Page 97 - Handbook of Adhesives and Sealants

Theories of Adhesion 65

cohesive rupture of a weak boundary layer is the real event. This the-
ory largely suggests that true interfacial failure seldom occurs. Failure
may occur so near to the interface that it is apparently at the interface,
but in most cases it is ductile plastic deformation or cohesive failure
of a weak boundary layer material. Weak boundary layers can origi-
nate from the adhesive, the adherend, the environment, or a combi-
nation of any of the three.
Weak boundary layers can occur on the adhesive or adherend if
an impurity concentrates near the bonding surface and forms a weak
attachment to the substrate. When failure occurs, it is the weak
boundary layer that fails, although failure may seem to occur at the
adhesive-adherend interface.
The history of a typical adhesive joint may be divided into three
time periods: application of the adhesive, setting, and the period it is
in service. Weak boundary layers could develop during any one of
these periods.
Before application of the adhesive, the most important task is to
remove the weak boundary layers. The most common material to be
removed is atmospheric air. Displacement of air by the adhesive is a
process already described: wetting. Two other examples of weak
boundary layers that are present at the time of adhesive application
can be seen in polyethylene and metal substrates. Polyethylene sub-
strates usually have a weak, low-molecular-weight constituent evenly
distributed throughout the polymer. This weak boundary layer is pres-
ent at the interface and contributes to low failing stress when un-
treated polyethylene is used as an adhesive or adherend. Some me-
tallic oxides, such as aluminum oxide, are very strong and do not
significantly impair joint strength. However other oxides, such as
those associated with copper and copper alloys, are weak and require
removal prior to application of the adhesive. Weak boundary layers
such as low molecular weight polyethylene constituents and copper
oxides can be removed or strengthened by various surface treatments.
During the second major time period in the life of an adhesive, so-
lidification of the adhesive is the primary process. In many instances,
new boundary layers may form during the setting stage, and if they
happen to be weak, the final joint will also be weak. An example of a
weak boundary layer formed during this stage may be a chemical re-
action by-product of the setting reaction. Certain active metal surfaces
(e.g., titanium) are known to react with some chemical hardeners used
in adhesive formulations. A clean, reactive substrate surface may also
become contaminated by the components in the adhesive itself. For
example, the adhesive could contain water or low molecular weight
constituents that preferentially adsorb on the substrate surface.