Handbook of Solvents - George Wypych - ChemTech - Ventech!

7.3 Drying of coated film 411
Figure 7.3.28. 90° Peel test configuration [After reference
12].
Croll illustrated that interfacial work
of adhesion could be found by peeling test. 12
If the coating is peeled at a constant rate, the
rate of change of total internal energy of
coating is equal to zero. Thus the peeling
strength is related to the interfacial work of
adhesion as follows (Figure 7.3.28),
F
b
= γ −t
U [7.3.14]
C R
where:
γ interfacial work of adhesion
b sample width (Figure 7.3.28)
F measured force of peeling
tC thickness of sample (Figure 7.3.28)
UR recoverable strain energy per unit volume stored in the coating
In a case of spontaneous peeling,
γ=tU
p R [7.3.15]
where:
tp critical thickness for the spontaneous peeling
Interfacial work of adhesion, γ, can be measured by spontaneous peeling test using
above equation. Spontaneous peeling thickness, tp, can be directly measured, and the recoverable
strain energy, UR, for an elastic material under a one-dimensional strain is,
UR = E i
1 2
ε [7.3.16]
2
Figure 7.3.29. Graphical representation of the equations governing
the 90° peel test. [Adapted, by permission, from Croll, S.G., J. Coating
Technol., 52, 35 (1980)].
where:
E modulus of coating
εi internal strain in coating
As shown in Figure 7.3.29
the interfacial work of adhesion
and recoverable strain can also be
found by measuring peeling
strength at several different coating
thickness. If the adhesive
strength of the interface exceeds
the cohesive strength of one of the
coating component materials,
there occurs cohesive failure - the
coating layer loses the adhesive
strength within the layer itself. At
a small thickness, cohesive failure
often occurs during the peeling
test illustrated in Figure 7.3.28.