Handbook of Solvents - George Wypych - ChemTech - Ventech!

412 Seung Su Kim, Jae Chun Hyun
The spontaneous peeling thickness can be found by extrapolating the experimental data to
the zero peeling strength. The validity of interfacial work of adhesion and recoverable strain
which is obtained from this experiment can be tested by comparing this spontaneous peeling
thickness with obtained from independent measuring of spontaneous peeling thickness.
To enhance the adhesive strength, the surface properties of substrate are very important.
The surface treatments are often used to enhance the adhesive strength between the
coating and the substrate, as well as to enhance the surface energy of substrate - improving
wettability of coating. If adhesive problem occurs, check out the surface treatment processes
such as flame, plasma, or corona. 21 Corona treatment does not persist permanently,
therefore it should be done in-line, the corona is often applied directly before the coating
station.
Sometimes a thin, high surface energy adhesive or subbing layer is coated on the substrate
to improve the adhesion of coating. 21 As often the case, this layer is coated in-line during
manufacturing of substrate - such as in-line coating of acrylic resin during bi-axial
extension of PET film. If the adhesive failure occurred when one used these kind of treated
substrate, one should check that the correct side was used where the subbing layer was
coated and the status of coated subbing layer was perfect. Simple peel tests with adhesive
tape can give a clue to the coating status of subbing layer. First laminating the adhesive tape
with substrate where the coated material is applied, then pull off the tape rapidly. If there are
problems with the subbing layer such as the partial un-coating of subbing layer, then the
coating will be peeled off according to the un-coated pattern. By analyzing un-coated pattern,
one can find the steps of process which have problems. If the substrate is suspected,
change the substrate lots.
7.3.4.2.2 Surface tension driven defects
Defects can arise during drying of a coated film by building non-uniform surface tension
gradient over the coating. They include convection or Benard cells, fat edges or picture
framing, etc.
Figure 7.3.30. Surface tension driven defect: Convection cells.
Convection cells
Convection or Benard cells can arise when the coating is heated from the bottom of the
coating, and the density or surface tension gradients are built across the coating thickness.
These gradients lead to convection cells which look like regular close-packed hexagonal
surface patterns (Figure 7.3.30). 10,21 The evaporative cooling can also be the cause of the
temperature gradient. This gradient arises the density and surface tension gradient, which
brings about the internal flows within the coating to form the convection or Benard cells.