Abstract
In coatings, the use of extenders above the critical pigment volume concentration (CPVC), similar to their use below the CPVC, is driven by cost savings resulting from the replacement of resin or TiO2 with less expensive alternatives. These less expensive alternatives are limited below the CPVC to resin, but above the CPVC air voids can replace both resin and pigment. Air voids, unlike extender particles, scatter light on their own. They also improve the light scattering strength of the TiO2 by increasing the refractive index difference between the TiO2 particles and their surroundings. Formulating above the CPVC, however, is more complex than below the CPVC due to the presence of this additional film component.
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Notes
- 1.
There are two additional factors contributing to darkening on drying for waterborne paints below the CPVC. First, as these paints lose water during drying, the average refractive index of the matrix increases. This is because water has a lower RI (1.33) than other non-TiO2 paint constituents (1.45–1.55). This increase in average RI on drying lowers the scattering power of the TiO2 pigment. Second, extenders scatter light in water but not in the dry film, and scattering by extenders, therefore, decreases as water evaporates from the film.
- 2.
Once above the CPVC, extender replaces type 2 resin.
- 3.
We can alternatively define the porosity index as the ratio of air volume to the combined volumes of air and resin.
- 4.
The nonfilm-forming particles present in paints, such as TiO2 pigment or extender, generally have a positive effect on film integrity. Unlike soft organic resin, the inorganic pigment and extender particles are too tough to be abraded and so lend mechanical strength to the paint film. This same effect is seen in plastics and rubbers.
- 5.
Note that a TiO2 PVC value of 40 is in far excess from that of most paints, particularly paints formulated above the CPVC. However, the effects under discussion here are more evident at the higher TiO2 PVC values and so paints made under these conditions will be used to exemplify the important relationships between TiO2 PVC, CPVC, porosity index, and hiding power.
- 6.
The CPVC of the paints made with large extender particles increases with increasing extender content whereas the CPVC of the paints made with the small extender particles remains essentially constant.
- 7.
Technically, this is true only as long as the TiO2 PVC is held below the maximum PVC in the opacity versus TiO2 PVC curve (Fig. 7.1). This restriction is satisfied in virtually all commercial coatings and so does not restrict the argument here.
References
Stieg F.B., Ensminger R.I.: The production and control of high dry hiding. Off. Dig. 33, 792 (1961)
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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 Above the CPVC. In: Application of Light Scattering to Coatings. Springer, Cham. https://doi.org/10.1007/978-3-319-12015-7_7
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DOI: https://doi.org/10.1007/978-3-319-12015-7_7
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