Лако-красочные материалы — производство

As a rule of thumb, pigments whose intermolecular interactions are mainly defined by p-p-interactions within stacks tend to form needles or rods parallel to the stacking direction. Typical examples are the phthalocyanines (Figure 8.12a).

Only P. B.15:3 may be obtained in the form of platelets by defined control of super­saturation during the pigment synthesis [59].

According to attachment energy calculations, the equilibrium crystal habit of P. R.179 is represented by a prismatic rod with an aspect ratio of ca. 3 (Fig­ure 8.12b). These results correspond with experimental observations on crystals grown from xylene or phenol (see right-bottom part of Figure 8.15). As expected, the main growth direction is parallel to the stacking direction along the я-axis. Consequently, the {011} and {011} face accounts for more than 80% of the crystal surface.

The presence of one other directional interaction in addition to p-stacking leads to the formation of “platy” morphologies. Prominent examples are P. R. 255 [7],

P. V. 19 [9], and P. R. 122, which show pronounced growth rates in the directions of p-stacks and hydrogen bonds (Figure 8.13). For these pigments the platefaces represent ca. 60% of the crystal’s total surface area. Similarly for P. R. 255 and for P. V. 19, aromatic C-H groups are exposed on the platefaces giving them a pre­dominantly hydrophobic character. As a consequence, both pigments interact with polymer melts via these hydrophobic faces. As an example, P. V. 19 nucleates the b-form of isotactic polypropylene [60]. Again, P. R. 255 causes warping of injec-

Figure 8.13 Crystal habits of quinacridone pigments:

(a) calculated habit and surface structure of y-P. V. 19;

(b) comparison of calculated and experimentally observed crystal morphology of P. R. 122.

tion-molded polyethylene parts, which may be lowered by treating the surface with appropriate additives, as has been shown for P. R. 254 [61].

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