Page 121 - Handbook of Adhesives and Sealants

Theories of Adhesion 89

Figure 2.23 Examples of weak boundary layers. 24

Entrapment of air
Impurities, or low molecular weight species that concentrate near
the surface
Cohesively weak oxide layers on the substrate surface
Chemical deterioration of the coating, adhesive polymer, or sub-
strate during the bonding process (e.g., catalytic air oxidation dur-
ing heating)
Chemical deterioration and/or corrosion between the adhesive and
the substrate. (In some cases, for example with metals, the forma-
tion of brittle inter-metallic compounds can lead to a weak boundary
layer.)

2.7.2 Interphase region
Sharpe 25 extended Bikerman’s two-dimensional weak boundary layer
concept into a three-dimensional interphase concept. Interphases are
thin regions of the joint that have properties that are widely different
from the bulk materials. These regions are thick enough to have prop-
erties, such as modulus, Poisson’s ratio, tensile strength, etc., that will
affect the final properties of the joint.
Interphase regions can be formed by solidification of certain poly-
mers on certain high energy substrates. The interfacial structure will
be characteristic of the composition and structure of both the polymer
and the substrate, as well as the ambient conditions. Solidification
preserves this organization to create an interphase with unique prop-
erties that becomes a permanent part of the joint, influencing its me-
chanical response. Examples include semicrystalline polymers, such
as polyethylene. When this polymer is solidified from the melt while