Figure 2—Top figures represent the surface topography and RMS roughness of substrates S3 (left) and S2 (right) obtained using
optical profilometer. Bottom insets represent the linear roughness of the two surfaces over a lateral distance of 30 microns. The
S3 RMS Roughness = 426 nm S2 RMS Roughness = 178 nm
RMS roughness was also measured for substrate S1 and was 235 nm.
S3 RMS Roughness = 426 nm S2 RMS Roughness = 178 nm
- 3.00
1. 40
S3 RMS Roughness = 426 nm S2 RMS Roughness = 178 nm
S3 RMS Roughness = 426 nm S2 RMS Roughness = 178 nm
1.00
0.50
0.00
-0.50
- 1.00
- 1.50
- 2.00
- 2.50
0.80
0.80
0.60
0.60
0.40
0.40
Roughness µm
0.20
Roughness µm
0.20
0.00
0.00
-0.20
-0.20
-0.40
-0.40
-0.60
-0.60
-0.80
-0.80
- 1.00
- 1.00
- 1. 20
- 1. 20
0 5 10 15 20 25 30 0 5 10 15 20 25 30
Lateral Distance µm Lateral Distance µm
of weathered PVDF finishes become enriched in PVDF
homopolymer. 4, 5 This lower advancing contact angle of
S3 is no doubt at least partly attributable to the presence
of photooxidized species on the surface.
Another likely reason for the S2-S3 contact angle
difference is the surface roughness. Optical profilom-etry showed that the aged substrate, S3, had a much
higher root mean surface roughness compared to the
freshly prepared substrate S2 (Figure 2), consistent
with the generally accepted mechanism for weathering-induced gloss loss in coatings, and also with specific
published results for PVDF finishes. 6 The contact angle
hysteresis value (the difference between advancing and
receding values), which tends to be correlated with
surface roughness, was higher for substrate S3 than S2,
and was in line with the trend of RMS roughness for
the two panels. So, the high surface roughness may also
contribute in lowering the advancing contact angle of
substrate S3 well below the PVDF homopolymer levels.
Compared with lab-prepared substrates S2 and S3,
the commercially prepared substrate S1 had a much
higher advancing water contact angle, and a larger
value for the contact angle hysteresis. Since the gloss
and surface roughness for substrate S1 are roughly
comparable to those of S2, it is postulated that the sur-
face of the commercially produced substrate also has
present on it some additional, nonbinder hydrophobic species: for instance, some kind of low molecular
weight silicone species like those which were recently
reported in another study, 7 which were traced back in
that case to the primer formulation. The high value of
the contact angle hysteresis for substrate S1 also suggests that this additional hydrophobic material is inhomogeneously distributed on the surface. Both these
factors would be expected to make it more difficult to
achieve adhesion to S1, compared to S2.
RESTORATION COATING ADHESION
RESULTS AND DISCUSSION
Formulation Effects
The dry and wet adhesion results for coating C1 over
the three PVDF substrates showed striking differences.
The adhesion, both dry and wet, was poor over S1, the
fresh commercial PVDF finish. Also, the coating showed
a high density of water blisters during the water soak
test. By contrast, the same formulation had excellent dry
and wet adhesion with no water blisters over the freshly
prepared substrate, S2. Over the aged lab prepared sub-
