Abstract
Color is an important property of nearly every paint or plastic and is primarily controlled using color pigment particles. A wide range of particles is available to the paint or plastics formulator. These particles vary in performance in terms of the exact color that they provide as well as tint strength (how much colorant is required to give a certain color or, more technically, to absorb a certain portion of light radiated onto it), cost and stability against heat, UV light, solvents, and other chemicals. Color pigments can be conveniently classified in a number of ways—by color, chemical nature, ease of use, particle size, etc. Because many of these properties are determined by the chemical nature of the particles, in this chapter we will classify pigments broadly by their chemical type (organic or inorganic), with further subclassification according to their specific chemical family. In addition, we will review the most important properties of a color pigment—color, of course, as well as particle size, lightfastness, crystal form (both phase and shape), and dispersibility. Manufacturing methods are outlined to indicate how different pigment properties are conferred to specific families of pigments, and a logical system for naming color pigments is discussed.
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Notes
- 1.
In the case of interference pigments, color is generated by selective reflection of light according to wavelength and viewing angle. This is discussed in a later section of this chapter.
- 2.
The energy of an orbital is always given as a negative number, indicating energy is needed to remove the electron from the atom or molecule.
- 3.
ℓ is sometimes called the azimuthal quantum number.
- 4.
Strictly speaking, the orbitals shown in Fig. 8.1 are mixtures of the mℓ = −1, 0, and 1 orbitals.
- 5.
These are sometimes called the frontier orbitals.
- 6.
In this orbital designation, the numeral refers to the principal quantum number n and the letter to the angular momentum quantum number ℓ.
- 7.
A dipole is the separation of electric charges. A dipole moment is the measure of the electric field created by a dipole.
- 8.
Note that only those ring systems that satisfy the Hückel rule (4n + 2 electrons in the p-type orbitals, where n is an integer) can benefit energetically from these systems. For benzene, n equals one and there are six electrons in the delocalized orbitals.
- 9.
Based on quantum mechanical principles (more specifically, the “particle in a box” analysis), the spatial confinement of electrons generally increases their energies. By delocalizing molecular orbitals over a number of atoms, electrons have a greater region of occupancy and so their energies decrease. This added energetic stability generally increases with the number of atoms participating in the delocalized network, since this increases the occupancy region of the electrons (i.e., the size of the “box”).
- 10.
C.I. stands for Colour Index™. This system of naming pigments is described later in this chapter.
- 11.
Ideally the three benzene rings in crystal violet and malachite green would be coplanar, as this maximizes delocalized bonding, but due to steric constraints these molecules have a propeller or paddlewheel structure.
- 12.
Note that this is fundamentally different from the resonance structures of aromatic molecules. In that case a single true structure exists that is an average of the resonance structures, while each form of tautomer is a unique molecule. That said, interconversion of tautomers is often facile.
- 13.
Color is produced in some inorganic pigments through light interference effects, as discussed later.
- 14.
Colorless transparent inorganic pigments can be made from colorless materials such as TiO2 and ZnO. These are often used as UV light absorbers in applications such as sunscreens.
- 15.
Note that, to derive Eq. 8.1, the change in light wavelength that occurs when the light crosses from the medium into the platelet must be taken into account. It is, in fact, not the absolute distances of the two pathways that are important, but rather the time it takes to traverse those distances. If the difference in time traveled is an integer multiple of the time it takes for one cycle (that is, the inverse of frequency), then the waves combine constructively.
- 16.
Equation 1 omits any phase shifts that may occur on reflection. When such shifts occur, they must be account for in this equation. In particular, the time required for the light to undergo such a phase shift must be included in the travel times referred to in the previous footnote.
- 17.
Although n1 does not explicitly appear in Eq. 8.1, it is a part of the relationship between the angles α and β and so does play a role in this determination.
- 18.
- 19.
Luster is defined as the contrast between the light specularly reflected from one part of a surface and the light diffusely reflected from an adjacent area [28]. That is, high luster is seen when light is reflected strongly at the mirror angle (specularly), but weakly slightly off of this angle (diffusely).
- 20.
Somewhat confusingly, the term “% absorbance” is sometimes used to describe the percent of light that is not transmitted (i.e., 100 − %T). This is not what we are referring to here and elsewhere in this chapter.
- 21.
The units of ε depend on the units of b and c and are chosen such that A is unitless. b is typically given in units of cm and c is typically given in units of mol/l.
- 22.
The linear response between absorbance and concentration can break down for strongly absorbing systems and in situations where paint components chemically interact with one another.
- 23.
In Chap. 15 we will quantify the effect of colorant concentration on color.
- 24.
For reference, germanium metal has perhaps the highest refractive index in the visible spectrum, with a value of 4.05.
- 25.
This is a manifestation of surface energy. The thermodynamic driver for this behavior is a decrease in overall surface area, with the attendant decrease in surface energy.
- 26.
Both of these regions are the same size, since they contain the same volume of particles and have the same concentration.
- 27.
This is not true when molecules within the particles affect one another’s electronic structure, as was described in the previous section on crystal structure, or when the dissolved molecules interact electronically with solvent molecules.
- 28.
Factory colors are typically limited to a dozen or so while POS paints can be made in hundreds, if not thousands, of colors.
- 29.
Note that this is fundamentally different from systems that define colors (see Chap. 6). Here we are defining the materials that create colors, not the colors themselves.
- 30.
There are ten hues used for generic names—red, orange, yellow, green, blue, violet, black, white, brown, and metal.
- 31.
Because blue, brown, and black all begin with the letter b, they are abbreviated as B, Br, and Bk. For example, PBr is a brown pigment whereas PB is a blue pigment.
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Diebold, M., Backer, S.D., Niedenzu, P.M., Hester, B.R., Vanhecke, F.A.C. (2022). Color Pigments. In: Pigments, Extenders, and Particles in Surface Coatings and Plastics. Springer, Cham. https://doi.org/10.1007/978-3-030-99083-1_8
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