Handbook of Solvents - George Wypych - ChemTech - Ventech!

7.3 Drying of coated film 409
non-solvent. But it is difficult to
handle or store the water laden
coating solution, since the state of
solution is very unstable. Therefore
to supply water to the coating
the highly humidified air is sometimes
used at the early stages of
drying (in most cases the first and
the second zone of dryer). The water
was condensed at the coating
surface and it acted as non-solvent.
By adjusting air humidity
various kind of coating structures
can be obtained as in Figure
7.3.25.
The phase separation phenomena in polymer-polymer-solvent system can be called as
polymer incompatibility. 41,42,56,58 The relative solubility of two polymer in the common solvent
determines the surface structure of dried coating. Diverse kinds of surface structure can
be obtained by adjusting drying rate, substrate surface properties, relative solubility of solvent.
56,58 The shapes of surface structure can be simulated by Cell Dynamic System. 41,42
Figure 7.3.25. Surface structure of coatings with different drying
conditions (SEM ×500). Humidity of air [kg water/kg dry air] :A-
0.062, B - 0.036, Initial solid content : 10.0 wt%.
7.3.4.2 Drying defects
The maximum line speed of drying and operating conditions of dryer are often restricted by
the onset of defects. The drying related defects could be classified according to the cause of
defects - stress induced defects, surface tension driven defects, defects caused by air motion.
7.3.4.2.1 Internal stress induced defects
Figure 7.3.26. The origin of internal stress in a conventional polymer
solution coating.
Evaporation of solvent is necessarily
accompanied with the
shrinkage of coating in volume as
illustrated in Figure 7.3.26. This
shrinkage can be only allowed in
the coating thickness direction,
because the adhesion of coating to
the substrate prevents coating
from shrinking in the plane of
coating. Thus the stress is developed
in the plane of coating. This
stress brings about the defects
which is known as curling, cracking
and peeling. 10-15,21-23,34,46
Curling and cracking
The internal stresses are build up with drying of wet coating. When the coating has
enough mobility, the stresses that are developed during drying can be relieved by flow. After
the coating solidified stresses will build up in the lateral dimensions. The solidification
point equals the concentration at which the glass transition temperature of coating has risen