2 MATTE PAINT: SURFACE ROUGHNESS AND PIGMENT VOLUME CONCENTRATION

The paint industry describes the concentration of vehicle and pigment in a coating based on the volume percent instead of the weight percent, because many of the properties of coatings vary according to the ratio of the pigment volume to the vehicle volume, not with the ratio of the pigment mass to the vehicle mass. An important consideration is the critical pigment volume concentration (CPVC).

Above the CPVC void spaces are present. Below the CPVC the pigment particles are entirely surrounded by binder. Abrupt changes in many paint properties tend to occur at the CPVC (Patton 1979). In addition to becoming lighter in color with increasing void space, tensile properties decrease and porosity increases as the pigment volume concentration increases above the CPVC.

The general reasons why paint becomes lighter as the pigment volume concentration rises above the CPVC are illustrated in figures 1–3. A film below the CPVC has an excess of resin and may exhibit a smooth surface that specularly reflects light (fig. 1). This light is perceived as gloss. Above the CPVC there is less vehicle present, and, if the pigment particles protrude through the surface and the surface roughness increases (fig. 2), the light that is diffusely reflected will not be perceived as gloss. It will be additively mixed as the “white light” of the illuminant, and the resulting effect is perceived as a lighter color. This phenomenon—as well as the instrumental analysis of lightening due to surface roughness in contrast to “lightening” due to fading or loss of colorant—has been described by Johnston and Feller (1965). In general, because surface roughness causes diffuse reflectance and perceived lightening due to the additive mixing of the scattered illuminant radiation, a similar paint with a smoother surface that reflects more light specularly appears darker or glossier, depending on the angle of viewing.

Fig. 1. Idealized glossy surface

Fig. 2. Idealized matte surface

Fig. 3. Idealized matte/porous surface

When the volume of pigment is increased sufficiently, scattering also increases due to the greater volume of air/pigment interfaces (fig. 3). This increased amount of scattered, or diffuse, light is further additively mixed as white.

Feller and Kunz (1981) documented the increase in surface reflectance at 440 nm for ultramarine blue paint formulated at various PVCs (fig. 4). This is the wavelength at which the minimum reflectance will occur when the paint is saturated with vehicle or consolidant. Simply put, with increased void space (and therefore greater scattering at the air/pigment interface) the amount of reflected, diffusely scattered light increases, resulting in a light color. To avoid darkening of porous, matte paint by the addition of a consolidant, filling of void spaces and smoothing of the surface must be minimized.

Fig. 4. Percent reflectance (specular component included) vs. pigment volume concentration, ultramarine in dammar (from Feller and Kunz 1981)