Pigment volume concentration

The pigment volume concentration (PVC) or packing density is a key figure for describing paints and varnishes .

definition

The pigment volume concentration reflects the volume ratio between pigments or fillers and the binder in the cured paint film. The volumes of the pigments and fillers contained in the formulation are divided by the volumes of all solid raw materials, which in addition to pigments and fillers also include the binders:

${\displaystyle {\begin{aligned}PVK&={\frac {\sum {V_{P}}+\sum {V_{F}}}{\sum {V_{P}}+\sum {V_{F}}+\sum {V_{B}}}}\\&={\frac {1}{1+{\frac {\sum {V_{B}}}{\sum {V_{P}}+\sum {V_{F}}}}}}<100\%\end{aligned}}}$

With

• Volume of pigments ${\displaystyle V_{P}}$
• Volume of fillers ${\displaystyle V_{F}}$
• Volume of binder. ${\displaystyle V_{B}}$

To simplify matters, the additives also contained in the recipe are usually not taken into account in the calculation; Solvent and water are no longer contained in the hardened film anyway and are therefore also eliminated.

The PVC is usually expressed in% and lies between 0% ( clear lacquer without any pigments or fillers) and 100% (only theoretically possible, as it does not contain a binder).

For the calculation, the volumes V of the individual pigments, fillers and binders are first calculated from their masses  m (known from the formulation) and densities (from manufacturer information): ${\displaystyle \rho }$

${\displaystyle V={\frac {m}{\rho }}}$

The mass and density of the binders always relate to the crosslinked state; the masses of dissolved binders must therefore be reduced to their non-volatile content  (nfA) if necessary .

Critical pigment volume concentration

definition

The critical pigment volume concentration (CPVK) indicates the pigment volume concentration at which the pigments and fillers are still just wetted by the binder.

Below the CPVK, i.e. with subcritical formulations, the surfaces are completely covered. There are only interfaces between pigment or filler and binder.

If you continue to increase the proportion of pigment in the formulation, you are above the CPVK. With these supercritical formulations, the surfaces are not completely covered. In addition to the interface between pigment or filler and binder, there are additional interfaces between pigment or filler and air .

calculation

The CPVK can only be precisely determined experimentally, as it depends to a large extent on the respective formulation, more precisely on the wetting of the pigments and fillers by the binder.

A rough calculation can be made from the bulk volume of the pigment-filler mixture used.

Influence on the paint properties

When the PVC is changed, the properties of the paint also change. At the KPVK, some properties of the paint change dramatically. In detail these are:

shine

Because there is no longer a closed paint film, pigment and filler particles protrude from the surface. A loss of gloss occurs. The film surface appears matt.

Weathering behavior

At the interface between pigment and air, the former is no longer protected by the binder and the additives it contains. This makes the paint's weather stability significantly worse. This is forced by the possible penetration of water into the pores.

Mechanical properties

The higher the PVC, the lower the cohesion within the film-forming phase. This significantly reduces the flexibility of the film.

Hiding power

The additional interface between pigment or filler and air provides additional scattering due to the greater difference in the refractive indices compared to the interface between pigment or filler and binder . This increases the opacity of the paint layer.

application

Sub-critical formulations

Subcritical formulations are the rule, since the properties produced there such as weathering stability, high gloss and / or good mechanical properties are normally required. This is e.g. B. the case with:

• Automotive paints (OEM and refinish)
• Industrial paints
• Powder coatings
• Facade emulsion paints
• Wood varnishes

Supercritical formulations

In certain cases, however, it is precisely the properties of the supercritical formulation that are desired.

The most important example are interior dispersion paints . Good mechanical properties, high gloss and good weathering behavior are undesirable or at least not relevant here. However, increasing the hiding power is desirable because the applied layer is more hiding. In the case of the cheapest qualities, almost all of the titanium dioxide used as a white pigment is also replaced with filler . Since these formulations do not yet appear opaque when they are wet, they are also referred to as the dry hiding effect (dry deck effect). Such formulations can have a PVC of over 95%.

Individual evidence

1. ↑ A. Goldschmidt, H.-J. Streitberger; BASF Painting Technology Handbook; 2002; Vincentz Publishing House
2. ↑ A. Goldschmidt, H.-J. Streitberger; BASF Painting Technology Handbook; 2002; Vincentz Publishing House