Chemical and Physical Properties of Inorganic Pigments

20180723

Inorganic pigments play double-duty as fillers that provide a greater benefit than simple coloration of a formulation; they also impact physical properties of the film during application and throughout the product lifecycle. Pigments in coatings protect the resins and binders from electromagnetic or thermal degradation due to their reflectance of short-wave IR radiation, which also helps to keep the materials containing said pigments cooler.1

Why We Use Pigments

Before we can understand best practices for dispersing materials containing pigments, it is important to understand what a pigment is, and the chemical and physical reasons why we use pigments. Inorganic pigments are transition metal complexes,2 primarily oxides of crystalline or semi-crystalline repeating units of ceramic crystal lattice structure.

The d-orbital of the metal ions is responsible for a multitude of inorganic pigment properties, including color, reactivity, strength (as in Mohs hardness) and weatherability. Pigments are unique as fillers in that they are composed of transition metals surrounded by ligands (functional groups). The way in which the d-orbital of the metal ion interacts with the various ligands to which it is bonded also influences pigment properties; ligand substitution results in modified pigment characteristics.

As an interesting aside, the metal ion-ligand coordination complexes of pigments used in coatings function (that is to say, provide a visible output color) much like light-harvesting complexes in photosynthetic pigments. Drawing from the Stark-Einstein law, we will take this full circle. The law states that an absorbed photon will initiate a primary chemical or physical reaction within the system.3 For coating pigments, this means that the d-orbital of the transition metal experiences excitation; the degree to which this excitation increases the energy gap dictates corresponding perceived color of the material. For example, the transition metal Vanadium can form complexes of four different ionization states (i.e., V2+, V3+, V4+, V5+), which offer pigments of different hues from purple (V2+) to yellow (V5+).4

That is to say, to reduce agglomerates to aggregates requires one-tenth the energy of reducing aggregates to primary particles; aggregates are chemically bound. Surfactants physically bond to aggregates/primary particles and prevent the reformation of pigment agglomerates by disruption of the London-Van der Waal forces. It must be noted that the geometry of the particles plays a role in the extent to which these forces are felt over a specific distance. Surface defects and aspect ratios other than one result in an increased surface area-to-volume ratio, which entails a greater Van der Waal force of attraction than simple spheres; the probability of the geometry leading to mechanical interlocking is also increased.

Understanding Dispersion

Dispersion is a physical process that tends to increase the entropy of a system. A poorly stabilized dispersion will tend to flocculate; a state that decreases the potential number of conformations of the system (i.e., reduced entropy or randomness). This is largely due to the randomized Brownian motion of the dispersed particles, which are attracted (i.e., tend to agglomerate) via short-range London-Van der Waal forces.

For adequate dispersion it is essential that the surface tension of the liquid(s) be less than the surface free energy of the pigment (and other solids, such as fillers). If a specific solvent, resin or other liquid is to be used with a solid that it does not have an affinity for -in the sense of it being difficult to incorporate and wet out -surfactants are utilized to mitigate de-wetting and prevent floccules from forming.

This article comes from pcimag edit released

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