Abstract
The bulk of a dry paint film can consist of as many as four fundamentally different components: resin, pigment particles, extender particles, and, in some cases, air (the presence of air will be considered in detail in Chap. 7). Many physical properties of the film—including opacity—are determined by complex interactions between these different materials. Below the critical pigment volume concentration (CPVC), opacity is primarily affected by the concentration of TiO2 particles, and by the way that the nonpigmentary components of the paint, particularly extender particles, alter the distribution of the TiO2 particles in the dry film. The effects of extender particles on optics below the CPVC generally range from none to negative, depending on the paint formula and the type and amount of extender used. Having said that, in special cases, the extender may improve opacity. The primary physical characteristics of the extender that affect opacity are extender particle size, size distribution, and concentration. The overall effect of extender on paint opacity can be understood on the basis of the dependent scattering phenomenon described previously.
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Notes
- 1.
This is nonetheless an important role since it increases the volume available for pigment occupation and so affords better particle spacing and better opacity.
- 2.
For reference, nanoparticles are smaller than 100 nm (0.1 µm).
- 3.
The types 1 and 2 resins demand of large extender particles should also be taken into account when calculating effective PVC. However, because we do not know what these amounts are, we cannot account for them. This effect is generally small compared to the TiO2 particle crowding caused by large extender particles and so introduces minimal error to the effective PVC calculation.
- 4.
Note that this experiment is different from those described in the previous section. There we were replacing TiO2 or resin with large extender particles and keeping the total volume of the film constant. In Fig. 5.14, we are adding the large extender particles and making a thicker film. Note that in doing so, we are not changing the TiO2 coverage rate (e.g., grams of TiO2 per square meter covered).
- 5.
For geometric reasons, flat or platy particles have a lower impact on TiO2 particle spacing—and so the overlap of scattering volumes—than spherical or cubic particles of the same volume.
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© 2014 E. I. du Pont de Nemours and Company
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Diebold, M. (2014). Effects of Extenders on Paint Optics Below the CPVC. In: Application of Light Scattering to Coatings. Springer, Cham. https://doi.org/10.1007/978-3-319-12015-7_5
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DOI: https://doi.org/10.1007/978-3-319-12015-7_5
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