Page 73 - Handbook of Adhesives and Sealants

An Introduction to Adhesives and Sealants 41

Figure 1.11 Relations between the forces involved in adhesion. 12
achieve in practice theoretical adhesion values. It also shows clearly
the factors that contribute to the reduction of adhesive strength.
If the adhesive does not wet the surface of the substrate, the max-
imum possible joint strength will be degraded. Internal stresses occur
in the adhesive joint during production because of the different phys-
ical characteristics of the adhesive and substrate. For example, the
coefficient of thermal expansion of adhesive and substrate should be
as close as possible to limit stresses that develop during thermal cy-
cling or after cooling from an elevated-temperature cure. Adhesives
can be formulated with various fillers to modify their thermal-
expansion characteristics and limit such internal stresses. A relatively
elastic adhesive, capable of accommodating internal stress, may also
be useful when thermal-expansion differences are of concern.
Once an adhesive bond is made and placed in service, other forces
are at work weakening the bond. The type of stress, its orientation to
the adhesive, and the rate of loading are important. The external
stress could either reduce the measured bond strength further, or it
could actually increase the measured bond strength by counteracting
degrading internal stresses in the joint as shown in Fig. 1.11.