 coatings technology: Pigments
Most pigments used in paints are inorganic in nature and many are derived from natural sources by mining and processing (grinding and classifying). Many synthetic pigments have been developed that are chemically similar to the natural material but the process allows for tight property and quality control. One example is iron oxide. This is available from naturally mined and processed material. The synthetic version can use scrap metals mixed with acids and heat to control color. Further processing yields a uniform colored and controlled particle size material.
Pigments, which are dry particles, can vary in their particle shape. The most common inorganic pigments are generally:
- Spherical: Round, relatively regular shape
- Lamellar: Platy: flat, with a length and width magnitudes greater than the depth.
- Nodular: Rounded, irregular or crystalline
- Acicular: Needle-like, length far greater with width and depth equal
Organic pigments can appear as very fine, intricately-shaped structures or in structures typical of inorganic pigments.
Pigment Classes:
- Prime White Pigments: Highly refractive, white, opaque; used to opacify paint
- Colored Pigments: Organic and inorganic pigments; used to create color.
- Metallic Pigments: Ground or flaked metal; used for color or to enhance appearance.
- Extender Pigments: Low refractive; used to control cost and modify physical properties.
- Functional Pigments: Used to add desirable characteristics; examples include anti-corrosive, flame-retardant and stain-blocking types.
The refractive index (RI) is a scale used to rate the ability of materials to refract or bend light. The refractive index of air is taken as 1.00 and all materials are rated relative to it. A mixture of two materials with differing refractive indexes will distort the light passing through it and create opaqueness. A good example of this is the appearance in air of an extender pigment like calcium carbonate. In this situation the pigment appears bright white due to the difference of the refractive index of air (1.00) and the calcium carbonate (1.59). If the pigment is then mixed in a latex emulsion at a volume lower than the volume of latex solids, there is little change to the appearance in the liquid mixture; when the mixture is dried on a glass panel, it will look relatively clear. This is due to the very little difference between the polymer’s refractive index (1.50 - 1.70) and the calcium carbonate (1.59). Add more pigment to the point that it exceeds the volume of latex solids, and the dried film appears opaque because now there is insufficient resin to completely encapsulate the pigment particles, and air voids will appear between them. The refractive index of air is very different from the RI of the pigment/resin blend, so light is bent. This effect is known as “dry hiding” and relates to the critical pigment volume concentration or CPVC.
The particle size of the prime white or opaque pigments is critical to the efficiency in light scattering. A large particle of titanium dioxide (2 - 3 microns) has little opacity because titanium dioxide is a crystal and, as such, is clear. Reducing the particle size to between 0.2 and 0.4 microns maximizes the light scattering properties or reflection between particles relative to the wavelength of visible light. Further reduction in size would reduce the particle size to below the average wavelength of visible light into the ultra violet range and would appear transparent to the eye. Ultra fine titanium dioxide pigments are currently being evaluated as transparent UV inhibitors for coatings applications.
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