© 2006 by Taylor & Francis Group, LLC

62 Corrosion Control Through Organic Coatings
around the pigment particles, but voids still occur because there simply is not enough
binder.
The bottom part of Figure 3.2 shows the ideal scenario: the PVC is lower, and
the surrounding black binder is able to not only cover the pigment particles but also
leave no void between them.
3.3.3.2 Additives
In real waterborne paints, the film formation process can result in a nonhomogeneous
layer of cured paint. Tzitzinou and colleagues, for example, have shown that the
composition of a cured paint layer can be expected to vary through the depth of the
coating. They studied an anionic surfactant in an acrylic latex film. Using AFM and
Rutherford backscattering spectrometry on cured films, they found a higher concentration
of surfactant at the air surface than in the bulk of the coating [18]. Wegmann
has also studied the inhomogeneity of waterborne films after cure, but attributes his
findings mainly to insufficient coalescence during cure [19].
The chemistry of real latex formations is complex and currently defies predictive
modeling. A reported problem for waterborne modelers is that an increase in curing
temperature can affect various coating components differently. Snuparek and
colleagues added a nonionic emulsifier to a dispersion of copolymer butyl methacrylate/butyl
acrylate/acrylic acid. When cure took place at room temperature, the
water resistance of the films increased with the amount of emulsifier added. When
cure happened at 60°C, however, the water resistance of the films decreased with
the amount of emulsifier added [4].
3.4 MINIMUM FILM FORMATION TEMPERATURE
Minimum film formation temperature (MFFT) is the minimum temperature needed
for a binder to form a coherent film. This measurement is based on, although not
identical to, the glass transition temperature (T g) of the polymer.
If a coating is applied below the MFFT, the water evaporates as described for
Stage 1 (see Section 3.3). However, because the ambient temperature is below the
MFFT, the particles are too hard to deform. Particles do not coalesce as the interstitial
water evaporates in stage 2. A honeycomb structure, with Van der Waals bonding
between the particles and polymer molecules diffused across particle boundaries,
does not occur.
The MFFT can be measured in the laboratory as the minimum temperature at
which a cast latex film becomes clear. This is simply because if the coating has not
formed a coherent film, it will contain many voids between polymer particles. These
voids create internal surfaces within the film, which cause the opacity.
Latexes must always be applied at a temperature above the MFFT. This is
more difficult than it sounds, because the MFFT is a dynamic value, changing
over time. In a two-component system, the MFFT begins increasing as soon as
the components are mixed. Two-component waterborne paints must be applied
and dried before the MFFT has increased enough to reach room temperature. When
© 2006 by Taylor & Francis Group, LLC