Table 2—Details of Waterborne Test Formulations Using
PVDF-Acrylic Hybrid Emulsions
C1
C2
C3
C4
Water............................... 58.2
58.2
58.2 59.4
Rutile TiO2.......................215.0
Extender ............................ 0.0
125.4 125.4 215.0
89.6
89.6
0.0
Dispersant ........................ 13. 5
13. 5
13. 5 13. 4
Defoamer ........................... 0.5
0.5
0.5
0.5
Co-surfactant ...................... 2. 2
2. 2
2. 2
2. 2
PVDF-Acrylic Hybrid A .......734.0 458.0 458.0
0.0
PVDF-Acrylic Hybrid B ..........0.0
0.0
0.0 653.3
Coalescent (DPM) .............. 53.3
47. 5
47. 5 16. 4
Acrylic coresin .................183.5 115.0 115.0
0.0
HEUR thickener ................... 4. 1
High boiling
coalescent (NMP) ................0.0
2. 6
2. 6
1. 9
0.0
11.0
0.0
Pigment Volume
Concentrationa .................. 15
21
21
18
(a) Pigment Volume Concentration (PVC) = (Volume of Pigment + Filler)/
(Volume of Pigment + Filler + Binder).
described in Table 2. Commercially available new PVDF
finished coil stock was obtained from McElroy Metal
and designated as S1. The other two substrates, S2 and
S3, were prepared in our laboratory, using a simple
lab formulation without any coating additives. For
substrates S2 and S3, a PVDF finish (based on 70 wt%
PVDF polymer, 30 wt% acrylic copolymer) was applied directly over chromated aluminum, without the
use of a primer, and baked using a coil bake schedule.
Substrate S2 was prepared fresh in the lab a few days
before applying the waterborne test formulations, while
substrate S3 had been exposed in southern Florida at
south facing 45o for 14 years prior to applying the waterborne test formulation.
The three substrates were characterized by dynamic
water contact angle measurements, using a Kruss Contact
Angle Measuring System G10. The panels were cleaned
with water and allowed to air dry before measuring the
D.I. water contact angle. In addition, their surface roughness was determined by an optical interferometry technique, using the WYKO NT1100 optical profiling system
from Veeco. The measurements were performed in the
vertical scanning interferometry mode, and an area of
242 × 184 microns was analyzed at 25× magnification.
All four waterborne test formulations were ap-
plied over the three PVDF-coated substrates using a
draw down blade, to obtain a dry film thickness of 75
microns. Before the waterborne formulations were applied, the substrates were cleaned with soapy water,
thoroughly rinsed with tap water, and allowed to dry.
All coatings were air-dried under ambient conditions
for two weeks before testing adhesion. Adhesion was
tested using a cross-hatch tape pull method according to ASTM standard D3359. Ratings of 0B and 5B
implied poor and excellent adhesion, respectively. Dry
adhesion was tested before soaking the coated panels
in water. Samples were tested for water blisters and
wet adhesion immediately after immersion of panels
in water for 24 hr. Water blisters were rated according
to ASTM D714, with 10 being no blisters and 0 being
very large blisters. Also, the number of blisters on the
surface was rated on a “few” to ”dense” scale, in accordance with the standard.
SUBSTRATE CHARACTERIZATION
Dynamic water contact angle measurements of the
three substrates used in this study showed differences,
even though they were based on the same materials
(Figure 1). The advancing water contact angle of substrate
S1 was at 94.8o, which was considerably higher than a
contact angle measured for PVDF homopolymer, which
was at 85o (not shown). In comparison, substrate S2,
which was prepared fresh in the lab, showed an advancing water contact angle of 76.4o. This value was between
the contact angles of the PVDF homopolymer and acrylic
copolymers used in the finish formulation (the latter being in the 70o range), and suggests that the surface contains at least 30% or even higher levels of the acrylic copolymer component, consistent with other reports in the
literature. 4 The aged substrate, S3, had a lower advancing
water contact angle, 62.3o, despite the fact that surfaces
Figure 1—Advancing and receding water contact angles of the three
substrates. The contact angle hysteresis, which is the difference
between the advancing and receding contact angles, was 48. 1°,
31. 8°, and 35. 4° for substrates S1, S2, and S3, respectively.
120
Advancing Water Contact
Receding Water Contact
100
Water Contact Angle (deg)
80
60
40
20
0
S1
S2
