Table 3—Dry, Wet Adhesion, and Water Blister Resistance of Waterborne Restoration
Coating Formulations (C1–C4) over Three PVDF-Coated Substrates (S1–S3).
Comm-PVDF (S1)
Lab-PVDF (S2)
S. FL weathered
Lab-PVDF (S3)
Dry Wet
Water
Blisters
Dry Wet
Water
Blisters
C1 ..... 0B 0B 4/Medium
5B 5B
10
C2 ..... 0B 0B
6/Few
5B 5B
10
C3 ..... 0B 0B
6/Few
5B 5B
10
C4 ..... 0B 0B
10
5B 5B
10
in the dried coatings. 12 Such a
distribution can create microscopic channels in the coating,
which will make it more permeable and thus allow easier
pathways for the dissolved
impurities at the interface, and
hydrophilic moieties in the
coating, to diffuse out into the
water phase. This will reduce
osmotic pressure buildup and
reduce blister size and density.
Formulation C3 was similar
to C1, except that it contained
an additional quantity of a
second coalescent—N-methyl
pyrrolidone (NMP)—which was
also an active solvent for the
PVDF in the substrate. Formulation C3 not only showed
excellent water blistering resistance over both the fresh
and the weathered lab PVDF substrates (S2 and S3),
but also excellent wet adhesion (Table 3, Row 3). In additional studies, two separate factors were found to be
important for this second added coalescent: it needed
to have a low evaporation rate, and it needed to have
good solvency for PVDF polymers. Alternative formulations where the NMP was replaced with other good
coalescents for the binder, such as DPM, which were not
active solvents for PVDF homopolymer, did not see a
significant improvement of the adhesion or blister resistance. Likewise, the replacement of NMP with acetone,
a fast-evaporating active solvent for PVDF polymers, did
not improve the adhesion and water blistering properties
to the same extent as NMP. These results point to the
idea that adhesion to PVDF finishes can be significantly
enhanced by choosing a topcoat coalescent or cosolvent
which can effectively cause some softening or plasticization of the substrate.
In other studies not reported in detail here, a two-pack, solvent-based epoxy primer was also evaluated as
a primer layer between the coating C1 and PVDF aged
panel S3. The solvent primer also gave excellent system
wet and dry adhesion, and excellent blister resistance,
when used with the C1 topcoat. This would represent
another potential approach for restoration coatings
over PVDF finishes in regions of the world where such
primers could be used.
Increasing the pigment loading and the incorporation of active cosolvents definitely improved the water
blistering and wet adhesion of PVDF-Acrylic Hybrid A
coatings over new and aged PVDF finishes, but the incorporation of additional solvents inevitably increases
coating VOC levels, which goes in a direction opposite the trend in today’s coatings markets. Two other
alternative practical approaches to improve the adhe-
Water
Dry Wet Blisters
5B 0B 4/Medium
5B 0B 6/Few
5B 5B 10
5B 5B 10
Note: Adhesion of 0B and 5B indicates poor and excellent adhesion respectively. The ranking of water blistering is on a scale
of 0 to 10 with 10 being no blistering and 0 being large blister. Quantity of water blisters are graded from “few” to “dense.”
strate, S3, it had excellent dry adhesion, but numerous
water blisters and poor wet adhesion were observed af-
ter the water soak ( Table 3, Row 1).
The phenomenon of water blistering, although very
well covered in the literature, can be complicated due
to the influence of numerous factors. Osmotic pressure
is often cited in the literature as the main mechanism8, 9
for blistering. Most organic coatings are semi-perme-able membranes. Minute pockets of water-soluble organic and inorganic compounds present at the coating-substrate interface, such as chloride and sulfate salts
and corrosion products, draw in water by the process of
osmosis. As more and more water is drawn into these
pockets, blisters appear when the adhesive forces at the
interface are weaker than the force created by osmotic
pressure. Subsequently, the film delaminates. In water-based latex or dispersion coatings, another source of
osmotic pressure buildup may be operative. Typically,
water–based systems are formulated with additives
and solvents, which are hydrophilic in nature. As these
coatings dry, mobile hydrophilic materials can be concentrated, and possibly trapped, in small interstitial regions that are created during the particle packing phase
of the film-formation process. 10 These regions can then
become a second osmotic sink for water absorption.
These two factors may both be operative in the case of
the water blistering observed here.
Coating C2, which incorporated a filler and had a
higher pigment volume concentration than C1, had
similar adhesion results, but improved greatly in water
blistering characteristics ( Table 3, Row 2). The higher
pigment loading of coating C2 gives it a higher elastic
modulus than C1, as measured by mechanical testing,
and this is thought to contribute to an improved ability to mechanically resist the formation of blisters due
to osmotic pressure. 11 In addition, fillers and pigments
often tend to aggregate and distribute heterogeneously
