ChE EL 22: PAINTS AND COATINGS
OPTICAL PROPERTIES OF PAINTS AND COATINGS
Why is it important to understand optics when studying paints?
– We are surrounded by surface coatings, man made and natural. The
development of coatings and paints is ruled by the empirical approach.
Fabrication would be simplified if one could quantitatively calculate the
angular as well as color reflectance properties of mixtures from a knowledge
of the fundamental optical properties of the constituents
REFLECTANCE
– The reflectance of binary powder mixtures may be divided into three classes
by characterizing opacity of each of the two phases.
1. Opaque- Opaque
– For a surface consisting of opaque particles, the reflectance primarily
depends upon the particle reflectance, multiple scattering being small. Thus,
for a substance consisting of a mixture of two kinds of opaque particles, the
reflectance will depend upon the relative surface areas of the two
components
2. Opaque- Transparent
– For opaque- transparent mixtures, the analysis is not as simple because the
transparent particle scatters radiation
3. Transparent- Transparent
TAILORING REFLECTANCE
– Tailoring depends upon the availability of pigments having the desired
absorbing properties as a function of wavelength.
– The primary considerations for a paint or coating are the color (ie. The
wavelength dependence of reflectance) and the angular scattering
properties (i.e. the diffuseness or glossiness)
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ChE EL 22: PAINTS AND COATINGS
COLOR
– The impression of color is produced by the absorbance of light by colorants
(pigments) in the paint film
– The color is produced by the absorption and scattering properties of the
surface arising from two surfaces, a surface reflection component and a body
and surface scattering component
– The surface reflection component is due to the difference in the complex
index of refraction between the coating and the ambient medium
– The body and surface scattering can be dominant contributor of reflectance.
The magnitude of reflectance due to body scattering depends on the ratio of
the absorption coefficient to the scattering coefficient
– Pigments are used to give colour to a coating and/or to obtain hiding
power of the coating. A coating has full hiding power, also called full opacity,
when the substrate cannot be seen through the coating. Two principles are
mainly used to obtain full opacity: absorption and/or scattering of visible
light by pigment particles.
Color Testing
– Metamerism- is the standard and test panels contain different pigment
mixtures, their colors may match under standard light source but not under
another source.
– Color comparison is most successful if the two paint films have the same or
only slightly different gloss.
– In testing of coatings with special optical effects (e.g. metallic finishes) the
flop effect also has to be included. Flop is the change in color observed when
the angle of observation is varied.
– Colors can be defined with the CIELAB color space in which all colors are
arranged in a circle around a central vertical lightness axis. The center of the
hue circle is considered to be neutral gray.
– ISO 7724 describes methods for the instrumental determination of the color
coordinates and color differences. ISO 7724 specifications are satisfied by
many color measuring devices: tristimulus colorimeters,
spectrophotometers, and abridged spectrophotometers.
– Spectrophotometers can measure the spectral reflection of a sample ; they
are a useful aid in the analysis and synthesis of color samples.
GLOSS
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ChE EL 22: PAINTS AND COATINGS
– Gloss as defined by the Commission Internationale de l’Eclairage (CIE) is the
mode of appearance by which reflected highlights of objects are perceived
as superimposed on the surface to the directionally selective properties of
that surface.
– Specular gloss denotes the degree of mirror reflection and is the primary
visual gloss impression.
****Classification of specular gloss according to ISO 4628 System:
- high gloss, gloss, semi-gloss, semimatt, matt, and dull matt.
Gloss Measurement
– Arbitrary gloss scales were initially used.
– Psychometric scaling of gloss has been recently proposed and describes a
method devised for the multidimensional assessment of gloss.
– Human eye recognizes additional gloss phenomena described by RS Hunter
who proposed six types of gloss: specular gloss, sheen, contrast gloss/luster,
absence of bloom/reflection haze, distinctness of image/image clarity, and
directional or surface uniformity.
– Hunter and Judd, in 1939, found that specular gloss measured at 60⁰ in a
reflectometer provided a useful classification of paint finishes according to
glossy appearance.
– The instrument was standardized as ASTM D 523, improved and
subsequently standardized as ISO 2813.
– The 60⁰ reflectometer is now used worldwide as the standard instrument for
measuring specular gloss.
– The 60⁰reflectometer has been supplemented by the 20⁰ and 85⁰
reflectometers.
ABSORPTION
– Whether or not a pigment is able to absorb visible light depends on the
chemical composition of the molecules the pigment particles are composed
of.
– Carbon black absorbs all wavelengths λ that are present in visible light (λ ≈
400 – 800 nm). Only a low percentage of fine carbon black pigment is needed
to obtain full opacity.
– Clean fillers, like synthetic barium sulphate (BaSO4), and white pigments like
TiO2 do not absorb visible light at all.
– Colour pigments absorb only part of the wavelengths of visible light.
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ChE EL 22: PAINTS AND COATINGS
– Each molecule in a pigment particle participates in absorption. This implies
that molecules in the core of a big particle do not have the ability to absorb
light because the light was already absorbed by the molecules that are in the
shell of the particle.
– A pigment that consists of small particles contains more surface molecules
per gram than a coarse pigment. The amount of light that can be absorbed
per gram of pigment goes up when the particle size of the pigment goes
down because of that.
– Apart from this, it is more profitable for the producer because more colour
strength can be obtained from a gram of an expensive pigment when the
particles are fully separated from each other during the dispersion process
and are stabilised against flocculation1.
– The colour strength of fine and well dispersed pigments is higher than the
colour strength of coarse pigments that are not dispersed well.
SCATTERING
– Solid white particles in a coating can change the direction of light when the
particles and the matrix, that surrounds the particles, have a different
refractive index n. This phenomenon, called scattering, results in both white
colour and hiding power of the coating. Scattering efficiency is governed by
a few properties.
– First, scattering is strong when the difference in refractive index of particle
and matrix, Δn = np – nm, is big. The refractive index of a material is governed
by its chemical composition. Secondly, for a specific wavelength of light, λ,
there is an optimum with respect to particle size. The optimal particle
diameter d for scattering light is about half the wavelength of the light.
– The refractive index of binders, as used in coatings, is around 1.6. White
pigment titanium dioxide, TiO2, is preferably used as a scattering source
because the pigment has a high refractive index2. It is important to realise
that scattering is not a surface effect like absorption: scattering involves the
whole particle.
– TiO2 is a pigment that gives hiding power and whiteness because of
scattering all wavelengths in visible light. It will come as no surprise that the
particles of TiO2 pigments, as used in coatings and inks, have a diameter of
around 300 nm.
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