Page 65 - Handbook of Surface Improvement and Modification

60 Surface Matting (Flattening)

Figure 4.2. SEM pictures using topology mode (BET) (a, c, e) and composition mode (BEC) (b, d, f) of the pan-
els prepared with uncoated particles (a) and (b), 3-cycle particles (c) and (d), and 9-cycle particles (e) and (f). The
pictures for each sample represent the same area of the panel. [Adapted, by permission, from Valdesueiro, D;
Hettinga, H; Drijfhout, JP; Lips, P; Meesters, GMH; Kreutzer, MT; van Ommen, JR, Powder Technol., 318,
401-10, 2017.]
4.3 APPLICATION DATA

The polyester-based powder coating was changed from gloss to matt, by depositing ultra-
thin films of Al O on the surface of the powder coating particles performed in a fluidized
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2
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bed reactor. When the average alumina shell was thicker than 6 nm, there was no flow of
the core particle material, even though the powder particles did soften above the glass
2
transition temperature. This resulted in a rough and matte surface finish of the coating
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after curing. The matte finish coating had mechanical properties similar to the uncoated
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powder particles. The roughness formation on the panels was investigated using a SEM
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microscope (Figure 4.2). Two SEM modes were used to look at the surface: the back-
scattering-topology BET mode (Figures 4.2 a, c and e), and the back-scattering-composi-
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tion, BEC mode (Figures 4.2 b, d and f). The panels sprayed with the uncoated particles
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(Figure 4.2a) did not show roughness. The composition mode picture revealed darker and
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brighter areas (Figure 4.2b). As we expected, BET-mode images of particles sprayed by 3
cycles (Figure 4.2c) and by 9 cycles (thicker coating, rougher surface) (Figure 4.2e)
showed surface roughness, which corresponded to the darker areas observed in BEC mode
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pictures (Figures 4.2 d and f). The thickness of particle layer grew by 3.5 nm of Al O per
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3
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cycle. It was found that two spraying cycles were sufficient to alter the flow of the parti-
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cles and the appearance of the paint. The suppression of flow was not caused by delayed
softening but the films acted as a hard physical shell that prevented material release from
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the core that softened irrespective of the coating around it. The thickness of the film was
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not important as long as it covered the entire particle.
The decorative and functional reasons make matte coating attractive because they
minimize reflective light, hide defects, or create an aesthetic appearance for industrial and
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consumer coated items. Semi-matte finishes are popular in architectural applications.
Automotive dull black powder coatings hide interior trim components and contrast with
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high gloss body paints. A dull matte finish is required on military vehicles to resist detec-
3
tion. Two powder coatings with different levels of carboxyl functionality are crosslinked