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.
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.
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.
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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Titanium Dioxide: An Introduction by Marc Hirsch
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