Page 235 - Handbook of Adhesives and Sealants

204 Chapter Six

resulting bond strength without having a significant influence on the
bulk properties of the material.
Components within the polymeric bulk material can also migrate to
the surface. It is common to find low molecular weight polymers or
oligomers, plasticizers, pigments, mold release agents, shrink control
agents, and other processing aids as well as adsorbed contaminants
in the surface region. More so than with metals, the surface regions
of plastics are dynamic regions, continuously establishing new equi-
librium internally with the bulk material and externally with the sur-
roundings. In flexible amorphous plastics above the glass transition
temperature, low molecular weight components are able to diffuse out
of the bulk and to the surface region while elements of the surround-
ings can diffuse into the adherend.
A well-known example of this effect is the migration of plasticizer
from flexible polyvinyl chloride. The plasticizer can migrate from the
bulk adherend to the joint region and then to the interface. If the
adhesive is an effective barrier to plasticizer migration, this will create
a weak boundary layer at the interface. If the adhesive is not an ef-
fective barrier, then the plasticizer will migrate through the interface
and into the adhesive and thereby possibly change the physical prop-
erties of the adhesive.
The nature of the polymeric surface can change rapidly in response
to its surroundings. Even when the bulk material is in the glassy state
(below its glass transition temperature), the surface region can be
quite mobile owing to the presence of low molecular weight polymer
constituents and contaminants. Polymers, having both polar and
non-polar regions in their molecular chain, can present different chain
segments at the surface depending on whether the surroundings are
polar or not. Wiping a surface with an ionic solution will cause the
polar groups to orient toward the surface. While the same treatment
with a non-polar solvent, such as hexane, can bring the non-polar com-
ponents to the surface. Exposure to heat after surface treatment could
cause fresh, untreated molecular species to appear on the surface,
thereby losing the beneficial characteristics of the surface treatment.
As a result of these dynamic reactions, it is difficult to be confident
about the surface of any polymeric material. The actual surface to
which we are bonding is not always the surface that we anticipate. It
is also possible that the surface could change once the bond is made
and the assembled joint is placed into service. Thus, a weak boundary
layer that is not present during the bonding process may form during
the joint’s operating life and contribute to a weakening of the inter-
face. Although these dynamic processes are not always damaging to
the bond strength or to the integrity of the assembled joint, they need
to be considered early in the assembly design process. If such surface