Page 38 - Handbook of Surface Improvement and Modification

2.4 Properties and Application data 33

Figure 2.24. Optically scanned images of scratch-induced surfaces of differently weathered acrylic coatings with
increasing normal load rage of 10–2000 mN. [Adapted, by permission, from Seo, T-W; Weon, J-I, J. Mater. Sci.,
47, 2234-40, 2012.]
An increase in weathering time contributed to the increase in scratch resistance of
acrylic coatings (Figure 2.24), due to the embrittlement of coating surface and the
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enhancement of crosslinking density by UV irradiation. A rougher substrate surface also
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caused the improved interfacial adhesion.
Acrylic/melamine clearcoats containing different loads of nanosilica were artificially
weathered in a xenon tester, and their tribological properties after various exposure times
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were studied. The scratch resistance of coatings decreased at initial exposure to weather-
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ing, followed by a slight increase at later times of weathering. The increased scratch
resistance at later times was caused by an increase in hardness and elastic recovery of the
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coatings.
Well-developed clearcoat formulations are relatively resistant to weathering and for
this reason, they perform very well in these two studies. Many common polymers are sub-
stantially less resistant to UV exposure and they will show that weathering decreases sur-
face resistance to scratch.

2.4 PROPERTIES AND APPLICATION DATA
A comparative evaluation of electrostatic spray and ‘hot dipping’ fluidized bed to deposit
two different organic paints belonging to the class of thermoplastic and thermoset powders
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was performed. Hot dipping fluidized bed is a fast deposition technique (after substrate
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pre-heating, it takes just a few seconds to have the part completely powder coated). The
electrostatic spray deposition lasts longer (6.15 s) and the coated parts must be post-cured
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o
for a long time (e.g., 15 min at 150-200 C). The ‘hot dipping’ fluidized bed is particu-
larly suitable for the deposition of thick and smooth thermoplastic coatings, whereas the
electrostatic spray deposition is the best for thinner and highly scratch and wear resistant
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thermoset coatings. Figure 2.25 shows scratching maps of polyester and polyphthal-
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amide coatings obtained by hot dipping and electrostatic spray. The thermoplastic coat-
ings deform much more (maximum penetration depth of 90 μm – through thickness – and
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125 μm after ESD and HD fluidized bed coating process, respectively). The electrostatic