Aa. Roche et al., MEASUREMENT OF THE PRACTICAL ADHESION OF PAINT COATINGS TO METALLIC SHEETS BY THE PULL-OFF AND 3-POINT FLEXURE TESTS, Journal of adhesion science and technology, 8(6), 1994, pp. 587-609
In this paper, we compare the practical adhesion measurements of alkyd
, vinyl, epoxy, and polyurethane paints on cold-rolled steel using the
pull-off and three-point flexure tests. It has been shown that (a) in
the pull-off test, the ultimate load and ultimate strain values depen
d on the stud area. During trimming, cracks are created within the sys
tem which are responsible for a large decrease in the parameters measu
red. On the other hand, it is impossible to differentiate between the
failure initiation and the failure propagation zones; (b) in the three
-point flexure test, the area (W) subtended by the load/displacement c
urves corresponding to samples with and without stiffener does not dep
end on the bonded width or on the substrate compliance. After carrying
out such a mechanical test, it is also possible to discriminate betwe
en failure initiation and failure propagation using appropriate tools
for observation, such as scanning electron microscopy, and for analysi
s, such as electron microprobe analysis and Fourier transform infrared
spectroscopy. W is shown to be representative of the failure initiati
on energy. To give a clear indication of the formation of the paint/su
bstrate interphasial zone, we have studied the practical adhesion of d
ifferent types of acrylic paint (thermoplastic acrylic binder filled w
ith TiO2, BaSO4, or CaCO3, with or without crosslinked polyurethane) a
pplied onto aluminium substrates (treated in a sulphochromic acid bath
). Five different types of failure initiation were observed, each type
corresponding to quite different energies (W). Failure initiation tak
es place within the interphasial layer for thin coatings (thickness le
ss-than-equal-to 70 mum). For thicker paint layers, cohesive failures
were observed. A model showing the composition of the paint/metal inte
rphase is proposed.