Such systems can have good outdoor weathering
performance, but suffer from relatively high VOC
levels, which restricts their use in regions where VOC
level regulations are tougher to achieve. In addition,
isocyanate-crosslinked two-pack systems in general entail additional health and safety considerations, have
a relatively short pot life (unless plural component
equipment is used), and can be sensitive to application conditions—in particular, relative humidity.
To address some of the issues in the restoration coatings market, especially for metal restoration, we have
developed water-based, PVDF-acrylic hybrid dispersions. 2
The morphology of individual latex particles in the dispersion enables the dried coating to have long-term durability and color retention characteristics similar to those
of conventional solvent-based PVDF finishes. 3 Such a
system allows for economical, lower-VOC, durable water-based coatings to restore existing PVDF finishes and extend the useful life of the building or structure.
Adhesion, which is a primary property for restoration coatings, can be challenging over aged/weathered
PVDF coated metal roofs. Conventional PVDF finishes
are 70–80% PVDF and 20–30% acrylic copolymer by
resin weight. Studies have shown that exposure to UV
selectively erodes away the acrylic component near the
coating surface, leaving the surface rough and enriched
in fluorine content. 4-6 Low energy and topographical
features presented by the weathered surface can pose
difficulties for adhesion of water-based restoration coatings. This article discusses one-pack formulations with
PVDF-acrylic hybrid dispersions for restoration paints.
Table 1—PVDF-Acrylic Hybrid Physical Properties
PVDF Hybrid A
PVDF Hybrid B
PVDF: acrylic
mass ratio ............... 70: 30
50:50
Acrylic Tg................. Higher
MFFT, °C.................. 26
Lower
Approaches to achieve excellent adhesive properties of
the PVDF-acrylic hybrid coatings over new and weathered PVDF finishes are presented. In addition, the role
of surface energy and features of new and weathered
PVDF finishes on adhesion of topcoats are investigated.
MATERIAL AND TEST CONDITIONS
In this study, two PVDF-acrylic hybrid emulsions,
designated Hybrid A and Hybrid B, were used. The dispersions were made by a seeded emulsion polymerization process, using the same PVDF copolymer seed. The
final latex particles consisted essentially of a physical
alloy of a PVDF copolymer (with a Tg below 0°C) and
a thermodynamically miscible acrylic copolymer. The
final latex material properties were adjusted by varying
the acrylic comonomer ratio to adjust the acrylic Tg, as
well as the weight ratio of PVDF to acrylic, such that
PVDF-Acrylic Hybrid B coatings were softer and lower
in tensile-modulus than Hybrid A. The acrylic copolymer included a small amount of copolymerized acid-functional monomer, and the dispersions were neutralized with an organic amine base to a pH of 8–9, so that
in each case the materials used in coating formulations
were anionically stabilized. The resulting hybrid dispersions were unimodal in particle size, with an average
particle size in the 130–190 nm range depending on
the acrylic content. The properties of the two dispersions used in this study are summarized in Table 1.
The hybrid dispersions were formulated into white
paints, using standard grades of pigments, dispersants,
coalescents, and other additives such as are commonly
used with acrylic latex products. Specifically, coatings based on four white formulations were studied
(Table 2). Coatings C1, C2, and C3 were prepared
from PVDF-Acrylic Hybrid A with rutile TiO2, and
also contained some blended low Tg acrylic coresin to
bring the final PVDF copolymer content of the binder
into the 50–60 wt% range, for more direct comparison
with Coating C4, which was made with PVDF-Acrylic
Hybrid B. Coating C1 contained TiO2 only at 15 PVC,
while Coating C2, in addition to a similar level of
TiO2, also contained a barium sulfate extender, which
increased the total pigment volume concentration
to 21%. Coating C3 had the same PVC and pigment
package as C2, but incorporated some additional coalescent, in particular, N-methyl pyrrolidone (NMP),
which is not only a rather high boiling point coalescent, but also is an active solvent for PVDF. Finally,
Coating C4 was prepared from the lower Tg PVDF-Acrylic Hybrid B, using TiO2 as the only pigment. The
coatings formulations are provided in Table 2.
Three types of PVDF finish substrate were used to
test the adhesion of the four waterborne formulations
