Canadian Finishing & Coatings Manufacturing Magazine Jan/Feb 2023 - 40

PAINT AND COATINGS MANUFACTURING: POWDER RESINS - FEVE
Table 2: Topcoat Powder Systems in
Accelerated Weathering. The acrylic and
polyester systems were removed at over 60%
gloss failure.
coverage over the part before cure. Particles that are too
large can affect transfer efficiency during electrostatic spray
application and the build-up of waste material at the bottom of
the spray booth. When dealing with different particle sizes and
distributions, selecting proper electrostatic spray settings to
match the powder's characteristics is important. Adjustments
in voltage, airflow, and velocity can affect the surface and
mechanical properties of the final cured coating. However,
electrostatic spray settings may not always resolve performance
issues and defects observed in the coating. These defects may be
influenced by the powder's particle size or distribution.
High surface smoothness, opacity, and gloss selection for a
powder topcoat can be achieved with d(50) particle sizes around
25-40 microns. Surface smoothness and opacity begin to decrease
as particle sizes increase over d(50) of 70 microns. As the powder
particles reach larger sizes, the surface appearance and gloss
dramatically reduce. In addition, mechanical properties, such as
adhesion, flexibility, and impact resistance fail performance with
increasing particle size.
A wider particle size distribution range has a dramatic
influence on surface appearance. Topcoats with more than 50
microns particle distribution begin to develop surface defects
such as orange peel, pinholes, or voids. Microscopy imaging also
revealed aggregated particles embedded in the cured film. These
aggregates change surface smoothness and gloss. Furthermore,
systems with wide particle distributions failed mechanical
testing compared to systems with more narrow particle
distributions. In contrast, systems with particle distributions
that are less than ten microns also struggle to achieve higher
gloss readings on the film. These narrower distributions also
struggled to achieve surface smoothness, hiding power, and
higher gloss readings.
Consideration of particle size and distribution for a topcoat
powder system can provide the necessary protection required for
exterior structures. Early results show the observable influence of
particle sizes and distributions behaving differently in corrosive
environments. FEVE powder systems with very narrow,
uniform particle sizes and large distributions are expected
to have less success in accelerated weathering and corrosive
environments. Potentiostat readings on EIS (electrochemical
impedance spectroscopy) show higher water uptake in topcoat
systems containing larger particle sizes and wider distributions.
Additional investigations into how particle size and distribution
can affect both initial and long-term performance and appearance
are ongoing in long-term weathering and testing.
While raw material and formulation chemistry are the
foundation of creating a successful coating for exterior
performance, it is critical to select the correct processing
parameters for an optimal coating. Changes in milling/sieving
to achieve ideal particle size and distribution with adjustments
to spray settings can help eliminate surface defects, improve
mechanical properties, and increase the weatherability of longterm
topcoats.
Connie Przeslawski is a technical service chemist at AGC
Chemicals America Inc. She would like to acknowledge the support
provided by NETZSCH Premier Technologies for the cooperation
and support through the project LP-Coating at AGC Chemicals
Americas.
Table 3:Example of Particle Size Analysis of FEVE Powder Post-Mill and Sieve using Particle Size Analysis.
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Canadian Finishing & Coatings Manufacturing Magazine Jan/Feb 2023

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