Original article published Feb. 10, 2017. Updated Mar. 9, 2022.
Introduction
Pigments are used to give color to a coating and/or to obtain hiding power of the coating. A coating gives complete hiding power, also called full opacity, when the substrate cannot be seen through the coating. Two phenomena govern both color and opacity of a coating: absorption and/or scattering of visible light by the pigment particles that are within that coating.
Opacity of a coating judged on a black-and-white card
Absorption
Whether or not a pigment is able to absorb visible light depends on the chemical composition of the particles. Carbon black absorbs all wavelengths λ that are present in visible light (λ ≈ 380 – 750 nm). Only a low percentage of fine carbon black pigment is needed to obtain full opacity. Clean fillers, like synthetic barium sulfate (BaSO4), and white pigments like TiO2 do not, or hardly, absorb visible light. Color pigments absorb only part of the wavelengths of visible light. Red pigments, for example, are perceived as red because the particles absorb the blue and green part of light that is visible for humans.
Absorption of light by a pigment particle
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 close to the surface, 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 delivers a lot of money when the particles of expensive color pigments are fully separated from each other during the dispersion process and are stabilized against flocculation1,2. Finally, a pigment is more able to absorb visible light when the particles are distributed over the coating. So, the color strength of fine, fully separated, stabilized and distributed pigment is higher than the color strength of a coarse pigment that is only partly separated and/or poorly stabilized and/or not fully distributed in the coating.
The absorption efficiency of a color pigment
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 color and hiding power of the coating. The scattering efficiency of particles is governed by a few properties.
First, scattering is strong when the difference in the 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.
Scattering of light by particles in a coating
The refractive index of binders, as used in coatings, is around 1.55. White pigment titanium dioxide, TiO2, is preferably used as a scattering source because the pigment has a high refractive index3,4. It is important to realize that scattering is not a surface effect like absorption; scattering involves the whole particle.
The optimal particle size for scattering visible light
TiO2 is a pigment that gives hiding power and whiteness because titanium dioxide particles scatter all wavelengths in visible light. It will come as no surprise that the particles of TiO2 pigments, as used in paints, have a diameter of around 300 nm.
References
- Article Dispersants – The Understood Misunderstood Additive, Marc Hirsch, 10 September 2021.
- Article The Basics of Dispersion and Stabilization of Pigments and Fillers, Jochum Beetsma, 31 January 2020.
- Article Titanium Dioxide: The Misunderstood Pigment, Marc Hirsch, 27 March 2020.
- Article Efficient Use of TiO2 Pigment, Jochum Beetsma, 1 August 2019.
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