UNDERSTANDING CPVC: PIGMENTS AND RESINS, A COMPLEX RELATIONSHIP
There is a complex relationship that influences all characteristics of paint. In fact, the pigment system (pigments and fillers) and the binder, or resin, are together responsible for a range of properties, including the brightness of the colour, as well as the acceptance of colouring pastes, opaqueness and covering power, permeability, abrasion resistance and chemical and mechanical resistance.
A COMPARISON OF PVC AND CPVC
Traditionally, the pigment volume concentration (PVC) has been defined as the volumetric fraction of the pigment with respect to total volume of the paint film. In 1949, Asbeck and Van Loo described the critical pigment volume concentration (CPVC) as the point in which there is sufficient binder to wet all pigment particles and fill all the interstitial spaces between them. In other words, it is a situation in which there are no empty spaces in the coating. It is therefore important to always know how the paint formulation compares to the CPVC, since this effects the characteristics of the coating.
From a practical perspective, the CPVC is a transition point in which significant changes occur in the coating in terms of brightness, sheen, covering power, colouring strength, waterproofness, breathability, oxidation, abrasion resistance, adherence, flexibility, mud cracking and cold cracking.
The CPVC can have three values in relation to the PVC:
- When the CPVC is greater than the PVC, the coating has more resin than pigments. This excess can produce high levels of gloss.
- When the PVC is greater than the CPVC, the coating has empty spaces or voids due to the insufficient amount of polymer compared to pigments. The presence of air may cause a loss of local polymer connectivity, resulting in changes in the properties of the dry coating (for example, increased porosity). Coatings of this type are recommended for interior applications.
- When the PVC and CPVC are equal, the pigment is wet with the right amount of polymer to fill the empty spaces between the particles. In this case, the polymer and pigment are interconnected in a continuous form.
VARIABLES INFLUENCING THE CPVC
In general it should be remembered that, for both water-based and solvent-based paints, the CPVC is influenced by a series of variables. Here are the 13 most common:
- The absorption index of the pigment system
- The packing capacity of the pigment system
- The size and nature of the particles
- The degree of pigment dispersion
- The Tg value of the polymer (referring to thermoplastic polymers)
- The flow limit of the binder
- The rheology of the system
- The degree of polymerization of the resin
- The physical state of the binder
- The effect of particle size on binder dispersion (for water-based systems)
- The type of binder used
- The type and quantity of additives in the formulation
- The presence, or non-presence, of coalescing agents (for water-based systems)
METHODS FOR DETERMINING THE CPVC
Over the past 50 years, different methods have been proposed for determining the CPVC. Many of these methods have been based on changes in the coating properties (for example, film density). In practice, two methods are primarily used: the Gilsonite method and the film tension method. The Gilsonite method is based on the ability to stain the coating using a 10% solution of Gilsonite (commercial name of a natural asphalt) in white spirits. The intensity of the stain (obtained by immersing the paint sample in the Gilsonite solution) increases as the PVC value increases. The greater the PVC value, the more intense the stain. Visually, the CPVC corresponds to the PVC when the immersed section is the same colour as the unimmersed section. Alternatively, the film tension method is based on the tension produced in a paint film during solvent evaporation. This tension causes a contraction of the film that varies depending on binder hardness and is greatest when the PVC equals the CPVC.