Particle Orientation in the Film

Last but not least, the role of the particle orientation within the dry film has to be considered. Preferably, all the flakes are oriented parallel to the substrate, because the resulting parallel light reflection yields a maximum of lightness, brilliance and flop.

On the other hand, a poor orientation results in an irregular reflection, causing “salt and pepper effects”, “cloudiness” and a poor metallic effect and flop (Figure 5.33). A disorientation of the flakes can be created if either too wet films (or too slow-drying solvents) or too dry films (or too fast-drying solvents) are applied.

droplets arrive statistically at the substrate pigments are mobile at spray viscosity

Подпись:film too wet/solvents too «slow”: metal pigments de-leaf disorientation by Brownian movement formation of clouds («wet clouds”)

film too dry/solvents too «fast”:

reduced mobility/orientation of the flakes formation of clouds («dry clouds”)

«shrinkage” during solvent evaporation is pressing the flakes into a parallel situation

Подпись: dry filmhigh solids: less shrinkage — poorer orientation — poorer effect extremely with powder coatings!

the pigment orientation is supported by: fast drying (formulation) additives (e. g. wax dispersions, microgels)

Fig. 5.33 The influence of particle orientation on the metallic effect.

Furthermore, the shrinkage during solvent evaporation presses the flakes into a parallel situation. This fact also explains, why in high solid content coatings and, even more extremely, in powder coatings (Figure 5.34) (less or no shrinkage) the pigment

orientation is poorer than in low solid or medium high solid content coatings. The pigment orientation can be supported by a fast solvent release of the resin (e. g. in CAB-systems) and/or by using special additives (e. g. wax dispersions, microgels etc.).

Particle Orientation in the FilmПодпись: before baking (schematic)Подпись: after baking (schematic) powder coatings are dry during application

the pigment flakes are statistically oriented

-►no pigment orientation during application

only a short liquid phase during bake (at rather high viscosity)

no shrinking through solvent evaporation -►hindered pigment orientation/poorer effect the lower the baking viscosity (Tg) and

the longer the liquid phase (baking), the better is the metallic effect

Fig. 5.34 Pigment orientation (metallic effect) in powder coatings.

Вы можете оставить комментарий, или ссылку на Ваш сайт.

Оставить комментарий