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Pigments are insoluble colorants which are not physically or chemically affected in an incorporated substrate.
Pigments, especially those which occur in nature, have been used as colorants for hundreds of thousands of years. The development of synthetic pigments and dyes in the 19th century led to the widespread production of
manufactured pigments for use in printing, coating, and finishing industries.
Pigments appear to the human eye to be colored because white light — which contains all the colors of the visible spectrum — is selectively absorbed by the pigment itself. Reflected light from the pigment creates the appearance of color. For this reason, pigments (and all colors in general) appear slightly different when exposed to different types of light, such as sunlight, incandescent light, and fluorescent light.
Identical colors can be perceived as radically different when viewed under different light sources.
Image credit: Jonas Tullus
For the reasons listed above, the specification and standardization of pigments is dependent upon the light source used to test it. For example, the L*a*b* color specification system is based on a D65 (6500 K) light source, which is nearly equivalent to sunlight.
While "raw" pigments reflect light which is highly saturated in color, they are more typically combined with binders and other additives which influence physical properties of the pigment. As such, the addition of a binder results in the perception of a slightly dull color because some of the pure light naturally reflects off of the binder instead of the pigment molecules.
The image below shows the reflection and absorption characteristics of a hypothetical pigment. By reflecting the majority of wavelengths between 500 and 600 nm, the pigment is perceived as green.
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Pigments vs. Dyes
Pigments and dyes are used similarly as colorants but have fundamental differences and applications. The primary difference is solubility: pigments are not soluble within the vehicle, resulting in a suspension. Dyes, on the other hand, are either liquids or fully soluble in its vehicle, resulting in a solution. A single colorant may be considered either a dye or a pigment depending upon the vehicle or substrate it is used with.
Pigment colorants are typically longer-lasting and resist fading due to the fact that the pigment molecules tend to bond directly to the substrate. In industrial printing applications, pigment inks are typically used to color slick surfaces, such as transparencies and stickers, which would wick off dye ink. Dyes do have the advantage of lower cost and improved color vibrancy when compared with pigments.
The image below shows the difference in light reflection from two different printed surfaces. Note that the pigment ink results in a rough surface due to the suspension of dry pigment within the ink; this surface causes scattered reflection, resulting in the perception of a less-vibrant color.
Image credit: rihac
Like all colorants, pigments are useful in any application requiring the coloring of a product or substrate. Modern uses for industrial pigments include:
Paints and coatings
Masterbatches for plastics manufacturing
Instrument indicator needles (fluorescent pigments)
Eyeglass lenses (photochromic and thermochromic pigments)
The largest global pigment industries by value are:
Inks - publishing, packaging, and printing
Paints and coatings - automotive and architectural
Plastics and rubber
Inorganic pigments do not contain carbon and typically refer to metallic oxides and synthetic pigments. They may be naturally occurring and can be processed by simply cleaning, pulverizing, and mixing. Popular inorganic pigments include lead oxide (red), chromium oxide (green), cadmium yellow, and nickel titanate (yellow).
Classes of inorganic pigments. The circled section highlights complex inorganic color pigments (CICP).
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Organic pigments contain carbon and typically do not occur naturally. They are used as mass colorants in diverse applications such as surface coating, opto-electronic displays, and plastic manufacturing.
Organic pigments share several key features and advantages, some of which are listed below.
Excellent resistance and stability to solvents, light, heat, and environmental wear
Consistent coloring with bright, pure shades
Organic pigments can be further broken down into classes based on chemical makeup; they are generally classified into six types:
Acid/base dye pigments
Miscellaneous polycyclic pigments
Pigments are sometimes referred to by a slightly antiquated common name instead of their chemical name. These names typically refer to the location where the pigment mineral was mined or produced; for example, sienna and umber pigments (from Siena and Umbria, both in Italy) are common pigments as well as color formulations. Other pigments referred to by common names include Egyptian blue (calcium copper silicate), cerulean blue (cobalt[II] stannate), and Alazarin crimson (also known as Turkey red, or 1,2-dihydroxyanthraquinone).
A selection of color swatches, including umbers and siennas.
Image credit: Letraset
Pigments may be manufactured, tested, and used according to various standards. Important pigment standards include:
ISO 787 (General test methods for pigments; SERIES)
ISO 8780 (Pigments and extenders - methods of dispersion for assessment of dispersion characteristics; SERIES)
Standardcon - Types of Pigments