Page 70 - Handbook of Adhesives and Sealants

38 Chapter One

Figure 1.10 Illustration of poor (top) and good (bottom) wetting by an adhesive spread-
ing over a surface.

In the case of adhesives or sealants, it is important that the material
does not shrink excessively. Otherwise, undesirable internal stresses
could develop in the joint.

1.6.1.4 Forming an impervious joint. Once solidified, the adhesive or
sealant must have adequate strength and toughness to resist failure
under all expected service conditions. To determine the effect of the
environment on the performance of the joint, one must consider the
adhesive or sealant material, the substrate, and the interphase
regions that are formed before, during, and after the bonding process.
The initial performance and the durability of the joint are strongly
dependent on how the substrates are prepared and on the severity of
the service environment. The structure and chemistry of the surface
region of the parts to be joined and their response to service environ-
ments may well govern bond performance. If these surface regions
change significantly during processing of the joint or during service