By Clifford K. Schoff, Schoff Associates
Wet Adhesion and Its Measurement
The importance of good adhesion
of coatings to substrates is well known,
but most adhesion evaluation is done
on panels as produced. The panel
may be allowed to stand for a time in
case further cure occurs, but that is
all. However, coatings used in exterior
applications, especially primers, should
have their adhesion checked after
water immersion or exposure to high
humidity. Good wet adhesion is important for coatings over all substrates,
but is critical to corrosion resistance of
coatings over steel. Corrosion can be
prevented or greatly reduced by having excellent primer wet adhesion. If
corrosion does begin, having excellent
wet adhesion prevents or reduces its
spread. If water and electrolyte cannot
lift the coating and form a layer next to
the substrate, there will be little or no additional corrosion. Some coatings, such as cationic electrodeposition primers, rely entirely on excellent wet adhesion
to prevent corrosion. Primer adhesion, especially wet
adhesion, is helped by:
•
A clean, uniform substrate or pretreatment
surface to which to anchor.
•
Presence of components in the coating that
tend to adsorb on metals and displace water.
•
Ability of the paint to flow out and achieve
intimate contact with the pretreatment or
substrate.
•
Crosslinking after application with development of a rigid structure–rigid bonds resist
water displacement.
I never have been satisfied with the few wet
adhesion testing methods in the literature. My personal experience with such testing has been based
on the crosshatch and tape test (ASTM D3359, ISO
2409) after a panel has been in a water soak for a
number of hours. The surface was patted dry with a
paper towel and then the procedure was carried out.
That adhesion test is only good for ranking coatings
or comparing pairs of panels, not for providing absolute adhesion values. ASTM D6900 adds scrub testing after cross-cutting.
Scrape and scratch tests (see ASTM D2197) also
have been used, but a new one looks even better. I
recently came upon an excellent paper on measuring
wet adhesive strength via a scratch test [H. Jiang, R.
Browning, P. Liu, and H.-J. Sue, J. Coat. Technol. Res.,
60
March 2015
COATINGSTECH
8 (2), 255-263 (2011)] that I missed when it was first
published. The authors were particularly interested in
evaluating fusion-bonded epoxy coatings for oil and
gas pipelines, but their techniques could be applied
to more conventional coatings as well. They used a
relatively new test method for scratch resistance of
polymeric coatings and plastics using an instrumented
scratch machine (ASTM D7027, ISO19252:2008).
They exposed coated panels to hot water (80°C, 95°C)
for times ranging from one day to two weeks. Each
specimen was removed from the tank, rinsed with tap
water and wiped dry. Immediately after that, scratch
testing was run with a Surface Machine Systems (www.
surfacemachines.com) Model G4 scratch test machine,
which applies a progressively increasing load to the
scratch tip. The authors were able to measure critical
scratching loads for coating disbonding for different
immersion times, film thickness, and substrate surface
roughness. The details of how this was done are contained in their paper. Based on my reading of ASTM
D7027 and experience with past wet adhesion tests, I
would think that the critical load for disbonding would
be sufficient for at least ranking coatings and comparing pairs of panels for wet adhesion and might possibly, produce data that are more useful than that (better
than crosshatch and tape). However, the authors did
not stop with the critical load. They indicate that it is
necessary to calculate the interfacial stresses that
have been overcome by scratching in order to determine the wet-adhesive strength. They did so by finite
element analysis, the details of which are omitted, but
they do give a reference for similar calculations in an
earlier publication. There are a number of valuable references listed in this paper.
The authors point out that a thicker coating
reduces early damage to the coating/steel interface by
lowering the rate of diffusion of water and “lowers the
stress magnitude generated at the interface under the
same applied scratch normal load level. Consequently,
a higher scratching load is needed to achieve similar
stress level at the interface to induce debonding for
thicker coatings.” Their results showed decreases in
wet-adhesive strength with longer immersion time, but
the values began to level off after a week. They also
found that a rougher substrate gave higher wetadhesive strength (metal phosphate pretreatments
have provided roughness and adhesion for coatings over steel for many years). My conclusions from
this paper are that the best wet adhesion test so
far involves testing coatings at equal film thickness
after water immersion for at least a week using ASTM
D7027 and the techniques developed by the authors. CT