In Part I of this article, expert Jochum Beetsma discussed the definition of PVC, including examples of PVC calculations and the PVC of various types of coatings. In Part II, we will discuss Critical Pigment Volume Concentration (CPVC), pigment oil absorption (OA), and the impact that pigment shape and density have on coatings properties.
Critical Pigment Volume Concentration
As the PVC increases, many properties of a coating change abruptly. These changes occur at the CPVC. CPVC can be defined as the point at which there is just sufficient binder to provide a completely absorbed layer on the pigment surface as well as all the interstitial spaces between the pigment particles in a close-packed system.
Diagram of Paint at CPVC
The CPVC for a pigment combination can be calculated from the oil absorption (OA) provided that the OA value is based on a non-flocculated dispersion. OA is expressed as grams of linseed oil per 100 grams of pigment. ρ is the density of the pigment(s), and 93.5 is 100 times the density of linseed oil (EU). Both OA and CPVC are expressed as percentages and not as fractions. The definitions of both OA and CPVC are based on close-packed pigment-binder with just sufficient binder to absorb at the pigment’s surface and fill all the interstices between the pigment particles. An example of the calculation of CPVC of a white alkyd (EU) finish using rutile titanium dioxide (EU) with an oil absorption value of 20 (# of grams of linseed oil/100 grams of pigment) and a pigment density of 4.2 g/cc follows: As the pigment density and/or the OA increases, the CPVC decreases. Above the CPVC, air voids are present (film density decreases) and below the CPVC, the pigment particles are separated. The dramatic and abrupt change in the behavior of paint that occurs when passing through the CPVC can be used to determine the CPVC. The abrupt changes in properties include: physical (adhesion, tensile strength/elongation and paint density), durability (resistance to moisture, rust, moisture penetration, blistering, wet adhesion, stain resistance), and appearance (hiding, gloss, tint strength). Other factors that effect water and oxygen permeation include particle shape and particle size. Pigment particles vary in size and shape. Some of the terms used to describe pigment particle structure in increasing order that they depart from sphericity to an increasing degree are as follows: Pigments with platelet shaped particles can reduce permeability especially if they are aligned parallel to the coating surface. Mica (EU), micaceous iron oxide (EU) and metal flakes (EU) are a few examples of such pigments. The smaller the average pigment particle size, the more resistant pigments are to dense packing. For example, the dense packing factor for fine (precipitated) calcium carbonate (EU) is on the order of half that of coarser calcium carbonate. The surface area of a unit weight of pigment varies inversely with the particle diameter. This relationship is especially true for most pigment particles that do not vary greatly in shape from a sphere, nodule or rectangle. Thus, for a given weight of pigment particles, halving the diameter doubles the surface area, and the greater the surface area for a given pigment, the greater the vehicle demands.
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