Page 156 - Handbook of Surface Improvement and Modification

10

Anti-cratering and Leveling

10.1 METHODS AND MECHANISMS OF ANTI-CRATERING AND
LEVELING

Coating uniformity is one of the most important requirements in coating applications.
Coating defects encountered in many coating processes such as ribbing instabilities (in
roll coating), particle scratching (knife coating), roll patterns (gravure coating), or painting
1
brush marks must significantly level-off and disappear before drying or setting. On a
microscopic level, the absence of leveling abilities may influence properties such as gloss
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and smoothness of coating.
Good wetting is a fundamental prerequisite for the optimum adhesion. Surface
defects in coatings such as cratering and poor leveling can be minimized or improved with
properly selected additives. Even the thinnest films are expected to wet the substrate com-
pletely, possess optimal flow, and not be susceptible to cratering.
In addition to the rheology, the electrostatic charges that accumulate on substrates
and at liquid-air interfaces in various coating processes can affect liquid flow leading to
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defects. Electrostatic potential differences on the order of tens to hundreds volts can be
created by the static charges on non-conductive webs which may take days or weeks to
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dissipate. Static charges can be damaging to liquid coatings because they attract dust par-
ticles, create defects, or rupture coatings through electrohydrodynamic interaction with
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the liquid.
The effect of thixotropy on the hydrodynamic behavior of thin films has been studied
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in the relationship to leveling on a vertical plane. Thixotropic behavior is a typical feature
3
of paints, coatings, and inks. The thixotropic liquids have gel-like properties at rest which
3
disappear when sheared, but reappear when put to rest again. During brush application of
a thixotropic paint, the viscosity should be sufficiently low to obtain good spreading. 3
3
Then, the paint must have an optimal rate of structural build-up for good leveling. If the
build-up rate is too slow, the viscosity remains low and the paint has good leveling but the
3
too low viscosity will cause sagging on vertical surfaces. If opposite is the case, namely
the build-up rate is too high, the viscosity will rapidly become large, therefore, the sagging
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will not cause problems but the leveling will be too poor to produce even surface. A
hydrodynamic model of the leveling of thixotropic liquids has been proposed based on the
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lubrication approximation.
The cationic dispersant (polyethyleneimine) enhanced incorporation of nanoparticles
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(45-55 nm) based on SiC into electrodeposited films. The pre-coating method increased
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the SiC incorporation to values as high as 23 vol%.