This article is the follow-up of article Optical Properties of Pigments: Absorption and Scattering1. Absorption and scattering of light by pigments in a coating is governed by the chemical composition of the pigment and the size of the particles.
Each molecule in a pigment particle participates in absorption. 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. This implies that absorption efficiency of solid particles goes up when the particle size goes down. Scattering of visible light by particles depends upon the size of the particles as well. With respect to scattering, there is an optimum with respect to particle size.
Transparent Titanium Dioxide
In most coatings, titanium dioxide (TiO2) is used as white pigment, giving high white colour strength as well as hiding power, also called opacity1. The underlying principle is two-fold.
First, microscopic TiO2 particles scatter visible light — electromagnetic radiation with wavelength λ ≈ 400 – 700 nm — effectively because TiO2 has a high refractive index ‘n’. Optimum scattering of visible light by particles in a system is obtained when the particles have a diameter of about half the wavelength of the light. The diameter of white opaque TiO2 pigments is in the range of 280 nm for that reason.
Secondly, titanium dioxide hardly absorbs visible light. That is why the human eye sees the particles as white.
TiO2 absorbs UV radiation effectively, especially when the pigment particles are small. Small particles have a high specific surface area and therefore contain many surface molecules that are most effective in absorption of radiation. Nano-sized titanium dioxide (nano-TiO2) pigments, with a particle diameter far below 100 nm, are used as an UV absorber in systems ranging from coatings to sun creams. A coating containing nano-TiO2 is transparent because the particles hardly scatter visible light.
As nanoparticles are difficult to disperse in a liquid, many companies choose to buy ready-to-use dispersions of nano-TiO2. An example of such a product is CCR 110 of Cinkarna2. CCR 110 is a dispersion of TiO2 particles, with an average diameter of around 10 nm, in water. The material can be used as an UV absorber in coatings, plastics and cosmetic products like sun cream.
Transparent Iron Oxide Pigments
Iron oxide red pigments (Fe2O3, colour index: PR 101) and iron oxide yellow pigments (FeOOH, colour index: PY 42) are used as inorganic colour pigments in coatings. These materials have a high refractive index and therefore the particles scatter visible light. Iron oxide pigments are coloured, implying that the particles absorb part of the visible light. For some applications, coloured pigments are desired that absorb part of the visible light but that scatter visible light as little as possible.
Minor scattering is desired in, for example, transparent wood stain, also called wood varnish. These coatings must protect the wood and give colour to the object. On the other hand, the beautiful wood structure must remain visible. To achieve this combination of properties, the coating must contain pigment(s) that absorb part of the visible light to obtain colour but that hardly scatter visible light to maintain transparency.
An example of a transparent iron oxide red pigment is TRAFe® BTR-201W of Chemsfield3. The particle size is not given in the Technical Data Sheet of this product but the high specific surface area value (SSA = 100 – 120 m2/g) indicates that the pigment has a huge amount of surface molecules. Transparent iron oxide pigments are difficult to disperse, especially because many of these products consist of needle-shaped particles. For this reason, paint producing companies often choose to buy ready-to-use dispersions of transparent iron oxide pigments.
- Optical Properties of Pigments: Absorption and Scattering, Jochum Beetsma, 10 February 2017
- Technical Data Sheet of CCR 110
- Technical Data Sheet of TRAFe® BTR-201W Transparent Iron Oxide Pigment Red
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