Effect of Dispersants on Pigment Dispersions

The process of functionalization of any pigment, including those which provide solar absorptance of TSSS and TISS paint coatings (colored or black), is performed by intense mixing or ball milling of the pigment in the presence of various dispersant molecules. The pigments should not be over-ground below the diameter of the elemental particle size. The corresponding elemental particles consist of strongly connected smaller crystallites, which cannot be de-agglomerated further without intense ball milling. The size of the pigment particles in solar paints is about 200-300 nm and represents the limit of grinding because grinding the pigments to a smaller size has usually resulted in unwanted changes of color and a decrease of the concealment power of the applied coating. Namely, when the size of the particles becomes smaller than the wavelength of the visible radiation (300-600 nm), the ratio between the scattering and absorption of visible radiation changes, leading to an unwanted drop in the concealment power of the coating and an increase in transparency. The consequence ofthese changes is a higher consumption ofpigment to achieve the desired solar absorptance.

To obtain stable pigment dispersion, it is extremely important that the resin binder is firmly anchored onto the pigment surface. This cannot be achieved directly but via a thin layer of dispersant firmly attached to the surface of the pigment, which assures the compatibility of the pigment particles with the resin binder system. When the resin binder is not attached sufficiently strongly, the addition of a large amount of solvent could wash off the polymeric binder, which usually leads to the re-agglom­eration of the pigment particles and their flocculation. Dispersants therefore dra­matically affect the state of dispersion and simultaneously influence the application properties of the paints and the optical properties of the deposited coatings.

In the case of spectrally selective coatings, a thin and homogeneous layer of paint deposited onto the metal or polymeric substrate is desired. To achieve such a layer, the pigment particles must be arranged, if possible, into a well-defined layer, laterally assembled and linked one to another with a sufficient (below the critical pigment-to-

volume ratio) amount of polymer binder to bond the particles together without excessive parts at which no pigment covers the substrate (Figure 9.2.1).

The situation shown in Figure 9.2.1 should be avoided at all costs, since it leads to uneven pigmentation and undesired coating colors. Moreover, coatings should have the proper thickness in order to have the right level of infrared reflectivity, solar transmittance, and absorptance. The size of the pigment particles influencing the scattering of solar radiation is obviously also important but it will not be treated here [26]. The preparation of such pigment dispersions, with non-agglomerated and finely distributed, uniformly sized particles, is not easy and requires careful selection of dispersants capable of providing appropriate interactions with the pigment particle surface, as well as adequate compatibility with the polymer binder system. The effect of dispersants is quite general, as shown for the uniform spherical silica particles treated with trisilanol T7 POSS dispersant discussed below (Figure 9.2.2).