Michael Patrick Diebold

Mechanism of TiO2/ZnO instability in latex paints

A strong interaction between titanium dioxide and zinc oxide in latex paints has been recognized for decades, yet the harmful consequence of this interaction—paint instability—remains an issue for many coatings manufacturers. An investigation into the chemical and physical changes that occur during paint aging has revealed the mechanism of this instability. The role of the TiO2 pigment grade was further explored, providing an understanding of the gel mechanism and stability performance.

TiO2 Pigments in Liquid Paints

TiO2 pigment particles must be well spaced in the dry paint film if they are to scatter light at optimal efficiency. This spacing is controlled by a number of factors, and here we consider those that are determined by the state of the pigment in the liquid paint. These factors can be split into two categories—those that enhance the attractions between pigment particles, and those that enhance their repulsion. The first category is primarily limited to van der Waals forces, although a suspension of particles with opposite surface charges will also experience attractive forces. Repulsions fall into two classifications—electrostatic repulsion, seen when all particles have a similar surface charge, and steric repulsion, which prevents bulky absorbed dispersant molecules on different TiO2 particles from co-occupying the same region in space. These forces are important during the production of a paint, during its storage (which can be under extreme temperature conditions), and during paint drying, with each of these stages representing a different challenge to the formulator. The degree to which these forces affect TiO2 particle dispersion can be measured by optical methods (especially on the liquid paint before and during drying) and microscopy.

Cost-Effective Paint Formulation

TiO2 is among the costliest ingredients in many paints and as such its efficient use is critical for the commercial success of a given formulation. Many formulators are unsure of the optimal level of TiO2 in a paint from a cost standpoint. Determining this optimal pigment volume concentration (PVC) is not straightforward, as it depends not only on the cost of TiO2 but also on the cost of other ingredients such as resin and extender, and on the relationship between TiO2 scattering power and paint composition. While it seems sensible that the most cost-effective paint would have the least amount TiO2, it is in fact the case that too little TiO2 can result in a higher cost of coverage, just as too much TiO2 can. In this chapter, we detail methods for determining the most cost-effective TiO2 concentration in various types of paints.

Volume Relationships and the CPVC

The simplicity of the critical pigment volume concentration (CPVC) concept belies its importance in coatings formulations, and for this reason, we will devote an entire chapter to it. The CPVC represents an important transition point for a paint as it changes from a solid film to a porous one. The CPVC value is not constant across all paints, and important determinants of the CPVC for any given formula include the particle size and size distribution of nonfilm-forming particles (particularly extender particles), the size of the latex particles, and the presence of highly porous materials (either as separate extender particles or as coatings attached to the TiO2 pigment particles). These shifts in CPVC are the direct result of changes in the way that the nonfilm-forming particles pack in the paint film.

Scattering by a Single Particle

The interactions of light with particles are of critical importance to the coatings industry. Understanding these interactions provides the paint formulator with the information needed to develop a cost- and performance-optimized coatings system.

Scattering by Groups of Particles

The interactions between light and groups of particles in paint films are highly complex. While we can accurately calculate the scattering of an individual photon by a single particle in a resin matrix, these results are difficult to scale-up to scattering by groups of particles in a paint film. This is due to a number of complicating factors, the most important of which is that particles that are separated by short distances interfere with one another’s ability to scatter light. This phenomenon, termed “dependent scattering,” occurs when the pigment particles are so concentrated that close contacts are unavoidable (crowding), or when liquid dispersions of particles, such as a paint, are unstable and form flocculates or agglomerates (nonideal spacing). In this chapter, we review the dependent scattering phenomenon and describe ways of minimizing close pigment particle contacts and maximizing light scattering from paint films.

Effects of Extenders on Paint Optics Below the CPVC

The bulk of a dry paint film can consist of as many as four fundamentally different components: resin, pigment particles, extender particles, and, in some cases, air (the presence of air will be considered in detail in Chap. 7). Many physical properties of the film—including opacity—are determined by complex interactions between these different materials. Below the critical pigment volume concentration (CPVC), opacity is primarily affected by the concentration of TiO2 particles, and by the way that the nonpigmentary components of the paint, particularly extender particles, alter the distribution of the TiO2 particles in the dry film. The effects of extender particles on optics below the CPVC generally range from none to negative, depending on the paint formula and the type and amount of extender used. Having said that, in special cases, the extender may improve opacity. The primary physical characteristics of the extender that affect opacity are extender particle size, size distribution, and concentration. The overall effect of extender on paint opacity can be understood on the basis of the dependent scattering phenomenon described previously.

The Kubelka–Munk Framework and Measuring Opacity

So far, we have considered the scattering phenomenon itself—Mie’s theory of individual particles scattering light and the effects that different particle attributes have on maximum scattering strength, and the theory of groups of particles scattering light and the finding that close proximity of one particle to another leads to a loss of scattering strength (dependent scattering). We have, however, neglected a crucial aspect of this phenomenon—how to actually measure light scattering in the paint films of interest to us. To do this, we will consider scattering from the entire paint film, rather than from individual particles or groups of particles. Using this framework, we can transform intensity measurements of white light reflected from a paint film drawn down on a black and white chart into scattering values (S TiO2 and S coat) of the paint. Using Kubelka–Munk equations, we can calculate from the reflectance data the area that a liter of paint will cover at complete hide as formulated, and we can directly compare the light scattering and absorption efficiencies of different paints. In addition, we can use these equations as a guide to different ways of increasing the opacity of the paint and ensuring full advantage is taken of the pigment used.