The author points out the main aspects related to the critical concentration of pigments. Specify the causes and consequences of this defect in the formulation.by: Julián A. Restrepo.*
The volumetric concentration of the pigment system, or the so-called volume pigment concentration (PVC), has been shown to have a very significant effect on the performance and physical properties of pigmented coatings.
PVC is defined as the ratio of the fractional volume of the pigment system in the dry coating (i.e. the solids in volume of the paint) and is expressed mathematically as:
(1) PVC= VP / VP + VR
Being:
VP: Volume of the pigment system of the coating
VR: Volume of binder or resin solids
In this work, the pigment system is considered as the set of all the pigments present in the paint, that is, active and inactive pigments (the latter, commonly called fillers).
The effect of PVC on the properties of pigmented coatings was jointly explained by Walter Asbeck and Maurice Van Loo in 1949 [1], who showed that as this pigment content increases, the properties of the coating change to a critical point where they undergo a marked change; thus the PVC where this change occurs is called the critical PVC, critical concentration of pigment in volume or simply CPVC (critical pigment volume concentration).
In Spanish we refer to THE CPVC as CCPV (critical concentration of pigment in volume), but in the present work the acronyms in English have been used to refer to PVC and CPVC, to avoid confusion to the reader and the unnecessary introduction of different acronyms for the same concepts in Spanish.
It should be mentioned that, analyzed as a parameter of interaction between the pigment and the vehicle, there is also a theoretical definition of CPVC, being interpreted as the physical condition where there is just enough polymer of the coating to moisten and completely fill the spaces between the particles of the pigment system.
In subsequent studies, researchers Bierwagen and Hay divided the properties that are affected by CPVC into three main groups [2]:
a.Transport properties: Permeability, washability, corrosion resistance, blistering, penetration into porous substrates and electrical resistance. All of these depend on the flow of material or electric current through the coating.
b.Mechanical properties: Flexibility, tensile stress, glass transition temperature (Tg), wet rubbing resistance and cold crack resistance. These properties indicate the degree of resistance to external forces and depend on the performance of the binder, which can be modified by the addition of pigment.
c.Optical properties: Brightness, opacity and tint acceptance ("tint aceptnce"). These are all properties of the appearance of dry film.
In Figure 1 (Graphical representation of the variation of some of the properties of a coating with PVC: in blue, the gloss; in brown, tendency to blistering; in red, tendency to oxidation; in green, permeability) you can see how some of the properties of the dry film change as the PVC increases, observing that the greatest changes are obtained to the same PVC, this point is graphically interpreted as the critical PVC [3].
The experimental determination of the CPVC therefore requires the recording of the variation of a property of interest as the PVC is increased, obtaining the critical PVC as the one in which the turning point of the graph is presented, which can be determined visually (approximately), or more precisely mathematically, calculating the root of the second-order derivative or drawing tangent lines to the curves in the vicinity of the value of the CPVC, being the point of intersection of the same the critical PVC.
It should be clarified that Figure 1 is intended for illustrative purposes, since it is a simplification of the experimental results that are commonly obtained, due to the fact that depending on the property selected to determine the location of the CPVC, its measurement method and equipment, not a single value can be obtained, but in practice values are obtained with each measurement method and property chosen, but normally they must be close enough, so that if we keep in mind the experimental deviation of each method used we could affirm that we have obtained the CPVC in a single value.
At this point it is important to emphasize that the CPVC is physically a point and not a region, and the fact that it is erroneously considered as an area is due to what was discussed above, where the experimenter, observing that he can obtain different CPVC's very close to each other, is tempted to consider that the CPVC corresponds to a region of variation of the PVC.
Although the concept of CPVC was originally developed for solvent-based coatings, naturally its concept also applies to systems in aqueous dispersion (emulsion paints), although the particular differences of each of these coating systems must be borne in mind: In the case of emulsion paints the mechanism by which a wet film evolves to the dry state is totally different from that presented by solvent-based paints, and the latter makes it necessary to introduce a modified concept of CPVC for latices [4]. Therefore, the CPVC of water-based paints is more precisely referred to as CPVC latex (LCPVC).
Some factors that affect CPVC
The CPVC of solvent-based paints is mainly affected by the following variables [1] (for an analysis of the variables considered in aqueous systems you can consult the refs. [3-6]):
- The oil absorption index of the pigmentary system
- The packaging factor of the pigment system
- The degree of dispersion of the pigment system
- The hardness (Tg) and type of binder used
- The type and amount of additives present
The concept of the UPVC was defined by Asbeck in 1977 [7], as the maximum value that CPVC can have in a pigmented coating depending on the degree of dispersion. At this degree of dispersion, CPVC = UPVC and if this degree of dispersion is increased the CPVC will not change anymore.
In his work, Asbeck expressed the CPVC as a function of the degree of dispersion, considering the average size of the agglomerate present in the painting, through the expression:
(2) CPVC = UPVC - UPVC2 {1- d/D}3
Being
d: It is the average particle size of the pigment system
D: It is the average size of the chipboard.
The ratio d/D is interpreted as the relative size of the degree of agglomeration, so when D = d, there is no agglomeration and the term d/D = 1; while in cases where agglomeration occurs, this term will be less than 1.
Note in Figure 2 (Example of the variation of the CPVC with the degree of dispersion of the pigment system) that, as the degree of dispersion increases, the CPVC increases, until reaching the point where there is the condition of "ideal dispersion" (d / D = 1), where the CPVC = UPVC, while if there were no ideal dispersion the CPVC would be less than the value of the UPVC.Final comments
The CPVC proves to be a key parameter for the prediction and interpretation of the performance of coatings, being considered one of the most important variables to take into account when formulating any pigmented coating [8].
When the value of the CPVC of the formulation is taken into account in relation to the value of the PVC, its adequate proportion can lead to the formulation of pigmented coatings with excellent performance, for example in:
a) Architectural paints: Improve their covering, resistance to wet rubbing and flexibility, prevent the formation of cracks in the paint during the drying process, penetration into porous substrates
b) Anticorrosive paints: the formulation of paints less permeable to water (providing a correct protection of the substrate), with greater anticorrosive effect, less tendency to blistering, among others.
Thus, knowing the location of the CPVC allows the formulator to develop coatings with an appreciable performance, and if we also take into account the environmental impact factor, we will say that from its analysis coatings with an excellent cost/benefit/environmental impact ratio can be formulated [9].
References
[1] Asbeck, W.K. and Van Loo, M. "Critical pigment volume relationships". Ind. Eng. Chem., Vol. 41, No. 7 (1949), pp. 1470-1475.
[2] Bierwagen, G.P. and Hay, T.K. "The reduced pigment volume concentration as an important parameter in interpreting and predicting the properties of organic coatings". Prog. Org. Coat., 281, No. 3, (1975), pp. 281-303.
[3] Restrepo, J.A. "Predictive calculations of CPVC in water-based paints". Paper presented at Andina Paint: "Francisco Martínez", Medellín (Colombia), March 2005.
[4] Giudice, C.A. and Pereyra, A.M. "Critical concentration of pigment". Inpralatina, Vol. 13, July-August, 2008. pp. 19–27.
[5] Bierwagen, G.P. and Rich, D.C. "The critical pigment volume concentration in latex coatings". Prog. Org. Coat., 11 (1983), pp. 339-352.
[6] Berardi, P. "Parameters affecting the CPVC of resins in aqueous dispersions". Paint technology, 27, 24, July (1963), p. 24.
[7] Asbeck, W.K. "Dispersion and agglomeration effects on coatings performance". Jour. Coat. Tech.. Vol. 49, No. 635 (1977), pp. 59-70.
[8] Restrepo, J.A. "A technical look at the theoretical determination of CPVC in solvent-based coatings". REC Magazine, No 11, December 2006, pp. 7-14.
[9] Visit www.invesa.com, products that are friends of quality and the environment.
* PINTURAS SAPOLÍN - INVESA S.A.
[email protected]
Girardota, Colombia
