When pigments are added to a particular coating that
are added based on the PVC. What is PVC you say? It is the
pigment volume concentration. The pigment volume concentration is
essential in determining the amount of a particular pigment that can be
added to the polymer of the coating. The pigment has to have sufficient
"wetting" by the polymer to create a protective coating. By
wetting I mean, that there must be sufficient polymer, or binder, to completely
wet or surround all the pigment particles. There must be enough polymer
to completely fill the voids between the pigment particles. How do
you calculate the PVC? The following equation is used:
The point at which there is just sufficient polymer
to wet the pigment particles is known as the critical pigment volume concentration
(CPVC). Below the CPVC there is sufficient polymer for pigment wetting
and above the CPVC there is not. There are abrupt changes in the
coatings properties at the CPVC. Below is a depiction of PVC and
property changes that occur at the CPVC.
As you can see by the picture above that both gloss and blistering properties decrease as one reaches the CPVC, while permeability increase above the CPVC. Permeability properties increase because above the CPVC there are voids in the coating filled by air and the coating becomes discontinuous. Some of the properties that can be evaluated above and below the CPVC are blistering, gloss, rusting, permeability, enamel hold out, scrub resistance, tensile strength, and contrast ratio.
The filling of pigments in a coating is similar to
fillers in composites. In a composite the polymer matrix must be
in intimate contact with the fiber reinforcing material. If there
is not intimate contact then the fiber acts as a defect. The same
theory applies here. If there is not enough polymer to wet the pigment
then the pigment becomes the defect and the properties of the coating decrease.
return to the Pigments page