Mechanism
of Film Formation
Film formation is the conversion of a coating film from a liquid
or fluid form into a solid. A key difference between waterborne
acrylic coatings and solventborne coatings such as oil-based alkyds
is their mechanism of film formation. An understanding of the
process by which waterborne acrylic latex paints are converted
from a wet paint into a dried film also helps to explain why,
if used improperly, problems could occur.
A typical solventborne resin, such as an oil-based
alkyd, deposits the binder on the surface through a simple evaporation
of the carrier solvent. This is often referred to as “lacquer-dry,”
and is a straightforward process with only slight dependence
on factors like temperature and humidity. During the evaporation
process, entanglement of the binder molecules (polymer chains)
occurs, forming a tight-knit matrix. After film formation, further
reactions through oxidation (e.g., alkyds) or with cross-linkers
(e.g., two-component epoxies and urethanes) are required to
generate higher molecular weight, cross-linked films. The film
formation process for a solventborne coating is depicted below
(pigment is removed from picture for simplicity):
Film formation with waterborne acrylic latex binders is slightly
more complicated and occurs via a multi-step process, as depicted
in the figure. Recall that each spherical latex particle is
composed of several high molecular weight acrylic polymer chains.
Water evaporates from the film and the spherical particles form
a closely packed layer. Continued evaporation and diffusion
of water leads to deformation of the particles. Because many
acrylic binders are relatively hard, some co-solvents or coalescents
are necessary to soften the latex particles in order for them
to deform and flow (or coalesce) together. The disappearance
of the individual particles occurs as the polymer chains diffuse
across the particle boundaries and become entangled with one
another, finally forming a continuous film.
The development of a continuous film that
can act as a good barrier to water and salts is key to the performance
of a waterborne acrylic latex paint for industrial applications.
If the latex particles fail to fuse together, microscopic channels
will remain in the final paint film, allowing water and electrolytes
to move through the film with greater ease. The coalescence
process can be hindered if the paint is not properly formulated
for the application conditions, i.e., if the level of coalescing
co-solvent is insufficient to soften the latex particles so
that they can deform and coalesce at the intended application
temperature.
If not properly formulated, or if applied under the wrong environmental
conditions, a coating with poor performance will result. In
severe cases, after drying the film may crack and will not act
as a protective barrier. For more info on how environmental
conditions during application can affect film formation, and
ultimately the paint’s performance, click
here.
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