Page 44 - Handbook of Adhesion Promoters

2.11 Reversible hydrolysis 37

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humidity on adhesion. Two ranges of
humidity are apparent (one marked by the
line parallel to x-axis for the lower levels of
humidity, and the rapidly descending rela-
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tionship for the higher humidities). This
graph suggests the existence of a critical
humidity above which the joint is
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affected. One of the obvious factors
responsible for the existence of this critical
humidity is the water presence at interface
and its effect on the durability of bonds
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between steel and silane. The plasticiza-
tion and swelling effects of the water on the
adhesive are some of the suggested influ-
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ences.
Figure 2.42. Time to adhesion loss of epoxy coating on Primed and polished steel/epoxy was
steel vs. layer thickness. [Adapted, by permission, from
o
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Legghe, E; Aragon, E; Belec, L; Margaillan, A; Melot, immersed in water at 60 C. Figure 2.42
D, Prog. Org. Coat., 66, 276-80, 2009.] shows the relationship between coating
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thickness and time to adhesion loss. The
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loss of adhesion depends on the arrival of water at the interface. The diffusion time esti-
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mated on free films corresponds to the diffusion time in the supported coating. The dif-
fusion coefficient permits estimation of the time needed for water to saturate the epoxy
film as a function of thickness: 74
ΔM ∞ ΔM ∞ 16D t sat π 2
------------ = ------------ --------------------- so that t = ----------h [2.11]
M 0 M 0 π h sat 16D
where:
ΔM the amount of water absorbed at equilibrium
∞
M 0 the initial mass of the film
D diffusion coefficient
time to saturation
t sat
h film thickness
The time needed to saturate a supported coating of thickness l can thus be estimated,
considering that it equals the time needed to saturate a free film of thickness h = 2l (free
films experience diffusion from both sides): 74
π 2
t sat = -------l [2.12]
4D
The failure mechanisms of an epoxy varnish coating and an epoxy glass flake coat-
ing were investigated under ordinary pressure (1 atm) and high hydrostatic pressure
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(35 atm). The water diffusion was greatly accelerated by the increased pressure and the
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time to saturation was drastically decreased (from 55 to 30 h).
The diffusion of water in adhesive joints is a complex process because of the hetero-
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geneous microstructure of the composite and its interface with the adhesive. The diffu-
sivity parameter of the adhesive interphase was one order of magnitude lower than the