Introduction
Most coatings consist of a binder matrix filled with solid particles, being pigments and/or fillers. The particles must be dispersed, stabilized and distributed in the paint. The uniform distribution of the solid particles should stay intact during film formation, when the wet paint transforms into a dry coating.
Pigment Volume Concentration
Coating properties are governed by, among others, the degree of loading of the system with solid particles. Some coatings contain a high amount of solid particles and other coatings, like clearcoats, are free of solid particles. The particle loading of a coating is quantified by its Pigment Volume Concentration (PVC). The PVC of a system is defined as the volume percentage of solid particles in the system after film formation, when all volatile ingredients (water and solvents), have evaporated.
A system of high PVC has a high degree of particle loading and a system of low PVC contains a low volume percentage of solid particles.
PVC is a property of a system after film formation. This implies that the PVC is calculated by excluding the volatile components, being water and solvents. The volume of the non-volatile components should be used in the calculation. The weights must be converted to volumes by using the density of each of the components.
An example
Consider a white gloss paint based on alkyd resin, titanium dioxide (TiO2) pigment and de-aromated white spirit (D40):
For the ease of calculation, we forget about the additives. The total volume of solid particles in 100 grams of the paint can be calculated by dividing the weight of TiO2 by its density. This gives: Vp = 6.00 cm3. The paint does not contain filler, so: Vf = 0.
Now we look at the binder, consisting of alkyd resin dissolved in D40. The weight of the non-volatile part of the binder is 30.05 grams. From the supplier of the binder we know that the density of the pure alkyd resin is 1.10 g/cm3. So, the volume of the non-volatile part of the binder is: Vb = 27.32 cm3.
By using the definition, it follows that the PVC of this paint is 18.0 %. This value tells us that 82 volume-% of the coating, the system after film formation, consists of resin and that the pigment particles account for 18 volume-%.
The density of resins
Often, only the density of the binder as supplied, and not of the pure resin, is given in the technical data sheet (TDS). An estimate of the density of the pure resin, the non-volatile part of the binder as supplied, can be made by calculation. This is how it goes, using the binder from the example above.
We know that solvent D40 has a density of 0.78 g/cm3. The density of the alkyd resin solution is 0.91 g/cm3; this is not the density of the alkyd resin. The volume of the 60.1 grams of binder is 66.04 cm3.
The 60.1 grams of binder contains 30.05 grams of alkyd resin and 30.05 grams of D40. From the density of D40, it follows that the 60.1 grams of binder contains, by estimation, 38.53 cm3 of solvent. The rest of the volume of the binder is, by estimation, the alkyd resin: Vb = 27.51 cm3. From this analysis it follows that the estimated density of the alkyd resin is 1.09 g/cm3. By inserting the volume of the solid particles and the estimated volume of the non-volatile part of the binder in the equation, it follows that the estimated PVC of the coating is 17.9 %.
Even though the calculation method is not strictly correct, it most often gives PVC values that are close to the real PVC. The best thing to do, however, is to use the measured density of the non-volatile part of the resins that are in a system.
Additives
The additives were not taken into consideration in the calculation as given above. For a more accurate calculation of the PVC of a system, the additives should be included. Several pieces of information about the additives are needed to do so. First, the weight percentage of non-volatile materials, often called the ‘solid’ content, of each additive must be known. Secondly, the density of the non-volatile part of each additive should be obtained or estimated. Finally, the system developer must know if the additive will behave as film-forming polymer or as solid particles.
Reading this, it will come as no surprise that often the additives are not taken into consideration when the PVC of a coating is calculated.
Part 2 of PVC
The properties Critical Pigment Volume Concentration (CPVC) and Oil Absorption (OA) value, as well as the influence of particle size and particle shape, are discussed by Ron Lewarchik in another article about PVC1.
Reference
- Effect of Pigment Volume Concentration on Coatings Properties – Part II, Ron Lewarchik, 15 August 2014.
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Very nice article Jochum!
Marvelous day reading through your write up on paint pigment. Thanks.