Is there an experimental way of measuring the critical concentration of pigment in paint?

Question

With an assigned, generally paint-based, experimental project I've been looking up CPVC or critical pigment volume concentration in binders. CPVC defines the point of addition of pigments after which follows a severe drop in blistering and increase in rusting on the applied surface due to the pigment not while binding with the binder having voids in it. But going past the critical point can also make the paint fragment and crumble or maybe even lose just some of its pigment, in conclusion lose efficiency.

Paint manufacturers use this to make paints a certain type by knowing the CPVC of the mixture with which they can adjust to make the fitting paint for the job but the way they know is by applying the general formula for determining the point which is

$$CPVC = \frac{1}{ 1 + OA * ρ / K }$$

With the 'ρ' standing for the specific gravity of the pigment, 'K' specific gravity of the binder and 'OA' which is the oil absorbent number of the pigment depended of the exact pigment and the binder. But excluding it is there a way that a clear mixture of paint without any additions like extenders for example and just acrylic polymer binder and an inorganic pigment type can be used to calculate the near CPVC of it. Can it maybe be done by centrifuging a dry paint sample in a centrifuge tube and later sampling the super natant that will contain an ammount of pigment and binder that would dissolve in it and fall apart from the rest of the paint and quantitatively analysing it in a high performance liquid chromatograher? By increasing temperature or the pH it could also affect the stability (the CPVC) so can that method be used effectevly? Is there a better one and not by just assuming the CPVC by observing the paint on a wall?

Any help could be really helpful sense im young and do not know the process well, only by researching the concept itself, and it's hard to find details and think of a valid project idea.

Answer 1

An image of the CPVC effect is shown in the diagram below. The red color is the binder, the black is the pigment and the white is air (because there is not enough binder to coat all the pigment). The lines depict the change in properties you would expect, and you already have the formula to correlate the paint properties with the raw material properties.

BTW, economics generally suggests maximizing the inexpensive pigment/filler concentration, but adding too much increases permeability to water, which is bad in the case of metals which might corrode. On the other hand, blistering can be caused by corrosion of the substrate which then disadheres some of the film if it is too rich in binder; blistering is minimized by allowing the film to break a little. Of course, then corrosion continues - you just don't see it as much until it is too far gone. The best paint will protect from corrosion with low permeability and without disadhering.

So, how to measure a property related to CPVC if you can't pick one of these? First, you need some of the paint film; I assume you can start with the plain binder and add different amounts of the pigment/filler. Mix well, do not incorporate unnecessary air bubbles when mixing. Cast a film of constant thickness on aluminum foil by using a threaded rod; let the paint dry.

Then do your experiments!

1. Flexibility. If you bend the painted aluminum foil over a rod of defined radius, the rich binder film will not crack, but as more filler is added more and more cracking tendency will be noticed.

2. Permeability. If you put a drop of water (or perhaps alcohol) on the dried film, it will wet and spread. You may be able to differentiate between different levels of added pigment. If the aluminum foil substrate does not give good results, you can apply the paint to a decent grade of paper. Then a drop of water or other solvent might wick all the way thru and appear on the other side of the paper.

3. Glossiness. Ideally, you should have some kind of electronic measurement, but ranking the specimens subjectively (with your own eyes) is better than nothing. In practice, flat paints have a high pigment concentration and are not durable when washed; semi-gloss paints have less pigment, more binder, and can be washed more often. (I always wondered why they didn't go all the way to "full gloss", but I guess the binder was never so good as to give much better performance.)

4. Washability. Apply a water stream to the painted aluminum foil (like a dripping faucet: constant height, constant volume per minute), watch the paint film disappear.

And try coming up with some more tests!