Pigment - Part 1: What is Color

Pigment
Part 1: What is Color
Color plays an essential role in our day-to-day lives. From birth we are
taught to react to colors logically or emotionally. Colors have meaning, which
vary from culture to culture and continent to continent. It governs and
controls traffic, triggers strong emotions, and is used to describe moods.
Light, perceived by the human eye, is the product of electromagnetic waves
in a small range of wavelengths. Different wavelengths are perceived as
different colors. Color is therefore a perceptual phenomenon, which depends on the observer and
the conditions in which the color is observed (our eye-brain is very accommodating in adjusting
for varying environmental conditions).
Three things are required for the presence of color:
- an object,
- a light source (illuminant), and
- an observer.
1.1/ The Illuminant
Color has been successfully used for object tracking and recognition. However, the color of an
object changes if the illuminant's color changes.
To see colors, energy in the form of light is required. Color sensation is produced by physical
stimuli associated with the various wavelengths in the visible portion of the electromagnetic
spectrum. To understand color better, we must recognize the origin of light. Light comes from a
wide variety of sources and consists of electromagnetic radiation, a form of energy that spreads
in a wave motion.

Figure 1: Visual color spectrum
All visible light is made up of a mixture of colors, which combine in different proportions to make
up each distinctive light. The way we measure light is by a Spectral Power Distribution. In Figure
1, the visual color spectrum begins at 400 nm and finishes at 700 nm. Everything below 400 nm
is called ultraviolet (UV) and everything above 700 nm is referred to as infrared (IR). It is not
possible for the naked human eye to see ultraviolet or infrared light.

Average North Sky Daylight
(Illuminant D65)


Spectrum of Fluorescent Light




Incandescent Light (Illuminant
A)

Figure 2: Sources of light
Note: (Vertical axes: spectral distribution)

White light is composed of a select group of colors; each one characterized by a specific range of
wavelengths, which it absorbs. These are the colors of the spectrum - red, orange, yellow,
green, blue and violet.
Incandescence and luminescence are two main ways of creating light. Incandescence is light
from heat energy. Heating the source of a light bulb to a sufficiently high temperature will cause it
to glow. The stars and sun glow via incandescence. Luminescence, also known as cold light, is
light from other sources of energy independent of heating. It can be generated at room or even
lower temperatures. Quantum physics explains luminescence as movements of electrons from
their ground-state (lowest-energy level) into a state of high energy. When returning to its ground-
state, the electron gives back the energy in the form of a photon of light. If the time interval
between the two steps is short (few microseconds), the process is called fluorescence; if the
interval is long (some hours), the process is called phosphorescence.
The combination of these wavelengths in light can change according to the light source. For this
reason, colors can look different when compared under the influence of daylight, fluorescent light

or sodium lamps. Natural sunlight varies widely. It can be very blue, particularly around midday,
looking north. Direct sunlight usually is seen as golden, but, at sunset, it can be bright red.
Artificial light can be yellow, from sodium vapor, blue-green from mercury vapor, or it can be
yellow, from an incandescent light bulb, or varying colors from fluorescent light. The graphs in
Figure 2 show average north sky daylight (Illuminant D65), a cool white fluorescent light
(Illuminant F), and an incandescent light (Illuminant A).
Several phenomena can occur when light hits an object. Transmission occurs if the light passes
through the object, which is the case with transparent colors. It is referred to as reflection if, for
example, a blue object reflects the part of the color spectrum that represents blue and the
remaining light is absorbed. The reflection curve of white will show roughly equal intensities close
to 100% reflection in all wavelengths of the spectrum. Refraction or scattering is when light
changes direction as it passes from one medium to another, like from the polymer to a pigment or
filler particle in a plastic part. Scattering is influenced by the difference in refractive index between
a particle and its surroundings, particle size, and wavelength of light. An opaque color provides a
high scattering performance. A translucent color shows a combination of transmission and
scattering. Absorption happens if most wavelengths of the visible spectrum are absorbed. Black
surfaces absorb almost all light.
1.2/ The Object
An object appears in a particular color because the light, which is reflected from its surface, is
made up exactly of wavelengths, which combine to create the color observed. The object absorbs
all other wavelengths. For example, a blue object reflects the blue light spectrum, but absorbs
red, orange, yellow, green and violet, which are most of the other wavelengths. A red object
reflects the red spectrum but absorbs most of the orange, yellow, green, blue and violet.

Figure 1 : Absorption and Reflection of Light
Black and white colors are different to other colors in terms of the way that they reflect and
absorb light. A white object reflects almost all colors while a black object absorbs most colors
completely.
Other significant influences on the color of an object are shape and surface effects. For instance,
an object can be spherical or square, dull or glossy, transparent, opaque or translucent. It may

also appear metallic, pearlescent, fluorescent, or phosphorescent. Viewing angles also affect our
perception of color
1.3/ The Observer

The human eye is the defining observer of color. An observer nearly always bases acceptance of
a color on visual judgment. For this reason, a color match can become highly subjective, as color
vision varies widely from person to person. Characteristics such as age, gender, inherited traits,
and even mood, can affect color vision.
Figure 1 : Human Eye








Part 2: Basics on Color Measurement

People who believe that the eye is the most important observer of color, argue that judgments of
color can be made purely by reference to color cards by visual matching. The clear criticism of
this technique of judging color is that everyone's perception of color differs.
Besides genetic abnormalities, color vision changes with age due to the build up of yellow
macular pigmentation in the eye. For this reason, it is argued that all judgments of color must be
based on physical measurements. However, these measurements and their interpretation must
be related closely to the responses of visual observers.
Therefore, color control is split into two segments: visual and instrumental.
In the following pages, color measurement methods will be described:
- the equipment that can be used like colorimeters and spectrophotometers
- the CIELAB method of measurement

- and the Munsell color measurement system


2.1./ Equipment
Although the human eye can control color, there is a need for instruments in order to provide
objective color measuring and evaluation as well as help in matching colors.
There are two basic methods for measuring surfaces' color:
• The first is to imitate the analysis made by the eye in terms of responses to three stimuli.
This technique, known as "tristimulus colorimetry", sets out to measure X, Y, and Z
directly.
• The second method is to determine reflectance (R) for each wavelength band across the
range of the spectrum to which the eye is sensitive, and then to calculate the visual
responses by summing products of R and the standard values for distribution of the
sensitivity of the three-color responses (2. J, and Z).
The tristimulus method has theoretical advantages where the materials to be measured are
fluorescent, but there are serious practical problems in assuming that a tristimulus colorimeter
exactly matches human vision, that is, in eliminating color blindness from the instrument.
Two commonly used types of color measurement equipments are a colorimeter and a
spectrophotometer.
Colorimeters
A tristimulus colorimeter has three main components:
• a source of illumination (usually a lamp functioning at a constant voltage);
• a combination of filters used to modify the energy distribution of the incident / reflected
light;
• a photoelectric detector that converts the reflected light into an electrical output.
Each color has a fingerprint reflectance pattern in the spectrum. The colorimeter measures color
through three wide-band filters corresponding to the spectral sensitivity curves. Measurements
made on a tristimulus colorimeter are comparative, the instrument being standardized on glass or
ceramic standards. To achieve the most accurate measurements it is necessary to use calibrated
standards of similar colors to the measured materials. This "hitching post" technique enables

reasonably accurate tristimulus values to be obtained even when the colorimeter is demonstrably
colorblind.
Tristimulus colorimeters are most useful for quick comparison of near-matching colors.
They are not very accurate. Large differences are evident between the various instrument
manufacturers. However, colorimeters are less expensive than spectrophotometers.
Spectrophotometers
To get a precise measurement of color, it is advisable to use a spectrophotometer. A
spectrophotometer measures the reflectance for each wavelength, and allows to calculate
tristimulus values.
The advantage over tristimulus colorimetry is that adequate information is obtained to calculate
color values for any illuminant and that metamerism is automatically detected.
The negative is that high quality spectrophotometers are very expensive and measurements
take longer (although this disadvantage has been greatly reduced by instrument development).
In a spectrophotometer, the light is usually split into a spectrum by a prism or a diffraction grating
before each wavelength band is selected for measurement. Instruments have also been
developed in which narrow bands are selected by interference filters. The spectral resolution of
the instrument depends on the narrowness of the bands used for each successive measurement.
In theory, a spectrophotometer could be set up to compare reflected light directly with incident
light, but it is more usual to calibrate it against an opal glass standard itself previously calibrated
by an internationally reknown laboratory. Checks must also be made on the optical zero (e.g. by
measurements with a black light trap) because dust or other problems can give rise to stray light
in an instrument giving then false readings.
Today's spectrophotometers contain monochromators and photodiodes that measure the
reflectance curve of a product's color every 10 nm or less. The analysis generates typically 30 or
more data-points, with which a precise color composition can be calculated.
2.2./ CIELAB Method
An organization called CIE (Commission Internationale de l'Eclairage) determined standard
values that are used worldwide to measure color.
The values used by CIE are called L*, a* and b* and the color measurement method is called
CIELAB.

L* represents the difference between light (where L*=100) and dark (where L*=0). A* represents
the difference between green (-a*) and red (+a*), and b* represents the difference between
yellow (+b*) and blue (-b*). Using this system any color corresponds to a place on the graph
shown in Figure 3. Variables of L*, a*, b* or E* are represented as delta L*, delta a*, delta b* or
delta E*, where delta E* = delta (delta L*
2
+delta a*
2
+delta b*
2
). It represents the magnitude of the
difference in color, but does not indicate the direction of the color difference.




2.3./ Munsell Color System

The Munsell Color System, developed in 1898 by American artist A. Munsell, is another
commonly used color measurement system. Munsell aimed to create a "rational way to describe
color" that would use clear decimal notation rather than color names. In 1905 he published a
color notation, which has been reprinted several times and is still a standard for colorimetry.
Munsell modeled his system as an orb around whose equator runs a band of colors. The axis of
the orb is a scale of neutral gray values with white as the North Pole and black as the South Pole.
Extending horizontally from the axis at each gray value is a gradation of color progressing from
neutral gray to full saturation. With these three defining aspects, any of thousands of colors could
be fully described. Munsell named these aspects, or qualities: hue, value, and chroma.
Hue
Munsell defined hue as the quality by which we
distinguish one color from another. He selected

five principle colors: red, yellow, green, blue, and
purple; and five intermediate colors: yellow-red,
green-yellow, blue-green, purple-blue, and red-
purple. He arranged these in a wheel measured off
in 100 compass points. The colors were identified
as R for red, YR for red-yellow, Y for yellow etc.
Each primary and intermediate color was allotted
ten degrees around the compass and then further
identified by its place in the segment.
Value
Munsell defined value as the quality by which we
determine light colors from dark ones. Value is a
neutral axis that refers to the gray level of the
color, ranging from white to black.
Chroma
Chroma is the quality that distinguishes a pure hue from a gray shade. The chroma axis extends
from the value axis at a right angle and the amount of chroma is noted after the value
designation. Therefore, 7.5YR 7/12 indicates a yellow-red hue tending toward yellow with a value
of 7 and a chroma of 12. However, chroma is not uniform for every hue at every value.
Mussel saw that full chroma for individual hues might be achieved at very different places in the
color sphere. In the Munsell System, reds, blues, and purples tend to be stronger hues that
average higher chroma values at full saturation, while yellows and greens are weaker hues that
average fullest chroma saturation relatively close to the neutral axis.
In the "Munsell Book of Color", you will find the complete system in 40 pages. Each page has a
different hue running around the spectrum to red and on through purple back to violet (PB in the
Munsell notation). The colors on each page are arranged in rows of equal Value and in columns
of equal Chroma. Each color has three references corresponding to hue, value, and chroma (ex:
5YR/5/10 is a saturated orange).

Figure 1 : The Munsell System

Part 3: Differences of Pigment versus Dyes

Pigments
Pigments are organic or inorganic, colored, white or black materials that are practically insoluble
in the medium in which they are dispersed. They are distinct particles, which gives the medium
their color and opacity.

Figure 1 : Pigments Dispersion
The smallest units are called primary particles. The structure and shape of these particles
depends on the cristallinity of the pigment. During the pigment production process, primary
particles generally aggregate and generate agglomerates. During the dispersion of the pigment
into the polymer, high shear is generally needed to break up these agglomerates (improved
tinting strength).
Pigments are thus required to resist dissolving in solvents that they may contact during
application, otherwise problems such as "bleeding" and migration may occur. In addition,
depending on the demands of the particular application, pigments are required to be resistant to
light, weathering, heat and chemicals such as acids and alkalis.
Polymer Soluble Dyes
Polymer Soluble Dyes are soluble in the medium in which they are dispersed. This means that
there are no visible particles and the transparency of the medium is unchanged.

Figure 2 : Dyes Dispersion
Dye is a substance that is applied in order to impart color with some degree of permanence.
Part 4: Pigment Performances
Organic pigments are mainly used for applications needing high tinting strength and brilliant
shades while inorganic pigments are mainly useful where high opacity is needed.
Pigment performances and properties mainly depend on its chemical structures, surface
properties, crystallinity, particle size and size distribution.

Click on the image above to access more details on the required performance for your coating.

4.1./ Color

The color of a pigment is mainly dependent on its chemical structure, which is determined by the
selective absorption and reflection of various wavelengths of light at the surface of the pigment.
Colored pigments absorb part of all the wavelengths of light. For example, a blue pigment
reflects the blue wavelengths of the incident white light and absorbs all of the other wavelengths.
Hence, a blue car in orange sodium light looks black, because sodium light contains virtually no
blue component.
Black pigments absorb almost all the light, whereas white pigments reflect virtually all the
visible light falling on their surfaces.
Fluorescent pigments have an interesting characteristic. As well as having high reflection in
specific areas of the visible spectrum, they also absorb light in areas outside the visible spectrum
(ultra-violets that human eye can not detect), splitting the energy up, and re-emitting it in the
visible spectrum. Hence, they appear to emit more light than actually falls upon them, producing
their brilliant color.

Pigment Color
Titanium Dioxide Excellent
Iron Oxide Fair
Prussian blue Excellent
Lead chromate Excellent
Carbon black Excellent
Monoazo Excellent
Disazo Excellent
Phthalocyanine Excellent
4.2./ Color Strength


As well as color, color strength (or tinctorial strength) must be considered when choosing a

pigment. Color strength is the facility with which a colored pigment maintains its
characteristic color when mixed with another pigment. The higher the color strength, the less
pigment is required to achieve a standard depth of shade.

Chemical structure is one of the factors that influence the color strength of a pigment.
• In organic pigments, color strength depends on the ability to absorb certain
wavelengths of light. Highly conjugated molecules and highly aromatic ones show
increased color strength.
• Inorganic pigments that are colored due to having metals in two valency states, show
high color strength. In contrast, those that have a cation trapped in a crystal lattice are
weakly colored.
Particle size also influences the color strength of a pigment. Higher color strength is obtained
with smaller particles. Manufacturing conditions are the main factor that influences the particle
size of pigment crystals. Pigment manufacturers play a crucial role:
• They can reduce the size of the particles by preventing the growth of crystals during
synthesis,
• and they can increase color strength by efficient dispersion.
Pigment dispersion will also take a major role in the color strengh of the paint. Indeed, it will
impart colloidal stability to the finer particles, avoiding their flocculation and using their full intrinsic
color strength.

Pigment Color Strength
Titanium Dioxide Excellent
Iron Oxide Poor-Fair
Prussian blue Good
Lead chromate Fair
Carbon black Excellent
Monoazo Good-Excellent
Disazo Excellent
Phthalocyanine Excellent

4.3./ Heat Resistance


Few pigments degrade at temperatures normally associated with coatings. However, at higher
temperatures, pigments become more soluble and shading can occur. Thus, for organic
pigments, heat stability is closely related to solvent resistance.
Pigments that prove to be satisfactory at a certain stoving temperature may be totally inadequate
in an application requiring 10°C more.
Chemical stability is also likely to be critical at elevated temperatures. This is typically the case
in powder coating systems. Another key area is coil coatings, as metal complex pigments may
react with stabilizers at elevated temperatures, causing major shifts in shade.
Modifications can also occur in the crystal structure of pigments when subjected to elevated
temperatures. Pigments with a highly crystalline structure are usually more heat resistant than
polymorphic pigments, where the different crystal modifications may respond differently to heat.
Typically, inorganic pigments have enhanced heat stability, though an exception is yellow iron
oxide, which loses water from the crystal at high temperatures.
Heat stability is system dependant and this must be reflected in any test. All tests assess color at
various temperature intervals and evaluate the color difference between the sample in question
and a standard that has been processed at the minimum temperature.
Pigment Color
Titanium Dioxide Excellent
Iron Oxide Good-Excellent
Prussian blue Good
Lead chromate Good
Carbon black Good
Monoazo Poor-Fair
Disazo Good
Phthalocyanine Excellent

4.4./ Light Fastness
Light fastness is evaluated in relation to the whole pigmented system, not just the pigment. The
binder imparts a varying degree of protection to the pigment, so the same pigment will tend to
have better light fastness in a polymer than it will in paint.
Pigments will nearly always have a much poorer light fastness in a printing ink system, where
there is less resin to protect the pigment, and where there is a double effect of light passing
through the pigmented layer, being reflected by the substrate and back through the pigmented
layer.
Other pigments may also influence light fastness in a pigmented system:
- Titanium dioxide promotes the photodegradation of most organic pigments. Therefore, high
ratios of titanium dioxide lead to poorer levels of light fastness.
- Iron oxide can improve the light fastness of organic pigments, due to the fact that it is an
effective absorber of UV light.When the association of two pigments gives a better light fastness,
it is called a synergistic effect.
When the light fastness obtained is lower, it is called an antagonistic effect.
Some inorganic pigments are unchanged by exposure to light, but most pigments, and all organic
pigments, are changed in some way: darkening or complete fading can occur.
A Pigment's ability to resist light is influenced considerably by chemical constitution. Other less
significant influences are pigment concentration, the crystal modification, and particle size
distribution. Additionally, factors in the environment can dramatically affect results, such as the
presence of water and chemicals in the atmosphere or in the paint system.
The light fastness of a pigmented system can only truly be tested in the final formulation and
application. Light fastness tests must be carried out only under carefully controlled test
conditions.
4.5./ Weather Stability
For outdoor applications, pigments used for coloring should be selected for their weather
resistance characteristics. Closely related to light fastness, weatherability adds the extra
dimension of atmospheric conditions (including salt from the sea, waste gases from industrial
areas, or very low humidity from desert conditions). Weather resistant pigments are usually

lightfast but the reverse is not always the case.
The selection of pigments for outdoor use depends on :
• outdoor performance required (life time, climatic region/ Kilo Langley)
• binder type
• concentration of the pigment
• presence of titanium dioxide (which typically accelerates fading)
• concentration and type of light stabilizers used.
Performance can also be influenced by the surface of the painted object and by the processing
heat history.
Once the above variables have been defined, the best way to assess weathering resistance in
service is by using outdoor exposure tests in the climatic region(s) concerned. This is clearly not
always feasible. The widely used alternative is accelerated testing. Machines are available which
in addition to a xenon lamp, include wet cycles interspersed between longer dry cycles.
Weatherability is designated in terms of the 1-5 Grey Scale. 5 represents no change and 1 a
severe change.

4.6./ Insolubility
A pigment must be insoluble in the vehicle (the medium in which it is dispersed), and it must not
react with any of the components of the paint, such as crosslinking agents. Pigments are required
to retain these properties even when the paint is being dried, which is frequently carried out at
elevated temperatures. Once in the dried film, the pigment must also remain unaffected by the
substrate and to agents with which it comes into contact, including water, which may simply be in
the form of condensation, or acidic industrial atmospheres.
Under certain conditions, pigments may dissolve, leading to application problems. Organic
pigments may dissolve to a limited extent in organic solvents, and inorganic pigments may be
affected by other components. Solubility of a pigment generates the following problems:
Blooming
If the pigment dissolves in the solvent, as the paint dries, the solvent comes to the surface and
evaporates, leaving crystals of the pigment on the surface in the form of a fine powder. As
solubility increases with temperature, this phenomenon is made worse at elevated temperatures.

Plate out
The effect of plate out looks similar to blooming, but occurs in plastics and powder coatings.
However, it is not due to the pigment dissolving, but rather to the surface of the pigment not being
properly wetted out. It usually occurs mainly with complex pigments and once wiped from the
surface does not reappear.
Bleeding
Pigments in a dried paint film may dissolve in the solvent contained in a new coat of paint applied
on top of the original film. If the topcoat is a different color, particularly a white or pale color, the
result can be disastrous. Again elevated temperatures exacerbate the problem.
Recrystallization
This phenomenon was almost unknown until the introduction of beadmills. During the milling
stage, heat is generated, which dissolves a portion of the pigment. Over a period of time, the
dissolved "pigment" starts to precipitate out, losing brilliance and color strength. This becomes
especially noticeable in the case of paints containing two differently colored pigments that have
different solubility characteristics. The more soluble pigment dissolves and then as it comes out of
solution and precipitates, the paint will take the shade of the second pigment. Recrystallization
can even take place in aqueous systems. It can be avoided by using less soluble pigments and/or
by controlling the temperature during the dispersion process.
4.7./ Opacity
Hiding power is the ability of a pigmented coating to obliterate the surface. It is dependent on the
ability of the film to absorb and scatter light. Naturally, the thickness of the film and the
concentration of the pigment play a fundamental role. The color is also important.

Figure 1: Hiding power
Dark, saturated colors, such as blacks and deep blues, absorb most light falling upon them,
whereas yellows do not. However, carbon black and most organic blue pigments are fairly
transparent because they do not scatter the light that falls on them. In contrast, titanium dioxide
absorbs almost no light, yet its capacity to scatter light ensures that at a sufficiently high
concentration it will cover the substrate being coated. It is common practice to use a combination
of pigments to achieve the best results.

A key factor in the opacity of a pigment is its refractive index (RI), which measures the ability of a
substance to bend light. The opacifying effect is proportional to the difference between the
refractive index of the pigment and that of the medium in which it is dispersed. This is one of the
main reasons why titanium dioxide is now almost universally used as the white pigment in paint.
(see Table)
Medium RI
Air 1.0
Water 1.33
Film Formers 1.4-1.6
Pigment / Filler RI
Calcium carbonate 1.58
China clay (aluminium silicate) 1.56
Talc (magnesium silicate) 1.55
Barytes (barium sulphate) 1.64
Lithopone 30% (zinc sulphide/barium sulphate) 1.84
Zinc oxide 2.01
Zinc sulphide 2.37
Titanium dioxide:
Anatase
Rutile

2.55
2.76
Inorganic pigments have a high refractive index and organic pigments have much lower values.
Consequently, most inorganic pigments are opaque, whereas organic pigments are transparent.
The particle size distribution of the pigment is another factor that also plays an important role in
opacity. As the particle size increases, the ability of the particle to scatter light increases, up to a
maximum (see figure 6). It then starts to decrease. This ability to scatter light increases the hiding
power of the pigment, and therefore the hiding power also reaches a maximum and then
decreases as the particle size increases.

Figure 2 : Effect of particle size on scattering
Whereas the refractive index of a compound cannot be altered, the pigment manufacturer can
influence the particle size of pigments; consequently particle size selection has become one of
the principal developments in pigment technology in recent years.
Measurement of opacity
The coating is applied in a wedge shape over a contrast chart. The film thickness is built up over
the length of the chart, which is attached to a metal panel. The point at which complete
obliteration is observed is noted and the film thickness at that point measured.

4.8./ Transparency
Usually, transparency is obtained by reducing pigment particle size as possible. This is achieved
by surrounding the particles as soon as they are formed with a coating, which prevents the
growth of crystals. The most common products used for this coating are rosin or rosin derivatives.
This is particularly useful for printing ink pigments that are required to have high transparency and
it has the added advantage that such pigments are more easily dispersed.
Iron oxide pigments can be opaque or transparent. The transparent variety are an important
group of inorganic pigments as they are widely used for metallic finishes, where their high level of
transparency gives an attractive finish, and their weatherability resistance improves the
weatherability of pigments with which they can be combined. This is known as a synergistic
effect. Transparent iron oxides depend on the particles being unusually small, and also having a
crystal shape.
The dispersion process can influence transparency, as it involves breaking up agglomerates of
particles to individual primary particles. However, primary particles are not split up by the
dispersion process. All one can do is to make full use of the pigments original particle size. Good
dispersion will maximize the transparency of a small particle.
Measurement of transparency
Transparency is simply assessed by applying the coating over a black and white contrast chart
and measuring the color difference. The greater the color difference, the higher the transparency.

4.9./ Chemical Stability
Resin, crosslinking agents, UV-initiators, and any other additive may react with the pigment and
alter its performance. At the time when UV-cured coatings were new to the market, additives
significantly reduced storage stability, causing the coating to gel in the can. A great deal of care
must be taken when selecting pigments for powder coatings, as the initiator can change the
pigment shade and reduce fastness properties. Reputable pigment manufacturers publish data
on such systems and can often offer assistance in the case of difficulties.
Another adverse effect can come from chemicals that the coating gets in contact with. Water, in
the form of condensation, can seriously affect a paint film, particularly in bathrooms and kitchens.
Many of the detergents used for cleaning paintwork are harsh and have an abrasive affect upon
the pigment. Should the coating come into contact with food, it is essential firstly, that the coating
is unaffected and secondly, that the food remains unchanged.
Many testing processes concerning chemical stability consist of applying the chemical to the
surface of the coating, keeping them in contact for a given time, then measuring the discoloration
of the coating and/or the staining of the chemical concerned.
Pigment Color
Titanium Dioxide Excellent
Iron Oxide Excellent
Prussian blue Poor
Lead chromate Good
Carbon black Excellent
Monoazo Excellent
Disazo Excellent
Phthalocyanine Excellent


Part 5: Pigment Selection

Performances of paint are deeply linked to properties of used pigments.
Crystal structure, particle size and shape, dispersion and hardness are the mainly important

properties that need to be taken into account to obtain required performances.
Once the desired pigment is identified, the next step consists in establishing the right binder and
the concentration which can be determined by two different concepts: the pigment volume
concentration (PVC) or pigment to binder ratio (P:B).
Finally, the maximum price that can be tolerated for improved performance depends on the
awareness of the end use of the paint.
All these aspects to choose the right pigment are developed in the following pages.
5.1./ Pigments Characteristics
5.1.1.1/ pigment structure
Pigments can be crystalline or non-crystalline (amorphous). In crystalline pigments the atoms
within each molecule are arranged in a well structured pattern, however, in amorphorous
pigments the atoms are randomly arranged. It is also possible for materials to have several
different crystalline forms - known as polymorphism.
Color is dependent on these different structures. There exists pigments which have chemically
identical entities in different crystal forms, yet these polymorphic pigments are not suitable for use
as a pigment. Titanium dioxide, phthalocyanine blue, and linear trans quinacridone are examples
of such polymorphic pigments.
Techniques for influencing the formation of a desired crystal form and particle distribution, for the
purpose of optimizing the commercial product for end applications, are currently being developed
by pigment manufacturers
5.1.2/ Particale shape
The chemical structure, the crystalline structure or the synthesis of a pigment determine the
shape of particles. The primary particles of a pigment may be nodular, spherical, prismatic,
acicular or lamellar.

Figure 1 : Particle shapes
Primary particles are composed of single particles. The smaller these particles, the greater their
surface energy and therefore the more likely it is that they will clump together during
manufacturing. It is not practical to supply pigments in the form of primary particles as they would
be more like smoke than a powder. In practice, they only exist as the pigment is synthesized.

When the particles clump together during the manufacturing process they form either aggregates
or agglomerates.
Aggregates are connected along crystal boundaries during synthesis or drying. Due to the
difficulty of separating them, pigment manufacturers attempt to avoid their formation during the
pigment's production. Agglomerates are loose clusters of primary particles which can be broken
down via an efficient dispersion process.
Following the dispersion process it is still possible for particles to re-agglomerate into loosely held
groups, known as flocculates. This commonly occurs when there is a rapid change of state, ie. a
too rapid dilution, or the addition of an incompatible substance. Flocculation results in a loss of
tinctorial strength. However, flocculates are usually easier to separate than true agglomerates,
and even normal shear such as brushing out is sufficient. This results in an uneven increase in
tinctorial strength, depending on how much shear has been developed during brushing out. Small
particles are more susceptible to flocculation than larger ones, so pigments most at risk are
grades of carbon black and organic pigments, such as phthalocyanine and dioxazine violet
pigments. There are an increasing number of flocculation-stable grades being released on the
market.
Particle shape can influence the shade of a pigment and properties of the paint.
5.1.3/ Particle size
Pigment particles are not usually spherical. They can have different dimensions depending on
whether one measures the length, width or height. Particle size is an average diameter of primary
particles. Typical ranges are:
• carbon black - 0.01 to 0.08 µm;
• titanium dioxide - 0.22 to 0.24 µm.
• organics - 0.01 to 1.00 µm;
• inorganics - 0.10 to 5.00 µm;
Extender pigments can be among the coarsest pigment particles, up to 50 µm, but other types
can be exceptionally fine (e.g. the precipitated silicas).
The pigment's particle size can affect its color, hide and settling characteristics. Large particles
usually settle faster than smaller ones, and smaller ones are harder to disperse. Light scattering
is also often influenced by pigment size. And the distribution will also affect the colloidal stability

and color.
5.1.4/ Surface Area and oil absorption
The surface area is the total area of the solid surface. It is measured in squared units (m2) and is
usually defined for 1 gram of pigment (typical values for organic pigments are between 10 and
130m2). This surface area is determined by an accepted measurement technique such as the
BET (Brunauer, Emmett, and Teller) method using nitrogen adsorption. This technique consists in
calculating the adsorption properties of the pigment.
The surface area is closely linked to the pigment's demand for binder. Larger particles have a
smaller surface area and therefore a lower demand for binder. As the size of particle of pigment is
small, the area of surface become large. As a result, the paint need large amount of binder to wet
each of pigment particles during the dispersion process.
The amount of oil that is required to "wet out" 100 grams of pigment and to make paint with a
pigment is called oil absorption. Oil Absorption is expressed in number of grams of oil per 100
grams of pigment (or volume relationship from weight). This value varies depending upon the
pigments physical nature and particle size.
The amount of oil affects the time of dryness. In general, large amount of oil causes yellowing
and delay of dryness.
5.1.5/ Hardness
Hardness is usually based on Mohs Hardness Scale (a non-linear scale, used as a comparison
chart). The hardness of the pigment is measured by comparison with the ten classes of the Mohs
scale.
In the absolute scale of the hardness (of Rosiwal), the abrasion resistance is measured with
proofs from laboratory, and by attributing to the corindone the value 1000.
Also for the Knoop scale, the values of hardness are absolute. They depend on the depth of the
signs engraved on the minerals due to a special ustensil with a diamond point, with which a
standard of force is applied.
Mineral Mohs Scale Rosiwal Scale Knoop Scale
Gold
0 - -
Talc

1 0.03 1
Gypsum
2 1.25 32
Calcite
3 4.5 135
Fluorite
4 5 163
Apatite
5 6.5 430
Orthoclase
6 37 560
Quartz
7 120 820
Topaz
8 175 1340
Corundum
9 1000 1800
Diamond
10 140000 7000
These scales help define how hard a pigment is and if it will be easily abraded. The hardness of
the pigment can affect the durability and abrasion resistance of the film.
The hardness scales also allow the formulator to better define milling equipment needs and end
use. Some pigments are soft and can be damaged by milling, especially when placed in a ball mill
for extended periods of time.
Another important point to consider is the pigment's solubility and what effect the solvent will have
on the pigment's hardness and structure.

5.2./ Quantity of pigment
The amount of pigment used in a paint is determined by:
• its intensity and tinctorial strength;

• the required opacity;
• the gloss required;
• the resistance and durability specified.
The paint technologist works on one of the two main concepts, either pigment volume
concentration (PVC) or pigment to binder ratio (P:B).
The PVC is of fundamental concern when formulating paints that are required to have optimum
performance with respect to durability. It is known that there is a critical point that represents the
densest packing of the pigment particles commensurate with the degree of dispersion of the
system. It is a complex calculation but essential for paints that have to meet the highest
performance standards with respect to durability.
The P:B ratio, by weight or occasionally by volume, is a much simpler calculation, often used to
assist in formulating a good millbase and for balancing a formulation for gloss and opacity.
For systems requiring high gloss, a low PVC is required, whereas primers and undercoats can
have a much higher PVC - up to 90%.

5.3./ Paint type - Binder
The binder in the paint system plays a key role in terms of determining the pigment and the type
of solvent in which it is dissolved. A common choice for a solvent is water as it is compatible with
most polymers, except some toners.
White spirit is a commonly used solvent for long oil alkyd paints, which are widely used in
decorative gloss paints. A large majority of pigments are insoluble, or almost insoluble in white
spirit, so it rarely narrows the choice of pigments.
Industrial finishes can be based on broad variety of solvents. To take an example, solvents such
as xylene, ketones, and esters are very powerful and can dissolve pigments with poor or only
moderate resistance to solvents. If such paints are dried by stoving, the high temperature poses
even greater demands on the pigment used.
It is also necessary to consider whether the coating will be overcoated. For example, in the case
of a car getting repaired, the pigment used on the original finish will have to be fast to
overcoating.
In powder coatings crosslinking agents can affect the pigment. For this reason, the pigments

must be compatible with these agents at temperatures employed during application.
It is therefore evident that the type of resin and solvent used remain key factors in the choice of
pigment

5.4./ End use
An awareness of the end use of the paint is essential, as durability and chemical resistances
requirements. The maximmum price that can be tolerated for improved performance depends on
this knowledge. For example, a low-quality pigment would be insufficient in an automotive finish,
just as a high-quality pigment would be unnecessary for use in a gardening tool.
Paints can be classified according to the market in which they are used. Common classifications
set apart paints used for building, architectural or decorative, automotive finishes, OEM (original
equipment manufacturers) or VR (vehicle refinishes)and those used for industrial finishes. In the
same way, pigments used in paint do not require same properties than pigment used for inks
applications.
The price difference between a cheap organic pigment and a high performace pigment can be a
factor of around 20. High performance specialty pigments are often complex in composition and
have limited application, which accounts for their high pric
Part:6: Pigment Dispersion
Efficient and effective pigment dispersion is necessary in order to obtain optimum tinctorial
strength, cleanliness of shade and good gloss from the final coating. High quality coatings of high
brilliance and color strength are characterized by a perfect pigment dispersion, optimal pigment
particle size and long-term stabilization of the dispersed particle in the formulation. Most organic
pigments show better transparency as dispersion improves, while in the case of the larger particle
size inorganic pigments, opacity is improved by good dispersion.
The dispersion process consists of the permanent breaking down of agglomerates into, as far as
possible, primary particles.
There are four aspects to the dispersion process:
1. Deagglomeration is the breaking down of the agglomerates and agregates by the shear
forces of the equipment being employed. A mixture of crushing action and mechanical
shearing force is necessary.

2. Wetting out occurs at the surface of a pigment when a binder (or surface active agent)
sticks to the pigment's surface and acts as a connection between the pigment and the
binder. The air and moisture are displaced from the surface. Between the particles of the
pigment aggregates, agglomerates are replaced by the resin solution.
Wetting out time depends on the viscosity. Heat produced by the mechanical shearing
process causes the temperature of the mixture to rise, thus helping the wetting out
process. This increase in temperature reduces the viscosity as well as the effectiveness
of deagglomeration which is a well known phenomenon.
The wetting step consists of replacing the adsorbed materials on the surface of the pigments and
inside the agglomerates (water, oxygen, air, and/or processing media) by the resin solution.

The complete wetting out of the primary sized pigments particle helps to enhance the technical
performance of a liquid coating that depends very much on interaction between the pigment
particles and the binder system. Dispersing additives, which adsorb on the pigment surface,
facilitate liquid/solid interfacial interactions and help to replace the air/solid interface by a liquid
medium/solid interface.

Figure 1 : Replacement of air and water by the resin
The efficiency of the wetting depends primarly on the comparative surface tension properties of
the pigment and the vehicle, as well as the vicscosity of the resultant mix. The adsorption
mechanism depends on the chemical nature of the pigment and the types of dispersing agents
used. (visit dispersants familes)
Thermodynamic consideration:

The spontaneous wetting process (on wetting solid surfaces) is driven by minimization of the free
surface energy. Forced wetting processes (in non-wetting conditions) require the application of
external force, and spontaneous de-wetting will take place when the force is removed.

Thermodynamic condition for wetting requires the work of liquid/solid adhesion (Wa) to be as high
as possible and, for unlimited wetting, at least more than a half of the work of cohesion (Wk) is

required: Wa> Wk.

Velocity of penetration of a liquid into a powder can be explained in terms of the Washburn
equation (1921):

Figure 2 :Washburn equation
where h is the depth (or height) of penetration during the time t, - is the surface tension of the wetting liquid, - its
viscosity,
- the wetting angle, r - mean radius of capillaries, C - structural coefficient, associated with parameters of the
porous structure, W - energy (heat) of wetting.
The wetting step of dispersing processes can be intensified by the use of wetting agents and/or
binders with lower viscosity and surface tension. On the other hand, a resting of pigment/binder
premixes prior to their dissolving or grinding helps to accomplish the wetting stage and always
eases and accelerates dispersing processes.

3. Distribution demands the pigment to be equally dispersed throughout the binder system.
A lower viscosity tends to lead to a more even pigment distribution.
4. Stabilization prevents the pigments from re-agglomerating. The pigment dispersion is
stabilized by dispersing agents in order to prevent the formation of uncontrolled
flocculates. The resultant suspension is stabilized due to the adsorption of binder species
or molecules at the pigment surface.
The aim of stabilization is to keep the pigment particles separated as achieved in the last step,
and to control the degree of pigment particle size through the let-down and filling phase, storage
and and later in coating films during film formation.

Flocculated pigment suspensions are characterized by the non-uniform spatial distribution of
particles, which are allowed for immediate interparticle contacts. This results in worse rheology
(structural viscosity, blob-flow), low storage stability (in paints) and poor optical and colour
properties (in coatings).
It is well known, that even well grinded but not stabilized, fine particle size pigment suspension

can easily be destroyed by the letting down into a non-suitable paint base: flocculation typically
breaks down when shear is applied and will form again, when the shear is removed.
Therefore, immediately after grinding pigment suspensions must be stabilized by the addition of
additives, whether they are intended to be used immediately in let-down or as pigment
preparations (colorants).

Figure 1 : Dispersing agents avoid flocculation
Stabilization is achieved through adsorbtion of stabilizing molecules on the pigment surface, so
that repulsive forces prevent other particles from approaching close enough for the attractive van
der Waals forces to cause agglomeration. To know more about the factors influencing the
stability, take a look at the colloïdal stabilization.

There are two principal mechanisms for the stabilisation of pigmented dispersions:
5. Electrostatic stabilization: electrostatic stabilization occurs when equally-charged local
sites on the pigment surface come into contact with one another. Two particles having
the same charges give a repelling effect. The resulting Coulomb-repulsion of the charged
particles allows the system to remain stable.
6. Steric stabilization A pigment is said to be sterically stabilized when the surface of the
solid particles are completely covered by polymers, making particle-to-particle contact
impossible. Strong interactions between polymers and solvents (organic solvent or water)
prevent the polymers from coming too closely into contact with one another (flocculation).
7.

Part 7: Main Families of Pigments
Colored pigments are categorised as either organic or inorganic. Each have distinct
characteristics which, in the past, were used to distinguish one from the other. For example,
organic pigments are traditionally transparent. However, modern manufacturing techniques are
capable of imparting properties not previously associated with the chemical type: it is now
possible to produce high opacity organic pigments.
The use of inorganic pigments dates back to the early cave paintings that are 30,000 years old.

Although they occur naturally, for the manufacturing of paint they usually require modification. All
white pigments are inorganic as are a wide range of colored pigments.
Organic pigments are relatively new. Although natural dyes have been precipitated on to
inorganic bases (known as lakes) and used in artists' colors since the middle ages (e.g. madder
lake and crimson lake), true organic pigments have only been known since the early years of the
twentieth century. They divide in two sub-groups: one of vegetable and the other of animal origin.
The properties that have traditionally been associated with inorganic and organic pigments are
summarized in the table below.
Compared Pigments Properties
Organics
Pigment properties Inorganics
Classical with High Performances
Color, Purity
Often dull Usually bright
Opacity
High More or less transparent
Color strength
Middle to Low Normally High
Light Fastness
(Blue scale)
Good to High
(7-8)
Low to Middle
(< 7)
Good to High (7-8)
Weather Resistance
Varies (depending on
chemistry)
Insufficient Middle to High
Heat resistance

In general > 500 °C
Rarely < 200 ° C
150 to 220 ° C 200 to 300 ° C
Fastness to solvents -
Bleed resistance
High Middle to Good Good to High
Resistance to chemicals
Varies (depending on
chemistry)
High (except for salts) High
Price
Low to Middle Middle High

In this section, we have selected for you the main families of pigments. You will find all the
product, physical, structural and use information needed:


White pigments
Black pigments
Brown pigments
Yellow pigments
Orange pigments
Red pigments


Violet pigments
Blue pigments
Green pigments
Special effect pigments
Extender pigments

Corrosion-inhibiting pigments
7.1./ White pigments
All white pigments are inorganic. The more used white pigment is Titanium Dioxide. It became the
dominant white pigment after the Second World War.