Лако-красочные материалы — производство

That crystal faces differ with respect to rates of growth and dissolution as well as wetting and adsorption behavior is vital for the processing and coloristic proper­ties of pigments. Light scattering, which is dependent on particle size and refrac­tive index, also plays a significant role in determining the optical properties of pig­ments. It is therefore important to stabilize primary particles from flocculation, as this increases the effective particle size, and induces a color shift and loss of brilli­ance. To avoid flocculation, modern high-performance pigments are enhanced with surface modifiers, which counteract the attraction between the particles.

In order to be effective, it is important that surface modifiers are strongly adsorbed onto an available crystal face of a particle [62]. Nature uses such mecha­nisms in biomineralization, e. g., in the formation of mussel shells or in the pro­tection of hardy plants against frost. In both cases, additives control the crystal growth of either calcium carbonate [63] or ice through selective adsorption of tai­lor-made proteins on certain crystal faces [64].

This principle can be copied if the structures of the crystal faces are known. Then it is possible to search for or to design molecules which fit into the face according to the “lock and key” principle, thus stabilizing it (Figure 8.14) and actively inducing changes in morphology by the use of such tailor-made additives [65]. In the field of pigment technology, tailor-made additives have been designed to fit surface topologies and successively applied to control crystal growth of pig-

adsorption
of face-selective
additives

ments [66] to stabilize metastable polymorphs, e. g., P. B. 15:6 [67], and to prevent warping of pigmented polyolefin parts [62].

Face-selective additives can today be designed to fit into the structures of single­crystal faces. Their effectiveness is determined by the energy of interaction be­tween the additive and the crystal face. In simple cases, this energy can be calcu­lated for different positions of the additive on the crystal face.

As an example, the crystal growth of P. R.179 can be drastically inhibited (Fig­ure 8-15) by the addition of a few percent of a perylene dicarboxylic acid imide (PDCI)-type additive [68]. The changes in particle size are accompanied by signifi­cant modifications of the crystal habit. Crystals grown or ripened in the presence of MePDCI have the overall shape of ricegrains. Assuming that the elongated axis

with 5% additive: without additive:

SO3H

u’h’O

CH3

Figure 8.15 Inhibition of crystal growth of P. R. 179 by a PDCI-type additive (insert lower left corner); minimum energy structure of the additive embedded in the [011]-face of P. R. 179 (left); samples of P. R. 179 grown in the presence (top) and absence (bottom) of a PDCI-type additive (right).

in these grain-like crystals corresponds to the needle axis of P. R.179, the rounded ends should be terminated by a series of medium energy faces, presumably of the type h, k, l > 0. These faces are rough on the molecular scale and are covered, by their nature, with plenty of “step and kink” sites supporting a strong adsorption of the PDCI additive onto the surface. Conversely, the PDCI additive causes the appearance of these rough faces by stabilizing them. Usually only molecularly (more or less) flat surfaces are stable; the rough faces are unstable and would grow fast and disappear. One can now look at the phenomenon from two sides: the growth in the direction perpendicular to the face is hindered or the face, rough as it is, is stabilized.

If a sulfonated PDCI is used, then the sulfonate anion acts as an electrostatically stabilizing agent on pigment faces by attributing like charges to the pigment par­ticles. With a sulfonated PDCI-based additive, Paliogen® Red L 3875, a transpar­ent form of P. R. 179 has been developed for use in solvent — and waterborne coat­ing systems.

Shear-stress measurements reveal an extremely low yield value for Paliogen Red L 3875 combined with standard dispersants in a typical high-solids mill base formulation (Figure 8.16). In addition to the higher solids (pigments) content at application viscosity, the improved colloidal stability of the pigment particles in Paliogen Red L 3875 offers a yellowish shade and a high chroma. The humidity resistance, overspray fastness, and weatherability are not influenced by the strongly adsorbing additive and maintain the same high level, characteristic of other perylene pigments.

Further Reading