Handbook of Solvents - George Wypych - ChemTech - Ventech!

7.3 Drying of coated film 413
The convection cells, which come from the surface tension gradient, can arise when
the Marangoni number (Ma) exceeds 80. 10 The Marangoni number is
( / )( / )
d dT dT dy h
Ma = σ
μκ
2
[7.3.17]
where:
σ surface tension
µ viscosity
κ heat conductivity
T temperature
h thickness of coating
dT/dy temperature gradient in the thickness direction
Convection cells can arise at a lower Marangoni number when the coating thickness is
above 2 mm. 21 Convection or Benard cells can be reduced or eliminated by adjusting operating
conditions and formulation of coating solution. The possibility of having convection
cells is reduced at the following conditions 21
• Lower surface tension of coating liquid
• Reduce the thickness of the wet coating
• Increase the viscosity of coating liquid
• Reduce the drying rate by adding low volatile liquid or reducing the temperature of
drying air
When the thickness of wet layer of coating is less than 1 mm, as is often the case with
almost all coatings, convection cells are almost always due to the surface tension gradients.
10,21
However, it is not desirable to lower the drying rate because it decreases the line speed
of coating process. Frequently it is helpful to use surfactant to reduce the convection cells
due to the surface tension gradients. Care should be given in selection of the surfactant and
the amount of it not to deteriorate the final quality of products. The amount of surfactant
should be minimized, excess surfactants can migrate to the coating surface and react with
humidity at a coating station to form a haze coating surface. Moreover, the remaining excess
surfactant, which is in the final products, can migrate to the coating surface while the products
in use in a some environmental conditions to ruin the final quality of the products.
Fat edges
Fat edges can be built during drying of coating with non-uniform surface tension distribution
at the edges of coating. Figure 7.3.31 illustrates the mechanisms of fat edge defects.
The edges are usually
Figure 7.3.31. Formation of fat edge or picture framing.
thinner than the bulk of coating,
thus as the evaporation proceeded
the concentration of polymer is increased
faster at edges than in the
bulk of coating. Usually the surface
tension of solid is much
higher than that of polymer, the
surface tension is higher at the
edges than the bulk. The higher
surface tension at the edges will