Лако-красочные материалы — производство

DPP pigments with different particle sizes can be prepared by several methods. Classical conditioning (particle growth) in different solvents give rise to larger par­ticles with different morphology depending on the solvent characteristics [36]. Large particles can also be achieved directly in the synthesis through a heat treat­ment after the protonation of the intermediate pigment sodium salt. Small parti­cles are obtained either through classical mechanical diminution of the pigment particles through wet milling, dry milling or salt kneading, or through an acidic or basic reprecipitation [37]. In addition, new methods are now known, by which small particle size DPP pigments can be formed in situ [38-43].

Typical examples of an opaque and a transparent DPP are shown in Figure 11.8.

. У*

1 um *

11.5.3

Polymorphism

As in many other pigment classes such as copper phthalocyanines and quin — acridones [44], different crystal modifications of DPP pigments have been identi­fied [3, 45-48].

C. I. Pigment Red 254, for example, exists in two different crystal modifications as illustrated in Figure 11.9. The a modification is of a medium to slightly bluish shade of red and the thermodynamically more stable modification, whereas the b modification is of a more yellowish red shade.

11.5.4