Page 36 - Handbook of Adhesives and Sealants

6 Chapter One

part of the assembly made up of adherends; adhesive or sealant; prim-
ers, if present; and all associated interphase regions as shown in Fig.
1.2.

1.2.3 Important factors for successfully
using adhesives and sealants
From the complexity of the joint as described above, it should be evi-
dent that what is necessary to successfully understand and use ad-
hesives or sealants is far broader than simply a knowledge of certain
materials. The quality of the resulting application will depend on
many factors, some of which are very entangled and complicated.
One of the principal factors in the success of either an adhesive or
sealant is adhesion. Table 1.1 lists some of the external and internal
factors that influence adhesion. An understanding of how these factors
affect adhesion will determine the success of the bonding or sealing
operation. Knowledge of production processes, economics, and envi-
ronment and safety factors is also important.
Anyone intending to use adhesives or sealants faces the formidable
tasks of selecting the correct materials and determining proper pro-
cesses. The adhesive or sealant must flow onto the substrate surface
and then change from a flowable liquid to a structural solid without
creating harmful internal stresses in the joint. The substrate surface
must have previously been cleaned and, possibly, prepared specially
for maximum adhesion. The joint geometry must be correctly designed
with regard to the materials selected and to the expected loads to
avoid undesirable local stresses that could lead to early and premature
failure. Also, the physical and chemical characteristics of the joint
(adhesive/sealant, adherends, and interphase regions) must be un-
derstood and forecast in relation to the expected operating environ-
ment.
The end-user should not only be concerned with the performance of
the joint immediately after bonding or sealing. The performance of the
joint must also be considered throughout its practical service life. Al-
most all adhesive or sealant systems will undergo some change during
their life. These changes could have a profound effect on the strength
and permanence of the joint.
Unfortunately, substrates and adhesive/sealant materials tend to
change due to external influences from the environment. These
changes could occur: (a) during formation of the joint; and (b) during
aging in service. Not only is the adhesive and adherend subject to
change, but the interphase region could be subject to transformation
as well. These simultaneously occurring, dynamic processes are one
reason why it is so difficult to predict the life of a bonded joint. It may