Handbook of Solvents - George Wypych - ChemTech - Ventech!

410 Seung Su Kim, Jae Chun Hyun
to the experimental temperature. 13-15 These
stresses bring about the defects such as
cracking, curling and peeling. If the local
strength is overwhelmed by the local stress,
the response is cracking. It is often called as
‘mud cracking’, because it looks like a field
of mud in a dry period. And if the local
stresses exceed the adhesion strength of
coating to a substrate, the response is peeling
or delamination as shown in Figure 7.3.27. At the edge of the coating the internal residual
strength rises sharply, hence the stresses bring about the curling of coating at the edge if the
adhesion strength of coating is sufficient to endure the peeling or delamination. The curling
and peeling are also used to measure the internal stress of coating. 12,14,15,22
With solvent evaporation, the coating becomes to be concentrated with polymers, and
the coating becomes to have solid like nature (viscoelasticity). Thus the stresses depend not
only on the strain rate, but also strain. Strain is deformation from the stress-free state. 49 After
the solidification point, the solvent evaporation continues, so the stresses persist. Croll analyzed
the origin of residual internal stress during drying and correlated the internal stress
with the coating properties. 13-15 Figure 7.3.27. Internal stress related defects.
The residual internal stress, σ, for a coating is
E φs −φr
σ =
1−ν3 [7.3.13]
where:
E Young’s modulus
ν Poisson’s ratio
φ s volume fraction of solvent at the point of solidification
φ r volume fraction of solvent in the dried coating
As shown in the Equation 7.3.13, the internal stress depends on the difference of solvent
volume fraction before and after solidification point. Croll’s experiment confirms that
the internal stress does not depend on the coating thickness until the coating thickness is so
large that the net force on the interface exceeds adhesion. At this point stress is relieved by
peeling. 12-15
Adding plasticizer can often be helpful to reduce the internal residual stress of coating,
because it makes coating more flexible to the later stages of drying. 13-15 The residual internal
stress depends on the solvent volume loss from the solidification point, we can shift the solidification
point to the later stages of drying by using plasticizers. Adding plasticizers
makes the coating more flexible, but it is not always the desirable property of final products.
The residual internal stress is a result of combined action of stress and stress relaxation
process. Therefore to give a sufficient relaxation time for the coating, we often dry a coated
film in a mild operating conditions, e.g., dry at lower temperature and velocity of air.
Peeling
If the local stresses exceed the adhesive strength of coating to a substrate, then the
coating is delaminated from the substrate. Peeling easily occurs when the coating is thick.
With increasing thickness, one can find a critical thickness where the peeling occurs spontaneously.
This critical thickness for the spontaneous peeling can be used as a method of measuring
internal residual stress. 12