Page 120 - Handbook of Adhesives and Sealants
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88 Chapter Two
Figure 2.22 Schematic diagram of the effect of the rheological state on adhesive lap
shear and tensile joint strengths. 8
layers can occur. Examination of weak boundary layers and investi-
gation of phenomena occurring at the interphase regions of the joint
provides valuable information.
2.7.1 Boundary layer theory
One interpretation of adhesion that has been the most useful in de-
scribing adhesion phenomenon is the boundary layer theory set forth
by Bikerman. 23 This theory proposes the existence at an interface of
a finite boundary layer composed of adsorbed molecules that differ in
nature from those in the bulk phases. A schematic diagram showing
examples of weak boundary layers is presented in Fig. 2.23.
The criterion for good adhesion is merely that the boundary layer
be strong enough to withstand the effects of external stress. According
to Bikerman, rupture of an adhesive joint practically always proceeds
through a single material phase rather than between two materials.
The rupture is initiated at a point where the local stress exceeds the
local strength. When failure occurs in the boundary stratum, a weak
boundary layer is present.
Weak boundary layers may form due to a variety of causes. Often
the formation is unpredictable, and it is difficult or impossible to de-
termine the actual composition of the boundary layer. Examples of
common weak boundary layers include:
