© 2006 by Taylor & Francis Group, LLC

Weathering and Aging of Paint 105
contracts, causing tensile stress. These compression and tension forces have adverse
effects on the film’s cohesive integrity and on its adhesion to the substrate. Of the
two types of stresses, the tensile stresses formed as the coating dries have the greater
effect [9, 11, 21].
Coating stress is a dynamic phenomenon; it changes drastically during water
uptake and desorption. Sato and Inoue [22] have reported that the initial tensile
stresses (left over from shrinkage during film formation) of the dry film decrease to
zero as moisture is absorbed. Once the initial tensile stresses have been negated by
water uptake, further uptake leads to build-up of compressive stresses. If the film is
dried, tensile (shrinkage) stresses redevelop, but to a lower degree than originally
seen. Some degree of permanent creep was seen in Sato and Inoue’s study; it was
attributed to breaking and reforming valency associations in the epoxy polymer. The
same trend of initial tensile stress reduction, followed by compressive stress buildup
was seen by Perera and Vanden Eynde [23] with a polyurethane and a thermoplastic
latex coating.
Hygroscopic stresses are interrelated with ambient temperature [11, 20]. They
also depend heavily on the glass transition temperature (T g) of the coating [24]. In
immersion studies, Perera and Vanden Eynde examined the stress of an epoxy coating
whose T g was near — even below — the ambient temperature [25]. The films in
question initially had tensile stress from the film formation. Upon immersion, this
stress gradually disappeared. As in the previously cited studies, compressive stresses
built up. The difference was that these stresses then dissipated over several days
even though immersion continued. Hare also noted dissipation of compressive
stresses as the difference between T ambient and T g is reduced; he attributes it to a
reduced modulus and a flexibilizing of the film [11]. Because of the low T g of the
film, stress relaxation occurred and the compressive stresses due to water uptake
disappeared.
Hygroscopic stresses have a very real effect on coating performance. If a coating
forms high levels of internal stress during cure — not uncommon in thick, highly
crosslinked coatings — then applying other stresses during water uptake or desorption
can lead to cracking or delamination. Hare has reported another problem: cases
where the film expansion during water uptake created a strain beyond the film’s
yield point. Deformation here is irreversible; during drying, permanent wrinkles are
left in the dried paint [17]. Perera has pointed out that hygroscopic stress can be
critical to designing accelerated tests for coatings. For example, a highly crosslinked
coating can undergo more damage in the few hours it dries after the salt-spray test
has ended than it did in the entire time (hundreds of hours) of the test itself [26].
6.2.4 BLISTERING/ADHESION LOSS
Blistering is not, strictly speaking, brought about by aging of the coating. It would
be more correct to say that blistering is a sign of failure of the coating-substrate
system. Blistering occurs when moisture penetrates through the film and accumulates
at the coating-metal interface in sufficient numbers to force the film up from the
metal substrate. The two types of blistering in anticorrosion paints — alkaline and
neutral — are caused by different mechanisms.
© 2006 by Taylor & Francis Group, LLC