to black
and passing through a range of tones: golden, ash, reddish, mahogany, violets,
and so forth. The number of shades constituting such a range can exceed
60, although most consumer products include only 20-30 shades in a given
product line. All these products use and are based on strictly limited technical
factors, a summary of which follows.

Commercial Products Are Mixtures

The dyeing solutions contain mixtures of several single dyestuffs, as few as 3
but usually 5 to 6 with a maximum of 10-12. In fact, each particular shade is
the overall result of the blending of individual colorations (red, yellow, violet,
blue, etc.) supplied by each of the dyestuffs or each intermediate or coupler
combination.
Concentrations of Dyestuffs
The total quantity of all the dyestuffs used to obtain a shade is limited and
can range roughly between 0.01% and 5% by weight of the tinting medium
applied to the head. The concentration is a function of the dyestuffs used,
shade desired, the procedure involved, and any regulatory considerations. For
 Hair Colorants 673

instance, levels of specific dyestuffs in European products are regulated by

COLIPA.
Duration of Coloring Process
The time of contact of the dyeing solution with the scalp and the hair is on the
order of 5-40 minutes. Shorter dyeing times usually necessitate higher dye
loads but do not have the durability of products with longer application times
because of less thorough penetration. Additionally, shorter times allow only
minimal bleaching of base color by peroxide-containing products.

Quantity of Solution Applied

The amount of dyeing solution applied varies between 15 and 100 ml.

Frequency of Application
Temporary colorants based on acid dyes are used generally only when desired,
especially for a specific event or occasion. Their poor resistance to perspiration
and precipitation, along with complete removal by shampoo, makes usage
more episodic than routine. This includes products containing very low levels
of violet colorants used to counteract the yellowness of completely gray hair.
Although they are generally formulated as shampoos and meant to be used each
time the hair is washed, the user customarily shampoos less frequently than
the general population. Products based on the more substantive basic dyes can
be used at every shampooing, usually to "freshen" a previously applied, more
permanent colorant. Semipermanent products are formulated to last through
six to eight shampoos rather than a specific time, so they are usually reapplied
when needed. Oxidative colorants usually last a minimum of 24 shampoos,
so the appearance of new hair growth (the "roots") determines the time for
reapplication. Generally, this is three to six weeks, with once per month on
average.
Treatment After Coloration
Colorants must be formulated to minimize the staining of the scalp while
maximizing the hair dyeing. This is with semipermanent and permanent prod-
ucts, assisted by abundant rinsing with water that is obligatory after every
application of dye and also by shampooing one or more times to wash away
any dye that has not been absorbed.
Many products include a conditioning after-treatment to ease both wet and
dry combability and enhance the feel of the hair. They must accomplish this
task while not leaching out any of the colorant. In fact, after-treatments often
embellish the durability of the color by helping to "close" the cuticle layer
back to its preswollen configuration.
674 Harry's Cosmeticology

TEMPORARY HAIR DYES

DYESTUFFS
The dyestuffs used are generally acid dyes or basic dyes and may belong
to any chemical class, although azo and anthraquinone compounds predomi-
nate. Table 31.1 lists some of the common temporary dyestuffs along with
Color Index Number and Name, and FDA designation where applicable.
This list is representative, not exhaustive. The names in boldface are the
recognized International Nomenclature for Cosmetic Ingredients identifica-
tions; some colorants can have two identifications, depending on whether FDA
certification is obtained.

TYPES OF COMMERCIAL TEMPORARY PRODUCTS AND

THEIR FORMULATION
Temporary hair coloration products may be rinses, shampoos, or colored fixa-
tives. In rinses the dyestuffs are used in the form of simple aqueous or
aqueous-alcoholic solutions. To increase the substantivity to hair, various assis-
tants (organic acids such as citric or tartaric, certain solvents such as benzyl
alcohol or propylene glycol, etc.) are added, or the hair may be pretreated with
cationic compounds. Alternatively, coloration may be achieved by precipi-
tating the dye onto hair with a cationic polymer, although it should be noted
that such tinting solutions may not be stable long term. These rinses are gener-
ally ready-to-use and are more fluid than the thickened products in other hair
dye categories.

Table 31.1 Common Temporary Colorants

CI Name CI Number FDA Designation Chemical Class

Acid Yellow 3 47005 D&C Yellow #10 Quinoline

Acid Orange 7 15510 D&C Orange #4 Monoazo
Acid Orange 24 20170 Disazo
Direct Red 80 35780 Polyazo
Food Red 1 14700 FD&C Red #4 Monoazo
Acid Red 33 17200 D&C Red #33 Monoazo
Acid Violet 43 60730 Ext. D&C Violet #2 Anthraquinone
Acid Blue 9 42090 FD&C Blue #1 Triphenylmethane
Acid Green 25 61570 D&C Green #5 Anthraquinone
Direct Black 51 31720 Disazo
Basic Yellow 57 13119 Monoazo
Basic Red 76 12245 Monoazo
Basic Blue 99 56059 Naphthoquinoneimine
Basic Brown 16 12250 Monoazo
Basic Brown 17 12251 Monoazo
 Hair Colorants 675

Temporary colorants formulated solely with D& C and/or FD&C dyes have
the significant advantage of not requiring a preliminary patch test, usually
performed 48 hours prior to application of the product to hair, to check for
allergenic response. Some products in this category have been claimed to be
hypoallergenic.
The more substantive basic dyes have found a successful niche as treat-
ments between application of longer lasting colorants. These "color refresher"
products are usually formulated as shampoos and may be employed each time
the hair is washed.
It should be noted that application of heat after treatment with temporary
colorants may significantly increase the durability of the dyes.
The second approach, that of colored fixatives, consists in applying the
dyestuffs to the hair in the vehicle of a setting or styling aid. This is usually in
the form of a gel or aerosol mousse and is a leave-in product. Alternatively,
insoluble pigments rather than more traditional colorants may be used as the
coloring agents in this type of product. This should be thought of more as a
deposition of particles rather than a dyeing and will be far more susceptible
to flaking and abrasive removal. Also, formulation of a stable, homogeneous
suspension may make formulation, especially of an aerosol, problematical.
Pigments may also be used as temporary hair colorants and are usually
formulated in a waxy base and rubbed directly on hair. This type of product
is quite simple to apply, but the result is not particularly natural-looking or
appealing and suffers from the same drawbacks as other pigment deposition
products. This is an extremely minor player in hair dyeing.
A typical formulation for a temporary coloring lotion follows:

Ingredient Weight %
Propylene glycol 3.0
Hydroxyethylcellulose 1.0
Sodium hydroxide 0.04
Propyl paraben 0.3
Preservative 0.4
FD&C Blue 1 0.04
Ext. D&C Violet 2 0.05
D&C Orange 4 0.2
D&C Yellow 10 0.03
D&C Green 5 0.02
FD&C Red 4 0.05
Water q.s. 100
676 Harry's Cosmeticology

SEMIPERMANENT COLORANTS

Semipermanent colorants are the first step in hair coloring for many consumers.
Positioned largely as "gray coverage" products, they offer the advantages of
efficacy, simplicity, and reversibility. They require no mixing and contain no
ammonia or peroxide—all appealing attributes to the newcomer who wants
to cover the initial gray that appears with aging. Additionally, an undesirable
result can be removed by repeated shampooings. These are usually called
Level 1 products.

DYESTUFFS
Dyes used in semipermanent hair coloring are generally smaller molecules;
most are classed as nitro dyes. Their colors are bright and sharp, and many
also find use in permanent products in which vivid dyes in the yellow to
orange range are difficult to obtain. Perhaps their only drawback is their very
uniqueness: they are generally not useful in other dyeing applications, so the
limited quantities required by the industry rarely result in economies of scale.
Consequently, some of these compounds are quite expensive.
The great majority of these dyes belong to the following chemical classes:

Nitrophenylenediamines
Nitroaminophenols
Aminoanthraquinones

The first two classes provide yellow to violet colors, while the
aminoanthraquinones are necessary for the violet to blue tints required to
develop a comprehensive palette.

NITROPHENYLENEDIAMINES
Nitrophenylenediamines are by far the most important class of semipermanent
dyes because of their ease of synthesis and, more significantly, the range of
colors available. Depending on isomer and substitution, a spectrum of colors
from yellow to violet with a range of 140 nm is available. Nitrophenylenedi-
amines can be described by the general formula:

N0 2
 Hair Colorants 677

Table 31.2 Common Nitrophenylenediamine Semipermanent

Colorants
Compound Color

4-nitro-m-phenylenediamine Yellow
HC Yellow No. 10 Yellow
4-nitro-o-phenylenediamine Orange-yellow
HC Yellow No. 5 Orange
2-nitro- p-phenylenediamine Orange-red
HC Red 14 Orange-red
HC Red 10 Orange-red
HC Red 11 Red
HC Red 1 Violet-red
HC Red 3 Violet-red
HC Red 13 Red-violet
N,N'-fcii(2-hydroxyethyl)-2-nitro-p-phenylenediamine Red-violet
HC Violet 2 Violet
HC Blue 2 Violet

where Ri—R4 may vary considerably but are generally H, methyl, 2-

hydroxyethyl, 3-hydroxypropyl, 2,3-dihydroxypropyl, and the like. Although
there are six possible isomers, only three are used; the remaining three
offer no advantages and are far more difficult to synthesize. In order of
importance, the isomers utilized are 2-nitro-p-phenylenediamine, 4-nitro-o-
phenylenediamine, and 4-nitro-m-phenylenediamine. The predominance of the
2-nitro ppd's is due mostly to the 65 nm range, from orange-red to violet,
achievable from the unsubstituted parent [1] to the trisubstituted HC Blue
2 [2]. This particular isomer also has the advantage of an inexpensive and
readily available starting material, 4-fluoro-3-nitroaniline. Numerous degrees
and patterns of substitution are obtainable with this base skeleton.

NH 2 N(CH CH OH)
2 2 z

1 2

Another factor in the wider popularity of this isomer is the capability of

obtaining the shades provided by the 4-nitro-orthophenylenediamine (opd)
and 4-nitro-metaphenylinediamines (mpd) with nitroaminophenols, a far more
populous and useful class. This will be discussed in the following section.
678 Harry's Cosmeticology

Versatility and low cost also play a role in placing 4-nitro opd's next
in order of significance. The high reactivity and minimal cost of 2,4-
dinitrochlorobenzene make this an ideal starting material for reaction with
many amines, which then is followed by the equally straightforward selective
reduction of the 2-nitro group with sulfide.
Finally, the less versatile and less easily manipulated l,3-dichloro-4-
nitrobenzene, the usual starting material for 4-nitro mpd's, make this isomer
the least used of the nitrophenylenediamines.
Table 31.2 lists some of the available compounds (INCI names are used) in
the nitrophenylenediamine series and their colors.

Nitroaminophenols
Nitroaminophenols also represent a widely used class of semipermanent hair
colorants. The range of available colors is quite substantial, ~ 80 nm, even
more than that of nitrophenylenediamines. The wide variety of isomers and
substitution possibilities give the formulator wide leverage in obtaining the
yellow to orange-red tints supplied by these compounds. While the nitro-
phenylenediamine series nominally includes the same spectral range as the
nitroaminophenols, the extensive number of the latter compounds provides
much more flexibility for these colors than the limited number of compounds
in the former series. Thus the nitroaminophenols are quite necessary for semi-
permanent products, rather than simply being an enhancement. In fact, most of
the yellow to orange dyes used are nitroaminophenols, with nitrophenylene-
diamines supplying orange to violet colors.
The generic structure is given below (including nitroaminophenolic ethers
in this class).

Nr\R 3

As with the nitrophenylenediamines, R i — R3 may have many possibilities

but generally use the same hydrogen, lower alkyl, and hydroxy- and di-
hydroxyalkyl moieties as the previous class. Unlike the nitrophenylenediamine
class, which really uses only three starting materials, this class presents far
more options for the synthetic chemist developing new colors for the formu-
lator. There are numerous isomers of unsubstituted nitroaminophenol itself,
and these lead to many reaction possibilities. Added to this, the usual stable
of reactions such as replacement of activated halogens by amines, hydroxy or
 Hair Colorants 679

Table 31.3 Common Nitroaminophenol Semipermanent

Colorants
Compound Color

2-amino-4-nitrophenol Yellow
2-amino-5-nitrophenol Yellow
HC Yellow No. 4 Yellow
HC Yellow No. 9 Yellow
2-nitro-5-glyceryl methylaniline Yellow
2-amino-3-nitrophenol Yellow-orange
2-nitro-N-hydroxyethyl-p-anisidine Yellow-orange
HC Orange No. 2 Orange
HC Orange No. 3 Orange
4-amino-3-nitrophenol Orange
4-hydroxypropylamino-3-nitrophenol Orange-red
3-nitro-p-hydroxyethylaminophenol Orange-red

hydroxyl compounds, nitration, and selective reductions, and the accessibility

of many compounds becomes obvious. Table 31.3 lists some of these dyes;
again, this list is representative, not exhaustive.

Aminoanthraquinones
Aminoanthraquinones (AQ) are used to complete the color palette of semi-
permanent dyes; they contribute the violet and blue colors and extend the
spectrum approximately 100 nm. These molecules are significantly larger than
the nitro dyes, but this is somewhat mitigated by their planarity and the
nonbulky substituents of the AQs used for hair dyes. These dyes are used
generally as disperse dyes, consisting of 40-60% colorant and the remainder
a dispersing agent, usually a very hydrophilic lignosulfonate. There is some
controversy over whether such dyes are truly in solution or in an ultrafine
dispersion when in product, but certainly their use is necessary for the formu-
lator since attempts to dissolve the solvent (i.e., nondisperse colorant) form of
most AQs results in a nondissoluble residue in the bottom of the container.
Interestingly, neither the large size nor the dispersant prevents thorough pene-
tration and diffusion throughout the hair shaft.
The AQ colorants have traditionally been borrowed from other dyeing indus-
tries, especially textiles. Cost is thus minimal and the selection wide, although
factors such as size and nature of the substituents usually restrict these choices.
Recently, however, some AQs are being specifically designed and manu-
factured for hair dyes. This allows particular attributes, such as increased
hydrophilicity, to be built into the molecule and removes dependence of the
hair color companies on the vagaries of the textile dyeing industry.
680 Harry's Cosmeticology

Table 31.4 Common Aminoanthraquinone

Semipermanent Colorants
Compound Color

Disperse Red 15 Red-violet

Disperse Red 11 Red-violet
Disperse Violet 1 Violet
Disperse Violet 4 Blue-violet
Disperse Blue 3 Blue
HC Blue No. 14 Blue

The range of AQs used in hair dyeing need not be as extensive as those
of other classes because of their high intensity and wider spectral bandwidth.
Representative examples are listed in Table 31.4.

Other Dyes
The preceding three dye classes represent by far the most significant
semipermanent hair colorants, but selected other dyes are also used for
this application. Reasons generally include molecular size and hydrophilic
character rather than unique color properties. Nitroanilines, dinitroanilines,
and azo dyes are among these colorants; Table 31.5 lists some of these.

COMMERCIAL SEMIPERMANENT PRODUCTS AND THEIR

FORMULATION
General practice among formulators of semipermanent hair colorants is to
use several dyes of the same color but different molecular sizes. The smaller
molecules will penetrate the entire hair shaft from root to tip, including the
tighter, less porous area near the scalp, but will wash out of the more porous

Table 31.5 Miscellaneous Semipermanent Colorants

Compound Color Type

HC Yellow No. 6 Yellow Nitroaniline

HC Yellow No. 15 Yellow Nitroaniline
HC Yellow No. 2 Orange Nitroaniline
HC Orange No. 1 Orange Nitroaniline
HC Yellow No. 7 Orange Monoazo
Disperse Black 9 Orange Monoazo
2-Hydroxyethyl picramic acid Red-orange Dinitroaniline
 Hair Colorants 681

tip end. Conversely, larger molecules will not enter the hair near the root
but will permeate and remain in the tip end. Examples of this type of "color
pairs" include HC Red Numbers 1 and 3 (both nitro-p-phenylenediamines, but
the former having aromatic rather than aliphatic substitution, i.e., a dipheny-
lamine), and HC Yellow Number 2/HC Orange 1 (the former a nitroaniline,
the latter a diphenylamine).
Perhaps the most significant aspect of a semipermanent vehicle is its ability
to temporarily swell the hair strand to allow penetration of the dyestuffs.
The cuticle must open enough to allow thorough diffusion of the dyes into
and through the cortex, then close back down to retain the dyes, and this
must occur at ambient temperature and in a limited time. Optimum condi-
tions for this process include mild alkalinity; thus most products are formu-
lated in a pH range of 8.5-9.5. Ammonia is never used as the alkalizer,
since it is too powerful and many dyes are not stable to NH3. Secondary
amines such as diethanolamine and methylethanolamine are generally avoided
because of concerns about potential nitrosamine formation, and tertiary amines
such as triethanol- or triisopropanolamine are not strong enough alkalizers.
Thus primary amines, especially sterically hindered ones such as aminomethyl
propanol and aminoethyl propanediol, are the alkalizers of choice.
Semipermanent products are usually lotions or aerosol mousses, and a cap
may be used to slightly raise ambient temperature and thus enhance dyetake.
Browns, reds, and blacks are by far the most popular shades since the gray
hair that semipermanents are meant to cover appears most dramatically in
these base colors. Product lines include, of course, tints for lighter shades and
blondes, but these are not as popular because of the less noticeable nature of
gray in these shades.
Most semipermanent products are applied to damp hair and then rinsed
or shampooed off after 15-40 minutes; so formulations may contain mild
surfactants in addition to solvents and swelling agents necessary for dye
solubility and diffusion. This type of product does little or no damage to
hair, but a conditioning agent may be added to the formula to enhance both
the feel and the wet and dry combability of the hair. Such a conditioner may
be alternatively included as an after-treatment.
One other application for semipermanent products is for tinting chemically
treated hair that may not tolerate more rigorous oxidation dyeing. This includes
hair that has been permed or relaxed, and formulations with low levels of
semipermanent colorants and mildly alkaline pH are excellent choices for
tinting such hair. The increased porosity of this hair will enhance dyetake but
will also increase washout; in this case, however, gentleness is preferable to
durability.
682 Harry's Cosmeticology

A typical semipermanent lotion formulation is given below:

Ingredient Weight %
Ethoxydiglycol 3.2
PEG-8 tallow amide 1.5
Hydroxyethylcellulose 1.0
Cocamide DEA 2.0
Aminoethylpropanediol 1.5
Oleic acid 1.8
TEA-Dodecylbenzenesulfonate 0.9
HC Blue No. 2 0.4
Disperse Blue 3 0.3
Disperse Black 9 0.05
HC Yellow No. 4 0.1
HC Red No. 1 0.07
Disperse Violet 1 0.02
HC Orange No. 1 0.01
Water q.s. 100

PERMANENT HAIR DYES

As stated previously, this class of hair colorants is by far the most popular.
The advantages of these products lie not only in their durability but also in
their versatility: these are the only products that allow the user to lighten the
base hair color. Most products have traditionally done this to some extent, and
the appeal of lightening remains strong. In recent years, however, a subclass
of permanent products has arisen, frequently called demipermanents. These
products rely on the same oxidative chemistry and have similar durability to
permanents but lighten the base color little or not at all. Coverage of gray
is not quite as thorough but the results are often more natural looking than
the more evenly dyeing traditional products. Both types of products offer the
user the most effective hair coloration and maximum flexibility. The general
theory of oxidative dyeing holds that small colorless molecules are allowed to
diffuse into and through the hair, then undergo a series of chemical reactions
to generate large colored molecules incapable of being washed out. While this
is simplistic, it is nevertheless accurate enough to describe the process.
Oxidative hair coloring chemistry requires three constituents:

1. An ortho or para substituted aminoaromatic, usually single ring, with the substituent
being hydroxy, amine, or substituted amine. This is called the primary intermediate;
 Hair Colorants 683
2. A second aromatic, again usually single ring, with at least one but usually two
electron-donating groups arranged meta to each other. This is called the coupler
or color modifier.
3. An oxidant, almost exclusively hydrogen peroxide but possibly perborate, percar-
bonate, and the like.

There are several similarities between oxidative hair dye and color photo-
graphic chemistries. Both rely on oxidative processes, silver salts being the
oxidants in the latter case and peroxide in the former. Both use closely related
primary intermediates, and some of the couplers, especially for blue tints,
are also alike. The similarities mostly end there, however, and the different
priorities dictate the variations. Hair dyes are periodically reapplied, so fast-
ness to light is less crucial (although fastness to aqueous surfactant washing,
insignificant to the photographic chemist, is more so). Photographic chem-
istry occurs in discrete layers sandwiched between protective layers; there-
fore, competing reactions in mixtures need not be considered. Nor are the
chemistries completely alike: some of the couplers and/or finished dyes of
color photography are not stable to alkaline peroxide. This is especially unfor-
tunate, since some of the photographic colors would be excellent tools for the
hair colorist.

DYESTUFFS
Many of the compounds in this hair dye class, perhaps more accurately called
dye precursors rather than dyestuffs, are used in large quantities in other, often
unrelated, industries. Thus they can frequently be obtained in very high purity
for very low cost. Examples include p-phenylenediamine itself, used in other
dyeing applications but also as an antioxidant and as a monomer for synthetic
polymers, and p-aminophenol, precursor to the analgesic acetaminophen.
Several compounds, however, are unique to hair dyeing and thus are custom-
manufactured.

Primary Intermediates
These compounds are easily oxidized to form an intermediate reactive species,
hence the name. There are many theoretical possibilities for this class,
but almost every compound used is a p-phenylenediamine, p-aminophenol,
or some variation thereof. The most common compounds are shown in
Figure 31.1.
Although all of the examples above are para disubstituted benzenes, ortho
compounds such as oaminophenol have been used for many years and, more
recently, heterocycles such as pyridine and pyrimidine compounds have gained
wider acceptance.
684 Harry's Cosmeticology

NH 2 NH 2

OH OH

Figure 31.1. Common primary intermediates

Couplers
These compounds are the substrate to which the oxidized intermediate attaches,
or couples, to give a colorless diphenylamine, the leuco form of the final indo
dye. There are also many possibilities for this class, but almost all are resorci-
nols, m-aminophenols, m-phenylenediamines, or naphthols. The common ones
are shown in Figure 31.2.
Heterocycles and fused ring compounds such as 2,6-dimethoxy-3,5-
pyridinediamine, 6-hydroxyindole, and hydroxybenzomorpholine are also
growing in usage.

Figure 31.2. Common oxidative couplers

 Hair Colorants 685

FORMATION OF COLORS IN THE HAIR

Oxidative hair dyeing relies on a three-step process to form the final dyes
from the colorless precursors:

1. Oxidation o f the primary to form a reactive intermediate, either a quinonediimine

(from diamines) or a quinoneimine (from aminophenols), by alkaline peroxide.
This is a slow step;
2. Addition of the imine to the coupler to give a colorless diphenylamine, referred to
as the leuco form of the dye;
3. Oxidation o f the diphenylamine to the final dye; this is quite rapid. These dyestuffs
are called indo dyes and bear a structural similarity to the azomethine class.

This sequence can be exemplified by the reaction of p-phenylenediamine and

resorcinol, shown in Figure 31.3.
The chemistry is not quite as simple as might first appear, however. Further
coupling can occur, giving rise to tri- and multinuclear species of different
tints. This effect may be enhanced, if desired, by manipulating the relative
amounts of each compound or class. Also, rates of reaction, both of the initial
oxidation and coupling, will vary considerably between each intermediate and
each intermediate or coupler combination.
Additionally, the imines can self-couple to form multinuclear compounds;
the trimer formed by p-phenylenediamine in known as Bandrowski's base.

Stepi:

NH 2 NH

NH OH OH

Step 2:
OH
NH

OH OH

Step 3:

HgN OH Ό

Figure 31.3. Formation of an oxidative hair dye

686 Harry's Cosmeticology

However, this sequence is far less favored in the presence of couplers than
straightforward addition to form the diphenylamine, so it is of no consequence.
It can be clearly seen that quite a range of colors is attainable by the
formulator who has four to six primaries and six to eight couplers available.
In fact, many products contain a combination of two to three primaries and
three to five couplers, the resulting shade being a smooth blend of color and
nuance.
Shade descriptions of just 12 combinations, two primaries and six couplers,
are given in Table 31.6.
The attainable color range of oxidative hair dyes does have some limitations.
Neither a bright yellow nor a bright orange is available. This lack is somewhat
mitigated by a number of factors: first, peroxide will lighten, to some extent,
the natural color of hair. This has a brightening effect that lessens the need
for yellow and orange colors. Second, semipermanent dyes that are stable
to ammonia and peroxide may be used to add these colors. This is not an
elegant solution, however, since the nuances and brightness they add will
be less durable. Their levels must be kept low to prevent significant shade
changes over short time periods. Standard oxidative dyes with this coloration
would be useful additions to the palette, rather than the "band-aid" addition
of semipermanent colorants.

TOXICOLOGY
Some of the oxidation dye precursors, notably p-phenylenediamine but also
some couplers, are known to be potential sensitizers. Although the occur-
rence of dermatitis or other allergic response is quite rare with respect to the
use of hair dye products, many countries have introduced laws or regulations
requiring users to carry out prophetic patch tests before dyeing and speci-
fying limits to concentrations of ingredients in the formulae, and so forth. For
example, products in the United States must show in a prominent and conspic-
uous place the statement: "Caution. This product contains ingredients that may

Table 31.6 Shade Descriptions of Various Primary or Coupler

Combinations
ρ -phenylenediamine p-aminophenol

m -Phenylenediamine Blue Violet

m-Aminophenol Red-brown Orange
4-Amino-2-hydroxytoluene Red-violet Orange-red
Resorcinol Green-brown Yellow-brown
2-Methylresorcinol Violet-brown Orange-brown
1-Naphthol Blue-violet Red-orange
 Hair Colorants 687

cause skin irritation on certain individuals, and a preliminary test according

to the accompanying directions should first be made. This product must not
be used for dyeing the eyelashes or eyebrows; to do so may cause blindness."
The safety of hair dye ingredients of all classes is thoroughly reviewed in the
United States by the Cosmetic Ingredient Review (CIR), and the findings are
published in the International Journal of Toxicology. Further discussion of the
safety and toxicology of hair dye compounds can be found in Kalopissis [1]
and Corbett, et al. [2].

TYPES AND FORMULATION OF PERMANENT HAIR DYES

Oxidative hair coloring products come in many forms: liquid, creme, gel,
powder, and aerosol. Except for a powder that uses a solid oxidant, all other
types require separation of the dye-containing portion usually called the dye
base, and the oxidant usually called the developer. In the case of an aerosol,
this is accomplished with separate compartments whose contents are mixed
when dispensed.
The concentration of hydrogen peroxide is measured in volume rather than
percentage and refers to the volume of oxygen liberated on peroxide decom-
position. This nomenclature is somewhat archaic but is firmly entrenched;
therefore it is used here. For hair dyeing purposes, concentration usually varies
between 3% (10 volume) and 9% (30 volume), with up to 12% (40 volume)
available to the professional colorist.
The selection of alkalizing agent and peroxide concentration are the most
significant factors in determining the product character. Traditional products,
often referred to as Level 3 colorants, use ammonia and 30 volume H 2 O 2 ;
these are both the longest lasting and most complete gray coverage prod-
ucts. They also offer significant lightening during the coloring process, which
increases both the brightness and the levelness of the final result. These advan-
tageous properties require a somewhat longer application time to achieve,
20-45 minutes, but generally 20-30 minutes suffice. Effective results rely on
application near the roots first, to allow more exposure time for this undyed
new growth, followed by application to the remainder of the hair. These prod-
ucts comprise the largest share of this category and thus the largest selling types
of hair colorants. Recent improvements to this category include the use of an
alkanolamine, usually monoethanolamine, to replace some of the ammonia.
This significantly reduces the strong ammonia odor to where it can be almost
completely masked by fragrance without decreasing the amount of lightening.
Present technology still requires the use of some ammonia to lighten, with
peroxide, the base hair color.
688 Harry's Cosmeticology

Recent years have seen the introduction and fast growth of a related
series of hair colorants generally referred to as demipermanents or Level 2
colorants. The chemistry and precursors remain the same, but the peroxide
concentration is lower (10-12 volume) and a nonammonia alkalizer, typically
monoethanolamine or aminomethylpropanol, is used. Little or no lightening
is achieved and gray coverage is somewhat less thorough, but this lack of
complete evenness gives the appearance of highlights and thus may be a
more natural result. They may be thought of as gray-blending rather than
gray-covering, dyeing the gray hair to a tone quite similar to the base hair
color rather than an even dyeing of all the hair. An additional advantage is a
shorter dyeing time, usually ~10 minutes, since no extended bleaching period
is required. However, these products do not perform very well on hair with
extensive amounts of gray or on hair that may be more resistant to dyeing
and thus requires the enhanced swelling that ammonia provides. Also, the less
rigorous dyeing may require more frequent application. Colorants of this type
also find use in professional salons as mild, longer lasting hair dyes used after
a strenuous hair treatment (e.g., bleaching, relaxing). These products are used
to tone rather than heavily dye the bleached hair.
One final type of oxidative product is designed to have an application
time of five minutes; these are usually but not exclusively targeted at the
men's market. These usually contain high levels of dye precursors to obtain
significant color formation in the abbreviated time, and lower concentrations of
peroxide, since no lightening is achievable anyway. Alkalizer is alkanolamine
and/or ammonia. Although the shorter time has great appeal, the results are
not as aesthetically pleasing as those from other oxidative products. Dyetake
may be quite uneven and significant washout, because of less complete dye
penetration, usually occurs in the first few shampoos. Durability is also
sacrificed, again because of less thorough diffusion. These products have
therefore not generated mass appeal.
Formulation of oxidative hair colorants has shifted in recent years from
vehicles with high levels of organic solvents to those that are largely aqueous.
Problems with dye solubilization and long-term emulsion stability have been
overcome, and new products make far more efficient use of the dyes. Optimiza-
tion of the vehicles has given equal or better coloring results with lower dye
concentrations and has allowed development of products with shortened dyeing
times. The most convenient medium for coloring the hair is a shampoo; thus
many products are formulated around an aqueous surfactant composition modi-
fied to contain the dye precursors and the required adjuvants. These include
antioxidants, generally sodium sulfite and erythorbic acid, for protection during
initial solubilization or manufacture and long term in product. Another neces-
sary additive is EDTA or one of its salts, used to chelate any metals that
 Hair Colorants 689

would catalyze peroxide decomposition upon mixing with developer. This is,
in fact, found often in both the dye base and the developer. Biological preser-
vatives are usually not required because of the high pH. Similarly, products in
creme, gel, and lotion forms may be devised with other additives such as fatty
alcohol sulfates or ether sulfates, fatty acid alkanolamides, nonionic, cationic,
or amphoteric surfactants, fatty alcohols and/or amines, amine oxides, alco-
holic solvents, thickeners, and so forth. Generally, the only required property
of a hair dye formulation is a thick viscosity after mixing with developer, such
that the applied colorant will not drip down out of the hair and onto the face
and neck of the user.
Conditioners are frequently included in hair dye products and offer smooth
combability, both wet and dry, and enhanced feel and luster. Modern perma-
nent dye formulations keep oxidative damage to a minimum, but conditioners
add improved aesthetics of high value to the user. Conditioners may be
included in the dye base or as a separate package after-treatment. While this
later option requires another formulation and more packaging components, it
may offer multiple uses to the consumer and could therefore be perceived as
an added benefit. The developer formulation may be as simple or as complex
as the product chemist desires. It may consist of only peroxide and water, it
may have a stabilizing acid, such as phosphoric or etidronic, or it may be
a complicated mixture in its own right. Ingredients may include thickeners,
silicon-based conditioners, surfactants, and so forth. Developers containing
acrylate-based thickeners have become especially popular in recent years.
A sample formulation found in Brown and Pohl [3] is as follows:

Ingredient Weight %
Oleic acid 21.0
Sodium lauryl sulfate 2.0
Oleyl alcohol 10.0
Ammonia (28%) 12.0
Isopropyl alcohol 10.0
Propylene glycol 4.0
Sodium sulfite 0.2
EDTA 0.05
p-Phenylenediamine 0.2
Resorcinol 0.3
4-Amino-2-hydroxytoluene 0.2
m-Aminophenol 0.1
Water q.s. 100
690 Harry's Cosmeticology

Another exemplary formulation, that gives a dark brown color on hair, is

in U.S. 5,851,237 [4]:

Ingredient Weight %
Cocamidopropylbetaine 10
Monoethanolamine 2
Citric acid 1
Ammonium hydroxide 10
Sodium sulfite 0.1
1 -(4-aminophenyl)pyrrolidine sulfate 1.0
m-Aminophenol 0.5
Resorcinol 0.5
1-Naphthol 0.1
Water q.s. 100

OTHER DYES FOR HAIR

Hair coloring products based on other technologies have been developed to fill
specific consumer needs; some of these have been available for many years.
Although such products continue to be sold, improvements on and expansions
of common hair dye technology, especially permanent colorants, have largely
fulfilled these needs. Additionally, the fact remains that modern hair coloring
products are better than anything else in accomplishing that goal. The limited
appeal of niche materials pales beside the performance and convenience of
today's colorants based on synthetic dyestuffs. Thus the market for products
using alternative approaches is quite small.

AUTOOXIDATIVE DYES
This class of dyes relies quite heavily on standard oxidation dyestuffs but
uses the slow oxidation by air rather than the fast peroxide oxidation to
cause reaction. "Autooxidative" is a somewhat misleading term, since it is
atmospheric oxygen that is the active oxidant. Color development is therefore
very slow, on the order of days, and appeals chiefly to the user who wants
gradual color change. These products are for gray coverage only, generally
come in shades of medium or light brown and darker, and are usually aimed
at the men's market. Dyestuffs include the same primary intermediates and
many of the same couplers as permanent products and some tri-substituted
compounds. These later compounds are benzenes with oxidizable groups in
the 1,2,4-positions. Two are most common: 2,4-diaminophenol and 1,2,4-
trihydroxybenzene. The development of five-minute products targeted at men,
 Hair Colorants 691

and especially of demipermanent products, has largely pushed this class out
of the market.

VEGETABLE HAIR DYES

As noted in the introduction, henna has a long history of use in tinting hair.
It is still used in some areas today, often in combination with one or more
other plant materials. Little or no product formulation is required: ground or
powdered plant material is prepared in a poltice, using usually hot water and
a mild acid for pH adjustment at the time of application. The plant mate-
rial may include other species, such as indigo, for additional nuances. The
paste is applied to the hair for some period of time and then rinsed off. This
type of product has the appeal of a purely natural colorant, but the severe
disadvantages largely outweigh this. These include highly variable results,
limited shades, and messiness. Far superior and controllable results could be
obtained by simply incorporating synthetic lawsone into a commercial hair
dye formulation.

METALLIC HAIR DYES

Hair colorants based on metals have been around for decades. These are
usually based on lead and rely on the slow formation of lead sulfide from the
lead acetate salt in the product. As with autooxidative colorants, the appeal
is gradual color formation over several days. Disadvantages include limited
shades, potentially variable results, uneven shade development, and possible
sulfide odors. This technology, as with the other alternative methods, has
become even less appealing with the advent of modern coloring formulations.

INDOLIC DYES
The natural hair and skin colorant for non-red shades is eumelanin, a
biopolymer whose structure is not strictly defined. Its monomelic precursor in
the biological pathway is dihydroxyindole [3]:

Products seeking to mimic eumelanin coloration have included this indole

and/or its analogues and usually rely on air oxidation of this unstable com-
pound. Despite intensive effort, the structure and characteristics of eumel-
anin are not yet completely understood, so attempts to imitate it have been
692 Harry's Cosmeticology

premature. Products formulated around [3] have demonstrated poor fastness

properties, with severe off-shade wearing. Even a successful indolic product,
however, would suffer from the same drawbacks seen in other alternatives,
namely, limited shade range, slow color development, potentially variable
results, and no lightening possible. Additionally, [3] is difficult to synthesize
and is therefore quite expensive. A protected, stable precursor that gener-
ates [3] in situ may be used. The claim of a natural melanin-based colorant,
the presumed market hook for such a product, might be suspect in light of
the chemically synthesized active component. For now technical limitations
preclude an effective indole-based hair colorant.

HAIR DYE REMOVERS

The removal of hair dyes is sometimes necessary because of either a mistake
or an undesirable result. The degree of difficulty in removal is directly related
to the intended durability of the product.
Temporary products can be removed by one or more shampoos.
Semipermanent products can be removed by multiple vigorous shampooings.
These products are designed to last anywhere from four to eight shampoos,
depending on the specific formulation. Complete removal will require at least
this number, since there is no environmental wearing between the shampoos.
Oxidative products, whether permanent or demipermanent, cannot be
removed by shampooing. There are two options for corrective action: chemical
treatment, or redyeing with another shade, usually using a demipermanent, to
obtain the desired tonality. Chemical treatments will not remove oxidative
colorants completely but will significantly reduce color. The result may be
acceptable to the user or, if not, more conducive to corrective recoloration.
Treatment with ascorbic and/or isoascorbic acid will accomplish this [5]. More
stringent chemical treatments, such as those using strong bleaching agents, will
also bleach both the dyes and the underlying hair color but will risk damaging
hair already treated with an oxidative dye.
Corrective recoloration, preferably with a demipermanent and with or
without a prior nonrigorous chemical color reduction, remains the treatment
of choice. Either approach should only be undertaken by a professional.

BLEACHING AND LIGHTENING

The depth of shade of color is often described by a range of 12 levels, with
level 1 being black and level 12 being ultralight blonde. This nomenclature is
not to be confused with the Level rating of a product, which is a measure of
 Hair Colorants 693

its durability. This repetition of term is unfortunate, but the designations are
generally accepted by the industry.
Temporary and semipermanent products do not lighten the natural color at
all. It may sometimes appear that this is the case, but the effect is because
of lighter, brighter dyes that give the impression of highlighting, or selective
lightening.
Demipermanent products usually lighten the natural hair color about 1 level.
These products can be formulated not to lighten at all, or possibly up to 2
levels.
The highest amount of lightening that can be achieved with simultaneous
dyeing is about 3-4 levels; these require higher concentrations of ammonia
and peroxide and longer application times.
Thus rigorous lightening cannot be achieved with simultaneous dyeing and
requires a separate procedure. This is "double process" an initial bleaching
followed by dyeing, usually with a mild demipermanent. A semipermanent
might also be appropriate, but the greater porosity of the treated hair would
result in faster, and possibly uneven, washout. In practice, bleaching is rarely
followed by an intense dyeing, and the coloration is more of a toning with a
product containing low dye concentrations.
Bleaching the hair more than 3-4 levels allows the user to produce blonde
shades from darker hair, although a reduction of 6-7 levels is the maximum
practicable. This strong bleaching is only accomplished with ammonia,
high, volume peroxide, and peroxy salts. These are sodium, potassium, and
ammonium persulfate, and most bleaching formulations have at least two and
usually all three. Ammonium persulfate is the ammonia source. Peroxide and
persulfate work synergistically in the bleaching process, which seems to be a
two-step sequence: breaking apart of the melanin granules ("melanosomes")
followed by chemical decolorization.
The practice of bleaching usually consists of mixing peroxide with the
preblended persulfate (often containing silicates for flow characteristics)
powder to the consistency of a thick paste, then application to the hair for
a period of time that depends on the desired result. The degree of lightening
is controlled by the peroxide concentration, contact time, and to some extent
the amount of ammonium persulfate. A variation of this procedure is the
application of the mixture only to selected portions of hair, followed by the
wrapping of these portions in foil. This results in highlighting rather than
overall lightening.
Recent improvements in this area have focused on making the persulfates
easier to use. Especially noteworthy is the addition of an inert oil to the powder
to prevent dusting [6]. Lightening kits are available to the home user, including
usually a mild shampoo and a conditioning after-treatment. However, most
694 Harry's Cosmeticology

bleaching and/or lightening, especially if followed by dyeing, is performed

by a trained professional in the more controlled salon environment. This is
because of the somewhat rigorous nature of the bleaching process, the possi-
bility of damage if done incorrectly, and the convenience of having it done
by a professional.
Very mild lighteners consisting of low-level peroxide are also available to
the consumer. These are meant to mimic the lightening effects of sun exposure
and are leave-in products, usually aerosols.
REFERENCES
1. Kalopissis, G., Toxicology and hair dyes, pp. 2 8 7 - 3 0 8 , in The Science of Hair
Care, Zviak, C. ed., Marcel Dekker, New York, 1986.
2. Corbett, J., et al., Cosmetic Toxicology In: Toxicology, Marquardt, H., et al., eds.,
Academic Press, San Diego »(in preparation).
3. Brown, K., and Pohl, S., Permanent Hair Dyes, Society of Cosmetic Chemists
Monograph SCC, New York, 1996.
4. U.S. Patent 5,851,237, Oxidative hair dye compositions and methods containing
l-(4-Aminophenyl) pyrrolidines, Anderson, J., and Wong, M., December 22, 1998.
5. U.S. Patent 5,782,933, Ascorbic and isoascorbic acids to remove or adjust oxidative
color in hair, Wis-Surel, G., et al., July 2 1 , 1998; Assignee: Bristol-Myers Squibb
Company.
6. U.S. Patent 5,698,186 Methods of controlling dust and compositions produced
thereby, Weeks, G., December 16, 1997; Assignee: Clairol.